From a644efe9ff55cb8269244d514018a7c0527b815d Mon Sep 17 00:00:00 2001
From: clawbot <clawbot@noreply.example.org>
Date: Mon, 10 Aug 2026 15:47:49 +0200
Subject: [PATCH 01/22] chore: root .editorconfig and hardened .gitignore
 (closes #15)

---
 backend/.editorconfig => .editorconfig |  0
 .gitignore                             | 27 ++++++++++++++++++++++++--
 TODO.md                                |  7 +++++++
 3 files changed, 32 insertions(+), 2 deletions(-)
 rename backend/.editorconfig => .editorconfig (100%)

diff --git a/backend/.editorconfig b/.editorconfig
similarity index 100%
rename from backend/.editorconfig
rename to .editorconfig
diff --git a/.gitignore b/.gitignore
index 9451024..ee4128a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,27 @@
-node_modules/
-dist/
+# OS
 .DS_Store
+Thumbs.db
+
+# Editors
+*.swp
+*.swo
+*~
+*.bak
+.idea/
+.vscode/
+*.sublime-*
+
+# Node
+node_modules/
+
+# Environment / secrets
+.env
+.env.*
+*.pem
+*.key
+
+# Build output
+dist/
+
+# Logs
 *.log
diff --git a/TODO.md b/TODO.md
index 587c805..2545dc2 100644
--- a/TODO.md
+++ b/TODO.md
@@ -22,6 +22,13 @@ files, so merging it also closes most compliance gaps.
 
 # Completed Steps
 
+- 2026-08-09: dotfile compliance — lifted `backend/.editorconfig` to the repo
+  root so `root = true` covers the frontend too, and replaced `.gitignore` with
+  the org model (OS, editor, node, and environment/secrets sections) plus this
+  repo's `dist/` and `*.log`. `.env`, `.env.*`, `*.pem`, and `*.key` are now
+  ignored repo-wide, not just under `backend/`. Excluding `.git` from
+  `.dockerignore` stays deferred: both images read git metadata at build time
+  (`COPY .git` in `Dockerfile.backend`, `git rev-parse` in `vite.config.js`)
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints, Makefile
   shims, README Entrypoints section
 - 2026-02-27: backend with buffered zstd-compressed report storage; CI workflow
-- 
2.54.0


From 25a852d35c0f2ff29381221db9af8daf0245bd36 Mon Sep 17 00:00:00 2001
From: clawbot <clawbot@noreply.example.org>
Date: Mon, 10 Aug 2026 16:04:48 +0200
Subject: [PATCH 02/22] build: Dockerfile.backend multistage lint stage (closes
 #17)

---
 Dockerfile.backend | 41 +++++++++++++++++++++++++++++++++--------
 TODO.md            |  4 ++++
 backend/Makefile   | 21 +++++++++------------
 3 files changed, 46 insertions(+), 20 deletions(-)

diff --git a/Dockerfile.backend b/Dockerfile.backend
index d97bbbd..e06a617 100644
--- a/Dockerfile.backend
+++ b/Dockerfile.backend
@@ -1,25 +1,50 @@
+# Lint stage — fast feedback on formatting and lint issues. The
+# golangci/golangci-lint image ships Go, gofmt, make and the linter, so
+# nothing is installed here.
+# golangci/golangci-lint:v2.7.2 (2026-08-09)
+FROM golangci/golangci-lint@sha256:5d6d5c70a61f1356adfd9dd6316ce286799fefc9d743421356ff1b00842368ba AS lint
+
+WORKDIR /src
+COPY backend/go.mod backend/go.sum ./
+RUN go mod download
+COPY backend/ .
+RUN make fmt-check
+RUN make lint
+
+# Build stage
 # golang:1.25-alpine (2026-02-27)
 FROM golang:1.25-alpine@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
 
-RUN apk add --no-cache git make gcc musl-dev
+RUN apk add --no-cache make
 
-# golangci-lint v2.7.2 (2026-02-27)
-RUN CGO_ENABLED=0 go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@9f61b0f53f80672872fced07b6874397c3ed197b
+WORKDIR /src
+
+# Force BuildKit to run the lint stage before proceeding. BuildKit runs
+# stages in parallel by default; without this no-op copy a lint failure
+# would not gate compilation.
+COPY --from=lint /src/go.sum /dev/null
 
-WORKDIR /repo/backend
 COPY backend/go.mod backend/go.sum ./
 RUN go mod download
-COPY .git /repo/.git
 COPY backend/ .
 
-RUN make check
-RUN make build
+RUN make test
 
+# The build is driven through the Makefile so there is exactly one
+# definition of the build command; it expands to
+# CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.Version=... -X main.Buildarch=..."
+# VERSION is handed over in the environment rather than as a make
+# variable so it still reaches the build if the target is ever turned
+# into a shim around a script.
+ARG VERSION=dev
+RUN VERSION="${VERSION}" make build
+
+# Runtime stage
 # alpine:3.23 (2026-02-27)
 FROM alpine:3.23@sha256:25109184c71bdad752c8312a8623239686a9a2071e8825f20acb8f2198c3f659
 
 RUN apk add --no-cache ca-certificates
-COPY --from=builder /repo/backend/netwatch-server /usr/local/bin/netwatch-server
+COPY --from=builder /src/netwatch-server /usr/local/bin/netwatch-server
 
 EXPOSE 8080
 ENTRYPOINT ["netwatch-server"]
diff --git a/TODO.md b/TODO.md
index 2545dc2..bed8e9d 100644
--- a/TODO.md
+++ b/TODO.md
@@ -22,6 +22,10 @@ files, so merging it also closes most compliance gaps.
 
 # Completed Steps
 
+- 2026-08-09: `Dockerfile.backend` reworked to the mandated Go multistage
+  lint-stage pattern: separate `lint` stage on the hash-pinned
+  `golangci/golangci-lint` image, `COPY --from=lint` stage dependency,
+  `CGO_ENABLED=0` static build driven by `ARG VERSION`, and no more `COPY .git`
 - 2026-08-09: dotfile compliance — lifted `backend/.editorconfig` to the repo
   root so `root = true` covers the frontend too, and replaced `.gitignore` with
   the org model (OS, editor, node, and environment/secrets sections) plus this
diff --git a/backend/Makefile b/backend/Makefile
index 38cebcf..4660353 100644
--- a/backend/Makefile
+++ b/backend/Makefile
@@ -1,25 +1,22 @@
-UNAME_S := $(shell uname -s)
-VERSION := $(shell git describe --always --dirty)
+# VERSION is overridable (the Dockerfile passes its ARG VERSION in) and
+# degrades to "dev" when git is unavailable or there is no .git — the
+# build must not depend on the repository history being in the build
+# context.
+VERSION ?= $(shell { git describe --always --dirty; } 2>/dev/null || echo dev)
 BUILDARCH := $(shell uname -m)
 BINARY := netwatch-server
 
+GOLDFLAGS += -s -w
 GOLDFLAGS += -X main.Version=$(VERSION)
 GOLDFLAGS += -X main.Buildarch=$(BUILDARCH)
 
-ifeq ($(UNAME_S),Darwin)
-	GOFLAGS := -ldflags "$(GOLDFLAGS)"
-else
-	GOFLAGS = -ldflags "-linkmode external -extldflags -static $(GOLDFLAGS)"
-endif
-
 .PHONY: all build test lint fmt fmt-check check docker hooks run clean
 
 all: build
 
-build: ./$(BINARY)
-
-./$(BINARY): $(shell find . -name '*.go' -type f) go.mod go.sum
-	go build -o $@ $(GOFLAGS) ./cmd/netwatch-server/
+build:
+	CGO_ENABLED=0 go build -trimpath -ldflags "$(GOLDFLAGS)" \
+		-o ./$(BINARY) ./cmd/netwatch-server/
 
 test:
 	timeout 30 go test ./...
-- 
2.54.0


From 852a11eec200f3a3e587a53dc1b339d0aa21e357 Mon Sep 17 00:00:00 2001
From: clawbot <clawbot@noreply.example.org>
Date: Mon, 10 Aug 2026 16:07:28 +0200
Subject: [PATCH 03/22] fix: wrap per-host status line so 320px viewport does
 not scroll (closes #42)

---
 TODO.md     |  2 ++
 src/main.js | 57 +++++++++++++++++++++++++++++++++++------------------
 2 files changed, 40 insertions(+), 19 deletions(-)

diff --git a/TODO.md b/TODO.md
index bed8e9d..0caaa52 100644
--- a/TODO.md
+++ b/TODO.md
@@ -22,6 +22,8 @@ files, so merging it also closes most compliance gaps.
 
 # Completed Steps
 
+- 2026-08-10: per-host status line wraps below the 768px breakpoint instead of
+  forcing horizontal page scroll at 320px
 - 2026-08-09: `Dockerfile.backend` reworked to the mandated Go multistage
   lint-stage pattern: separate `lint` stage on the hash-pinned
   `golangci/golangci-lint` image, `COPY --from=lint` stage dependency,
diff --git a/src/main.js b/src/main.js
index 3ab6bb8..aaa3cdf 100644
--- a/src/main.js
+++ b/src/main.js
@@ -537,6 +537,12 @@ class SparklineRenderer {
 
 // --- UI Renderer -------------------------------------------------------------
 
+// The per-host status line must stay wrappable: its populated content is
+// wider than the host column at a 320px viewport, and `whitespace-nowrap`
+// here overflows the element and forces the whole document to scroll
+// horizontally.
+const STATUS_TEXT_CLASS = "status-text text-xs text-right col-span-2 mt-5";
+
 function hostRowHTML(host, index, showPin = true) {
     const pinColor = host.pinned
         ? "text-blue-500"
@@ -562,7 +568,7 @@ function hostRowHTML(host, index, showPin = true) {
             <span class="text-gray-500">---</span>
           </div>
           <a href="${host.url}" target="_blank" rel="noopener" class="text-xs text-gray-500 truncate block col-span-2 -mt-2">${host.url}</a>
-          <div class="status-text text-xs text-gray-500 whitespace-nowrap text-right col-span-2 mt-5" data-host="${index}">waiting...</div>
+          <div class="${STATUS_TEXT_CLASS} text-gray-500" data-host="${index}">waiting...</div>
         </div>
         <div class="flex-grow sparkline-container rounded overflow-hidden border border-gray-700/30">
           <canvas class="sparkline-canvas w-full" data-host="${index}" height="${CONFIG.canvasHeight}"></canvas>
@@ -688,6 +694,26 @@ function buildUI(state) {
 
 // --- UI Updaters -------------------------------------------------------------
 
+// Renders `min 1ms / med 2ms / avg 3ms / max 4ms`. Each label, value and
+// trailing separator is one unbreakable unit, so wrapping only ever happens
+// between stats and a wrapped line never starts with a separator.
+function statusStatsHTML(stats) {
+    return stats
+        .map(([label, value], i) => {
+            const sep =
+                i < stats.length - 1
+                    ? ` <span class="text-gray-500">/</span>`
+                    : "";
+            return (
+                `<span class="whitespace-nowrap">` +
+                `<span class="text-gray-400">${label} </span>` +
+                `<span class="${latencyClass(value, "online")}">${value}ms</span>` +
+                `${sep}</span>`
+            );
+        })
+        .join(" ");
+}
+
 function updateHostRow(host, index) {
     const latencyEl = document.querySelector(
         `.latency-value[data-host="${index}"]`,
@@ -712,28 +738,22 @@ function updateHostRow(host, index) {
     const min = host.minLatency();
     const max = host.maxLatency();
     if (host.status === "online" && avg !== null) {
-        statusEl.innerHTML =
-            `<span class="text-gray-400">min </span><span class="${latencyClass(min, "online")}">${min}ms</span>` +
-            ` <span class="text-gray-500">/</span> ` +
-            `<span class="text-gray-400">med </span><span class="${latencyClass(med, "online")}">${med}ms</span>` +
-            ` <span class="text-gray-500">/</span> ` +
-            `<span class="text-gray-400">avg </span><span class="${latencyClass(avg, "online")}">${avg}ms</span>` +
-            ` <span class="text-gray-500">/</span> ` +
-            `<span class="text-gray-400">max </span><span class="${latencyClass(max, "online")}">${max}ms</span>`;
-        statusEl.className =
-            "status-text text-xs whitespace-nowrap text-right col-span-2 mt-5";
+        statusEl.innerHTML = statusStatsHTML([
+            ["min", min],
+            ["med", med],
+            ["avg", avg],
+            ["max", max],
+        ]);
+        statusEl.className = STATUS_TEXT_CLASS;
     } else if (host.status === "offline") {
         statusEl.textContent = "unreachable";
-        statusEl.className =
-            "status-text text-xs text-red-400 whitespace-nowrap text-right col-span-2 mt-5";
+        statusEl.className = `${STATUS_TEXT_CLASS} text-red-400`;
     } else if (host.status === "error") {
         statusEl.textContent = "timeout";
-        statusEl.className =
-            "status-text text-xs text-orange-400 whitespace-nowrap text-right col-span-2 mt-5";
+        statusEl.className = `${STATUS_TEXT_CLASS} text-orange-400`;
     } else {
         statusEl.textContent = "connecting...";
-        statusEl.className =
-            "status-text text-xs text-gray-500 whitespace-nowrap text-right col-span-2 mt-5";
+        statusEl.className = `${STATUS_TEXT_CLASS} text-gray-500`;
     }
 
     SparklineRenderer.draw(canvas, host.history);
@@ -1044,8 +1064,7 @@ function greyOutUI(state) {
         }
         if (statusEl) {
             statusEl.textContent = "paused";
-            statusEl.className =
-                "status-text text-xs text-gray-500 whitespace-nowrap text-right col-span-2 mt-5";
+            statusEl.className = `${STATUS_TEXT_CLASS} text-gray-500`;
         }
         // Grey out the status dot
         const row = document.querySelector(`.host-row[data-index="${i}"]`);
-- 
2.54.0


From f7c7f92e27ac3bf6598c832f8f4166fddc30236e Mon Sep 17 00:00:00 2001
From: clawbot <clawbot@noreply.example.org>
Date: Mon, 10 Aug 2026 16:12:00 +0200
Subject: [PATCH 04/22] fix(frontend): meet the 44x44 minimum tap target on
 every control (closes #43)

---
 TODO.md        |  4 ++++
 src/main.js    |  2 +-
 src/styles.css | 35 ++++++++++++++++++++++++++++++++++-
 3 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/TODO.md b/TODO.md
index 0caaa52..1da8930 100644
--- a/TODO.md
+++ b/TODO.md
@@ -22,6 +22,10 @@ files, so merging it also closes most compliance gaps.
 
 # Completed Steps
 
+- 2026-08-10: every interactive control now meets the 44x44 CSS px minimum tap
+  target (`.pin-btn`, `#interval-select`, the debug-log label and, on narrow
+  viewports, `#pause-btn`). The pin button's hit area grows via matching
+  negative margins, so its layout footprint and row density are unchanged
 - 2026-08-10: per-host status line wraps below the 768px breakpoint instead of
   forcing horizontal page scroll at 320px
 - 2026-08-09: `Dockerfile.backend` reworked to the mandated Go multistage
diff --git a/src/main.js b/src/main.js
index aaa3cdf..135d36a 100644
--- a/src/main.js
+++ b/src/main.js
@@ -677,7 +677,7 @@ function buildUI(state) {
         </p>
         <p class="mt-2"><a href="https://git.eeqj.de/sneak/netwatch/commit/${__COMMIT_FULL__}" target="_blank" rel="noopener" class="text-gray-600 hover:text-gray-400">${__COMMIT_HASH__}</a></p>
         <p class="mt-2">
-          <label class="cursor-pointer">
+          <label class="debug-toggle-label cursor-pointer">
             <input type="checkbox" id="debug-toggle" class="mr-1">
             <span>Debug log</span>
           </label>
diff --git a/src/styles.css b/src/styles.css
index 600e453..a232397 100644
--- a/src/styles.css
+++ b/src/styles.css
@@ -14,6 +14,38 @@ body {
         ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
 }
 
+/* ---- Minimum tap targets ----------------------------------------------
+   Every interactive control is at least 44x44 CSS px (Apple HIG, WCAG 2.2
+   SC 2.5.5). Not scoped to a breakpoint or to `pointer: coarse`: a large
+   phone in landscape is above the 768px breakpoint and still a touch
+   device. */
+
+/* The button grows to 44x44 while the negative margins keep its layout
+   footprint at the 16x16 of the icon inside it, so row height and the
+   icon's position are unchanged. */
+.pin-btn {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    width: 2.75rem;
+    height: 2.75rem;
+    margin: -0.875rem;
+}
+
+/* A select paints its own background and border, so it has to actually be
+   44 tall rather than borrow the trick above. */
+#interval-select {
+    min-height: 2.75rem;
+}
+
+/* The tappable target for #debug-toggle is the label wrapping it. */
+.debug-toggle-label {
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    min-height: 2.75rem;
+}
+
 .sparkline-container {
     background: linear-gradient(
         to bottom,
@@ -38,9 +70,10 @@ body {
         gap: 0.75rem;
     }
 
-    /* Pause button: smaller on mobile */
+    /* Pause button: smaller on mobile, but not below the tap-target floor */
     #pause-btn {
         padding: 0.5rem 1rem;
+        min-height: 2.75rem;
     }
 
     #pause-btn svg {
-- 
2.54.0


From f3895789d20883352920fe0bd10beae360152e76 Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Mon, 21 Sep 2026 15:05:25 +0200
Subject: [PATCH 05/22] feat(backend): server hardening: timeouts, security
 headers, trusted-proxy client IP (closes #19)

Add ReadHeaderTimeout and IdleTimeout to the http.Server as named constants beside the existing timeouts. Add a SecurityHeaders middleware (HSTS, a JSON-API CSP of default-src 'none'; frame-ancestors 'none', X-Frame-Options DENY, nosniff, Referrer-Policy, Permissions-Policy), registered before CORS so preflight responses carry it. Resolve the client IP from X-Forwarded-For / X-Real-IP only when the direct peer is in the trusted-proxy allowlist (loopback plus RFC1918 by default, configurable via TRUSTED_PROXIES); an untrusted peer's forwarded headers are ignored. Uses net/netip; no new dependency.

Model: opus-4-8 (implementation and review); claude-fable-5 (merge)
---
 TODO.md                                       |   5 +
 backend/README.md                             |  16 +-
 backend/internal/config/config.go             |  28 ++++
 backend/internal/middleware/export_test.go    |  21 +++
 backend/internal/middleware/middleware.go     | 133 ++++++++++++++++-
 .../internal/middleware/middleware_test.go    | 139 ++++++++++++++++++
 backend/internal/server/http.go               |  20 ++-
 backend/internal/server/routes.go             |   1 +
 8 files changed, 347 insertions(+), 16 deletions(-)
 create mode 100644 backend/internal/middleware/export_test.go
 create mode 100644 backend/internal/middleware/middleware_test.go

diff --git a/TODO.md b/TODO.md
index 1da8930..b284278 100644
--- a/TODO.md
+++ b/TODO.md
@@ -22,6 +22,11 @@ files, so merging it also closes most compliance gaps.
 
 # Completed Steps
 
+- 2026-09-21: backend HTTP hardening (issue #19): added `ReadHeaderTimeout` and
+  `IdleTimeout` to the server, a `SecurityHeaders` middleware (HSTS, tight CSP,
+  frame/sniff/referrer/permissions headers) registered before CORS, and
+  trusted-proxy client IP resolution honouring `X-Forwarded-For` / `X-Real-IP`
+  only from a `TRUSTED_PROXIES` allowlist (loopback plus RFC1918 by default)
 - 2026-08-10: every interactive control now meets the 44x44 CSS px minimum tap
   target (`.pin-btn`, `#interval-select`, the debug-log label and, on narrow
   viewports, `#pause-btn`). The pin button's hit area grows via matching
diff --git a/backend/README.md b/backend/README.md
index a7de988..3cad737 100644
--- a/backend/README.md
+++ b/backend/README.md
@@ -42,11 +42,17 @@ Internal packages in `internal/` follow standard Go project layout:
 
 ### Configuration
 
-| Variable   | Default            | Description                       |
-| ---------- | ------------------ | --------------------------------- |
-| `PORT`     | `8080`             | HTTP listen port                  |
-| `DATA_DIR` | `./data/reports`   | Directory for compressed reports  |
-| `DEBUG`    | `false`            | Enable debug logging              |
+| Variable          | Default              | Description                                                                                              |
+| ----------------- | -------------------- | -------------------------------------------------------------------------------------------------------- |
+| `PORT`            | `8080`               | HTTP listen port                                                                                         |
+| `DATA_DIR`        | `./data/reports`     | Directory for compressed reports                                                                         |
+| `DEBUG`           | `false`              | Enable debug logging                                                                                     |
+| `TRUSTED_PROXIES` | loopback + RFC1918   | Comma-separated CIDRs whose `X-Forwarded-For` / `X-Real-IP` headers are trusted for client IP resolution |
+
+`TRUSTED_PROXIES` defaults to `127.0.0.1/32,::1/128,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`.
+The loopback entries cover the reverse proxy that shares the container; the
+RFC1918 ranges match `nginx.conf`. A request whose direct peer is outside this
+set has its forwarded headers ignored, and the direct peer is logged instead.
 
 ### Report storage
 
diff --git a/backend/internal/config/config.go b/backend/internal/config/config.go
index 6966d57..02a6db9 100644
--- a/backend/internal/config/config.go
+++ b/backend/internal/config/config.go
@@ -5,6 +5,7 @@ package config
 import (
 	"errors"
 	"log/slog"
+	"strings"
 
 	"sneak.berlin/go/netwatch/internal/globals"
 	"sneak.berlin/go/netwatch/internal/logger"
@@ -14,6 +15,14 @@ import (
 	"go.uber.org/fx"
 )
 
+// defaultTrustedProxies lists the networks whose forwarded
+// headers are honoured by default. It covers the RFC1918
+// ranges (to match nginx.conf) plus IPv4 and IPv6 loopback,
+// because the reverse proxy shares the container and reaches
+// the backend over loopback.
+const defaultTrustedProxies = "127.0.0.1/32,::1/128," +
+	"10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"
+
 // Params defines the dependencies for Config.
 type Params struct {
 	fx.In
@@ -30,6 +39,7 @@ type Config struct {
 	MetricsUsername string
 	Port            int
 	SentryDSN       string
+	TrustedProxies  []string
 	log             *slog.Logger
 	params          *Params
 }
@@ -56,6 +66,7 @@ func New(
 	viper.SetDefault("SENTRY_DSN", "")
 	viper.SetDefault("METRICS_USERNAME", "")
 	viper.SetDefault("METRICS_PASSWORD", "")
+	viper.SetDefault("TRUSTED_PROXIES", defaultTrustedProxies)
 
 	err := viper.ReadInConfig()
 	if err != nil {
@@ -73,6 +84,7 @@ func New(
 		MetricsUsername: viper.GetString("METRICS_USERNAME"),
 		Port:            viper.GetInt("PORT"),
 		SentryDSN:       viper.GetString("SENTRY_DSN"),
+		TrustedProxies:  splitList(viper.GetString("TRUSTED_PROXIES")),
 		log:             log,
 		params:          &params,
 	}
@@ -84,3 +96,19 @@ func New(
 
 	return s, nil
 }
+
+// splitList turns a comma-separated setting into a trimmed
+// slice, dropping empty entries.
+func splitList(raw string) []string {
+	parts := strings.Split(raw, ",")
+
+	out := make([]string, 0, len(parts))
+	for _, p := range parts {
+		p = strings.TrimSpace(p)
+		if p != "" {
+			out = append(out, p)
+		}
+	}
+
+	return out
+}
diff --git a/backend/internal/middleware/export_test.go b/backend/internal/middleware/export_test.go
new file mode 100644
index 0000000..d1961c0
--- /dev/null
+++ b/backend/internal/middleware/export_test.go
@@ -0,0 +1,21 @@
+package middleware
+
+import (
+	"net/http"
+	"net/netip"
+)
+
+// Test-only wrappers exposing unexported helpers to the
+// external middleware_test package.
+
+func ClientIP(
+	remoteAddr string,
+	header http.Header,
+	trusted []netip.Prefix,
+) string {
+	return clientIP(remoteAddr, header, trusted)
+}
+
+func ParseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
+	return parseTrustedProxies(cidrs)
+}
diff --git a/backend/internal/middleware/middleware.go b/backend/internal/middleware/middleware.go
index 01cbe8d..5f5194b 100644
--- a/backend/internal/middleware/middleware.go
+++ b/backend/internal/middleware/middleware.go
@@ -3,9 +3,12 @@
 package middleware
 
 import (
+	"fmt"
 	"log/slog"
 	"net"
 	"net/http"
+	"net/netip"
+	"strings"
 	"time"
 
 	"sneak.berlin/go/netwatch/internal/config"
@@ -19,6 +22,15 @@ import (
 
 const corsMaxAgeSec = 300
 
+// Security header values. The backend is a JSON API with no
+// HTML surface, so the CSP forbids every resource type and
+// framing outright.
+const (
+	hstsValue              = "max-age=31536000; includeSubDomains"
+	cspValue               = "default-src 'none'; frame-ancestors 'none'"
+	permissionsPolicyValue = "camera=(), microphone=(), geolocation=()"
+)
+
 // Params defines the dependencies for Middleware.
 type Params struct {
 	fx.In
@@ -30,8 +42,9 @@ type Params struct {
 
 // Middleware holds shared state for middleware factories.
 type Middleware struct {
-	log    *slog.Logger
-	params *Params
+	log            *slog.Logger
+	params         *Params
+	trustedProxies []netip.Prefix
 }
 
 // New creates a Middleware instance.
@@ -39,13 +52,38 @@ func New(
 	_ fx.Lifecycle,
 	params Params,
 ) (*Middleware, error) {
+	trusted, err := parseTrustedProxies(params.Config.TrustedProxies)
+	if err != nil {
+		return nil, err
+	}
+
 	s := new(Middleware)
 	s.params = &params
 	s.log = params.Logger.Get()
+	s.trustedProxies = trusted
 
 	return s, nil
 }
 
+// parseTrustedProxies converts CIDR strings into prefixes,
+// failing fast on any malformed entry.
+func parseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
+	prefixes := make([]netip.Prefix, 0, len(cidrs))
+
+	for _, cidr := range cidrs {
+		prefix, err := netip.ParsePrefix(cidr)
+		if err != nil {
+			return nil, fmt.Errorf(
+				"trusted proxy %q: %w", cidr, err,
+			)
+		}
+
+		prefixes = append(prefixes, prefix.Masked())
+	}
+
+	return prefixes, nil
+}
+
 type loggingResponseWriter struct {
 	http.ResponseWriter
 
@@ -72,6 +110,70 @@ func ipFromHostPort(hostPort string) string {
 	return host
 }
 
+// clientIP resolves the caller's address. X-Forwarded-For and
+// X-Real-IP are honoured only when the direct peer is a
+// trusted proxy; otherwise the direct peer is returned so a
+// spoofed header cannot forge the logged address.
+func clientIP(
+	remoteAddr string,
+	header http.Header,
+	trusted []netip.Prefix,
+) string {
+	peer := ipFromHostPort(remoteAddr)
+
+	if !addrInAny(peer, trusted) {
+		return peer
+	}
+
+	if xff := firstForwardedFor(header.Get("X-Forwarded-For")); xff != "" {
+		return xff
+	}
+
+	if xr := strings.TrimSpace(header.Get("X-Real-IP")); validIP(xr) {
+		return xr
+	}
+
+	return peer
+}
+
+// firstForwardedFor returns the left-most valid address in an
+// X-Forwarded-For list (the original client), or "" if none.
+func firstForwardedFor(value string) string {
+	for part := range strings.SplitSeq(value, ",") {
+		candidate := strings.TrimSpace(part)
+		if validIP(candidate) {
+			return candidate
+		}
+	}
+
+	return ""
+}
+
+func validIP(s string) bool {
+	_, err := netip.ParseAddr(s)
+
+	return err == nil
+}
+
+// addrInAny reports whether s parses as an address contained
+// in any of the trusted prefixes.
+func addrInAny(s string, trusted []netip.Prefix) bool {
+	addr, err := netip.ParseAddr(s)
+	if err != nil {
+		return false
+	}
+
+	addr = addr.Unmap()
+
+	for _, prefix := range trusted {
+		if prefix.Contains(addr) {
+			return true
+		}
+	}
+
+	return false
+}
+
 // Logging returns middleware that logs each request with
 // timing, status code, and client information.
 func (s *Middleware) Logging() func(http.Handler) http.Handler {
@@ -96,7 +198,11 @@ func (s *Middleware) Logging() func(http.Handler) http.Handler {
 						"referer", r.Referer(),
 						"proto", r.Proto,
 						"remote_ip",
-						ipFromHostPort(r.RemoteAddr),
+						clientIP(
+							r.RemoteAddr,
+							r.Header,
+							s.trustedProxies,
+						),
 						"status", lrw.statusCode,
 						"latency_ms",
 						latency.Milliseconds(),
@@ -109,6 +215,27 @@ func (s *Middleware) Logging() func(http.Handler) http.Handler {
 	}
 }
 
+// SecurityHeaders returns middleware that sets response
+// security headers. It runs before CORS so the headers are
+// present on preflight responses the CORS handler writes.
+func (s *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(
+			func(w http.ResponseWriter, r *http.Request) {
+				h := w.Header()
+				h.Set("Strict-Transport-Security", hstsValue)
+				h.Set("Content-Security-Policy", cspValue)
+				h.Set("X-Frame-Options", "DENY")
+				h.Set("X-Content-Type-Options", "nosniff")
+				h.Set("Referrer-Policy", "no-referrer")
+				h.Set("Permissions-Policy", permissionsPolicyValue)
+
+				next.ServeHTTP(w, r)
+			},
+		)
+	}
+}
+
 // CORS returns middleware that adds permissive CORS headers.
 func (s *Middleware) CORS() func(http.Handler) http.Handler {
 	return cors.Handler(cors.Options{
diff --git a/backend/internal/middleware/middleware_test.go b/backend/internal/middleware/middleware_test.go
new file mode 100644
index 0000000..2965593
--- /dev/null
+++ b/backend/internal/middleware/middleware_test.go
@@ -0,0 +1,139 @@
+package middleware_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"testing"
+
+	"sneak.berlin/go/netwatch/internal/middleware"
+)
+
+func mustPrefixes(t *testing.T, cidrs ...string) []netip.Prefix {
+	t.Helper()
+
+	prefixes, err := middleware.ParseTrustedProxies(cidrs)
+	if err != nil {
+		t.Fatalf("ParseTrustedProxies(%v): %v", cidrs, err)
+	}
+
+	return prefixes
+}
+
+func TestParseTrustedProxiesRejectsMalformed(t *testing.T) {
+	t.Parallel()
+
+	_, err := middleware.ParseTrustedProxies([]string{"not-a-cidr"})
+	if err == nil {
+		t.Fatal("expected error for malformed CIDR, got nil")
+	}
+}
+
+type clientIPCase struct {
+	name       string
+	remoteAddr string
+	xff        string
+	xRealIP    string
+	want       string
+}
+
+func clientIPCases() []clientIPCase {
+	return []clientIPCase{
+		{
+			name:       "trusted proxy uses forwarded-for",
+			remoteAddr: "127.0.0.1:5000",
+			xff:        "203.0.113.7",
+			want:       "203.0.113.7",
+		},
+		{
+			name:       "trusted proxy uses left-most of chain",
+			remoteAddr: "10.1.2.3:5000",
+			xff:        "203.0.113.7, 10.1.2.3",
+			want:       "203.0.113.7",
+		},
+		{
+			name:       "trusted proxy falls back to x-real-ip",
+			remoteAddr: "127.0.0.1:5000",
+			xRealIP:    "203.0.113.9",
+			want:       "203.0.113.9",
+		},
+		{
+			name:       "untrusted peer ignores forwarded-for",
+			remoteAddr: "198.51.100.4:5000",
+			xff:        "203.0.113.7",
+			want:       "198.51.100.4",
+		},
+		{
+			name:       "untrusted peer ignores x-real-ip",
+			remoteAddr: "198.51.100.4:5000",
+			xRealIP:    "203.0.113.9",
+			want:       "198.51.100.4",
+		},
+		{
+			name:       "trusted proxy with no headers uses peer",
+			remoteAddr: "10.1.2.3:5000",
+			want:       "10.1.2.3",
+		},
+		{
+			name:       "trusted proxy with garbage header uses peer",
+			remoteAddr: "127.0.0.1:5000",
+			xff:        "not-an-ip",
+			want:       "127.0.0.1",
+		},
+	}
+}
+
+func TestClientIP(t *testing.T) {
+	t.Parallel()
+
+	trusted := mustPrefixes(t, "127.0.0.1/32", "::1/128", "10.0.0.0/8")
+
+	for _, tc := range clientIPCases() {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			header := http.Header{}
+			if tc.xff != "" {
+				header.Set("X-Forwarded-For", tc.xff)
+			}
+
+			if tc.xRealIP != "" {
+				header.Set("X-Real-IP", tc.xRealIP)
+			}
+
+			got := middleware.ClientIP(tc.remoteAddr, header, trusted)
+			if got != tc.want {
+				t.Errorf("ClientIP() = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestSecurityHeaders(t *testing.T) {
+	t.Parallel()
+
+	handler := (&middleware.Middleware{}).SecurityHeaders()(
+		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		}),
+	)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/", http.NoBody)
+	handler.ServeHTTP(rec, req)
+
+	want := map[string]string{
+		"Strict-Transport-Security": "max-age=31536000; includeSubDomains",
+		"Content-Security-Policy":   "default-src 'none'; frame-ancestors 'none'",
+		"X-Frame-Options":           "DENY",
+		"X-Content-Type-Options":    "nosniff",
+		"Referrer-Policy":           "no-referrer",
+		"Permissions-Policy":        "camera=(), microphone=(), geolocation=()",
+	}
+
+	for name, value := range want {
+		if got := rec.Header().Get(name); got != value {
+			t.Errorf("header %s = %q, want %q", name, got, value)
+		}
+	}
+}
diff --git a/backend/internal/server/http.go b/backend/internal/server/http.go
index d3824c6..a0d3c68 100644
--- a/backend/internal/server/http.go
+++ b/backend/internal/server/http.go
@@ -8,20 +8,24 @@ import (
 )
 
 const (
-	readTimeout    = 10 * time.Second
-	writeTimeout   = 10 * time.Second
-	maxHeaderBytes = 1 << 20 // 1 MiB
+	readTimeout       = 10 * time.Second
+	readHeaderTimeout = 5 * time.Second
+	writeTimeout      = 10 * time.Second
+	idleTimeout       = 60 * time.Second
+	maxHeaderBytes    = 1 << 20 // 1 MiB
 )
 
 func (s *Server) serveUntilShutdown() {
 	listenAddr := fmt.Sprintf(":%d", s.params.Config.Port)
 
 	s.httpServer = &http.Server{
-		Addr:           listenAddr,
-		Handler:        s,
-		MaxHeaderBytes: maxHeaderBytes,
-		ReadTimeout:    readTimeout,
-		WriteTimeout:   writeTimeout,
+		Addr:              listenAddr,
+		Handler:           s,
+		MaxHeaderBytes:    maxHeaderBytes,
+		ReadTimeout:       readTimeout,
+		ReadHeaderTimeout: readHeaderTimeout,
+		WriteTimeout:      writeTimeout,
+		IdleTimeout:       idleTimeout,
 	}
 
 	s.SetupRoutes()
diff --git a/backend/internal/server/routes.go b/backend/internal/server/routes.go
index da74cf6..5f2dd8b 100644
--- a/backend/internal/server/routes.go
+++ b/backend/internal/server/routes.go
@@ -17,6 +17,7 @@ func (s *Server) SetupRoutes() {
 	s.router.Use(middleware.Recoverer)
 	s.router.Use(middleware.RequestID)
 	s.router.Use(s.mw.Logging())
+	s.router.Use(s.mw.SecurityHeaders())
 	s.router.Use(s.mw.CORS())
 	s.router.Use(middleware.Timeout(requestTimeout))
 
-- 
2.54.0


From 14eb376d792eaa83ccbca6637800979f2231bc3d Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Mon, 21 Sep 2026 18:29:20 +0200
Subject: [PATCH 06/22] test: automated responsive-layout harness (closes #13)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

`make frontend-viewport-test` builds `dist/`, serves it from the same
digest-pinned nginx image and nginx.conf the shipping container uses, and
drives a digest-pinned headless Chrome over CDP. Viewport widths are derived
from the app's own @media breakpoints rather than a list of phone models: each
breakpoint is tested one pixel below, on, and above, plus four anchor
viewports. Assertions are on computed layout — horizontal overflow, off-screen
elements, clipped text, 44x44 tap targets, host-row reflow — not screenshots,
and each check declares the minimum elements it must find so a stale selector
fails instead of passing blind against a page it is not measuring. Kept out of
`make check`: it needs Docker and takes minutes. Proven able to fail before
being trusted.

Model: opus-4-8
---
 .dockerignore                 |   1 +
 .gitignore                    |   1 +
 .prettierignore               |   1 +
 Makefile                      |   9 +-
 README.md                     |  14 ++
 TODO.md                       |  11 ++
 package.json                  |   1 +
 script/frontend-viewport-test | 102 ++++++++++++++
 test/viewport/README.md       | 112 +++++++++++++++
 test/viewport/checks.js       | 245 ++++++++++++++++++++++++++++++++
 test/viewport/facts.js        | 184 ++++++++++++++++++++++++
 test/viewport/harness.js      | 258 ++++++++++++++++++++++++++++++++++
 test/viewport/viewports.js    | 224 +++++++++++++++++++++++++++++
 yarn.lock                     | 154 +++++++++++++++++++-
 14 files changed, 1315 insertions(+), 2 deletions(-)
 create mode 100755 script/frontend-viewport-test
 create mode 100644 test/viewport/README.md
 create mode 100644 test/viewport/checks.js
 create mode 100644 test/viewport/facts.js
 create mode 100644 test/viewport/harness.js
 create mode 100644 test/viewport/viewports.js

diff --git a/.dockerignore b/.dockerignore
index 27fac04..416281f 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -1,5 +1,6 @@
 node_modules
 dist
+tmp
 .DS_Store
 *.log
 .claude
diff --git a/.gitignore b/.gitignore
index ee4128a..e676a31 100644
--- a/.gitignore
+++ b/.gitignore
@@ -22,6 +22,7 @@ node_modules/
 
 # Build output
 dist/
+tmp/
 
 # Logs
 *.log
diff --git a/.prettierignore b/.prettierignore
index d1a0b78..d0374bc 100644
--- a/.prettierignore
+++ b/.prettierignore
@@ -1,5 +1,6 @@
 backend/
 dist/
 node_modules/
+tmp/
 yarn.lock
 .claude/
diff --git a/Makefile b/Makefile
index 353f708..20f313f 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,5 @@
-.PHONY: bootstrap setup dev test lint fmt fmt-check check docker hooks
+.PHONY: bootstrap setup dev test lint fmt fmt-check check \
+	frontend-viewport-test docker hooks
 
 # Standard targets are thin shims; the implementations live in script/
 # per the scripts-to-rule-them-all pattern (see the Entrypoints section
@@ -28,6 +29,12 @@ fmt-check:
 check:
 	@script/check
 
+# Responsive-layout verification in a containerised browser. Kept out of
+# check: it needs Docker and takes minutes, where make test has to stay
+# under 20 seconds.
+frontend-viewport-test:
+	@script/frontend-viewport-test
+
 docker:
 	@script/docker
 
diff --git a/README.md b/README.md
index d4dbdd4..1ecaa46 100644
--- a/README.md
+++ b/README.md
@@ -41,11 +41,25 @@ provide:
 - `script/fmt` — format all files (writes)
 - `script/fmt-check` — check formatting (read-only)
 - `script/check` — run test, lint, and fmt-check
+- `script/frontend-viewport-test` — responsive-layout verification of the built
+  frontend in a containerised headless Chrome (see
+  [test/viewport/README.md](test/viewport/README.md)). Not part of
+  `script/check`: it needs Docker and takes minutes.
 - `script/docker` — build the Docker image tagged via `script/projectname`
 - `script/cibuild` — CI entrypoint: plain `docker build .`
 - `script/precommit` — run by the git pre-commit hook; runs `script/check`
 - `script/install-precommit` — install the git pre-commit hook
 
+## Responsive layout
+
+The narrow-viewport layout lives in the `max-width: 768px` media block in
+`src/styles.css`. It is verified automatically by `make frontend-viewport-test`,
+which drives a digest-pinned headless Chrome against the built `dist/` and
+asserts on computed layout at widths derived from that CSS — one pixel either
+side of every breakpoint it declares, plus a 320px floor, a desktop baseline and
+two landscape sizes. See [test/viewport/README.md](test/viewport/README.md) for
+what it covers and what it genuinely cannot.
+
 ## Rationale
 
 When debugging network issues, it's useful to have a persistent at-a-glance view
diff --git a/TODO.md b/TODO.md
index b284278..29d1782 100644
--- a/TODO.md
+++ b/TODO.md
@@ -44,6 +44,14 @@ files, so merging it also closes most compliance gaps.
   ignored repo-wide, not just under `backend/`. Excluding `.git` from
   `.dockerignore` stays deferred: both images read git metadata at build time
   (`COPY .git` in `Dockerfile.backend`, `git rev-parse` in `vite.config.js`)
+- 2026-08-09: automated responsive-layout harness
+  (`make frontend-viewport-test`): digest-pinned headless Chrome driven over CDP
+  against the built `dist/`, viewport widths derived from the breakpoints in
+  `src/styles.css` ([#13](https://git.eeqj.de/sneak/netwatch/issues/13)). Every
+  check carries a presence guard so none of them can pass against a page it is
+  not actually measuring. Found two real layout defects, filed as
+  [#42](https://git.eeqj.de/sneak/netwatch/issues/42) and
+  [#43](https://git.eeqj.de/sneak/netwatch/issues/43)
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints, Makefile
   shims, README Entrypoints section
 - 2026-02-27: backend with buffered zstd-compressed report storage; CI workflow
@@ -61,6 +69,9 @@ files, so merging it also closes most compliance gaps.
 
 # Future Steps
 
+- Wire `script/frontend-viewport-test` into CI as its own step (deliberately not
+  part of `make check` today; the decision has real CI-runtime cost and is
+  tracked separately)
 - Compliance top-up as one small commit: add .editorconfig and add the hooks
   target to the Makefile
 - After merge, confirm .gitea/workflows/check.yml is on main and CI is green
diff --git a/package.json b/package.json
index a5e3225..f4cfeaf 100644
--- a/package.json
+++ b/package.json
@@ -14,6 +14,7 @@
         "autoprefixer": "^10.4.23",
         "postcss": "^8.5.6",
         "prettier": "^3.8.1",
+        "puppeteer-core": "25.5.0",
         "tailwindcss": "^4.1.18",
         "vite": "^7.3.1"
     }
diff --git a/script/frontend-viewport-test b/script/frontend-viewport-test
new file mode 100755
index 0000000..aa1b2f6
--- /dev/null
+++ b/script/frontend-viewport-test
@@ -0,0 +1,102 @@
+#!/bin/sh
+# script/frontend-viewport-test: verify the responsive layout of the built
+# frontend in a real browser engine.
+#
+# Builds dist/, serves it with the same nginx image and the same nginx.conf
+# the shipping container uses, points a containerised headless Chrome at it
+# over CDP, and asserts on computed layout at every viewport width derived
+# from the app's own CSS. See test/viewport/README.md for what this covers
+# and what it cannot.
+#
+# Deliberately not part of script/check: it needs Docker and takes far
+# longer than the 20s budget make test has to stay inside.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+# chromedp/headless-shell 151.0.7922.109, 2026-08-09
+BROWSER_IMAGE="chromedp/headless-shell@sha256:2d349b544a1ea6b5b5fd7c0fe99215ff662339c57407ee2e8c0a11af93516b04"
+# nginx:stable-alpine, 2026-02-22 (the digest Dockerfile ships)
+SERVER_IMAGE="nginx@sha256:15e96e59aa3b0aada3a121296e3bce117721f42d88f5f64217ef4b18f458c6ab"
+# node:22-alpine, 2026-02-22 (the digest Dockerfile builds with)
+NODE_IMAGE="node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34"
+
+RUN_ID="$$-$(date +%s)"
+NETWORK="netwatch-viewport-$RUN_ID"
+SERVER="netwatch-viewport-server-$RUN_ID"
+BROWSER="netwatch-viewport-browser-$RUN_ID"
+HARNESS="netwatch-viewport-harness-$RUN_ID"
+ARTIFACT_DIR="$ROOT/tmp/viewport"
+
+# Every container is named and removed here, including the harness itself:
+# `timeout` below kills the `docker run` client, not the container it
+# started, and an unnamed survivor keeps the --internal network in use so
+# `docker network rm` fails too. This host runs many sessions at once and
+# neither may be left behind.
+cleanup() {
+    docker rm -f "$HARNESS" > /dev/null 2>&1 || true
+    docker rm -f "$BROWSER" > /dev/null 2>&1 || true
+    docker rm -f "$SERVER" > /dev/null 2>&1 || true
+    docker network rm "$NETWORK" > /dev/null 2>&1 || true
+}
+trap cleanup EXIT INT TERM
+
+main() {
+    cd "$ROOT"
+
+    # Test what ships: the production build, not a dev server.
+    "$ROOT/script/test"
+    if [ ! -f "$ROOT/dist/index.html" ]; then
+        echo "frontend-viewport-test: dist/index.html missing after build" >&2
+        exit 1
+    fi
+
+    mkdir -p "$ARTIFACT_DIR"
+
+    # An --internal network has no route off the host, so the browser
+    # cannot reach the real internet no matter what the page asks for.
+    # Latency probes are answered by the harness instead. This also means
+    # no port can be published from it, which is why the harness itself
+    # runs as a third container on the same network rather than on the
+    # host.
+    docker network create --internal "$NETWORK" > /dev/null
+
+    docker run -d --rm --name "$SERVER" \
+        --network "$NETWORK" --network-alias netwatch \
+        -v "$ROOT/dist:/usr/share/nginx/html:ro" \
+        -v "$ROOT/nginx.conf:/etc/nginx/conf.d/default.conf:ro" \
+        "$SERVER_IMAGE" > /dev/null
+
+    # The image's own entrypoint already exposes CDP on 9222 and passes
+    # --no-sandbox, so only extra flags belong here; re-specifying the
+    # debugging port collides with it and leaves the endpoint bound to
+    # loopback only. --hide-scrollbars keeps innerWidth equal to
+    # clientWidth, so the overflow assertion has no scrollbar-sized slack
+    # to hide behind, and matches the overlay scrollbars phones use.
+    docker run -d --rm --name "$BROWSER" --init --shm-size=1g \
+        --network "$NETWORK" \
+        "$BROWSER_IMAGE" \
+        --hide-scrollbars \
+        > /dev/null
+
+    # Chrome refuses DevTools requests whose Host header is neither
+    # localhost nor an IP address, so dial the container by address rather
+    # than by its network alias.
+    browser_ip="$(docker inspect \
+        -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' \
+        "$BROWSER")"
+
+    timeout 900 docker run --rm --init --name "$HARNESS" \
+        --network "$NETWORK" \
+        --user "$(id -u):$(id -g)" \
+        -v "$ROOT:/app" \
+        -w /app \
+        -e NETWATCH_ROOT=/app \
+        -e NETWATCH_BASE_URL=http://netwatch:8080 \
+        -e "NETWATCH_CDP_URL=http://$browser_ip:9222" \
+        -e NETWATCH_ARTIFACT_DIR=/app/tmp/viewport \
+        "$NODE_IMAGE" \
+        node test/viewport/harness.js
+}
+
+main "$@"
diff --git a/test/viewport/README.md b/test/viewport/README.md
new file mode 100644
index 0000000..d37a688
--- /dev/null
+++ b/test/viewport/README.md
@@ -0,0 +1,112 @@
+# Responsive-layout harness
+
+Automated verification of the responsive layout that landed in #5. Run it with:
+
+```bash
+make frontend-viewport-test
+```
+
+It builds `dist/`, serves it from the same digest-pinned `nginx` image and the
+same `nginx.conf` the shipping container uses, drives a digest-pinned headless
+Chrome against it over CDP, and asserts on computed layout at every viewport
+width derived from the app's own CSS. Screenshots land in `tmp/viewport/`
+alongside a `results.json`; they are artifacts for a human to look at when
+something fails, not the evidence. The assertions are the evidence.
+
+The target is deliberately outside `make check`: it needs Docker and takes
+minutes, and `make test` has to stay under 20 seconds.
+
+## How the widths are chosen
+
+Not from a list of phone models. `viewports.js` parses the `@media` conditions
+out of `src/styles.css` and scans `src/main.js` and `index.html` for Tailwind
+responsive prefixes, then tests every breakpoint it finds at one pixel below it,
+exactly on it, and one pixel above it. A generic 375px "phone" test sails
+straight past an off-by-one at a media query boundary; `max-width: 768px`
+matches _at_ 768, and the sweep pins down which side of that line each layout is
+on.
+
+Nothing hardcodes 768. Add a second media block or start using `md:` classes and
+the new breakpoint is covered without this directory being touched. The app is
+desktop-first today (all narrow rules live in `max-width` blocks); a
+`min-width`-only, mobile-first set is handled as its inverse, and a set that
+mixes the two makes the run fail loudly rather than test the right widths with
+the wrong expectation. Four further viewports are fixed anchors, each with a
+stated reason: a 320px floor, a 1280px desktop baseline, and two phone-landscape
+sizes straddling the breakpoint for the rotation case.
+
+## What it asserts
+
+- **app-rendered** — enough host rows exist and enough of them show a numeric
+  latency. This one exists so the rest cannot pass vacuously against a blank
+  page.
+- **no-horizontal-overflow** — `documentElement.scrollWidth` fits the layout
+  viewport, with the widest offending element named.
+- **nothing-past-viewport-edge** — no visible element's box extends past the
+  viewport edge.
+- **no-clipped-text** — nothing hides text behind `overflow: hidden`. Deliberate
+  ellipsis truncation (Tailwind's `truncate`, used on host names and URLs) is
+  excluded: it is a design choice, not breakage.
+- **tap-targets-44px** — every interactive control is at least 44x44 CSS px on
+  touch viewports, _and_ each selector in the control list matched at least the
+  number of visible elements it declares. The second half is what stops the
+  check passing vacuously: with size alone, a renamed class would take its
+  controls out of the measured set and the check would report "all 0 controls
+  are at least 44x44" and pass. See below.
+- **host-rows-stacked / host-rows-side-by-side** — the rows genuinely reflow.
+  Computed `flex-direction` _and_ the actual geometry are checked, and in the
+  narrow layout the info block and the sparkline must each occupy essentially
+  the full row width. A row that merely shrank its 420px column would fail.
+- **probing-still-runs / gateway-detection-still-runs** — narrow viewports keep
+  probing and keep detecting the gateway. The mobile early-return path proposed
+  in #8 was rejected; this is what would catch it coming back.
+
+### The tap-target threshold
+
+44x44 CSS px. That is the figure in Apple's Human Interface Guidelines and in
+WCAG 2.2 SC 2.5.5 "Target Size (Enhanced)". WCAG 2.2 SC 2.5.8 (level AA) sets a
+lower 24x24 floor, but that floor comes with a spacing exception these controls
+do not qualify for — the pin buttons sit directly against the host name they
+belong to.
+
+## Determinism
+
+The browser container runs on an `--internal` docker network and has no route to
+the internet, so the app's latency probes cannot reach anything real. The
+harness answers them itself from a fixed delay table, with a deterministic
+fraction failed outright, so the rows render a realistic spread of one-, two-
+and three-digit latencies plus some unreachable rows. That spread is what the
+layout has to survive; 24 identical `---` placeholders would not exercise it.
+
+## What this cannot verify
+
+Real limits, so nobody re-parks this issue as needing hardware:
+
+- **Non-Chromium engines.** This is Chrome. iOS Safari is WebKit and cannot be
+  emulated by it; Safari-specific bugs (viewport units under a collapsing URL
+  bar, `-webkit-fill-available`, form control metrics) will not show up here.
+- **Real touch input.** `hasTouch` emulation changes what the page is told, not
+  how a finger behaves. Gesture handling, scroll momentum, double-tap zoom and
+  hover-state fallbacks on touch are out of scope.
+- **Physical pixel density and rendering.** `deviceScaleFactor` is set, but
+  subpixel antialiasing, OLED colour rendering and actual legibility at a given
+  physical size are not measurable here.
+- **Fonts.** The container has DejaVu, not the platform's own UI monospace. Text
+  metrics are therefore close to, but not identical to, a real device — a layout
+  that fits here by a few pixels might not there.
+- **On-device performance.** Canvas sparkline redraw cost, battery, and
+  behaviour on a slow radio are not measured.
+- **Browser chrome.** The address bar, safe-area insets and notch cutouts are
+  not simulated.
+
+Everything else this issue was actually about — does the layout reflow, does
+anything overflow, is content clipped, are the controls big enough — is a
+function of viewport width and CSS, and is covered above.
+
+## Relation to the unit test framework (#21)
+
+Complementary layers, not two stacks. `vitest` (#21) will exercise module-level
+logic in-process with no browser. This harness exercises rendered layout in a
+real engine and is the only thing here that can see a media query. Neither
+replaces the other; assertions about computed styles and element geometry belong
+here, assertions about functions belong in `vitest`.
diff --git a/test/viewport/checks.js b/test/viewport/checks.js
new file mode 100644
index 0000000..14c7ebf
--- /dev/null
+++ b/test/viewport/checks.js
@@ -0,0 +1,245 @@
+// Pass/fail decisions for the responsive-layout harness.
+//
+// Kept in node rather than in the page so that a failure can be reported
+// with the measurements that produced it. Every check runs at every
+// viewport; none of them short-circuits, so one failure does not hide the
+// rest.
+
+// Minimum tap target, in CSS pixels. 44x44 is the figure in Apple's Human
+// Interface Guidelines and in WCAG 2.2 SC 2.5.5 "Target Size (Enhanced)".
+// WCAG 2.2 SC 2.5.8 (level AA) sets a lower 24x24 floor, but that floor
+// comes with a spacing exception these controls do not qualify for: the
+// pin buttons sit directly against the host name they belong to. Held at
+// 44 deliberately.
+export const MIN_TAP_TARGET_PX = 44;
+
+// The controls named in the definition of done, plus the pause button.
+// Each carries the smallest number of *visible* instances the page has to
+// contain for the tap-target oracle to be measuring anything at all.
+//
+// Without those floors the check is inert: `undersized` is empty both when
+// every control is large enough and when the selectors have gone stale and
+// matched nothing, and the pass condition cannot tell those apart. A single
+// combined floor would not be enough either — 26 pin buttons would cover
+// for all three singleton controls vanishing at once — so the floor is per
+// selector, and one stale selector out of four fails the check.
+export const INTERACTIVE_CONTROLS = [
+    { selector: "#pause-btn", minCount: 1 },
+    { selector: "#interval-select", minCount: 1 },
+    // One per pinnable host row. `app-rendered` already requires at least
+    // 10 host rows, so a count below that means the pin buttons stopped
+    // being rendered per row rather than that there were fewer hosts.
+    { selector: ".pin-btn", minCount: 10 },
+    { selector: "#debug-toggle", minCount: 1 },
+];
+
+export const INTERACTIVE_SELECTORS = INTERACTIVE_CONTROLS.map(
+    (control) => control.selector,
+);
+
+// A host row is only "reflowed" if it stacked *and* went full width.
+// A row that merely shrank its 420px info column would keep
+// flex-direction: row, and a row that stacked but left the info column at
+// its fixed width would fail the width test.
+const FULL_WIDTH_FRACTION = 0.9;
+
+function summarise(items, format, limit = 3) {
+    const shown = items.slice(0, limit).map(format).join("; ");
+    const rest = items.length > limit ? ` (+${items.length - limit} more)` : "";
+    return shown + rest;
+}
+
+// Collapse an overflow report to the elements actually responsible.
+// Identical elements (24 host rows all doing the same thing) are counted
+// rather than listed, and the deepest ones come first, since every
+// ancestor of an overflowing element also reports as overflowing.
+function deepestOffenders(entries) {
+    const byElement = new Map();
+    for (const entry of entries) {
+        const reach = entry.reach ?? entry.right;
+        const existing = byElement.get(entry.el);
+        if (existing) {
+            existing.count += 1;
+            existing.reach = Math.max(existing.reach, reach);
+        } else {
+            byElement.set(entry.el, { ...entry, reach, count: 1 });
+        }
+    }
+    return [...byElement.values()].sort(
+        (a, b) => b.depth - a.depth || b.reach - a.reach,
+    );
+}
+
+function checkRowLayout(row, expectStacked) {
+    if (expectStacked) {
+        if (row.flexDirection !== "column") {
+            return `row ${row.index}: flex-direction is ${row.flexDirection}, expected column`;
+        }
+        if (row.sparkline.top < row.info.bottom - 1) {
+            return `row ${row.index}: sparkline top ${row.sparkline.top} is above info bottom ${row.info.bottom} — still side by side`;
+        }
+        const minWidth = row.containerWidth * FULL_WIDTH_FRACTION;
+        if (row.info.width < minWidth) {
+            return `row ${row.index}: info block is ${row.info.width}px of ${row.containerWidth}px — shrunk, not reflowed`;
+        }
+        if (row.sparkline.width < minWidth) {
+            return `row ${row.index}: sparkline is ${row.sparkline.width}px of ${row.containerWidth}px — shrunk, not reflowed`;
+        }
+        return null;
+    }
+
+    if (row.flexDirection !== "row") {
+        return `row ${row.index}: flex-direction is ${row.flexDirection}, expected row`;
+    }
+    if (row.sparkline.left < row.info.right - 1) {
+        return `row ${row.index}: sparkline left ${row.sparkline.left} overlaps info right ${row.info.right} — not side by side`;
+    }
+    return null;
+}
+
+export function evaluateChecks(facts, viewport, probes) {
+    const checks = [];
+    const check = (name, ok, detail) => checks.push({ name, ok, detail });
+
+    // Guard against the whole harness passing vacuously because the page
+    // never rendered. Everything below is only meaningful if this holds.
+    check(
+        "app-rendered",
+        facts.rowCount >= 10 && facts.numericLatencies >= 5,
+        `${facts.rowCount} host rows, ${facts.numericLatencies} showing a numeric latency`,
+    );
+
+    const viewportWidth = Math.min(facts.innerWidth, facts.documentClientWidth);
+    const culprits = deepestOffenders([
+        ...facts.overflowing,
+        ...facts.contentOverflowing,
+    ]);
+    check(
+        "no-horizontal-overflow",
+        facts.documentScrollWidth <= viewportWidth,
+        `documentElement.scrollWidth ${facts.documentScrollWidth} vs viewport ${viewportWidth}` +
+            (culprits.length === 0
+                ? ""
+                : "; widest content: " +
+                  summarise(
+                      culprits,
+                      (c) =>
+                          `${c.el} reaches ${Math.round(c.reach)}px${c.count > 1 ? ` (x${c.count})` : ""}`,
+                  )),
+    );
+
+    check(
+        "nothing-past-viewport-edge",
+        facts.overflowing.length === 0,
+        facts.overflowing.length === 0
+            ? "no element extends past the viewport"
+            : `${facts.overflowing.length} element(s) past the edge: ` +
+                  summarise(
+                      facts.overflowing,
+                      (o) => `${o.el} spans ${o.left}..${o.right}`,
+                  ),
+    );
+
+    check(
+        "no-clipped-text",
+        facts.clipped.length === 0,
+        facts.clipped.length === 0
+            ? "no element hides text behind overflow (deliberate ellipsis excluded)"
+            : `${facts.clipped.length} element(s) clipping text: ` +
+                  summarise(
+                      facts.clipped,
+                      (c) =>
+                          `${c.el} scrollWidth ${c.scrollWidth} > clientWidth ${c.clientWidth}`,
+                  ),
+    );
+
+    if (viewport.touch) {
+        // Presence first: a selector that matches nothing contributes no
+        // undersized targets, so without this the check would report
+        // "all 0 controls are at least 44x44" and pass.
+        const seen = new Map();
+        for (const target of facts.tapTargets) {
+            seen.set(target.selector, (seen.get(target.selector) ?? 0) + 1);
+        }
+        const missing = INTERACTIVE_CONTROLS.filter(
+            (control) => (seen.get(control.selector) ?? 0) < control.minCount,
+        );
+
+        const undersized = facts.tapTargets.filter(
+            (t) => t.width < MIN_TAP_TARGET_PX || t.height < MIN_TAP_TARGET_PX,
+        );
+        const bySelector = new Map();
+        for (const target of undersized) {
+            const existing = bySelector.get(target.selector);
+            if (!existing || target.width * target.height < existing.area) {
+                bySelector.set(target.selector, {
+                    ...target,
+                    area: target.width * target.height,
+                    count: (existing?.count ?? 0) + 1,
+                });
+            } else {
+                existing.count += 1;
+            }
+        }
+        const detail = [];
+        if (missing.length > 0) {
+            detail.push(
+                "oracle is not measuring the page: " +
+                    summarise(
+                        missing,
+                        (c) =>
+                            `${c.selector} matched ${seen.get(c.selector) ?? 0} visible element(s), expected at least ${c.minCount}`,
+                        4,
+                    ),
+            );
+        }
+        detail.push(
+            undersized.length === 0
+                ? `${facts.tapTargets.length} controls measured, all at least ${MIN_TAP_TARGET_PX}x${MIN_TAP_TARGET_PX}`
+                : `${undersized.length} of ${facts.tapTargets.length} controls below ${MIN_TAP_TARGET_PX}x${MIN_TAP_TARGET_PX}: ` +
+                      summarise(
+                          [...bySelector.values()],
+                          (t) =>
+                              `${t.selector} ${t.width}x${t.height}${t.count > 1 ? ` (x${t.count})` : ""}`,
+                          4,
+                      ),
+        );
+        check(
+            `tap-targets-${MIN_TAP_TARGET_PX}px`,
+            missing.length === 0 && undersized.length === 0,
+            detail.join("; "),
+        );
+    }
+
+    const badRows = facts.rows
+        .map((row) => checkRowLayout(row, viewport.expectStacked))
+        .filter(Boolean);
+    check(
+        viewport.expectStacked ? "host-rows-stacked" : "host-rows-side-by-side",
+        facts.rows.length > 0 && badRows.length === 0,
+        facts.rows.length === 0
+            ? "no host rows were measured"
+            : badRows.length === 0
+              ? `all ${facts.rows.length} rows laid out as expected`
+              : `${badRows.length} of ${facts.rows.length} rows wrong: ` +
+                summarise(badRows, (r) => r),
+    );
+
+    // The mobile early-return path proposed in #8 was rejected: narrow
+    // viewports must keep probing and keep detecting the gateway, not
+    // quietly skip work.
+    check(
+        "probing-still-runs",
+        probes.attempted > 0,
+        `${probes.attempted} outbound probe requests issued`,
+    );
+    check(
+        "gateway-detection-still-runs",
+        facts.gatewayDetected,
+        facts.gatewayDetected
+            ? "Local Gateway row present"
+            : "no Local Gateway row — gateway detection did not run or did not complete",
+    );
+
+    return checks;
+}
diff --git a/test/viewport/facts.js b/test/viewport/facts.js
new file mode 100644
index 0000000..f266e6c
--- /dev/null
+++ b/test/viewport/facts.js
@@ -0,0 +1,184 @@
+// Layout facts collected from inside the page.
+//
+// This function is serialised and evaluated in the browser, so it must be
+// entirely self-contained: no imports, no closures over module scope. It
+// only *measures*; every pass/fail decision is made back in node by
+// checks.js, so failures can be reported with real numbers attached.
+
+export function collectLayoutFacts(options) {
+    const describe = (el) => {
+        const id = el.id ? "#" + el.id : "";
+        const classes =
+            typeof el.className === "string" && el.className.trim()
+                ? "." + el.className.trim().split(/\s+/).slice(0, 3).join(".")
+                : "";
+        return el.tagName.toLowerCase() + id + classes;
+    };
+
+    const round = (n) => Math.round(n * 10) / 10;
+
+    // Overflow propagates up every ancestor, so a single wide element
+    // reports as body, #app, the row, and so on. Depth lets the report
+    // name the deepest — that is, the actual — offender.
+    const depthOf = (el) => {
+        let depth = 0;
+        for (let node = el.parentElement; node; node = node.parentElement) {
+            depth++;
+        }
+        return depth;
+    };
+
+    const isVisible = (el) => {
+        const style = getComputedStyle(el);
+        if (style.display === "none") return false;
+        if (style.visibility === "hidden") return false;
+        const rect = el.getBoundingClientRect();
+        return rect.width > 0 && rect.height > 0;
+    };
+
+    const innerWidth = window.innerWidth;
+    const clientWidth = document.documentElement.clientWidth;
+    // Under mobile emulation Chrome lets window.innerWidth *grow* to the
+    // width of overflowing content, exactly as a phone zooms out to fit a
+    // too-wide page. Measuring against it would therefore hide the
+    // overflow it is supposed to expose: at a 320px device width a page
+    // that spills to 350 reports innerWidth 350 and looks clean. Every
+    // comparison below is against the layout viewport instead.
+    const viewportWidth = Math.min(innerWidth, clientWidth);
+    const elements = Array.from(document.querySelectorAll("body *"));
+
+    // Elements sticking out past the right (or left) edge of the viewport.
+    // The document-level scrollWidth check says *that* the page overflows;
+    // this says *what* is doing it.
+    const overflowing = [];
+    // Elements clipping their own text. Deliberate ellipsis truncation
+    // (Tailwind's `truncate`) is opt-in and excluded: it is a design
+    // choice, not breakage.
+    const clipped = [];
+    // Elements whose content spills out of their own box without being
+    // clipped, past the right edge of the viewport. A block element is
+    // only ever as wide as its container, so text overflowing it has no
+    // element rect of its own to catch — but it is exactly what drags
+    // documentElement.scrollWidth past the viewport width, so without
+    // this the page-level overflow failure has nothing to point at.
+    const contentOverflowing = [];
+
+    for (const el of elements) {
+        if (!isVisible(el)) continue;
+        const rect = el.getBoundingClientRect();
+        if (rect.right > viewportWidth + 1 || rect.left < -1) {
+            overflowing.push({
+                el: describe(el),
+                depth: depthOf(el),
+                left: round(rect.left),
+                right: round(rect.right),
+            });
+        }
+        const style = getComputedStyle(el);
+        const clips =
+            style.overflowX === "hidden" || style.overflowX === "clip";
+        const ellipsis = style.textOverflow === "ellipsis";
+        const hasText = el.textContent.trim().length > 0;
+        const spills =
+            el.clientWidth > 0 && el.scrollWidth > el.clientWidth + 1;
+        if (clips && !ellipsis && hasText && spills) {
+            clipped.push({
+                el: describe(el),
+                scrollWidth: el.scrollWidth,
+                clientWidth: el.clientWidth,
+            });
+        }
+        if (
+            !clips &&
+            spills &&
+            rect.left + el.scrollWidth > viewportWidth + 1
+        ) {
+            contentOverflowing.push({
+                el: describe(el),
+                depth: depthOf(el),
+                scrollWidth: el.scrollWidth,
+                clientWidth: el.clientWidth,
+                reach: round(rect.left + el.scrollWidth),
+            });
+        }
+    }
+
+    // Interactive controls. The measured target is the nearest thing that
+    // is genuinely tappable — for a checkbox that is the <label> wrapping
+    // it, which is larger than the box itself and is what a finger hits.
+    const tapTargets = [];
+    for (const selector of options.interactiveSelectors) {
+        for (const el of document.querySelectorAll(selector)) {
+            if (!isVisible(el)) continue;
+            const target = el.closest("button, a, label, select") || el;
+            const rect = target.getBoundingClientRect();
+            tapTargets.push({
+                selector,
+                el: describe(target),
+                width: round(rect.width),
+                height: round(rect.height),
+            });
+        }
+    }
+
+    // Host rows. The question is not "did it get narrower" but "did it
+    // reflow": the info block and the sparkline must end up stacked
+    // vertically and full width in the narrow layout, and side by side in
+    // the wide one. Both the computed flex-direction and the actual
+    // geometry are recorded so a row that claims to be a column but is
+    // still laid out side by side cannot slip through.
+    const rows = [];
+    for (const row of document.querySelectorAll(".host-row")) {
+        const inner = row.firstElementChild;
+        if (!inner) continue;
+        const sparkline = inner.querySelector(".sparkline-container");
+        const info = sparkline ? sparkline.previousElementSibling : null;
+        if (!sparkline || !info) continue;
+        const innerStyle = getComputedStyle(inner);
+        const innerRect = inner.getBoundingClientRect();
+        const infoRect = info.getBoundingClientRect();
+        const sparkRect = sparkline.getBoundingClientRect();
+        rows.push({
+            index: row.dataset.index,
+            flexDirection: innerStyle.flexDirection,
+            containerWidth: round(innerRect.width),
+            info: {
+                left: round(infoRect.left),
+                right: round(infoRect.right),
+                bottom: round(infoRect.bottom),
+                width: round(infoRect.width),
+            },
+            sparkline: {
+                left: round(sparkRect.left),
+                top: round(sparkRect.top),
+                width: round(sparkRect.width),
+            },
+        });
+    }
+
+    const localRows = Array.from(
+        document.querySelectorAll("#local-hosts .host-row"),
+    );
+
+    return {
+        innerWidth,
+        // innerWidth includes any classic scrollbar, clientWidth does not.
+        // Reported separately so the overflow check can hold itself to the
+        // narrower of the two rather than to whichever one is more
+        // forgiving.
+        documentClientWidth: document.documentElement.clientWidth,
+        documentScrollWidth: document.documentElement.scrollWidth,
+        overflowing,
+        contentOverflowing,
+        clipped,
+        tapTargets,
+        rows,
+        rowCount: document.querySelectorAll(".host-row").length,
+        numericLatencies: Array.from(
+            document.querySelectorAll(".latency-value"),
+        ).filter((el) => /\d/.test(el.textContent)).length,
+        gatewayDetected: localRows.some((row) =>
+            row.textContent.includes("Local Gateway"),
+        ),
+    };
+}
diff --git a/test/viewport/harness.js b/test/viewport/harness.js
new file mode 100644
index 0000000..52a4483
--- /dev/null
+++ b/test/viewport/harness.js
@@ -0,0 +1,258 @@
+// Responsive-layout harness.
+//
+// Drives the built frontend in a real, containerised, digest-pinned Chrome
+// over CDP and asserts on computed layout at every viewport width derived
+// from the app's own CSS. Screenshots are written alongside as artifacts;
+// they are not the evidence, the assertions are.
+//
+// This is not meant to be run by hand. `make frontend-viewport-test` brings
+// up the browser and the web server and then runs this; every input it
+// needs arrives in the environment.
+
+import { mkdirSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+import puppeteer from "puppeteer-core";
+import { collectLayoutFacts } from "./facts.js";
+import { evaluateChecks, INTERACTIVE_SELECTORS } from "./checks.js";
+import { deriveViewports } from "./viewports.js";
+
+function required(name) {
+    const value = process.env[name];
+    if (!value) {
+        throw new Error(
+            `${name} is not set; run this via script/frontend-viewport-test`,
+        );
+    }
+    return value;
+}
+
+const ROOT = required("NETWATCH_ROOT");
+const BASE_URL = required("NETWATCH_BASE_URL");
+const CDP_URL = required("NETWATCH_CDP_URL");
+const ARTIFACT_DIR = required("NETWATCH_ARTIFACT_DIR");
+
+const BROWSER_TIMEOUT_MS = 60000;
+const PAGE_TIMEOUT_MS = 30000;
+
+// Canned responses for the app's outbound latency probes. The browser
+// container sits on an --internal docker network and physically cannot
+// reach the internet, so nothing here is about blocking traffic; it is
+// about determinism. Real probes would render 24 rows of whatever the
+// network happened to be doing. These delays make the rows show a
+// realistic spread of value widths — one, two and three digit latencies,
+// plus some unreachable rows — because that spread is what the layout has
+// to survive.
+const PROBE_DELAYS_MS = [2, 45, 123, 456, 780];
+// One in every UNREACHABLE_MODULUS probes is failed outright so that the
+// offline row rendering is exercised too.
+const UNREACHABLE_MODULUS = 7;
+// The gateway candidate that "answers", so gateway detection succeeds and
+// the Local Gateway row renders. Matches GATEWAY_CANDIDATES in src/main.js.
+const RESPONSIVE_GATEWAY = "http://192.168.1.1";
+
+const sleep = (ms) => new Promise((resolve) => setTimeout(resolve, ms));
+
+function stableHash(text) {
+    let hash = 0;
+    for (let i = 0; i < text.length; i++) {
+        hash = (hash * 31 + text.charCodeAt(i)) | 0;
+    }
+    return Math.abs(hash);
+}
+
+async function connectBrowser() {
+    const deadline = Date.now() + BROWSER_TIMEOUT_MS;
+    let lastError;
+    for (;;) {
+        try {
+            const response = await fetch(`${CDP_URL}/json/version`);
+            const info = await response.json();
+            // The endpoint advertises whatever Host it was reached on;
+            // pin it back to the address we actually dialled.
+            const endpoint = new URL(info.webSocketDebuggerUrl);
+            endpoint.host = new URL(CDP_URL).host;
+            const browser = await puppeteer.connect({
+                browserWSEndpoint: endpoint.toString(),
+                protocolTimeout: BROWSER_TIMEOUT_MS,
+            });
+            return { browser, version: info.Browser };
+        } catch (error) {
+            lastError = error;
+            if (Date.now() > deadline) {
+                throw new Error(`browser never came up: ${lastError}`);
+            }
+            await sleep(250);
+        }
+    }
+}
+
+function installProbeResponder(page, probes) {
+    const respond = (request, delayMs) =>
+        sleep(delayMs).then(() =>
+            request.respond({
+                status: 200,
+                contentType: "text/plain",
+                body: "",
+            }),
+        );
+
+    page.on("request", (request) => {
+        const url = request.url();
+        const settle = async () => {
+            if (url.startsWith(BASE_URL) || url.startsWith("data:")) {
+                return request.continue();
+            }
+            probes.attempted++;
+            if (url.startsWith(RESPONSIVE_GATEWAY)) {
+                probes.fulfilled++;
+                return respond(request, 5);
+            }
+            const hash = stableHash(url);
+            if (hash % UNREACHABLE_MODULUS === 0) {
+                probes.failed++;
+                return request.abort("connectionfailed");
+            }
+            probes.fulfilled++;
+            return respond(
+                request,
+                PROBE_DELAYS_MS[hash % PROBE_DELAYS_MS.length],
+            );
+        };
+        // The page may be torn down while a delayed response is pending;
+        // that is not a harness failure.
+        settle().catch(() => {});
+    });
+}
+
+async function runViewport(browser, viewport) {
+    const page = await browser.newPage();
+    const probes = { attempted: 0, fulfilled: 0, failed: 0 };
+    try {
+        page.setDefaultTimeout(PAGE_TIMEOUT_MS);
+        await page.setRequestInterception(true);
+        installProbeResponder(page, probes);
+        await page.setViewport({
+            width: viewport.width,
+            height: viewport.height,
+            deviceScaleFactor: viewport.deviceScaleFactor,
+            isMobile: viewport.touch,
+            hasTouch: viewport.touch,
+            isLandscape: viewport.width > viewport.height,
+        });
+
+        await page.goto(BASE_URL, { waitUntil: "load" });
+        await page.waitForSelector(".host-row");
+
+        // The app discards its first tick as a cold start, so rows only
+        // carry real values from the second one. Changing the interval
+        // restarts the loop at 1s, which reaches a populated UI without
+        // waiting out two default 3s intervals — and exercises the
+        // interval dropdown while we are at it.
+        await page.select("#interval-select", "1000");
+        await page.waitForFunction(
+            () =>
+                Array.from(document.querySelectorAll(".latency-value")).filter(
+                    (el) => /\d/.test(el.textContent),
+                ).length >= 5,
+        );
+        // Let the resize/redraw handlers settle before measuring.
+        await page.evaluate(
+            () =>
+                new Promise((resolve) =>
+                    requestAnimationFrame(() => requestAnimationFrame(resolve)),
+                ),
+        );
+
+        const facts = await page.evaluate(collectLayoutFacts, {
+            interactiveSelectors: INTERACTIVE_SELECTORS,
+        });
+
+        const screenshot = join(
+            ARTIFACT_DIR,
+            `${viewport.width}x${viewport.height}-${viewport.name}.png`,
+        );
+        await page.screenshot({ path: screenshot, fullPage: true });
+
+        return {
+            viewport,
+            probes,
+            facts,
+            screenshot,
+            checks: evaluateChecks(facts, viewport, probes),
+        };
+    } finally {
+        await page.close().catch(() => {});
+    }
+}
+
+function report(results, conditions, browserVersion) {
+    const label = (viewport) =>
+        `${viewport.width}x${viewport.height}`.padEnd(9) +
+        " " +
+        viewport.name.padEnd(24);
+
+    console.log(`browser:     ${browserVersion}`);
+    console.log(`served from: ${BASE_URL} (built dist/)`);
+    console.log(
+        "breakpoints: " +
+            conditions
+                .map((c) => `${c.type}-width ${c.px}px (${c.source})`)
+                .join(", "),
+    );
+    console.log("");
+
+    let passed = 0;
+    let failed = 0;
+    for (const result of results) {
+        const bad = result.checks.filter((c) => !c.ok);
+        passed += result.checks.length - bad.length;
+        failed += bad.length;
+        // Passing viewports get one line. Detail is for failures.
+        console.log(
+            `${bad.length === 0 ? "PASS" : "FAIL"} ${label(result.viewport)} ` +
+                `${result.checks.length - bad.length}/${result.checks.length} checks` +
+                `${result.viewport.expectStacked ? "  [narrow layout expected]" : ""}`,
+        );
+        for (const check of bad) {
+            console.log(`       ${check.name}: ${check.detail}`);
+        }
+        if (bad.length > 0) {
+            console.log(`       why this width: ${result.viewport.why}`);
+            console.log(`       screenshot: ${result.screenshot}`);
+        }
+    }
+
+    console.log("");
+    console.log(
+        `${results.length} viewports, ${passed + failed} checks: ` +
+            `${passed} passed, ${failed} failed`,
+    );
+    console.log(`artifacts: ${ARTIFACT_DIR}`);
+    return failed;
+}
+
+async function main() {
+    const { conditions, viewports } = deriveViewports(ROOT);
+    mkdirSync(ARTIFACT_DIR, { recursive: true });
+
+    const { browser, version } = await connectBrowser();
+    const results = [];
+    try {
+        for (const viewport of viewports) {
+            results.push(await runViewport(browser, viewport));
+        }
+    } finally {
+        await browser.disconnect().catch(() => {});
+    }
+
+    writeFileSync(
+        join(ARTIFACT_DIR, "results.json"),
+        JSON.stringify({ browser: version, conditions, results }, null, 2) +
+            "\n",
+    );
+
+    const failed = report(results, conditions, version);
+    process.exitCode = failed === 0 ? 0 : 1;
+}
+
+await main();
diff --git a/test/viewport/viewports.js b/test/viewport/viewports.js
new file mode 100644
index 0000000..23a20b2
--- /dev/null
+++ b/test/viewport/viewports.js
@@ -0,0 +1,224 @@
+// Viewport derivation for the responsive-layout harness.
+//
+// The widths tested are read out of the CSS the application actually
+// ships, not taken from a list of popular phone models. A generic 375px
+// "phone" test sails straight past an off-by-one error at a media query
+// boundary, which is the classic way a responsive layout breaks, so
+// every breakpoint found in the sources is probed three times: one pixel
+// below it, exactly on it, and one pixel above it.
+//
+// Nothing here hardcodes 768. If someone adds a second media block or
+// starts using Tailwind responsive prefixes, that breakpoint starts
+// being covered without this file being edited.
+
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+
+// Tailwind CSS v4 default breakpoints, in rem. The app currently uses
+// none of these prefixes, so the whole table is inert until someone
+// writes an `md:`-prefixed utility class.
+const TAILWIND_BREAKPOINT_REM = {
+    sm: 40,
+    md: 48,
+    lg: 64,
+    xl: 80,
+    "2xl": 96,
+};
+
+// The app does not override the root font size, so rem and em in media
+// queries resolve against the browser default.
+const ROOT_FONT_SIZE_PX = 16;
+
+// Extract every min-width / max-width condition from the @media blocks in
+// a stylesheet. Returns e.g. [{ type: "max", px: 768, source: "..." }].
+export function mediaConditionsFromCss(css, source) {
+    const conditions = [];
+    for (const block of css.matchAll(/@media([^{]+)\{/g)) {
+        const features = block[1].matchAll(
+            /\(\s*(min|max)-width\s*:\s*([\d.]+)(px|rem|em)\s*\)/g,
+        );
+        for (const feature of features) {
+            const scale = feature[3] === "px" ? 1 : ROOT_FONT_SIZE_PX;
+            conditions.push({
+                type: feature[1],
+                px: Math.round(Number(feature[2]) * scale),
+                source,
+            });
+        }
+    }
+    return conditions;
+}
+
+// Extract the breakpoints implied by Tailwind responsive prefixes used in
+// markup. A prefix only counts when it opens a utility class, so `text-sm`
+// does not masquerade as the `sm:` breakpoint.
+export function mediaConditionsFromMarkup(sources) {
+    const conditions = [];
+    for (const { path, text } of sources) {
+        for (const [name, rem] of Object.entries(TAILWIND_BREAKPOINT_REM)) {
+            const used = new RegExp(
+                `(^|["'\\s])${name}:[a-z0-9[\\](),_./%-]+`,
+                "m",
+            ).test(text);
+            if (used) {
+                conditions.push({
+                    type: "min",
+                    px: rem * ROOT_FONT_SIZE_PX,
+                    source: path,
+                });
+            }
+        }
+    }
+    return conditions;
+}
+
+// Whether a given width should be rendering the app's narrow (stacked)
+// layout.
+//
+// Two breakpoint styles can be answered from the condition list alone:
+//
+//   - Desktop-first, which is what the app ships today: the wide layout is
+//     unconditional and every narrow rule lives in a `max-width` block, so
+//     a width is narrow exactly when one of those blocks matches. Note that
+//     `max-width: 768px` matches *at* 768 — getting this inclusive boundary
+//     wrong in either direction is what the three-widths-per-breakpoint
+//     sweep exists to catch.
+//   - Mobile-first, which is what Tailwind's `sm:`/`md:` prefixes are: the
+//     stacked layout is the unconditional base and a `min-width` block is
+//     what widens it, so a width is narrow exactly when it sits below every
+//     `min-width` breakpoint.
+//
+// A mix of the two cannot be resolved from the breakpoints alone — which
+// block owns the host-row reflow is a property of the rules inside it, not
+// of the condition — so this throws rather than guessing. Guessing is how
+// the wrong expectation gets applied at the right widths and the whole
+// sweep quietly verifies nothing.
+export function expectsStackedLayout(width, conditions) {
+    const kinds = new Set(conditions.map((c) => c.type));
+    for (const kind of kinds) {
+        if (kind !== "max" && kind !== "min") {
+            throw new Error(
+                `unsupported media condition type "${kind}" in ` +
+                    "expectsStackedLayout (test/viewport/viewports.js)",
+            );
+        }
+    }
+    if (kinds.has("max") && kinds.has("min")) {
+        throw new Error(
+            "the app now mixes max-width and min-width breakpoints (" +
+                conditions
+                    .map((c) => `${c.type}-width ${c.px}px in ${c.source}`)
+                    .join(", ") +
+                "), so which layout a width should be showing can no longer " +
+                "be inferred from the breakpoint list; teach " +
+                "expectsStackedLayout in test/viewport/viewports.js which " +
+                "block owns the host-row reflow",
+        );
+    }
+    if (kinds.has("min")) {
+        return !conditions.some((c) => width >= c.px);
+    }
+    return conditions.some((c) => width <= c.px);
+}
+
+// Viewports that are not derived from a breakpoint. Each one is here for
+// a stated reason; none of them is a stand-in for "a phone".
+const ANCHOR_VIEWPORTS = [
+    {
+        name: "floor-portrait",
+        width: 320,
+        height: 568,
+        deviceScaleFactor: 2,
+        touch: true,
+        why: "320px is the narrowest viewport still in mainstream use; nothing has to work below it",
+    },
+    {
+        name: "phone-landscape-narrow",
+        width: 667,
+        height: 375,
+        deviceScaleFactor: 2,
+        touch: true,
+        why: "phone rotated to landscape, still inside the narrow layout",
+    },
+    {
+        name: "phone-landscape-wide",
+        width: 844,
+        height: 390,
+        deviceScaleFactor: 3,
+        touch: true,
+        why: "large phone rotated to landscape: crosses into the wide layout while still being a touch device",
+    },
+    {
+        name: "desktop",
+        width: 1280,
+        height: 800,
+        deviceScaleFactor: 1,
+        touch: false,
+        why: "desktop baseline",
+    },
+];
+
+export function deriveViewports(root) {
+    const conditions = [
+        ...mediaConditionsFromCss(
+            readFileSync(join(root, "src/styles.css"), "utf8"),
+            "src/styles.css",
+        ),
+        ...mediaConditionsFromMarkup([
+            {
+                path: "src/main.js",
+                text: readFileSync(join(root, "src/main.js"), "utf8"),
+            },
+            {
+                path: "index.html",
+                text: readFileSync(join(root, "index.html"), "utf8"),
+            },
+        ]),
+    ];
+
+    if (conditions.length === 0) {
+        throw new Error(
+            "no responsive breakpoints found in src/styles.css, src/main.js or " +
+                "index.html — either the responsive layout was deleted or this " +
+                "derivation has stopped matching the sources",
+        );
+    }
+
+    const viewports = new Map();
+    const add = (viewport) => {
+        const key = `${viewport.width}x${viewport.height}`;
+        if (!viewports.has(key)) viewports.set(key, viewport);
+    };
+
+    for (const condition of conditions) {
+        for (const [offset, label] of [
+            [-1, "below"],
+            [0, "at"],
+            [+1, "above"],
+        ]) {
+            const width = condition.px + offset;
+            add({
+                name: `${condition.type}-width-${condition.px}-${label}`,
+                width,
+                // Tall enough that the whole app is laid out in one column
+                // without the viewport height influencing wrapping.
+                height: 1024,
+                deviceScaleFactor: 2,
+                touch: true,
+                why: `${offset === 0 ? "exactly on" : `1px ${label}`} the ${condition.type}-width: ${condition.px}px breakpoint declared in ${condition.source}`,
+            });
+        }
+    }
+
+    for (const anchor of ANCHOR_VIEWPORTS) add(anchor);
+
+    return {
+        conditions,
+        viewports: [...viewports.values()]
+            .map((viewport) => ({
+                ...viewport,
+                expectStacked: expectsStackedLayout(viewport.width, conditions),
+            }))
+            .sort((a, b) => a.width - b.width || a.height - b.height),
+    };
+}
diff --git a/yarn.lock b/yarn.lock
index d342c47..980d6a4 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -197,6 +197,14 @@
     "@emnapi/runtime" "^1.7.1"
     "@tybys/wasm-util" "^0.10.1"
 
+"@puppeteer/browsers@3.1.0":
+  version "3.1.0"
+  resolved "https://registry.yarnpkg.com/@puppeteer/browsers/-/browsers-3.1.0.tgz#5728ae0bc649263133ac1f8bd5d360eb75d11748"
+  integrity sha512-RDLpio3fH/qrj5k4DVY6eyiN8tCS0Zovd/6jW//n605oeqkWcUjn+3k+9ZtZBnbwMpsu0F7xDIiKXvVmG5c5Bw==
+  dependencies:
+    modern-tar "^0.7.6"
+    yargs "^18.0.0"
+
 "@rollup/rollup-android-arm-eabi@4.57.0":
   version "4.57.0"
   resolved "https://registry.yarnpkg.com/@rollup/rollup-android-arm-eabi/-/rollup-android-arm-eabi-4.57.0.tgz#f762035679a6b168138c94c960fda0b0cdb00d98"
@@ -441,6 +449,16 @@
   resolved "https://registry.yarnpkg.com/@types/estree/-/estree-1.0.8.tgz#958b91c991b1867ced318bedea0e215ee050726e"
   integrity sha512-dWHzHa2WqEXI/O1E9OjrocMTKJl2mSrEolh1Iomrv6U+JuNwaHXsXx9bLu5gG7BUWFIN0skIQJQ/L1rIex4X6w==
 
+ansi-regex@^6.2.2:
+  version "6.2.2"
+  resolved "https://registry.yarnpkg.com/ansi-regex/-/ansi-regex-6.2.2.tgz#60216eea464d864597ce2832000738a0589650c1"
+  integrity sha512-Bq3SmSpyFHaWjPk8If9yc6svM8c56dB5BAtW4Qbw5jHTwwXXcTLoRMkpDJp6VL0XzlWaCHTXrkFURMYmD0sLqg==
+
+ansi-styles@^6.2.1:
+  version "6.2.3"
+  resolved "https://registry.yarnpkg.com/ansi-styles/-/ansi-styles-6.2.3.tgz#c044d5dcc521a076413472597a1acb1f103c4041"
+  integrity sha512-4Dj6M28JB+oAH8kFkTLUo+a2jwOFkuqb3yucU0CANcRRUbxS0cP0nZYCGjcc3BNXwRIsUVmDGgzawme7zvJHvg==
+
 autoprefixer@^10.4.23:
   version "10.4.23"
   resolved "https://registry.yarnpkg.com/autoprefixer/-/autoprefixer-10.4.23.tgz#c6aa6db8e7376fcd900f9fd79d143ceebad8c4e6"
@@ -473,16 +491,43 @@ caniuse-lite@^1.0.30001759, caniuse-lite@^1.0.30001760:
   resolved "https://registry.yarnpkg.com/caniuse-lite/-/caniuse-lite-1.0.30001766.tgz#b6f6b55cb25a2d888d9393104d14751c6a7d6f7a"
   integrity sha512-4C0lfJ0/YPjJQHagaE9x2Elb69CIqEPZeG0anQt9SIvIoOH4a4uaRl73IavyO+0qZh6MDLH//DrXThEYKHkmYA==
 
+chromium-bidi@17.0.2:
+  version "17.0.2"
+  resolved "https://registry.yarnpkg.com/chromium-bidi/-/chromium-bidi-17.0.2.tgz#921a586deecd0c2d8b9242c4c1b73c3aa39ff77c"
+  integrity sha512-5v9GQFhTktFvotn/OFNJBmKLKRAb6n9r0bVCwf7sHgWc3/JryK0bj1nn93L3pHFrfgcsu6Be6EWsDi+1XHTGDg==
+  dependencies:
+    mitt "^3.0.1"
+    zod "^3.24.1"
+
+cliui@^9.0.1:
+  version "9.0.1"
+  resolved "https://registry.yarnpkg.com/cliui/-/cliui-9.0.1.tgz#6f7890f386f6f1f79953adc1f78dec46fcc2d291"
+  integrity sha512-k7ndgKhwoQveBL+/1tqGJYNz097I7WOvwbmmU2AR5+magtbjPWQTS1C5vzGkBC8Ym8UWRzfKUzUUqFLypY4Q+w==
+  dependencies:
+    string-width "^7.2.0"
+    strip-ansi "^7.1.0"
+    wrap-ansi "^9.0.0"
+
 detect-libc@^2.0.3:
   version "2.1.2"
   resolved "https://registry.yarnpkg.com/detect-libc/-/detect-libc-2.1.2.tgz#689c5dcdc1900ef5583a4cb9f6d7b473742074ad"
   integrity sha512-Btj2BOOO83o3WyH59e8MgXsxEQVcarkUOpEYrubB0urwnN10yQ364rsiByU11nZlqWYZm05i/of7io4mzihBtQ==
 
+devtools-protocol@0.0.1653615:
+  version "0.0.1653615"
+  resolved "https://registry.yarnpkg.com/devtools-protocol/-/devtools-protocol-0.0.1653615.tgz#c600e0c619612156b2422a66d958ba188d87dbe8"
+  integrity sha512-pGVkY3T/qXxAp2nFPodwYqOevk6ncNMSmvL8QfRCx5ZWGd6Vor7AFNmyaA8Zs6uJyP1QAfjuLandCgvSix1BNA==
+
 electron-to-chromium@^1.5.263:
   version "1.5.282"
   resolved "https://registry.yarnpkg.com/electron-to-chromium/-/electron-to-chromium-1.5.282.tgz#6695816e5b170210d6aa07561546ed7d97347630"
   integrity sha512-FCPkJtpst28UmFzd903iU7PdeVTfY0KAeJy+Lk0GLZRwgwYHn/irRcaCbQQOmr5Vytc/7rcavsYLvTM8RiHYhQ==
 
+emoji-regex@^10.3.0:
+  version "10.6.0"
+  resolved "https://registry.yarnpkg.com/emoji-regex/-/emoji-regex-10.6.0.tgz#bf3d6e8f7f8fd22a65d9703475bc0147357a6b0d"
+  integrity sha512-toUI84YS5YmxW219erniWD0CIVOo46xGKColeNQRgOzDorgBi1v4D71/OFzgD9GO2UGKIv1C3Sp8DAn0+j5w7A==
+
 enhanced-resolve@^5.18.3:
   version "5.18.4"
   resolved "https://registry.yarnpkg.com/enhanced-resolve/-/enhanced-resolve-5.18.4.tgz#c22d33055f3952035ce6a144ce092447c525f828"
@@ -523,7 +568,7 @@ esbuild@^0.27.0:
     "@esbuild/win32-ia32" "0.27.2"
     "@esbuild/win32-x64" "0.27.2"
 
-escalade@^3.2.0:
+escalade@^3.1.1, escalade@^3.2.0:
   version "3.2.0"
   resolved "https://registry.yarnpkg.com/escalade/-/escalade-3.2.0.tgz#011a3f69856ba189dffa7dc8fcce99d2a87903e5"
   integrity sha512-WUj2qlxaQtO4g6Pq5c29GTcWGDyd8itL8zTlipgECz3JesAiiOKotd8JU6otB3PACgG6xkJUyVhboMS+bje/jA==
@@ -543,6 +588,16 @@ fsevents@~2.3.2, fsevents@~2.3.3:
   resolved "https://registry.yarnpkg.com/fsevents/-/fsevents-2.3.3.tgz#cac6407785d03675a2a5e1a5305c697b347d90d6"
   integrity sha512-5xoDfX+fL7faATnagmWPpbFtwh/R77WmMMqqHGS65C3vvB0YHrgF+B1YmZ3441tMj5n63k0212XNoJwzlhffQw==
 
+get-caller-file@^2.0.5:
+  version "2.0.5"
+  resolved "https://registry.yarnpkg.com/get-caller-file/-/get-caller-file-2.0.5.tgz#4f94412a82db32f36e3b0b9741f8a97feb031f7e"
+  integrity sha512-DyFP3BM/3YHTQOCUL/w0OZHR0lpKeGrxotcHWcqNEdnltqFwXVfhEBQ94eIo34AfQpo0rGki4cyIiftY06h2Fg==
+
+get-east-asian-width@^1.0.0, get-east-asian-width@^1.5.0:
+  version "1.6.0"
+  resolved "https://registry.yarnpkg.com/get-east-asian-width/-/get-east-asian-width-1.6.0.tgz#216900f91df11a8b2c198c3e1d93d6c035a776b9"
+  integrity sha512-QRbvDIbx6YklUe6RxeTeleMR0yv3cYH6PsPZHcnVn7xv7zO1BHN8r0XETu8n6Ye3Q+ahtSarc3WgtNWmehIBfA==
+
 graceful-fs@^4.2.4:
   version "4.2.11"
   resolved "https://registry.yarnpkg.com/graceful-fs/-/graceful-fs-4.2.11.tgz#4183e4e8bf08bb6e05bbb2f7d2e0c8f712ca40e3"
@@ -634,6 +689,16 @@ magic-string@^0.30.21:
   dependencies:
     "@jridgewell/sourcemap-codec" "^1.5.5"
 
+mitt@^3.0.1:
+  version "3.0.1"
+  resolved "https://registry.yarnpkg.com/mitt/-/mitt-3.0.1.tgz#ea36cf0cc30403601ae074c8f77b7092cdab36d1"
+  integrity sha512-vKivATfr97l2/QBCYAkXYDbrIWPM2IIKEl7YPhjCvKlG3kE2gm+uBo6nEXK3M5/Ffh/FLpKExzOQ3JJoJGFKBw==
+
+modern-tar@^0.7.6:
+  version "0.7.7"
+  resolved "https://registry.yarnpkg.com/modern-tar/-/modern-tar-0.7.7.tgz#ca71d79603630076b10733b0751ccab284bbc1ef"
+  integrity sha512-t9VmxaqrmANnEOBhpSDI6HD192Ge48k8vmWqQQL7hSFEqHEYwZbbsu49+aKLWZeRvFs3j1pMhXOqqF4kPlvjkQ==
+
 nanoid@^3.3.11:
   version "3.3.11"
   resolved "https://registry.yarnpkg.com/nanoid/-/nanoid-3.3.11.tgz#4f4f112cefbe303202f2199838128936266d185b"
@@ -673,6 +738,18 @@ prettier@^3.8.1:
   resolved "https://registry.yarnpkg.com/prettier/-/prettier-3.8.1.tgz#edf48977cf991558f4fcbd8a3ba6015ba2a3a173"
   integrity sha512-UOnG6LftzbdaHZcKoPFtOcCKztrQ57WkHDeRD9t/PTQtmT0NHSeWWepj6pS0z/N7+08BHFDQVUrfmfMRcZwbMg==
 
+puppeteer-core@25.5.0:
+  version "25.5.0"
+  resolved "https://registry.yarnpkg.com/puppeteer-core/-/puppeteer-core-25.5.0.tgz#a41b14d582056b998e0bc3561ac47c7615d38a53"
+  integrity sha512-XPNT0dQJtphqQ4I29zxlG4IIPbg1iEHAQKWuQgtMJGXjACV77pZSmJvDi51IIIfd+DTKICcopJwUx4upVQ4XbA==
+  dependencies:
+    "@puppeteer/browsers" "3.1.0"
+    chromium-bidi "17.0.2"
+    devtools-protocol "0.0.1653615"
+    typed-query-selector "^2.12.2"
+    webdriver-bidi-protocol "0.4.2"
+    ws "^8.21.1"
+
 rollup@^4.43.0:
   version "4.57.0"
   resolved "https://registry.yarnpkg.com/rollup/-/rollup-4.57.0.tgz#9fa13c1fb779d480038f45708b5e01b9449b6853"
@@ -712,6 +789,30 @@ source-map-js@^1.2.1:
   resolved "https://registry.yarnpkg.com/source-map-js/-/source-map-js-1.2.1.tgz#1ce5650fddd87abc099eda37dcff024c2667ae46"
   integrity sha512-UXWMKhLOwVKb728IUtQPXxfYU+usdybtUrK/8uGE8CQMvrhOpwvzDBwj0QhSL7MQc7vIsISBG8VQ8+IDQxpfQA==
 
+string-width@^7.0.0, string-width@^7.2.0:
+  version "7.2.0"
+  resolved "https://registry.yarnpkg.com/string-width/-/string-width-7.2.0.tgz#b5bb8e2165ce275d4d43476dd2700ad9091db6dc"
+  integrity sha512-tsaTIkKW9b4N+AEj+SVA+WhJzV7/zMhcSu78mLKWSk7cXMOSHsBKFWUs0fWwq8QyK3MgJBQRX6Gbi4kYbdvGkQ==
+  dependencies:
+    emoji-regex "^10.3.0"
+    get-east-asian-width "^1.0.0"
+    strip-ansi "^7.1.0"
+
+string-width@^8.2.1:
+  version "8.2.2"
+  resolved "https://registry.yarnpkg.com/string-width/-/string-width-8.2.2.tgz#7310516493df575742fe98af6fae87d85d5ed0ac"
+  integrity sha512-GaPUh5gfdrYzqeVNZvUfT23vYYxXzKYidUcnMtJg/3rxRV63EFZy3k6xfKlmfeJD0176lnUV/Usr3XcwSvFzpg==
+  dependencies:
+    get-east-asian-width "^1.5.0"
+    strip-ansi "^7.1.2"
+
+strip-ansi@^7.1.0, strip-ansi@^7.1.2:
+  version "7.2.0"
+  resolved "https://registry.yarnpkg.com/strip-ansi/-/strip-ansi-7.2.0.tgz#d22a269522836a627af8d04b5c3fd2c7fa3e32e3"
+  integrity sha512-yDPMNjp4WyfYBkHnjIRLfca1i6KMyGCtsVgoKe/z1+6vukgaENdgGBZt+ZmKPc4gavvEZ5OgHfHdrazhgNyG7w==
+  dependencies:
+    ansi-regex "^6.2.2"
+
 tailwindcss@4.1.18, tailwindcss@^4.1.18:
   version "4.1.18"
   resolved "https://registry.yarnpkg.com/tailwindcss/-/tailwindcss-4.1.18.tgz#f488ba47853abdb5354daf9679d3e7791fc4f4e3"
@@ -735,6 +836,11 @@ tslib@^2.4.0:
   resolved "https://registry.yarnpkg.com/tslib/-/tslib-2.8.1.tgz#612efe4ed235d567e8aba5f2a5fab70280ade83f"
   integrity sha512-oJFu94HQb+KVduSUQL7wnpmqnfmLsOA/nAh6b6EH0wCEoK0/mPeXU6c3wKDV83MkOuHPRHtSXKKU99IBazS/2w==
 
+typed-query-selector@^2.12.2:
+  version "2.12.2"
+  resolved "https://registry.yarnpkg.com/typed-query-selector/-/typed-query-selector-2.12.2.tgz#65e2462ac6b0aecfae1bfac1a4f3027070dbabaa"
+  integrity sha512-EOPFbyIub4ngnEdqi2yOcNeDLaX/0jcE1JoAXQDDMIthap7FoN795lc/SHfIq2d416VufXpM8z/lD+WRm2gfOQ==
+
 update-browserslist-db@^1.2.0:
   version "1.2.3"
   resolved "https://registry.yarnpkg.com/update-browserslist-db/-/update-browserslist-db-1.2.3.tgz#64d76db58713136acbeb4c49114366cc6cc2e80d"
@@ -756,3 +862,49 @@ vite@^7.3.1:
     tinyglobby "^0.2.15"
   optionalDependencies:
     fsevents "~2.3.3"
+
+webdriver-bidi-protocol@0.4.2:
+  version "0.4.2"
+  resolved "https://registry.yarnpkg.com/webdriver-bidi-protocol/-/webdriver-bidi-protocol-0.4.2.tgz#f51bb71c2606e90e3d5727607c728b25d617b58b"
+  integrity sha512-VSV+fzfChirL3e7jay2yUC7B4HQCGtEWEg/MSSQbK+qWbqeGlRLlXTzPpYr3XGUvbpDHumWZBJxgesg4N7dbtA==
+
+wrap-ansi@^9.0.0:
+  version "9.0.2"
+  resolved "https://registry.yarnpkg.com/wrap-ansi/-/wrap-ansi-9.0.2.tgz#956832dea9494306e6d209eb871643bb873d7c98"
+  integrity sha512-42AtmgqjV+X1VpdOfyTGOYRi0/zsoLqtXQckTmqTeybT+BDIbM/Guxo7x3pE2vtpr1ok6xRqM9OpBe+Jyoqyww==
+  dependencies:
+    ansi-styles "^6.2.1"
+    string-width "^7.0.0"
+    strip-ansi "^7.1.0"
+
+ws@^8.21.1:
+  version "8.21.3"
+  resolved "https://registry.yarnpkg.com/ws/-/ws-8.21.3.tgz#660b4faddb6a3e575c86e078126919961f4de4fc"
+  integrity sha512-201TZ/kPWxoPr/OKWjquZR1SWKXcvxdH+e1xrx89b3YbmzLMFCLfnaG1HFIgWzJOEWZ7MvpK++odZufgYR50Rw==
+
+y18n@^5.0.5:
+  version "5.0.8"
+  resolved "https://registry.yarnpkg.com/y18n/-/y18n-5.0.8.tgz#7f4934d0f7ca8c56f95314939ddcd2dd91ce1d55"
+  integrity sha512-0pfFzegeDWJHJIAmTLRP2DwHjdF5s7jo9tuztdQxAhINCdvS+3nGINqPd00AphqJR/0LhANUS6/+7SCb98YOfA==
+
+yargs-parser@^22.0.0:
+  version "22.0.0"
+  resolved "https://registry.yarnpkg.com/yargs-parser/-/yargs-parser-22.0.0.tgz#87b82094051b0567717346ecd00fd14804b357c8"
+  integrity sha512-rwu/ClNdSMpkSrUb+d6BRsSkLUq1fmfsY6TOpYzTwvwkg1/NRG85KBy3kq++A8LKQwX6lsu+aWad+2khvuXrqw==
+
+yargs@^18.0.0:
+  version "18.1.0"
+  resolved "https://registry.yarnpkg.com/yargs/-/yargs-18.1.0.tgz#cd7e98c703ef51695bbbf062ed58f28e94291b56"
+  integrity sha512-2rAgRKu54VsHkqI0/tYkmluGXHD4KW7yZoycuqDQ15QOTnc2VVfy0nN/1eMhnQLO00A+dwtK20xuCnc1YGeUyg==
+  dependencies:
+    cliui "^9.0.1"
+    escalade "^3.1.1"
+    get-caller-file "^2.0.5"
+    string-width "^8.2.1"
+    y18n "^5.0.5"
+    yargs-parser "^22.0.0"
+
+zod@^3.24.1:
+  version "3.25.76"
+  resolved "https://registry.yarnpkg.com/zod/-/zod-3.25.76.tgz#26841c3f6fd22a6a2760e7ccb719179768471e34"
+  integrity sha512-gzUt/qt81nXsFGKIFcC3YnfEAx5NkunCfnDlvuBSSFS02bcXu4Lmea0AFIUwbLWxWPx3d9p8S5QoaujKcNQxcQ==
-- 
2.54.0


From d7cf010e002ba81c7bdef4237d069617f58b139e Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Mon, 21 Sep 2026 18:47:12 +0200
Subject: [PATCH 07/22] fix(server): shut down through fx so buffered reports
 flush (closes #22)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

The server ran os.Exit at the end of its own goroutine, racing fx's
teardown and sometimes killing the process before reportbuf's OnStop
flushed — silently losing a full flush window of telemetry on every
restart, at exit 0. Shutdown now goes through fx.Shutdowner, so every
OnStop runs in order.

The http.Server is built synchronously in OnStart before the serving
goroutine, so shutdown can no longer race or nil-deref it. A listen
failure exits non-zero via fx.ExitCode(1). reportbuf's OnStop is guarded
by sync.Once. writeTimeout now exceeds the chi per-request budget so that
budget is reachable. Dead startupTime, exitCode, and cancelFunc fields
are gone. A new test asserts a buffered report reaches disk after the
lifecycle stops.

Model: opus-4-8
---
 TODO.md                                      |   9 ++
 backend/internal/reportbuf/reportbuf.go      |  20 ++--
 backend/internal/reportbuf/reportbuf_test.go |  74 ++++++++++++-
 backend/internal/server/http.go              |  41 +++++--
 backend/internal/server/server.go            | 110 ++++++-------------
 5 files changed, 154 insertions(+), 100 deletions(-)

diff --git a/TODO.md b/TODO.md
index 29d1782..49f0492 100644
--- a/TODO.md
+++ b/TODO.md
@@ -22,6 +22,15 @@ files, so merging it also closes most compliance gaps.
 
 # Completed Steps
 
+- 2026-09-21: shutdown lifecycle correctness. The process now shuts down through
+  fx instead of `os.Exit`, so every component's `OnStop` runs and buffered
+  reports are flushed to disk on `SIGTERM` — previously a full flush window of
+  telemetry was silently lost on every restart. The `http.Server` is now built
+  before its serving goroutine starts, so shutdown can no longer race or
+  nil-deref it; a listen failure exits non-zero via `fx.Shutdowner`; `reportbuf`
+  `OnStop` is idempotent; and `writeTimeout` now exceeds the chi per-request
+  budget so that budget is actually reachable. Dead `startupTime`, `exitCode`,
+  and `cancelFunc` fields were removed
 - 2026-09-21: backend HTTP hardening (issue #19): added `ReadHeaderTimeout` and
   `IdleTimeout` to the server, a `SecurityHeaders` middleware (HSTS, tight CSP,
   frame/sniff/referrer/permissions headers) registered before CORS, and
diff --git a/backend/internal/reportbuf/reportbuf.go b/backend/internal/reportbuf/reportbuf.go
index ff4cbad..1ef3b19 100644
--- a/backend/internal/reportbuf/reportbuf.go
+++ b/backend/internal/reportbuf/reportbuf.go
@@ -40,11 +40,12 @@ type Params struct {
 // Buffer accumulates JSON lines in memory and flushes them
 // to zstd-compressed files on disk.
 type Buffer struct {
-	buf     bytes.Buffer
-	dataDir string
-	done    chan struct{}
-	log     *slog.Logger
-	mu      sync.Mutex
+	buf      bytes.Buffer
+	dataDir  string
+	done     chan struct{}
+	log      *slog.Logger
+	mu       sync.Mutex
+	stopOnce sync.Once
 }
 
 // New creates a Buffer and registers lifecycle hooks to
@@ -76,8 +77,13 @@ func New(
 			return nil
 		},
 		OnStop: func(_ context.Context) error {
-			close(b.done)
-			b.flushLocked()
+			// stopOnce makes OnStop idempotent: a second
+			// invocation must not close an already-closed channel
+			// (which would panic) or flush again.
+			b.stopOnce.Do(func() {
+				close(b.done)
+				b.flushLocked()
+			})
 
 			return nil
 		},
diff --git a/backend/internal/reportbuf/reportbuf_test.go b/backend/internal/reportbuf/reportbuf_test.go
index 24a9ae9..4b6b550 100644
--- a/backend/internal/reportbuf/reportbuf_test.go
+++ b/backend/internal/reportbuf/reportbuf_test.go
@@ -1,13 +1,77 @@
 package reportbuf_test
 
 import (
+	"os"
+	"strings"
 	"testing"
 
-	_ "sneak.berlin/go/netwatch/internal/reportbuf"
+	"sneak.berlin/go/netwatch/internal/config"
+	"sneak.berlin/go/netwatch/internal/globals"
+	"sneak.berlin/go/netwatch/internal/logger"
+	"sneak.berlin/go/netwatch/internal/reportbuf"
+
+	"go.uber.org/fx"
+	"go.uber.org/fx/fxtest"
 )
 
-func TestImport(t *testing.T) {
-	t.Parallel()
-	// Compilation check — verifies the package parses
-	// and all imports resolve.
+// TestFlushOnShutdown proves the flush-on-shutdown path: a
+// report appended after start but before the periodic flush
+// window must reach disk when the fx lifecycle stops. This is
+// the exact case that silent data loss on restart used to
+// destroy.
+func TestFlushOnShutdown(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("DATA_DIR", dir)
+
+	var buf *reportbuf.Buffer
+
+	app := fxtest.New(t,
+		fx.Provide(
+			globals.New,
+			logger.New,
+			config.New,
+			reportbuf.New,
+		),
+		fx.Populate(&buf),
+	)
+
+	app.RequireStart()
+
+	err := buf.Append(map[string]string{"probe": "shutdown"})
+	if err != nil {
+		t.Fatalf("append report: %v", err)
+	}
+
+	// RequireStop runs the reportbuf OnStop hook, which is the
+	// only code path that flushes buffered reports on shutdown.
+	app.RequireStop()
+
+	if !hasReportFile(t, dir) {
+		t.Fatal("no report file on disk after shutdown; " +
+			"the buffered report was lost")
+	}
+}
+
+func hasReportFile(t *testing.T, dir string) bool {
+	t.Helper()
+
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatalf("read data dir: %v", err)
+	}
+
+	for _, e := range entries {
+		if strings.HasSuffix(e.Name(), ".jsonl.zst") {
+			info, statErr := e.Info()
+			if statErr != nil {
+				t.Fatalf("stat %s: %v", e.Name(), statErr)
+			}
+
+			if info.Size() > 0 {
+				return true
+			}
+		}
+	}
+
+	return false
 }
diff --git a/backend/internal/server/http.go b/backend/internal/server/http.go
index a0d3c68..e621ce7 100644
--- a/backend/internal/server/http.go
+++ b/backend/internal/server/http.go
@@ -5,20 +5,31 @@ import (
 	"fmt"
 	"net/http"
 	"time"
+
+	"go.uber.org/fx"
 )
 
 const (
 	readTimeout       = 10 * time.Second
 	readHeaderTimeout = 5 * time.Second
-	writeTimeout      = 10 * time.Second
 	idleTimeout       = 60 * time.Second
 	maxHeaderBytes    = 1 << 20 // 1 MiB
+
+	// requestTimeout (routes.go) is the single per-request
+	// processing budget, enforced by chi's middleware.Timeout.
+	// writeTimeout must exceed that budget so a handler can write
+	// its 503 when the chi timeout fires; if it were shorter the
+	// server would abort the write first and the chi budget would
+	// be unreachable dead configuration.
+	writeTimeout = requestTimeout + 5*time.Second
 )
 
-func (s *Server) serveUntilShutdown() {
+// newHTTPServer constructs the http.Server. It performs no I/O
+// and does not start listening.
+func (s *Server) newHTTPServer() *http.Server {
 	listenAddr := fmt.Sprintf(":%d", s.params.Config.Port)
 
-	s.httpServer = &http.Server{
+	return &http.Server{
 		Addr:              listenAddr,
 		Handler:           s,
 		MaxHeaderBytes:    maxHeaderBytes,
@@ -27,21 +38,29 @@ func (s *Server) serveUntilShutdown() {
 		WriteTimeout:      writeTimeout,
 		IdleTimeout:       idleTimeout,
 	}
+}
 
-	s.SetupRoutes()
-
+// listenAndServe runs the listener until the server is shut
+// down. A genuine listen failure (not the expected
+// ErrServerClosed from a clean shutdown) requests process
+// shutdown through fx with a non-zero exit code, so the failure
+// is visible to any supervisor.
+func (s *Server) listenAndServe() {
 	s.log.Info("http begin listen",
-		"listenaddr", listenAddr,
+		"listenaddr", s.httpServer.Addr,
 		"version", s.params.Globals.Version,
 		"buildarch", s.params.Globals.Buildarch,
 	)
 
 	err := s.httpServer.ListenAndServe()
-	if err != nil && !errors.Is(err, http.ErrServerClosed) {
-		s.log.Error("listen error", "error", err)
+	if err == nil || errors.Is(err, http.ErrServerClosed) {
+		return
+	}
 
-		if s.cancelFunc != nil {
-			s.cancelFunc()
-		}
+	s.log.Error("listen error", "error", err)
+
+	shutdownErr := s.shutdowner.Shutdown(fx.ExitCode(1))
+	if shutdownErr != nil {
+		s.log.Error("request shutdown failed", "error", shutdownErr)
 	}
 }
diff --git a/backend/internal/server/server.go b/backend/internal/server/server.go
index a67bd1e..d14dd25 100644
--- a/backend/internal/server/server.go
+++ b/backend/internal/server/server.go
@@ -1,16 +1,14 @@
 // Package server provides the HTTP server lifecycle,
-// including startup, routing, signal handling, and graceful
-// shutdown.
+// including startup, routing, and graceful shutdown. The
+// process lifetime is owned by fx: shutdown is requested
+// through fx.Shutdowner so every component's OnStop hook runs
+// in dependency order.
 package server
 
 import (
 	"context"
 	"log/slog"
 	"net/http"
-	"os"
-	"os/signal"
-	"syscall"
-	"time"
 
 	"sneak.berlin/go/netwatch/internal/config"
 	"sneak.berlin/go/netwatch/internal/globals"
@@ -31,19 +29,18 @@ type Params struct {
 	Handlers   *handlers.Handlers
 	Logger     *logger.Logger
 	Middleware *middleware.Middleware
+	Shutdowner fx.Shutdowner
 }
 
 // Server is the top-level HTTP server orchestrator.
 type Server struct {
-	cancelFunc  context.CancelFunc
-	exitCode    int
-	h           *handlers.Handlers
-	httpServer  *http.Server
-	log         *slog.Logger
-	mw          *middleware.Middleware
-	params      Params
-	router      *chi.Mux
-	startupTime time.Time
+	h          *handlers.Handlers
+	httpServer *http.Server
+	log        *slog.Logger
+	mw         *middleware.Middleware
+	params     Params
+	router     *chi.Mux
+	shutdowner fx.Shutdowner
 }
 
 // New creates a Server and registers lifecycle hooks for
@@ -57,23 +54,25 @@ func New(
 	s.mw = params.Middleware
 	s.h = params.Handlers
 	s.log = params.Logger.Get()
+	s.shutdowner = params.Shutdowner
 
 	lc.Append(fx.Hook{
 		OnStart: func(_ context.Context) error {
-			s.startupTime = time.Now().UTC()
+			// Build the router and http.Server synchronously
+			// here, before spawning the serving goroutine, so
+			// httpServer is fully constructed by the time OnStop
+			// (or an early signal) can read it. fx guarantees
+			// OnStart returns before OnStop runs, so no
+			// synchronization or nil check is needed at shutdown.
+			s.SetupRoutes()
+			s.httpServer = s.newHTTPServer()
 
-			go func() { //nolint:contextcheck // fx OnStart ctx is startup-only; run() creates its own
-				s.run()
-			}()
+			go s.listenAndServe()
 
 			return nil
 		},
-		OnStop: func(_ context.Context) error {
-			if s.cancelFunc != nil {
-				s.cancelFunc()
-			}
-
-			return nil
+		OnStop: func(ctx context.Context) error {
+			return s.shutdown(ctx)
 		},
 	})
 
@@ -88,60 +87,17 @@ func (s *Server) ServeHTTP(
 	s.router.ServeHTTP(w, r)
 }
 
-func (s *Server) run() {
-	exitCode := s.serve()
-	os.Exit(exitCode)
-}
-
-func (s *Server) serve() int {
-	var ctx context.Context //nolint:wsl // ctx must be declared before multi-assign
-
-	ctx, s.cancelFunc = context.WithCancel(
-		context.Background(),
-	)
-
-	go func() {
-		c := make(chan os.Signal, 1)
-
-		signal.Ignore(syscall.SIGPIPE)
-		signal.Notify(c, os.Interrupt, syscall.SIGTERM)
-
-		sig := <-c
-		s.log.Info("signal received", "signal", sig)
-
-		if s.cancelFunc != nil {
-			s.cancelFunc()
-		}
-	}()
-
-	go func() {
-		s.serveUntilShutdown()
-	}()
-
-	<-ctx.Done()
-	s.cleanShutdown()
-
-	return s.exitCode
-}
-
-const shutdownTimeout = 5 * time.Second
-
-func (s *Server) cleanShutdown() {
-	s.exitCode = 0
-
-	ctxShutdown, shutdownCancel := context.WithTimeout(
-		context.Background(),
-		shutdownTimeout,
-	)
-	defer shutdownCancel()
-
-	err := s.httpServer.Shutdown(ctxShutdown)
+// shutdown gracefully stops the HTTP server within the
+// deadline of the context fx provides for OnStop.
+func (s *Server) shutdown(ctx context.Context) error {
+	err := s.httpServer.Shutdown(ctx)
 	if err != nil {
-		s.log.Error(
-			"server clean shutdown failed",
-			"error", err,
-		)
+		s.log.Error("server clean shutdown failed", "error", err)
+
+		return err
 	}
 
 	s.log.Info("server stopped")
+
+	return nil
 }
-- 
2.54.0


From 7a1ee6e5a8d1ed203adbebc7ac3c1251e22f0faa Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Mon, 21 Sep 2026 19:30:06 +0200
Subject: [PATCH 08/22] lint: adopt org-standard .golangci.yml and
 golangci-lint v2.12.2 (closes #14)

The old backend/.golangci.yml declared version "2" but used v1 schema
keys, so under v2 it never validated and its thresholds were inert: the
linter ran at defaults. Replace it verbatim with the org-standard file,
repin the Dockerfile.backend lint stage to golangci-lint v2.12.2, and
assert the config's sha256 as the first step of the backend lint target
so it cannot silently drift again -- a local hash check, no network.

The standard config surfaces findings only in the tests: the repeated
IP literals in middleware_test.go become named constants (goconst) and
its request switches to NewRequestWithContext (noctx). reportbuf.go's
gosec suppression gains a plain justification comment. The rest of the
backend, including the fx-based server lifecycle, is already clean.
TODO.md updated.

Model: opus-4-8
---
 Dockerfile.backend                            |  4 +--
 TODO.md                                       | 29 ++++++++++++----
 backend/.golangci.yml                         | 26 ++++++++-------
 backend/Makefile                              | 19 +++++++++++
 .../internal/middleware/middleware_test.go    | 33 ++++++++++++-------
 backend/internal/reportbuf/reportbuf.go       |  4 ++-
 6 files changed, 81 insertions(+), 34 deletions(-)

diff --git a/Dockerfile.backend b/Dockerfile.backend
index e06a617..191a65e 100644
--- a/Dockerfile.backend
+++ b/Dockerfile.backend
@@ -1,8 +1,8 @@
 # Lint stage — fast feedback on formatting and lint issues. The
 # golangci/golangci-lint image ships Go, gofmt, make and the linter, so
 # nothing is installed here.
-# golangci/golangci-lint:v2.7.2 (2026-08-09)
-FROM golangci/golangci-lint@sha256:5d6d5c70a61f1356adfd9dd6316ce286799fefc9d743421356ff1b00842368ba AS lint
+# golangci/golangci-lint:v2.12.2 (2026-08-10)
+FROM golangci/golangci-lint@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240 AS lint
 
 WORKDIR /src
 COPY backend/go.mod backend/go.sum ./
diff --git a/TODO.md b/TODO.md
index 49f0492..d4ff3d6 100644
--- a/TODO.md
+++ b/TODO.md
@@ -10,15 +10,16 @@
 
 # Status
 
-pre-1.0. No git tags. Backend work in flight on feat/reportbuf-storage (dirty:
-src/main.js). Frontend is functional; backend is new and unmerged.
+pre-1.0. No git tags. `feat/reportbuf-storage` is merged; the backend, the CI
+workflow, and the backend repo standard files are all on `main`. Frontend and
+backend are both functional. Working toward the 1.0.0 milestone by closing the
+remaining repo-compliance issues on the tracker.
 
 # Next Step
 
-Land feat/reportbuf-storage: finish the in-progress src/main.js change, get make
-check green, and merge the branch to main. The branch adds the backend (buffered
-zstd-compressed report storage), the CI workflow, and backend repo standard
-files, so merging it also closes most compliance gaps.
+Confirm the `.gitea/workflows/check.yml` run is green (main always green
+policy). The workflow file is already on `main`; what is unverified is that its
+latest run passes.
 
 # Completed Steps
 
@@ -36,6 +37,14 @@ files, so merging it also closes most compliance gaps.
   frame/sniff/referrer/permissions headers) registered before CORS, and
   trusted-proxy client IP resolution honouring `X-Forwarded-For` / `X-Real-IP`
   only from a `TRUSTED_PROXIES` allowlist (loopback plus RFC1918 by default)
+- 2026-08-10: adopted the org-standard `backend/.golangci.yml` verbatim and
+  moved the pinned golangci-lint from v2.7.2 to v2.12.2 (the `lint` stage of
+  `Dockerfile.backend` now pins the `golangci/golangci-lint:v2.12.2` image by
+  digest); the previous config declared `version: "2"` but used v1 schema keys,
+  so every threshold in it was inert and its green result was meaningless.
+  `backend/Makefile`'s `lint` target now asserts the config's sha256 against the
+  canonical file first, so drift from the org standard fails the build instead
+  of silently degrading to defaults
 - 2026-08-10: every interactive control now meets the 44x44 CSS px minimum tap
   target (`.pin-btn`, `#interval-select`, the debug-log label and, on narrow
   viewports, `#pause-btn`). The pin button's hit area grows via matching
@@ -65,7 +74,7 @@ files, so merging it also closes most compliance gaps.
   shims, README Entrypoints section
 - 2026-02-27: backend with buffered zstd-compressed report storage; CI workflow
   and backend repo standard files; backend Dockerfile fixed (Go 1.25,
-  golangci-lint) and moved to repo root (feat/reportbuf-storage, unmerged)
+  golangci-lint) and moved to repo root (feat/reportbuf-storage)
 - 2026-02-26: host row layout redesigned with CSS grid; overflow and spacing
   fixes; nginx config extracted; port hardcoded to 8080
 - 2026-02-26: debug log panel, median stats, recovery probe, Docker build fix,
@@ -87,3 +96,9 @@ files, so merging it also closes most compliance gaps.
   (main always green policy)
 - Decide what to do with untracked resume.sh: commit it, gitignore it, or delete
   it
+- Upstream fix needed in `sneak/prompts`: the org-standard `.golangci.yml`
+  enables `gomodguard`, which golangci-lint v2.12.2 reports as deprecated since
+  v2.12.0 and replaced by `gomodguard_v2`, so every backend lint run prints a
+  deprecation warning. The file is standardized and must never be edited in this
+  repo, so nothing can be done here beyond tracking it — tracked at
+  <https://git.eeqj.de/sneak/netwatch/issues/41>
diff --git a/backend/.golangci.yml b/backend/.golangci.yml
index 34a8e31..26b1610 100644
--- a/backend/.golangci.yml
+++ b/backend/.golangci.yml
@@ -1,5 +1,9 @@
 version: "2"
 
+# Config schema uses the golangci-lint v2 layout (settings live under
+# linters.settings, not top-level linters-settings) so that the
+# thresholds below are actually applied by golangci-lint >= v2.
+
 run:
   timeout: 5m
   modules-download-mode: readonly
@@ -14,19 +18,17 @@ linters:
     - wsl              # Deprecated, replaced by wsl_v5
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go
-
-linters-settings:
-  lll:
-    line-length: 88
-  funlen:
-    lines: 80
-    statements: 50
-  cyclop:
-    max-complexity: 15
-  dupl:
-    threshold: 100
+  settings:
+    lll:
+      line-length: 88
+    funlen:
+      lines: 80
+      statements: 50
+    cyclop:
+      max-complexity: 15
+    dupl:
+      threshold: 100
 
 issues:
-  exclude-use-default: false
   max-issues-per-linter: 0
   max-same-issues: 0
diff --git a/backend/Makefile b/backend/Makefile
index 4660353..96edfb9 100644
--- a/backend/Makefile
+++ b/backend/Makefile
@@ -10,6 +10,17 @@ GOLDFLAGS += -s -w
 GOLDFLAGS += -X main.Version=$(VERSION)
 GOLDFLAGS += -X main.Buildarch=$(BUILDARCH)
 
+# macOS ships shasum rather than sha256sum.
+SHA256SUM := $(shell command -v sha256sum >/dev/null 2>&1 && echo sha256sum || echo shasum -a 256)
+
+# .golangci.yml is standardized org-wide and must never be edited here
+# (REPO_POLICIES.md). Its last silent drift replaced the v2 schema with
+# v1 keys, which left every threshold in the file inert while the build
+# stayed green. The lint target therefore asserts the file still matches
+# the canonical copy byte for byte. The check is a local hash comparison:
+# no network, no remote schema, nothing unpinned in the build path.
+GOLANGCI_CONFIG_SHA256 := 021cc83f4e6fc7c31b95b34b846723dfcf20b66b7baeea1dc40406e643346bcb
+
 .PHONY: all build test lint fmt fmt-check check docker hooks run clean
 
 all: build
@@ -22,6 +33,14 @@ test:
 	timeout 30 go test ./...
 
 lint:
+	@actual=$$($(SHA256SUM) .golangci.yml | cut -d' ' -f1); \
+	if [ "$$actual" != "$(GOLANGCI_CONFIG_SHA256)" ]; then \
+		echo ".golangci.yml has drifted from the org standard."; \
+		echo "  expected $(GOLANGCI_CONFIG_SHA256)"; \
+		echo "  actual   $$actual"; \
+		echo "Restore it verbatim from sneak/prompts; do not edit it."; \
+		exit 1; \
+	fi
 	golangci-lint run ./...
 
 fmt:
diff --git a/backend/internal/middleware/middleware_test.go b/backend/internal/middleware/middleware_test.go
index 2965593..a664ee6 100644
--- a/backend/internal/middleware/middleware_test.go
+++ b/backend/internal/middleware/middleware_test.go
@@ -9,6 +9,15 @@ import (
 	"sneak.berlin/go/netwatch/internal/middleware"
 )
 
+const (
+	// loopbackPeer is a remote address inside the trusted-proxy allowlist.
+	loopbackPeer = "127.0.0.1:5000"
+	// forwardedIP is the client address presented via X-Forwarded-For.
+	forwardedIP = "203.0.113.7"
+	// realIP is the client address presented via X-Real-IP.
+	realIP = "203.0.113.9"
+)
+
 func mustPrefixes(t *testing.T, cidrs ...string) []netip.Prefix {
 	t.Helper()
 
@@ -41,32 +50,32 @@ func clientIPCases() []clientIPCase {
 	return []clientIPCase{
 		{
 			name:       "trusted proxy uses forwarded-for",
-			remoteAddr: "127.0.0.1:5000",
-			xff:        "203.0.113.7",
-			want:       "203.0.113.7",
+			remoteAddr: loopbackPeer,
+			xff:        forwardedIP,
+			want:       forwardedIP,
 		},
 		{
 			name:       "trusted proxy uses left-most of chain",
 			remoteAddr: "10.1.2.3:5000",
-			xff:        "203.0.113.7, 10.1.2.3",
-			want:       "203.0.113.7",
+			xff:        forwardedIP + ", 10.1.2.3",
+			want:       forwardedIP,
 		},
 		{
 			name:       "trusted proxy falls back to x-real-ip",
-			remoteAddr: "127.0.0.1:5000",
-			xRealIP:    "203.0.113.9",
-			want:       "203.0.113.9",
+			remoteAddr: loopbackPeer,
+			xRealIP:    realIP,
+			want:       realIP,
 		},
 		{
 			name:       "untrusted peer ignores forwarded-for",
 			remoteAddr: "198.51.100.4:5000",
-			xff:        "203.0.113.7",
+			xff:        forwardedIP,
 			want:       "198.51.100.4",
 		},
 		{
 			name:       "untrusted peer ignores x-real-ip",
 			remoteAddr: "198.51.100.4:5000",
-			xRealIP:    "203.0.113.9",
+			xRealIP:    realIP,
 			want:       "198.51.100.4",
 		},
 		{
@@ -76,7 +85,7 @@ func clientIPCases() []clientIPCase {
 		},
 		{
 			name:       "trusted proxy with garbage header uses peer",
-			remoteAddr: "127.0.0.1:5000",
+			remoteAddr: loopbackPeer,
 			xff:        "not-an-ip",
 			want:       "127.0.0.1",
 		},
@@ -119,7 +128,7 @@ func TestSecurityHeaders(t *testing.T) {
 	)
 
 	rec := httptest.NewRecorder()
-	req := httptest.NewRequest(http.MethodGet, "/", http.NoBody)
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/", http.NoBody)
 	handler.ServeHTTP(rec, req)
 
 	want := map[string]string{
diff --git a/backend/internal/reportbuf/reportbuf.go b/backend/internal/reportbuf/reportbuf.go
index 1ef3b19..7b89266 100644
--- a/backend/internal/reportbuf/reportbuf.go
+++ b/backend/internal/reportbuf/reportbuf.go
@@ -169,7 +169,9 @@ func (b *Buffer) writeFile(data []byte) {
 	name := fmt.Sprintf("reports-%s.jsonl.zst", ts)
 	path := filepath.Join(b.dataDir, name)
 
-	f, err := os.OpenFile( //nolint:gosec // path built from controlled dataDir + timestamp
+	// path is built from the operator-supplied dataDir plus a
+	// generated timestamp, so it carries no external input.
+	f, err := os.OpenFile( //nolint:gosec // see comment above
 		path,
 		os.O_WRONLY|os.O_CREATE|os.O_EXCL,
 		filePerms,
-- 
2.54.0


From 503399e020acdb10cbc0cb7d8d13540b7c84775a Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Mon, 28 Sep 2026 20:39:39 +0200
Subject: [PATCH 09/22] fix(backend): report ingest correctness: 500 on a
 refused report, 413 on oversize, global body cap (closes #23)

A report the buffer refuses now returns 500 instead of a false `ok`.
Reports reach disk later, so a failed disk write is still answered 200
and shows in the log, and at shutdown as a failed stop with a non-zero
exit. An over-limit body returns 413; malformed JSON stays 400. A
MaxBodyBytes middleware caps every route at 1 MiB; a route group can
only lower that limit. The raw geo blob is no longer logged; client_id,
timestamp and decode error text are cut to 128 bytes before logging.
Panic recovery logs the panic value and stack through slog.

Model: opus-5-5
---
 TODO.md                                       |  10 +
 backend/internal/handlers/export_test.go      |  13 ++
 backend/internal/handlers/handlers.go         |  21 +-
 backend/internal/handlers/report.go           |  96 +++++---
 backend/internal/handlers/report_test.go      | 212 ++++++++++++++++++
 backend/internal/middleware/export_test.go    |   7 +
 backend/internal/middleware/middleware.go     |  84 +++++++
 .../internal/middleware/middleware_test.go    | 154 +++++++++++++
 backend/internal/reportbuf/reportbuf.go       |  60 +++--
 backend/internal/reportbuf/reportbuf_test.go  |  42 ++++
 backend/internal/server/export_test.go        |   5 +
 backend/internal/server/routes.go             |  12 +-
 backend/internal/server/routes_test.go        |  76 +++++++
 13 files changed, 737 insertions(+), 55 deletions(-)
 create mode 100644 backend/internal/handlers/export_test.go
 create mode 100644 backend/internal/handlers/report_test.go
 create mode 100644 backend/internal/server/export_test.go
 create mode 100644 backend/internal/server/routes_test.go

diff --git a/TODO.md b/TODO.md
index d4ff3d6..29e4877 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,16 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-28: report ingest correctness (issue #23): a storage failure now
+  returns 500 instead of a false `ok`; oversize bodies return 413 (distinguished
+  from malformed JSON, which stays 400); a `MaxBodyBytes` middleware caps every
+  route, not just the report route; the raw attacker-controlled `geo` blob is no
+  longer logged (only its length), and `client_id`, `timestamp` and decode error
+  text are length-bounded before logging; a `decodeJSON` handler helper was
+  added; panic recovery now routes the stack through slog instead of chi's
+  plain-text stderr; and writing a report file now returns its error, so a
+  failed final flush on shutdown makes the process exit non-zero instead of
+  losing the buffered reports silently
 - 2026-09-21: shutdown lifecycle correctness. The process now shuts down through
   fx instead of `os.Exit`, so every component's `OnStop` runs and buffered
   reports are flushed to disk on `SIGTERM` — previously a full flush window of
diff --git a/backend/internal/handlers/export_test.go b/backend/internal/handlers/export_test.go
new file mode 100644
index 0000000..7de02a5
--- /dev/null
+++ b/backend/internal/handlers/export_test.go
@@ -0,0 +1,13 @@
+package handlers
+
+import "log/slog"
+
+// MaxLoggedFieldBytes exposes the log bound to the external tests.
+const MaxLoggedFieldBytes = maxLoggedFieldBytes
+
+// NewForTest builds a Handlers around a report sink and logger,
+// bypassing the fx graph so handler behaviour (including the
+// storage failure path) is exercisable in unit tests.
+func NewForTest(buf reportAppender, log *slog.Logger) *Handlers {
+	return &Handlers{buf: buf, log: log}
+}
diff --git a/backend/internal/handlers/handlers.go b/backend/internal/handlers/handlers.go
index a327fcd..db51059 100644
--- a/backend/internal/handlers/handlers.go
+++ b/backend/internal/handlers/handlers.go
@@ -18,6 +18,13 @@ import (
 
 const jsonContentType = "application/json; charset=utf-8"
 
+// reportAppender is the subset of the report buffer the handlers
+// depend on. Defining it here keeps the storage failure path
+// exercisable with a stub in tests.
+type reportAppender interface {
+	Append(v any) error
+}
+
 // Params defines the dependencies for Handlers.
 type Params struct {
 	fx.In
@@ -30,7 +37,7 @@ type Params struct {
 
 // Handlers provides HTTP handler factories for all endpoints.
 type Handlers struct {
-	buf    *reportbuf.Buffer
+	buf    reportAppender
 	hc     *healthcheck.Healthcheck
 	log    *slog.Logger
 	params *Params
@@ -72,3 +79,15 @@ func (s *Handlers) respondJSON(
 		}
 	}
 }
+
+// decodeJSON decodes the request body into v. The body is
+// expected to already be bounded by the body-size middleware, so
+// a caller can distinguish an over-limit body from malformed
+// JSON by testing the returned error for *http.MaxBytesError.
+func (s *Handlers) decodeJSON(
+	_ http.ResponseWriter,
+	r *http.Request,
+	v any,
+) error {
+	return json.NewDecoder(r.Body).Decode(v)
+}
diff --git a/backend/internal/handlers/report.go b/backend/internal/handlers/report.go
index d4b54ed..81954a2 100644
--- a/backend/internal/handlers/report.go
+++ b/backend/internal/handlers/report.go
@@ -2,10 +2,14 @@ package handlers
 
 import (
 	"encoding/json"
+	"errors"
 	"net/http"
 )
 
-const maxReportBodyBytes = 1 << 20 // 1 MiB
+// maxLoggedFieldBytes bounds untrusted text (string fields,
+// decode error text) before it is logged, so a caller cannot
+// inflate log volume with an oversized value.
+const maxLoggedFieldBytes = 128
 
 type reportSample struct {
 	T       int64   `json:"t"`
@@ -35,48 +39,80 @@ func (s *Handlers) HandleReport() http.HandlerFunc {
 	}
 
 	return func(w http.ResponseWriter, r *http.Request) {
-		r.Body = http.MaxBytesReader(
-			w, r.Body, maxReportBodyBytes,
-		)
-
 		var rpt report
 
-		err := json.NewDecoder(r.Body).Decode(&rpt)
+		err := s.decodeJSON(w, r, &rpt)
 		if err != nil {
-			s.log.Error("failed to decode report",
-				"error", err,
-			)
 			s.respondJSON(w, r,
 				&response{Status: "error"},
-				http.StatusBadRequest,
+				s.decodeErrorStatus(err),
 			)
 
 			return
 		}
 
-		totalSamples := 0
-		for _, h := range rpt.Hosts {
-			totalSamples += len(h.History)
-		}
+		s.logReportReceived(rpt)
 
-		s.log.Info("report received",
-			"client_id", rpt.ClientID,
-			"timestamp", rpt.Timestamp,
-			"host_count", len(rpt.Hosts),
-			"total_samples", totalSamples,
-			"geo", string(rpt.Geo),
-		)
-
-		bufErr := s.buf.Append(rpt)
-		if bufErr != nil {
-			s.log.Error("failed to buffer report",
-				"error", bufErr,
+		err = s.buf.Append(rpt)
+		if err != nil {
+			s.log.Error("failed to buffer report", "error", err)
+			s.respondJSON(w, r,
+				&response{Status: "error"},
+				http.StatusInternalServerError,
 			)
+
+			return
 		}
 
-		s.respondJSON(w, r,
-			&response{Status: "ok"},
-			http.StatusOK,
-		)
+		s.respondJSON(w, r, &response{Status: "ok"}, http.StatusOK)
 	}
 }
+
+// decodeErrorStatus logs a report decode failure and returns the
+// status to send: 413 when the body exceeded the size limit,
+// otherwise 400 for malformed JSON.
+func (s *Handlers) decodeErrorStatus(err error) int {
+	var tooLarge *http.MaxBytesError
+	if errors.As(err, &tooLarge) {
+		s.log.Warn("report body too large", "limit_bytes", tooLarge.Limit)
+
+		return http.StatusRequestEntityTooLarge
+	}
+
+	// The decoder's error text can quote request bytes (a whole
+	// oversized number, for example), so it is bounded too.
+	s.log.Error("failed to decode report",
+		"error", boundedForLog(err.Error()),
+	)
+
+	return http.StatusBadRequest
+}
+
+// logReportReceived logs an accepted report. Untrusted fields are
+// bounded (client_id, timestamp) or reduced to a length
+// (geo_bytes) so the raw attacker-controlled body never reaches
+// the log.
+func (s *Handlers) logReportReceived(rpt report) {
+	totalSamples := 0
+	for _, h := range rpt.Hosts {
+		totalSamples += len(h.History)
+	}
+
+	s.log.Info("report received",
+		"client_id", boundedForLog(rpt.ClientID),
+		"timestamp", boundedForLog(rpt.Timestamp),
+		"host_count", len(rpt.Hosts),
+		"total_samples", totalSamples,
+		"geo_bytes", len(rpt.Geo),
+	)
+}
+
+// boundedForLog truncates an untrusted string to a fixed byte
+// bound so an attacker-controlled field cannot dominate the log.
+func boundedForLog(s string) string {
+	if len(s) > maxLoggedFieldBytes {
+		return s[:maxLoggedFieldBytes]
+	}
+
+	return s
+}
diff --git a/backend/internal/handlers/report_test.go b/backend/internal/handlers/report_test.go
new file mode 100644
index 0000000..68e9c71
--- /dev/null
+++ b/backend/internal/handlers/report_test.go
@@ -0,0 +1,212 @@
+package handlers_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/netwatch/internal/handlers"
+	"sneak.berlin/go/netwatch/internal/middleware"
+)
+
+var errStorageFailed = errors.New("storage failed")
+
+// stubAppender drives the storage success/failure path without a
+// real buffer or disk.
+type stubAppender struct {
+	err error
+}
+
+func (s stubAppender) Append(any) error { return s.err }
+
+func newTestHandlers(buf stubAppender, out io.Writer) *handlers.Handlers {
+	return handlers.NewForTest(buf, slog.New(slog.NewJSONHandler(out, nil)))
+}
+
+func decodeStatus(t *testing.T, body []byte) string {
+	t.Helper()
+
+	var resp struct {
+		Status string `json:"status"`
+	}
+
+	err := json.Unmarshal(body, &resp)
+	if err != nil {
+		t.Fatalf("response body not JSON: %v (%q)", err, body)
+	}
+
+	return resp.Status
+}
+
+func TestHandleReportStorageFailureIsNon2xx(t *testing.T) {
+	t.Parallel()
+
+	h := newTestHandlers(stubAppender{err: errStorageFailed}, io.Discard)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(`{"clientId":"c1","hosts":[]}`),
+	)
+
+	h.HandleReport().ServeHTTP(rec, req)
+
+	if rec.Code < 500 {
+		t.Fatalf("storage failure status = %d, want a 5xx", rec.Code)
+	}
+
+	if got := decodeStatus(t, rec.Body.Bytes()); got != "error" {
+		t.Fatalf("status field = %q, want %q", got, "error")
+	}
+}
+
+func TestHandleReportMalformedJSONIs400(t *testing.T) {
+	t.Parallel()
+
+	h := newTestHandlers(stubAppender{}, io.Discard)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(`{not json`),
+	)
+
+	h.HandleReport().ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("malformed status = %d, want %d",
+			rec.Code, http.StatusBadRequest)
+	}
+}
+
+func TestHandleReportOversizeIs413(t *testing.T) {
+	t.Parallel()
+
+	const limit = 32
+
+	h := newTestHandlers(stubAppender{}, io.Discard)
+	handler := (&middleware.Middleware{}).MaxBodyBytes(limit)(
+		h.HandleReport(),
+	)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(`{"clientId":"`+strings.Repeat("x", 200)+`"}`),
+	)
+	// No declared length, so only the middleware's read cap can
+	// stop this body.
+	req.ContentLength = -1
+
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusRequestEntityTooLarge {
+		t.Fatalf("oversize status = %d, want %d",
+			rec.Code, http.StatusRequestEntityTooLarge)
+	}
+}
+
+func TestHandleReportDoesNotLogRawGeo(t *testing.T) {
+	t.Parallel()
+
+	const sentinel = "SENSITIVE-GEO-BLOB"
+
+	var logbuf bytes.Buffer
+
+	h := newTestHandlers(stubAppender{}, &logbuf)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(
+			`{"clientId":"c1","geo":{"raw":"`+sentinel+`"},"hosts":[]}`,
+		),
+	)
+
+	h.HandleReport().ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	if strings.Contains(logbuf.String(), sentinel) {
+		t.Fatal("raw geo bytes were written to the log")
+	}
+
+	if !strings.Contains(logbuf.String(), "geo_bytes") {
+		t.Fatal("expected a bounded geo_bytes field in the log")
+	}
+}
+
+func TestHandleReportLogsClientIDCutToBound(t *testing.T) {
+	t.Parallel()
+
+	long := strings.Repeat("c", 2*handlers.MaxLoggedFieldBytes)
+
+	var logbuf bytes.Buffer
+
+	h := newTestHandlers(stubAppender{}, &logbuf)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(
+			`{"clientId":"`+long+`","timestamp":"`+long+`","hosts":[]}`,
+		),
+	)
+
+	h.HandleReport().ServeHTTP(rec, req)
+
+	var logged map[string]any
+
+	err := json.Unmarshal(logbuf.Bytes(), &logged)
+	if err != nil {
+		t.Fatalf("log line not JSON: %v (%q)", err, logbuf.String())
+	}
+
+	want := long[:handlers.MaxLoggedFieldBytes]
+
+	if logged["client_id"] != want {
+		t.Fatalf("logged client_id not cut to %d bytes: %q",
+			handlers.MaxLoggedFieldBytes, logged["client_id"])
+	}
+
+	if logged["timestamp"] != want {
+		t.Fatalf("logged timestamp not cut to %d bytes: %q",
+			handlers.MaxLoggedFieldBytes, logged["timestamp"])
+	}
+}
+
+func TestHandleReportDecodeErrorLogIsBounded(t *testing.T) {
+	t.Parallel()
+
+	// A number too large for its int64 field makes the decoder's
+	// error text quote the whole number.
+	huge := strings.Repeat("9", 2*handlers.MaxLoggedFieldBytes)
+
+	var logbuf bytes.Buffer
+
+	h := newTestHandlers(stubAppender{}, &logbuf)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(`{"hosts":[{"history":[{"t":`+huge+`}]}]}`),
+	)
+
+	h.HandleReport().ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
+	}
+
+	if strings.Contains(logbuf.String(), huge) {
+		t.Fatal("the whole oversized number was written to the log")
+	}
+}
diff --git a/backend/internal/middleware/export_test.go b/backend/internal/middleware/export_test.go
index d1961c0..03722f4 100644
--- a/backend/internal/middleware/export_test.go
+++ b/backend/internal/middleware/export_test.go
@@ -1,6 +1,7 @@
 package middleware
 
 import (
+	"log/slog"
 	"net/http"
 	"net/netip"
 )
@@ -8,6 +9,12 @@ import (
 // Test-only wrappers exposing unexported helpers to the
 // external middleware_test package.
 
+// NewWithLogger builds a Middleware around a logger for tests
+// that exercise the logging paths without the fx graph.
+func NewWithLogger(log *slog.Logger) *Middleware {
+	return &Middleware{log: log}
+}
+
 func ClientIP(
 	remoteAddr string,
 	header http.Header,
diff --git a/backend/internal/middleware/middleware.go b/backend/internal/middleware/middleware.go
index 5f5194b..b74092f 100644
--- a/backend/internal/middleware/middleware.go
+++ b/backend/internal/middleware/middleware.go
@@ -3,11 +3,14 @@
 package middleware
 
 import (
+	"errors"
 	"fmt"
+	"io"
 	"log/slog"
 	"net"
 	"net/http"
 	"net/netip"
+	"runtime/debug"
 	"strings"
 	"time"
 
@@ -22,6 +25,14 @@ import (
 
 const corsMaxAgeSec = 300
 
+// jsonErrorBody is the body written for errors raised inside
+// middleware, matching the {"status":"error"} shape the handlers
+// return so clients see one error contract across the API.
+const (
+	jsonContentType = "application/json; charset=utf-8"
+	jsonErrorBody   = "{\"status\":\"error\"}\n"
+)
+
 // Security header values. The backend is a JSON API with no
 // HTML surface, so the CSP forbids every resource type and
 // framing outright.
@@ -236,6 +247,79 @@ func (s *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
 	}
 }
 
+// writeJSONError writes the shared JSON error body with the
+// given status. Used where middleware must reject a request
+// before it reaches a handler.
+func writeJSONError(w http.ResponseWriter, status int) {
+	w.Header().Set("Content-Type", jsonContentType)
+	w.WriteHeader(status)
+	_, _ = io.WriteString(w, jsonErrorBody)
+}
+
+// MaxBodyBytes returns middleware that caps the request body at
+// limit bytes. A declared Content-Length over the limit is
+// rejected immediately with 413. Bodies without a declared
+// length (or that understate it) are capped as they are read, so
+// a handler that reads the body sees a *http.MaxBytesError it can
+// map to 413. Mounted again on a route group, it can only lower
+// the limit: a cap applied earlier in the chain still holds.
+func (s *Middleware) MaxBodyBytes(
+	limit int64,
+) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(
+			func(w http.ResponseWriter, r *http.Request) {
+				if r.ContentLength > limit {
+					writeJSONError(
+						w,
+						http.StatusRequestEntityTooLarge,
+					)
+
+					return
+				}
+
+				r.Body = http.MaxBytesReader(w, r.Body, limit)
+
+				next.ServeHTTP(w, r)
+			},
+		)
+	}
+}
+
+// Recoverer returns middleware that recovers from a panic in a
+// downstream handler, logs the panic and stack trace through
+// slog, and responds 500 with no body. http.ErrAbortHandler is
+// re-panicked so the server can abort the response as intended.
+func (s *Middleware) Recoverer() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(
+			func(w http.ResponseWriter, r *http.Request) {
+				defer func() {
+					rec := recover()
+					if rec == nil {
+						return
+					}
+
+					err, ok := rec.(error)
+					if ok && errors.Is(err, http.ErrAbortHandler) {
+						panic(rec)
+					}
+
+					s.log.ErrorContext(r.Context(),
+						"panic recovered",
+						"panic", fmt.Sprintf("%v", rec),
+						"stack", string(debug.Stack()),
+					)
+
+					w.WriteHeader(http.StatusInternalServerError)
+				}()
+
+				next.ServeHTTP(w, r)
+			},
+		)
+	}
+}
+
 // CORS returns middleware that adds permissive CORS headers.
 func (s *Middleware) CORS() func(http.Handler) http.Handler {
 	return cors.Handler(cors.Options{
diff --git a/backend/internal/middleware/middleware_test.go b/backend/internal/middleware/middleware_test.go
index a664ee6..8016256 100644
--- a/backend/internal/middleware/middleware_test.go
+++ b/backend/internal/middleware/middleware_test.go
@@ -1,9 +1,14 @@
 package middleware_test
 
 import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"log/slog"
 	"net/http"
 	"net/http/httptest"
 	"net/netip"
+	"strings"
 	"testing"
 
 	"sneak.berlin/go/netwatch/internal/middleware"
@@ -146,3 +151,152 @@ func TestSecurityHeaders(t *testing.T) {
 		}
 	}
 }
+
+// TestMaxBodyBytesRejectsOversizeOnNonReadingRoute confirms the
+// limit is enforced even for a handler that never reads the body
+// (for example the health check), via the Content-Length check.
+func TestMaxBodyBytesRejectsOversizeOnNonReadingRoute(t *testing.T) {
+	t.Parallel()
+
+	const limit = 16
+
+	called := false
+	handler := (&middleware.Middleware{}).MaxBodyBytes(limit)(
+		http.HandlerFunc(func(_ http.ResponseWriter, _ *http.Request) {
+			called = true
+		}),
+	)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodPost, "/.well-known/healthcheck",
+		strings.NewReader(strings.Repeat("x", limit+1)),
+	)
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusRequestEntityTooLarge {
+		t.Fatalf("status = %d, want %d",
+			rec.Code, http.StatusRequestEntityTooLarge)
+	}
+
+	if called {
+		t.Fatal("handler ran despite oversize body")
+	}
+
+	if got := rec.Body.String(); got != "{\"status\":\"error\"}\n" {
+		t.Errorf("body = %q, want %q", got, "{\"status\":\"error\"}\n")
+	}
+
+	got := rec.Header().Get("Content-Type")
+	if got != "application/json; charset=utf-8" {
+		t.Errorf("Content-Type = %q, want a JSON content type", got)
+	}
+}
+
+func TestMaxBodyBytesAllowsWithinLimit(t *testing.T) {
+	t.Parallel()
+
+	const limit = 64
+
+	handler := (&middleware.Middleware{}).MaxBodyBytes(limit)(
+		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		}),
+	)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(`{"clientId":"c1"}`),
+	)
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+}
+
+func TestRecovererReturns500AndLogsThroughSlog(t *testing.T) {
+	t.Parallel()
+
+	var logbuf bytes.Buffer
+
+	mw := middleware.NewWithLogger(
+		slog.New(slog.NewJSONHandler(&logbuf, nil)),
+	)
+
+	handler := mw.Recoverer()(
+		http.HandlerFunc(func(_ http.ResponseWriter, _ *http.Request) {
+			panic("boom")
+		}),
+	)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/", http.NoBody)
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusInternalServerError {
+		t.Fatalf("status = %d, want %d",
+			rec.Code, http.StatusInternalServerError)
+	}
+
+	var record map[string]any
+
+	err := json.Unmarshal(logbuf.Bytes(), &record)
+	if err != nil {
+		t.Fatalf("panic log is not one JSON record: %v (%q)", err, logbuf.String())
+	}
+
+	if record["msg"] != "panic recovered" || record["level"] != "ERROR" {
+		t.Errorf("log record = %v, want msg %q at level ERROR",
+			record, "panic recovered")
+	}
+
+	if record["panic"] != "boom" {
+		t.Errorf("panic field = %v, want %q", record["panic"], "boom")
+	}
+
+	stack, _ := record["stack"].(string)
+	if !strings.HasPrefix(stack, "goroutine ") {
+		t.Errorf("stack field = %q, want a stack trace", stack)
+	}
+}
+
+// TestRecovererRepanicsOnAbortHandler checks that a handler aborting
+// with http.ErrAbortHandler is not treated as a crash: Recoverer
+// panics again so the server aborts the response, and logs nothing.
+func TestRecovererRepanicsOnAbortHandler(t *testing.T) {
+	t.Parallel()
+
+	var logbuf bytes.Buffer
+
+	mw := middleware.NewWithLogger(
+		slog.New(slog.NewJSONHandler(&logbuf, nil)),
+	)
+
+	handler := mw.Recoverer()(
+		http.HandlerFunc(func(_ http.ResponseWriter, _ *http.Request) {
+			panic(http.ErrAbortHandler)
+		}),
+	)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/", http.NoBody)
+
+	var recovered any
+
+	func() {
+		defer func() { recovered = recover() }()
+
+		handler.ServeHTTP(rec, req)
+	}()
+
+	err, _ := recovered.(error)
+	if !errors.Is(err, http.ErrAbortHandler) {
+		t.Errorf("Recoverer panicked with %v, want http.ErrAbortHandler", recovered)
+	}
+
+	if logbuf.Len() != 0 {
+		t.Errorf("abort was logged: %q", logbuf.String())
+	}
+}
diff --git a/backend/internal/reportbuf/reportbuf.go b/backend/internal/reportbuf/reportbuf.go
index 7b89266..4626770 100644
--- a/backend/internal/reportbuf/reportbuf.go
+++ b/backend/internal/reportbuf/reportbuf.go
@@ -80,12 +80,16 @@ func New(
 			// stopOnce makes OnStop idempotent: a second
 			// invocation must not close an already-closed channel
 			// (which would panic) or flush again.
+			var err error
+
 			b.stopOnce.Do(func() {
 				close(b.done)
-				b.flushLocked()
+				err = b.flushLocked()
 			})
 
-			return nil
+			// A failed final flush fails the stop, so the process
+			// exits non-zero.
+			return err
 		},
 	})
 
@@ -109,7 +113,12 @@ func (b *Buffer) Append(v any) error {
 		data := b.drainBuf()
 		b.mu.Unlock()
 
-		go b.writeFile(data)
+		go func() {
+			writeErr := b.writeFile(data)
+			if writeErr != nil {
+				b.log.Error("flush reports failed", "error", writeErr)
+			}
+		}()
 
 		return nil
 	}
@@ -128,7 +137,10 @@ func (b *Buffer) flushLoop() {
 	for {
 		select {
 		case <-ticker.C:
-			b.flushLocked()
+			err := b.flushLocked()
+			if err != nil {
+				b.log.Error("flush reports failed", "error", err)
+			}
 		case <-b.done:
 			return
 		}
@@ -137,19 +149,19 @@ func (b *Buffer) flushLoop() {
 
 // flushLocked acquires the lock, drains the buffer, and
 // writes the data to a compressed file.
-func (b *Buffer) flushLocked() {
+func (b *Buffer) flushLocked() error {
 	b.mu.Lock()
 
 	if b.buf.Len() == 0 {
 		b.mu.Unlock()
 
-		return
+		return nil
 	}
 
 	data := b.drainBuf()
 	b.mu.Unlock()
 
-	b.writeFile(data)
+	return b.writeFile(data)
 }
 
 // drainBuf copies the buffer contents and resets it.
@@ -164,7 +176,7 @@ func (b *Buffer) drainBuf() []byte {
 
 // writeFile creates a timestamped zstd-compressed JSONL file
 // in the data directory.
-func (b *Buffer) writeFile(data []byte) {
+func (b *Buffer) writeFile(data []byte) error {
 	ts := time.Now().UTC().Format("2006-01-02T15-04-05.000Z")
 	name := fmt.Sprintf("reports-%s.jsonl.zst", ts)
 	path := filepath.Join(b.dataDir, name)
@@ -177,31 +189,35 @@ func (b *Buffer) writeFile(data []byte) {
 		filePerms,
 	)
 	if err != nil {
-		b.log.Error("create report file", "error", err)
-
-		return
+		return fmt.Errorf("create report file: %w", err)
 	}
 
+	// Closes the file on the early returns below. The success
+	// path closes it explicitly to check the error; closing it
+	// a second time here is harmless.
 	defer func() { _ = f.Close() }()
 
 	enc, err := zstd.NewWriter(f)
 	if err != nil {
-		b.log.Error("create zstd encoder", "error", err)
-
-		return
+		return fmt.Errorf("create zstd encoder: %w", err)
 	}
 
-	_, writeErr := enc.Write(data)
-	if writeErr != nil {
-		b.log.Error("write compressed data", "error", writeErr)
-
+	_, err = enc.Write(data)
+	if err != nil {
 		_ = enc.Close()
 
-		return
+		return fmt.Errorf("write compressed data: %w", err)
 	}
 
-	closeErr := enc.Close()
-	if closeErr != nil {
-		b.log.Error("close zstd encoder", "error", closeErr)
+	err = enc.Close()
+	if err != nil {
+		return fmt.Errorf("close zstd encoder: %w", err)
 	}
+
+	err = f.Close()
+	if err != nil {
+		return fmt.Errorf("close report file: %w", err)
+	}
+
+	return nil
 }
diff --git a/backend/internal/reportbuf/reportbuf_test.go b/backend/internal/reportbuf/reportbuf_test.go
index 4b6b550..ade9457 100644
--- a/backend/internal/reportbuf/reportbuf_test.go
+++ b/backend/internal/reportbuf/reportbuf_test.go
@@ -1,6 +1,8 @@
 package reportbuf_test
 
 import (
+	"errors"
+	"io/fs"
 	"os"
 	"strings"
 	"testing"
@@ -52,6 +54,46 @@ func TestFlushOnShutdown(t *testing.T) {
 	}
 }
 
+// TestFailedFinalFlushFailsStop proves a final flush that cannot
+// write its file makes the stop fail, which makes the process
+// exit non-zero instead of dropping the buffered reports silently.
+func TestFailedFinalFlushFailsStop(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("DATA_DIR", dir)
+
+	var buf *reportbuf.Buffer
+
+	app := fxtest.New(t,
+		fx.Provide(
+			globals.New,
+			logger.New,
+			config.New,
+			reportbuf.New,
+		),
+		fx.Populate(&buf),
+	)
+
+	app.RequireStart()
+
+	err := buf.Append(map[string]string{"probe": "shutdown"})
+	if err != nil {
+		t.Fatalf("append report: %v", err)
+	}
+
+	// Removing the data directory leaves the final flush nowhere to
+	// write. A read-only directory would not do: tests run as root
+	// in the backend image, and root ignores the read-only bit.
+	err = os.RemoveAll(dir)
+	if err != nil {
+		t.Fatalf("remove data dir: %v", err)
+	}
+
+	err = app.Stop(t.Context())
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("stop error = %v, want the final flush's error", err)
+	}
+}
+
 func hasReportFile(t *testing.T, dir string) bool {
 	t.Helper()
 
diff --git a/backend/internal/server/export_test.go b/backend/internal/server/export_test.go
new file mode 100644
index 0000000..7279b22
--- /dev/null
+++ b/backend/internal/server/export_test.go
@@ -0,0 +1,5 @@
+package server
+
+// MaxRequestBodyBytes exposes the router-wide body limit to the
+// external tests.
+const MaxRequestBodyBytes = maxRequestBodyBytes
diff --git a/backend/internal/server/routes.go b/backend/internal/server/routes.go
index 5f2dd8b..9d4547d 100644
--- a/backend/internal/server/routes.go
+++ b/backend/internal/server/routes.go
@@ -7,18 +7,26 @@ import (
 	"github.com/go-chi/chi/v5/middleware"
 )
 
-const requestTimeout = 60 * time.Second
+const (
+	requestTimeout = 60 * time.Second
+
+	// maxRequestBodyBytes caps every request body. A route group
+	// can mount s.mw.MaxBodyBytes with a smaller value to lower
+	// its bound, but cannot raise it: this cap runs first.
+	maxRequestBodyBytes int64 = 1 << 20 // 1 MiB
+)
 
 // SetupRoutes configures the chi router with middleware and
 // all application routes.
 func (s *Server) SetupRoutes() {
 	s.router = chi.NewRouter()
 
-	s.router.Use(middleware.Recoverer)
+	s.router.Use(s.mw.Recoverer())
 	s.router.Use(middleware.RequestID)
 	s.router.Use(s.mw.Logging())
 	s.router.Use(s.mw.SecurityHeaders())
 	s.router.Use(s.mw.CORS())
+	s.router.Use(s.mw.MaxBodyBytes(maxRequestBodyBytes))
 	s.router.Use(middleware.Timeout(requestTimeout))
 
 	s.router.Get(
diff --git a/backend/internal/server/routes_test.go b/backend/internal/server/routes_test.go
new file mode 100644
index 0000000..1a50a7f
--- /dev/null
+++ b/backend/internal/server/routes_test.go
@@ -0,0 +1,76 @@
+package server_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/netwatch/internal/config"
+	"sneak.berlin/go/netwatch/internal/globals"
+	"sneak.berlin/go/netwatch/internal/handlers"
+	"sneak.berlin/go/netwatch/internal/healthcheck"
+	"sneak.berlin/go/netwatch/internal/logger"
+	"sneak.berlin/go/netwatch/internal/middleware"
+	"sneak.berlin/go/netwatch/internal/reportbuf"
+	"sneak.berlin/go/netwatch/internal/server"
+
+	"go.uber.org/fx"
+	"go.uber.org/fx/fxtest"
+)
+
+// TestHealthCheckRejectsOversizeBody sends the health check, which
+// never reads its body, a body one byte over the limit. Only the
+// router-wide body limit can reject it.
+func TestHealthCheckRejectsOversizeBody(t *testing.T) {
+	t.Parallel()
+
+	var srv *server.Server
+
+	// The same constructors as main, never started: SetupRoutes is
+	// called directly, so nothing listens.
+	app := fxtest.New(t,
+		fx.Provide(
+			config.New,
+			globals.New,
+			handlers.New,
+			healthcheck.New,
+			logger.New,
+			middleware.New,
+			reportbuf.New,
+			server.New,
+		),
+		fx.Populate(&srv),
+	)
+
+	err := app.Err()
+	if err != nil {
+		t.Fatalf("build server: %v", err)
+	}
+
+	srv.SetupRoutes()
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodGet, "/.well-known/healthcheck",
+		strings.NewReader(
+			strings.Repeat("x", int(server.MaxRequestBodyBytes)+1),
+		),
+	)
+
+	srv.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusRequestEntityTooLarge {
+		t.Fatalf("status = %d, want %d",
+			rec.Code, http.StatusRequestEntityTooLarge)
+	}
+
+	if got := rec.Body.String(); got != "{\"status\":\"error\"}\n" {
+		t.Errorf("body = %q, want %q", got, "{\"status\":\"error\"}\n")
+	}
+
+	got := rec.Header().Get("Content-Type")
+	if got != "application/json; charset=utf-8" {
+		t.Errorf("Content-Type = %q, want a JSON content type", got)
+	}
+}
-- 
2.54.0


From 45d2ad21bcc40f6ec3b3d42db2b865562a56dc3d Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Mon, 28 Sep 2026 23:39:11 +0200
Subject: [PATCH 10/22] feat(frontend): post collected samples to
 /api/v1/reports (closes #53)

A Reporter beside AppState POSTs each host's unreported, non-paused
samples to /api/v1/reports every reportInterval (default 60s).
buildReport is an exported pure function of host state; init() runs only
when #app exists, so a test can import the module. A per-host mark
advances only on a delivered POST. At most one report POST is pending at
a time and it is abandoned after half the interval, so a slow POST never
overlaps the next report and a mark never moves backwards. The samples of
an abandoned POST are sent again at the next interval, so a backend that
stored them but answered late receives them twice. Failure is quiet and
never blocks probing. vite.config.js proxies /api for yarn dev.

Model: opus-5-5
---
 README.md      |  15 ++++
 TODO.md        |  10 +++
 src/main.js    | 181 +++++++++++++++++++++++++++++++++++++++++++++++--
 vite.config.js |   7 ++
 4 files changed, 209 insertions(+), 4 deletions(-)

diff --git a/README.md b/README.md
index 1ecaa46..6924046 100644
--- a/README.md
+++ b/README.md
@@ -23,6 +23,9 @@ docker build -t netwatch .
 docker run -p 8080:8080 netwatch
 ```
 
+`yarn dev` proxies `/api` to `http://127.0.0.1:8080`, so a locally running
+`netwatch-server` (see `backend/`) receives the reports the page posts.
+
 ## Entrypoints
 
 This repository adheres to the
@@ -85,6 +88,18 @@ code lives in `src/main.js` with a class-based architecture:
 - **`tick()`**: Main loop — measures all hosts in parallel via `Promise.all`,
   pushes samples, redraws UI. When paused, pushes blank markers (no probes, no
   false outage)
+- **`Reporter`**: Posts collected samples to the backend
+
+### Reporting
+
+Every `reportInterval` (default 60s) the page POSTs a JSON report to the
+same-origin path `/api/v1/reports`: a random per-browser `clientId` kept in
+`localStorage`, `geo` sent as null, and each host's unreported, non-paused
+samples (timestamp, latency, error). A per-host high-water mark makes every
+report a delta, so only new samples are sent; the mark advances only on a
+delivered report, and while paused nothing is sent. Delivery failure is quiet —
+one debug-log line per outage, retried at the next interval, never blocking
+probing. The report-building step is a pure function of host state.
 
 ### Monitoring targets
 
diff --git a/TODO.md b/TODO.md
index 29e4877..ae23deb 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,16 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-28: frontend reporting client (issue #53): a `Reporter` class posts
+  collected samples to `/api/v1/reports` every `reportInterval` (default 60s) as
+  a per-host delta, with the report-building step a pure exported function of
+  host state; a per-host mark advances only on a delivered POST; at most one
+  report POST is pending at a time and it is abandoned after half the interval,
+  so a slow POST never overlaps the next report and a mark never moves
+  backwards; the samples of an abandoned POST are sent again at the next
+  interval, so a backend that stored them but answered late receives them twice;
+  the per-browser client id works in insecure (plain-HTTP) contexts;
+  `vite.config.js` proxies `/api` to the local backend for `yarn dev`
 - 2026-09-28: report ingest correctness (issue #23): a storage failure now
   returns 500 instead of a false `ok`; oversize bodies return 413 (distinguished
   from malformed JSON, which stays 400); a `MaxBodyBytes` middleware caps every
diff --git a/src/main.js b/src/main.js
index 135d36a..21b4a62 100644
--- a/src/main.js
+++ b/src/main.js
@@ -7,9 +7,11 @@ import "./styles.css";
 // graphMaxLatency — values above it pin to the top of the chart but still
 // display their real value in the latency figure. The history buffer holds
 // maxHistoryPoints samples (historyDuration / updateInterval).
+// reportInterval is how often collected samples are POSTed to the backend.
 const CONFIG = {
     updateInterval: 3000,
     maxHistoryPoints: 100,
+    reportInterval: 60000,
     get historyDuration() {
         return (this.maxHistoryPoints * this.updateInterval) / 1000;
     },
@@ -346,6 +348,159 @@ class AppState {
     }
 }
 
+// --- Reporting ---------------------------------------------------------------
+
+// A random UUIDv4. `crypto.randomUUID` exists only in secure contexts
+// (HTTPS or localhost); over plain HTTP to any other host — the normal LAN
+// deployment — it is undefined, so feature-detect it and otherwise build the
+// id from `crypto.getRandomValues`, which is available in insecure contexts.
+function randomId() {
+    if (typeof crypto !== "undefined" && crypto.randomUUID) {
+        return crypto.randomUUID();
+    }
+    const bytes = new Uint8Array(16);
+    crypto.getRandomValues(bytes);
+    bytes[6] = (bytes[6] & 0x0f) | 0x40; // version 4
+    bytes[8] = (bytes[8] & 0x3f) | 0x80; // variant 1
+    const hex = [...bytes].map((b) => b.toString(16).padStart(2, "0"));
+    return (
+        hex.slice(0, 4).join("") +
+        "-" +
+        hex.slice(4, 6).join("") +
+        "-" +
+        hex.slice(6, 8).join("") +
+        "-" +
+        hex.slice(8, 10).join("") +
+        "-" +
+        hex.slice(10, 16).join("")
+    );
+}
+
+// A random id identifying this browser across reports. Generated once and
+// kept in localStorage; if storage is unavailable (e.g. private mode) a
+// fresh id is used for this session only.
+function getClientId() {
+    const key = "netwatch-client-id";
+    try {
+        let id = localStorage.getItem(key);
+        if (!id) {
+            id = randomId();
+            localStorage.setItem(key, id);
+        }
+        return id;
+    } catch {
+        return randomId();
+    }
+}
+
+// Build the delta report body the backend decodes, plus the new per-host
+// high-water marks. Pure function of the passed state: `hosts` is an array
+// of { name, url, status, history }, `since` maps a host url to the Unix-ms
+// timestamp of the last sample already reported for it, and `now` is a Date.
+// Only non-paused samples newer than the mark are included. Returns null
+// when no host has an unreported sample.
+export function buildReport(hosts, clientId, now, since) {
+    const reportHosts = [];
+    const marks = new Map();
+    for (const host of hosts) {
+        const mark = since.get(host.url) ?? 0;
+        const samples = [];
+        let high = mark;
+        for (const p of host.history) {
+            if (p.paused) continue;
+            if (p.timestamp <= mark) continue;
+            samples.push({
+                t: p.timestamp,
+                latency: p.latency,
+                error: p.error ?? null,
+            });
+            if (p.timestamp > high) high = p.timestamp;
+        }
+        if (samples.length === 0) continue;
+        reportHosts.push({
+            name: host.name,
+            url: host.url,
+            status: host.status,
+            history: samples,
+        });
+        marks.set(host.url, high);
+    }
+    if (reportHosts.length === 0) return null;
+    return {
+        body: {
+            clientId,
+            geo: null,
+            hosts: reportHosts,
+            timestamp: now.toISOString(),
+        },
+        marks,
+    };
+}
+
+// Periodically POSTs unreported samples to the same-origin backend. Holds
+// the per-host high-water marks so each report is a delta; marks only
+// advance on a delivered report, so a failed POST simply re-sends those
+// samples next interval (bounded by the history window — whatever has since
+// fallen out is dropped). Failure is quiet: one debug line per outage, one
+// on recovery, never an alert, never a tight retry loop.
+//
+// Only one report is ever in flight, and it is abandoned after half the
+// interval, so a slow POST can neither overlap the next report (which would
+// carry the same samples) nor stall reporting for good.
+class Reporter {
+    constructor(state, clientId, intervalMs) {
+        this.state = state;
+        this.clientId = clientId;
+        this.intervalMs = intervalMs;
+        this.marks = new Map();
+        this.failing = false;
+        this.sending = false;
+        this.timerId = null;
+    }
+
+    start() {
+        if (this.timerId) return;
+        this.timerId = setInterval(() => this.flush(), this.intervalMs);
+    }
+
+    async flush() {
+        if (this.sending || this.state.paused) return;
+        const report = buildReport(
+            this.state.allHosts,
+            this.clientId,
+            new Date(),
+            this.marks,
+        );
+        if (!report) return;
+        this.sending = true;
+        try {
+            const resp = await fetch("/api/v1/reports", {
+                method: "POST",
+                headers: { "Content-Type": "application/json" },
+                credentials: "omit",
+                body: JSON.stringify(report.body),
+                signal: AbortSignal.timeout(this.intervalMs / 2),
+            });
+            if (!resp.ok) throw new Error(`HTTP ${resp.status}`);
+            for (const [url, t] of report.marks) {
+                const current = this.marks.get(url) ?? 0;
+                this.marks.set(url, Math.max(current, t));
+            }
+            if (this.failing) {
+                log.debug("Report delivery recovered");
+                this.failing = false;
+            }
+        } catch (err) {
+            if (!this.failing) {
+                log.debug(`Report delivery failed: ${err.message}`);
+                this.failing = true;
+            }
+        } finally {
+            this.sending = false;
+        }
+    }
+}
+
 // --- Latency Measurement -----------------------------------------------------
 
 async function measureLatency(url) {
@@ -1162,6 +1317,19 @@ async function init() {
     buildUI(state);
     log.info("UI built, starting tick loop");
 
+    // Reporting is best-effort: any failure setting it up (e.g. no usable
+    // crypto for the client id) must never stop the monitor from probing.
+    try {
+        const reporter = new Reporter(
+            state,
+            getClientId(),
+            CONFIG.reportInterval,
+        );
+        reporter.start();
+    } catch (err) {
+        log.error(`Reporting disabled: ${err.message}`);
+    }
+
     document
         .getElementById("pause-btn")
         .addEventListener("click", () => togglePause(state));
@@ -1274,8 +1442,13 @@ async function init() {
     setTimeout(() => handleResize(state), 100);
 }
 
-if (document.readyState === "loading") {
-    document.addEventListener("DOMContentLoaded", init);
-} else {
-    init();
+// Bootstrap only when loaded as the page: a real DOM containing the #app
+// mount point this module renders into. Importing the module in a unit test
+// (which has no #app) runs nothing, so buildReport can be tested in isolation.
+if (typeof document !== "undefined" && document.getElementById("app")) {
+    if (document.readyState === "loading") {
+        document.addEventListener("DOMContentLoaded", init);
+    } else {
+        init();
+    }
 }
diff --git a/vite.config.js b/vite.config.js
index 92cd874..0f362bd 100644
--- a/vite.config.js
+++ b/vite.config.js
@@ -7,6 +7,13 @@ const commitFull = execSync("git rev-parse HEAD").toString().trim();
 
 export default defineConfig({
     plugins: [tailwindcss()],
+    server: {
+        // Proxy /api to a locally running netwatch-server so `yarn dev`
+        // exercises the real report-posting path.
+        proxy: {
+            "/api": "http://127.0.0.1:8080",
+        },
+    },
     define: {
         __COMMIT_HASH__: JSON.stringify(commitHash),
         __COMMIT_FULL__: JSON.stringify(commitFull),
-- 
2.54.0


From de4e86c433dda8280d9478cfe76cfbc98a9ee60e Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Tue, 29 Sep 2026 01:22:08 +0200
Subject: [PATCH 11/22] build: unify the gate so root make check covers the
 backend (closes #16)

Root make check, and with it the pre-commit hook, now gates the Go
backend too. The backend's Makefile targets are shims over
backend/script/*; script/cibuild builds both images and is the
workflow's only build step. Root make test runs both halves within one
30-second timeout.

Root make lint runs golangci-lint only in Docker, by building the lint
stage of Dockerfile.backend without the cache; the .golangci.yml drift
check moved into backend/script/lint. script/bootstrap installs no
linter: it reuses a Go at least as new as backend/go.mod asks for,
otherwise installs the pinned, hash-verified release, linked into
~/.local/bin without replacing anything it did not create. With VERSION
unset or empty, the backend version falls back to git describe inside a
git checkout, then to dev.

Model: opus-5-5
---
 .gitea/workflows/check.yml    |   2 +-
 Dockerfile                    |  10 ++-
 Dockerfile.backend            |   9 +--
 Makefile                      |  10 ++-
 README.md                     |  36 ++++++---
 TODO.md                       |  12 +++
 backend/Makefile              |  67 ++++-------------
 backend/README.md             |  38 +++++++++-
 backend/script/build          |  21 ++++++
 backend/script/clean          |  12 +++
 backend/script/fmt            |  12 +++
 backend/script/fmt-check      |  18 +++++
 backend/script/lint           |  32 ++++++++
 backend/script/run            |  13 ++++
 backend/script/test           |  12 +++
 script/bootstrap              | 137 +++++++++++++++++++++++++++++++---
 script/cibuild                |  10 ++-
 script/docker                 |   9 ++-
 script/fmt                    |   6 +-
 script/fmt-check              |   7 +-
 script/frontend-check         |  18 +++++
 script/frontend-fmt           |  14 ++++
 script/frontend-fmt-check     |  13 ++++
 script/frontend-lint          |  12 +++
 script/frontend-test          |  14 ++++
 script/frontend-viewport-test |   2 +-
 script/lint                   |  13 +++-
 script/test                   |   7 +-
 28 files changed, 460 insertions(+), 106 deletions(-)
 create mode 100755 backend/script/build
 create mode 100755 backend/script/clean
 create mode 100755 backend/script/fmt
 create mode 100755 backend/script/fmt-check
 create mode 100755 backend/script/lint
 create mode 100755 backend/script/run
 create mode 100755 backend/script/test
 create mode 100755 script/frontend-check
 create mode 100755 script/frontend-fmt
 create mode 100755 script/frontend-fmt-check
 create mode 100755 script/frontend-lint
 create mode 100755 script/frontend-test

diff --git a/.gitea/workflows/check.yml b/.gitea/workflows/check.yml
index 860d9dd..a732557 100644
--- a/.gitea/workflows/check.yml
+++ b/.gitea/workflows/check.yml
@@ -6,5 +6,5 @@ jobs:
         steps:
             # actions/checkout v4.2.2, 2026-02-22
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+            # script/cibuild builds both images.
             - run: script/cibuild
-            - run: docker build -f Dockerfile.backend .
diff --git a/Dockerfile b/Dockerfile
index ed37836..492b41d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -5,10 +5,12 @@ COPY package.json yarn.lock ./
 RUN yarn install --frozen-lockfile
 RUN apk add --no-cache git make
 COPY . .
-# make check runs script/check (test + lint + fmt-check); its test step
-# is the production yarn build, so this both produces dist/ and gates the
-# image on lint/fmt-check/test regressions, not merely a broken build.
-RUN make check
+# make frontend-check is the frontend half of make check (test + lint +
+# fmt-check); its test step is the production yarn build, so this both
+# produces dist/ and gates the image on lint/fmt-check/test regressions.
+# This node stage has neither Go nor Docker for the other half, which
+# Dockerfile.backend gates; script/cibuild builds both images.
+RUN make frontend-check
 
 # nginx:stable-alpine as of 2026-02-22
 FROM nginx@sha256:15e96e59aa3b0aada3a121296e3bce117721f42d88f5f64217ef4b18f458c6ab
diff --git a/Dockerfile.backend b/Dockerfile.backend
index 191a65e..51a530f 100644
--- a/Dockerfile.backend
+++ b/Dockerfile.backend
@@ -30,12 +30,11 @@ COPY backend/ .
 
 RUN make test
 
-# The build is driven through the Makefile so there is exactly one
-# definition of the build command; it expands to
+# make build is a shim around backend/script/build, the one definition
+# of the build command:
 # CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.Version=... -X main.Buildarch=..."
-# VERSION is handed over in the environment rather than as a make
-# variable so it still reaches the build if the target is ever turned
-# into a shim around a script.
+# That script reads VERSION from the environment, so it is handed over
+# there rather than as a make variable.
 ARG VERSION=dev
 RUN VERSION="${VERSION}" make build
 
diff --git a/Makefile b/Makefile
index 20f313f..eb7bb43 100644
--- a/Makefile
+++ b/Makefile
@@ -1,9 +1,10 @@
-.PHONY: bootstrap setup dev test lint fmt fmt-check check \
+.PHONY: bootstrap setup dev test lint fmt fmt-check check frontend-check \
 	frontend-viewport-test docker hooks
 
 # Standard targets are thin shims; the implementations live in script/
 # per the scripts-to-rule-them-all pattern (see the Entrypoints section
-# of README.md).
+# of README.md). test, lint, fmt, fmt-check and check cover the whole
+# repo: the frontend here and the Go backend in backend/.
 
 bootstrap:
 	@script/bootstrap
@@ -29,6 +30,11 @@ fmt-check:
 check:
 	@script/check
 
+# The frontend half of check, for Dockerfile's node build stage, which
+# has neither Go nor Docker. Use check everywhere else.
+frontend-check:
+	@script/frontend-check
+
 # Responsive-layout verification in a containerised browser. Kept out of
 # check: it needs Docker and takes minutes, where make test has to stay
 # under 20 seconds.
diff --git a/README.md b/README.md
index 6924046..52e6382 100644
--- a/README.md
+++ b/README.md
@@ -31,25 +31,41 @@ docker run -p 8080:8080 netwatch
 This repository adheres to the
 [Scripts to Rule Them All](https://github.com/github/scripts-to-rule-them-all)
 standard: normalized scripts in `script/` are the entrypoints for the
-development workflow, and the Makefile targets are thin shims that call them. We
-provide:
+development workflow, and the Makefile targets are thin shims that call them.
+The Go backend in `backend/` has its own `script/` directory and shim Makefile
+(see [backend/README.md](backend/README.md)). The root scripts cover both
+halves, so the root `make check` fails if either one is broken. We provide:
 
 - `script/bootstrap` — install all dependencies (pinned node via nvm if needed,
-  yarn via corepack, `yarn install --frozen-lockfile`)
+  yarn via corepack, `yarn install --frozen-lockfile`, the pinned Go unless one
+  at least as new as `backend/go.mod` asks for is installed, and the Go
+  modules), linking what it installs itself into `~/.local/bin`, which has to be
+  on `PATH`. It installs no Go linter and not Docker: `make lint` runs the
+  linter in Docker
 - `script/setup` — make a fresh clone ready for development: bootstrap plus the
   git pre-commit hook
-- `script/projectname` — print the project name (used for the Docker image tag)
-- `script/test` — run the production build as the test (no unit tests yet)
-- `script/lint` — run prettier in check mode
-- `script/fmt` — format all files (writes)
-- `script/fmt-check` — check formatting (read-only)
+- `script/projectname` — print the project name (used for the Docker image tags)
+- `script/test` — run `script/frontend-test`, then the backend's Go tests, both
+  within one 30-second timeout
+- `script/lint` — run `script/frontend-lint`, then golangci-lint in Docker, by
+  building the lint stage of `Dockerfile.backend` without the cache
+- `script/fmt` — format all files (writes): prettier, then gofmt over `backend/`
+- `script/fmt-check` — check formatting (read-only): prettier, then gofmt
 - `script/check` — run test, lint, and fmt-check
+- `script/frontend-test` — run the production build as the frontend's test (no
+  unit tests yet)
+- `script/frontend-lint` — run prettier in check mode
+- `script/frontend-fmt` — format everything prettier understands (writes)
+- `script/frontend-fmt-check` — check prettier formatting (read-only)
+- `script/frontend-check` — the frontend half of `script/check`, for
+  `Dockerfile`, whose node build stage has neither Go nor Docker
 - `script/frontend-viewport-test` — responsive-layout verification of the built
   frontend in a containerised headless Chrome (see
   [test/viewport/README.md](test/viewport/README.md)). Not part of
   `script/check`: it needs Docker and takes minutes.
-- `script/docker` — build the Docker image tagged via `script/projectname`
-- `script/cibuild` — CI entrypoint: plain `docker build .`
+- `script/docker` — build both images, tagged via `script/projectname`:
+  `netwatch` from `Dockerfile` and `netwatch-server` from `Dockerfile.backend`
+- `script/cibuild` — CI entrypoint: builds both images
 - `script/precommit` — run by the git pre-commit hook; runs `script/check`
 - `script/install-precommit` — install the git pre-commit hook
 
diff --git a/TODO.md b/TODO.md
index ae23deb..ecb3301 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,18 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-28: unified the gate (issue #16): the root `make check` covers the Go
+  backend as well as the frontend, and the pre-commit hook with it; the backend
+  moved onto scripts-to-rule-them-all (`backend/script/*`, `backend/Makefile` as
+  shims, its duplicate hook installer removed); `script/cibuild` builds both
+  images and is the workflow's only build step. The root `make lint` runs
+  golangci-lint only in Docker, by building the lint stage of
+  `Dockerfile.backend` without the cache. `script/bootstrap` installs the pinned
+  Go unless the installed one is at least what `backend/go.mod` asks for, links
+  what it installs into `~/.local/bin` without replacing anything it did not
+  create, and installs no linter. Root `make test` runs both halves within one
+  30-second timeout. When `VERSION` is unset or empty, the backend binary's
+  version falls back to `git describe` inside a git checkout, then to `dev`
 - 2026-09-28: frontend reporting client (issue #53): a `Reporter` class posts
   collected samples to `/api/v1/reports` every `reportInterval` (default 60s) as
   a per-host delta, with the report-building step a pure exported function of
diff --git a/backend/Makefile b/backend/Makefile
index 96edfb9..5724459 100644
--- a/backend/Makefile
+++ b/backend/Makefile
@@ -1,69 +1,30 @@
-# VERSION is overridable (the Dockerfile passes its ARG VERSION in) and
-# degrades to "dev" when git is unavailable or there is no .git — the
-# build must not depend on the repository history being in the build
-# context.
-VERSION ?= $(shell { git describe --always --dirty; } 2>/dev/null || echo dev)
-BUILDARCH := $(shell uname -m)
-BINARY := netwatch-server
+# Thin shims; the implementations live in backend/script/ (see the
+# Entrypoints section of README.md). There is no check, hooks or docker
+# target here: the root Makefile's check covers this directory, its
+# hooks target installs the repo's only pre-commit hook, and its docker
+# target builds this image, whose build context is the repo root.
 
-GOLDFLAGS += -s -w
-GOLDFLAGS += -X main.Version=$(VERSION)
-GOLDFLAGS += -X main.Buildarch=$(BUILDARCH)
-
-# macOS ships shasum rather than sha256sum.
-SHA256SUM := $(shell command -v sha256sum >/dev/null 2>&1 && echo sha256sum || echo shasum -a 256)
-
-# .golangci.yml is standardized org-wide and must never be edited here
-# (REPO_POLICIES.md). Its last silent drift replaced the v2 schema with
-# v1 keys, which left every threshold in the file inert while the build
-# stayed green. The lint target therefore asserts the file still matches
-# the canonical copy byte for byte. The check is a local hash comparison:
-# no network, no remote schema, nothing unpinned in the build path.
-GOLANGCI_CONFIG_SHA256 := 021cc83f4e6fc7c31b95b34b846723dfcf20b66b7baeea1dc40406e643346bcb
-
-.PHONY: all build test lint fmt fmt-check check docker hooks run clean
+.PHONY: all build test lint fmt fmt-check run clean
 
 all: build
 
 build:
-	CGO_ENABLED=0 go build -trimpath -ldflags "$(GOLDFLAGS)" \
-		-o ./$(BINARY) ./cmd/netwatch-server/
+	@script/build
 
 test:
-	timeout 30 go test ./...
+	@script/test
 
 lint:
-	@actual=$$($(SHA256SUM) .golangci.yml | cut -d' ' -f1); \
-	if [ "$$actual" != "$(GOLANGCI_CONFIG_SHA256)" ]; then \
-		echo ".golangci.yml has drifted from the org standard."; \
-		echo "  expected $(GOLANGCI_CONFIG_SHA256)"; \
-		echo "  actual   $$actual"; \
-		echo "Restore it verbatim from sneak/prompts; do not edit it."; \
-		exit 1; \
-	fi
-	golangci-lint run ./...
+	@script/lint
 
 fmt:
-	go fmt ./...
+	@script/fmt
 
 fmt-check:
-	@test -z "$$(gofmt -l .)" || \
-		(echo "Files not formatted:"; gofmt -l .; exit 1)
+	@script/fmt-check
 
-check: test lint fmt-check
-
-docker:
-	timeout 300 docker build -t netwatch-server -f ../Dockerfile.backend ..
-
-hooks:
-	@printf '#!/bin/sh\ncd backend && make check\n' > \
-		$$(git rev-parse --show-toplevel)/.git/hooks/pre-commit
-	@chmod +x \
-		$$(git rev-parse --show-toplevel)/.git/hooks/pre-commit
-	@echo "Pre-commit hook installed"
-
-run: build
-	./$(BINARY)
+run:
+	@script/run
 
 clean:
-	rm -f ./$(BINARY)
+	@script/clean
diff --git a/backend/README.md b/backend/README.md
index 3cad737..1230c12 100644
--- a/backend/README.md
+++ b/backend/README.md
@@ -4,18 +4,50 @@ SPA and persists them as zstd-compressed JSONL files on disk.
 
 ## Getting Started
 
+From this directory:
+
 ```bash
 # Build and run locally
 make run
+```
 
-# Run tests, lint, and format check
+From the repo root, which is also the build context of `Dockerfile.backend`:
+
+```bash
+# Run tests, lint, and format check over the frontend and this backend
 make check
 
-# Docker
-docker build -t netwatch-server .
+# Build both images, including netwatch-server
+make docker
 docker run -p 8080:8080 netwatch-server
 ```
 
+## Entrypoints
+
+This directory follows the same
+[Scripts to Rule Them All](https://github.com/github/scripts-to-rule-them-all)
+pattern as the repo root: the targets in `backend/Makefile` are thin shims over
+`backend/script/`. `Dockerfile.backend` runs them, and the root scripts call
+`test`, `fmt` and `fmt-check`:
+
+- `script/build` — compile the static `netwatch-server` binary with its version
+  and architecture stamped in. The version is `VERSION` from the environment;
+  when that is unset or empty, it falls back to `git describe` inside a git
+  checkout, then to `dev`
+- `script/test` — run the Go tests under a 30-second timeout
+- `script/lint` — check `.golangci.yml` against its pinned sha256, then run
+  golangci-lint. It runs inside the golangci-lint image of the lint stage of
+  `Dockerfile.backend`; from a checkout, run `make lint` at the repo root, which
+  builds that stage
+- `script/fmt` — format the Go sources (writes)
+- `script/fmt-check` — check Go formatting (read-only)
+- `script/run` — build and run the server locally
+- `script/clean` — remove build artifacts
+
+There is no `check`, `hooks` or `docker` target here: the root `make check`
+covers this directory, the root `make hooks` installs the repo's only pre-commit
+hook, and the root `make docker` builds this image.
+
 ## Rationale
 
 The NetWatch frontend collects latency measurements from the browser but has no
diff --git a/backend/script/build b/backend/script/build
new file mode 100755
index 0000000..36b3e96
--- /dev/null
+++ b/backend/script/build
@@ -0,0 +1,21 @@
+#!/bin/sh
+# script/build: compile the static netwatch-server binary into the
+# backend project root, with its version and architecture stamped in.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+
+    # VERSION comes from the environment (Dockerfile.backend passes its
+    # ARG VERSION in). Unset or empty, it is git describe, or "dev" where
+    # there is no git or no repository history.
+    version="${VERSION:-$(git describe --always --dirty 2>/dev/null || echo dev)}"
+
+    CGO_ENABLED=0 go build -trimpath \
+        -ldflags "-s -w -X main.Version=$version -X main.Buildarch=$(uname -m)" \
+        -o netwatch-server ./cmd/netwatch-server/
+}
+
+main "$@"
diff --git a/backend/script/clean b/backend/script/clean
new file mode 100755
index 0000000..1f4e638
--- /dev/null
+++ b/backend/script/clean
@@ -0,0 +1,12 @@
+#!/bin/sh
+# script/clean: remove build artifacts.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    rm -f netwatch-server
+}
+
+main "$@"
diff --git a/backend/script/fmt b/backend/script/fmt
new file mode 100755
index 0000000..9fa78bf
--- /dev/null
+++ b/backend/script/fmt
@@ -0,0 +1,12 @@
+#!/bin/sh
+# script/fmt: format the Go sources (writes).
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    go fmt ./...
+}
+
+main "$@"
diff --git a/backend/script/fmt-check b/backend/script/fmt-check
new file mode 100755
index 0000000..57405d0
--- /dev/null
+++ b/backend/script/fmt-check
@@ -0,0 +1,18 @@
+#!/bin/sh
+# script/fmt-check: check Go formatting (read-only). Same scope as
+# script/fmt, but fails instead of writing.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    unformatted="$(gofmt -l .)"
+    if [ -n "$unformatted" ]; then
+        echo "Files not formatted:" >&2
+        echo "$unformatted" >&2
+        exit 1
+    fi
+}
+
+main "$@"
diff --git a/backend/script/lint b/backend/script/lint
new file mode 100755
index 0000000..d00c5b0
--- /dev/null
+++ b/backend/script/lint
@@ -0,0 +1,32 @@
+#!/bin/sh
+# script/lint: run golangci-lint over the backend. This runs inside the
+# lint stage of Dockerfile.backend, whose digest-pinned golangci-lint
+# image provides the linter; nothing installs golangci-lint on the host.
+# From a checkout, run `make lint` at the repo root, which builds that
+# stage.
+#
+# .golangci.yml is standardized org-wide and must never be edited here
+# (REPO_POLICIES.md). Its last silent drift replaced the v2 schema with
+# v1 keys, which left every threshold in the file inert while the build
+# stayed green. So the file is first checked against the canonical
+# copy's sha256: a local comparison, no network, nothing unpinned.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+GOLANGCI_CONFIG_SHA256="021cc83f4e6fc7c31b95b34b846723dfcf20b66b7baeea1dc40406e643346bcb"
+
+main() {
+    cd "$ROOT"
+    actual="$(sha256sum .golangci.yml | cut -d' ' -f1)"
+    if [ "$actual" != "$GOLANGCI_CONFIG_SHA256" ]; then
+        echo ".golangci.yml has drifted from the org standard." >&2
+        echo "  expected $GOLANGCI_CONFIG_SHA256" >&2
+        echo "  actual   $actual" >&2
+        echo "Restore it verbatim from sneak/prompts; do not edit it." >&2
+        exit 1
+    fi
+    golangci-lint run ./...
+}
+
+main "$@"
diff --git a/backend/script/run b/backend/script/run
new file mode 100755
index 0000000..5ea87ba
--- /dev/null
+++ b/backend/script/run
@@ -0,0 +1,13 @@
+#!/bin/sh
+# script/run: build and run netwatch-server locally.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    "$ROOT/script/build"
+    exec ./netwatch-server "$@"
+}
+
+main "$@"
diff --git a/backend/script/test b/backend/script/test
new file mode 100755
index 0000000..cbfc889
--- /dev/null
+++ b/backend/script/test
@@ -0,0 +1,12 @@
+#!/bin/sh
+# script/test: run the backend test suite.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    timeout 30 go test ./...
+}
+
+main "$@"
diff --git a/script/bootstrap b/script/bootstrap
index 4df1d8c..f94201d 100755
--- a/script/bootstrap
+++ b/script/bootstrap
@@ -5,7 +5,18 @@
 # or apk (detected in that order); assumes nothing is present. Node is
 # used directly if installed; otherwise it is installed at a pinned
 # version via nvm (installing nvm itself first, from a hash-verified
-# release archive, never curl | sh).
+# release archive, never curl | sh). Go, with its gofmt, is used
+# directly if it is at least the version backend/go.mod asks for;
+# otherwise the pinned Go release is installed from its hash-verified
+# archive.
+#
+# What this script installs outside the system package manager lives
+# under $HOME and is linked into ~/.local/bin, where make and the git
+# hook find it once that directory is on PATH. Nothing in ~/.local/bin
+# that this script did not create is ever replaced.
+#
+# golangci-lint is not installed: make lint runs it in Docker, which
+# this script does not install either.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -16,6 +27,12 @@ NVM_VERSION="0.40.3"
 # sha256 of https://github.com/nvm-sh/nvm/archive/refs/tags/v0.40.3.tar.gz
 NVM_SHA256="5f4d6aaa04a177dc93c985e31dbc411ab6b8c6e1e21d8015dbc1372625fcd1d0"
 YARN_VERSION="1.22.22"
+# The Go inside the golang:1.25-alpine image Dockerfile.backend builds
+# with, 2026-08-09. The archive hashes are in ensure_go.
+GO_VERSION="1.25.7"
+
+BIN_DIR="$HOME/.local/bin"
+TOOLCHAIN="$HOME/.local/share/$("$ROOT/script/projectname")/toolchain"
 
 PKGMGR=""
 SUDO=""
@@ -79,6 +96,26 @@ verify_sha256() {
     fi
 }
 
+# link_bin <target> <name>: make an installed tool reachable as
+# $BIN_DIR/<name>. Only a symlink this script made, one pointing into
+# $TOOLCHAIN or ~/.nvm, is ever replaced; if anything else is already
+# there, bootstrap stops.
+link_bin() {
+    link="$BIN_DIR/$2"
+    if [ -L "$link" ] || [ -e "$link" ]; then
+        case "$(readlink "$link" || true)" in
+            "$TOOLCHAIN"/* | "$HOME"/.nvm/*) ;;
+            *)
+                echo "bootstrap: $link was not created by this script;" >&2
+                echo "  remove or rename it, then re-run bootstrap" >&2
+                exit 1
+                ;;
+        esac
+    fi
+    mkdir -p "$BIN_DIR"
+    ln -sf "$1" "$link"
+}
+
 # nvm is a bash script; run a command in a bash with nvm loaded
 nvm_sh() {
     bash -c ". \"\$HOME/.nvm/nvm.sh\" && $*"
@@ -103,39 +140,117 @@ ensure_node() {
     if ! missing node; then return 0; fi
     ensure_nvm
     nvm_sh "nvm install $NODE_VERSION"
+    link_bin "$HOME/.nvm/versions/node/v$NODE_VERSION/bin/node" node
 }
 
+# ensure_yarn: corepack writes its shims (pnpm and yarnpkg as well as
+# yarn) into $TOOLCHAIN rather than next to itself, and the npm fallback
+# installs there too; only yarn is linked.
 ensure_yarn() {
     if ! missing yarn; then return 0; fi
+    shims="$TOOLCHAIN/corepack-shims"
+    mkdir -p "$shims"
     if ! missing corepack; then
-        corepack enable
+        corepack enable --install-directory "$shims"
         corepack prepare "yarn@$YARN_VERSION" --activate
     elif [ -s "$HOME/.nvm/nvm.sh" ]; then
-        nvm_sh "nvm use $NODE_VERSION >/dev/null && corepack enable && \
+        nvm_sh "nvm use $NODE_VERSION >/dev/null && \
+            corepack enable --install-directory \"$shims\" && \
             corepack prepare yarn@$YARN_VERSION --activate"
     else
-        npm install -g "yarn@$YARN_VERSION"
+        npm install -g --prefix "$TOOLCHAIN/npm-global" "yarn@$YARN_VERSION"
+        shims="$TOOLCHAIN/npm-global/bin"
     fi
+    link_bin "$shims/yarn" yarn
 }
 
-install_js_deps() {
-    if missing yarn && [ -s "$HOME/.nvm/nvm.sh" ]; then
-        nvm_sh "nvm use $NODE_VERSION >/dev/null && cd \"$ROOT\" && \
-            yarn install --frozen-lockfile"
-    else
-        yarn install --frozen-lockfile
+# go_ok: the go on PATH has its gofmt beside it (a Go release ships the
+# two together) and is at least the version backend/go.mod asks for.
+# GOTOOLCHAIN=local makes an older go fail here instead of fetching a
+# newer toolchain for itself.
+go_ok() {
+    if missing go; then return 1; fi
+    [ -x "$(dirname "$(command -v go)")/gofmt" ] || return 1
+    (cd "$ROOT/backend" && GOTOOLCHAIN=local go list -m >/dev/null 2>&1)
+}
+
+# ensure_go: unless go_ok, install GO_VERSION and link its go and gofmt.
+# They are linked on every run that needs them, so a deleted link is put
+# back, and the archive is unpacked again if either binary is missing.
+ensure_go() {
+    if go_ok; then return 0; fi
+    go_dir="$TOOLCHAIN/go-$GO_VERSION"
+    if [ ! -x "$go_dir/bin/go" ] || [ ! -x "$go_dir/bin/gofmt" ]; then
+        # sha256 of each archive, from https://go.dev/dl/?mode=json
+        case "$(uname -s)-$(uname -m)" in
+            Linux-x86_64)
+                plat="linux-amd64"
+                sha="12e6d6a191091ae27dc31f6efc630e3a3b8ba409baf3573d955b196fdf086005"
+                ;;
+            Linux-aarch64)
+                plat="linux-arm64"
+                sha="ba611a53534135a81067240eff9508cd7e256c560edd5d8c2fef54f083c07129"
+                ;;
+            Darwin-x86_64)
+                plat="darwin-amd64"
+                sha="bf5050a2152f4053837b886e8d9640c829dbacbc3370f913351eb0904cb706f5"
+                ;;
+            Darwin-arm64)
+                plat="darwin-arm64"
+                sha="ff18369ffad05c57d5bed888b660b31385f3c913670a83ef557cdfd98ea9ae1b"
+                ;;
+            *)
+                echo "bootstrap: no pinned Go release for this platform" >&2
+                exit 1
+                ;;
+        esac
+        if missing curl; then pkg_install curl curl curl curl; fi
+        mkdir -p "$TOOLCHAIN"
+        curl -fsSL -o "$go_dir.tar.gz" \
+            "https://go.dev/dl/go$GO_VERSION.$plat.tar.gz"
+        verify_sha256 "$go_dir.tar.gz" "$sha"
+        # Unpacked beside its final place and then moved there, so an
+        # interrupted run never leaves a partial Go that looks complete.
+        rm -rf "$go_dir.partial"
+        mkdir "$go_dir.partial"
+        tar -xzf "$go_dir.tar.gz" -C "$go_dir.partial" --strip-components=1
+        rm -rf "$go_dir" "$go_dir.tar.gz"
+        mv "$go_dir.partial" "$go_dir"
     fi
+    link_bin "$go_dir/bin/go" go
+    link_bin "$go_dir/bin/gofmt" gofmt
 }
 
 main() {
     cd "$ROOT"
 
+    # Tools linked on an earlier run count as installed, and tools linked
+    # on this run are found by the steps after it.
+    path_hint=""
+    case ":$PATH:" in
+        *":$BIN_DIR:"*) ;;
+        *) path_hint=yes ;;
+    esac
+    PATH="$BIN_DIR:$PATH"
+
     if missing make; then pkg_install gnumake make make make; fi
     if missing git; then pkg_install git git git git; fi
 
     ensure_node
     ensure_yarn
-    install_js_deps
+    yarn install --frozen-lockfile
+
+    ensure_go
+    (cd "$ROOT/backend" && go mod download)
+
+    if missing docker; then
+        echo "bootstrap: docker not found; make lint, and so make check" >&2
+        echo "  and the pre-commit hook, need it to run the Go linter" >&2
+    fi
+    if [ -n "$path_hint" ] && [ -d "$BIN_DIR" ]; then
+        echo "bootstrap: add $BIN_DIR to the front of your PATH, e.g." >&2
+        echo "  export PATH=\"\$HOME/.local/bin:\$PATH\"" >&2
+    fi
 
     echo "bootstrap complete"
 }
diff --git a/script/cibuild b/script/cibuild
index 966f51d..d3eb606 100755
--- a/script/cibuild
+++ b/script/cibuild
@@ -1,13 +1,17 @@
 #!/bin/sh
-# script/cibuild: run the CI build. The Dockerfile runs make check, so
-# a successful build implies all checks pass.
+# script/cibuild: run the CI build. It builds both images: the frontend
+# from Dockerfile and the backend from Dockerfile.backend. Each runs its
+# half of the checks as build steps, so a successful cibuild implies the
+# whole repo is green. This is the only build step the Gitea workflow
+# runs.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build .
+    timeout 300 docker build .
+    timeout 300 docker build -f Dockerfile.backend .
 }
 
 main "$@"
diff --git a/script/docker b/script/docker
index aa9387f..6476087 100755
--- a/script/docker
+++ b/script/docker
@@ -1,6 +1,7 @@
 #!/bin/sh
-# script/docker: build the Docker image tagged with the project name.
-# The tag comes from script/projectname.
+# script/docker: build both Docker images, tagged with the project name
+# from script/projectname: the frontend as <name>, from Dockerfile, and
+# the backend as <name>-server, from Dockerfile.backend.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -8,7 +9,9 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    timeout 300 docker build -t "$("$SCRIPT_DIR/projectname")" .
+    name="$("$SCRIPT_DIR/projectname")"
+    timeout 300 docker build -t "$name" .
+    timeout 300 docker build -t "$name-server" -f Dockerfile.backend .
 }
 
 main "$@"
diff --git a/script/fmt b/script/fmt
index e7d63e2..7270800 100755
--- a/script/fmt
+++ b/script/fmt
@@ -1,12 +1,14 @@
 #!/bin/sh
-# script/fmt: format all files (writes).
+# script/fmt: format the whole repo (writes): prettier over everything
+# it understands, then gofmt over the Go backend.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    yarn prettier --write .
+    "$ROOT/script/frontend-fmt"
+    "$ROOT/backend/script/fmt"
 }
 
 main "$@"
diff --git a/script/fmt-check b/script/fmt-check
index 07ad5ea..28518f7 100755
--- a/script/fmt-check
+++ b/script/fmt-check
@@ -1,13 +1,14 @@
 #!/bin/sh
-# script/fmt-check: check formatting (read-only). Same scope as
-# script/fmt, but fails instead of writing.
+# script/fmt-check: check formatting across the whole repo (read-only).
+# Same scope as script/fmt, but fails instead of writing.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    yarn prettier --check .
+    "$ROOT/script/frontend-fmt-check"
+    "$ROOT/backend/script/fmt-check"
 }
 
 main "$@"
diff --git a/script/frontend-check b/script/frontend-check
new file mode 100755
index 0000000..c2b8314
--- /dev/null
+++ b/script/frontend-check
@@ -0,0 +1,18 @@
+#!/bin/sh
+# script/frontend-check: run the frontend half of the checks only (test,
+# lint, fmt-check). This exists for the frontend Dockerfile, whose build
+# stage is a node image with neither Go nor Docker; the backend half is
+# gated by Dockerfile.backend. Everywhere else, use script/check, which
+# covers the whole repo. Must not modify any files.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    "$ROOT/script/frontend-test"
+    "$ROOT/script/frontend-lint"
+    "$ROOT/script/frontend-fmt-check"
+}
+
+main "$@"
diff --git a/script/frontend-fmt b/script/frontend-fmt
new file mode 100755
index 0000000..1af33b9
--- /dev/null
+++ b/script/frontend-fmt
@@ -0,0 +1,14 @@
+#!/bin/sh
+# script/frontend-fmt: format the frontend and every other file prettier
+# understands, repo-wide (writes). backend/ is in .prettierignore; Go
+# sources are formatted by backend/script/fmt.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    yarn prettier --write .
+}
+
+main "$@"
diff --git a/script/frontend-fmt-check b/script/frontend-fmt-check
new file mode 100755
index 0000000..1501fce
--- /dev/null
+++ b/script/frontend-fmt-check
@@ -0,0 +1,13 @@
+#!/bin/sh
+# script/frontend-fmt-check: check prettier formatting (read-only). Same
+# scope as script/frontend-fmt, but fails instead of writing.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    yarn prettier --check .
+}
+
+main "$@"
diff --git a/script/frontend-lint b/script/frontend-lint
new file mode 100755
index 0000000..12b5b29
--- /dev/null
+++ b/script/frontend-lint
@@ -0,0 +1,12 @@
+#!/bin/sh
+# script/frontend-lint: run the frontend linter (prettier in check mode).
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    yarn prettier --check .
+}
+
+main "$@"
diff --git a/script/frontend-test b/script/frontend-test
new file mode 100755
index 0000000..4c74c65
--- /dev/null
+++ b/script/frontend-test
@@ -0,0 +1,14 @@
+#!/bin/sh
+# script/frontend-test: run the frontend test suite. The frontend has no
+# unit tests; the production build serves as the test (fails on broken
+# code).
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    timeout 30 yarn build
+}
+
+main "$@"
diff --git a/script/frontend-viewport-test b/script/frontend-viewport-test
index aa1b2f6..53a2ee1 100755
--- a/script/frontend-viewport-test
+++ b/script/frontend-viewport-test
@@ -45,7 +45,7 @@ main() {
     cd "$ROOT"
 
     # Test what ships: the production build, not a dev server.
-    "$ROOT/script/test"
+    "$ROOT/script/frontend-test"
     if [ ! -f "$ROOT/dist/index.html" ]; then
         echo "frontend-viewport-test: dist/index.html missing after build" >&2
         exit 1
diff --git a/script/lint b/script/lint
index 054682a..8eea8a2 100755
--- a/script/lint
+++ b/script/lint
@@ -1,12 +1,21 @@
 #!/bin/sh
-# script/lint: run the linter (prettier in check mode).
+# script/lint: lint the whole repo: prettier over the frontend, then the
+# Go linter over backend/.
+#
+# The Go linter runs only in Docker: this builds the lint stage of
+# Dockerfile.backend, the digest-pinned golangci-lint image, which runs
+# the backend's fmt-check and lint targets. --no-cache makes the linter
+# really run every time rather than reuse an earlier result, and the
+# stage is built for its checks alone, so no image is kept.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    yarn prettier --check .
+    "$ROOT/script/frontend-lint"
+    timeout 300 docker build --no-cache --target lint \
+        --output type=cacheonly -f Dockerfile.backend .
 }
 
 main "$@"
diff --git a/script/test b/script/test
index 4e98401..f0ad7e8 100755
--- a/script/test
+++ b/script/test
@@ -1,13 +1,14 @@
 #!/bin/sh
-# script/test: run the test suite. This repo has no unit tests; the
-# production build serves as the test (fails on broken code).
+# script/test: run the test suite for the whole repo: the frontend at
+# the repo root, then the Go backend in backend/. Both halves together
+# get 30 seconds; each also keeps its own limit for the Dockerfiles.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    timeout 30 yarn build
+    timeout 30 sh -c 'script/frontend-test && backend/script/test'
 }
 
 main "$@"
-- 
2.54.0


From bbcc7d921d4a640b7995b1d013e43ff34163e2e3 Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Tue, 29 Sep 2026 02:59:33 +0200
Subject: [PATCH 12/22] build: one image, nginx in front of the backend on
 loopback (closes #52)

The root Dockerfile builds the only image; Dockerfile.backend is gone.
Its stages: lint, a Go stage that runs the tests and builds
netwatch-server, the node stage, and an nginx runtime. nginx serves
dist/ on 8080 and proxies /api/ and /.well-known/healthcheck to the
backend on 127.0.0.1:8081. bin/entrypoint.sh starts both, turns TERM or
INT into a stop of both, and exits non-zero when either exits on its
own. The backend runs as user netwatch and keeps reports on the /data
volume. New setting BIND_ADDRESS (empty: every interface). STOPSIGNAL is
SIGTERM, since the nginx image's SIGQUIT would miss the entrypoint.
script/docker is the org model verbatim.

Model: opus-5-5
---
 .gitea/workflows/check.yml             |  2 +-
 Dockerfile                             | 73 ++++++++++++++++++++++++--
 Dockerfile.backend                     | 49 -----------------
 README.md                              | 20 ++++---
 TODO.md                                |  8 +++
 backend/Makefile                       |  2 +-
 backend/README.md                      | 25 ++++++---
 backend/internal/config/config.go      |  4 ++
 backend/internal/server/export_test.go |  6 +++
 backend/internal/server/http.go        |  8 ++-
 backend/internal/server/http_test.go   | 32 +++++++++++
 backend/internal/server/routes_test.go | 22 +++++---
 backend/script/build                   |  2 +-
 backend/script/lint                    |  2 +-
 bin/entrypoint.sh                      | 59 +++++++++++++++++++++
 nginx.conf                             | 19 +++++++
 script/bootstrap                       |  4 +-
 script/cibuild                         | 10 ++--
 script/docker                          | 20 ++++---
 script/frontend-check                  |  8 +--
 script/lint                            |  6 +--
 21 files changed, 280 insertions(+), 101 deletions(-)
 delete mode 100644 Dockerfile.backend
 create mode 100644 backend/internal/server/http_test.go
 create mode 100755 bin/entrypoint.sh

diff --git a/.gitea/workflows/check.yml b/.gitea/workflows/check.yml
index a732557..06b80af 100644
--- a/.gitea/workflows/check.yml
+++ b/.gitea/workflows/check.yml
@@ -6,5 +6,5 @@ jobs:
         steps:
             # actions/checkout v4.2.2, 2026-02-22
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
-            # script/cibuild builds both images.
+            # script/cibuild builds the image, whose stages run every check.
             - run: script/cibuild
diff --git a/Dockerfile b/Dockerfile
index 492b41d..3046f01 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,5 +1,51 @@
+# The one image netwatch ships: nginx serves the built frontend and
+# passes /api/ and /.well-known/healthcheck to netwatch-server, the Go
+# backend, which runs in the same container on loopback only.
+# bin/entrypoint.sh starts and watches both.
+
+# Lint stage — fast feedback on formatting and lint issues. The
+# golangci/golangci-lint image ships Go, gofmt, make and the linter, so
+# nothing is installed here. The root make lint builds this stage alone.
+# golangci/golangci-lint:v2.12.2 (2026-08-10)
+FROM golangci/golangci-lint@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240 AS lint
+
+WORKDIR /src
+COPY backend/go.mod backend/go.sum ./
+RUN go mod download
+COPY backend/ .
+RUN make fmt-check
+RUN make lint
+
+# Backend build stage
+# golang:1.25-alpine (2026-02-27)
+FROM golang:1.25-alpine@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
+
+RUN apk add --no-cache make
+
+WORKDIR /src
+
+# Force BuildKit to run the lint stage before proceeding. BuildKit runs
+# stages in parallel by default; without this no-op copy a lint failure
+# would not gate compilation.
+COPY --from=lint /src/go.sum /dev/null
+
+COPY backend/go.mod backend/go.sum ./
+RUN go mod download
+COPY backend/ .
+
+RUN make test
+
+# make build is a shim around backend/script/build, the one definition
+# of the build command:
+# CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.Version=... -X main.Buildarch=..."
+# That script reads VERSION from the environment, so it is handed over
+# there rather than as a make variable.
+ARG VERSION=dev
+RUN VERSION="${VERSION}" make build
+
+# Frontend stage
 # node:22-alpine as of 2026-02-22
-FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34 AS build
+FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34 AS frontend
 WORKDIR /app
 COPY package.json yarn.lock ./
 RUN yarn install --frozen-lockfile
@@ -8,16 +54,33 @@ COPY . .
 # make frontend-check is the frontend half of make check (test + lint +
 # fmt-check); its test step is the production yarn build, so this both
 # produces dist/ and gates the image on lint/fmt-check/test regressions.
-# This node stage has neither Go nor Docker for the other half, which
-# Dockerfile.backend gates; script/cibuild builds both images.
+# This node stage has neither Go nor Docker; the lint and builder stages
+# above gate the backend half.
 RUN make frontend-check
 
+# Runtime stage
 # nginx:stable-alpine as of 2026-02-22
 FROM nginx@sha256:15e96e59aa3b0aada3a121296e3bce117721f42d88f5f64217ef4b18f458c6ab
+
+# netwatch-server runs as this user, which owns the report directory.
+# nginx keeps the image's own arrangement: its main process runs as
+# root, its worker processes as the nginx user.
+RUN addgroup -g 1000 -S netwatch && \
+    adduser -u 1000 -S netwatch -G netwatch
+
 RUN rm /etc/nginx/conf.d/default.conf
 COPY nginx.conf /etc/nginx/conf.d/netwatch.conf
-COPY --from=build /app/dist /usr/share/nginx/html
+COPY --from=frontend /app/dist /usr/share/nginx/html
+COPY --from=builder /src/netwatch-server /usr/local/bin/netwatch-server
+COPY bin/entrypoint.sh /usr/local/bin/entrypoint.sh
+
+ENV DATA_DIR=/data/reports
+RUN mkdir -p /data/reports && chown -R netwatch:netwatch /data
+VOLUME /data
 
 EXPOSE 8080
 
-CMD ["nginx", "-g", "daemon off;"]
+# The nginx image stops its container with SIGQUIT; the entrypoint
+# acts on TERM and INT.
+STOPSIGNAL SIGTERM
+ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]
diff --git a/Dockerfile.backend b/Dockerfile.backend
deleted file mode 100644
index 51a530f..0000000
--- a/Dockerfile.backend
+++ /dev/null
@@ -1,49 +0,0 @@
-# Lint stage — fast feedback on formatting and lint issues. The
-# golangci/golangci-lint image ships Go, gofmt, make and the linter, so
-# nothing is installed here.
-# golangci/golangci-lint:v2.12.2 (2026-08-10)
-FROM golangci/golangci-lint@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240 AS lint
-
-WORKDIR /src
-COPY backend/go.mod backend/go.sum ./
-RUN go mod download
-COPY backend/ .
-RUN make fmt-check
-RUN make lint
-
-# Build stage
-# golang:1.25-alpine (2026-02-27)
-FROM golang:1.25-alpine@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
-
-RUN apk add --no-cache make
-
-WORKDIR /src
-
-# Force BuildKit to run the lint stage before proceeding. BuildKit runs
-# stages in parallel by default; without this no-op copy a lint failure
-# would not gate compilation.
-COPY --from=lint /src/go.sum /dev/null
-
-COPY backend/go.mod backend/go.sum ./
-RUN go mod download
-COPY backend/ .
-
-RUN make test
-
-# make build is a shim around backend/script/build, the one definition
-# of the build command:
-# CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.Version=... -X main.Buildarch=..."
-# That script reads VERSION from the environment, so it is handed over
-# there rather than as a make variable.
-ARG VERSION=dev
-RUN VERSION="${VERSION}" make build
-
-# Runtime stage
-# alpine:3.23 (2026-02-27)
-FROM alpine:3.23@sha256:25109184c71bdad752c8312a8623239686a9a2071e8825f20acb8f2198c3f659
-
-RUN apk add --no-cache ca-certificates
-COPY --from=builder /src/netwatch-server /usr/local/bin/netwatch-server
-
-EXPOSE 8080
-ENTRYPOINT ["netwatch-server"]
diff --git a/README.md b/README.md
index 52e6382..aa90734 100644
--- a/README.md
+++ b/README.md
@@ -44,11 +44,11 @@ halves, so the root `make check` fails if either one is broken. We provide:
   linter in Docker
 - `script/setup` — make a fresh clone ready for development: bootstrap plus the
   git pre-commit hook
-- `script/projectname` — print the project name (used for the Docker image tags)
+- `script/projectname` — print the project name (used for the Docker image tag)
 - `script/test` — run `script/frontend-test`, then the backend's Go tests, both
   within one 30-second timeout
 - `script/lint` — run `script/frontend-lint`, then golangci-lint in Docker, by
-  building the lint stage of `Dockerfile.backend` without the cache
+  building the lint stage of `Dockerfile` without the cache
 - `script/fmt` — format all files (writes): prettier, then gofmt over `backend/`
 - `script/fmt-check` — check formatting (read-only): prettier, then gofmt
 - `script/check` — run test, lint, and fmt-check
@@ -63,9 +63,9 @@ halves, so the root `make check` fails if either one is broken. We provide:
   frontend in a containerised headless Chrome (see
   [test/viewport/README.md](test/viewport/README.md)). Not part of
   `script/check`: it needs Docker and takes minutes.
-- `script/docker` — build both images, tagged via `script/projectname`:
-  `netwatch` from `Dockerfile` and `netwatch-server` from `Dockerfile.backend`
-- `script/cibuild` — CI entrypoint: builds both images
+- `script/docker` — build the image from `Dockerfile` without the build cache,
+  tagged `netwatch` via `script/projectname`
+- `script/cibuild` — CI entrypoint: builds the image
 - `script/precommit` — run by the git pre-commit hook; runs `script/check`
 - `script/install-precommit` — install the git pre-commit hook
 
@@ -178,13 +178,21 @@ After running `yarn build`, deploy the contents of the `dist/` directory to any
 static file host (S3, GCS, Cloudflare Pages, Vercel, Netlify, GitHub Pages) or
 use the Docker image behind a reverse proxy.
 
-The Docker image:
+The Docker image, built from `Dockerfile`, is the whole service in one
+container: nginx serves the built frontend and passes `/api/` and
+`/.well-known/healthcheck` to the Go backend, `netwatch-server`, which listens
+only inside the container, on `127.0.0.1:8081`. The image:
 
 - Listens on port 8080 by default (override with `PORT` env var)
 - Trusts `X-Forwarded-For` from RFC1918 reverse proxies (10/8, 172.16/12,
   192.168/16)
 - Sends access logs to stdout
 - Caches static assets with immutable headers
+- Stores reports in `DATA_DIR`, `/data/reports` by default, on the `/data`
+  volume. The backend runs as user `netwatch` (uid 1000), so a directory
+  bind-mounted at `/data` must be writable by uid 1000
+- Writes buffered reports to disk on `docker stop`, and exits non-zero if nginx
+  or the backend exits on its own, so the platform restarts it
 
 ## Browser Compatibility
 
diff --git a/TODO.md b/TODO.md
index ecb3301..f96341a 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,14 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-28: one container image (issue #52): the root `Dockerfile` builds the
+  only image, and `Dockerfile.backend` is gone. nginx serves the frontend on
+  port 8080 and proxies `/api/` and `/.well-known/healthcheck` to the backend,
+  which listens on `127.0.0.1:8081` in the same container; the new
+  `BIND_ADDRESS` setting sets its listen address. `bin/entrypoint.sh` starts
+  both, passes TERM and INT on to both, and exits non-zero when either exits on
+  its own. The backend runs as user `netwatch` and stores reports on the `/data`
+  volume. `script/docker` is the org model again
 - 2026-09-28: unified the gate (issue #16): the root `make check` covers the Go
   backend as well as the frontend, and the pre-commit hook with it; the backend
   moved onto scripts-to-rule-them-all (`backend/script/*`, `backend/Makefile` as
diff --git a/backend/Makefile b/backend/Makefile
index 5724459..faf9ec5 100644
--- a/backend/Makefile
+++ b/backend/Makefile
@@ -2,7 +2,7 @@
 # Entrypoints section of README.md). There is no check, hooks or docker
 # target here: the root Makefile's check covers this directory, its
 # hooks target installs the repo's only pre-commit hook, and its docker
-# target builds this image, whose build context is the repo root.
+# target builds the one image, which contains this backend.
 
 .PHONY: all build test lint fmt fmt-check run clean
 
diff --git a/backend/README.md b/backend/README.md
index 1230c12..fa4af1b 100644
--- a/backend/README.md
+++ b/backend/README.md
@@ -11,15 +11,16 @@ From this directory:
 make run
 ```
 
-From the repo root, which is also the build context of `Dockerfile.backend`:
+From the repo root, whose `Dockerfile` builds the one image that ships this
+backend behind nginx (see [Container image](#container-image)):
 
 ```bash
 # Run tests, lint, and format check over the frontend and this backend
 make check
 
-# Build both images, including netwatch-server
+# Build the image: nginx, the frontend and this backend
 make docker
-docker run -p 8080:8080 netwatch-server
+docker run -p 8080:8080 netwatch
 ```
 
 ## Entrypoints
@@ -27,7 +28,7 @@ docker run -p 8080:8080 netwatch-server
 This directory follows the same
 [Scripts to Rule Them All](https://github.com/github/scripts-to-rule-them-all)
 pattern as the repo root: the targets in `backend/Makefile` are thin shims over
-`backend/script/`. `Dockerfile.backend` runs them, and the root scripts call
+`backend/script/`. The root `Dockerfile` runs them, and the root scripts call
 `test`, `fmt` and `fmt-check`:
 
 - `script/build` — compile the static `netwatch-server` binary with its version
@@ -37,8 +38,8 @@ pattern as the repo root: the targets in `backend/Makefile` are thin shims over
 - `script/test` — run the Go tests under a 30-second timeout
 - `script/lint` — check `.golangci.yml` against its pinned sha256, then run
   golangci-lint. It runs inside the golangci-lint image of the lint stage of
-  `Dockerfile.backend`; from a checkout, run `make lint` at the repo root, which
-  builds that stage
+  the root `Dockerfile`; from a checkout, run `make lint` at the repo root,
+  which builds that stage
 - `script/fmt` — format the Go sources (writes)
 - `script/fmt-check` — check Go formatting (read-only)
 - `script/run` — build and run the server locally
@@ -46,7 +47,7 @@ pattern as the repo root: the targets in `backend/Makefile` are thin shims over
 
 There is no `check`, `hooks` or `docker` target here: the root `make check`
 covers this directory, the root `make hooks` installs the repo's only pre-commit
-hook, and the root `make docker` builds this image.
+hook, and the root `make docker` builds the image that contains this backend.
 
 ## Rationale
 
@@ -76,6 +77,7 @@ Internal packages in `internal/` follow standard Go project layout:
 
 | Variable          | Default              | Description                                                                                              |
 | ----------------- | -------------------- | -------------------------------------------------------------------------------------------------------- |
+| `BIND_ADDRESS`    | empty                | IP address to listen on; empty listens on every interface                                                |
 | `PORT`            | `8080`               | HTTP listen port                                                                                         |
 | `DATA_DIR`        | `./data/reports`     | Directory for compressed reports                                                                         |
 | `DEBUG`           | `false`              | Enable debug logging                                                                                     |
@@ -86,6 +88,15 @@ The loopback entries cover the reverse proxy that shares the container; the
 RFC1918 ranges match `nginx.conf`. A request whose direct peer is outside this
 set has its forwarded headers ignored, and the direct peer is logged instead.
 
+### Container image
+
+The root `Dockerfile` builds one image in which nginx listens on the public port
+8080, serves the frontend, and proxies `/api/` and `/.well-known/healthcheck` to
+this server. The image's entrypoint, `bin/entrypoint.sh`, starts the server as
+user `netwatch` (uid 1000) with `BIND_ADDRESS=127.0.0.1` and `PORT=8081`, so
+only nginx reaches it. `DATA_DIR` is `/data/reports`, on the `/data` volume,
+which `netwatch` owns.
+
 ### Report storage
 
 Reports are written as `reports-<timestamp>.jsonl.zst` files in `DATA_DIR`.
diff --git a/backend/internal/config/config.go b/backend/internal/config/config.go
index 02a6db9..4f60de4 100644
--- a/backend/internal/config/config.go
+++ b/backend/internal/config/config.go
@@ -33,6 +33,7 @@ type Params struct {
 
 // Config holds the resolved application configuration.
 type Config struct {
+	BindAddress     string
 	DataDir         string
 	Debug           bool
 	MetricsPassword string
@@ -62,6 +63,8 @@ func New(
 
 	viper.SetDefault("DATA_DIR", "./data/reports")
 	viper.SetDefault("DEBUG", "false")
+	// An empty BIND_ADDRESS listens on every interface.
+	viper.SetDefault("BIND_ADDRESS", "")
 	viper.SetDefault("PORT", "8080")
 	viper.SetDefault("SENTRY_DSN", "")
 	viper.SetDefault("METRICS_USERNAME", "")
@@ -78,6 +81,7 @@ func New(
 	}
 
 	s := &Config{
+		BindAddress:     viper.GetString("BIND_ADDRESS"),
 		DataDir:         viper.GetString("DATA_DIR"),
 		Debug:           viper.GetBool("DEBUG"),
 		MetricsPassword: viper.GetString("METRICS_PASSWORD"),
diff --git a/backend/internal/server/export_test.go b/backend/internal/server/export_test.go
index 7279b22..84cdd90 100644
--- a/backend/internal/server/export_test.go
+++ b/backend/internal/server/export_test.go
@@ -3,3 +3,9 @@ package server
 // MaxRequestBodyBytes exposes the router-wide body limit to the
 // external tests.
 const MaxRequestBodyBytes = maxRequestBodyBytes
+
+// ListenAddr exposes the address the server listens on to the
+// external tests.
+func (s *Server) ListenAddr() string {
+	return s.newHTTPServer().Addr
+}
diff --git a/backend/internal/server/http.go b/backend/internal/server/http.go
index e621ce7..46f4a67 100644
--- a/backend/internal/server/http.go
+++ b/backend/internal/server/http.go
@@ -2,8 +2,9 @@ package server
 
 import (
 	"errors"
-	"fmt"
+	"net"
 	"net/http"
+	"strconv"
 	"time"
 
 	"go.uber.org/fx"
@@ -27,7 +28,10 @@ const (
 // newHTTPServer constructs the http.Server. It performs no I/O
 // and does not start listening.
 func (s *Server) newHTTPServer() *http.Server {
-	listenAddr := fmt.Sprintf(":%d", s.params.Config.Port)
+	listenAddr := net.JoinHostPort(
+		s.params.Config.BindAddress,
+		strconv.Itoa(s.params.Config.Port),
+	)
 
 	return &http.Server{
 		Addr:              listenAddr,
diff --git a/backend/internal/server/http_test.go b/backend/internal/server/http_test.go
new file mode 100644
index 0000000..d1370df
--- /dev/null
+++ b/backend/internal/server/http_test.go
@@ -0,0 +1,32 @@
+package server_test
+
+import "testing"
+
+// TestListenAddress checks that the server listens on BIND_ADDRESS
+// and PORT, and on port 8080 on every interface when neither is set.
+// The container image sets both, to keep the backend on loopback
+// behind nginx.
+func TestListenAddress(t *testing.T) {
+	tests := []struct {
+		bindAddress string
+		port        string
+		want        string
+	}{
+		{bindAddress: "", port: "", want: ":8080"},
+		{bindAddress: "127.0.0.1", port: "8081", want: "127.0.0.1:8081"},
+		{bindAddress: "::1", port: "8081", want: "[::1]:8081"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.want, func(t *testing.T) {
+			// t.Setenv rules out t.Parallel.
+			t.Setenv("BIND_ADDRESS", tt.bindAddress)
+			t.Setenv("PORT", tt.port)
+
+			got := newServer(t).ListenAddr()
+			if got != tt.want {
+				t.Errorf("listen address = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
diff --git a/backend/internal/server/routes_test.go b/backend/internal/server/routes_test.go
index 1a50a7f..71f5117 100644
--- a/backend/internal/server/routes_test.go
+++ b/backend/internal/server/routes_test.go
@@ -19,16 +19,14 @@ import (
 	"go.uber.org/fx/fxtest"
 )
 
-// TestHealthCheckRejectsOversizeBody sends the health check, which
-// never reads its body, a body one byte over the limit. Only the
-// router-wide body limit can reject it.
-func TestHealthCheckRejectsOversizeBody(t *testing.T) {
-	t.Parallel()
+// newServer builds a Server from the same constructors as main,
+// configured from the environment. It is never started, so nothing
+// listens.
+func newServer(t *testing.T) *server.Server {
+	t.Helper()
 
 	var srv *server.Server
 
-	// The same constructors as main, never started: SetupRoutes is
-	// called directly, so nothing listens.
 	app := fxtest.New(t,
 		fx.Provide(
 			config.New,
@@ -48,6 +46,16 @@ func TestHealthCheckRejectsOversizeBody(t *testing.T) {
 		t.Fatalf("build server: %v", err)
 	}
 
+	return srv
+}
+
+// TestHealthCheckRejectsOversizeBody sends the health check, which
+// never reads its body, a body one byte over the limit. Only the
+// router-wide body limit can reject it.
+func TestHealthCheckRejectsOversizeBody(t *testing.T) {
+	t.Parallel()
+
+	srv := newServer(t)
 	srv.SetupRoutes()
 
 	rec := httptest.NewRecorder()
diff --git a/backend/script/build b/backend/script/build
index 36b3e96..40d4d01 100755
--- a/backend/script/build
+++ b/backend/script/build
@@ -8,7 +8,7 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 main() {
     cd "$ROOT"
 
-    # VERSION comes from the environment (Dockerfile.backend passes its
+    # VERSION comes from the environment (the root Dockerfile passes its
     # ARG VERSION in). Unset or empty, it is git describe, or "dev" where
     # there is no git or no repository history.
     version="${VERSION:-$(git describe --always --dirty 2>/dev/null || echo dev)}"
diff --git a/backend/script/lint b/backend/script/lint
index d00c5b0..256f805 100755
--- a/backend/script/lint
+++ b/backend/script/lint
@@ -1,6 +1,6 @@
 #!/bin/sh
 # script/lint: run golangci-lint over the backend. This runs inside the
-# lint stage of Dockerfile.backend, whose digest-pinned golangci-lint
+# lint stage of the root Dockerfile, whose digest-pinned golangci-lint
 # image provides the linter; nothing installs golangci-lint on the host.
 # From a checkout, run `make lint` at the repo root, which builds that
 # stage.
diff --git a/bin/entrypoint.sh b/bin/entrypoint.sh
new file mode 100755
index 0000000..3dece27
--- /dev/null
+++ b/bin/entrypoint.sh
@@ -0,0 +1,59 @@
+#!/bin/sh
+# The container's entrypoint: runs netwatch-server and nginx side by
+# side. TERM or INT stops both, and the container exits 0 if both exit
+# cleanly. If either exits on its own, the other is stopped too and the
+# container exits non-zero, so the platform restarts it instead of
+# leaving it half up.
+#
+# No set -e: kill and wait return non-zero here in normal operation.
+set -u
+
+# A stop signal is only noted here; the loop below acts on it.
+stop_requested=""
+trap 'stop_requested=yes' TERM INT
+
+# netwatch-server runs as the netwatch user and listens on loopback
+# only, on a port other than the public one; nginx.conf proxies to this
+# address. The netwatch user has no login shell, hence -s /bin/sh.
+# busybox su replaces itself with the command instead of staying on as
+# its parent, so $! is the server's own PID.
+BIND_ADDRESS=127.0.0.1 PORT=8081 \
+    su -s /bin/sh netwatch -c 'exec netwatch-server' &
+backend=$!
+
+# nginx starts through the nginx image's own entrypoint, which applies
+# the image's start-up configuration and then replaces itself with
+# nginx.
+/docker-entrypoint.sh nginx -g 'daemon off;' &
+nginx=$!
+
+running() {
+    kill -0 "$1" 2>/dev/null
+}
+
+# POSIX sh cannot wait for whichever of two children exits first, so
+# look once a second. The shell collects a child that has exited while
+# it runs sleep, and running() is false for that child from then on.
+while [ -z "$stop_requested" ] && running "$backend" && running "$nginx"; do
+    sleep 1
+done
+
+# Stop both, then wait until neither is left.
+kill -TERM "$backend" "$nginx" 2>/dev/null
+while running "$backend" || running "$nginx"; do
+    sleep 1
+done
+
+wait "$backend"
+backend_status=$?
+wait "$nginx"
+nginx_status=$?
+echo "entrypoint: netwatch-server exited $backend_status," \
+    "nginx exited $nginx_status"
+
+# Success is a requested stop that both processes exited cleanly from.
+if [ -n "$stop_requested" ] && [ "$backend_status" -eq 0 ] &&
+    [ "$nginx_status" -eq 0 ]; then
+    exit 0
+fi
+exit 1
diff --git a/nginx.conf b/nginx.conf
index cb176d4..65e8aaf 100644
--- a/nginx.conf
+++ b/nginx.conf
@@ -25,4 +25,23 @@ server {
         expires 1y;
         add_header Cache-Control "public, immutable";
     }
+
+    # netwatch-server, the Go backend, runs in the same container and
+    # listens on loopback only: bin/entrypoint.sh starts it on
+    # 127.0.0.1:8081. These headers go with every request passed to it.
+    # X-Forwarded-For carries only the client address, as resolved by
+    # the real IP settings above, and not the chain the request came
+    # with: the backend takes the first entry, which a client can write.
+    proxy_set_header Host $host;
+    proxy_set_header X-Real-IP $remote_addr;
+    proxy_set_header X-Forwarded-For $remote_addr;
+    proxy_set_header X-Forwarded-Proto $scheme;
+
+    location /api/ {
+        proxy_pass http://127.0.0.1:8081;
+    }
+
+    location = /.well-known/healthcheck {
+        proxy_pass http://127.0.0.1:8081;
+    }
 }
diff --git a/script/bootstrap b/script/bootstrap
index f94201d..66e5873 100755
--- a/script/bootstrap
+++ b/script/bootstrap
@@ -27,8 +27,8 @@ NVM_VERSION="0.40.3"
 # sha256 of https://github.com/nvm-sh/nvm/archive/refs/tags/v0.40.3.tar.gz
 NVM_SHA256="5f4d6aaa04a177dc93c985e31dbc411ab6b8c6e1e21d8015dbc1372625fcd1d0"
 YARN_VERSION="1.22.22"
-# The Go inside the golang:1.25-alpine image Dockerfile.backend builds
-# with, 2026-08-09. The archive hashes are in ensure_go.
+# The Go inside the golang:1.25-alpine image Dockerfile builds the
+# backend with, 2026-08-09. The archive hashes are in ensure_go.
 GO_VERSION="1.25.7"
 
 BIN_DIR="$HOME/.local/bin"
diff --git a/script/cibuild b/script/cibuild
index d3eb606..d860bf5 100755
--- a/script/cibuild
+++ b/script/cibuild
@@ -1,9 +1,8 @@
 #!/bin/sh
-# script/cibuild: run the CI build. It builds both images: the frontend
-# from Dockerfile and the backend from Dockerfile.backend. Each runs its
-# half of the checks as build steps, so a successful cibuild implies the
-# whole repo is green. This is the only build step the Gitea workflow
-# runs.
+# script/cibuild: run the CI build: build the one image from Dockerfile,
+# whose stages run the checks as build steps (the backend's fmt-check,
+# lint and tests, and the frontend's test, lint and fmt-check). This is
+# the only build step the Gitea workflow runs.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -11,7 +10,6 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 main() {
     cd "$ROOT"
     timeout 300 docker build .
-    timeout 300 docker build -f Dockerfile.backend .
 }
 
 main "$@"
diff --git a/script/docker b/script/docker
index 6476087..c4688e8 100755
--- a/script/docker
+++ b/script/docker
@@ -1,7 +1,8 @@
 #!/bin/sh
-# script/docker: build both Docker images, tagged with the project name
-# from script/projectname: the frontend as <name>, from Dockerfile, and
-# the backend as <name>-server, from Dockerfile.backend.
+# script/docker: build the Docker image tagged with the project name.
+# Identical in all repos; the tag comes from script/projectname.
+# --no-cache because the gate phases the final stage depends on are RUN
+# steps, and a cached one is a check that did not run.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -9,9 +10,16 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    name="$("$SCRIPT_DIR/projectname")"
-    timeout 300 docker build -t "$name" .
-    timeout 300 docker build -t "$name-server" -f Dockerfile.backend .
+    # Own line: a failing command substitution inside an argument does
+    # not trip `set -e`, so the inline form degrades silently to an
+    # empty constant. VERSION is computed here because .dockerignore
+    # excludes .git, so `git describe` in a build stage yields an empty
+    # version without failing.
+    version="$(git describe --tags --always --dirty 2>/dev/null || true)"
+    [ -n "$version" ] || version="unknown"
+    docker build --no-cache \
+        --build-arg VERSION="$version" \
+        -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"
diff --git a/script/frontend-check b/script/frontend-check
index c2b8314..0819860 100755
--- a/script/frontend-check
+++ b/script/frontend-check
@@ -1,9 +1,9 @@
 #!/bin/sh
 # script/frontend-check: run the frontend half of the checks only (test,
-# lint, fmt-check). This exists for the frontend Dockerfile, whose build
-# stage is a node image with neither Go nor Docker; the backend half is
-# gated by Dockerfile.backend. Everywhere else, use script/check, which
-# covers the whole repo. Must not modify any files.
+# lint, fmt-check). This exists for the frontend stage of Dockerfile, a
+# node image with neither Go nor Docker; the Dockerfile's lint and
+# backend build stages gate the backend half. Everywhere else, use
+# script/check, which covers the whole repo. Must not modify any files.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
diff --git a/script/lint b/script/lint
index 8eea8a2..1b3783d 100755
--- a/script/lint
+++ b/script/lint
@@ -3,8 +3,8 @@
 # Go linter over backend/.
 #
 # The Go linter runs only in Docker: this builds the lint stage of
-# Dockerfile.backend, the digest-pinned golangci-lint image, which runs
-# the backend's fmt-check and lint targets. --no-cache makes the linter
+# Dockerfile, the digest-pinned golangci-lint image, which runs the
+# backend's fmt-check and lint targets. --no-cache makes the linter
 # really run every time rather than reuse an earlier result, and the
 # stage is built for its checks alone, so no image is kept.
 set -eu
@@ -15,7 +15,7 @@ main() {
     cd "$ROOT"
     "$ROOT/script/frontend-lint"
     timeout 300 docker build --no-cache --target lint \
-        --output type=cacheonly -f Dockerfile.backend .
+        --output type=cacheonly .
 }
 
 main "$@"
-- 
2.54.0


From ea66caf3388dcb5117b613f8dea413a827ca8c32 Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Tue, 29 Sep 2026 04:22:19 +0200
Subject: [PATCH 13/22] fix(backend): rate-limit and cap report ingest, drop
 wildcard CORS (closes #20)

POST /api/v1/reports stays unauthenticated but is bounded. Each client
address, as the trusted-proxy logic resolves it, may send
REPORTS_PER_MINUTE reports a minute (default 60, counted by
go-chi/httprate over a sliding minute); past that it gets 429 with
Retry-After. reportbuf refuses a report that would take the report
files past DATA_DIR_MAX_BYTES (default 1 GiB), counting the files
already in DATA_DIR and unwritten reports at their uncompressed size;
the handler answers 507. CORS adds nothing unless CORS_ALLOWED_ORIGINS
lists origins. A limit that is not a positive number, or an origin
that is not a plain scheme://host[:port], stops the server from
starting.

Model: opus-5-5
---
 TODO.md                                       |  10 +
 backend/README.md                             |  52 +++-
 backend/go.mod                                |   5 +-
 backend/go.sum                                |  12 +-
 backend/internal/config/config.go             | 102 +++++--
 backend/internal/config/config_test.go        |  64 +++++
 backend/internal/handlers/report.go           |  20 +-
 backend/internal/handlers/report_test.go      |  27 ++
 backend/internal/middleware/export_test.go    |   7 +
 backend/internal/middleware/middleware.go     |  49 +++-
 .../internal/middleware/middleware_test.go    | 203 ++++++++++++++
 backend/internal/reportbuf/export_test.go     |   7 +
 backend/internal/reportbuf/reportbuf.go       |  88 +++++-
 backend/internal/reportbuf/reportbuf_test.go  | 253 ++++++++++++++++++
 backend/internal/server/routes.go             |   5 +-
 backend/internal/server/routes_test.go        |  58 ++++
 16 files changed, 908 insertions(+), 54 deletions(-)
 create mode 100644 backend/internal/config/config_test.go
 create mode 100644 backend/internal/reportbuf/export_test.go

diff --git a/TODO.md b/TODO.md
index f96341a..adbc424 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,16 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: bounded the report endpoint (issue #20): `POST /api/v1/reports`
+  still needs no credentials, but each client address, as resolved through
+  `TRUSTED_PROXIES`, may send `REPORTS_PER_MINUTE` (default 60) reports a
+  minute, counted by `go-chi/httprate`, and past that gets 429 with
+  `Retry-After`; the report files in `DATA_DIR`, counted from start with those
+  already there, may total at most `DATA_DIR_MAX_BYTES` (default 1 GiB), past
+  which reports get 507; and the wildcard CORS is gone: no CORS headers unless
+  `CORS_ALLOWED_ORIGINS` lists origins, and an entry that is not a plain
+  `scheme://host[:port]` origin, `*` included, stops the server from starting.
+  Deleting report files frees room only at the next start; pruning is issue #54
 - 2026-09-28: one container image (issue #52): the root `Dockerfile` builds the
   only image, and `Dockerfile.backend` is gone. nginx serves the frontend on
   port 8080 and proxies `/api/` and `/.well-known/healthcheck` to the backend,
diff --git a/backend/README.md b/backend/README.md
index fa4af1b..095af11 100644
--- a/backend/README.md
+++ b/backend/README.md
@@ -75,13 +75,16 @@ Internal packages in `internal/` follow standard Go project layout:
 
 ### Configuration
 
-| Variable          | Default              | Description                                                                                              |
-| ----------------- | -------------------- | -------------------------------------------------------------------------------------------------------- |
-| `BIND_ADDRESS`    | empty                | IP address to listen on; empty listens on every interface                                                |
-| `PORT`            | `8080`               | HTTP listen port                                                                                         |
-| `DATA_DIR`        | `./data/reports`     | Directory for compressed reports                                                                         |
-| `DEBUG`           | `false`              | Enable debug logging                                                                                     |
-| `TRUSTED_PROXIES` | loopback + RFC1918   | Comma-separated CIDRs whose `X-Forwarded-For` / `X-Real-IP` headers are trusted for client IP resolution |
+| Variable               | Default              | Description                                                                                              |
+| ---------------------- | -------------------- | -------------------------------------------------------------------------------------------------------- |
+| `BIND_ADDRESS`         | empty                | IP address to listen on; empty listens on every interface                                                |
+| `PORT`                 | `8080`               | HTTP listen port                                                                                         |
+| `DATA_DIR`             | `./data/reports`     | Directory for compressed reports                                                                         |
+| `DATA_DIR_MAX_BYTES`   | `1073741824` (1 GiB) | Largest total size of the report files in `DATA_DIR`; see [Report limits](#report-limits)                |
+| `DEBUG`                | `false`              | Enable debug logging                                                                                     |
+| `TRUSTED_PROXIES`      | loopback + RFC1918   | Comma-separated CIDRs whose `X-Forwarded-For` / `X-Real-IP` headers are trusted for client IP resolution |
+| `REPORTS_PER_MINUTE`   | `60`                 | Reports each client address may send a minute; see [Report limits](#report-limits)                       |
+| `CORS_ALLOWED_ORIGINS` | empty                | Comma-separated origins whose pages may call the API; see [CORS](#cors)                                  |
 
 `TRUSTED_PROXIES` defaults to `127.0.0.1/32,::1/128,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`.
 The loopback entries cover the reverse proxy that shares the container; the
@@ -103,6 +106,41 @@ Reports are written as `reports-<timestamp>.jsonl.zst` files in `DATA_DIR`.
 Each file contains one JSON object per line, compressed with zstd. Files are
 created with `O_EXCL` to prevent overwrites.
 
+### Report limits
+
+`POST /api/v1/reports` takes reports from anyone who can reach it, without
+credentials, so it is bounded instead. Both refusals below answer with the same
+`{"status":"error"}` body as any other error.
+
+- **Rate limit.** Each client address, resolved through `TRUSTED_PROXIES`, may
+  send `REPORTS_PER_MINUTE` reports a minute; past that it gets 429 with
+  `Retry-After: 60`. The minute slides: reports from the minute before still
+  count, fading out over the current one, so an address is sure never to be
+  refused only while it sends at most half of `REPORTS_PER_MINUTE` in any 60
+  seconds. The page sends one report a minute from each open tab, so the default
+  of 60 refuses nothing from up to 30 tabs behind one address, such as a
+  household or an office sharing it, however their reports bunch up. Report
+  responses also carry `X-RateLimit-Limit`, `X-RateLimit-Remaining` and
+  `X-RateLimit-Reset` headers.
+- **Size cap.** The report files in `DATA_DIR` may total at most
+  `DATA_DIR_MAX_BYTES`, counting the files already there at start. Reports
+  waiting in memory count at their uncompressed size until they are written, so
+  a report that would take the total past the cap is refused with 507, and
+  nothing of it is stored. Deleting report files frees room only at the next
+  start, when the files are counted again. The default of 1 GiB is small enough
+  for any host; set it to the space you can give `DATA_DIR`.
+
+### CORS
+
+The page calls the API from the origin it is served from, so by default the
+server sends no CORS headers, and browsers let no other origin's pages call it.
+To serve the page from elsewhere, list that origin in `CORS_ALLOWED_ORIGINS`
+(for example `https://netwatch.example.com`); pages from a listed origin may
+`GET` and `POST` with a `Content-Type` header. Each entry must be a plain
+origin, `scheme://host` with an optional `:port`, as browsers send it: no path,
+not even a trailing `/`, and no `*`. Any other entry stops the server from
+starting, with an error naming `CORS_ALLOWED_ORIGINS`.
+
 ## TODO
 
 - Add integration test that POSTs a report and verifies the compressed output
diff --git a/backend/go.mod b/backend/go.mod
index 89e3ef4..a2462e8 100644
--- a/backend/go.mod
+++ b/backend/go.mod
@@ -5,6 +5,7 @@ go 1.25.5
 require (
 	github.com/go-chi/chi/v5 v5.2.5
 	github.com/go-chi/cors v1.2.2
+	github.com/go-chi/httprate v0.16.0
 	github.com/joho/godotenv v1.5.1
 	github.com/klauspost/compress v1.18.4
 	github.com/spf13/viper v1.21.0
@@ -14,6 +15,7 @@ require (
 require (
 	github.com/fsnotify/fsnotify v1.9.0 // indirect
 	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
+	github.com/klauspost/cpuid/v2 v2.2.10 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
 	github.com/sagikazarmark/locafero v0.11.0 // indirect
 	github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8 // indirect
@@ -21,10 +23,11 @@ require (
 	github.com/spf13/cast v1.10.0 // indirect
 	github.com/spf13/pflag v1.0.10 // indirect
 	github.com/subosito/gotenv v1.6.0 // indirect
+	github.com/zeebo/xxh3 v1.0.2 // indirect
 	go.uber.org/dig v1.19.0 // indirect
 	go.uber.org/multierr v1.10.0 // indirect
 	go.uber.org/zap v1.26.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
-	golang.org/x/sys v0.29.0 // indirect
+	golang.org/x/sys v0.30.0 // indirect
 	golang.org/x/text v0.28.0 // indirect
 )
diff --git a/backend/go.sum b/backend/go.sum
index de6ddf2..cac5e22 100644
--- a/backend/go.sum
+++ b/backend/go.sum
@@ -8,6 +8,8 @@ github.com/go-chi/chi/v5 v5.2.5 h1:Eg4myHZBjyvJmAFjFvWgrqDTXFyOzjj7YIm3L3mu6Ug=
 github.com/go-chi/chi/v5 v5.2.5/go.mod h1:X7Gx4mteadT3eDOMTsXzmI4/rwUpOwBHLpAfupzFJP0=
 github.com/go-chi/cors v1.2.2 h1:Jmey33TE+b+rB7fT8MUy1u0I4L+NARQlK6LhzKPSyQE=
 github.com/go-chi/cors v1.2.2/go.mod h1:sSbTewc+6wYHBBCW7ytsFSn836hqM7JxpglAy2Vzc58=
+github.com/go-chi/httprate v0.16.0 h1:8V5DH9j6pSK6UQoBsTpvMyFxycqaKEIToyPKzHJjUa8=
+github.com/go-chi/httprate v0.16.0/go.mod h1:A8lo+qRhk+s9LiuP5saS7XCGDXRXMcrueq0NfIuCa/I=
 github.com/go-viper/mapstructure/v2 v2.4.0 h1:EBsztssimR/CONLSZZ04E8qAkxNYq4Qp9LvH92wZUgs=
 github.com/go-viper/mapstructure/v2 v2.4.0/go.mod h1:oJDH3BJKyqBA2TXFhDsKDGDTlndYOZ6rGS0BRZIxGhM=
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
@@ -16,6 +18,8 @@ github.com/joho/godotenv v1.5.1 h1:7eLL/+HRGLY0ldzfGMeQkb7vMd0as4CfYvUVzLqw0N0=
 github.com/joho/godotenv v1.5.1/go.mod h1:f4LDr5Voq0i2e/R5DDNOoa2zzDfwtkZa6DnEwAbqwq4=
 github.com/klauspost/compress v1.18.4 h1:RPhnKRAQ4Fh8zU2FY/6ZFDwTVTxgJ/EMydqSTzE9a2c=
 github.com/klauspost/compress v1.18.4/go.mod h1:R0h/fSBs8DE4ENlcrlib3PsXS61voFxhIs2DeRhCvJ4=
+github.com/klauspost/cpuid/v2 v2.2.10 h1:tBs3QSyvjDyFTq3uoc/9xFpCuOsJQFNPiAhYdw2skhE=
+github.com/klauspost/cpuid/v2 v2.2.10/go.mod h1:hqwkgyIinND0mEev00jJYCxPNVRVXFQeu1XKlok6oO0=
 github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
 github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
@@ -42,6 +46,10 @@ github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu
 github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
 github.com/subosito/gotenv v1.6.0 h1:9NlTDc1FTs4qu0DDq7AEtTPNw6SVm7uBMsUCUjABIf8=
 github.com/subosito/gotenv v1.6.0/go.mod h1:Dk4QP5c2W3ibzajGcXpNraDfq2IrhjMIvMSWPKKo0FU=
+github.com/zeebo/assert v1.3.0 h1:g7C04CbJuIDKNPFHmsk4hwZDO5O+kntRxzaUoNXj+IQ=
+github.com/zeebo/assert v1.3.0/go.mod h1:Pq9JiuJQpG8JLJdtkwrJESF0Foym2/D9XMU5ciN/wJ0=
+github.com/zeebo/xxh3 v1.0.2 h1:xZmwmqxHZA8AI603jOQ0tMqmBr9lPeFwGg6d+xy9DC0=
+github.com/zeebo/xxh3 v1.0.2/go.mod h1:5NWz9Sef7zIDm2JHfFlcQvNekmcEl9ekUZQQKCYaDcA=
 go.uber.org/dig v1.19.0 h1:BACLhebsYdpQ7IROQ1AGPjrXcP5dF80U3gKoFzbaq/4=
 go.uber.org/dig v1.19.0/go.mod h1:Us0rSJiThwCv2GteUN0Q7OKvU7n5J4dxZ9JKUXozFdE=
 go.uber.org/fx v1.24.0 h1:wE8mruvpg2kiiL1Vqd0CC+tr0/24XIB10Iwp2lLWzkg=
@@ -54,8 +62,8 @@ go.uber.org/zap v1.26.0 h1:sI7k6L95XOKS281NhVKOFCUNIvv9e0w4BF8N3u+tCRo=
 go.uber.org/zap v1.26.0/go.mod h1:dtElttAiwGvoJ/vj4IwHBS/gXsEu/pZ50mUIRWuG0so=
 go.yaml.in/yaml/v3 v3.0.4 h1:tfq32ie2Jv2UxXFdLJdh3jXuOzWiL1fo0bu/FbuKpbc=
 go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=
-golang.org/x/sys v0.29.0 h1:TPYlXGxvx1MGTn2GiZDhnjPA9wZzZeGKHHmKhHYvgaU=
-golang.org/x/sys v0.29.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+golang.org/x/sys v0.30.0 h1:QjkSwP/36a20jFYWkSue1YwXzLmsV5Gfq7Eiy72C1uc=
+golang.org/x/sys v0.30.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
 golang.org/x/text v0.28.0 h1:rhazDwis8INMIwQ4tpjLDzUhx6RlXqZNPEM0huQojng=
 golang.org/x/text v0.28.0/go.mod h1:U8nCwOR8jO/marOQ0QbDiOngZVEBB7MAiitBuMjXiNU=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
diff --git a/backend/internal/config/config.go b/backend/internal/config/config.go
index 4f60de4..35000af 100644
--- a/backend/internal/config/config.go
+++ b/backend/internal/config/config.go
@@ -4,7 +4,9 @@ package config
 
 import (
 	"errors"
+	"fmt"
 	"log/slog"
+	"net/url"
 	"strings"
 
 	"sneak.berlin/go/netwatch/internal/globals"
@@ -23,6 +25,20 @@ import (
 const defaultTrustedProxies = "127.0.0.1/32,::1/128," +
 	"10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"
 
+// Default limits on stored reports; backend/README.md gives the
+// reasons for these values.
+const (
+	defaultReportsPerMinute = 60
+	defaultDataDirMaxBytes  = 1 << 30 // 1 GiB
+)
+
+var (
+	errNotPositive = errors.New("must be a positive whole number")
+	errNotOrigin   = errors.New(
+		"must be an origin, scheme://host with an optional port",
+	)
+)
+
 // Params defines the dependencies for Config.
 type Params struct {
 	fx.In
@@ -33,16 +49,19 @@ type Params struct {
 
 // Config holds the resolved application configuration.
 type Config struct {
-	BindAddress     string
-	DataDir         string
-	Debug           bool
-	MetricsPassword string
-	MetricsUsername string
-	Port            int
-	SentryDSN       string
-	TrustedProxies  []string
-	log             *slog.Logger
-	params          *Params
+	BindAddress        string
+	CORSAllowedOrigins []string
+	DataDir            string
+	DataDirMaxBytes    int64
+	Debug              bool
+	MetricsPassword    string
+	MetricsUsername    string
+	Port               int
+	ReportsPerMinute   int
+	SentryDSN          string
+	TrustedProxies     []string
+	log                *slog.Logger
+	params             *Params
 }
 
 // New loads configuration from env, .env files, and config
@@ -61,11 +80,15 @@ func New(
 
 	viper.AutomaticEnv()
 
+	// An empty CORS_ALLOWED_ORIGINS allows no other origin.
+	viper.SetDefault("CORS_ALLOWED_ORIGINS", "")
 	viper.SetDefault("DATA_DIR", "./data/reports")
+	viper.SetDefault("DATA_DIR_MAX_BYTES", defaultDataDirMaxBytes)
 	viper.SetDefault("DEBUG", "false")
 	// An empty BIND_ADDRESS listens on every interface.
 	viper.SetDefault("BIND_ADDRESS", "")
 	viper.SetDefault("PORT", "8080")
+	viper.SetDefault("REPORTS_PER_MINUTE", defaultReportsPerMinute)
 	viper.SetDefault("SENTRY_DSN", "")
 	viper.SetDefault("METRICS_USERNAME", "")
 	viper.SetDefault("METRICS_PASSWORD", "")
@@ -81,16 +104,36 @@ func New(
 	}
 
 	s := &Config{
-		BindAddress:     viper.GetString("BIND_ADDRESS"),
-		DataDir:         viper.GetString("DATA_DIR"),
-		Debug:           viper.GetBool("DEBUG"),
-		MetricsPassword: viper.GetString("METRICS_PASSWORD"),
-		MetricsUsername: viper.GetString("METRICS_USERNAME"),
-		Port:            viper.GetInt("PORT"),
-		SentryDSN:       viper.GetString("SENTRY_DSN"),
-		TrustedProxies:  splitList(viper.GetString("TRUSTED_PROXIES")),
-		log:             log,
-		params:          &params,
+		BindAddress:        viper.GetString("BIND_ADDRESS"),
+		CORSAllowedOrigins: splitList(viper.GetString("CORS_ALLOWED_ORIGINS")),
+		DataDir:            viper.GetString("DATA_DIR"),
+		DataDirMaxBytes:    viper.GetInt64("DATA_DIR_MAX_BYTES"),
+		Debug:              viper.GetBool("DEBUG"),
+		MetricsPassword:    viper.GetString("METRICS_PASSWORD"),
+		MetricsUsername:    viper.GetString("METRICS_USERNAME"),
+		Port:               viper.GetInt("PORT"),
+		ReportsPerMinute:   viper.GetInt("REPORTS_PER_MINUTE"),
+		SentryDSN:          viper.GetString("SENTRY_DSN"),
+		TrustedProxies:     splitList(viper.GetString("TRUSTED_PROXIES")),
+		log:                log,
+		params:             &params,
+	}
+
+	// viper reads a value that is not a number as 0, so this also
+	// catches a mistyped setting.
+	if s.ReportsPerMinute <= 0 {
+		return nil, fmt.Errorf("REPORTS_PER_MINUTE %q: %w",
+			viper.GetString("REPORTS_PER_MINUTE"), errNotPositive)
+	}
+
+	if s.DataDirMaxBytes <= 0 {
+		return nil, fmt.Errorf("DATA_DIR_MAX_BYTES %q: %w",
+			viper.GetString("DATA_DIR_MAX_BYTES"), errNotPositive)
+	}
+
+	err = checkOrigins(s.CORSAllowedOrigins)
+	if err != nil {
+		return nil, err
 	}
 
 	if s.Debug {
@@ -101,6 +144,25 @@ func New(
 	return s, nil
 }
 
+// checkOrigins fails on the first CORS_ALLOWED_ORIGINS entry that is
+// not a plain origin, scheme://host with an optional port, as browsers
+// send it; anything more, such as a trailing "/", would match no page.
+// go-chi/cors reads a "*" anywhere in an entry as a wildcard, so no
+// entry may contain one.
+func checkOrigins(origins []string) error {
+	for _, origin := range origins {
+		u, err := url.Parse(origin)
+		if err != nil || u.Scheme == "" || u.Host == "" ||
+			strings.Contains(origin, "*") ||
+			origin != u.Scheme+"://"+u.Host {
+			return fmt.Errorf("CORS_ALLOWED_ORIGINS %q: %w",
+				origin, errNotOrigin)
+		}
+	}
+
+	return nil
+}
+
 // splitList turns a comma-separated setting into a trimmed
 // slice, dropping empty entries.
 func splitList(raw string) []string {
diff --git a/backend/internal/config/config_test.go b/backend/internal/config/config_test.go
new file mode 100644
index 0000000..935a9d7
--- /dev/null
+++ b/backend/internal/config/config_test.go
@@ -0,0 +1,64 @@
+package config_test
+
+import (
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/netwatch/internal/config"
+	"sneak.berlin/go/netwatch/internal/globals"
+	"sneak.berlin/go/netwatch/internal/logger"
+
+	"go.uber.org/fx"
+)
+
+// requireConfigError builds the config as main does and fails the
+// test unless that fails with an error naming setting. It uses
+// fx.New, because fxtest.New fails the test itself on an error.
+func requireConfigError(t *testing.T, setting string) {
+	t.Helper()
+
+	app := fx.New(
+		fx.NopLogger,
+		fx.Provide(globals.New, logger.New, config.New),
+		fx.Invoke(func(*config.Config) {}),
+	)
+
+	err := app.Err()
+	if err == nil || !strings.Contains(err.Error(), setting) {
+		t.Fatalf("config error = %v, want one naming %s", err, setting)
+	}
+}
+
+// TestReportsPerMinuteMustBePositive: unchecked, zero would panic
+// when the routes are built, and a negative rate would lift the
+// limit.
+func TestReportsPerMinuteMustBePositive(t *testing.T) {
+	t.Setenv("REPORTS_PER_MINUTE", "0")
+
+	requireConfigError(t, "REPORTS_PER_MINUTE")
+}
+
+// TestDataDirMaxBytesMustBeANumber: viper reads a value that is not
+// a number, such as "1GB", as 0, which would refuse every report.
+func TestDataDirMaxBytesMustBeANumber(t *testing.T) {
+	t.Setenv("DATA_DIR_MAX_BYTES", "1GB")
+
+	requireConfigError(t, "DATA_DIR_MAX_BYTES")
+}
+
+// TestCORSAllowedOriginsMustBeOrigins: "*" would let every origin in,
+// and an entry that is not a plain origin would match no page.
+func TestCORSAllowedOriginsMustBeOrigins(t *testing.T) {
+	for _, entry := range []string{
+		"*",
+		"https://*.netwatch.example",
+		"netwatch.example",
+		"https://netwatch.example/",
+	} {
+		t.Run(entry, func(t *testing.T) {
+			t.Setenv("CORS_ALLOWED_ORIGINS", entry)
+
+			requireConfigError(t, "CORS_ALLOWED_ORIGINS")
+		})
+	}
+}
diff --git a/backend/internal/handlers/report.go b/backend/internal/handlers/report.go
index 81954a2..859a3a7 100644
--- a/backend/internal/handlers/report.go
+++ b/backend/internal/handlers/report.go
@@ -4,6 +4,8 @@ import (
 	"encoding/json"
 	"errors"
 	"net/http"
+
+	"sneak.berlin/go/netwatch/internal/reportbuf"
 )
 
 // maxLoggedFieldBytes bounds untrusted text (string fields,
@@ -55,10 +57,9 @@ func (s *Handlers) HandleReport() http.HandlerFunc {
 
 		err = s.buf.Append(rpt)
 		if err != nil {
-			s.log.Error("failed to buffer report", "error", err)
 			s.respondJSON(w, r,
 				&response{Status: "error"},
-				http.StatusInternalServerError,
+				s.appendErrorStatus(err),
 			)
 
 			return
@@ -88,6 +89,21 @@ func (s *Handlers) decodeErrorStatus(err error) int {
 	return http.StatusBadRequest
 }
 
+// appendErrorStatus logs a failure to store a report and returns
+// the status to send: 507 when the report files are at their size
+// cap, otherwise 500.
+func (s *Handlers) appendErrorStatus(err error) int {
+	if errors.Is(err, reportbuf.ErrFull) {
+		s.log.Warn("report refused: report files at their size cap")
+
+		return http.StatusInsufficientStorage
+	}
+
+	s.log.Error("failed to buffer report", "error", err)
+
+	return http.StatusInternalServerError
+}
+
 // logReportReceived logs an accepted report. Untrusted fields are
 // bounded (client_id, timestamp) or reduced to a length
 // (geo_bytes) so the raw attacker-controlled body never reaches
diff --git a/backend/internal/handlers/report_test.go b/backend/internal/handlers/report_test.go
index 68e9c71..996ce5c 100644
--- a/backend/internal/handlers/report_test.go
+++ b/backend/internal/handlers/report_test.go
@@ -13,6 +13,7 @@ import (
 
 	"sneak.berlin/go/netwatch/internal/handlers"
 	"sneak.berlin/go/netwatch/internal/middleware"
+	"sneak.berlin/go/netwatch/internal/reportbuf"
 )
 
 var errStorageFailed = errors.New("storage failed")
@@ -66,6 +67,32 @@ func TestHandleReportStorageFailureIsNon2xx(t *testing.T) {
 	}
 }
 
+// TestHandleReportFullIs507 checks the answer when the report files
+// are at their size cap: 507 and the usual error body, which tells
+// the client nothing more.
+func TestHandleReportFullIs507(t *testing.T) {
+	t.Parallel()
+
+	h := newTestHandlers(stubAppender{err: reportbuf.ErrFull}, io.Discard)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(`{"clientId":"c1","hosts":[]}`),
+	)
+
+	h.HandleReport().ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusInsufficientStorage {
+		t.Fatalf("status = %d, want %d",
+			rec.Code, http.StatusInsufficientStorage)
+	}
+
+	if got := rec.Body.String(); got != "{\"status\":\"error\"}\n" {
+		t.Errorf("body = %q, want %q", got, "{\"status\":\"error\"}\n")
+	}
+}
+
 func TestHandleReportMalformedJSONIs400(t *testing.T) {
 	t.Parallel()
 
diff --git a/backend/internal/middleware/export_test.go b/backend/internal/middleware/export_test.go
index 03722f4..22e1e7e 100644
--- a/backend/internal/middleware/export_test.go
+++ b/backend/internal/middleware/export_test.go
@@ -15,6 +15,13 @@ func NewWithLogger(log *slog.Logger) *Middleware {
 	return &Middleware{log: log}
 }
 
+// NewWithTrustedProxies builds a Middleware that honours forwarded
+// headers from the given networks, for tests of the client address
+// paths without the fx graph.
+func NewWithTrustedProxies(trusted []netip.Prefix) *Middleware {
+	return &Middleware{trustedProxies: trusted}
+}
+
 func ClientIP(
 	remoteAddr string,
 	header http.Header,
diff --git a/backend/internal/middleware/middleware.go b/backend/internal/middleware/middleware.go
index b74092f..2e685fc 100644
--- a/backend/internal/middleware/middleware.go
+++ b/backend/internal/middleware/middleware.go
@@ -20,6 +20,7 @@ import (
 
 	"github.com/go-chi/chi/v5/middleware"
 	"github.com/go-chi/cors"
+	"github.com/go-chi/httprate"
 	"go.uber.org/fx"
 )
 
@@ -320,21 +321,43 @@ func (s *Middleware) Recoverer() func(http.Handler) http.Handler {
 	}
 }
 
-// CORS returns middleware that adds permissive CORS headers.
-func (s *Middleware) CORS() func(http.Handler) http.Handler {
+// CORS returns middleware that lets pages served from the given
+// origins call the API. With no origins it adds no CORS headers at
+// all, so only same-origin pages can use the API. That case must not
+// reach cors.Handler, which treats an empty origin list as "allow
+// every origin".
+func (s *Middleware) CORS(
+	origins []string,
+) func(http.Handler) http.Handler {
+	if len(origins) == 0 {
+		return func(next http.Handler) http.Handler { return next }
+	}
+
 	return cors.Handler(cors.Options{
-		AllowedOrigins: []string{"*"},
-		AllowedMethods: []string{
-			"GET", "POST", "PUT", "DELETE", "OPTIONS",
-		},
-		AllowedHeaders: []string{
-			"Accept",
-			"Authorization",
-			"Content-Type",
-			"X-CSRF-Token",
-		},
-		ExposedHeaders:   []string{"Link"},
+		AllowedOrigins:   origins,
+		AllowedMethods:   []string{http.MethodGet, http.MethodPost},
+		AllowedHeaders:   []string{"Content-Type"},
 		AllowCredentials: false,
 		MaxAge:           corsMaxAgeSec,
 	})
 }
+
+// RateLimit returns middleware that allows each client address
+// perMinute requests a minute and answers the rest with 429, the
+// Retry-After header httprate sets, and the usual error body. The
+// address is the one clientIP resolves, so clients behind the reverse
+// proxy are limited one by one, not together as the proxy.
+func (s *Middleware) RateLimit(
+	perMinute int,
+) func(http.Handler) http.Handler {
+	return httprate.LimitBy(perMinute, time.Minute,
+		func(r *http.Request) (string, error) {
+			return clientIP(r.RemoteAddr, r.Header, s.trustedProxies), nil
+		},
+		httprate.WithLimitHandler(
+			func(w http.ResponseWriter, _ *http.Request) {
+				writeJSONError(w, http.StatusTooManyRequests)
+			},
+		),
+	)
+}
diff --git a/backend/internal/middleware/middleware_test.go b/backend/internal/middleware/middleware_test.go
index 8016256..6ae6ded 100644
--- a/backend/internal/middleware/middleware_test.go
+++ b/backend/internal/middleware/middleware_test.go
@@ -10,6 +10,8 @@ import (
 	"net/netip"
 	"strings"
 	"testing"
+	"testing/synctest"
+	"time"
 
 	"sneak.berlin/go/netwatch/internal/middleware"
 )
@@ -300,3 +302,204 @@ func TestRecovererRepanicsOnAbortHandler(t *testing.T) {
 		t.Errorf("abort was logged: %q", logbuf.String())
 	}
 }
+
+// okHandler stands in for the route a middleware guards.
+func okHandler() http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
+}
+
+// TestRateLimitRefusesPastAllowanceThenResets checks one client
+// address: it may use its whole allowance at once, the next request
+// is refused with 429, and later it may send again.
+func TestRateLimitRefusesPastAllowanceThenResets(t *testing.T) {
+	t.Parallel()
+
+	// synctest runs this on a fake clock: time.Sleep returns at once,
+	// with the clock moved on.
+	synctest.Test(t, func(t *testing.T) {
+		const perMinute = 2
+
+		handler := (&middleware.Middleware{}).RateLimit(perMinute)(okHandler())
+
+		post := func() *httptest.ResponseRecorder {
+			rec := httptest.NewRecorder()
+			req := httptest.NewRequestWithContext(t.Context(),
+				http.MethodPost, "/api/v1/reports", http.NoBody)
+			handler.ServeHTTP(rec, req)
+
+			return rec
+		}
+
+		for i := range perMinute {
+			if code := post().Code; code != http.StatusOK {
+				t.Fatalf("request %d: status = %d, want %d",
+					i+1, code, http.StatusOK)
+			}
+		}
+
+		rec := post()
+		if rec.Code != http.StatusTooManyRequests {
+			t.Fatalf("request past the allowance: status = %d, want %d",
+				rec.Code, http.StatusTooManyRequests)
+		}
+
+		if got := rec.Body.String(); got != "{\"status\":\"error\"}\n" {
+			t.Errorf("body = %q, want %q", got, "{\"status\":\"error\"}\n")
+		}
+
+		if got := rec.Header().Get("Retry-After"); got != "60" {
+			t.Fatalf("Retry-After = %q, want %q", got, "60")
+		}
+
+		// httprate also counts the previous minute's requests, fading
+		// them out over the current one, so two minutes on the whole
+		// allowance is back.
+		time.Sleep(2 * time.Minute)
+
+		for i := range perMinute {
+			if code := post().Code; code != http.StatusOK {
+				t.Fatalf("two minutes later, request %d: status = %d, want %d",
+					i+1, code, http.StatusOK)
+			}
+		}
+	})
+}
+
+// postForwarded sends handler a report from peer that names client in
+// X-Forwarded-For, and returns the status.
+func postForwarded(
+	t *testing.T,
+	handler http.Handler,
+	peer, client string,
+) int {
+	t.Helper()
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodPost, "/api/v1/reports", http.NoBody)
+	req.RemoteAddr = peer
+	req.Header.Set("X-Forwarded-For", client)
+	handler.ServeHTTP(rec, req)
+
+	return rec.Code
+}
+
+// TestRateLimitIsPerForwardedClient checks that clients behind a
+// trusted proxy each get their own allowance: the limit is keyed on
+// the client address clientIP resolves, not on the proxy's.
+func TestRateLimitIsPerForwardedClient(t *testing.T) {
+	t.Parallel()
+
+	const otherClient = "203.0.113.8"
+
+	mw := middleware.NewWithTrustedProxies(mustPrefixes(t, "127.0.0.1/32"))
+	handler := mw.RateLimit(1)(okHandler())
+
+	code := postForwarded(t, handler, loopbackPeer, forwardedIP)
+	if code != http.StatusOK {
+		t.Fatalf("first request: status = %d, want %d", code, http.StatusOK)
+	}
+
+	code = postForwarded(t, handler, loopbackPeer, forwardedIP)
+	if code != http.StatusTooManyRequests {
+		t.Fatalf("same client again: status = %d, want %d",
+			code, http.StatusTooManyRequests)
+	}
+
+	code = postForwarded(t, handler, loopbackPeer, otherClient)
+	if code != http.StatusOK {
+		t.Fatalf("other client behind the same proxy: status = %d, want %d",
+			code, http.StatusOK)
+	}
+}
+
+// TestRateLimitIgnoresForwardedForFromUntrustedPeer checks that a
+// peer that is not a trusted proxy cannot get a fresh allowance by
+// naming a different client in X-Forwarded-For on each request.
+func TestRateLimitIgnoresForwardedForFromUntrustedPeer(t *testing.T) {
+	t.Parallel()
+
+	const untrustedPeer = "198.51.100.4:5000"
+
+	mw := middleware.NewWithTrustedProxies(mustPrefixes(t, "127.0.0.1/32"))
+	handler := mw.RateLimit(1)(okHandler())
+
+	code := postForwarded(t, handler, untrustedPeer, "203.0.113.8")
+	if code != http.StatusOK {
+		t.Fatalf("first request: status = %d, want %d", code, http.StatusOK)
+	}
+
+	code = postForwarded(t, handler, untrustedPeer, "203.0.113.9")
+	if code != http.StatusTooManyRequests {
+		t.Fatalf("same peer naming another client: status = %d, want %d",
+			code, http.StatusTooManyRequests)
+	}
+}
+
+// preflight sends cors the preflight request a browser makes before
+// it POSTs JSON from origin.
+func preflight(
+	t *testing.T,
+	cors func(http.Handler) http.Handler,
+	origin string,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodOptions, "/api/v1/reports", http.NoBody)
+	req.Header.Set("Origin", origin)
+	req.Header.Set("Access-Control-Request-Method", http.MethodPost)
+	req.Header.Set("Access-Control-Request-Headers", "content-type")
+	cors(okHandler()).ServeHTTP(rec, req)
+
+	return rec
+}
+
+// TestCORSWithoutOriginsAddsNoHeaders checks the default: with no
+// origins configured, no origin is given any CORS header.
+func TestCORSWithoutOriginsAddsNoHeaders(t *testing.T) {
+	t.Parallel()
+
+	rec := preflight(t,
+		(&middleware.Middleware{}).CORS(nil), "https://elsewhere.example")
+
+	for name := range rec.Header() {
+		if strings.HasPrefix(name, "Access-Control-") {
+			t.Errorf("CORS header %s set with no origins configured", name)
+		}
+	}
+}
+
+func TestCORSAllowsOnlyListedOrigins(t *testing.T) {
+	t.Parallel()
+
+	const listed = "https://netwatch.example"
+
+	cors := (&middleware.Middleware{}).CORS([]string{listed})
+
+	cases := []struct {
+		name   string
+		origin string
+		want   string
+	}{
+		{name: "listed origin allowed", origin: listed, want: listed},
+		{name: "other origin refused", origin: "https://elsewhere.example"},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			rec := preflight(t, cors, tc.origin)
+
+			got := rec.Header().Get("Access-Control-Allow-Origin")
+			if got != tc.want {
+				t.Errorf("Access-Control-Allow-Origin = %q, want %q",
+					got, tc.want)
+			}
+		})
+	}
+}
diff --git a/backend/internal/reportbuf/export_test.go b/backend/internal/reportbuf/export_test.go
new file mode 100644
index 0000000..6440af9
--- /dev/null
+++ b/backend/internal/reportbuf/export_test.go
@@ -0,0 +1,7 @@
+package reportbuf
+
+// Flush writes the buffered reports to a file now, as the periodic
+// flush does, so tests need not wait a minute for it.
+func (b *Buffer) Flush() error {
+	return b.flushLocked()
+}
diff --git a/backend/internal/reportbuf/reportbuf.go b/backend/internal/reportbuf/reportbuf.go
index 4626770..318b77f 100644
--- a/backend/internal/reportbuf/reportbuf.go
+++ b/backend/internal/reportbuf/reportbuf.go
@@ -6,11 +6,13 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io/fs"
 	"log/slog"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync"
 	"time"
 
@@ -27,8 +29,16 @@ const (
 	defaultDataDir                 = "./data/reports"
 	dirPerms           fs.FileMode = 0o750
 	filePerms          fs.FileMode = 0o640
+
+	// Report files are named filePrefix + timestamp + fileSuffix.
+	filePrefix = "reports-"
+	fileSuffix = ".jsonl.zst"
 )
 
+// ErrFull is returned by Append when storing the report would
+// take the report files past the configured maximum size.
+var ErrFull = errors.New("report files at their size cap")
+
 // Params defines the dependencies for Buffer.
 type Params struct {
 	fx.In
@@ -44,8 +54,13 @@ type Buffer struct {
 	dataDir  string
 	done     chan struct{}
 	log      *slog.Logger
+	maxBytes int64
 	mu       sync.Mutex
 	stopOnce sync.Once
+	// usedBytes is what Append checks against maxBytes: the size
+	// of the report files in dataDir, plus the reports not yet
+	// written to one at their uncompressed size.
+	usedBytes int64
 }
 
 // New creates a Buffer and registers lifecycle hooks to
@@ -60,9 +75,10 @@ func New(
 	}
 
 	b := &Buffer{
-		dataDir: dir,
-		done:    make(chan struct{}),
-		log:     params.Logger.Get(),
+		dataDir:  dir,
+		done:     make(chan struct{}),
+		log:      params.Logger.Get(),
+		maxBytes: params.Config.DataDirMaxBytes,
 	}
 
 	lc.Append(fx.Hook{
@@ -72,6 +88,12 @@ func New(
 				return fmt.Errorf("create data dir: %w", err)
 			}
 
+			// Report files left by earlier runs count too.
+			b.usedBytes, err = reportFilesSize(b.dataDir)
+			if err != nil {
+				return err
+			}
+
 			go b.flushLoop()
 
 			return nil
@@ -97,15 +119,27 @@ func New(
 }
 
 // Append marshals v as a single JSON line and appends it to
-// the buffer. If the buffer reaches the size threshold, it is
-// drained and written to disk asynchronously.
+// the buffer. It stores nothing and returns ErrFull if the line
+// would take usedBytes past maxBytes. If the buffer reaches the
+// size threshold, it is drained and written to disk
+// asynchronously.
 func (b *Buffer) Append(v any) error {
 	line, err := json.Marshal(v)
 	if err != nil {
 		return fmt.Errorf("marshal report: %w", err)
 	}
 
+	lineBytes := int64(len(line)) + 1 // with its newline
+
 	b.mu.Lock()
+
+	if b.usedBytes+lineBytes > b.maxBytes {
+		b.mu.Unlock()
+
+		return ErrFull
+	}
+
+	b.usedBytes += lineBytes
 	b.buf.Write(line)
 	b.buf.WriteByte('\n')
 
@@ -178,8 +212,7 @@ func (b *Buffer) drainBuf() []byte {
 // in the data directory.
 func (b *Buffer) writeFile(data []byte) error {
 	ts := time.Now().UTC().Format("2006-01-02T15-04-05.000Z")
-	name := fmt.Sprintf("reports-%s.jsonl.zst", ts)
-	path := filepath.Join(b.dataDir, name)
+	path := filepath.Join(b.dataDir, filePrefix+ts+fileSuffix)
 
 	// path is built from the operator-supplied dataDir plus a
 	// generated timestamp, so it carries no external input.
@@ -214,10 +247,51 @@ func (b *Buffer) writeFile(data []byte) error {
 		return fmt.Errorf("close zstd encoder: %w", err)
 	}
 
+	info, err := f.Stat()
+	if err != nil {
+		return fmt.Errorf("stat report file: %w", err)
+	}
+
 	err = f.Close()
 	if err != nil {
 		return fmt.Errorf("close report file: %w", err)
 	}
 
+	// The reports counted at their uncompressed size while they
+	// waited; now they count as the file. After a failed write they
+	// stay counted as they were, which errs toward refusing reports
+	// early rather than letting the files pass the cap.
+	b.mu.Lock()
+	b.usedBytes += info.Size() - int64(len(data))
+	b.mu.Unlock()
+
 	return nil
 }
+
+// reportFilesSize returns the total size of the report files in
+// dir.
+func reportFilesSize(dir string) (int64, error) {
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return 0, fmt.Errorf("read data dir: %w", err)
+	}
+
+	var total int64
+
+	for _, entry := range entries {
+		name := entry.Name()
+		if !strings.HasPrefix(name, filePrefix) ||
+			!strings.HasSuffix(name, fileSuffix) {
+			continue
+		}
+
+		info, err := entry.Info()
+		if err != nil {
+			return 0, fmt.Errorf("stat report file: %w", err)
+		}
+
+		total += info.Size()
+	}
+
+	return total, nil
+}
diff --git a/backend/internal/reportbuf/reportbuf_test.go b/backend/internal/reportbuf/reportbuf_test.go
index ade9457..90d3a94 100644
--- a/backend/internal/reportbuf/reportbuf_test.go
+++ b/backend/internal/reportbuf/reportbuf_test.go
@@ -1,11 +1,17 @@
 package reportbuf_test
 
 import (
+	"encoding/json"
 	"errors"
 	"io/fs"
 	"os"
+	"path/filepath"
+	"strconv"
 	"strings"
+	"sync"
+	"sync/atomic"
 	"testing"
+	"time"
 
 	"sneak.berlin/go/netwatch/internal/config"
 	"sneak.berlin/go/netwatch/internal/globals"
@@ -94,6 +100,253 @@ func TestFailedFinalFlushFailsStop(t *testing.T) {
 	}
 }
 
+// startBuffer starts a Buffer through fx, as main does, with the
+// DATA_DIR and DATA_DIR_MAX_BYTES the calling test has set.
+func startBuffer(t *testing.T) *reportbuf.Buffer {
+	t.Helper()
+
+	var buf *reportbuf.Buffer
+
+	app := fxtest.New(t,
+		fx.Provide(
+			globals.New,
+			logger.New,
+			config.New,
+			reportbuf.New,
+		),
+		fx.Populate(&buf),
+	)
+
+	app.RequireStart()
+	t.Cleanup(app.RequireStop)
+
+	return buf
+}
+
+// lineBytes is what one report takes in the buffer: its JSON and a
+// newline.
+func lineBytes(t *testing.T, report any) int {
+	t.Helper()
+
+	line, err := json.Marshal(report)
+	if err != nil {
+		t.Fatalf("marshal report: %v", err)
+	}
+
+	return len(line) + 1
+}
+
+func TestAppendPastCapIsRefused(t *testing.T) {
+	report := map[string]string{"id": "cap"}
+
+	t.Setenv("DATA_DIR", t.TempDir())
+	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(lineBytes(t, report)))
+
+	buf := startBuffer(t)
+
+	err := buf.Append(report)
+	if err != nil {
+		t.Fatalf("report that fills the cap exactly: %v", err)
+	}
+
+	err = buf.Append(report)
+	if !errors.Is(err, reportbuf.ErrFull) {
+		t.Fatalf("report past the cap: error = %v, want ErrFull", err)
+	}
+}
+
+// TestCapCountsReportFilesAlreadyInDataDir starts on a data
+// directory holding a report file from an earlier run, and a file
+// that is not a report, which must not count.
+func TestCapCountsReportFilesAlreadyInDataDir(t *testing.T) {
+	const earlierBytes = 100
+
+	report := map[string]string{"id": "cap"}
+	dir := t.TempDir()
+
+	writeBytes(t, filepath.Join(dir, "reports-2026-01-01T00-00-00.000Z.jsonl.zst"),
+		earlierBytes)
+	writeBytes(t, filepath.Join(dir, "notes.txt"), 10*earlierBytes)
+
+	t.Setenv("DATA_DIR", dir)
+	t.Setenv("DATA_DIR_MAX_BYTES",
+		strconv.Itoa(earlierBytes+lineBytes(t, report)))
+
+	buf := startBuffer(t)
+
+	err := buf.Append(report)
+	if err != nil {
+		t.Fatalf("report that fills the cap exactly: %v", err)
+	}
+
+	err = buf.Append(report)
+	if !errors.Is(err, reportbuf.ErrFull) {
+		t.Fatalf("report past the cap: error = %v, want ErrFull", err)
+	}
+}
+
+// TestWrittenReportsCountAtFileSize checks that once reports are
+// written, they count as their compressed file, not their
+// uncompressed size, which frees room under the cap.
+func TestWrittenReportsCountAtFileSize(t *testing.T) {
+	// Repetitive, so its file is far smaller than its JSON.
+	report := map[string]string{"id": strings.Repeat("a", 1000)}
+	size := lineBytes(t, report)
+
+	t.Setenv("DATA_DIR", t.TempDir())
+	// Room for the report twice over only if the first one counts
+	// at its file's size by the time the second arrives.
+	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(2*size-1))
+
+	buf := startBuffer(t)
+
+	err := buf.Append(report)
+	if err != nil {
+		t.Fatalf("first report: %v", err)
+	}
+
+	err = buf.Flush()
+	if err != nil {
+		t.Fatalf("flush: %v", err)
+	}
+
+	// The second report is written at shutdown, and must not land in
+	// the first file's millisecond (see TestWrittenReportsKeepCounting).
+	time.Sleep(time.Millisecond)
+
+	err = buf.Append(report)
+	if err != nil {
+		t.Fatalf("second report, after the first was written: %v", err)
+	}
+}
+
+// TestWrittenReportsKeepCounting writes one report file after another
+// under a small cap: each report must be taken while the files on disk
+// leave room for it, and refused once they do not.
+func TestWrittenReportsKeepCounting(t *testing.T) {
+	const maxBytes = 200
+
+	report := map[string]string{"id": "written"}
+	size := int64(lineBytes(t, report))
+	dir := t.TempDir()
+
+	t.Setenv("DATA_DIR", dir)
+	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(maxBytes))
+
+	buf := startBuffer(t)
+
+	// Every file takes at least a byte, so they fill the cap within
+	// maxBytes rounds.
+	for range maxBytes {
+		used := reportFilesBytes(t, dir)
+
+		err := buf.Append(report)
+		if used+size > maxBytes {
+			if !errors.Is(err, reportbuf.ErrFull) {
+				t.Fatalf("with %d bytes of report files: error = %v, "+
+					"want ErrFull", used, err)
+			}
+
+			return
+		}
+
+		if err != nil {
+			t.Fatalf("with %d bytes of report files: %v", used, err)
+		}
+
+		// Report files are named to the millisecond; two in the same
+		// one collide (https://git.eeqj.de/sneak/netwatch/issues/61).
+		time.Sleep(time.Millisecond)
+
+		err = buf.Flush()
+		if err != nil {
+			t.Fatalf("flush: %v", err)
+		}
+	}
+
+	t.Fatal("the report files never filled the cap")
+}
+
+// TestConcurrentAppendsStopAtCap appends from many goroutines at once
+// with room for exactly roomFor reports: exactly that many must be
+// taken, which holds only if Append checks and counts each report
+// under one lock.
+func TestConcurrentAppendsStopAtCap(t *testing.T) {
+	const (
+		roomFor = 5
+		senders = 50
+	)
+
+	// Large, so each Append takes long enough for the senders to
+	// overlap while the cap is reached.
+	report := map[string]string{"id": strings.Repeat("a", 1_000_000)}
+
+	t.Setenv("DATA_DIR", t.TempDir())
+	t.Setenv("DATA_DIR_MAX_BYTES",
+		strconv.Itoa(roomFor*lineBytes(t, report)))
+
+	buf := startBuffer(t)
+
+	var (
+		taken atomic.Int64
+		wg    sync.WaitGroup
+	)
+
+	start := make(chan struct{})
+
+	for range senders {
+		wg.Go(func() {
+			<-start
+
+			err := buf.Append(report)
+			if err == nil {
+				taken.Add(1)
+			} else if !errors.Is(err, reportbuf.ErrFull) {
+				t.Errorf("append: %v", err)
+			}
+		})
+	}
+
+	close(start)
+	wg.Wait()
+
+	if got := taken.Load(); got != roomFor {
+		t.Fatalf("%d reports taken, want %d", got, roomFor)
+	}
+}
+
+// reportFilesBytes returns the total size of the report files in dir.
+func reportFilesBytes(t *testing.T, dir string) int64 {
+	t.Helper()
+
+	paths, err := filepath.Glob(filepath.Join(dir, "reports-*.jsonl.zst"))
+	if err != nil {
+		t.Fatalf("list report files: %v", err)
+	}
+
+	var total int64
+
+	for _, path := range paths {
+		info, statErr := os.Stat(path)
+		if statErr != nil {
+			t.Fatalf("stat %s: %v", path, statErr)
+		}
+
+		total += info.Size()
+	}
+
+	return total
+}
+
+func writeBytes(t *testing.T, path string, n int) {
+	t.Helper()
+
+	err := os.WriteFile(path, make([]byte, n), 0o600)
+	if err != nil {
+		t.Fatalf("write %s: %v", path, err)
+	}
+}
+
 func hasReportFile(t *testing.T, dir string) bool {
 	t.Helper()
 
diff --git a/backend/internal/server/routes.go b/backend/internal/server/routes.go
index 9d4547d..4c87f59 100644
--- a/backend/internal/server/routes.go
+++ b/backend/internal/server/routes.go
@@ -25,7 +25,7 @@ func (s *Server) SetupRoutes() {
 	s.router.Use(middleware.RequestID)
 	s.router.Use(s.mw.Logging())
 	s.router.Use(s.mw.SecurityHeaders())
-	s.router.Use(s.mw.CORS())
+	s.router.Use(s.mw.CORS(s.params.Config.CORSAllowedOrigins))
 	s.router.Use(s.mw.MaxBodyBytes(maxRequestBodyBytes))
 	s.router.Use(middleware.Timeout(requestTimeout))
 
@@ -35,6 +35,7 @@ func (s *Server) SetupRoutes() {
 	)
 
 	s.router.Route("/api/v1", func(r chi.Router) {
-		r.Post("/reports", s.h.HandleReport())
+		r.With(s.mw.RateLimit(s.params.Config.ReportsPerMinute)).
+			Post("/reports", s.h.HandleReport())
 	})
 }
diff --git a/backend/internal/server/routes_test.go b/backend/internal/server/routes_test.go
index 71f5117..972debd 100644
--- a/backend/internal/server/routes_test.go
+++ b/backend/internal/server/routes_test.go
@@ -49,6 +49,64 @@ func newServer(t *testing.T) *server.Server {
 	return srv
 }
 
+// TestReportsAreRateLimited checks that POST /api/v1/reports is
+// behind the per-address rate limit, set here to two a minute.
+func TestReportsAreRateLimited(t *testing.T) {
+	t.Setenv("REPORTS_PER_MINUTE", "2")
+
+	srv := newServer(t)
+	srv.SetupRoutes()
+
+	post := func() int {
+		rec := httptest.NewRecorder()
+		req := httptest.NewRequestWithContext(t.Context(),
+			http.MethodPost, "/api/v1/reports",
+			strings.NewReader(`{"clientId":"c1","hosts":[]}`),
+		)
+		srv.ServeHTTP(rec, req)
+
+		return rec.Code
+	}
+
+	for i := range 2 {
+		if code := post(); code != http.StatusOK {
+			t.Fatalf("report %d: status = %d, want %d",
+				i+1, code, http.StatusOK)
+		}
+	}
+
+	if code := post(); code != http.StatusTooManyRequests {
+		t.Fatalf("third report in a minute: status = %d, want %d",
+			code, http.StatusTooManyRequests)
+	}
+}
+
+// TestCORSAllowedOriginsReachTheRouter checks that an origin listed in
+// CORS_ALLOWED_ORIGINS is allowed by the router, not only when handed
+// to the CORS middleware directly.
+func TestCORSAllowedOriginsReachTheRouter(t *testing.T) {
+	const origin = "https://netwatch.example:8443"
+
+	t.Setenv("CORS_ALLOWED_ORIGINS", origin)
+
+	srv := newServer(t)
+	srv.SetupRoutes()
+
+	// The preflight a browser sends before it POSTs JSON from origin.
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodOptions, "/api/v1/reports", http.NoBody)
+	req.Header.Set("Origin", origin)
+	req.Header.Set("Access-Control-Request-Method", http.MethodPost)
+	req.Header.Set("Access-Control-Request-Headers", "content-type")
+	srv.ServeHTTP(rec, req)
+
+	got := rec.Header().Get("Access-Control-Allow-Origin")
+	if got != origin {
+		t.Fatalf("Access-Control-Allow-Origin = %q, want %q", got, origin)
+	}
+}
+
 // TestHealthCheckRejectsOversizeBody sends the health check, which
 // never reads its body, a body one byte over the limit. Only the
 // router-wide body limit can reject it.
-- 
2.54.0


From ced1956b06ec4880f8fef8bb62fc9071c15238ac Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Tue, 29 Sep 2026 04:55:48 +0200
Subject: [PATCH 14/22] nginx: listen on PORT, default 8080; server_tokens off
 (closes #26)

nginx.conf is now a template the nginx image renders into conf.d at
container start. bin/entrypoint.sh sets PORT to 8080 when unset or
empty, and stops with an error before starting anything when PORT is
not digits only: nginx would take a value such as localhost or
unix:/tmp/x.sock as an address and start anyway. NGINX_ENVSUBST_FILTER
limits the rendering to PORT, so $uri, $host and every other nginx
variable pass through unchanged. server_tokens off drops the version
from the Server header and error pages. script/frontend-viewport-test
renders the template the same way. EXPOSE still documents 8080; the
backend stays on 127.0.0.1:8081.

Model: opus-5-5
---
 Dockerfile                    |  5 ++++-
 TODO.md                       |  6 ++++++
 bin/entrypoint.sh             | 21 +++++++++++++++++++--
 nginx.conf                    |  8 +++++++-
 script/frontend-viewport-test |  5 ++++-
 5 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/Dockerfile b/Dockerfile
index 3046f01..d9b8879 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -68,8 +68,10 @@ FROM nginx@sha256:15e96e59aa3b0aada3a121296e3bce117721f42d88f5f64217ef4b18f458c6
 RUN addgroup -g 1000 -S netwatch && \
     adduser -u 1000 -S netwatch -G netwatch
 
+# At start-up the nginx image renders every template here into
+# conf.d; bin/entrypoint.sh says how.
 RUN rm /etc/nginx/conf.d/default.conf
-COPY nginx.conf /etc/nginx/conf.d/netwatch.conf
+COPY nginx.conf /etc/nginx/templates/netwatch.conf.template
 COPY --from=frontend /app/dist /usr/share/nginx/html
 COPY --from=builder /src/netwatch-server /usr/local/bin/netwatch-server
 COPY bin/entrypoint.sh /usr/local/bin/entrypoint.sh
@@ -78,6 +80,7 @@ ENV DATA_DIR=/data/reports
 RUN mkdir -p /data/reports && chown -R netwatch:netwatch /data
 VOLUME /data
 
+# The default public port; PORT changes it.
 EXPOSE 8080
 
 # The nginx image stops its container with SIGQUIT; the entrypoint
diff --git a/TODO.md b/TODO.md
index adbc424..c275f1e 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,12 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: nginx listens on `PORT` (issue #26), 8080 when unset or empty: the
+  nginx image renders `nginx.conf` as a template at container start, filling in
+  `PORT` and no other variable. `bin/entrypoint.sh` refuses to start when `PORT`
+  is not digits only. `server_tokens off` keeps the nginx version out of
+  responses. `script/frontend-viewport-test` renders the template the same way.
+  Gzip and a `50x.html` error page are not added
 - 2026-09-29: bounded the report endpoint (issue #20): `POST /api/v1/reports`
   still needs no credentials, but each client address, as resolved through
   `TRUSTED_PROXIES`, may send `REPORTS_PER_MINUTE` (default 60) reports a
diff --git a/bin/entrypoint.sh b/bin/entrypoint.sh
index 3dece27..b100e75 100755
--- a/bin/entrypoint.sh
+++ b/bin/entrypoint.sh
@@ -8,6 +8,18 @@
 # No set -e: kill and wait return non-zero here in normal operation.
 set -u
 
+# PORT is the public port nginx listens on, 8080 when unset or empty.
+# nginx would take a value such as localhost or unix:/tmp/x.sock as an
+# address and start anyway, so anything but digits stops the container
+# here, before either process starts.
+export PORT="${PORT:-8080}"
+case "$PORT" in
+    *[!0-9]*)
+        echo "entrypoint: PORT must be a port number, not '$PORT'" >&2
+        exit 1
+        ;;
+esac
+
 # A stop signal is only noted here; the loop below acts on it.
 stop_requested=""
 trap 'stop_requested=yes' TERM INT
@@ -23,8 +35,13 @@ backend=$!
 
 # nginx starts through the nginx image's own entrypoint, which applies
 # the image's start-up configuration and then replaces itself with
-# nginx.
-/docker-entrypoint.sh nginx -g 'daemon off;' &
+# nginx. Part of that start-up configuration renders nginx.conf into
+# conf.d with nginx listening on PORT. NGINX_ENVSUBST_FILTER limits
+# that rendering to PORT: a variable nginx itself uses, such as $uri,
+# would otherwise be replaced by an environment variable of the same
+# name.
+NGINX_ENVSUBST_FILTER='^PORT$' \
+    /docker-entrypoint.sh nginx -g 'daemon off;' &
 nginx=$!
 
 running() {
diff --git a/nginx.conf b/nginx.conf
index 65e8aaf..697340b 100644
--- a/nginx.conf
+++ b/nginx.conf
@@ -1,7 +1,13 @@
+# A template: the nginx image renders it into conf.d at container start,
+# filling in PORT and nothing else. bin/entrypoint.sh sets PORT and that
+# limit.
 server {
-    listen 8080;
+    listen ${PORT};
     server_name _;
 
+    # Keep the nginx version out of the Server header and error pages.
+    server_tokens off;
+
     root /usr/share/nginx/html;
     index index.html;
 
diff --git a/script/frontend-viewport-test b/script/frontend-viewport-test
index 53a2ee1..5a97c99 100755
--- a/script/frontend-viewport-test
+++ b/script/frontend-viewport-test
@@ -61,10 +61,13 @@ main() {
     # host.
     docker network create --internal "$NETWORK" > /dev/null
 
+    # nginx.conf is a template: the image renders it over its own
+    # default.conf, with the same port and limit bin/entrypoint.sh uses.
     docker run -d --rm --name "$SERVER" \
         --network "$NETWORK" --network-alias netwatch \
+        -e PORT=8080 -e NGINX_ENVSUBST_FILTER='^PORT$' \
         -v "$ROOT/dist:/usr/share/nginx/html:ro" \
-        -v "$ROOT/nginx.conf:/etc/nginx/conf.d/default.conf:ro" \
+        -v "$ROOT/nginx.conf:/etc/nginx/templates/default.conf.template:ro" \
         "$SERVER_IMAGE" > /dev/null
 
     # The image's own entrypoint already exposes CDP on 9222 and passes
-- 
2.54.0


From d2f219ca19fc1375abb4db4c5481294966cd1fc0 Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Tue, 29 Sep 2026 06:39:10 +0200
Subject: [PATCH 15/22] upaas: health check, settings checked at start, README
 section (closes #59)

The image's HEALTHCHECK requests /.well-known/healthcheck through
nginx on the port from PORT, so it fails unless both processes answer.
The backend reads PORT and DEBUG with strconv instead of viper, which
turned a bad PORT into 0 and a bad DEBUG into false. Those, and a
BIND_ADDRESS that is not an IP address, now stop the start with an
error naming the variable; the TRUSTED_PROXIES error names it too.
bin/entrypoint.sh also refuses a container PORT outside 1 to 65535,
or 8081, where the backend listens, naming PORT. README.md gains
"Running under upaas". Its first-run steps create the host directory
owned by uid 1000, so the image changes no ownership.

Model: opus-5-5
---
 Dockerfile                                    |  6 ++
 README.md                                     | 40 +++++++++++
 TODO.md                                       | 10 +++
 backend/README.md                             |  4 ++
 backend/internal/config/config.go             | 67 ++++++++++++++-----
 backend/internal/config/config_test.go        | 57 ++++++++++++++++
 backend/internal/middleware/middleware.go     |  6 +-
 .../internal/middleware/middleware_test.go    |  4 +-
 bin/entrypoint.sh                             | 16 ++++-
 9 files changed, 187 insertions(+), 23 deletions(-)

diff --git a/Dockerfile b/Dockerfile
index d9b8879..87435d2 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -83,6 +83,12 @@ VOLUME /data
 # The default public port; PORT changes it.
 EXPOSE 8080
 
+# Requests the backend's health check through nginx, on the port from
+# PORT, so it fails unless both answer. upaas reads the result 60
+# seconds after a deploy and fails the deploy unless it is healthy.
+HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+    CMD wget -q -O /dev/null "http://127.0.0.1:${PORT:-8080}/.well-known/healthcheck"
+
 # The nginx image stops its container with SIGQUIT; the entrypoint
 # acts on TERM and INT.
 STOPSIGNAL SIGTERM
diff --git a/README.md b/README.md
index aa90734..89e5c5c 100644
--- a/README.md
+++ b/README.md
@@ -194,6 +194,46 @@ only inside the container, on `127.0.0.1:8081`. The image:
 - Writes buffered reports to disk on `docker stop`, and exits non-zero if nginx
   or the backend exits on its own, so the platform restarts it
 
+## Running under upaas
+
+What the [upaas](https://git.eeqj.de/sneak/upaas) app for netwatch needs:
+
+- **Port:** container port `8080`.
+- **Volume:** container path `/data`; the reports are kept in `/data/reports`.
+- **First run:** upaas bind-mounts the host directory it is given and does not
+  create it, and the backend, which runs as uid 1000, does not start unless it
+  can write there. Create the directory, owned by uid 1000, before the first
+  deploy:
+
+    ```bash
+    mkdir -p /path/to/data
+    chown 1000:1000 /path/to/data
+    ```
+
+- **Environment variables:** none is required. An empty one counts as unset, and
+  one set to a value netwatch cannot use stops the container at start, with the
+  reason in its log.
+    - `PORT`, default `8080`: the container port, from 1 to 65535. `8081` cannot
+      be used: the backend listens on it inside the container
+    - `REPORTS_PER_MINUTE`, default `60`: reports each client address may send a
+      minute
+    - `DATA_DIR_MAX_BYTES`, default `1073741824` (1 GiB): the most room the
+      report files may take
+    - `CORS_ALLOWED_ORIGINS`, default empty: other origins whose pages may call
+      the API
+    - `DEBUG`, default `false`: debug logging
+    - `DATA_DIR`, default `/data/reports`: leave unset; reports kept outside
+      `/data` do not survive a redeploy
+    - `TRUSTED_PROXIES`, default loopback and RFC1918: leave unset. The
+      backend's only client is nginx, on loopback, which passes on the client
+      address; nginx takes it from `X-Forwarded-For` only from RFC1918
+      addresses.
+- **Health check:** the image's `HEALTHCHECK` requests
+  `/.well-known/healthcheck` through nginx every 30 seconds, so it fails unless
+  both nginx and the backend answer. upaas reads the container's health 60
+  seconds after a deploy and fails the deploy unless it is `healthy`. The
+  container also stops when either process exits.
+
 ## Browser Compatibility
 
 Requires a modern browser with ES modules, Fetch API, Canvas API, and CSS custom
diff --git a/TODO.md b/TODO.md
index c275f1e..caa9dce 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,16 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: ready to run under upaas (issue #59): the image has a
+  `HEALTHCHECK` that requests `/.well-known/healthcheck` through nginx on the
+  port from `PORT`. The backend no longer reads a bad `PORT` as 0 or a bad
+  `DEBUG` as false: those, and a `BIND_ADDRESS` that is not an IP address, stop
+  it from starting with an error naming the variable, as the limits,
+  `CORS_ALLOWED_ORIGINS` and, now by name, `TRUSTED_PROXIES` already did.
+  `bin/entrypoint.sh` also refuses a `PORT` outside 1 to 65535, and `8081`,
+  where the backend listens inside the container, naming `PORT`. `README.md` has
+  a "Running under upaas" section, whose first-run steps create the host
+  directory for `/data` owned by uid 1000; the image does not change its owner
 - 2026-09-29: nginx listens on `PORT` (issue #26), 8080 when unset or empty: the
   nginx image renders `nginx.conf` as a template at container start, filling in
   `PORT` and no other variable. `bin/entrypoint.sh` refuses to start when `PORT`
diff --git a/backend/README.md b/backend/README.md
index 095af11..435647f 100644
--- a/backend/README.md
+++ b/backend/README.md
@@ -91,6 +91,10 @@ The loopback entries cover the reverse proxy that shares the container; the
 RFC1918 ranges match `nginx.conf`. A request whose direct peer is outside this
 set has its forwarded headers ignored, and the direct peer is logged instead.
 
+A variable set to a value the server cannot use, such as `PORT=abc`,
+`DEBUG=maybe` or a `BIND_ADDRESS` that is not an IP address, stops it from
+starting, with an error naming the variable. An empty variable counts as unset.
+
 ### Container image
 
 The root `Dockerfile` builds one image in which nginx listens on the public port
diff --git a/backend/internal/config/config.go b/backend/internal/config/config.go
index 35000af..c8236d0 100644
--- a/backend/internal/config/config.go
+++ b/backend/internal/config/config.go
@@ -6,7 +6,10 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"math"
+	"net/netip"
 	"net/url"
+	"strconv"
 	"strings"
 
 	"sneak.berlin/go/netwatch/internal/globals"
@@ -37,6 +40,9 @@ var (
 	errNotOrigin   = errors.New(
 		"must be an origin, scheme://host with an optional port",
 	)
+	errNotPort = errors.New("must be a port number, 1 to 65535")
+	errNotBool = errors.New("must be true or false")
+	errNotIP   = errors.New("must be an IP address, or empty")
 )
 
 // Params defines the dependencies for Config.
@@ -65,7 +71,8 @@ type Config struct {
 }
 
 // New loads configuration from env, .env files, and config
-// files, returning a fully resolved Config.
+// files, returning a fully resolved Config. It fails, with an error
+// naming the setting, on a value the server cannot use.
 func New(
 	_ fx.Lifecycle,
 	params Params,
@@ -103,15 +110,29 @@ func New(
 		}
 	}
 
+	// Read with strconv: viper's GetInt and GetBool would read a value
+	// they cannot parse as 0 or false instead of failing.
+	port, err := strconv.Atoi(viper.GetString("PORT"))
+	if err != nil || port < 1 || port > math.MaxUint16 {
+		return nil, fmt.Errorf("PORT %q: %w",
+			viper.GetString("PORT"), errNotPort)
+	}
+
+	debug, err := strconv.ParseBool(viper.GetString("DEBUG"))
+	if err != nil {
+		return nil, fmt.Errorf("DEBUG %q: %w",
+			viper.GetString("DEBUG"), errNotBool)
+	}
+
 	s := &Config{
 		BindAddress:        viper.GetString("BIND_ADDRESS"),
 		CORSAllowedOrigins: splitList(viper.GetString("CORS_ALLOWED_ORIGINS")),
 		DataDir:            viper.GetString("DATA_DIR"),
 		DataDirMaxBytes:    viper.GetInt64("DATA_DIR_MAX_BYTES"),
-		Debug:              viper.GetBool("DEBUG"),
+		Debug:              debug,
 		MetricsPassword:    viper.GetString("METRICS_PASSWORD"),
 		MetricsUsername:    viper.GetString("METRICS_USERNAME"),
-		Port:               viper.GetInt("PORT"),
+		Port:               port,
 		ReportsPerMinute:   viper.GetInt("REPORTS_PER_MINUTE"),
 		SentryDSN:          viper.GetString("SENTRY_DSN"),
 		TrustedProxies:     splitList(viper.GetString("TRUSTED_PROXIES")),
@@ -119,19 +140,7 @@ func New(
 		params:             &params,
 	}
 
-	// viper reads a value that is not a number as 0, so this also
-	// catches a mistyped setting.
-	if s.ReportsPerMinute <= 0 {
-		return nil, fmt.Errorf("REPORTS_PER_MINUTE %q: %w",
-			viper.GetString("REPORTS_PER_MINUTE"), errNotPositive)
-	}
-
-	if s.DataDirMaxBytes <= 0 {
-		return nil, fmt.Errorf("DATA_DIR_MAX_BYTES %q: %w",
-			viper.GetString("DATA_DIR_MAX_BYTES"), errNotPositive)
-	}
-
-	err = checkOrigins(s.CORSAllowedOrigins)
+	err = s.check()
 	if err != nil {
 		return nil, err
 	}
@@ -144,6 +153,32 @@ func New(
 	return s, nil
 }
 
+// check fails with an error naming the first setting here whose value
+// the server cannot use. New checks PORT and DEBUG as it reads them,
+// and the middleware checks TRUSTED_PROXIES as it parses it.
+func (s *Config) check() error {
+	// viper reads a value that is not a number as 0, so this also
+	// catches a mistyped setting.
+	if s.ReportsPerMinute <= 0 {
+		return fmt.Errorf("REPORTS_PER_MINUTE %q: %w",
+			viper.GetString("REPORTS_PER_MINUTE"), errNotPositive)
+	}
+
+	if s.DataDirMaxBytes <= 0 {
+		return fmt.Errorf("DATA_DIR_MAX_BYTES %q: %w",
+			viper.GetString("DATA_DIR_MAX_BYTES"), errNotPositive)
+	}
+
+	if s.BindAddress != "" {
+		_, err := netip.ParseAddr(s.BindAddress)
+		if err != nil {
+			return fmt.Errorf("BIND_ADDRESS %q: %w", s.BindAddress, errNotIP)
+		}
+	}
+
+	return checkOrigins(s.CORSAllowedOrigins)
+}
+
 // checkOrigins fails on the first CORS_ALLOWED_ORIGINS entry that is
 // not a plain origin, scheme://host with an optional port, as browsers
 // send it; anything more, such as a trailing "/", would match no page.
diff --git a/backend/internal/config/config_test.go b/backend/internal/config/config_test.go
index 935a9d7..a75e31e 100644
--- a/backend/internal/config/config_test.go
+++ b/backend/internal/config/config_test.go
@@ -29,6 +29,63 @@ func requireConfigError(t *testing.T, setting string) {
 	}
 }
 
+// TestSettingsLoadAsGiven: valid values pass the checks and are used
+// as given. bin/entrypoint.sh starts the server with these
+// BIND_ADDRESS and PORT values.
+func TestSettingsLoadAsGiven(t *testing.T) {
+	t.Setenv("BIND_ADDRESS", "127.0.0.1")
+	t.Setenv("PORT", "8081")
+	t.Setenv("DEBUG", "true")
+
+	var cfg *config.Config
+
+	app := fx.New(
+		fx.NopLogger,
+		fx.Provide(globals.New, logger.New, config.New),
+		fx.Populate(&cfg),
+	)
+
+	err := app.Err()
+	if err != nil {
+		t.Fatalf("config error = %v", err)
+	}
+
+	if cfg.BindAddress != "127.0.0.1" || cfg.Port != 8081 || !cfg.Debug {
+		t.Fatalf("BindAddress, Port, Debug = %q, %d, %t; "+
+			"want \"127.0.0.1\", 8081, true",
+			cfg.BindAddress, cfg.Port, cfg.Debug)
+	}
+}
+
+// TestPortMustBeAPortNumber: viper reads a value that is not a number
+// as 0, on which the server would listen on a random port.
+func TestPortMustBeAPortNumber(t *testing.T) {
+	for _, value := range []string{"abc", "0", "65536", "8080.5"} {
+		t.Run(value, func(t *testing.T) {
+			t.Setenv("PORT", value)
+
+			requireConfigError(t, "PORT")
+		})
+	}
+}
+
+// TestDebugMustBeTrueOrFalse: viper reads any other value, such as
+// "yes", as false.
+func TestDebugMustBeTrueOrFalse(t *testing.T) {
+	t.Setenv("DEBUG", "yes")
+
+	requireConfigError(t, "DEBUG")
+}
+
+// TestBindAddressMustBeAnIPAddress: a host name would be looked up
+// only once the server starts listening, and a mistyped one would stop
+// it then with an error that does not name the setting.
+func TestBindAddressMustBeAnIPAddress(t *testing.T) {
+	t.Setenv("BIND_ADDRESS", "localhost")
+
+	requireConfigError(t, "BIND_ADDRESS")
+}
+
 // TestReportsPerMinuteMustBePositive: unchecked, zero would panic
 // when the routes are built, and a negative rate would lift the
 // limit.
diff --git a/backend/internal/middleware/middleware.go b/backend/internal/middleware/middleware.go
index 2e685fc..d59ecdf 100644
--- a/backend/internal/middleware/middleware.go
+++ b/backend/internal/middleware/middleware.go
@@ -77,8 +77,8 @@ func New(
 	return s, nil
 }
 
-// parseTrustedProxies converts CIDR strings into prefixes,
-// failing fast on any malformed entry.
+// parseTrustedProxies converts the TRUSTED_PROXIES entries into
+// prefixes, failing fast on any malformed entry.
 func parseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
 	prefixes := make([]netip.Prefix, 0, len(cidrs))
 
@@ -86,7 +86,7 @@ func parseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
 		prefix, err := netip.ParsePrefix(cidr)
 		if err != nil {
 			return nil, fmt.Errorf(
-				"trusted proxy %q: %w", cidr, err,
+				"TRUSTED_PROXIES %q: %w", cidr, err,
 			)
 		}
 
diff --git a/backend/internal/middleware/middleware_test.go b/backend/internal/middleware/middleware_test.go
index 6ae6ded..559c308 100644
--- a/backend/internal/middleware/middleware_test.go
+++ b/backend/internal/middleware/middleware_test.go
@@ -40,8 +40,8 @@ func TestParseTrustedProxiesRejectsMalformed(t *testing.T) {
 	t.Parallel()
 
 	_, err := middleware.ParseTrustedProxies([]string{"not-a-cidr"})
-	if err == nil {
-		t.Fatal("expected error for malformed CIDR, got nil")
+	if err == nil || !strings.Contains(err.Error(), "TRUSTED_PROXIES") {
+		t.Fatalf("error = %v, want one naming TRUSTED_PROXIES", err)
 	}
 }
 
diff --git a/bin/entrypoint.sh b/bin/entrypoint.sh
index b100e75..e0d4fb3 100755
--- a/bin/entrypoint.sh
+++ b/bin/entrypoint.sh
@@ -10,8 +10,9 @@ set -u
 
 # PORT is the public port nginx listens on, 8080 when unset or empty.
 # nginx would take a value such as localhost or unix:/tmp/x.sock as an
-# address and start anyway, so anything but digits stops the container
-# here, before either process starts.
+# address and start anyway, and reports a bad port without naming
+# PORT, so a value that is not a usable port stops the container here,
+# before either process starts.
 export PORT="${PORT:-8080}"
 case "$PORT" in
     *[!0-9]*)
@@ -19,6 +20,17 @@ case "$PORT" in
         exit 1
         ;;
 esac
+# The length is checked first because, for a number too big for it,
+# the shell's test prints an error and is false, so the range checks
+# alone would let it through.
+if [ "${#PORT}" -gt 5 ] || [ "$PORT" -lt 1 ] || [ "$PORT" -gt 65535 ]; then
+    echo "entrypoint: PORT must be from 1 to 65535, not '$PORT'" >&2
+    exit 1
+fi
+if [ "$PORT" -eq 8081 ]; then
+    echo "entrypoint: PORT cannot be 8081, netwatch-server listens there" >&2
+    exit 1
+fi
 
 # A stop signal is only noted here; the loop below acts on it.
 stop_requested=""
-- 
2.54.0


From 6022cc8b02f2d9e5fd7672ead37a7bab9b1d7a65 Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Tue, 29 Sep 2026 08:05:26 +0200
Subject: [PATCH 16/22] fix(backend): give each report file a name of its own
 (closes #61)

Report files were named by a millisecond timestamp and created with
O_EXCL, so two flushes in the same millisecond, such as a flush for
size and the final flush at shutdown, got the same name and the second
failed, losing its reports. Each name now carries a number after the
timestamp that goes up by one for each file the server starts to
write, so names still sort by time and never repeat within a run. A
failed write uses up its number, leaving a gap if the file could not
be created and otherwise a file under that number that may be
incomplete.

Model: opus-5-5
---
 TODO.md                                      |  7 ++
 backend/README.md                            | 11 ++-
 backend/internal/reportbuf/export_test.go    |  8 ++
 backend/internal/reportbuf/reportbuf.go      | 20 ++++-
 backend/internal/reportbuf/reportbuf_test.go | 85 ++++++++++++++++++--
 5 files changed, 116 insertions(+), 15 deletions(-)

diff --git a/TODO.md b/TODO.md
index caa9dce..da6fdcd 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,13 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: report file names can no longer collide (issue #61): each is
+  `reports-<timestamp>-<number>.jsonl.zst`, where the number goes up by one for
+  each file the server starts to write, so two flushes in the same millisecond,
+  such as a flush for size and the final flush at shutdown, each get a file of
+  their own instead of the second one failing. A failed write uses up its
+  number, leaving a gap if the file could not be created and otherwise a file
+  under that number that may be incomplete.
 - 2026-09-29: ready to run under upaas (issue #59): the image has a
   `HEALTHCHECK` that requests `/.well-known/healthcheck` through nginx on the
   port from `PORT`. The backend no longer reads a bad `PORT` as 0 or a bad
diff --git a/backend/README.md b/backend/README.md
index 435647f..1cb73ca 100644
--- a/backend/README.md
+++ b/backend/README.md
@@ -106,9 +106,14 @@ which `netwatch` owns.
 
 ### Report storage
 
-Reports are written as `reports-<timestamp>.jsonl.zst` files in `DATA_DIR`.
-Each file contains one JSON object per line, compressed with zstd. Files are
-created with `O_EXCL` to prevent overwrites.
+Reports are written as `reports-<timestamp>-<number>.jsonl.zst` files in
+`DATA_DIR`. The timestamp is in UTC to the millisecond, so the names sort by
+time. The number starts at 1 when the server starts and goes up by one for each
+file the server starts to write, so two files written in the same millisecond
+still get different names. A failed write uses up its number, leaving a gap in
+the numbers if the file could not be created and otherwise a file under that
+number that may be incomplete. Each file contains one JSON object per line,
+compressed with zstd. Files are created with `O_EXCL` to prevent overwrites.
 
 ### Report limits
 
diff --git a/backend/internal/reportbuf/export_test.go b/backend/internal/reportbuf/export_test.go
index 6440af9..efe8133 100644
--- a/backend/internal/reportbuf/export_test.go
+++ b/backend/internal/reportbuf/export_test.go
@@ -1,7 +1,15 @@
 package reportbuf
 
+import "time"
+
 // Flush writes the buffered reports to a file now, as the periodic
 // flush does, so tests need not wait a minute for it.
 func (b *Buffer) Flush() error {
 	return b.flushLocked()
 }
+
+// StopClock makes every report file the buffer writes from now on
+// carry the timestamp at, as if all were written in one millisecond.
+func (b *Buffer) StopClock(at time.Time) {
+	b.now = func() time.Time { return at }
+}
diff --git a/backend/internal/reportbuf/reportbuf.go b/backend/internal/reportbuf/reportbuf.go
index 318b77f..eeb0184 100644
--- a/backend/internal/reportbuf/reportbuf.go
+++ b/backend/internal/reportbuf/reportbuf.go
@@ -14,6 +14,7 @@ import (
 	"path/filepath"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"sneak.berlin/go/netwatch/internal/config"
@@ -30,7 +31,8 @@ const (
 	dirPerms           fs.FileMode = 0o750
 	filePerms          fs.FileMode = 0o640
 
-	// Report files are named filePrefix + timestamp + fileSuffix.
+	// Report files are named filePrefix + timestamp + "-" + number +
+	// fileSuffix; see writeFile.
 	filePrefix = "reports-"
 	fileSuffix = ".jsonl.zst"
 )
@@ -56,6 +58,12 @@ type Buffer struct {
 	log      *slog.Logger
 	maxBytes int64
 	mu       sync.Mutex
+	// now is the clock report files are named by: time.Now, except
+	// in tests that need two flushes to share a timestamp.
+	now func() time.Time
+	// seq numbers the report files, so that two named in the same
+	// millisecond still get different names.
+	seq      atomic.Uint64
 	stopOnce sync.Once
 	// usedBytes is what Append checks against maxBytes: the size
 	// of the report files in dataDir, plus the reports not yet
@@ -79,6 +87,7 @@ func New(
 		done:     make(chan struct{}),
 		log:      params.Logger.Get(),
 		maxBytes: params.Config.DataDirMaxBytes,
+		now:      time.Now,
 	}
 
 	lc.Append(fx.Hook{
@@ -211,11 +220,14 @@ func (b *Buffer) drainBuf() []byte {
 // writeFile creates a timestamped zstd-compressed JSONL file
 // in the data directory.
 func (b *Buffer) writeFile(data []byte) error {
-	ts := time.Now().UTC().Format("2006-01-02T15-04-05.000Z")
-	path := filepath.Join(b.dataDir, filePrefix+ts+fileSuffix)
+	// The timestamp comes first, so the names sort by time; the number
+	// after it tells apart files named in the same millisecond.
+	ts := b.now().UTC().Format("2006-01-02T15-04-05.000Z")
+	name := fmt.Sprintf("%s%s-%d%s", filePrefix, ts, b.seq.Add(1), fileSuffix)
+	path := filepath.Join(b.dataDir, name)
 
 	// path is built from the operator-supplied dataDir plus a
-	// generated timestamp, so it carries no external input.
+	// generated timestamp and number, so it carries no external input.
 	f, err := os.OpenFile( //nolint:gosec // see comment above
 		path,
 		os.O_WRONLY|os.O_CREATE|os.O_EXCL,
diff --git a/backend/internal/reportbuf/reportbuf_test.go b/backend/internal/reportbuf/reportbuf_test.go
index 90d3a94..0b56b7e 100644
--- a/backend/internal/reportbuf/reportbuf_test.go
+++ b/backend/internal/reportbuf/reportbuf_test.go
@@ -3,9 +3,11 @@ package reportbuf_test
 import (
 	"encoding/json"
 	"errors"
+	"fmt"
 	"io/fs"
 	"os"
 	"path/filepath"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -18,6 +20,7 @@ import (
 	"sneak.berlin/go/netwatch/internal/logger"
 	"sneak.berlin/go/netwatch/internal/reportbuf"
 
+	"github.com/klauspost/compress/zstd"
 	"go.uber.org/fx"
 	"go.uber.org/fx/fxtest"
 )
@@ -210,10 +213,6 @@ func TestWrittenReportsCountAtFileSize(t *testing.T) {
 		t.Fatalf("flush: %v", err)
 	}
 
-	// The second report is written at shutdown, and must not land in
-	// the first file's millisecond (see TestWrittenReportsKeepCounting).
-	time.Sleep(time.Millisecond)
-
 	err = buf.Append(report)
 	if err != nil {
 		t.Fatalf("second report, after the first was written: %v", err)
@@ -254,10 +253,6 @@ func TestWrittenReportsKeepCounting(t *testing.T) {
 			t.Fatalf("with %d bytes of report files: %v", used, err)
 		}
 
-		// Report files are named to the millisecond; two in the same
-		// one collide (https://git.eeqj.de/sneak/netwatch/issues/61).
-		time.Sleep(time.Millisecond)
-
 		err = buf.Flush()
 		if err != nil {
 			t.Fatalf("flush: %v", err)
@@ -315,6 +310,43 @@ func TestConcurrentAppendsStopAtCap(t *testing.T) {
 	}
 }
 
+// TestTwoFlushesInOneMillisecond flushes twice within one millisecond,
+// as a flush for size and the final flush at shutdown can: each flush
+// must write a file of its own, and the files must hold every report.
+func TestTwoFlushesInOneMillisecond(t *testing.T) {
+	const flushes = 2
+
+	dir := t.TempDir()
+	t.Setenv("DATA_DIR", dir)
+
+	buf := startBuffer(t)
+	buf.StopClock(time.Date(2026, 1, 1, 0, 0, 0, 0, time.UTC))
+
+	for id := 1; id <= flushes; id++ {
+		err := buf.Append(map[string]int{"id": id})
+		if err != nil {
+			t.Fatalf("append report %d: %v", id, err)
+		}
+
+		err = buf.Flush()
+		if err != nil {
+			t.Fatalf("flush %d: %v", id, err)
+		}
+	}
+
+	files := readReportFiles(t, dir)
+	if len(files) != flushes {
+		t.Fatalf("%d report files after %d flushes", len(files), flushes)
+	}
+
+	for id := 1; id <= flushes; id++ {
+		want := fmt.Sprintf(`{"id":%d}`+"\n", id)
+		if !slices.Contains(files, want) {
+			t.Fatalf("no report file holds report %d alone", id)
+		}
+	}
+}
+
 // reportFilesBytes returns the total size of the report files in dir.
 func reportFilesBytes(t *testing.T, dir string) int64 {
 	t.Helper()
@@ -338,6 +370,43 @@ func reportFilesBytes(t *testing.T, dir string) int64 {
 	return total
 }
 
+// readReportFiles returns the decompressed contents of each report
+// file in dir.
+func readReportFiles(t *testing.T, dir string) []string {
+	t.Helper()
+
+	files := os.DirFS(dir)
+
+	names, err := fs.Glob(files, "reports-*.jsonl.zst")
+	if err != nil {
+		t.Fatalf("list report files: %v", err)
+	}
+
+	dec, err := zstd.NewReader(nil)
+	if err != nil {
+		t.Fatalf("create zstd decoder: %v", err)
+	}
+	defer dec.Close()
+
+	contents := make([]string, 0, len(names))
+
+	for _, name := range names {
+		compressed, readErr := fs.ReadFile(files, name)
+		if readErr != nil {
+			t.Fatalf("read %s: %v", name, readErr)
+		}
+
+		data, decErr := dec.DecodeAll(compressed, nil)
+		if decErr != nil {
+			t.Fatalf("decompress %s: %v", name, decErr)
+		}
+
+		contents = append(contents, string(data))
+	}
+
+	return contents
+}
+
 func writeBytes(t *testing.T, path string, n int) {
 	t.Helper()
 
-- 
2.54.0


From 8833603eff5495f0775293e4d753f09f352be887 Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Tue, 29 Sep 2026 08:55:47 +0200
Subject: [PATCH 17/22] nginx: trust X-Forwarded-For only from TRUSTED_PROXIES
 (closes #64)

nginx trusted X-Forwarded-For from every RFC1918 address, so a client
reaching it from one could write a new address on each request and
get a fresh rate-limit allowance. The container's TRUSTED_PROXIES now
names the reverse proxies nginx trusts, none by default.
bin/entrypoint.sh makes each entry a CIDR, checks it with the new
"netwatch-server check-cidr", which runs the server's own
TRUSTED_PROXIES parsing, and writes one set_real_ip_from line per
entry into /etc/nginx/trusted-proxies.conf, which nginx.conf includes.
The backend is started with TRUSTED_PROXIES=127.0.0.1/32, since nginx
is its only client. The viewport test mounts an empty file there.

Model: opus-5-5
---
 README.md                                     | 18 ++++++---
 TODO.md                                       |  9 +++++
 backend/README.md                             | 22 ++++++++---
 backend/cmd/netwatch-server/main.go           | 16 ++++++++
 backend/internal/config/config.go             |  9 +++--
 backend/internal/middleware/export_test.go    |  4 --
 backend/internal/middleware/middleware.go     | 10 +++--
 .../internal/middleware/middleware_test.go    | 23 +++++++++--
 bin/entrypoint.sh                             | 39 +++++++++++++++++--
 nginx.conf                                    | 10 +++--
 script/frontend-viewport-test                 |  3 ++
 11 files changed, 129 insertions(+), 34 deletions(-)

diff --git a/README.md b/README.md
index 89e5c5c..2f46400 100644
--- a/README.md
+++ b/README.md
@@ -184,8 +184,8 @@ container: nginx serves the built frontend and passes `/api/` and
 only inside the container, on `127.0.0.1:8081`. The image:
 
 - Listens on port 8080 by default (override with `PORT` env var)
-- Trusts `X-Forwarded-For` from RFC1918 reverse proxies (10/8, 172.16/12,
-  192.168/16)
+- Takes the client address from `X-Forwarded-For` only on requests from the
+  reverse proxies named in `TRUSTED_PROXIES`, and by default from none
 - Sends access logs to stdout
 - Caches static assets with immutable headers
 - Stores reports in `DATA_DIR`, `/data/reports` by default, on the `/data`
@@ -224,10 +224,16 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for netwatch needs:
     - `DEBUG`, default `false`: debug logging
     - `DATA_DIR`, default `/data/reports`: leave unset; reports kept outside
       `/data` do not survive a redeploy
-    - `TRUSTED_PROXIES`, default loopback and RFC1918: leave unset. The
-      backend's only client is nginx, on loopback, which passes on the client
-      address; nginx takes it from `X-Forwarded-For` only from RFC1918
-      addresses.
+    - `TRUSTED_PROXIES`, default empty: set it to the address the reverse proxy
+      in front of the container connects from, as an IP address or CIDR; several
+      are separated by commas. nginx takes the client address from
+      `X-Forwarded-For` only on a request from one of them, and the rate limit
+      counts that address. Unset, `X-Forwarded-For` is ignored and every client
+      behind the proxy shares the proxy's one allowance of `REPORTS_PER_MINUTE`.
+      Name only addresses nothing but the proxy connects from: any client that
+      connects from one can write its own `X-Forwarded-For`, and through a port
+      Docker publishes, every client may connect from the Docker network's
+      gateway, such as `172.17.0.1`.
 - **Health check:** the image's `HEALTHCHECK` requests
   `/.well-known/healthcheck` through nginx every 30 seconds, so it fails unless
   both nginx and the backend answer. upaas reads the container's health 60
diff --git a/TODO.md b/TODO.md
index da6fdcd..552f191 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,15 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: nginx takes the client address from `X-Forwarded-For` only on
+  requests from the reverse proxies named in the container's `TRUSTED_PROXIES`
+  (issue #64), and by default from none, where it trusted every RFC1918 address
+  before, so a client could write a new address on each request and escape the
+  rate limit. `bin/entrypoint.sh` writes one `set_real_ip_from` line per entry
+  into `/etc/nginx/trusted-proxies.conf`, which `nginx.conf` includes, refusing
+  an entry that is not an IP address or CIDR, as `netwatch-server check-cidr`
+  finds; it starts the backend with `TRUSTED_PROXIES=127.0.0.1/32`, since nginx
+  is its only client
 - 2026-09-29: report file names can no longer collide (issue #61): each is
   `reports-<timestamp>-<number>.jsonl.zst`, where the number goes up by one for
   each file the server starts to write, so two flushes in the same millisecond,
diff --git a/backend/README.md b/backend/README.md
index 1cb73ca..5f80ef2 100644
--- a/backend/README.md
+++ b/backend/README.md
@@ -87,9 +87,10 @@ Internal packages in `internal/` follow standard Go project layout:
 | `CORS_ALLOWED_ORIGINS` | empty                | Comma-separated origins whose pages may call the API; see [CORS](#cors)                                  |
 
 `TRUSTED_PROXIES` defaults to `127.0.0.1/32,::1/128,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`.
-The loopback entries cover the reverse proxy that shares the container; the
-RFC1918 ranges match `nginx.conf`. A request whose direct peer is outside this
-set has its forwarded headers ignored, and the direct peer is logged instead.
+The loopback entries cover a reverse proxy on the same host. A request whose
+direct peer is outside this set has its forwarded headers ignored, and the
+direct peer is logged and rate-limited instead. The container image does not use
+this default; see [Container image](#container-image).
 
 A variable set to a value the server cannot use, such as `PORT=abc`,
 `DEBUG=maybe` or a `BIND_ADDRESS` that is not an IP address, stops it from
@@ -101,8 +102,19 @@ The root `Dockerfile` builds one image in which nginx listens on the public port
 8080, serves the frontend, and proxies `/api/` and `/.well-known/healthcheck` to
 this server. The image's entrypoint, `bin/entrypoint.sh`, starts the server as
 user `netwatch` (uid 1000) with `BIND_ADDRESS=127.0.0.1` and `PORT=8081`, so
-only nginx reaches it. `DATA_DIR` is `/data/reports`, on the `/data` volume,
-which `netwatch` owns.
+only nginx reaches it, and with `TRUSTED_PROXIES=127.0.0.1/32`, so it takes the
+client address nginx passes on and no other. `DATA_DIR` is `/data/reports`, on
+the `/data` volume, which `netwatch` owns.
+
+The container's own `TRUSTED_PROXIES` goes to nginx instead: IP addresses or
+CIDRs, separated by commas, of the reverse proxies in front of the container.
+nginx takes the client address from `X-Forwarded-For` only on a request from one
+of them. Unset or empty, nginx trusts no proxy, and the client address is the
+one each request comes from, so every client behind a proxy shares one rate
+limit. An entry that is not an IP address or CIDR, such as a hostname or
+`1.2.3`, stops the container at start with an error naming `TRUSTED_PROXIES`:
+the entrypoint checks each entry with `netwatch-server check-cidr`, which parses
+it as this server parses its own `TRUSTED_PROXIES`.
 
 ### Report storage
 
diff --git a/backend/cmd/netwatch-server/main.go b/backend/cmd/netwatch-server/main.go
index 3f3d86b..01c77a4 100644
--- a/backend/cmd/netwatch-server/main.go
+++ b/backend/cmd/netwatch-server/main.go
@@ -2,6 +2,9 @@
 package main
 
 import (
+	"fmt"
+	"os"
+
 	"sneak.berlin/go/netwatch/internal/config"
 	"sneak.berlin/go/netwatch/internal/globals"
 	"sneak.berlin/go/netwatch/internal/handlers"
@@ -22,6 +25,19 @@ var (
 )
 
 func main() {
+	// "netwatch-server check-cidr CIDR" exits 1, with the error, if
+	// this server would refuse CIDR in its TRUSTED_PROXIES.
+	// bin/entrypoint.sh runs it on each entry it gives nginx.
+	if len(os.Args) == 3 && os.Args[1] == "check-cidr" {
+		_, err := middleware.ParseTrustedProxies(os.Args[2:])
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+
+		return
+	}
+
 	globals.Appname = Appname
 	globals.Version = Version
 	globals.Buildarch = Buildarch
diff --git a/backend/internal/config/config.go b/backend/internal/config/config.go
index c8236d0..a258978 100644
--- a/backend/internal/config/config.go
+++ b/backend/internal/config/config.go
@@ -21,10 +21,11 @@ import (
 )
 
 // defaultTrustedProxies lists the networks whose forwarded
-// headers are honoured by default. It covers the RFC1918
-// ranges (to match nginx.conf) plus IPv4 and IPv6 loopback,
-// because the reverse proxy shares the container and reaches
-// the backend over loopback.
+// headers are honoured by default: IPv4 and IPv6 loopback,
+// for a reverse proxy on the same host, and the RFC1918
+// ranges. The container image does not use it:
+// bin/entrypoint.sh gives the server 127.0.0.1/32, since
+// nginx is its only client there.
 const defaultTrustedProxies = "127.0.0.1/32,::1/128," +
 	"10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"
 
diff --git a/backend/internal/middleware/export_test.go b/backend/internal/middleware/export_test.go
index 22e1e7e..ed47271 100644
--- a/backend/internal/middleware/export_test.go
+++ b/backend/internal/middleware/export_test.go
@@ -29,7 +29,3 @@ func ClientIP(
 ) string {
 	return clientIP(remoteAddr, header, trusted)
 }
-
-func ParseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
-	return parseTrustedProxies(cidrs)
-}
diff --git a/backend/internal/middleware/middleware.go b/backend/internal/middleware/middleware.go
index d59ecdf..a962c85 100644
--- a/backend/internal/middleware/middleware.go
+++ b/backend/internal/middleware/middleware.go
@@ -64,7 +64,7 @@ func New(
 	_ fx.Lifecycle,
 	params Params,
 ) (*Middleware, error) {
-	trusted, err := parseTrustedProxies(params.Config.TrustedProxies)
+	trusted, err := ParseTrustedProxies(params.Config.TrustedProxies)
 	if err != nil {
 		return nil, err
 	}
@@ -77,9 +77,11 @@ func New(
 	return s, nil
 }
 
-// parseTrustedProxies converts the TRUSTED_PROXIES entries into
-// prefixes, failing fast on any malformed entry.
-func parseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
+// ParseTrustedProxies converts the TRUSTED_PROXIES entries into
+// prefixes, failing fast on any malformed entry. Each entry must be
+// a CIDR; a lone address is refused. "netwatch-server check-cidr"
+// runs it too.
+func ParseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
 	prefixes := make([]netip.Prefix, 0, len(cidrs))
 
 	for _, cidr := range cidrs {
diff --git a/backend/internal/middleware/middleware_test.go b/backend/internal/middleware/middleware_test.go
index 559c308..30e7cbe 100644
--- a/backend/internal/middleware/middleware_test.go
+++ b/backend/internal/middleware/middleware_test.go
@@ -36,15 +36,32 @@ func mustPrefixes(t *testing.T, cidrs ...string) []netip.Prefix {
 	return prefixes
 }
 
+// TestParseTrustedProxiesRejectsMalformed includes entries nginx would
+// read as another address or look up as a hostname, in the CIDR form
+// bin/entrypoint.sh gives "netwatch-server check-cidr".
 func TestParseTrustedProxiesRejectsMalformed(t *testing.T) {
 	t.Parallel()
 
-	_, err := middleware.ParseTrustedProxies([]string{"not-a-cidr"})
-	if err == nil || !strings.Contains(err.Error(), "TRUSTED_PROXIES") {
-		t.Fatalf("error = %v, want one naming TRUSTED_PROXIES", err)
+	for _, cidr := range []string{
+		"not-a-cidr", "10.0.0.1", "1.2.3/32", "172.30/32", "10/32",
+		"cafe/32", "999.1.1.1/32", "10.0.0.0/33", "::1/129",
+		"fe80::1%eth0/128",
+	} {
+		_, err := middleware.ParseTrustedProxies([]string{cidr})
+		if err == nil || !strings.Contains(err.Error(), "TRUSTED_PROXIES") {
+			t.Errorf("%q: error = %v, want one naming TRUSTED_PROXIES",
+				cidr, err)
+		}
 	}
 }
 
+func TestParseTrustedProxiesAcceptsCIDRs(t *testing.T) {
+	t.Parallel()
+
+	mustPrefixes(t, "172.17.0.1/32", "10.0.0.0/8", "2001:db8::1/128",
+		"2001:db8::/32", "::ffff:192.0.2.1/128")
+}
+
 type clientIPCase struct {
 	name       string
 	remoteAddr string
diff --git a/bin/entrypoint.sh b/bin/entrypoint.sh
index e0d4fb3..4458f9e 100755
--- a/bin/entrypoint.sh
+++ b/bin/entrypoint.sh
@@ -32,16 +32,47 @@ if [ "$PORT" -eq 8081 ]; then
     exit 1
 fi
 
+# TRUSTED_PROXIES names the reverse proxies in front of the container,
+# as IP addresses or CIDRs separated by commas. nginx takes the client
+# address from X-Forwarded-For only on a request from one of them, so
+# unset or empty, it trusts no one. nginx.conf includes the file written
+# here, one set_real_ip_from line per entry.
+#
+# nginx looks up an entry it cannot read as an address as a hostname,
+# and trusts what it finds (1.2.3 is found as 1.2.0.3). So each entry
+# is made a CIDR, a lone address getting /128 if it is IPv6 and /32 if
+# not, and netwatch-server checks it with the parsing it gives its own
+# TRUSTED_PROXIES. Its error, naming the CIDR, is dropped for the one
+# below, naming the entry as written. set -f keeps a * in an entry from
+# becoming a list of file names.
+TRUSTED_PROXIES="${TRUSTED_PROXIES:-}"
+set -f
+for proxy in $(printf '%s' "$TRUSTED_PROXIES" | tr ',' ' '); do
+    case "$proxy" in
+        */*) cidr="$proxy" ;;
+        *:*) cidr="$proxy/128" ;;
+        *) cidr="$proxy/32" ;;
+    esac
+    if ! netwatch-server check-cidr "$cidr" 2> /dev/null; then
+        echo "entrypoint: TRUSTED_PROXIES must be IP addresses or CIDRs" \
+            "separated by commas; '$proxy' is neither" >&2
+        exit 1
+    fi
+    echo "set_real_ip_from $cidr;"
+done > /etc/nginx/trusted-proxies.conf
+
 # A stop signal is only noted here; the loop below acts on it.
 stop_requested=""
 trap 'stop_requested=yes' TERM INT
 
 # netwatch-server runs as the netwatch user and listens on loopback
 # only, on a port other than the public one; nginx.conf proxies to this
-# address. The netwatch user has no login shell, hence -s /bin/sh.
-# busybox su replaces itself with the command instead of staying on as
-# its parent, so $! is the server's own PID.
-BIND_ADDRESS=127.0.0.1 PORT=8081 \
+# address. Its only client is nginx, so it takes the client address
+# nginx passes on from 127.0.0.1 alone, whatever TRUSTED_PROXIES the
+# container has. The netwatch user has no login shell, hence -s
+# /bin/sh. busybox su replaces itself with the command instead of
+# staying on as its parent, so $! is the server's own PID.
+BIND_ADDRESS=127.0.0.1 PORT=8081 TRUSTED_PROXIES=127.0.0.1/32 \
     su -s /bin/sh netwatch -c 'exec netwatch-server' &
 backend=$!
 
diff --git a/nginx.conf b/nginx.conf
index 697340b..0322c27 100644
--- a/nginx.conf
+++ b/nginx.conf
@@ -11,10 +11,12 @@ server {
     root /usr/share/nginx/html;
     index index.html;
 
-    # Trust RFC1918 reverse proxies for X-Forwarded-For
-    set_real_ip_from 10.0.0.0/8;
-    set_real_ip_from 172.16.0.0/12;
-    set_real_ip_from 192.168.0.0/16;
+    # The client address comes from X-Forwarded-For only on a request
+    # from the reverse proxies in TRUSTED_PROXIES: bin/entrypoint.sh
+    # writes one set_real_ip_from line for each into this file, and
+    # leaves it empty when TRUSTED_PROXIES is unset, so that by default
+    # the client address is the one each request comes from.
+    include /etc/nginx/trusted-proxies.conf;
     real_ip_header X-Forwarded-For;
     real_ip_recursive on;
 
diff --git a/script/frontend-viewport-test b/script/frontend-viewport-test
index 5a97c99..e4b53d1 100755
--- a/script/frontend-viewport-test
+++ b/script/frontend-viewport-test
@@ -63,11 +63,14 @@ main() {
 
     # nginx.conf is a template: the image renders it over its own
     # default.conf, with the same port and limit bin/entrypoint.sh uses.
+    # The empty file it includes trusts no proxy, as bin/entrypoint.sh
+    # writes it when TRUSTED_PROXIES is unset.
     docker run -d --rm --name "$SERVER" \
         --network "$NETWORK" --network-alias netwatch \
         -e PORT=8080 -e NGINX_ENVSUBST_FILTER='^PORT$' \
         -v "$ROOT/dist:/usr/share/nginx/html:ro" \
         -v "$ROOT/nginx.conf:/etc/nginx/templates/default.conf.template:ro" \
+        -v /dev/null:/etc/nginx/trusted-proxies.conf:ro \
         "$SERVER_IMAGE" > /dev/null
 
     # The image's own entrypoint already exposes CDP on 9222 and passes
-- 
2.54.0


From d74d1e311e92719f225ad8e6dd30b5149ccf5673 Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Tue, 29 Sep 2026 09:39:11 +0200
Subject: [PATCH 18/22] fix(backend): cut request log fields to the log bound
 (closes #60)

The request log wrote the URL, User-Agent, Referer and other
request-supplied strings with no length limit, and the server accepts
headers up to 1 MiB, so one request could put about 1 MiB per field
into a log line. Every string the request log takes from the request,
including the request ID chi copies from X-Request-Id, is now cut to
the 128-byte bound the report handler already used. That bound and
its helper moved from the handlers package to the logger package so
both use the one copy.

Model: opus-5-5
---
 TODO.md                                       |  6 +++
 backend/internal/handlers/export_test.go      |  3 --
 backend/internal/handlers/report.go           | 22 ++-------
 backend/internal/handlers/report_test.go      | 11 +++--
 backend/internal/logger/logger.go             | 15 ++++++
 backend/internal/middleware/middleware.go     | 23 ++++-----
 .../internal/middleware/middleware_test.go    | 49 +++++++++++++++++++
 7 files changed, 92 insertions(+), 37 deletions(-)

diff --git a/TODO.md b/TODO.md
index 552f191..623f8cf 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,12 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: the request log is bounded (issue #60): the method, URL, protocol,
+  `User-Agent`, `Referer`, request ID (which chi takes from the client's
+  `X-Request-Id` header) and client address it writes are each cut to 128 bytes,
+  the bound the report handler already used, so one request can no longer put
+  about 1 MiB per field into a log line. That bound and its helper now live in
+  the `logger` package, shared by both
 - 2026-09-29: nginx takes the client address from `X-Forwarded-For` only on
   requests from the reverse proxies named in the container's `TRUSTED_PROXIES`
   (issue #64), and by default from none, where it trusted every RFC1918 address
diff --git a/backend/internal/handlers/export_test.go b/backend/internal/handlers/export_test.go
index 7de02a5..220da99 100644
--- a/backend/internal/handlers/export_test.go
+++ b/backend/internal/handlers/export_test.go
@@ -2,9 +2,6 @@ package handlers
 
 import "log/slog"
 
-// MaxLoggedFieldBytes exposes the log bound to the external tests.
-const MaxLoggedFieldBytes = maxLoggedFieldBytes
-
 // NewForTest builds a Handlers around a report sink and logger,
 // bypassing the fx graph so handler behaviour (including the
 // storage failure path) is exercisable in unit tests.
diff --git a/backend/internal/handlers/report.go b/backend/internal/handlers/report.go
index 859a3a7..e7dd4e4 100644
--- a/backend/internal/handlers/report.go
+++ b/backend/internal/handlers/report.go
@@ -5,14 +5,10 @@ import (
 	"errors"
 	"net/http"
 
+	"sneak.berlin/go/netwatch/internal/logger"
 	"sneak.berlin/go/netwatch/internal/reportbuf"
 )
 
-// maxLoggedFieldBytes bounds untrusted text (string fields,
-// decode error text) before it is logged, so a caller cannot
-// inflate log volume with an oversized value.
-const maxLoggedFieldBytes = 128
-
 type reportSample struct {
 	T       int64   `json:"t"`
 	Latency *int    `json:"latency"`
@@ -83,7 +79,7 @@ func (s *Handlers) decodeErrorStatus(err error) int {
 	// The decoder's error text can quote request bytes (a whole
 	// oversized number, for example), so it is bounded too.
 	s.log.Error("failed to decode report",
-		"error", boundedForLog(err.Error()),
+		"error", logger.BoundedForLog(err.Error()),
 	)
 
 	return http.StatusBadRequest
@@ -115,20 +111,10 @@ func (s *Handlers) logReportReceived(rpt report) {
 	}
 
 	s.log.Info("report received",
-		"client_id", boundedForLog(rpt.ClientID),
-		"timestamp", boundedForLog(rpt.Timestamp),
+		"client_id", logger.BoundedForLog(rpt.ClientID),
+		"timestamp", logger.BoundedForLog(rpt.Timestamp),
 		"host_count", len(rpt.Hosts),
 		"total_samples", totalSamples,
 		"geo_bytes", len(rpt.Geo),
 	)
 }
-
-// boundedForLog truncates an untrusted string to a fixed byte
-// bound so an attacker-controlled field cannot dominate the log.
-func boundedForLog(s string) string {
-	if len(s) > maxLoggedFieldBytes {
-		return s[:maxLoggedFieldBytes]
-	}
-
-	return s
-}
diff --git a/backend/internal/handlers/report_test.go b/backend/internal/handlers/report_test.go
index 996ce5c..cd94b5c 100644
--- a/backend/internal/handlers/report_test.go
+++ b/backend/internal/handlers/report_test.go
@@ -12,6 +12,7 @@ import (
 	"testing"
 
 	"sneak.berlin/go/netwatch/internal/handlers"
+	"sneak.berlin/go/netwatch/internal/logger"
 	"sneak.berlin/go/netwatch/internal/middleware"
 	"sneak.berlin/go/netwatch/internal/reportbuf"
 )
@@ -174,7 +175,7 @@ func TestHandleReportDoesNotLogRawGeo(t *testing.T) {
 func TestHandleReportLogsClientIDCutToBound(t *testing.T) {
 	t.Parallel()
 
-	long := strings.Repeat("c", 2*handlers.MaxLoggedFieldBytes)
+	long := strings.Repeat("c", 2*logger.MaxLoggedFieldBytes)
 
 	var logbuf bytes.Buffer
 
@@ -197,16 +198,16 @@ func TestHandleReportLogsClientIDCutToBound(t *testing.T) {
 		t.Fatalf("log line not JSON: %v (%q)", err, logbuf.String())
 	}
 
-	want := long[:handlers.MaxLoggedFieldBytes]
+	want := long[:logger.MaxLoggedFieldBytes]
 
 	if logged["client_id"] != want {
 		t.Fatalf("logged client_id not cut to %d bytes: %q",
-			handlers.MaxLoggedFieldBytes, logged["client_id"])
+			logger.MaxLoggedFieldBytes, logged["client_id"])
 	}
 
 	if logged["timestamp"] != want {
 		t.Fatalf("logged timestamp not cut to %d bytes: %q",
-			handlers.MaxLoggedFieldBytes, logged["timestamp"])
+			logger.MaxLoggedFieldBytes, logged["timestamp"])
 	}
 }
 
@@ -215,7 +216,7 @@ func TestHandleReportDecodeErrorLogIsBounded(t *testing.T) {
 
 	// A number too large for its int64 field makes the decoder's
 	// error text quote the whole number.
-	huge := strings.Repeat("9", 2*handlers.MaxLoggedFieldBytes)
+	huge := strings.Repeat("9", 2*logger.MaxLoggedFieldBytes)
 
 	var logbuf bytes.Buffer
 
diff --git a/backend/internal/logger/logger.go b/backend/internal/logger/logger.go
index 9f30c1a..de6c884 100644
--- a/backend/internal/logger/logger.go
+++ b/backend/internal/logger/logger.go
@@ -11,6 +11,21 @@ import (
 	"go.uber.org/fx"
 )
 
+// MaxLoggedFieldBytes bounds untrusted text (request fields,
+// header values, decode error text) before it is logged, so a
+// caller cannot inflate log volume with an oversized value.
+const MaxLoggedFieldBytes = 128
+
+// BoundedForLog truncates an untrusted string to a fixed byte
+// bound so an attacker-controlled field cannot dominate the log.
+func BoundedForLog(s string) string {
+	if len(s) > MaxLoggedFieldBytes {
+		return s[:MaxLoggedFieldBytes]
+	}
+
+	return s
+}
+
 // Params defines the dependencies for Logger.
 type Params struct {
 	fx.In
diff --git a/backend/internal/middleware/middleware.go b/backend/internal/middleware/middleware.go
index a962c85..aff7864 100644
--- a/backend/internal/middleware/middleware.go
+++ b/backend/internal/middleware/middleware.go
@@ -189,7 +189,10 @@ func addrInAny(s string, trusted []netip.Prefix) bool {
 }
 
 // Logging returns middleware that logs each request with
-// timing, status code, and client information.
+// timing, status code, and client information. Every string
+// taken from the request is cut to logger.MaxLoggedFieldBytes,
+// including the request ID, which chi takes from the client's
+// X-Request-Id header when one is sent.
 func (s *Middleware) Logging() func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(
@@ -202,21 +205,19 @@ func (s *Middleware) Logging() func(http.Handler) http.Handler {
 					latency := time.Since(start)
 					s.log.InfoContext(ctx, "request",
 						"request_start", start,
-						"method", r.Method,
-						"url", r.URL.String(),
-						"useragent", r.UserAgent(),
+						"method", logger.BoundedForLog(r.Method),
+						"url", logger.BoundedForLog(r.URL.String()),
+						"useragent", logger.BoundedForLog(r.UserAgent()),
 						"request_id",
-						ctx.Value(
-							middleware.RequestIDKey,
-						),
-						"referer", r.Referer(),
-						"proto", r.Proto,
+						logger.BoundedForLog(middleware.GetReqID(ctx)),
+						"referer", logger.BoundedForLog(r.Referer()),
+						"proto", logger.BoundedForLog(r.Proto),
 						"remote_ip",
-						clientIP(
+						logger.BoundedForLog(clientIP(
 							r.RemoteAddr,
 							r.Header,
 							s.trustedProxies,
-						),
+						)),
 						"status", lrw.statusCode,
 						"latency_ms",
 						latency.Milliseconds(),
diff --git a/backend/internal/middleware/middleware_test.go b/backend/internal/middleware/middleware_test.go
index 30e7cbe..c916cf2 100644
--- a/backend/internal/middleware/middleware_test.go
+++ b/backend/internal/middleware/middleware_test.go
@@ -13,7 +13,10 @@ import (
 	"testing/synctest"
 	"time"
 
+	"sneak.berlin/go/netwatch/internal/logger"
 	"sneak.berlin/go/netwatch/internal/middleware"
+
+	chimiddleware "github.com/go-chi/chi/v5/middleware"
 )
 
 const (
@@ -320,6 +323,52 @@ func TestRecovererRepanicsOnAbortHandler(t *testing.T) {
 	}
 }
 
+// TestLoggingCutsRequestStringsToBound sends an over-long URL and
+// over-long header values, and checks the request log writes each
+// one cut to logger.MaxLoggedFieldBytes.
+func TestLoggingCutsRequestStringsToBound(t *testing.T) {
+	t.Parallel()
+
+	long := strings.Repeat("a", 2*logger.MaxLoggedFieldBytes)
+
+	var logbuf bytes.Buffer
+
+	mw := middleware.NewWithLogger(
+		slog.New(slog.NewJSONHandler(&logbuf, nil)),
+	)
+
+	handler := chimiddleware.RequestID(mw.Logging()(okHandler()))
+
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodGet, "/"+long, http.NoBody)
+	req.Header.Set("User-Agent", long)
+	req.Header.Set("Referer", long)
+	req.Header.Set("X-Request-Id", long)
+
+	handler.ServeHTTP(httptest.NewRecorder(), req)
+
+	var logged map[string]any
+
+	err := json.Unmarshal(logbuf.Bytes(), &logged)
+	if err != nil {
+		t.Fatalf("log line not JSON: %v (%q)", err, logbuf.String())
+	}
+
+	want := map[string]string{
+		"url":        ("/" + long)[:logger.MaxLoggedFieldBytes],
+		"useragent":  long[:logger.MaxLoggedFieldBytes],
+		"referer":    long[:logger.MaxLoggedFieldBytes],
+		"request_id": long[:logger.MaxLoggedFieldBytes],
+	}
+
+	for field, value := range want {
+		if logged[field] != value {
+			t.Errorf("logged %s = %q, want it cut to %d bytes",
+				field, logged[field], logger.MaxLoggedFieldBytes)
+		}
+	}
+}
+
 // okHandler stands in for the route a middleware guards.
 func okHandler() http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
-- 
2.54.0


From d81da05748ffb122682964da4e80e083de3ca326 Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Tue, 29 Sep 2026 10:22:12 +0200
Subject: [PATCH 19/22] nginx: security headers on every response (closes #18)

nginx sent none of the security headers REPO_POLICIES.md requires.
security-headers.conf now sets all six with always, included at server
level and again in /assets/, whose own add_header would otherwise drop
them. nginx hides the copies netwatch-server sets, so /api/ and the
health check carry each header once. The content security policy
allows no inline script or style; the host row's status dot took its
grey from a style attribute, now a class. connect-src is * because
several probed hosts redirect to other hosts and the browser checks
every redirect against it. Referrer-Policy is no-referrer, as the
backend already sends.

Model: opus-5-5
---
 Dockerfile                    |  1 +
 README.md                     |  2 ++
 TODO.md                       |  7 +++++++
 backend/README.md             |  4 +++-
 nginx.conf                    | 16 ++++++++++++++++
 script/frontend-viewport-test |  4 +++-
 security-headers.conf         | 24 ++++++++++++++++++++++++
 src/main.js                   |  2 +-
 8 files changed, 57 insertions(+), 3 deletions(-)
 create mode 100644 security-headers.conf

diff --git a/Dockerfile b/Dockerfile
index 87435d2..417fc97 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -72,6 +72,7 @@ RUN addgroup -g 1000 -S netwatch && \
 # conf.d; bin/entrypoint.sh says how.
 RUN rm /etc/nginx/conf.d/default.conf
 COPY nginx.conf /etc/nginx/templates/netwatch.conf.template
+COPY security-headers.conf /etc/nginx/security-headers.conf
 COPY --from=frontend /app/dist /usr/share/nginx/html
 COPY --from=builder /src/netwatch-server /usr/local/bin/netwatch-server
 COPY bin/entrypoint.sh /usr/local/bin/entrypoint.sh
diff --git a/README.md b/README.md
index 2f46400..415457e 100644
--- a/README.md
+++ b/README.md
@@ -188,6 +188,8 @@ only inside the container, on `127.0.0.1:8081`. The image:
   reverse proxies named in `TRUSTED_PROXIES`, and by default from none
 - Sends access logs to stdout
 - Caches static assets with immutable headers
+- Sends the security headers `REPO_POLICIES.md` requires on every response, as
+  `security-headers.conf` sets them, in place of the backend's own
 - Stores reports in `DATA_DIR`, `/data/reports` by default, on the `/data`
   volume. The backend runs as user `netwatch` (uid 1000), so a directory
   bind-mounted at `/data` must be writable by uid 1000
diff --git a/TODO.md b/TODO.md
index 623f8cf..e37242e 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,13 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: nginx sends the security headers `REPO_POLICIES.md` requires on
+  every response (issue #18), including errors, `/assets/` and what it passes on
+  from the backend, whose own copies it drops so each header goes out once. They
+  live in `security-headers.conf`, which `nginx.conf` includes. The content
+  security policy allows no inline script or style, so the status dot's grey in
+  `src/main.js` is now a class; `connect-src` is `*` because probed hosts
+  redirect to others, and the browser checks each redirect against it
 - 2026-09-29: the request log is bounded (issue #60): the method, URL, protocol,
   `User-Agent`, `Referer`, request ID (which chi takes from the client's
   `X-Request-Id` header) and client address it writes are each cut to 128 bytes,
diff --git a/backend/README.md b/backend/README.md
index 5f80ef2..ad29c08 100644
--- a/backend/README.md
+++ b/backend/README.md
@@ -104,7 +104,9 @@ this server. The image's entrypoint, `bin/entrypoint.sh`, starts the server as
 user `netwatch` (uid 1000) with `BIND_ADDRESS=127.0.0.1` and `PORT=8081`, so
 only nginx reaches it, and with `TRUSTED_PROXIES=127.0.0.1/32`, so it takes the
 client address nginx passes on and no other. `DATA_DIR` is `/data/reports`, on
-the `/data` volume, which `netwatch` owns.
+the `/data` volume, which `netwatch` owns. nginx replaces the security headers
+this server sets with those in the root `security-headers.conf`, so those are
+what clients of the image see.
 
 The container's own `TRUSTED_PROXIES` goes to nginx instead: IP addresses or
 CIDRs, separated by commas, of the reverse proxies in front of the container.
diff --git a/nginx.conf b/nginx.conf
index 0322c27..3ea963a 100644
--- a/nginx.conf
+++ b/nginx.conf
@@ -8,6 +8,11 @@ server {
     # Keep the nginx version out of the Server header and error pages.
     server_tokens off;
 
+    # The security headers, on every response. An add_header in a
+    # location drops every add_header from here, so a location with one
+    # of its own includes this file again.
+    include /etc/nginx/security-headers.conf;
+
     root /usr/share/nginx/html;
     index index.html;
 
@@ -32,6 +37,7 @@ server {
     location /assets/ {
         expires 1y;
         add_header Cache-Control "public, immutable";
+        include /etc/nginx/security-headers.conf;
     }
 
     # netwatch-server, the Go backend, runs in the same container and
@@ -45,6 +51,16 @@ server {
     proxy_set_header X-Forwarded-For $remote_addr;
     proxy_set_header X-Forwarded-Proto $scheme;
 
+    # netwatch-server sets the same security headers on its own
+    # responses. Its copies are dropped so that each header goes out
+    # once, as security-headers.conf sets it.
+    proxy_hide_header Strict-Transport-Security;
+    proxy_hide_header Content-Security-Policy;
+    proxy_hide_header X-Frame-Options;
+    proxy_hide_header X-Content-Type-Options;
+    proxy_hide_header Referrer-Policy;
+    proxy_hide_header Permissions-Policy;
+
     location /api/ {
         proxy_pass http://127.0.0.1:8081;
     }
diff --git a/script/frontend-viewport-test b/script/frontend-viewport-test
index e4b53d1..d26e0fe 100755
--- a/script/frontend-viewport-test
+++ b/script/frontend-viewport-test
@@ -64,13 +64,15 @@ main() {
     # nginx.conf is a template: the image renders it over its own
     # default.conf, with the same port and limit bin/entrypoint.sh uses.
     # The empty file it includes trusts no proxy, as bin/entrypoint.sh
-    # writes it when TRUSTED_PROXIES is unset.
+    # writes it when TRUSTED_PROXIES is unset. nginx.conf also includes
+    # the security headers, so the page runs under the shipped policy.
     docker run -d --rm --name "$SERVER" \
         --network "$NETWORK" --network-alias netwatch \
         -e PORT=8080 -e NGINX_ENVSUBST_FILTER='^PORT$' \
         -v "$ROOT/dist:/usr/share/nginx/html:ro" \
         -v "$ROOT/nginx.conf:/etc/nginx/templates/default.conf.template:ro" \
         -v /dev/null:/etc/nginx/trusted-proxies.conf:ro \
+        -v "$ROOT/security-headers.conf:/etc/nginx/security-headers.conf:ro" \
         "$SERVER_IMAGE" > /dev/null
 
     # The image's own entrypoint already exposes CDP on 9222 and passes
diff --git a/security-headers.conf b/security-headers.conf
new file mode 100644
index 0000000..6602d6e
--- /dev/null
+++ b/security-headers.conf
@@ -0,0 +1,24 @@
+# The security headers REPO_POLICIES.md requires on every response.
+# nginx.conf includes this file, which Dockerfile copies to
+# /etc/nginx/security-headers.conf. always sends each header on error
+# responses too.
+
+add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;
+
+# Scripts and styles load only from the page's own origin. Inline ones
+# are blocked, style attributes in markup included, so style elements
+# through classes or element.style. data: images are for the favicon
+# in index.html. connect-src is * because the browser checks each probe in
+# src/main.js against it, and also every redirect the probe follows,
+# and several of those hosts redirect to others; a list of hosts here
+# would block those probes. It also covers the reports the page sends
+# to its own origin.
+add_header Content-Security-Policy "default-src 'self'; connect-src *; img-src 'self' data:; object-src 'none'; base-uri 'none'; form-action 'none'; frame-ancestors 'none'" always;
+
+add_header X-Frame-Options DENY always;
+add_header X-Content-Type-Options nosniff always;
+
+# The probed hosts are not told where the page is served from.
+add_header Referrer-Policy no-referrer always;
+
+add_header Permissions-Policy "accelerometer=(), camera=(), display-capture=(), geolocation=(), gyroscope=(), magnetometer=(), microphone=(), midi=(), payment=(), usb=()" always;
diff --git a/src/main.js b/src/main.js
index 21b4a62..526a544 100644
--- a/src/main.js
+++ b/src/main.js
@@ -716,7 +716,7 @@ function hostRowHTML(host, index, showPin = true) {
         ${pinBtn}
         <div class="w-[420px] flex-shrink-0 grid grid-cols-[minmax(0,1fr)_auto] items-center">
           <div class="flex items-center gap-2 min-w-[200px]">
-            <div class="w-3 h-3 rounded-full flex-shrink-0" style="background-color: ${latencyHex(null)}"></div>
+            <div class="w-3 h-3 rounded-full flex-shrink-0 bg-[#6b7280]"></div>
             <span class="font-medium text-white truncate">${host.name}</span>
           </div>
           <div class="latency-value text-4xl font-bold tabular-nums text-right mt-3" data-host="${index}">
-- 
2.54.0


From c226ceee0133d826dfdd44c4085179a5767a79b9 Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Tue, 29 Sep 2026 10:56:00 +0200
Subject: [PATCH 20/22] chore(backend): re-vendor .golangci.yml with
 gomodguard_v2 (closes #41)

golangci-lint v2.12 deprecates gomodguard, which the org .golangci.yml
reached through "default: all", so every lint run printed a
deprecation warning. backend/.golangci.yml is now the current copy
from sneak/prompts, fetched unedited: gomodguard is disabled and
gomodguard_v2 enabled with the org block list. The new file also
turns depguard on with its test-support rule, which forbids
net/http/httptest outside test code. netwatch has no test-support
packages of its own to add to that rule, so the file is identical to
the canonical one. backend/script/lint checks the new sha256. The
backend raises no findings under the new rules.

Model: opus-5-5
---
 TODO.md               | 13 +++++----
 backend/.golangci.yml | 68 +++++++++++++++++++++++++++++++++++++++++--
 backend/script/lint   |  2 +-
 3 files changed, 74 insertions(+), 9 deletions(-)

diff --git a/TODO.md b/TODO.md
index e37242e..212faab 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,13 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: `backend/.golangci.yml` re-vendored from `sneak/prompts` (issue
+  #41): `gomodguard`, deprecated in golangci-lint v2.12.0, is disabled and its
+  successor `gomodguard_v2` enabled with the org block list, so lint runs print
+  no deprecation warning. The new file also turns `depguard` on with its
+  `test-support` rule, which keeps `net/http/httptest` out of non-test code;
+  netwatch adds no entries of its own to that rule. `backend/script/lint` checks
+  the new sha256
 - 2026-09-29: nginx sends the security headers `REPO_POLICIES.md` requires on
   every response (issue #18), including errors, `/assets/` and what it passes on
   from the backend, whose own copies it drops so each header goes out once. They
@@ -191,9 +198,3 @@ latest run passes.
   (main always green policy)
 - Decide what to do with untracked resume.sh: commit it, gitignore it, or delete
   it
-- Upstream fix needed in `sneak/prompts`: the org-standard `.golangci.yml`
-  enables `gomodguard`, which golangci-lint v2.12.2 reports as deprecated since
-  v2.12.0 and replaced by `gomodguard_v2`, so every backend lint run prints a
-  deprecation warning. The file is standardized and must never be edited in this
-  repo, so nothing can be done here beyond tracking it — tracked at
-  <https://git.eeqj.de/sneak/netwatch/issues/41>
diff --git a/backend/.golangci.yml b/backend/.golangci.yml
index 26b1610..a7a74c2 100644
--- a/backend/.golangci.yml
+++ b/backend/.golangci.yml
@@ -10,14 +10,20 @@ run:
 
 linters:
   default: all
+  enable:
+    # Successor to the deprecated gomodguard. Named explicitly, rather than
+    # left to `default: all`, because it carries the module policy below.
+    - gomodguard_v2
   disable:
     # Genuinely incompatible with project patterns
     - exhaustruct      # Requires all struct fields
-    - depguard         # Dependency allow/block lists
     - godot            # Requires comments to end with periods
-    - wsl              # Deprecated, replaced by wsl_v5
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go
+    # Deprecated: the warning is attached to the old name, so it is
+    # silenced by disabling that name, not by enabling the successor.
+    - wsl              # Deprecated, replaced by wsl_v5
+    - gomodguard       # Deprecated, replaced by gomodguard_v2
   settings:
     lll:
       line-length: 88
@@ -28,6 +34,64 @@ linters:
       max-complexity: 15
     dupl:
       threshold: 100
+    depguard:
+      # Test-support code must not be compiled into the shipped binary. A
+      # test-support package exists to hand a test privileges the program
+      # itself must never have, so a file that is not a test must not import
+      # one. Test files, and the files inside a package whose directory name
+      # ends in `test`, are where that code belongs, and are exempt.
+      #
+      # The deny list below is the one part of this file a repository is
+      # expected to extend, and the only part it may. depguard matches an
+      # import path against a list of prefixes, so it cannot be told "any path
+      # whose last segment ends in test"; a repository's own test-support
+      # packages have to be named here one at a time, by full import path,
+      # under a module path that differs from repository to repository. Add
+      # them; change nothing else.
+      rules:
+        test-support:
+          list-mode: lax
+          files:
+            - "$all"
+            - "!$test"
+            - "!**/*test/**"
+          deny:
+            - pkg: net/http/httptest
+              desc: >-
+                Test-support code belongs in test files and in packages whose
+                directory name ends in test, not in the shipped binary.
+    # Only decisions already recorded in the Go package defaults are
+    # listed here. Every entry matches the module path exactly.
+    gomodguard_v2:
+      blocked:
+        - module: github.com/rs/zerolog
+          recommendations:
+            - log/slog
+          reason: "Structured logging is stdlib log/slog."
+        # One entry per pre-fork module path, because the later releases
+        # are separate paths. A prefix match would be shorter but would
+        # also reach github.com/go-redis/redismock, the test double for
+        # the successor these entries recommend.
+        - module: github.com/go-redis/redis
+          recommendations:
+            - github.com/redis/go-redis/v9
+          reason: "Pre-fork module; use the maintained go-redis v9."
+        - module: github.com/go-redis/redis/v7
+          recommendations:
+            - github.com/redis/go-redis/v9
+          reason: "Pre-fork module; use the maintained go-redis v9."
+        - module: github.com/go-redis/redis/v8
+          recommendations:
+            - github.com/redis/go-redis/v9
+          reason: "Pre-fork module; use the maintained go-redis v9."
+        - module: github.com/sergi/go-diff
+          recommendations:
+            - github.com/aymanbagabas/go-udiff
+          reason: "No unified diff output; use go-udiff."
+        - module: github.com/hexops/gotextdiff
+          recommendations:
+            - github.com/aymanbagabas/go-udiff
+          reason: "Unmaintained fork; use go-udiff."
 
 issues:
   max-issues-per-linter: 0
diff --git a/backend/script/lint b/backend/script/lint
index 256f805..720b747 100755
--- a/backend/script/lint
+++ b/backend/script/lint
@@ -14,7 +14,7 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-GOLANGCI_CONFIG_SHA256="021cc83f4e6fc7c31b95b34b846723dfcf20b66b7baeea1dc40406e643346bcb"
+GOLANGCI_CONFIG_SHA256="a79b63a254602a5318db5d0e9a06bc71b84bf0c1d896305229d8bfed1d1b1776"
 
 main() {
     cd "$ROOT"
-- 
2.54.0


From f4237689757a2ede0160c1532b470cceb7f11da9 Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Tue, 29 Sep 2026 11:55:52 +0200
Subject: [PATCH 21/22] cibuild: the org model, which runs every check uncached
 (closes #37)

script/cibuild was a plain docker build ., so on a tree Docker had
seen before every check step came from the build cache and the build
still passed. It is now the org model from sneak/prompts, byte for
byte: script/bootstrap, script/check, then docker build --no-cache
with the git describe version as the VERSION build argument.

The workflow puts ~/.local/bin, where bootstrap links what it
installs, on the step's PATH. Bootstrap now installs its pinned node
when the installed one is older than 22.12.0, the oldest the
frontend's dependencies accept (puppeteer-core's engines field), as
it already does for Go against backend/go.mod.

Model: opus-5-5
---
 .gitea/workflows/check.yml |  6 ++++--
 README.md                  | 15 ++++++++-------
 TODO.md                    |  8 ++++++++
 script/bootstrap           | 32 +++++++++++++++++++++++++-------
 script/cibuild             | 26 ++++++++++++++++++++------
 5 files changed, 65 insertions(+), 22 deletions(-)

diff --git a/.gitea/workflows/check.yml b/.gitea/workflows/check.yml
index 06b80af..08c2ebc 100644
--- a/.gitea/workflows/check.yml
+++ b/.gitea/workflows/check.yml
@@ -6,5 +6,7 @@ jobs:
         steps:
             # actions/checkout v4.2.2, 2026-02-22
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
-            # script/cibuild builds the image, whose stages run every check.
-            - run: script/cibuild
+            # script/cibuild bootstraps, runs every check and builds the
+            # image. script/bootstrap links what it installs into
+            # ~/.local/bin, so that has to be on PATH for the rest.
+            - run: PATH="$HOME/.local/bin:$PATH" script/cibuild
diff --git a/README.md b/README.md
index 415457e..c7c826a 100644
--- a/README.md
+++ b/README.md
@@ -36,12 +36,12 @@ The Go backend in `backend/` has its own `script/` directory and shim Makefile
 (see [backend/README.md](backend/README.md)). The root scripts cover both
 halves, so the root `make check` fails if either one is broken. We provide:
 
-- `script/bootstrap` — install all dependencies (pinned node via nvm if needed,
-  yarn via corepack, `yarn install --frozen-lockfile`, the pinned Go unless one
-  at least as new as `backend/go.mod` asks for is installed, and the Go
-  modules), linking what it installs itself into `~/.local/bin`, which has to be
-  on `PATH`. It installs no Go linter and not Docker: `make lint` runs the
-  linter in Docker
+- `script/bootstrap` — install all dependencies (the pinned node via nvm unless
+  one new enough for the frontend's dependencies is installed, yarn via
+  corepack, `yarn install --frozen-lockfile`, the pinned Go unless one at least
+  as new as `backend/go.mod` asks for is installed, and the Go modules), linking
+  what it installs itself into `~/.local/bin`, which has to be on `PATH`. It
+  installs no Go linter and not Docker: `make lint` runs the linter in Docker
 - `script/setup` — make a fresh clone ready for development: bootstrap plus the
   git pre-commit hook
 - `script/projectname` — print the project name (used for the Docker image tag)
@@ -65,7 +65,8 @@ halves, so the root `make check` fails if either one is broken. We provide:
   `script/check`: it needs Docker and takes minutes.
 - `script/docker` — build the image from `Dockerfile` without the build cache,
   tagged `netwatch` via `script/projectname`
-- `script/cibuild` — CI entrypoint: builds the image
+- `script/cibuild` — CI entrypoint: runs `script/bootstrap` and `script/check`,
+  then builds the image as `script/docker` does, without the build cache
 - `script/precommit` — run by the git pre-commit hook; runs `script/check`
 - `script/install-precommit` — install the git pre-commit hook
 
diff --git a/TODO.md b/TODO.md
index 212faab..1135681 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,14 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: CI can no longer pass on checks that did not run (issue #37):
+  `script/cibuild` is now the org model, byte for byte. It runs
+  `script/bootstrap` and `script/check`, then builds the image with `--no-cache`
+  and the version from `git describe` as the `VERSION` build argument, where it
+  used to be a plain `docker build .` whose check steps could come from the
+  build cache. The workflow puts `~/.local/bin`, where bootstrap links what it
+  installs, on the step's `PATH`, and bootstrap now installs its pinned node
+  when the installed one is older than the frontend's dependencies need
 - 2026-09-29: `backend/.golangci.yml` re-vendored from `sneak/prompts` (issue
   #41): `gomodguard`, deprecated in golangci-lint v2.12.0, is disabled and its
   successor `gomodguard_v2` enabled with the org block list, so lint runs print
diff --git a/script/bootstrap b/script/bootstrap
index 66e5873..d807ee3 100755
--- a/script/bootstrap
+++ b/script/bootstrap
@@ -3,12 +3,12 @@
 # this repo. Idempotent: every install is guarded by a check so already
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes nothing is present. Node is
-# used directly if installed; otherwise it is installed at a pinned
-# version via nvm (installing nvm itself first, from a hash-verified
-# release archive, never curl | sh). Go, with its gofmt, is used
-# directly if it is at least the version backend/go.mod asks for;
-# otherwise the pinned Go release is installed from its hash-verified
-# archive.
+# used directly if it is at least NODE_MIN_VERSION; otherwise it is
+# installed at a pinned version via nvm (installing nvm itself first,
+# from a hash-verified release archive, never curl | sh). Go, with its
+# gofmt, is used directly if it is at least the version backend/go.mod
+# asks for; otherwise the pinned Go release is installed from its
+# hash-verified archive.
 #
 # What this script installs outside the system package manager lives
 # under $HOME and is linked into ~/.local/bin, where make and the git
@@ -23,6 +23,10 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 # Pinned versions, 2026-07-07
 NODE_VERSION="22.17.0"
+# The oldest node the frontend's dependencies accept: the "engines"
+# field of puppeteer-core 25.5.0, the most demanding of them, asks for
+# 22.12.0 or newer, 2026-09-29. An older installed node is not used.
+NODE_MIN_VERSION="22.12.0"
 NVM_VERSION="0.40.3"
 # sha256 of https://github.com/nvm-sh/nvm/archive/refs/tags/v0.40.3.tar.gz
 NVM_SHA256="5f4d6aaa04a177dc93c985e31dbc411ab6b8c6e1e21d8015dbc1372625fcd1d0"
@@ -136,8 +140,22 @@ ensure_nvm() {
     rm -rf "$tmp"
 }
 
+# node_ok: the node on PATH is at least NODE_MIN_VERSION. node itself
+# compares the two: major, then minor, then patch.
+node_ok() {
+    if missing node; then return 1; fi
+    node -e '
+        const have = process.versions.node.split(".").map(Number);
+        const want = process.argv[1].split(".").map(Number);
+        for (let i = 0; i < 3; i++) {
+            if (have[i] !== want[i]) process.exit(have[i] > want[i] ? 0 : 1);
+        }
+    ' "$NODE_MIN_VERSION"
+}
+
+# ensure_node: unless node_ok, install NODE_VERSION and link its node.
 ensure_node() {
-    if ! missing node; then return 0; fi
+    if node_ok; then return 0; fi
     ensure_nvm
     nvm_sh "nvm install $NODE_VERSION"
     link_bin "$HOME/.nvm/versions/node/v$NODE_VERSION/bin/node" node
diff --git a/script/cibuild b/script/cibuild
index d860bf5..688299f 100755
--- a/script/cibuild
+++ b/script/cibuild
@@ -1,15 +1,29 @@
 #!/bin/sh
-# script/cibuild: run the CI build: build the one image from Dockerfile,
-# whose stages run the checks as build steps (the backend's fmt-check,
-# lint and tests, and the frontend's test, lint and fmt-check). This is
-# the only build step the Gitea workflow runs.
+# script/cibuild: run the CI build. It bootstraps first: a CI runner
+# checks out and runs this and nothing else, and script/fmt-check runs
+# the formatter on the host, which a pristine checkout cannot do.
+# --no-cache for the same reason as script/docker: the gate phases the
+# final stage depends on are RUN steps, and a cached one is a check that
+# did not run.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    timeout 300 docker build .
+    "$SCRIPT_DIR/bootstrap"
+    "$SCRIPT_DIR/check"
+    # Own line: a failing command substitution inside an argument does
+    # not trip `set -e`, so the inline form degrades silently to an
+    # empty constant. VERSION is computed here because .dockerignore
+    # excludes .git, so `git describe` in a build stage yields an empty
+    # version without failing.
+    version="$(git describe --tags --always --dirty 2>/dev/null || true)"
+    [ -n "$version" ] || version="unknown"
+    docker build --no-cache \
+        --build-arg VERSION="$version" \
+        -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"
-- 
2.54.0


From a5ca73c585f9f4f6017d59c787185db00de59c98 Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Tue, 29 Sep 2026 12:03:59 +0200
Subject: [PATCH 22/22] Container sets up its own data directory (closes #75)
 (#76)

Closes https://git.eeqj.de/sneak/netwatch/issues/75.

`bin/entrypoint.sh`, which already runs as root, now makes the data directory usable before the backend starts: it creates `DATA_DIR` if missing, gives it and `/data` to the `netwatch` user (`chown -R`), and sets mode 750 on both, the mode the backend gives a directory it creates. The backend still runs as `netwatch`. The README "Running under upaas" section loses its first-run step that created and chowned the host directory and names only the path to mount. The Dockerfile's build-time `mkdir` and `chown` of `/data` are gone, since the entrypoint now does this on every start.

What the diff does not show:

- The host directory mounted at `/data` ends up owned by uid 1000 with mode 750, and everything under `DATA_DIR` is chowned to uid 1000 on every start.
- If the directory cannot be created or chowned, the container stops with that tool's error before either process starts.

Recorded runs with `--mount type=bind`: an empty directory owned by root (mode 755, and again mode 700), and one holding a `reports` directory and report file owned by uid 1001 with mode 700. Each time the container turned healthy, `netwatch-server` ran as `netwatch`, and a posted report was written to `DATA_DIR`; a second start on the root-owned and the uid 1001 directories did the same.

Judgement call: `/data` itself is given to `netwatch` as well as `DATA_DIR`, so the backend can reach `DATA_DIR` inside a host directory with mode 700.

Model: opus-5-5
Reviewed-on: https://git.eeqj.de/sneak/netwatch/pulls/76
Co-authored-by: clawbot <35+clawbot@noreply.example.org>
---
 Dockerfile        |  3 ++-
 README.md         | 15 +++------------
 TODO.md           |  9 +++++++++
 backend/README.md |  3 ++-
 bin/entrypoint.sh | 23 +++++++++++++++++++++++
 5 files changed, 39 insertions(+), 14 deletions(-)

diff --git a/Dockerfile b/Dockerfile
index 417fc97..d13e53e 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -77,8 +77,9 @@ COPY --from=frontend /app/dist /usr/share/nginx/html
 COPY --from=builder /src/netwatch-server /usr/local/bin/netwatch-server
 COPY bin/entrypoint.sh /usr/local/bin/entrypoint.sh
 
+# bin/entrypoint.sh creates DATA_DIR at start and gives it and /data to
+# the netwatch user, whatever is mounted there.
 ENV DATA_DIR=/data/reports
-RUN mkdir -p /data/reports && chown -R netwatch:netwatch /data
 VOLUME /data
 
 # The default public port; PORT changes it.
diff --git a/README.md b/README.md
index c7c826a..acbb62d 100644
--- a/README.md
+++ b/README.md
@@ -192,8 +192,9 @@ only inside the container, on `127.0.0.1:8081`. The image:
 - Sends the security headers `REPO_POLICIES.md` requires on every response, as
   `security-headers.conf` sets them, in place of the backend's own
 - Stores reports in `DATA_DIR`, `/data/reports` by default, on the `/data`
-  volume. The backend runs as user `netwatch` (uid 1000), so a directory
-  bind-mounted at `/data` must be writable by uid 1000
+  volume. Before the backend starts, the image creates `DATA_DIR` and gives it
+  and `/data` to user `netwatch` (uid 1000), which the backend runs as, so a
+  host directory bind-mounted at `/data` ends up owned by uid 1000
 - Writes buffered reports to disk on `docker stop`, and exits non-zero if nginx
   or the backend exits on its own, so the platform restarts it
 
@@ -203,16 +204,6 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for netwatch needs:
 
 - **Port:** container port `8080`.
 - **Volume:** container path `/data`; the reports are kept in `/data/reports`.
-- **First run:** upaas bind-mounts the host directory it is given and does not
-  create it, and the backend, which runs as uid 1000, does not start unless it
-  can write there. Create the directory, owned by uid 1000, before the first
-  deploy:
-
-    ```bash
-    mkdir -p /path/to/data
-    chown 1000:1000 /path/to/data
-    ```
-
 - **Environment variables:** none is required. An empty one counts as unset, and
   one set to a value netwatch cannot use stops the container at start, with the
   reason in its log.
diff --git a/TODO.md b/TODO.md
index 1135681..8b17dd2 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,6 +23,15 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: the container sets up its own data directory (issue #75):
+  `bin/entrypoint.sh`, still as root, creates `DATA_DIR` if missing and gives it
+  and `/data` to the `netwatch` user with mode 750 before starting the backend
+  as that user, so an empty host directory owned by root, or one holding files
+  from another uid, works with no step on the host. It stops the start instead
+  when a symbolic link is on the path to `DATA_DIR`, since root would change
+  whatever the link points to. The `README.md` first-run step that created and
+  chowned the host directory is gone, and the image no longer sets that
+  ownership at build time
 - 2026-09-29: CI can no longer pass on checks that did not run (issue #37):
   `script/cibuild` is now the org model, byte for byte. It runs
   `script/bootstrap` and `script/check`, then builds the image with `--no-cache`
diff --git a/backend/README.md b/backend/README.md
index ad29c08..9914eb0 100644
--- a/backend/README.md
+++ b/backend/README.md
@@ -104,7 +104,8 @@ this server. The image's entrypoint, `bin/entrypoint.sh`, starts the server as
 user `netwatch` (uid 1000) with `BIND_ADDRESS=127.0.0.1` and `PORT=8081`, so
 only nginx reaches it, and with `TRUSTED_PROXIES=127.0.0.1/32`, so it takes the
 client address nginx passes on and no other. `DATA_DIR` is `/data/reports`, on
-the `/data` volume, which `netwatch` owns. nginx replaces the security headers
+the `/data` volume; the entrypoint creates it and gives it and `/data` to
+`netwatch` before starting the server. nginx replaces the security headers
 this server sets with those in the root `security-headers.conf`, so those are
 what clients of the image see.
 
diff --git a/bin/entrypoint.sh b/bin/entrypoint.sh
index 4458f9e..6f3bdf8 100755
--- a/bin/entrypoint.sh
+++ b/bin/entrypoint.sh
@@ -61,6 +61,29 @@ for proxy in $(printf '%s' "$TRUSTED_PROXIES" | tr ',' ' '); do
     echo "set_real_ip_from $cidr;"
 done > /etc/nginx/trusted-proxies.conf
 
+# netwatch-server keeps its report files in DATA_DIR, on the /data
+# volume, which may be a host directory owned by root or by another
+# uid. Both are given to the netwatch user here, with the mode the
+# server gives a directory it creates, so the host directory needs no
+# preparing.
+#
+# chown and chmod, run as root, change whatever a symbolic link on the
+# path points to, anywhere in the container, and the netwatch user can
+# put one in /data. So the start stops unless readlink -f, which
+# follows every link on a path, gives /data and DATA_DIR back as they
+# are. It also writes a path in full, so a DATA_DIR with '.', '..' or
+# an extra '/' in it is refused too.
+export DATA_DIR="${DATA_DIR:-/data/reports}"
+mkdir -p "$DATA_DIR" || exit 1
+if [ "$(readlink -f /data)" != /data ] ||
+    [ "$(readlink -f "$DATA_DIR")" != "$DATA_DIR" ]; then
+    echo "entrypoint: DATA_DIR must be a full path with no '.', '..'," \
+        "extra '/' or symbolic link on it or on /data, not '$DATA_DIR'" >&2
+    exit 1
+fi
+chown -R netwatch:netwatch /data "$DATA_DIR" || exit 1
+chmod 750 /data "$DATA_DIR" || exit 1
+
 # A stop signal is only noted here; the loop below acts on it.
 stop_requested=""
 trap 'stop_requested=yes' TERM INT
-- 
2.54.0