docker: run as non-root, add health check, document upaas (closes #147)

The runtime image runs as uid 10001, which owns /var/lib/dnswatcher. The
working directory is /, so config loading finds no .env or dnswatcher
config file there; the binary lives in /usr/local/bin. A Docker
HEALTHCHECK probes /.well-known/healthcheck every 10 seconds with busybox
wget, so the container is healthy well before upaas reads its health at
60 seconds.

Startup now fails with an error naming the data directory when it cannot
be written, instead of running with every save failing. The check creates
the directory if needed and writes and removes the temp file Save uses.

README gains "Running under upaas": the prod branch, host directory
setup, network and port, environment and health check.

Model: opus-5-5

Dockerfile

@@ -41,31 +41,33 @@ FROM alpine@sha256:c3f8e73fdb79deaebaa2037150150191b9dcbfba68b4a46d70103204c53f4
 
 RUN apk add --no-cache ca-certificates tzdata
 
-# The binary lives in /usr/local/bin, not the working directory: config
-# loading searches the working directory, and with a YAML config type Viper
-# also matches an extension-less file named "dnswatcher" there, so a binary
-# named "dnswatcher" in that directory would be parsed as a config file.
 COPY --from=builder /src/bin/dnswatcher /usr/local/bin/dnswatcher
 
-# Run as an unprivileged user. The data directory is owned by that user so
-# writes succeed both on a bind mount and when Docker seeds a fresh named
-# volume from the image (a fresh volume inherits this directory's ownership).
-RUN addgroup -S dnswatcher \
+# Run as an unprivileged user that owns the data directory. A fresh named
+# volume inherits this ownership; a bind-mounted host directory must be
+# owned by uid 10001 (see "Running under upaas" in README.md), or startup
+# fails.
+RUN addgroup -S -g 10001 dnswatcher \
     && adduser -S -G dnswatcher -u 10001 dnswatcher \
     && mkdir -p /var/lib/dnswatcher \
     && chown dnswatcher:dnswatcher /var/lib/dnswatcher
 
 ENV DNSWATCHER_DATA_DIR=/var/lib/dnswatcher
 
-WORKDIR /var/lib/dnswatcher
+# Config loading also reads a `.env` file and a file named `dnswatcher`
+# (any config extension, or none) from the working directory. `/` holds
+# neither, so every setting comes from the environment. Do not make the
+# data directory, or the binary's directory, the working directory.
+WORKDIR /
 
 USER dnswatcher
 
 EXPOSE 8080
 
-# busybox wget (already in alpine) probes the health endpoint. PORT defaults
-# to 8080 and is honoured if the operator overrides it.
-HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+# busybox wget (already in alpine) probes the health endpoint. The first
+# probe runs one interval after start. upaas reads the container's health
+# 60 seconds after a deploy and fails the deploy unless it is healthy.
+HEALTHCHECK --interval=10s --timeout=5s --start-period=10s --retries=3 \
     CMD wget -q -O /dev/null "http://127.0.0.1:${PORT:-8080}/.well-known/healthcheck" || exit 1
 
 ENTRYPOINT ["/usr/local/bin/dnswatcher"]

README.md

@@ -203,6 +203,46 @@ are read, so a smaller value would sever the connection before a handler
 using its full budget could respond. `IdleTimeout` exceeds common
 Prometheus scrape intervals so the scraper reuses its connection.
 
+### Security Headers
+
+Every response — the dashboard, the static assets under `/s/...`, the
+healthchecks, the JSON API, and `/metrics` — carries the following
+headers, set by a global middleware:
+
+| Header                      | Value                                 |
+|-----------------------------|---------------------------------------|
+| `Strict-Transport-Security` | `max-age=31536000; includeSubDomains` |
+| `Content-Security-Policy`   | see below                             |
+| `X-Frame-Options`           | `DENY`                                |
+| `X-Content-Type-Options`    | `nosniff`                             |
+| `Referrer-Policy`           | `no-referrer`                         |
+| `Permissions-Policy`        | all unused browser features denied    |
+
+The content security policy is:
+
+```
+default-src 'self'; script-src 'none'; style-src 'self'; img-src 'self';
+font-src 'none'; connect-src 'none'; object-src 'none'; base-uri 'none';
+form-action 'none'; frame-ancestors 'none'
+```
+
+The dashboard ships no JavaScript (the 30-second refresh is a
+`<meta http-equiv="refresh">`), no inline styles, no inline event
+handlers, and no images; its only subresource is the embedded stylesheet
+at `/s/css/tailwind.min.css`, which `style-src 'self'` permits. The
+policy therefore needs neither `unsafe-inline` nor `unsafe-eval`.
+`frame-ancestors 'none'` is the primary anti-framing control, with
+`X-Frame-Options: DENY` retained as the legacy fallback.
+
+HSTS is emitted unconditionally, including over plain HTTP. dnswatcher is
+expected to run behind a TLS-terminating reverse proxy, and the browser
+must still be told to enforce HTTPS end to end, so the header is never
+gated on whether the request itself arrived over TLS.
+
+`Referrer-Policy: no-referrer` is stricter than the
+`strict-origin-when-cross-origin` baseline: the dashboard has no
+cross-origin navigation needs, and its URL may name internal hosts.
+
 ---
 
 ## Architecture
@@ -215,7 +255,8 @@ internal/
   globals/globals.go           Build-time variables (version)
   logger/logger.go             slog structured logging (TTY detection)
   healthcheck/healthcheck.go   Health check service
-  middleware/middleware.go      HTTP middleware (logging, CORS, metrics auth)
+  middleware/middleware.go      HTTP middleware (logging, CORS, security
+                               headers, metrics auth)
   handlers/handlers.go         HTTP request handlers
   server/
     server.go                  HTTP server lifecycle
@@ -467,40 +508,57 @@ docker run -d \
 
 ---
 
-## Deploying with upaas
+## Running under upaas
 
-upaas builds this repo's `Dockerfile` and runs the resulting image with the
-volume, port, and environment variables the operator enters. Configure:
+[upaas](https://git.eeqj.de/sneak/upaas) builds the image from this
+repository's `Dockerfile` and runs it. The app needs:
 
-- **Branch:** `main`. This is the recommended branch on the deployment
-  decision issue and is not yet confirmed by the owner.
-- **Volume:** mount persistent storage at `/var/lib/dnswatcher`. The image runs
-  as an unprivileged user (`dnswatcher`, uid 10001) that owns this directory; a
-  fresh named volume inherits that ownership, and a bind-mounted host directory
-  must be writable by uid 10001.
-- **Port:** the container listens on `8080`. Override with `PORT` if needed;
-  the healthcheck honours it.
-- **Required environment:** `DNSWATCHER_TARGETS`, a comma-separated list of the
-  domains and hostnames to watch. The process refuses to start without it.
-- **Recommended environment:** the notification endpoints
-  `DNSWATCHER_SLACK_WEBHOOK`, `DNSWATCHER_MATTERMOST_WEBHOOK`, and
-  `DNSWATCHER_NTFY_TOPIC` (without at least one, changes are only visible on the
-  dashboard); and `DNSWATCHER_METRICS_USERNAME` / `DNSWATCHER_METRICS_PASSWORD`
-  to enable the basic-auth-protected `/metrics` endpoint.
-- **Healthcheck:** the image already declares a Docker `HEALTHCHECK` against
-  `/.well-known/healthcheck`; no operator configuration is needed. The same
-  path is available for an external probe.
+- **Branch:** `prod`. `prod` is cut from `main`, and merging a `main` to
+  `prod` pull request is a deploy.
+- **Volume:** one host directory mounted at `/var/lib/dnswatcher`, where
+  the state file lives. upaas bind-mounts the host path it is given and
+  does not create it. The container runs as uid 10001 and does not start
+  unless it can write there. Create the directory before the first
+  deploy:
 
-The dashboard is unauthenticated and shows every watched name and recent alert;
-decide deliberately whether to expose it publicly.
+  ```sh
+  mkdir -p /path/to/data
+  chown 10001:10001 /path/to/data
+  chmod 700 /path/to/data
+  ```
+
+- **Network and port:** the dashboard is unauthenticated and shows every
+  watched name and recent alert, and upaas publishes every mapped port on
+  all interfaces of the host
+  ([upaas issue 113](https://git.eeqj.de/sneak/upaas/issues/113)). Add a
+  port mapping to container port `8080` only if the dashboard should be
+  public. Otherwise add none: set the app's Docker network in upaas to
+  your reverse proxy's Docker network, and the proxy reaches the app at
+  `upaas-` followed by the app name, port `8080`.
+- **Required environment:** `DNSWATCHER_TARGETS`, a comma-separated list
+  of the domains and hostnames to watch. dnswatcher refuses to start
+  without it.
+- **Recommended environment:** at least one notification endpoint
+  (`DNSWATCHER_SLACK_WEBHOOK`, `DNSWATCHER_MATTERMOST_WEBHOOK`,
+  `DNSWATCHER_NTFY_TOPIC`); without one, changes show only on the
+  dashboard. `DNSWATCHER_METRICS_USERNAME` and
+  `DNSWATCHER_METRICS_PASSWORD` serve `/metrics` behind basic auth.
+- **Leave unset:** `DNSWATCHER_DATA_DIR`, which the image sets to
+  `/var/lib/dnswatcher`, and `PORT`, which defaults to `8080`. Every
+  setting comes from the environment; the image holds no config file.
+- **Health check:** the image's own, which requests
+  `/.well-known/healthcheck` every 10 seconds. upaas reads the
+  container's health 60 seconds after a deploy and marks the deploy
+  failed unless it is `healthy`.
 
 ---
 
 ## Monitoring Lifecycle
 
-1. **Startup**: Load state from disk. If no state file exists, start
-   with empty state (first check will establish baseline without
-   triggering change notifications).
+1. **Startup**: Check that the data directory can be written, and exit
+   with an error naming it if not. Load state from disk. If no state
+   file exists, start with empty state (first check will establish
+   baseline without triggering change notifications).
 2. **Initial check**: Immediately perform all DNS, port, and TLS checks
    on startup.
 3. **Periodic checks** (DNS always runs first):

TODO.md

@@ -23,8 +23,11 @@ Rationale, Design, TODO, License, Author) if any are still missing.
 
 # Completed Steps
 
-- 2026-09-21: upaas deploy readiness — runtime image runs as unprivileged
-  `dnswatcher`, Docker `HEALTHCHECK`, README "Deploying with upaas" (closes #147).
+- 2026-09-28: upaas deploy readiness — runtime image runs as unprivileged
+  `dnswatcher`, Docker `HEALTHCHECK`, startup fails when the data directory is
+  not writable, README "Running under upaas" (closes #147).
+- 2026-09-21: added behavioural tests for `internal/globals`,
+  `internal/healthcheck`, and `internal/logger` (closes #110).
 - 2026-09-21: `go mod tidy` dropped the redundant `golang.org/x/sync`
   `// indirect` line so `script/bootstrap` leaves a clean tree (#132)
 - 2026-08-10: comment-only corrections to `script/bootstrap`,
@@ -113,6 +116,16 @@ Rationale, Design, TODO, License, Author) if any are still missing.
   than the 60s `chimw.Timeout` handler budget so that budget stays
   reachable, and tests in `internal/server` pin both the non-zero
   values and that relationship (#99)
+- 2026-08-09: security response headers middleware
+  (`SecurityHeaders()` in `internal/middleware/middleware.go`)
+  registered globally in `internal/server/routes.go`, so HSTS, CSP,
+  `X-Frame-Options`, `X-Content-Type-Options`, `Referrer-Policy`, and
+  `Permissions-Policy` are set on every response including `/s/...` and
+  `/metrics`; the CSP needs no `unsafe-inline`/`unsafe-eval` because the
+  dashboard ships no JavaScript and no inline styles; HSTS is emitted
+  unconditionally per policy (TLS-terminating proxy in front). Remaining
+  1.0 hardening items — `http.Server` timeouts, request body limits,
+  rate limiting, CORS scoping — are tracked separately
 - 2026-08-07: golangci-lint bumped to v2.12.2 (commit-pinned installs
   in `Dockerfile` and `script/bootstrap`); `.golangci.yml` set to the
   org-standard v2-schema config used across the org's repos

internal/globals/globals_test.go

@@ -0,0 +1,50 @@
+package globals_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"sneak.berlin/go/dnswatcher/internal/globals"
+)
+
+// TestGlobals exercises the package-level version and appname
+// variables through their setters and read-back via New. These are
+// shared package state, so the test mutates a global and must run
+// sequentially; it cannot use t.Parallel().
+//
+//nolint:paralleltest // mutates shared package-level globals, must run sequentially
+func TestGlobals(t *testing.T) {
+	versions := []string{"v1.2.3", "dev", "", "v1.2.3-4-gabcdef"}
+	for _, want := range versions {
+		globals.SetVersion(want)
+
+		g, err := globals.New(nil)
+		require.NoError(t, err)
+		assert.Equal(t, want, g.Version,
+			"New must surface the version set by SetVersion")
+	}
+
+	names := []string{"dnswatcher", "other", ""}
+	for _, want := range names {
+		globals.SetAppname(want)
+
+		g, err := globals.New(nil)
+		require.NoError(t, err)
+		assert.Equal(t, want, g.Appname,
+			"New must surface the appname set by SetAppname")
+	}
+
+	// New returns a snapshot: a later SetVersion must not mutate a
+	// Globals handed out earlier.
+	globals.SetVersion("first")
+
+	g, err := globals.New(nil)
+	require.NoError(t, err)
+
+	globals.SetVersion("second")
+	assert.Equal(t, "first", g.Version,
+		"a Globals returned by New must not change when the "+
+			"package variable is set again")
+}

internal/healthcheck/healthcheck_test.go

@@ -0,0 +1,134 @@
+package healthcheck_test
+
+import (
+	"context"
+	"encoding/json"
+	"testing"
+	"time"
+
+	"go.uber.org/fx"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"sneak.berlin/go/dnswatcher/internal/config"
+	"sneak.berlin/go/dnswatcher/internal/globals"
+	"sneak.berlin/go/dnswatcher/internal/healthcheck"
+	"sneak.berlin/go/dnswatcher/internal/logger"
+)
+
+// recordingLifecycle is a minimal fx.Lifecycle that records the hooks
+// appended to it, so healthcheck.New can be exercised through its real
+// constructor without standing up a whole fx application.
+type recordingLifecycle struct {
+	hooks []fx.Hook
+}
+
+func (l *recordingLifecycle) Append(hook fx.Hook) {
+	l.hooks = append(l.hooks, hook)
+}
+
+// newHealthcheck builds a Healthcheck through the real constructor and
+// runs the registered OnStart hook so StartupTime is set the same way
+// the fx lifecycle would set it.
+func newHealthcheck(
+	t *testing.T,
+	maintenance bool,
+	version string,
+) *healthcheck.Healthcheck {
+	t.Helper()
+
+	g := &globals.Globals{Appname: "dnswatcher", Version: version}
+
+	log, err := logger.New(nil, logger.Params{Globals: g})
+	require.NoError(t, err)
+
+	lifecycle := &recordingLifecycle{}
+
+	hc, err := healthcheck.New(lifecycle, healthcheck.Params{
+		Globals: g,
+		Config:  &config.Config{MaintenanceMode: maintenance},
+		Logger:  log,
+	})
+	require.NoError(t, err)
+
+	require.Len(t, lifecycle.hooks, 1,
+		"New must register exactly one lifecycle hook")
+	require.NotNil(t, lifecycle.hooks[0].OnStart)
+	require.NoError(t, lifecycle.hooks[0].OnStart(context.Background()))
+
+	return hc
+}
+
+func TestCheckStatusAndPayloadShape(t *testing.T) {
+	t.Parallel()
+
+	hc := newHealthcheck(t, false, "v9.9.9")
+	resp := hc.Check()
+
+	assert.Equal(t, "ok", resp.Status)
+
+	// The JSON shape and field names are part of the contract for the
+	// /health and /.well-known/healthcheck routes, so assert on the
+	// exact set of keys the response marshals to.
+	raw, err := json.Marshal(resp)
+	require.NoError(t, err)
+
+	var fields map[string]json.RawMessage
+	require.NoError(t, json.Unmarshal(raw, &fields))
+
+	wantKeys := []string{
+		"status",
+		"now",
+		"uptimeSeconds",
+		"uptimeHuman",
+		"version",
+		"appname",
+		"maintenanceMode",
+	}
+	assert.Len(t, fields, len(wantKeys),
+		"response must marshal to exactly the documented fields")
+
+	for _, key := range wantKeys {
+		assert.Contains(t, fields, key, "missing JSON field %q", key)
+	}
+
+	// The Now field is documented as RFC3339Nano; a change to the
+	// format constant should turn this red.
+	_, err = time.Parse(time.RFC3339Nano, resp.Now)
+	assert.NoError(t, err, "Now must be RFC3339Nano")
+}
+
+func TestCheckMaintenanceModeReflectsConfig(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name        string
+		maintenance bool
+	}{
+		{"maintenance off", false},
+		{"maintenance on", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			hc := newHealthcheck(t, tt.maintenance, "test")
+			resp := hc.Check()
+			assert.Equal(t, tt.maintenance, resp.Maintenance,
+				"maintenanceMode must mirror Config.MaintenanceMode")
+		})
+	}
+}
+
+func TestCheckSurfacesVersionAndAppname(t *testing.T) {
+	t.Parallel()
+
+	hc := newHealthcheck(t, false, "surfaced-version-123")
+	resp := hc.Check()
+
+	assert.Equal(t, "surfaced-version-123", resp.Version,
+		"version from globals must appear in the payload")
+	assert.Equal(t, "dnswatcher", resp.Appname)
+}

internal/logger/logger_test.go

@@ -0,0 +1,66 @@
+package logger_test
+
+import (
+	"context"
+	"log/slog"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"sneak.berlin/go/dnswatcher/internal/globals"
+	"sneak.berlin/go/dnswatcher/internal/logger"
+)
+
+func newTestLogger(t *testing.T) *logger.Logger {
+	t.Helper()
+
+	g := &globals.Globals{Appname: "dnswatcher", Version: "test"}
+
+	l, err := logger.New(nil, logger.Params{Globals: g})
+	require.NoError(t, err)
+
+	return l
+}
+
+// TestNewReturnsUsableLogger checks that the constructor yields a
+// working *slog.Logger.
+func TestNewReturnsUsableLogger(t *testing.T) {
+	t.Parallel()
+
+	l := newTestLogger(t)
+	require.NotNil(t, l.Get(), "Get must return a non-nil logger")
+}
+
+// TestDefaultLevelExcludesDebug verifies the default configuration
+// logs at info: debug records are suppressed, info records pass.
+func TestDefaultLevelExcludesDebug(t *testing.T) {
+	t.Parallel()
+
+	log := newTestLogger(t).Get()
+	ctx := context.Background()
+
+	assert.False(t, log.Enabled(ctx, slog.LevelDebug),
+		"debug must be suppressed at the default level")
+	assert.True(t, log.Enabled(ctx, slog.LevelInfo),
+		"info must be enabled at the default level")
+}
+
+// TestEnableDebugLoggingChangesLevel verifies the debug and non-debug
+// configurations differ as intended: enabling debug makes debug
+// records pass where they previously did not.
+func TestEnableDebugLoggingChangesLevel(t *testing.T) {
+	t.Parallel()
+
+	l := newTestLogger(t)
+	log := l.Get()
+	ctx := context.Background()
+
+	require.False(t, log.Enabled(ctx, slog.LevelDebug),
+		"debug must start disabled")
+
+	l.EnableDebugLogging()
+
+	assert.True(t, log.Enabled(ctx, slog.LevelDebug),
+		"debug must be enabled after EnableDebugLogging")
+}

internal/middleware/middleware.go

@@ -21,6 +21,60 @@ import (
 // corsMaxAge is the maximum age for CORS preflight responses.
 const corsMaxAge = 300
 
+// Security response header values applied to every response.
+//
+// The CSP is as strict as the dashboard allows: the template ships no
+// JavaScript, no inline styles, no inline event handlers and no images,
+// and its only subresource is the embedded stylesheet at
+// /s/css/tailwind.min.css, which style-src 'self' permits. Neither
+// unsafe-inline nor unsafe-eval is used. frame-ancestors 'none' is the
+// primary anti-framing control; X-Frame-Options is the legacy fallback.
+const (
+	// hstsValue is emitted unconditionally, including over plain HTTP,
+	// because the service runs behind a TLS-terminating proxy and the
+	// browser must still enforce HTTPS end to end.
+	hstsValue = "max-age=31536000; includeSubDomains"
+
+	cspValue = "default-src 'self'; " +
+		"script-src 'none'; " +
+		"style-src 'self'; " +
+		"img-src 'self'; " +
+		"font-src 'none'; " +
+		"connect-src 'none'; " +
+		"object-src 'none'; " +
+		"base-uri 'none'; " +
+		"form-action 'none'; " +
+		"frame-ancestors 'none'"
+
+	frameOptionsValue = "DENY"
+
+	contentTypeOptionsValue = "nosniff"
+
+	// referrerPolicyValue is stricter than the policy minimum of
+	// strict-origin-when-cross-origin: the dashboard has no
+	// cross-origin navigation needs and its URL may name internal
+	// hosts.
+	referrerPolicyValue = "no-referrer"
+
+	permissionsPolicyValue = "accelerometer=(), " +
+		"autoplay=(), " +
+		"camera=(), " +
+		"display-capture=(), " +
+		"encrypted-media=(), " +
+		"fullscreen=(), " +
+		"geolocation=(), " +
+		"gyroscope=(), " +
+		"magnetometer=(), " +
+		"microphone=(), " +
+		"midi=(), " +
+		"payment=(), " +
+		"picture-in-picture=(), " +
+		"publickey-credentials-get=(), " +
+		"screen-wake-lock=(), " +
+		"usb=(), " +
+		"xr-spatial-tracking=()"
+)
+
 // Params contains dependencies for Middleware.
 type Params struct {
 	fx.In
@@ -186,6 +240,37 @@ func (m *Middleware) CORS() func(http.Handler) http.Handler {
 	})
 }
 
+// SecurityHeaders returns middleware that sets the security response
+// headers required for production internet exposure on every response.
+//
+// The headers are set before the request reaches the next handler so
+// that they are present on every response, including panics recovered
+// by chi's Recoverer and timeouts produced by chi's Timeout.
+func (m *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(
+			writer http.ResponseWriter,
+			request *http.Request,
+		) {
+			header := writer.Header()
+			header.Set("Strict-Transport-Security", hstsValue)
+			header.Set("Content-Security-Policy", cspValue)
+			header.Set("X-Frame-Options", frameOptionsValue)
+			header.Set(
+				"X-Content-Type-Options",
+				contentTypeOptionsValue,
+			)
+			header.Set("Referrer-Policy", referrerPolicyValue)
+			header.Set(
+				"Permissions-Policy",
+				permissionsPolicyValue,
+			)
+
+			next.ServeHTTP(writer, request)
+		})
+	}
+}
+
 // MetricsAuth returns basic auth middleware for /metrics.
 func (m *Middleware) MetricsAuth() func(http.Handler) http.Handler {
 	if m.params.Config.MetricsUsername == "" {

internal/middleware/middleware_test.go

@@ -0,0 +1,334 @@
+package middleware_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+	"go.uber.org/fx/fxtest"
+
+	"sneak.berlin/go/dnswatcher/internal/config"
+	"sneak.berlin/go/dnswatcher/internal/globals"
+	"sneak.berlin/go/dnswatcher/internal/handlers"
+	"sneak.berlin/go/dnswatcher/internal/logger"
+	"sneak.berlin/go/dnswatcher/internal/middleware"
+	"sneak.berlin/go/dnswatcher/internal/notify"
+	"sneak.berlin/go/dnswatcher/internal/state"
+)
+
+// Expected security header values, spelled out literally so that any
+// change to the middleware has to be made deliberately here as well.
+const (
+	wantHSTS = "max-age=31536000; includeSubDomains"
+
+	wantCSP = "default-src 'self'; " +
+		"script-src 'none'; " +
+		"style-src 'self'; " +
+		"img-src 'self'; " +
+		"font-src 'none'; " +
+		"connect-src 'none'; " +
+		"object-src 'none'; " +
+		"base-uri 'none'; " +
+		"form-action 'none'; " +
+		"frame-ancestors 'none'"
+
+	wantFrameOptions = "DENY"
+
+	wantContentTypeOptions = "nosniff"
+
+	wantReferrerPolicy = "no-referrer"
+
+	wantPermissionsPolicy = "accelerometer=(), " +
+		"autoplay=(), " +
+		"camera=(), " +
+		"display-capture=(), " +
+		"encrypted-media=(), " +
+		"fullscreen=(), " +
+		"geolocation=(), " +
+		"gyroscope=(), " +
+		"magnetometer=(), " +
+		"microphone=(), " +
+		"midi=(), " +
+		"payment=(), " +
+		"picture-in-picture=(), " +
+		"publickey-credentials-get=(), " +
+		"screen-wake-lock=(), " +
+		"usb=(), " +
+		"xr-spatial-tracking=()"
+)
+
+// stylesheetPath is the only subresource the dashboard loads.
+const stylesheetPath = "/s/css/tailwind.min.css"
+
+// newTestLogger builds a logger for direct component construction.
+func newTestLogger(t *testing.T) *logger.Logger {
+	t.Helper()
+
+	glob, err := globals.New(nil)
+	if err != nil {
+		t.Fatalf("globals.New: %v", err)
+	}
+
+	log, err := logger.New(nil, logger.Params{Globals: glob})
+	if err != nil {
+		t.Fatalf("logger.New: %v", err)
+	}
+
+	return log
+}
+
+// newTestMiddleware builds a Middleware without an fx application.
+func newTestMiddleware(t *testing.T) *middleware.Middleware {
+	t.Helper()
+
+	glob, err := globals.New(nil)
+	if err != nil {
+		t.Fatalf("globals.New: %v", err)
+	}
+
+	mw, err := middleware.New(nil, middleware.Params{
+		Logger:  newTestLogger(t),
+		Globals: glob,
+		Config:  &config.Config{},
+	})
+	if err != nil {
+		t.Fatalf("middleware.New: %v", err)
+	}
+
+	return mw
+}
+
+// serveWithSecurityHeaders runs a GET through SecurityHeaders and
+// returns the recorded response.
+func serveWithSecurityHeaders(
+	t *testing.T,
+	target string,
+	handler http.Handler,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	mw := newTestMiddleware(t)
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, target, nil,
+	)
+
+	mw.SecurityHeaders()(handler).ServeHTTP(rec, req)
+
+	return rec
+}
+
+// okHandler writes a trivial 200 response.
+func okHandler() http.Handler {
+	return http.HandlerFunc(func(
+		writer http.ResponseWriter,
+		_ *http.Request,
+	) {
+		writer.WriteHeader(http.StatusOK)
+	})
+}
+
+func TestSecurityHeaders(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name   string
+		header string
+		want   string
+	}{
+		{
+			"hsts",
+			"Strict-Transport-Security",
+			wantHSTS,
+		},
+		{
+			"csp",
+			"Content-Security-Policy",
+			wantCSP,
+		},
+		{
+			"frame options",
+			"X-Frame-Options",
+			wantFrameOptions,
+		},
+		{
+			"content type options",
+			"X-Content-Type-Options",
+			wantContentTypeOptions,
+		},
+		{
+			"referrer policy",
+			"Referrer-Policy",
+			wantReferrerPolicy,
+		},
+		{
+			"permissions policy",
+			"Permissions-Policy",
+			wantPermissionsPolicy,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			rec := serveWithSecurityHeaders(t, "/", okHandler())
+
+			got := rec.Header().Get(tt.header)
+			if got != tt.want {
+				t.Errorf(
+					"%s = %q, want %q",
+					tt.header, got, tt.want,
+				)
+			}
+		})
+	}
+}
+
+// TestSecurityHeadersCSPDirectives guards the properties the repo
+// policy requires of the content security policy itself.
+func TestSecurityHeadersCSPDirectives(t *testing.T) {
+	t.Parallel()
+
+	rec := serveWithSecurityHeaders(t, "/", okHandler())
+	csp := rec.Header().Get("Content-Security-Policy")
+
+	forbidden := []string{"unsafe-inline", "unsafe-eval"}
+	for _, directive := range forbidden {
+		if strings.Contains(csp, directive) {
+			t.Errorf("CSP must not contain %q: %q", directive, csp)
+		}
+	}
+
+	required := []string{
+		"default-src 'self'",
+		"script-src 'none'",
+		"style-src 'self'",
+		"frame-ancestors 'none'",
+	}
+	for _, directive := range required {
+		if !strings.Contains(csp, directive) {
+			t.Errorf("CSP must contain %q: %q", directive, csp)
+		}
+	}
+}
+
+// TestSecurityHeadersOnErrorResponse verifies the headers are emitted
+// even when the wrapped handler fails, since they are set before the
+// handler runs.
+func TestSecurityHeadersOnErrorResponse(t *testing.T) {
+	t.Parallel()
+
+	failing := http.HandlerFunc(func(
+		writer http.ResponseWriter,
+		_ *http.Request,
+	) {
+		http.Error(
+			writer,
+			"boom",
+			http.StatusInternalServerError,
+		)
+	})
+
+	rec := serveWithSecurityHeaders(t, "/api/v1/status", failing)
+
+	if rec.Code != http.StatusInternalServerError {
+		t.Fatalf("status = %d, want 500", rec.Code)
+	}
+
+	if got := rec.Header().Get(
+		"X-Content-Type-Options",
+	); got != wantContentTypeOptions {
+		t.Errorf(
+			"X-Content-Type-Options = %q, want %q",
+			got, wantContentTypeOptions,
+		)
+	}
+
+	if got := rec.Header().Get(
+		"Strict-Transport-Security",
+	); got != wantHSTS {
+		t.Errorf(
+			"Strict-Transport-Security = %q, want %q",
+			got, wantHSTS,
+		)
+	}
+}
+
+// newTestHandlers builds real Handlers with empty monitoring state.
+func newTestHandlers(t *testing.T) *handlers.Handlers {
+	t.Helper()
+
+	glob, err := globals.New(nil)
+	if err != nil {
+		t.Fatalf("globals.New: %v", err)
+	}
+
+	log := newTestLogger(t)
+
+	notifier, err := notify.New(fxtest.NewLifecycle(t), notify.Params{
+		Logger: log,
+		Config: &config.Config{},
+	})
+	if err != nil {
+		t.Fatalf("notify.New: %v", err)
+	}
+
+	hnd, err := handlers.New(nil, handlers.Params{
+		Logger:  log,
+		Globals: glob,
+		State:   state.NewForTest(),
+		Notify:  notifier,
+	})
+	if err != nil {
+		t.Fatalf("handlers.New: %v", err)
+	}
+
+	return hnd
+}
+
+// TestDashboardRendersWithSecurityHeaders renders the real dashboard
+// through the middleware and checks that the policy still permits the
+// one stylesheet the page loads.
+func TestDashboardRendersWithSecurityHeaders(t *testing.T) {
+	t.Parallel()
+
+	mw := newTestMiddleware(t)
+	hnd := newTestHandlers(t)
+
+	router := chi.NewRouter()
+	router.Use(mw.SecurityHeaders())
+	router.Get("/", hnd.HandleDashboard())
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/", nil,
+	)
+
+	router.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rec.Code)
+	}
+
+	body := rec.Body.String()
+	if !strings.Contains(body, stylesheetPath) {
+		t.Errorf("dashboard does not reference %q", stylesheetPath)
+	}
+
+	if !strings.Contains(body, "dnswatcher") {
+		t.Errorf("dashboard body looks empty: %d bytes", len(body))
+	}
+
+	csp := rec.Header().Get("Content-Security-Policy")
+	if csp != wantCSP {
+		t.Errorf("CSP = %q, want %q", csp, wantCSP)
+	}
+
+	// The stylesheet is same-origin, so style-src 'self' allows it.
+	if !strings.Contains(csp, "style-src 'self'") {
+		t.Errorf("CSP would block %q: %q", stylesheetPath, csp)
+	}
+}

internal/server/routes.go

@@ -21,6 +21,7 @@ func (s *Server) SetupRoutes() {
 	// Global middleware
 	s.router.Use(chimw.Recoverer)
 	s.router.Use(chimw.RequestID)
+	s.router.Use(s.mw.SecurityHeaders())
 	s.router.Use(s.mw.Logging())
 	s.router.Use(s.mw.CORS())
 	s.router.Use(chimw.Timeout(requestTimeout))

internal/state/state.go

@@ -148,6 +148,11 @@ func New(
 
 	lifecycle.Append(fx.Hook{
 		OnStart: func(_ context.Context) error {
+			err := state.checkDataDirWritable()
+			if err != nil {
+				return err
+			}
+
 			return state.Load()
 		},
 		OnStop: func(_ context.Context) error {
@@ -345,3 +350,27 @@ func (s *State) GetCertificateState(
 
 	return cs, ok
 }
+
+// checkDataDirWritable creates the data directory if needed, then writes
+// and removes the temp file that Save uses. It runs at startup so that an
+// unwritable directory stops the process, instead of the process running
+// with every save failing and only logged.
+func (s *State) checkDataDirWritable() error {
+	dir := s.config.DataDir
+	tmpPath := s.config.StatePath() + ".tmp"
+
+	err := os.MkdirAll(dir, dirPermissions)
+	if err == nil {
+		err = os.WriteFile(tmpPath, nil, filePermissions)
+	}
+
+	if err == nil {
+		err = os.Remove(tmpPath)
+	}
+
+	if err != nil {
+		return fmt.Errorf("data directory %s is not writable: %w", dir, err)
+	}
+
+	return nil
+}

internal/state/state_test.go

@@ -4,10 +4,16 @@ import (
 	"encoding/json"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync"
 	"testing"
 	"time"
 
+	"go.uber.org/fx/fxtest"
+
+	"sneak.berlin/go/dnswatcher/internal/config"
+	"sneak.berlin/go/dnswatcher/internal/globals"
+	"sneak.berlin/go/dnswatcher/internal/logger"
 	"sneak.berlin/go/dnswatcher/internal/state"
 )
 
@@ -493,6 +499,82 @@ func TestSaveWritePermissionError(t *testing.T) {
 	}
 }
 
+// startState builds a State through the real constructor and runs its
+// startup hook against dataDir, returning the startup error.
+func startState(t *testing.T, dataDir string) error {
+	t.Helper()
+
+	g, err := globals.New(nil)
+	if err != nil {
+		t.Fatalf("globals.New: %v", err)
+	}
+
+	log, err := logger.New(nil, logger.Params{Globals: g})
+	if err != nil {
+		t.Fatalf("logger.New: %v", err)
+	}
+
+	lifecycle := fxtest.NewLifecycle(t)
+
+	_, err = state.New(lifecycle, state.Params{
+		Logger: log,
+		Config: &config.Config{DataDir: dataDir},
+	})
+	if err != nil {
+		t.Fatalf("state.New: %v", err)
+	}
+
+	return lifecycle.Start(t.Context())
+}
+
+// TestStartupFailsWhenDataDirNotWritable verifies that startup stops
+// with an error naming the data directory when it cannot be written.
+// The directory's parent is a regular file, which also fails as root.
+func TestStartupFailsWhenDataDirNotWritable(t *testing.T) {
+	t.Parallel()
+
+	parent := filepath.Join(t.TempDir(), "file")
+
+	err := os.WriteFile(parent, nil, 0o600)
+	if err != nil {
+		t.Fatalf("writing file: %v", err)
+	}
+
+	dataDir := filepath.Join(parent, "data")
+
+	err = startState(t, dataDir)
+	if err == nil {
+		t.Fatal("startup should fail when the data directory is not writable")
+	}
+
+	want := "data directory " + dataDir + " is not writable"
+	if !strings.Contains(err.Error(), want) {
+		t.Errorf("startup error %q does not contain %q", err, want)
+	}
+}
+
+// TestStartupCreatesDataDir verifies that startup creates a missing
+// data directory and leaves nothing behind in it.
+func TestStartupCreatesDataDir(t *testing.T) {
+	t.Parallel()
+
+	dataDir := filepath.Join(t.TempDir(), "data")
+
+	err := startState(t, dataDir)
+	if err != nil {
+		t.Fatalf("startup error: %v", err)
+	}
+
+	entries, err := os.ReadDir(dataDir)
+	if err != nil {
+		t.Fatalf("reading data directory: %v", err)
+	}
+
+	if len(entries) != 0 {
+		t.Errorf("startup left %d entries in the data directory", len(entries))
+	}
+}
+
 // TestPortStateUnmarshalJSON_NewFormat verifies deserialization of the
 // current multi-hostname format.
 func TestPortStateUnmarshalJSON_NewFormat(t *testing.T) {