From 4c2932d6d67a553e46f0435b9953487efa9d4bbb Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Fri, 2 Oct 2026 00:19:30 +0200
Subject: [PATCH] fmt: format and check Markdown with prettier in a container
 (closes #119)

make fmt and make fmt-check now cover every Markdown file with prettier
(4-space tabs, proseWrap always), as template-app-go does: prettier is
pinned by package.json and yarn.lock and runs in a docker build on a
digest-pinned node image, never on the host. The check is forced to run
with --no-cache-filter, as script/lint is.

script/fmt-check is split into a Go half and a Markdown half because the
Dockerfile lint stage cannot run docker: that stage now runs the Go half
and script/cibuild runs the Markdown half after the build. *.md leaves
.dockerignore so documents reach the build context.

README.md, TESTING.md and TODO.md are reformatted by make fmt; apart from
the README Entrypoints entries, its make fmt line under Building and the
TODO entry, that diff is mechanical.

Model: opus-5-5
---
 .dockerignore             |   5 +-
 .gitignore                |   1 +
 .prettierignore           |   5 +
 .prettierrc               |   4 +
 Dockerfile                |   6 +-
 Dockerfile.fmt            |  55 +++
 README.md                 | 692 +++++++++++++++++++-------------------
 TESTING.md                |  53 ++-
 TODO.md                   |  21 +-
 package.json              |   9 +
 script/bootstrap          |  14 +-
 script/cibuild            |  13 +-
 script/fmt                |  50 ++-
 script/fmt-check          |  23 +-
 script/fmt-check-go       |  31 ++
 script/fmt-check-markdown |  29 ++
 yarn.lock                 |   8 +
 17 files changed, 602 insertions(+), 417 deletions(-)
 create mode 100644 .prettierignore
 create mode 100644 .prettierrc
 create mode 100644 Dockerfile.fmt
 create mode 100644 package.json
 create mode 100755 script/fmt-check-go
 create mode 100755 script/fmt-check-markdown
 create mode 100644 yarn.lock

diff --git a/.dockerignore b/.dockerignore
index bae18ae..0e93e56 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -1,6 +1,9 @@
 .git/
 bin/
-*.md
+node_modules/
+# No .md may be excluded: Dockerfile.fmt checks every document with
+# prettier, and an exclusion here would drop a file from that check while
+# prettier still reports every file it was handed clean.
 LICENSE
 .editorconfig
 .gitignore
diff --git a/.gitignore b/.gitignore
index 9c22cd8..d0077e3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 bin/
+node_modules/
 vendor/
 data/
 .env
diff --git a/.prettierignore b/.prettierignore
new file mode 100644
index 0000000..c8d365b
--- /dev/null
+++ b/.prettierignore
@@ -0,0 +1,5 @@
+bin/
+data/
+node_modules/
+.claude/
+static/css/tailwind.min.css
diff --git a/.prettierrc b/.prettierrc
new file mode 100644
index 0000000..8af31cd
--- /dev/null
+++ b/.prettierrc
@@ -0,0 +1,4 @@
+{
+    "tabWidth": 4,
+    "proseWrap": "always"
+}
diff --git a/Dockerfile b/Dockerfile
index a76cbb6..c289c07 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,7 +1,9 @@
 # Lint stage - fast feedback on lint issues, before the build starts.
 # The linter is invoked directly rather than through `make lint`: that
 # target shells out to `docker build -f Dockerfile.lint`, and there is
-# no docker daemon inside a docker build.
+# no docker daemon inside a docker build. For the same reason this stage
+# runs only the Go half of `make fmt-check`; script/cibuild runs the
+# markdown half after this build.
 # script/cibuild and script/docker name this stage in --no-cache-filter.
 # golangci/golangci-lint:v2.12.2 (Debian-based), 2026-08-10
 FROM golangci/golangci-lint:v2.12.2@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240 AS lint
@@ -12,7 +14,7 @@ RUN go mod download
 
 COPY . .
 
-RUN make fmt-check
+RUN script/fmt-check-go
 RUN golangci-lint run --config .golangci.yml ./...
 
 # Build stage
diff --git a/Dockerfile.fmt b/Dockerfile.fmt
new file mode 100644
index 0000000..be7d0ba
--- /dev/null
+++ b/Dockerfile.fmt
@@ -0,0 +1,55 @@
+# prettier over the markdown, in a container, so it is never installed
+# on the host. script/fmt-check-markdown builds the fmt-check stage;
+# script/fmt builds fmt-out and takes the formatted files back.
+
+# node:22-bookworm-slim, 2026-09-05
+FROM node:22-bookworm-slim@sha256:83f487e0a63425e5b4d146fb5e5be574bcbe1b7b843d3ebafdd95eaf7767a7e5 AS nodedeps
+
+# prettier lives outside /src so that a `COPY . .` of the repo cannot
+# overwrite it, and so that node_modules never appears in the tree
+# prettier is about to walk.
+WORKDIR /tools
+
+# package.json pins the version and yarn.lock pins the bytes:
+# --frozen-lockfile installs exactly the lockfile's resolution and fails
+# if package.json disagrees with it, so the tool cannot float between
+# runs. yarn is the one in the image above.
+COPY package.json yarn.lock ./
+RUN yarn install --frozen-lockfile --non-interactive --no-progress
+
+ENV PATH="/tools/node_modules/.bin:${PATH}"
+
+WORKDIR /src
+
+# Read-only markdown check. Must match $stage in
+# script/fmt-check-markdown.
+FROM nodedeps AS fmt-check
+
+COPY . .
+
+# --config, not discovery: a .prettierrc that failed to arrive would
+# otherwise leave prettier on its defaults, where proseWrap is "preserve"
+# and every wrap this check exists to enforce passes. Missing the file is
+# a hard error instead. --no-editorconfig for the same reason in reverse:
+# .editorconfig is not in the build context, so honouring it here and on
+# a developer's machine would be two different answers.
+RUN prettier --config .prettierrc --no-editorconfig --check "**/*.md"
+
+# Write path. Not a check: script/fmt builds this and takes the files.
+FROM nodedeps AS fmt
+
+COPY . .
+
+RUN prettier --config .prettierrc --no-editorconfig --write "**/*.md"
+
+# Only the markdown leaves, with its paths intact, so that the export
+# below cannot put anything else back over the caller's working tree.
+RUN mkdir -p /out && cd /src && \
+    find . -name '*.md' -type f -exec cp --parents '{}' /out/ ';'
+
+# Export target: `docker build --target fmt-out --output type=local`
+# writes /out's tree into a directory on the client, which is how
+# script/fmt gets formatted markdown back without a bind mount.
+# Must match $stage in script/fmt.
+FROM scratch AS fmt-out
+COPY --from=fmt /out/ /
diff --git a/README.md b/README.md
index 4e8c13d..c5461fb 100644
--- a/README.md
+++ b/README.md
@@ -1,16 +1,20 @@
 # dnswatcher
 
-dnswatcher is an MIT-licensed, pre-1.0 Go daemon by [@sneak](https://sneak.berlin) that monitors DNS records, TCP port availability, and TLS certificates, delivering real-time change notifications via Slack, Mattermost, and ntfy webhooks.
+dnswatcher is an MIT-licensed, pre-1.0 Go daemon by
+[@sneak](https://sneak.berlin) that monitors DNS records, TCP port availability,
+and TLS certificates, delivering real-time change notifications via Slack,
+Mattermost, and ntfy webhooks.
 
-> ⚠️ Pre-1.0 software. APIs, configuration, and behavior may change without notice.
+> ⚠️ Pre-1.0 software. APIs, configuration, and behavior may change without
+> notice.
 
-dnswatcher watches configured DNS domains and hostnames for changes, monitors TCP
-port availability, tracks TLS certificate expiry, and delivers real-time
+dnswatcher watches configured DNS domains and hostnames for changes, monitors
+TCP port availability, tracks TLS certificate expiry, and delivers real-time
 notifications via Slack, Mattermost, and/or ntfy webhooks.
 
 It performs all DNS resolution itself via iterative (non-recursive) queries,
-tracing from root nameservers to authoritative servers directly—never relying
-on upstream recursive resolvers.
+tracing from root nameservers to authoritative servers directly—never relying on
+upstream recursive resolvers.
 
 State is persisted to a local JSON file so that monitoring survives restarts
 without requiring an external database.
@@ -19,21 +23,20 @@ without requiring an external database.
 
 ## No DNS mocking. Ever.
 
-**DNS is never mocked in this project — not in tests, not anywhere else.**
-No mock resolvers, no fake DNS servers, no stubbed lookups.
+**DNS is never mocked in this project — not in tests, not anywhere else.** No
+mock resolvers, no fake DNS servers, no stubbed lookups.
 
-dnswatcher's entire purpose is correct behavior against the real DNS.
-Tests exercise real iterative resolution against live nameservers by
-design; a test suite that passes against a mock proves nothing about the
-one thing this program exists to do.
+dnswatcher's entire purpose is correct behavior against the real DNS. Tests
+exercise real iterative resolution against live nameservers by design; a test
+suite that passes against a mock proves nothing about the one thing this program
+exists to do.
 
-When live tests are flaky, that is a robustness problem, and it gets
-fixed with robustness: retries with backoff, querying multiple
-independent nameservers, longer timeouts — or explicit opt-in gating
-decided by the project owner. Never with mocks.
+When live tests are flaky, that is a robustness problem, and it gets fixed with
+robustness: retries with backoff, querying multiple independent nameservers,
+longer timeouts — or explicit opt-in gating decided by the project owner. Never
+with mocks.
 
-Contributions that introduce mocked, faked, or stubbed DNS will be
-rejected.
+Contributions that introduce mocked, faked, or stubbed DNS will be rejected.
 
 ---
 
@@ -46,126 +49,123 @@ rejected.
 - Every **1 hour**, performs a full iterative trace from root servers to
   discover all authoritative nameservers (NS records) for each domain.
 - Queries **every** discovered authoritative nameserver independently.
-- Stores the NS record set as observed by the delegation chain, and the
-  IPv4 and IPv6 addresses each nameserver's name resolves to.
+- Stores the NS record set as observed by the delegation chain, and the IPv4 and
+  IPv6 addresses each nameserver's name resolves to.
 - Any change triggers a notification:
-  - NS added to or removed from the delegation.
-  - NS address change: a nameserver that stays in the delegation
-    resolves to different addresses than on the previous check. A
-    nameserver added or removed gets only the NS change notification.
-    When the lookup of a nameserver's addresses fails or finds none,
-    its previous addresses are kept and nothing is sent.
+    - NS added to or removed from the delegation.
+    - NS address change: a nameserver that stays in the delegation resolves to
+      different addresses than on the previous check. A nameserver added or
+      removed gets only the NS change notification. When the lookup of a
+      nameserver's addresses fails or finds none, its previous addresses are
+      kept and nothing is sent.
 
 ### DNS Hostname Monitoring (Subdomains)
 
-- Accepts a list of DNS hostnames (subdomains, distinguished from apex
-  domains via the Public Suffix List).
+- Accepts a list of DNS hostnames (subdomains, distinguished from apex domains
+  via the Public Suffix List).
 - Every **1 hour**, performs a full iterative trace to discover the
-  authoritative nameservers of the zone the hostname is in, which is not
-  always its last two labels (a name under `co.uk`, or in a delegated
-  subdomain).
-- Queries **each** authoritative nameserver independently for **all**
-  record types: A, AAAA, CNAME, MX, TXT, SRV, CAA, NS.
-- Stores results **per nameserver**. The state for a hostname is not a
-  merged view — it is a map from nameserver to record set.
-- DNS names inside record values (CNAME, MX, SRV and NS targets) are
-  stored in lower case, because names are case-insensitive and
-  nameservers may answer in any letter case. TXT and CAA values keep
-  their letter case; they are not lower-cased.
-- Any observable change in any nameserver's response triggers a
-  notification. This includes:
-  - **Record change**: A nameserver returns different records than it
-    did on the previous check (additions, removals, value changes).
-  - **NS query failure**: A nameserver that previously responded
-    becomes unreachable (timeout, SERVFAIL, REFUSED, network error).
-    This is distinct from "responded with no records": a nameserver
-    that answers NXDOMAIN or with no records has responded. The alert
-    is sent once, on the check where it starts failing. A failing
-    nameserver gives no records, so it is not reported as a record
-    change or compared for inconsistency. A nameserver that is already
-    failing on the first check that sees it is recorded silently.
-  - **NS recovery**: A previously-unreachable nameserver starts
-    responding again. Its records are not compared with those from
-    before it failed, so a change made while it was failing is not
-    reported as a record change.
-  - **Inconsistency detected**: Two nameservers return different record
-    sets for the same hostname and did not already differ on the previous
-    check. Every pair of nameservers is compared. The alert is sent once
-    for each such pair, on the check where they start to disagree, and not
-    again while they keep disagreeing, including after a restart. A
-    nameserver that was not in the previous check (newly added, or back
-    after dropping out), or failed on it, and answers differently is
-    reported on the check where it answers. If a pair agrees again and
-    later disagrees, the alert is sent again.
+  authoritative nameservers of the zone the hostname is in, which is not always
+  its last two labels (a name under `co.uk`, or in a delegated subdomain).
+- Queries **each** authoritative nameserver independently for **all** record
+  types: A, AAAA, CNAME, MX, TXT, SRV, CAA, NS.
+- Stores results **per nameserver**. The state for a hostname is not a merged
+  view — it is a map from nameserver to record set.
+- DNS names inside record values (CNAME, MX, SRV and NS targets) are stored in
+  lower case, because names are case-insensitive and nameservers may answer in
+  any letter case. TXT and CAA values keep their letter case; they are not
+  lower-cased.
+- Any observable change in any nameserver's response triggers a notification.
+  This includes:
+    - **Record change**: A nameserver returns different records than it did on
+      the previous check (additions, removals, value changes).
+    - **NS query failure**: A nameserver that previously responded becomes
+      unreachable (timeout, SERVFAIL, REFUSED, network error). This is distinct
+      from "responded with no records": a nameserver that answers NXDOMAIN or
+      with no records has responded. The alert is sent once, on the check where
+      it starts failing. A failing nameserver gives no records, so it is not
+      reported as a record change or compared for inconsistency. A nameserver
+      that is already failing on the first check that sees it is recorded
+      silently.
+    - **NS recovery**: A previously-unreachable nameserver starts responding
+      again. Its records are not compared with those from before it failed, so a
+      change made while it was failing is not reported as a record change.
+    - **Inconsistency detected**: Two nameservers return different record sets
+      for the same hostname and did not already differ on the previous check.
+      Every pair of nameservers is compared. The alert is sent once for each
+      such pair, on the check where they start to disagree, and not again while
+      they keep disagreeing, including after a restart. A nameserver that was
+      not in the previous check (newly added, or back after dropping out), or
+      failed on it, and answers differently is reported on the check where it
+      answers. If a pair agrees again and later disagrees, the alert is sent
+      again.
 
 ### TCP Port Monitoring
 
-- For every configured domain and hostname, constructs a deduplicated list
-  of all IPv4 and IPv6 addresses resolved via A, AAAA, and CNAME chain
-  resolution across all authoritative nameservers.
+- For every configured domain and hostname, constructs a deduplicated list of
+  all IPv4 and IPv6 addresses resolved via A, AAAA, and CNAME chain resolution
+  across all authoritative nameservers.
 - Checks TCP connectivity on ports **80** and **443** for each IP address.
 - Every **1 hour**, re-checks all ports.
 - Any change in port availability triggers a notification:
-  - Port transitioned from open to closed (or vice versa).
-  - New IP appeared (from DNS change) and its port state was recorded.
-  - IP disappeared (from DNS change) — noted in the DNS change
-    notification; port state for that IP is removed.
+    - Port transitioned from open to closed (or vice versa).
+    - New IP appeared (from DNS change) and its port state was recorded.
+    - IP disappeared (from DNS change) — noted in the DNS change notification;
+      port state for that IP is removed.
 
 ### TLS Certificate Monitoring
 
-- Every **12 hours**, for each IP address listening on port 443, connects
-  via TLS using the correct SNI hostname.
+- Every **12 hours**, for each IP address listening on port 443, connects via
+  TLS using the correct SNI hostname.
 - Records the certificate's Subject CN, SANs, issuer, and expiry date.
 - Any change triggers a notification:
-  - Certificate is expiring within **7 days** (warning, repeated each
-    check until renewed or expired).
-  - Certificate CN, issuer, or SANs changed (replacement detected,
-    reports old and new values).
-  - TLS connection failure to a previously-reachable IP:443 (handshake
-    error, timeout, connection refused after previously succeeding).
-  - TLS recovery: a previously-failing IP:443 now completes a
-    handshake again.
+    - Certificate is expiring within **7 days** (warning, repeated each check
+      until renewed or expired).
+    - Certificate CN, issuer, or SANs changed (replacement detected, reports old
+      and new values).
+    - TLS connection failure to a previously-reachable IP:443 (handshake error,
+      timeout, connection refused after previously succeeding).
+    - TLS recovery: a previously-failing IP:443 now completes a handshake again.
 
 ### Notifications
 
 **Every observable state change produces a notification.** dnswatcher is
-designed as a real-time change feed — degradations, failures, recoveries,
-and routine changes are all reported equally.
+designed as a real-time change feed — degradations, failures, recoveries, and
+routine changes are all reported equally.
 
 Supported notification backends:
 
-| Backend        | Configuration            | Payload Format               |
-|----------------|--------------------------|------------------------------|
-| **Slack**      | Incoming Webhook URL     | Attachments with color       |
-| **Mattermost** | Incoming Webhook URL     | Slack-compatible attachments |
-| **ntfy**       | Topic URL (e.g. `https://ntfy.sh/mytopic`) | Title + body + priority |
+| Backend        | Configuration                              | Payload Format               |
+| -------------- | ------------------------------------------ | ---------------------------- |
+| **Slack**      | Incoming Webhook URL                       | Attachments with color       |
+| **Mattermost** | Incoming Webhook URL                       | Slack-compatible attachments |
+| **ntfy**       | Topic URL (e.g. `https://ntfy.sh/mytopic`) | Title + body + priority      |
 
 All configured endpoints receive every notification. Notification content
 includes:
 
-- **DNS record changes**: Which hostname, which nameserver, what record
-  type, old values, new values.
+- **DNS record changes**: Which hostname, which nameserver, what record type,
+  old values, new values.
 - **DNS NS changes**: Which domain, which nameservers were added/removed.
-- **NS address changes**: Which domain, which nameserver, its old and
-  new addresses.
-- **NS query failures**: Which nameserver failed, error type (timeout,
-  SERVFAIL, REFUSED, network error), which hostname/domain affected.
+- **NS address changes**: Which domain, which nameserver, its old and new
+  addresses.
+- **NS query failures**: Which nameserver failed, error type (timeout, SERVFAIL,
+  REFUSED, network error), which hostname/domain affected.
 - **NS recoveries**: Which nameserver recovered, which hostname/domain.
-- **NS inconsistencies**: Which nameservers disagree, what each one
-  returned, which hostname affected.
+- **NS inconsistencies**: Which nameservers disagree, what each one returned,
+  which hostname affected.
 - **Port changes**: Which IP:port, old state, new state, all associated
   hostnames.
-- **TLS expiry warnings**: Which certificate, days remaining, CN,
-  issuer, associated hostname and IP.
-- **TLS certificate changes**: Old and new CN/issuer/SANs, associated
-  hostname and IP.
+- **TLS expiry warnings**: Which certificate, days remaining, CN, issuer,
+  associated hostname and IP.
+- **TLS certificate changes**: Old and new CN/issuer/SANs, associated hostname
+  and IP.
 - **TLS connection failures/recoveries**: Which IP:port, error details,
   associated hostname.
 
 ### State Management
 
-- All monitoring state is kept in memory and persisted to a JSON file on
-  disk (`DATA_DIR/state.json`).
+- All monitoring state is kept in memory and persisted to a JSON file on disk
+  (`DATA_DIR/state.json`).
 - State is loaded on startup to resume monitoring without triggering
   false-positive change notifications.
 - State is written atomically (write to temp file, then rename) to prevent
@@ -173,41 +173,39 @@ includes:
 
 ### Web Dashboard
 
-dnswatcher includes an unauthenticated, read-only web dashboard at the
-root URL (`/`). It displays:
+dnswatcher includes an unauthenticated, read-only web dashboard at the root URL
+(`/`). It displays:
 
-- **Summary counts** for monitored domains, hostnames, ports, and
-  certificates.
+- **Summary counts** for monitored domains, hostnames, ports, and certificates.
 - **Domains** with their discovered nameservers.
 - **Hostnames** with per-nameserver DNS records and status.
 - **Ports** with open/closed state and associated hostnames.
 - **TLS certificates** with CN, issuer, expiry, and status.
-- **Recent alerts** (last 100 notifications sent since the process
-  started), displayed in reverse chronological order.
+- **Recent alerts** (last 100 notifications sent since the process started),
+  displayed in reverse chronological order.
 
-Every data point shows its age (e.g. "5m ago") so you can tell at a
-glance how fresh the information is. The page auto-refreshes every 30
-seconds.
+Every data point shows its age (e.g. "5m ago") so you can tell at a glance how
+fresh the information is. The page auto-refreshes every 30 seconds.
 
-The dashboard intentionally does not expose any configuration details
-such as webhook URLs, notification endpoints, or API tokens.
+The dashboard intentionally does not expose any configuration details such as
+webhook URLs, notification endpoints, or API tokens.
 
-All assets (CSS) are embedded in the binary and served from the
-application itself. The dashboard makes zero external HTTP requests —
-no CDN dependencies or third-party resources are loaded at runtime.
+All assets (CSS) are embedded in the binary and served from the application
+itself. The dashboard makes zero external HTTP requests — no CDN dependencies or
+third-party resources are loaded at runtime.
 
 ### HTTP API
 
 dnswatcher exposes a lightweight HTTP API for operational visibility:
 
-| Endpoint                              | Description                    |
-|---------------------------------------|--------------------------------|
-| `GET /`                               | Web dashboard (HTML)           |
-| `GET /s/...`                          | Static assets (embedded CSS)   |
-| `GET /.well-known/healthcheck`        | Health check (JSON)            |
-| `GET /health`                         | Health check (JSON, legacy)    |
-| `GET /api/v1/status`                  | Current monitoring state       |
-| `GET /metrics`                        | Prometheus metrics (optional)  |
+| Endpoint                       | Description                   |
+| ------------------------------ | ----------------------------- |
+| `GET /`                        | Web dashboard (HTML)          |
+| `GET /s/...`                   | Static assets (embedded CSS)  |
+| `GET /.well-known/healthcheck` | Health check (JSON)           |
+| `GET /health`                  | Health check (JSON, legacy)   |
+| `GET /api/v1/status`           | Current monitoring state      |
+| `GET /metrics`                 | Prometheus metrics (optional) |
 
 #### Server timeouts
 
@@ -216,7 +214,7 @@ constants in `internal/server/server.go`, not configurable via environment
 variables.
 
 | Timeout             | Value | Purpose                                       |
-|---------------------|-------|-----------------------------------------------|
+| ------------------- | ----- | --------------------------------------------- |
 | `ReadHeaderTimeout` | 10s   | Bounds the request header read (slowloris)    |
 | `ReadTimeout`       | 15s   | Bounds the whole request read, headers + body |
 | `WriteTimeout`      | 75s   | Bounds handler execution plus response flush  |
@@ -224,20 +222,20 @@ variables.
 
 These are distinct from the 60s per-request handler budget applied by
 `chimw.Timeout` in `internal/server/routes.go`, which cancels the request
-context but does not touch the socket. `WriteTimeout` is deliberately
-larger than that budget: the write deadline is armed once request headers
-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.
+context but does not touch the socket. `WriteTimeout` is deliberately larger
+than that budget: the write deadline is armed once request headers 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:
+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`                                |
@@ -254,21 +252,21 @@ 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`.
+`<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.
+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.
+`strict-origin-when-cross-origin` baseline: the dashboard has no cross-origin
+navigation needs, and its URL may name internal hosts.
 
 ---
 
@@ -301,23 +299,23 @@ internal/
 ### Design Principles
 
 - **No recursive resolvers**: All DNS resolution is performed iteratively,
-  tracing from root nameservers through the delegation chain to
-  authoritative servers.
+  tracing from root nameservers through the delegation chain to authoritative
+  servers.
 - **No external database**: State is persisted as a single JSON file.
 - **Dependency injection**: All components are wired via
   [uber/fx](https://github.com/uber-go/fx).
-- **Structured logging**: All logs use `log/slog` with JSON output in
-  production (TTY detection for development).
-- **Graceful shutdown**: All background goroutines respect context
-  cancellation and the fx lifecycle. In-flight notification deliveries
-  are drained on shutdown, bounded by the shutdown timeout.
+- **Structured logging**: All logs use `log/slog` with JSON output in production
+  (TTY detection for development).
+- **Graceful shutdown**: All background goroutines respect context cancellation
+  and the fx lifecycle. In-flight notification deliveries are drained on
+  shutdown, bounded by the shutdown timeout.
 
 ---
 
 ## Configuration
 
-Configuration is loaded via [Viper](https://github.com/spf13/viper) with
-the following precedence (highest to lowest):
+Configuration is loaded via [Viper](https://github.com/spf13/viper) with the
+following precedence (highest to lowest):
 
 1. Environment variables (prefixed with `DNSWATCHER_`)
 2. `.env` file (loaded via godotenv)
@@ -327,46 +325,45 @@ the following precedence (highest to lowest):
 
 ### Environment Variables
 
-| Variable                        | Description                                | Default     |
-|---------------------------------|--------------------------------------------|-------------|
-| `PORT`                          | HTTP listen port                           | `8080`      |
-| `DNSWATCHER_DEBUG`              | Enable debug logging                       | `false`     |
-| `DNSWATCHER_DATA_DIR`           | Directory for state file                   | `/var/lib/dnswatcher` |
-| `DNSWATCHER_TARGETS`            | Comma-separated DNS names (auto-classified via PSL) | `""`  |
-| `DNSWATCHER_SLACK_WEBHOOK`      | Slack incoming webhook URL                 | `""`        |
-| `DNSWATCHER_MATTERMOST_WEBHOOK` | Mattermost incoming webhook URL            | `""`        |
-| `DNSWATCHER_NTFY_TOPIC`         | ntfy topic URL                             | `""`        |
-| `DNSWATCHER_DNS_INTERVAL`       | DNS check interval, a positive duration such as `30m`; empty means the default, anything else stops startup | `1h` |
-| `DNSWATCHER_TLS_INTERVAL`       | TLS check interval, a positive duration such as `6h`; empty means the default, anything else stops startup | `12h` |
-| `DNSWATCHER_TLS_EXPIRY_WARNING` | Days before expiry to warn                 | `7`         |
-| `DNSWATCHER_SENTRY_DSN`         | Sentry DSN for error reporting             | `""`        |
-| `DNSWATCHER_MAINTENANCE_MODE`   | Enable maintenance mode                    | `false`     |
-| `DNSWATCHER_METRICS_USERNAME`   | Basic auth username for /metrics           | `""`        |
-| `DNSWATCHER_METRICS_PASSWORD`   | Basic auth password for /metrics           | `""`        |
-| `DNSWATCHER_SEND_TEST_NOTIFICATION` | Send a test notification after first scan completes | `false` |
+| Variable                            | Description                                                                                                 | Default               |
+| ----------------------------------- | ----------------------------------------------------------------------------------------------------------- | --------------------- |
+| `PORT`                              | HTTP listen port                                                                                            | `8080`                |
+| `DNSWATCHER_DEBUG`                  | Enable debug logging                                                                                        | `false`               |
+| `DNSWATCHER_DATA_DIR`               | Directory for state file                                                                                    | `/var/lib/dnswatcher` |
+| `DNSWATCHER_TARGETS`                | Comma-separated DNS names (auto-classified via PSL)                                                         | `""`                  |
+| `DNSWATCHER_SLACK_WEBHOOK`          | Slack incoming webhook URL                                                                                  | `""`                  |
+| `DNSWATCHER_MATTERMOST_WEBHOOK`     | Mattermost incoming webhook URL                                                                             | `""`                  |
+| `DNSWATCHER_NTFY_TOPIC`             | ntfy topic URL                                                                                              | `""`                  |
+| `DNSWATCHER_DNS_INTERVAL`           | DNS check interval, a positive duration such as `30m`; empty means the default, anything else stops startup | `1h`                  |
+| `DNSWATCHER_TLS_INTERVAL`           | TLS check interval, a positive duration such as `6h`; empty means the default, anything else stops startup  | `12h`                 |
+| `DNSWATCHER_TLS_EXPIRY_WARNING`     | Days before expiry to warn                                                                                  | `7`                   |
+| `DNSWATCHER_SENTRY_DSN`             | Sentry DSN for error reporting                                                                              | `""`                  |
+| `DNSWATCHER_MAINTENANCE_MODE`       | Enable maintenance mode                                                                                     | `false`               |
+| `DNSWATCHER_METRICS_USERNAME`       | Basic auth username for /metrics                                                                            | `""`                  |
+| `DNSWATCHER_METRICS_PASSWORD`       | Basic auth password for /metrics                                                                            | `""`                  |
+| `DNSWATCHER_SEND_TEST_NOTIFICATION` | Send a test notification after first scan completes                                                         | `false`               |
 
 **`DNSWATCHER_TARGETS` is required.** dnswatcher will refuse to start if no
 monitoring targets are configured. A monitoring daemon with nothing to monitor
 is a misconfiguration, so dnswatcher fails fast with a clear error message
-rather than running silently. Set `DNSWATCHER_TARGETS` to a comma-separated
-list of DNS names before starting.
+rather than running silently. Set `DNSWATCHER_TARGETS` to a comma-separated list
+of DNS names before starting.
 
 **`/metrics` is rate limited.** Each client address may send it 30 requests a
 minute, failed logins included; beyond that it answers `429 Too Many Requests`
 without checking the password. A Prometheus server scraping every 15 seconds
 sends 4 a minute. IPv6 addresses in one /64 count as one client. When the
-request comes from a private or loopback address, such as a reverse proxy's,
-the client address is taken from the `X-Real-IP` header the proxy sets, or else
-from `X-Forwarded-For`, as the last address in it that is not private or
-loopback. A proxy that sets neither makes all its clients share one allowance.
+request comes from a private or loopback address, such as a reverse proxy's, the
+client address is taken from the `X-Real-IP` header the proxy sets, or else from
+`X-Forwarded-For`, as the last address in it that is not private or loopback. A
+proxy that sets neither makes all its clients share one allowance.
 
 **`DNSWATCHER_DNS_INTERVAL` and `DNSWATCHER_TLS_INTERVAL`** take a positive
 duration: a number followed by a unit such as `s`, `m` or `h`, for example
-`90s`, `30m`, `1h` or `1h30m`. There is no unit for days; write `24h`. An
-unset or empty variable (`DNSWATCHER_DNS_INTERVAL=`) means the default. If
-either is set to anything else, including a bare number or a zero or negative
-duration, dnswatcher refuses to start with an error naming the variable and
-the value.
+`90s`, `30m`, `1h` or `1h30m`. There is no unit for days; write `24h`. An unset
+or empty variable (`DNSWATCHER_DNS_INTERVAL=`) means the default. If either is
+set to anything else, including a bare number or a zero or negative duration,
+dnswatcher refuses to start with an error naming the variable and the value.
 
 **`DNSWATCHER_SENTRY_DSN` reports crashes in HTTP requests to Sentry.** When it
 is set, a panic in an HTTP request handler is sent to Sentry, and the request
@@ -392,107 +389,107 @@ DNSWATCHER_SEND_TEST_NOTIFICATION=true
 
 ## DNS Resolution Strategy
 
-dnswatcher never uses the system's configured recursive resolver.  Instead,
-it performs full iterative resolution:
+dnswatcher never uses the system's configured recursive resolver. Instead, it
+performs full iterative resolution:
 
-1. **Root servers**: Starts from the IANA root nameserver list (hardcoded,
-   with periodic refresh).
+1. **Root servers**: Starts from the IANA root nameserver list (hardcoded, with
+   periodic refresh).
 2. **TLD delegation**: Queries root servers for the TLD NS records.
-3. **Domain delegation**: Queries TLD nameservers for the domain's NS
-   records.
-4. **Authoritative query**: Queries all discovered authoritative
-   nameservers directly for the requested records.
+3. **Domain delegation**: Queries TLD nameservers for the domain's NS records.
+4. **Authoritative query**: Queries all discovered authoritative nameservers
+   directly for the requested records.
 
 This approach ensures:
+
 - Independence from any upstream resolver's cache or filtering.
-- Ability to detect split-horizon or inconsistent responses across
-  authoritative servers.
+- Ability to detect split-horizon or inconsistent responses across authoritative
+  servers.
 - Visibility into the full delegation chain.
 
-For hostname monitoring, the resolver follows CNAME chains (with a
-depth limit to prevent loops) before collecting terminal A/AAAA records.
+For hostname monitoring, the resolver follows CNAME chains (with a depth limit
+to prevent loops) before collecting terminal A/AAAA records.
 
 ---
 
 ## State File Format
 
 The state file (`DATA_DIR/state.json`) contains the complete monitoring
-snapshot.  Hostname records are stored **per authoritative nameserver**,
-not as a merged view, to enable inconsistency detection.
+snapshot. Hostname records are stored **per authoritative nameserver**, not as a
+merged view, to enable inconsistency detection.
 
 ```json
 {
-  "version": 1,
-  "lastUpdated": "2026-02-19T12:00:00Z",
-  "domains": {
-    "example.com": {
-      "nameservers": ["ns1.example.com.", "ns2.example.com."],
-      "nameserverAddresses": {
-        "ns1.example.com.": ["192.0.2.53", "2001:db8::53"],
-        "ns2.example.com.": ["198.51.100.53"]
-      },
-      "lastChecked": "2026-02-19T12:00:00Z"
-    }
-  },
-  "hostnames": {
-    "www.example.com": {
-      "recordsByNameserver": {
-        "ns1.example.com.": {
-          "records": {
-            "A": ["93.184.216.34"],
-            "AAAA": ["2606:2800:220:1:248:1893:25c8:1946"]
-          },
-          "status": "ok",
-          "lastChecked": "2026-02-19T12:00:00Z"
-        },
-        "ns2.example.com.": {
-          "records": {
-            "A": ["93.184.216.34"],
-            "AAAA": ["2606:2800:220:1:248:1893:25c8:1946"]
-          },
-          "status": "ok",
-          "lastChecked": "2026-02-19T12:00:00Z"
+    "version": 1,
+    "lastUpdated": "2026-02-19T12:00:00Z",
+    "domains": {
+        "example.com": {
+            "nameservers": ["ns1.example.com.", "ns2.example.com."],
+            "nameserverAddresses": {
+                "ns1.example.com.": ["192.0.2.53", "2001:db8::53"],
+                "ns2.example.com.": ["198.51.100.53"]
+            },
+            "lastChecked": "2026-02-19T12:00:00Z"
         }
-      },
-      "lastChecked": "2026-02-19T12:00:00Z"
-    }
-  },
-  "ports": {
-    "93.184.216.34:80": {
-      "open": true,
-      "hostnames": ["www.example.com"],
-      "lastChecked": "2026-02-19T12:00:00Z"
     },
-    "93.184.216.34:443": {
-      "open": true,
-      "hostnames": ["www.example.com"],
-      "lastChecked": "2026-02-19T12:00:00Z"
+    "hostnames": {
+        "www.example.com": {
+            "recordsByNameserver": {
+                "ns1.example.com.": {
+                    "records": {
+                        "A": ["93.184.216.34"],
+                        "AAAA": ["2606:2800:220:1:248:1893:25c8:1946"]
+                    },
+                    "status": "ok",
+                    "lastChecked": "2026-02-19T12:00:00Z"
+                },
+                "ns2.example.com.": {
+                    "records": {
+                        "A": ["93.184.216.34"],
+                        "AAAA": ["2606:2800:220:1:248:1893:25c8:1946"]
+                    },
+                    "status": "ok",
+                    "lastChecked": "2026-02-19T12:00:00Z"
+                }
+            },
+            "lastChecked": "2026-02-19T12:00:00Z"
+        }
+    },
+    "ports": {
+        "93.184.216.34:80": {
+            "open": true,
+            "hostnames": ["www.example.com"],
+            "lastChecked": "2026-02-19T12:00:00Z"
+        },
+        "93.184.216.34:443": {
+            "open": true,
+            "hostnames": ["www.example.com"],
+            "lastChecked": "2026-02-19T12:00:00Z"
+        }
+    },
+    "certificates": {
+        "93.184.216.34:443:www.example.com": {
+            "commonName": "www.example.com",
+            "issuer": "DigiCert TLS RSA SHA256 2020 CA1",
+            "notAfter": "2027-01-15T23:59:59Z",
+            "subjectAlternativeNames": ["www.example.com"],
+            "status": "ok",
+            "lastChecked": "2026-02-19T06:00:00Z"
+        }
     }
-  },
-  "certificates": {
-    "93.184.216.34:443:www.example.com": {
-      "commonName": "www.example.com",
-      "issuer": "DigiCert TLS RSA SHA256 2020 CA1",
-      "notAfter": "2027-01-15T23:59:59Z",
-      "subjectAlternativeNames": ["www.example.com"],
-      "status": "ok",
-      "lastChecked": "2026-02-19T06:00:00Z"
-    }
-  }
 }
 ```
 
-The `status` field for each per-nameserver entry and certificate entry
-tracks reachability:
+The `status` field for each per-nameserver entry and certificate entry tracks
+reachability:
 
-| Status      | Meaning                                                    |
-|-------------|------------------------------------------------------------|
-| `ok`        | Query succeeded, records are current                       |
-| `error`     | Query failed (timeout, SERVFAIL, REFUSED, network error)   |
+| Status  | Meaning                                                  |
+| ------- | -------------------------------------------------------- |
+| `ok`    | Query succeeded, records are current                     |
+| `error` | Query failed (timeout, SERVFAIL, REFUSED, network error) |
 
-A nameserver that answers NXDOMAIN or with no records has status `ok` and
-empty `records`. A nameserver whose query failed has status `error`, empty
-`records`, and the reason in `error`.
+A nameserver that answers NXDOMAIN or with no records has status `ok` and empty
+`records`. A nameserver whose query failed has status `error`, empty `records`,
+and the reason in `error`.
 
 `nameserverAddresses` lists, by nameserver, the sorted addresses its name
 resolves to. A state file without it loads, and the next check fills it in
@@ -505,31 +502,38 @@ without a notification.
 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. We
+provide:
 
-- `script/bootstrap` — install all dependencies (go, `go mod download`).
-  It does not install golangci-lint: see `script/lint` below.
-- `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 test suite (race detector, coverage). Caching
-  is waived for testing, exactly as it is for linting: `-count=1`
-  forces every invocation to execute, because the suite queries live
-  DNS and a cached pass queries nothing. Failures are rerun with `-v`
-  automatically, and the build fails even if that rerun passes.
+- `script/bootstrap` — install all dependencies (go, `go mod download`). It does
+  not install golangci-lint or prettier: both run in Docker, see `script/lint`
+  and `script/fmt` below.
+- `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 test suite (race detector, coverage). Caching is
+  waived for testing, exactly as it is for linting: `-count=1` forces every
+  invocation to execute, because the suite queries live DNS and a cached pass
+  queries nothing. Failures are rerun with `-v` automatically, and the build
+  fails even if that rerun passes.
 - `script/lint` — run golangci-lint, always inside Docker: it builds
-  `Dockerfile.lint`, which COPYs the repo into the digest-pinned
-  `golangci-lint` image and lints as a build step, so a successful
-  build is a clean lint. The linter is never installed or run on the
-  host, and Docker is the only prerequisite. Caching is waived for
-  linting: the lint stage is forced to execute on every run with
-  `--no-cache-filter`, because a cached build lints nothing.
-- `script/fmt` — format all code (gofmt -s, goimports). goimports runs
-  with `go run` at a pinned commit, never from your `PATH`.
-- `script/fmt-check` — check formatting (read-only) with the same tools,
-  failing on any file `script/fmt` would change
+  `Dockerfile.lint`, which COPYs the repo into the digest-pinned `golangci-lint`
+  image and lints as a build step, so a successful build is a clean lint. The
+  linter is never installed or run on the host, and Docker is the only
+  prerequisite. Caching is waived for linting: the lint stage is forced to
+  execute on every run with `--no-cache-filter`, because a cached build lints
+  nothing.
+- `script/fmt` — format all code (gofmt -s, goimports) and all Markdown
+  (prettier). goimports runs with `go run` at a pinned commit, never from your
+  `PATH`. prettier runs inside Docker, built from `Dockerfile.fmt` on a
+  digest-pinned node image, at the version pinned by `package.json` and
+  `yarn.lock`; it is never installed on the host.
+- `script/fmt-check` — check formatting (read-only) with the same tools, failing
+  on any file `script/fmt` would change. It runs the two scripts below.
+- `script/fmt-check-go` — the gofmt and goimports half, on the host. The
+  `Dockerfile` lint stage runs it.
+- `script/fmt-check-markdown` — the prettier half, inside Docker, forced to
+  execute on every run with `--no-cache-filter`
 - `script/check` — run test, lint, and fmt-check
 - `script/docker` — build the Docker image tagged via `script/projectname`, with
   `--no-cache-filter=lint,builder` so the lint stage and the builder stage,
@@ -538,9 +542,9 @@ them. We provide:
 - `script/cibuild` — CI entrypoint: `docker build` with
   `--no-cache-filter=lint,builder`, so the lint stage and the builder stage,
   which runs the tests, run on every invocation, because a cached build lints
-  nothing and queries no DNS
-- `script/precommit` — run by the git pre-commit hook; `go mod tidy`
-  guard, then `script/check`
+  nothing and queries no DNS; then `script/fmt-check-markdown`
+- `script/precommit` — run by the git pre-commit hook; `go mod tidy` guard, then
+  `script/check`
 - `script/install-precommit` — install the git pre-commit hook
 
 ## Building
@@ -549,21 +553,21 @@ them. We provide:
 make build      # Build binary to bin/dnswatcher
 make test       # Run tests with race detector
 make lint       # Run golangci-lint in Docker (requires docker)
-make fmt        # Format code
+make fmt        # Format code and Markdown (requires docker)
 make check      # Run all checks (test, lint, fmt-check)
 make clean      # Remove build artifacts
 ```
 
 ### Build-Time Variables
 
-`make build` sets the version with `-ldflags "-X main.Version=..."`, taking
-it from `git describe --tags --always --dirty`, or from `VERSION` when given
-on the command line (`make build VERSION=1.2.3`). The version appears in the
-startup log and in the health check response.
+`make build` sets the version with `-ldflags "-X main.Version=..."`, taking it
+from `git describe --tags --always --dirty`, or from `VERSION` when given on the
+command line (`make build VERSION=1.2.3`). The version appears in the startup
+log and in the health check response.
 
 The Docker image has no `.git`, so the `Dockerfile` takes the version as
-`--build-arg VERSION`. `make docker` passes it; a plain `docker build`
-passes none, and that image reports `dev`.
+`--build-arg VERSION`. `make docker` passes it; a plain `docker build` passes
+none, and that image reports `dev`.
 
 ---
 
@@ -584,84 +588,80 @@ docker run -d \
 
 ## Running under upaas
 
-[upaas](https://git.eeqj.de/sneak/upaas) builds the image from this
-repository's `Dockerfile` and runs it. The app needs:
+[upaas](https://git.eeqj.de/sneak/upaas) builds the image from this repository's
+`Dockerfile` and runs it. The app needs:
 
-- **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.
-- **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.
+- **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.
+- **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.
+  `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`.
+  `/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**: 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.
+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):
-   - DNS checks: every `DNSWATCHER_DNS_INTERVAL` (default 1h). Also
-     re-run before every TLS check cycle to ensure fresh IPs.
-   - Port checks: every `DNSWATCHER_DNS_INTERVAL`, after DNS completes.
-   - TLS checks: every `DNSWATCHER_TLS_INTERVAL` (default 12h), after
-     DNS completes.
-   - Port and TLS checks always use freshly resolved IP addresses from
-     the DNS phase that immediately precedes them — never stale IPs
-     from a previous cycle.
-4. **On change detection**: Send notifications to all configured
-   endpoints, update in-memory state, persist to disk.
-5. **Shutdown**: The watcher stops checking and saves the final state
-   to disk, and shutdown waits for that save before it goes on. Then it
-   waits for in-flight notification deliveries to complete. Both waits
-   share the fx shutdown timeout (15s by default): deliveries still
-   retrying against an unreachable endpoint when that expires are
-   abandoned, and the number abandoned is logged at warn level rather
-   than dropped silently. Notifications generated after shutdown has
-   begun are refused and logged, so a late burst cannot extend the
-   shutdown. A DNS lookup, port check or TLS check that shutdown cuts
-   short saves nothing and sends no notification.
+    - DNS checks: every `DNSWATCHER_DNS_INTERVAL` (default 1h). Also re-run
+      before every TLS check cycle to ensure fresh IPs.
+    - Port checks: every `DNSWATCHER_DNS_INTERVAL`, after DNS completes.
+    - TLS checks: every `DNSWATCHER_TLS_INTERVAL` (default 12h), after DNS
+      completes.
+    - Port and TLS checks always use freshly resolved IP addresses from the DNS
+      phase that immediately precedes them — never stale IPs from a previous
+      cycle.
+4. **On change detection**: Send notifications to all configured endpoints,
+   update in-memory state, persist to disk.
+5. **Shutdown**: The watcher stops checking and saves the final state to disk,
+   and shutdown waits for that save before it goes on. Then it waits for
+   in-flight notification deliveries to complete. Both waits share the fx
+   shutdown timeout (15s by default): deliveries still retrying against an
+   unreachable endpoint when that expires are abandoned, and the number
+   abandoned is logged at warn level rather than dropped silently. Notifications
+   generated after shutdown has begun are refused and logged, so a late burst
+   cannot extend the shutdown. A DNS lookup, port check or TLS check that
+   shutdown cuts short saves nothing and sends no notification.
 
 ---
 
 ## Planned Future Features (Post-1.0)
 
-- **DNSSEC validation**: Validate the DNSSEC chain of trust during
-  iterative resolution and report DNSSEC failures as notifications.
+- **DNSSEC validation**: Validate the DNSSEC chain of trust during iterative
+  resolution and report DNSSEC failures as notifications.
 
 ---
 
 ## Project Structure
 
 Follows the conventions defined in `REPO_POLICIES.md`, adapted from the
-[upaas](https://git.eeqj.de/sneak/upaas) project template.  Uses uber/fx
-for dependency injection, go-chi for HTTP routing, slog for logging, and
-Viper for configuration.
+[upaas](https://git.eeqj.de/sneak/upaas) project template. Uses uber/fx for
+dependency injection, go-chi for HTTP routing, slog for logging, and Viper for
+configuration.
 
 ---
 
diff --git a/TESTING.md b/TESTING.md
index 6b4f3d8..3971309 100644
--- a/TESTING.md
+++ b/TESTING.md
@@ -2,44 +2,43 @@
 
 ## DNS Resolution Tests
 
-DNS is never mocked in this project, not in tests and not anywhere
-else; see the README section "No DNS mocking. Ever." Every test that
-looks something up in DNS **MUST** query live DNS servers, never a
-stand-in. Logic that works on record data, such as comparing or
-formatting records, may be tested on that data directly with no
-lookup.
+DNS is never mocked in this project, not in tests and not anywhere else; see the
+README section "No DNS mocking. Ever." Every test that looks something up in DNS
+**MUST** query live DNS servers, never a stand-in. Logic that works on record
+data, such as comparing or formatting records, may be tested on that data
+directly with no lookup.
 
 ### Rationale
 
-The resolver performs iterative resolution from root nameservers through
-the full delegation chain. Mocked responses cannot faithfully represent
-the variety of real-world DNS behavior (truncation, referrals, glue
-records, DNSSEC, varied response times, EDNS, etc.). Testing against
-real servers ensures the resolver works correctly in production.
+The resolver performs iterative resolution from root nameservers through the
+full delegation chain. Mocked responses cannot faithfully represent the variety
+of real-world DNS behavior (truncation, referrals, glue records, DNSSEC, varied
+response times, EDNS, etc.). Testing against real servers ensures the resolver
+works correctly in production.
 
 ### Constraints
 
 - Tests hit real DNS infrastructure and require network access
-- Test duration depends on network conditions; timeout tuning keeps
-  the suite within the 60-second target
-- Query timeout is calibrated to 3× maximum antipodal RTT (~300ms)
-  plus processing margin
+- Test duration depends on network conditions; timeout tuning keeps the suite
+  within the 60-second target
+- Query timeout is calibrated to 3× maximum antipodal RTT (~300ms) plus
+  processing margin
 - Root server fan-out is limited to reduce parallel query load
-- Live lookups that expect an answer go through `internal/livednstest`,
-  which limits how many run at once in a test binary and retries a
-  lookup that got none
-- Flaky failures from transient network issues are acceptable and
-  should be investigated as potential resolver bugs, not papered over
-  with mocks or skip flags
+- Live lookups that expect an answer go through `internal/livednstest`, which
+  limits how many run at once in a test binary and retries a lookup that got
+  none
+- Flaky failures from transient network issues are acceptable and should be
+  investigated as potential resolver bugs, not papered over with mocks or skip
+  flags
 
 ### What NOT to do
 
-- **Do not mock, fake or stub DNS** anywhere: no stand-in `DNSClient`,
-  no stand-in for the watcher's `DNSResolver`, no fake DNS server, no
-  canned responses
+- **Do not mock, fake or stub DNS** anywhere: no stand-in `DNSClient`, no
+  stand-in for the watcher's `DNSResolver`, no fake DNS server, no canned
+  responses
 - **Do not add `-short` flags** to skip slow tests
 - **Do not increase `-timeout`** to hide hanging queries
-- **Do not remove `-count=1` from `script/test`** — Go's test cache
-  replays a previous run's output without querying anything, so a
-  cached pass is not evidence that live resolution works
+- **Do not remove `-count=1` from `script/test`** — Go's test cache replays a
+  previous run's output without querying anything, so a cached pass is not
+  evidence that live resolution works
 - **Do not modify linter configuration** to suppress findings
diff --git a/TODO.md b/TODO.md
index 897bda6..a7e12ce 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,12 +1,12 @@
 # Workflow
 
-* branch (from `next`)
-* do the work in Next Step
-* move Next Step to the top of Completed Steps
-* move the top item of Future Steps into Next Step
-* commit (`TODO.md` changes in the same commit as the work)
-* push
-* open a PR against `next`
+- branch (from `next`)
+- do the work in Next Step
+- move Next Step to the top of Completed Steps
+- move the top item of Future Steps into Next Step
+- commit (`TODO.md` changes in the same commit as the work)
+- push
+- open a PR against `next`
 
 # Status
 
@@ -15,11 +15,12 @@ on the 1.0 milestone: https://git.eeqj.de/sneak/dnswatcher/milestone/7
 
 # Next Step
 
-trial run of the finished image:
-https://git.eeqj.de/sneak/dnswatcher/issues/149
+trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-01: `make fmt` and `make fmt-check` cover Markdown with prettier, run
+  in Docker at the version pinned by `yarn.lock` (closes #119).
 - 2026-10-01: `make fmt-check` fails on a file `goimports` would change; both
   format scripts run `goimports` at its pinned commit, not from `PATH` (#119).
 - 2026-10-01: a hostname is queried at the servers of the zone it is in, found
@@ -114,8 +115,6 @@ https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 - 1.0 readiness: run it with a real config and read the logs:
   https://git.eeqj.de/sneak/dnswatcher/issues/66
-- Markdown formatting with prettier:
-  https://git.eeqj.de/sneak/dnswatcher/issues/119
 - README accuracy sweep: https://git.eeqj.de/sneak/dnswatcher/issues/108
 - README sections required by policy:
   https://git.eeqj.de/sneak/dnswatcher/issues/173
diff --git a/package.json b/package.json
new file mode 100644
index 0000000..53eca1f
--- /dev/null
+++ b/package.json
@@ -0,0 +1,9 @@
+{
+    "name": "dnswatcher-tooling",
+    "version": "0.0.0",
+    "private": true,
+    "description": "Pins the prettier that script/fmt and script/fmt-check run against this repo's markdown. Not a JavaScript project; nothing here is imported, published, or shipped.",
+    "devDependencies": {
+        "prettier": "3.9.6"
+    }
+}
diff --git a/script/bootstrap b/script/bootstrap
index a463d0a..dd76db4 100755
--- a/script/bootstrap
+++ b/script/bootstrap
@@ -3,11 +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.
-# goimports is not installed here: script/fmt and script/fmt-check run
-# it with `go run` at a pinned commit.
+# goimports is not installed here: script/fmt and script/fmt-check-go
+# run it with `go run` at a pinned commit.
 # The linter is NOT installed here: golangci-lint runs via docker only
 # (script/lint), pinned by image digest, so its only prerequisite is a
-# working docker.
+# working docker. Nor is prettier: script/fmt and
+# script/fmt-check-markdown run it in a container from Dockerfile.fmt.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -66,11 +67,12 @@ main() {
     if missing make; then pkg_install gnumake make make make; fi
     if missing go; then pkg_install go golang go go; fi
 
-    # Linting runs via docker only (script/lint). Warn, don't fail:
-    # everything except `make lint` works without it.
+    # Linting and the markdown formatter run via docker only. Warn,
+    # don't fail: building and testing work without it.
     if missing docker; then
         echo "bootstrap: WARNING: docker not found; install it to" \
-            "run make lint and make docker." >&2
+            "run make lint, make fmt, make fmt-check, make check" \
+            "and make docker." >&2
     fi
 
     go mod download
diff --git a/script/cibuild b/script/cibuild
index 29bea03..ad4ed76 100755
--- a/script/cibuild
+++ b/script/cibuild
@@ -1,18 +1,23 @@
 #!/bin/sh
 # script/cibuild: run the CI build. The Dockerfile's lint stage runs
-# make fmt-check and golangci-lint; its builder stage runs make test
-# and make build.
+# the Go half of make fmt-check and golangci-lint; its builder stage
+# runs make test and make build. The markdown half of make fmt-check
+# runs after that build, as its own build of Dockerfile.fmt, because
+# there is no docker inside a docker build.
 #
 # --no-cache-filter=lint,builder runs both stages on every invocation;
 # otherwise an unchanged tree is served from the layer cache and passes
-# without linting or querying live DNS.
+# without linting or querying live DNS. script/fmt-check-markdown busts
+# its own cache the same way.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
     docker build --no-cache-filter=lint,builder .
+    "$SCRIPT_DIR/fmt-check-markdown"
 }
 
 main "$@"
diff --git a/script/fmt b/script/fmt
index a0b28e6..d64453a 100755
--- a/script/fmt
+++ b/script/fmt
@@ -1,19 +1,65 @@
 #!/bin/sh
-# script/fmt: format all files (writes).
+# script/fmt: format all files (writes). Go with gofmt and goimports on
+# the host, markdown with the prettier pinned by Dockerfile.fmt.
 #
 # goimports runs with `go run` at a pinned commit, never from PATH, so
 # every machine formats with the same version and nothing installs it.
+#
+# The markdown pass is a `docker build --output type=local` rather than a
+# `docker run -v`, so it needs no bind mount and behaves the same against
+# a remote daemon; the formatted documents come back out of the build and
+# are copied over the tree here.
+#
+# Unlike script/fmt-check-markdown this does not bust the cache: it is
+# not a gate, and any edit to a document changes the COPY layer above the
+# prettier step, so a cached result is a result over this exact tree.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# goimports v0.42.0, 2026-08-07. Must match script/fmt-check.
+# goimports v0.42.0, 2026-08-07. Must match script/fmt-check-go.
 GOIMPORTS_REF="golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0"
 
+# Must match the export stage name in Dockerfile.fmt.
+stage=fmt-out
+
+die() {
+    echo "script/fmt: $*" >&2
+    exit 1
+}
+
 main() {
     cd "$ROOT"
     gofmt -s -w .
     go run "$GOIMPORTS_REF" -w .
+
+    tmp="$(mktemp -d "${TMPDIR:-/tmp}/dnswatcher-fmt.XXXXXX")"
+    trap 'rm -rf "$tmp"' EXIT INT TERM
+
+    docker build \
+        --target "$stage" \
+        --output "type=local,dest=$tmp/out" \
+        -f Dockerfile.fmt .
+
+    # An empty export means prettier was handed nothing, which must not
+    # read as "already formatted".
+    (cd "$tmp/out" && find . -type f -name '*.md') |
+        sed 's|^\./||' | LC_ALL=C sort >"$tmp/files"
+
+    [ -s "$tmp/files" ] ||
+        die "the formatting build produced no markdown; the build" \
+            "context reached prettier empty"
+
+    # Copied only where the bytes differ, so an already-formatted tree
+    # keeps its timestamps and says nothing.
+    while IFS= read -r f; do
+        [ -n "$f" ] || continue
+        if [ -f "$f" ] && cmp -s "$tmp/out/$f" "$f"; then
+            continue
+        fi
+        cp "$tmp/out/$f" "$f"
+        echo "prettier: reformatted $f"
+    done <"$tmp/files"
 }
 
 main "$@"
diff --git a/script/fmt-check b/script/fmt-check
index 40911dd..c5fb94f 100755
--- a/script/fmt-check
+++ b/script/fmt-check
@@ -1,27 +1,14 @@
 #!/bin/sh
 # script/fmt-check: check formatting (read-only). Same tools and scope
-# as script/fmt, but fails instead of writing.
+# as script/fmt, but fails instead of writing: the Go on the host, the
+# markdown with prettier in a container.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
-
-# goimports v0.42.0, 2026-08-07. Must match script/fmt.
-GOIMPORTS_REF="golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
 
 main() {
-    cd "$ROOT"
-    files="$(gofmt -s -l .)"
-    if [ -n "$files" ]; then
-        echo "gofmt: files not formatted:" >&2
-        echo "$files" >&2
-        exit 1
-    fi
-    files="$(go run "$GOIMPORTS_REF" -l .)"
-    if [ -n "$files" ]; then
-        echo "goimports: files not formatted:" >&2
-        echo "$files" >&2
-        exit 1
-    fi
+    "$SCRIPT_DIR/fmt-check-go"
+    "$SCRIPT_DIR/fmt-check-markdown"
 }
 
 main "$@"
diff --git a/script/fmt-check-go b/script/fmt-check-go
new file mode 100755
index 0000000..fa21fc5
--- /dev/null
+++ b/script/fmt-check-go
@@ -0,0 +1,31 @@
+#!/bin/sh
+# script/fmt-check-go: fail unless every Go source is formatted the way
+# script/fmt would leave it, and name the files that are not. Read-only.
+#
+# Its own script because the Dockerfile's lint stage runs this half
+# alone: there is no docker inside a docker build to run the markdown
+# half in.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+# goimports v0.42.0, 2026-08-07. Must match script/fmt.
+GOIMPORTS_REF="golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0"
+
+main() {
+    cd "$ROOT"
+    files="$(gofmt -s -l .)"
+    if [ -n "$files" ]; then
+        echo "gofmt: files not formatted:" >&2
+        echo "$files" >&2
+        exit 1
+    fi
+    files="$(go run "$GOIMPORTS_REF" -l .)"
+    if [ -n "$files" ]; then
+        echo "goimports: files not formatted:" >&2
+        echo "$files" >&2
+        exit 1
+    fi
+}
+
+main "$@"
diff --git a/script/fmt-check-markdown b/script/fmt-check-markdown
new file mode 100755
index 0000000..e849b59
--- /dev/null
+++ b/script/fmt-check-markdown
@@ -0,0 +1,29 @@
+#!/bin/sh
+# script/fmt-check-markdown: fail unless every .md is formatted the way
+# script/fmt would leave it. Read-only.
+#
+# prettier is never installed on the host: it runs in a container built
+# from Dockerfile.fmt, pinned by package.json and yarn.lock.
+# --no-cache-filter is here for the reason script/lint gives: a cached
+# build checks nothing.
+#
+# Its own script because script/cibuild runs this half alone, after the
+# Dockerfile's lint stage has checked the Go.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+# Must match the markdown check stage name in Dockerfile.fmt.
+stage=fmt-check
+
+main() {
+    cd "$ROOT"
+    docker build \
+        --progress=plain \
+        --no-cache-filter="$stage" \
+        --target "$stage" \
+        -f Dockerfile.fmt \
+        .
+}
+
+main "$@"
diff --git a/yarn.lock b/yarn.lock
new file mode 100644
index 0000000..8f3c21a
--- /dev/null
+++ b/yarn.lock
@@ -0,0 +1,8 @@
+# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
+# yarn lockfile v1
+
+
+prettier@3.9.6:
+  version "3.9.6"
+  resolved "https://registry.yarnpkg.com/prettier/-/prettier-3.9.6.tgz#b3ea5146515d40fc53f18aa63f74dfab1e10dbf6"
+  integrity sha512-OpN0zzVdiaiAhxpuuj5efpIS4sY9j7bY6uR5mnj5yPzGkdkjNKSJeUThPb60Jw29QuAZgA4o+/iB49kFiaBX6g==