web: /healthz is answered only to local probes (audit I6)

Security audit 2026-10-17, finding I6 (low), stage 3. /healthz is
registered outside auth and every proxy fragment passed it through: the
live server answered "ok" to the internet, and "unhealthy: postfix"
would have been a free tell.

Now handleHealth answers only a request with no X-Forwarded-For (nor
Forwarded) from loopback or a private address: the image's HEALTHCHECK
(127.0.0.1), the host through the loopback-published port (the Docker
gateway), a monitor on the Docker network. Every supported proxy appends
X-Forwarded-For, so through it the endpoint is 404. The plan named
"trusted-proxy peers"; that would have let the internet in again through
the trusted proxy, hence this shape. The e2e's readiness probe is the
host's, and stays answered.

Test: TestHealthzAnswersOnlyLocalProbes. Docs: guide § Container health,
architecture.md; CHANGELOG; plan.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture.md

@@ -63,7 +63,7 @@ The panel runs `postfix-reload` through supervisord's XML-RPC interface and
 waits for its exit status (`internal/supervisor`, `RunOnce`); the program
 rests EXITED after a run.
 
-**Liveness:** `GET /healthz` (unauthenticated) returns 200 when panel - whose
+**Liveness:** `GET /healthz` (unauthenticated; answered only to a request with no `X-Forwarded-For` from loopback or a private address, else 404) returns 200 when panel - whose
 roles include the DKIM signer - and postfix are RUNNING
 (`internal/health`, `Liveness`); Docker `HEALTHCHECK` uses the same probe (its
 90-second start period covers postfix's STARTING).

docs/guide.md

@@ -819,15 +819,18 @@ again whole and loses the keys of domains added under 2.8.x.
 ### Container health
 
 The image declares a Docker `HEALTHCHECK` that probes `GET /healthz` on port
-8080 (unauthenticated). It returns `200 ok` when the panel - whose roles
+8080 (unauthenticated, and answered only to a request that did not come
+through a reverse proxy - no `X-Forwarded-For` - from loopback or a private
+address; anyone else gets `404`). It returns `200 ok` when the panel - whose roles
 include the DKIM signer - and Postfix are both `RUNNING` under supervisord;
 otherwise `503 unhealthy`. This
 catches a dead mail path that would still leave the HTTP server up, but it
 does **not** verify TLS certificates, DNS records, or end-to-end delivery —
 use the authenticated [Overview and Health](#overview-and-health) pages for
-that. External monitoring
-can use the same endpoint through the reverse proxy if you expose it, or poll
-`docker inspect` health state on the host.
+that. External monitoring runs on the host - `curl -fsS
+http://127.0.0.1:8080/healthz` - or polls `docker inspect` health state;
+through the reverse proxy the endpoint is not found, so its answer tells a
+stranger nothing.
 
 ### Server-level DNS (PTR/rDNS)
 

docs/security-audit-2026-10-17.md

@@ -790,9 +790,12 @@ the argument.
 - [x] **A3.** After five unsolicited reset links in a day, one mail saying
   further requests are ignored for 24 h, then silence. — **Opus**, 1.5 h.
   *Done 2026-10-17:* TestResetLinksPauseForADayAfterFive.
-- [ ] **I6.** `/healthz` answered only to loopback and trusted-proxy peers;
+- [x] **I6.** `/healthz` answered only to loopback and trusted-proxy peers;
   the guide's "Container health" says the proxy may expose it on purpose
-  with the fragment line. — **Sonnet**, 0.5 h.
+  with the fragment line. — **Sonnet**, 0.5 h. *Done 2026-10-17,* in another
+  shape: answered to a request with no `X-Forwarded-For` from loopback or a
+  private address - a trusted proxy's peer would have let the internet in
+  again through that proxy; TestHealthzAnswersOnlyLocalProbes.
 
 ### Stage 4 — resources and imports (~5 h)