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[docs] Capture nginx stale-DNS trap in registry-vm runbook

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Discovered during the 2026-04-19 registry:2.8.3 pin deploy: nginx caches
its upstream DNS at startup and does NOT re-resolve after registry-*
containers are recreated. Symptom was /v2/_catalog returning
{"repositories": []} and /v2/ returning 200 without auth — nginx was
forwarding to a stale IP that a different backend container now owns.

Fix is always 'docker restart registry-nginx' after any registry-*
bounce. Captured in registry-vm.md so future manual operators and the
coming auto-sync pipeline (beads code-3vl) both encode the step.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Viktor Barzin 2026-04-19 17:24:09 +00:00
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docs/runbooks/registry-vm.md
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@ -140,6 +140,36 @@ ssh root@10.0.20.10 '
' '
``` ```
## Bouncing registry containers — the nginx DNS trap
`docker compose up -d` on `/opt/registry/docker-compose.yml` recreates
`registry-*` containers when their image tag changes, which assigns them
new IPs on the `registry` bridge network. **`registry-nginx` resolves its
upstream DNS names (`registry-private`, `registry-dockerhub`, …) ONCE at
startup and caches the results** — it does not re-resolve after a
recreate.
Symptom if you forget: `/v2/_catalog` on `:5050` returns
`{"repositories": []}`, `/v2/` returns 200 without auth, pulls return
the wrong image. nginx is forwarding to a stale IP that now belongs to a
different registry-* backend (commonly the pull-through ghcr or
dockerhub cache, which have empty catalogs from the htpasswd-auth user's
perspective).
**Always follow a registry-* bounce with `docker restart registry-nginx`.**
Or prevent the problem by setting a `resolver` directive in
`nginx_registry.conf` so upstream names are re-resolved per request.
```sh
ssh root@10.0.20.10 '
cd /opt/registry && docker compose up -d
docker restart registry-nginx
sleep 3
docker ps --format "{{.Names}}\t{{.Image}}\t{{.Status}}" \
| grep -E "registry-"
'
```
## Related docs ## Related docs
- `docs/architecture/dns.md` — resolver IP assignments per subnet. - `docs/architecture/dns.md` — resolver IP assignments per subnet.