docs: README, changelog, contributing, security, deployment, operations, CI

.gitea/workflows/build.yml

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+name: build
+
+on:
+  push:
+    branches:
+      - main
+    tags:
+      - 'v*'
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to Forgejo registry
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: git.ankh-morpork.discworld.network
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: git.ankh-morpork.discworld.network/laurence/dns-traefik-sync
+          tags: |
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=sha,prefix=sha-
+            type=raw,value=latest,enable={{is_default_branch}}
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}

.gitignore

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+__pycache__/
+*.pyc
+*.pyo
+*.keytab
+*.ccache
+/tmp/
+.env

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented here.
+
+Format follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
+Versioning follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+---
+
+## [1.0.0] - 2026-05-24
+
+### Added
+
+- Initial release
+- Poll Traefik API for docker-provider routers with Host() rules
+- Filter hostnames to configured DNS zone
+- Create and remove A records via GSS-TSIG authenticated RFC 2136 dynamic DNS update
+- Kerberos authentication using keytab (no passwords, no SSH)
+- State tracking across poll cycles to minimise DNS writes
+- Docker secret support for keytab
+- Configurable via environment variables
+- Debian bookworm-slim base image
+- Runs as nobody

CONTRIBUTING.md

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+# Contributing
+
+## Development setup
+
+Clone the repo and set up a venv:
+
+    git clone https://git.ankh-morpork.discworld.network/laurence/dns-traefik-sync.git
+    cd dns-traefik-sync
+    python3 -m venv venv
+    source venv/bin/activate
+
+No external Python dependencies - stdlib only by design.
+
+## Testing locally
+
+Set environment variables to point at a test Traefik instance and DNS zone, then run:
+
+    python3 dns_sync.py
+
+You will need a valid Kerberos ticket or keytab. For dry-run testing without a real Samba DC,
+set DNS_SERVER to a non-existent host and observe that nsupdate errors are caught and logged cleanly.
+
+## Building the image
+
+    docker build -t dns-traefik-sync:dev .
+
+## Releases
+
+Tag with vMAJOR.MINOR.PATCH - the Gitea Actions workflow will build and push automatically:
+
+    git tag v1.0.1
+    git push origin v1.0.1
+
+Update CHANGELOG.md before tagging.
+
+## Code style
+
+- stdlib only, no third-party dependencies
+- All configuration via environment variables
+- Errors caught per-hostname, never crash the whole sync cycle
+- Log every DNS write at INFO level

Dockerfile

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+FROM debian:bookworm-slim
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+        python3 \
+        krb5-user \
+        dnsutils \
+        ca-certificates && \
+    rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+COPY dns_sync.py .
+COPY krb5.conf /etc/krb5.conf
+RUN chmod +x dns_sync.py
+
+USER nobody
+ENTRYPOINT ["python3", "/app/dns_sync.py"]

README.md

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+# dns-traefik-sync
+
+Automatic DNS registration for Traefik-managed containers using Kerberos-authenticated
+RFC 2136 dynamic updates against a Samba Active Directory DNS server.
+
+Polls the Traefik API, extracts Host() rules from docker-provider routers, and
+creates/removes A records in Samba AD DNS using GSS-TSIG authentication.
+No SSH. No passwords. No broad privilege.
+
+## How it works
+
+    Traefik API (/api/http/routers)
+        └── docker provider routers with Host() rules
+                └── filtered to target zone
+                        └── diff against last known state
+                                └── nsupdate -g (GSS-TSIG via Kerberos keytab)
+                                        └── Samba AD DNS (vetinari)
+
+On each poll cycle:
+
+- Fetches all routers from the Traefik API
+- Filters to docker provider only, extracts hostnames matching the target zone
+- Compares against state from the previous cycle
+- Adds A records for new hostnames, removes A records for gone hostnames
+- Renews the Kerberos ticket before each cycle
+
+## Requirements
+
+- Traefik with API enabled
+- Samba AD DC with a service account in DnsAdmins
+- Keytab for the service account stored as a Docker secret
+- Docker Swarm mode (for secrets)
+
+## Configuration
+
+All configuration via environment variables:
+
+| Variable         | Default                            | Description                        |
+|------------------|------------------------------------|------------------------------------|
+| TRAEFIK_API      | http://192.168.88.2:8080           | Traefik API base URL               |
+| DNS_SERVER       | 192.168.88.1                       | Samba AD DNS server IP             |
+| DNS_ZONE         | ankh-morpork.discworld.network     | Zone to manage records in          |
+| TARGET_IP        | 192.168.88.2                       | IP address for created A records   |
+| TTL              | 300                                | TTL for created records (seconds)  |
+| POLL_INTERVAL    | 60                                 | Poll interval (seconds)            |
+| KEYTAB           | /run/secrets/dns-updater-keytab    | Path to Kerberos keytab            |
+| KRB_PRINCIPAL    | dns-updater@DISCWORLD.NETWORK      | Kerberos principal                 |
+| STATE_FILE       | /tmp/dns_state.json                | State persistence path             |
+
+## Deployment
+
+See docs/deployment.md for full setup instructions.
+See docs/operations.md for day-to-day operational guidance.
+
+## Security model
+
+- Service account scoped to DnsAdmins only
+- Keytab stored as a Docker secret, never on disk
+- GSS-TSIG: Kerberos mutual authentication, updates signed and verified by the KDC
+- No SSH, no HTTP webhooks, no plaintext credentials
+- Container runs as nobody
+
+## Limitations
+
+- A records only (no CNAME, no AAAA)
+- Single target IP (all records point to the same host)
+- Single zone
+- On restart, re-syncs from Traefik - safe and idempotent
+
+## Changelog
+
+See CHANGELOG.md

SECURITY.md

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+# Security
+
+## Reporting
+
+Raise a private issue in Forgejo or contact the repo owner directly.
+
+## Credentials
+
+- Never commit keytabs, .ccache files, or .env files - all are in .gitignore
+- The keytab must be stored as a Docker secret only
+- The service account (dns-updater) should have no other privileges beyond DnsAdmins
+- Rotate the keytab periodically - see docs/operations.md
+
+## Threat model
+
+| Threat | Mitigation |
+|---|---|
+| Keytab compromise | Scoped to DnsAdmins only; rotate via samba-tool user setpassword |
+| Container escape | Runs as nobody; no capabilities; no host mounts beyond the secret |
+| DNS spoofing | GSS-TSIG: updates are Kerberos-signed and verified by the KDC |
+| Unauthorised DNS writes | Only the dns-updater principal can authenticate with this keytab |
+| Traefik API abuse | API is read-only; no writes to Traefik from this service |

dns_sync.py

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+#!/usr/bin/env python3
+
+import time
+import re
+import subprocess
+import logging
+import os
+import json
+from pathlib import Path
+
+logging.basicConfig(
+    level=logging.INFO,
+    format='%(asctime)s %(levelname)s %(message)s'
+)
+log = logging.getLogger(__name__)
+
+TRAEFIK_API     = os.environ.get('TRAEFIK_API', 'http://192.168.88.2:8080')
+DNS_SERVER      = os.environ.get('DNS_SERVER', '192.168.88.1')
+DNS_ZONE        = os.environ.get('DNS_ZONE', 'ankh-morpork.discworld.network')
+TARGET_IP       = os.environ.get('TARGET_IP', '192.168.88.2')
+TTL             = int(os.environ.get('TTL', '300'))
+POLL_INTERVAL   = int(os.environ.get('POLL_INTERVAL', '60'))
+KEYTAB          = os.environ.get('KEYTAB', '/run/secrets/dns-updater-keytab')
+KRB_PRINCIPAL   = os.environ.get('KRB_PRINCIPAL', 'dns-updater@DISCWORLD.NETWORK')
+STATE_FILE      = os.environ.get('STATE_FILE', '/tmp/dns_state.json')
+
+def kinit():
+    result = subprocess.run(
+        ['kinit', '-kt', KEYTAB, KRB_PRINCIPAL],
+        capture_output=True, text=True
+    )
+    if result.returncode != 0:
+        raise RuntimeError(f"kinit failed: {result.stderr}")
+    log.info("Kerberos ticket obtained")
+
+def get_traefik_hostnames():
+    import urllib.request
+    import urllib.error
+    try:
+        with urllib.request.urlopen(f"{TRAEFIK_API}/api/http/routers", timeout=10) as r:
+            routers = json.loads(r.read())
+    except urllib.error.URLError as e:
+        log.error(f"Failed to reach Traefik API: {e}")
+        return set()
+
+    hostnames = set()
+    for router in routers:
+        if router.get('provider') != 'docker':
+            continue
+        rule = router.get('rule', '')
+        for match in re.finditer(r'Host\(`([^`]+)`\)', rule):
+            host = match.group(1)
+            if host.endswith(DNS_ZONE):
+                hostnames.add(host)
+    return hostnames
+
+def nsupdate(commands):
+    input_data = f"gss-tsig\n{commands}\n"
+    result = subprocess.run(
+        ['nsupdate', '-g'],
+        input=input_data, capture_output=True, text=True
+    )
+    if result.returncode != 0:
+        raise RuntimeError(f"nsupdate failed: {result.stderr}")
+
+def add_record(hostname):
+    log.info(f"Adding {hostname} -> {TARGET_IP}")
+    nsupdate(f"""server {DNS_SERVER}
+zone {DNS_ZONE}
+update add {hostname}. {TTL} A {TARGET_IP}
+send""")
+
+def remove_record(hostname):
+    log.info(f"Removing {hostname}")
+    nsupdate(f"""server {DNS_SERVER}
+zone {DNS_ZONE}
+update delete {hostname}. A
+send""")
+
+def load_state():
+    try:
+        return set(json.loads(Path(STATE_FILE).read_text()))
+    except Exception:
+        return set()
+
+def save_state(hostnames):
+    Path(STATE_FILE).write_text(json.dumps(list(hostnames)))
+
+def sync():
+    kinit()
+    current   = get_traefik_hostnames()
+    previous  = load_state()
+
+    to_add    = current - previous
+    to_remove = previous - current
+
+    for h in to_add:
+        try:
+            add_record(h)
+        except Exception as e:
+            log.error(f"Failed to add {h}: {e}")
+            current.discard(h)
+
+    for h in to_remove:
+        try:
+            remove_record(h)
+        except Exception as e:
+            log.error(f"Failed to remove {h}: {e}")
+
+    save_state(current)
+
+if __name__ == '__main__':
+    log.info("dns-traefik-sync starting")
+    while True:
+        try:
+            sync()
+        except Exception as e:
+            log.error(f"Sync failed: {e}")
+        time.sleep(POLL_INTERVAL)

docker-compose.yml

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+version: "3.8"
+
+services:
+  dns-traefik-sync:
+    image: git.ankh-morpork.discworld.network/laurence/dns-traefik-sync:latest
+    environment:
+      TRAEFIK_API: "http://192.168.88.2:8080"
+      DNS_SERVER: "192.168.88.1"
+      DNS_ZONE: "ankh-morpork.discworld.network"
+      TARGET_IP: "192.168.88.2"
+      TTL: "300"
+      POLL_INTERVAL: "60"
+      KEYTAB: "/run/secrets/dns-updater-keytab"
+      KRB_PRINCIPAL: "dns-updater@DISCWORLD.NETWORK"
+    secrets:
+      - dns-updater-keytab
+    restart: unless-stopped
+
+secrets:
+  dns-updater-keytab:
+    external: true

docs/deployment.md

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+# Deployment
+
+## Prerequisites
+
+- Samba AD DC
+- Docker host joined to the AD domain
+- Docker Swarm mode enabled
+- Traefik running with API enabled
+
+---
+
+## 1. Samba AD: Create service account and keytab
+
+On the Samba DC:
+
+    sudo samba-tool user create dns-updater --random-password
+    sudo samba-tool group addmembers DnsAdmins dns-updater
+    sudo samba-tool domain exportkeytab /etc/samba/dns-updater.keytab \
+      --principal=dns-updater@DISCWORLD.NETWORK
+    sudo kinit -kt /etc/samba/dns-updater.keytab dns-updater@DISCWORLD.NETWORK
+    klist
+
+---
+
+## 2. Join the Docker host to the domain
+
+    sudo apt install sssd-ad sssd-tools realmd adcli krb5-user \
+      packagekit samba-common-bin libnss-sss libpam-sss
+    sudo realm join discworld.network -U administrator
+    sudo realm list
+
+---
+
+## 3. Transfer keytab and create Docker secret
+
+On the DC make the keytab readable temporarily:
+
+    sudo cp /etc/samba/dns-updater.keytab /tmp/dns-updater.keytab
+    sudo chown <youradmin>:<youradmin> /tmp/dns-updater.keytab
+
+On your local machine:
+
+    scp <dcadmin>@<dc>:/tmp/dns-updater.keytab .
+    scp dns-updater.keytab <dockeradmin>@<dockerhost>:/tmp/
+    shred -u dns-updater.keytab
+
+On the DC clean up:
+
+    sudo shred -u /tmp/dns-updater.keytab
+
+On the Docker host:
+
+    sudo docker secret create dns-updater-keytab /tmp/dns-updater.keytab
+    sudo shred -u /tmp/dns-updater.keytab
+    sudo docker secret ls
+
+---
+
+## 4. Enable Docker Swarm
+
+    sudo docker swarm init --advertise-addr <docker-host-ip>
+
+---
+
+## 5. Build and push the image
+
+    git clone https://git.ankh-morpork.discworld.network/laurence/dns-traefik-sync.git
+    cd dns-traefik-sync
+    sudo docker build -t git.ankh-morpork.discworld.network/laurence/dns-traefik-sync:latest .
+    sudo docker login git.ankh-morpork.discworld.network
+    sudo docker push git.ankh-morpork.discworld.network/laurence/dns-traefik-sync:latest
+
+---
+
+## 6. Deploy the stack
+
+    sudo docker stack deploy -c docker-compose.yml dns-traefik-sync
+    sudo docker stack ps dns-traefik-sync
+    sudo docker service logs dns-traefik-sync_dns-traefik-sync
+
+---
+
+## 7. Verify DNS records are being created
+
+    dig @192.168.88.1 adguard.ankh-morpork.discworld.network
+
+Or on the DC:
+
+    sudo samba-tool dns query 127.0.0.1 ankh-morpork.discworld.network @ ALL -U administrator

docs/operations.md

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+# Operations
+
+## Checking service status
+
+    sudo docker service ls
+    sudo docker stack ps dns-traefik-sync
+    sudo docker service logs dns-traefik-sync_dns-traefik-sync --follow
+
+## What healthy logs look like
+
+    2026-05-24 18:00:00 INFO dns-traefik-sync starting
+    2026-05-24 18:00:00 INFO Kerberos ticket obtained
+    2026-05-24 18:00:00 INFO Adding adguard.ankh-morpork.discworld.network -> 192.168.88.2
+    2026-05-24 18:01:00 INFO Kerberos ticket obtained
+    2026-05-24 18:01:00 INFO No changes
+
+## Manually verifying DNS records
+
+    dig @192.168.88.1 grafana.ankh-morpork.discworld.network A
+    sudo samba-tool dns query 127.0.0.1 ankh-morpork.discworld.network @ ALL -U administrator
+
+## Forcing a re-sync
+
+    sudo docker service update --force dns-traefik-sync_dns-traefik-sync
+
+## Updating the image
+
+    cd dns-traefik-sync
+    git pull
+    sudo docker build -t git.ankh-morpork.discworld.network/laurence/dns-traefik-sync:latest .
+    sudo docker push git.ankh-morpork.discworld.network/laurence/dns-traefik-sync:latest
+    sudo docker service update --image \
+      git.ankh-morpork.discworld.network/laurence/dns-traefik-sync:latest \
+      dns-traefik-sync_dns-traefik-sync
+
+## Keytab rotation
+
+On the Samba DC:
+
+    sudo samba-tool user setpassword dns-updater --random-password
+    sudo samba-tool domain exportkeytab /etc/samba/dns-updater.keytab \
+      --principal=dns-updater@DISCWORLD.NETWORK
+
+Update the Docker secret:
+
+    sudo docker secret rm dns-updater-keytab
+    sudo docker secret create dns-updater-keytab /tmp/dns-updater.keytab
+    sudo shred -u /tmp/dns-updater.keytab
+    sudo docker stack deploy -c docker-compose.yml dns-traefik-sync
+
+## Removing a DNS record manually
+
+    sudo samba-tool dns delete 127.0.0.1 ankh-morpork.discworld.network \
+      <hostname> A 192.168.88.2 -U administrator
+
+## Troubleshooting
+
+### kinit fails - no suitable keys
+
+Keytab is stale. Re-export on the DC:
+
+    sudo samba-tool domain exportkeytab /etc/samba/dns-updater.keytab \
+      --principal=dns-updater@DISCWORLD.NETWORK
+
+### nsupdate fails with REFUSED
+
+dns-updater not in DnsAdmins:
+
+    sudo samba-tool group addmembers DnsAdmins dns-updater
+
+### nsupdate fails with NOTAUTH
+
+Wrong DNS_ZONE or DNS_SERVER environment variable.
+
+### Records not being created for a container
+
+Container must have a Host() rule in its Traefik label matching DNS_ZONE.
+Internal and non-docker providers are ignored.

krb5.conf

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+[libdefaults]
+    default_realm = DISCWORLD.NETWORK
+    dns_lookup_realm = false
+    dns_lookup_kdc = true
+
+[realms]
+    DISCWORLD.NETWORK = {
+        kdc = 192.168.88.1
+        admin_server = 192.168.88.1
+    }
+
+[domain_realm]
+    .discworld.network = DISCWORLD.NETWORK
+    discworld.network = DISCWORLD.NETWORK