**Self-hosted Dynamic DNS (DynDNS / DDNS) for FRITZ!Box, OpenWrt, pfSense, OPNsense, UniFi, MikroTik, Linux cron jobs, and any router that can call a custom DDNS update URL.**
KarlDNS is the friendly name for `karldns`: a small FastAPI Dynamic DNS provider you can run on your own VPS. It gives users accountless private update URLs, FRITZ!Box-ready DynDNS fields, public A/AAAA records through Cloudflare or RFC 2136, and generated hostnames that also work as CNAME targets for user-owned domains.
- **Self-hosted Dynamic DNS provider** for home labs, small ISPs, communities, and private infrastructure.
- **FRITZ!Box compatible**: copy the generated `Update-URL`, `Domainnamen`, `Benutzername`, and `Kennwort` straight into the German FRITZ!Box DynDNS form.
That is the normal Cloudflare setup. Use a least-privilege Cloudflare API token limited to the zone KarlDNS should update. `DDNS_TRUSTED_HOSTS` is optional because KarlDNS automatically trusts the host in `DDNS_PUBLIC_BASE_URL`; set it only when the same instance should answer for additional public hostnames.
`DDNS_DNS_ZONES` is the allowlist of DNS zones this service may publish. For the public KarlDNS model this is normally only your provider zone, such as `karldns.de`.
Create a hostname in the web UI. For the normal accountless flow, the generated `Update-URL` already contains the secret update slug. Put that URL into the router and use the generated hostname as `Domainnamen`.
FRITZ!Box replaces `<ipaddr>` and `<ip6addr>` with the current WAN IPv4/IPv6 address and sends an HTTP GET request to that URL whenever it detects an address change. KarlDNS returns DynDNS-style responses such as `good 203.0.113.10`, `nochg 203.0.113.10`, `badauth`, or `nohost`.
This URL is enough for routers that only expose a single update URL field and do not send separate credentials. KarlDNS also shows a `Full update URL` that embeds `hostname`, `username`, and `password` as query parameters for clients that require credential-style DynDNS URLs but do not expose separate credential fields. Treat both URLs as secrets.
Every generated hostname can be updated with curl, cron, a shell script, Home Assistant, OpenWrt, pfSense, or any client that can send an HTTP GET request:
For Linux servers, Proxmox hosts, NAS boxes, and small home-lab machines that are always on, KarlDNS ships a cron updater. It creates a hostname if you do not pass one, installs a tiny updater script, schedules it with cron, runs the first update, and prints the hostname plus private dashboard URL.
System-wide install:
```bash
curl -fsSL https://karldns.de/install.sh | sudo sh
```
The same script is hosted in GitHub and can be used directly:
```bash
curl -fsSL https://raw.githubusercontent.com/StasonJatham/karldns.de/main/scripts/install-linux.sh | sudo sh
```
User-level install without sudo:
```bash
curl -fsSL https://karldns.de/install.sh | sh -s -- --mode user
```
Use an existing KarlDNS update URL:
```bash
curl -fsSL https://karldns.de/install.sh | sudo sh -s -- \
The updater discovers the public address through `https://karldns.de/ip` and sends it to the private update URL. If explicit discovery fails, it falls back to calling the update URL without IP parameters so KarlDNS can use the request source address. Re-running the installer is safe: it replaces the managed cron block and keeps unrelated cron entries intact.
Installed files in system mode:
```text
/usr/local/bin/karldns-update
/etc/karldns/updater.env
/etc/cron.d/karldns-updater
/var/cache/karldns/last-ip
```
Installed files in user mode:
```text
~/.local/bin/karldns-update
~/.config/karldns/updater.env
~/.cache/karldns/last-ip
```
`updater.env` is written with mode `600` because it contains the private update URL and dashboard URL. The hostname itself is not secret. The update URL and dashboard URL are secrets.
Uninstall, keeping the private config by default:
```bash
curl -fsSL https://karldns.de/uninstall.sh | sudo sh
```
Remove the private config and last-IP cache too:
```bash
curl -fsSL https://karldns.de/uninstall.sh | sudo sh -s -- --delete-config
```
For Proxmox, install on the host if the host owns the public connection. If you install inside an LXC that has no IPv6 route, use `--ipv6 off` so the cron job only updates A records:
```bash
curl -fsSL https://karldns.de/install.sh | sudo sh -s -- --ipv6 off
Put the KarlDNS `Update-URL`, `Domainnamen`, `Benutzername`, and `Kennwort` into the router. The router updates `a1b2c3d4.karldns.de`; `home.example.com` follows through the CNAME.
Use a subdomain such as `home.example.com`. Root/apex domains like `example.com` usually cannot be plain CNAME records unless your DNS provider supports CNAME flattening or ALIAS/ANAME records.
KarlDNS does not verify, claim, or publish records inside user-owned domains. Your DNS provider keeps serving `home.example.com`; KarlDNS only maintains the generated `*.karldns.de` target.
The generated hostname is not a secret. Treat it like any public DNS name. The update URL slug, router password, private dashboard URL, and full credentialed update URL are the secrets. Dashboard forms use CSRF tokens, and dashboard password unlock cookies last 24 hours by default unless the user chooses the 30-day stay-unlocked option.
The example environment is intentionally short: admin password/path, dashboard cookie signing secret, public URL, managed hostname suffix, Cloudflare token, Cloudflare zone ID, and the SQLite path. The app creates the SQLite parent directory and all tables on first run. Re-running the container or starting the app twice is idempotent; schema creation uses `CREATE TABLE IF NOT EXISTS`, and the cleanup scheduler keeps a single durable task row.
The image and compose file default `DDNS_DATABASE` to `/data/ddns.sqlite3`, and the compose volume persists `/data`.
The default container command runs one Uvicorn worker. Keep that deployment shape unless you have tested multi-worker update behavior for your traffic pattern; rate-limit counters and per-host update locks are process-local.
- Keep `/docs` and `/redoc` available for API users if you want a public developer surface. Admin routes are hidden from OpenAPI and still require Basic auth.
- Leave cleanup enabled. The cleanup scheduler is stored in SQLite with a task row, next run time, status, last error, and a database lease so only one app process runs a due cleanup job.
- Run one Uvicorn worker with SQLite unless you have tested your deployment under load. Scheduled cleanup is lease-protected across workers, but rate-limit state and per-host update locks are process-local.
- Keep Uvicorn access logs disabled. Use `DDNS_REDACTED_ACCESS_LOG=1` or `karldns serve --access-log` only for KarlDNS' built-in redacted access log, which masks bearer URL paths and query strings.
| `DDNS_DASHBOARD_COOKIE_SECRET` | `DDNS_ADMIN_PASSWORD` fallback | HMAC signing secret for dashboard unlock cookies. Use a separate 32+ character random value in production. |
| `DDNS_REDACTED_ACCESS_LOG` | `0` | Enable KarlDNS request logs with bearer paths and update query strings redacted. |
KarlDNS is suitable for self-hosting and private/public beta use when a real DNS backend is configured. For a larger public free DynDNS provider, add external abuse controls, monitoring, alerting, and a tested backup/restore process before launch.
The repository intentionally contains only placeholders in `.env.example` and documentation. Do not commit a real `.env`, DNS token, TSIG secret, admin password, database, private dashboard URL, or router-generated update URL. Generated hostnames and CNAME targets are public identifiers, not authentication secrets.
KarlDNS does not need secret-bearing query strings in application logs. The bundled server command leaves Uvicorn access logs disabled by default; configure reverse-proxy logs the same way or redact full request URLs.
