# mdns-reflector — IPv4 + IPv6 mDNS reflector for MikroTik RouterOS

A lightweight mDNS reflector (multicast DNS repeater) designed to run as a container on MikroTik RouterOS 7.x. It relays mDNS packets (port 5353, groups `224.0.0.251` and `ff02::fb`) between all specified interfaces — both IPv4 **and** IPv6 — filling the gap where RouterOS' built-in `/ip dns set mdns-repeat-ifaces=` only supports IPv4.

## How it works

- Opens one IPv4 and one IPv6 UDP socket **per interface**, bound to that interface via `SO_BINDTODEVICE`
- Joins the mDNS multicast groups on each interface
- Spawns one reader thread per interface; packets received on any interface are forwarded to every other interface
- Writes a health heartbeat to `/tmp/mdns-health` every 10 seconds
- Runs as non-root user `mdns` with `cap_net_raw,cap_net_bind_service` capabilities

## Quick start

```bash
# Build
podman build -t mdns-reflector .

# Run (auto-detect non-loopback interfaces)
podman run --rm --network host mdns-reflector

# Run with explicit interfaces
podman run --rm --network host \
  -e INTERFACES="ether1 bridge" \
  mdns-reflector
```

## MikroTik RouterOS container setup

RB5009 running RouterOS 7.x:

```routeros
/container config set registry-url=https://git.sb20.xengi.de

/container add \
   remote-image=git.sb20.xengi.de/xengi/mdns-reflector:latest \
   interface=bridge \
   root-dir=mdns-reflector \
   envlist=mdns-env \
   start-on-boot=yes

/container env add name=mdns-env key=INTERFACES value="bridge iot private"

/container start [find where image~"mdns-reflector"]
```

> **Note:** MikroTik containers require `--network host`-like behavior, which you get by attaching the container to a single interface (usually your LAN bridge). The `INTERFACES` env var tells the reflector which interfaces *inside* the host to bridge mDNS across.

### Unprivileged container considerations

> This container already runs as non-root user `mdns`.
> On RouterOS, ensure the container has sufficient privileges:
>
> ```routeros
> /container set [find where image~"mdns-reflector"] \
>     nesting=yes \
>     hostname=mdns-reflector
> ```

## Environment variables

| Variable      | Default               | Description                                      |
|---------------|-----------------------|--------------------------------------------------|
| `INTERFACES`  | all non-lo interfaces  | Space-separated list of interface names to bridge |

## Health check

The container exposes a Docker `HEALTHCHECK` driven by `healthcheck.sh`. It verifies:

1. Health heartbeat file exists and is < 30 seconds old
2. `mdns-repeater` process is running
3. At least one bound UDP socket exists on port 5353

Then on RouterOS, pull the updated image:

```routeros
/container pull [find where image~"mdns-reflector"]
```

## Building from source

```bash
# Multi-stage build (produces ~3 MB image)
podman build -t mdns-reflector .

# Or compile the C binary directly
gcc -static -O2 -o mdns-repeater mdns-repeater.c -lpthread
```

## Files

| File                | Purpose                                       |
|---------------------|-----------------------------------------------|
| `mdns-repeater.c`   | IPv4/IPv6 mDNS reflector                     |
| `Dockerfile`        | Multi-stage Alpine build                      |
| `entrypoint.sh`     | Interface detection & launcher                |
| `healthcheck.sh`    | Docker HEALTHCHECK probe                     |
| `.dockerignore`     | Reduces build context size                    |

## License

MIT