computer made a thing

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+# 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
+