mdnsd - mDNS Repeater for Docker

mdnsd makes it easy to bridge mDNS packets between Docker networks and LAN-facing network interfaces. The instructions below make use of the pre-built containers on Docker Hub.

mdnsd is a Python wrapper around the powerful mdns-repeater by geekman. mdnsd provides a convenient abstraction for resolving Docker network names to host interfaces, and passing these to mdns-repeater for ...repeating. For advanced use cases, it's possible to pass command-line arguments directly to mdns-repeater.

Usage

Docker CLI

docker run -d \
  --name mdnsd \
  -e MDNSD_HOST_INTERFACES='[if_names]' \
  -e MDNSD_DOCKER_NETWORKS='[net_names]' \
  -v '/var/run/docker.sock:/var/run/docker.sock:ro'
  kjkent/mdnsd

Docker Compose

services:
  mdnsd:
    container_name: mdnsd
    environment:
      MDNSD_DOCKER_NETWORKS: '[net_names]'
      MDNSD_HOST_INTERFACES: '[if_names]'
    image: kjkent/mdnsd
    network_mode: host
    restart: always
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro

Advice

• MDNSD_HOST_INTERFACES and MDNSD_DOCKER_NETWORKS are space-delimited lists.
• Host interfaces can be found using a CLI tool such as ip link, and can look like wlan0, eth0, enp6s0, etc...
• By default, Docker names its bridge networks like docker0, but it's strongly recommended to set static names for the network(s) you wish to use with mdnsd. This precludes any issues with Docker unexpectedly using a different network name.
• Currently, mdnsd only supports resolving Docker bridge networks, though other types like macvlan will likely be added later.
• A maximum of five total interfaces (host interfaces + Docker networks) is supported.

Caution

Mounting the Docker socket (/var/run/docker.sock) to a container (even as read-only!) essentially gives it root access to your machine. Be aware of the security considerations this involves, and consider using a socket proxy.

mdnsd uses the Docker socket to resolve Docker network names to host interface names. Review the file src/python/entrypoint.py for the implementation details.

If you are using mdnsd to repeat traffic across host interfaces only, you do not need to mount the socket and can remove it from the configuration. In this scenario, ensure MDNSD_DOCKER_NETWORKS is not defined, otherwise the container will error-out.

Building locally

The provided build.yaml, by default, builds an image tagged as kjkent/mdnsd:local. Similarly, the sample docker-compose.yaml looks for an image named kjkent/mdnsd:latest (:latest being the default tag). These files are found in src/docker; edit them to fit your needs.

These commands are given as a loose guide; everybody's requirements and environments differ. If you're in doubt, use a prebuilt image as demonstrated above.

# Clone the repo
git clone --recursive https://github.com/kjkent/mdnsd.git

# Build the container
docker compose -f mdnsd/src/docker/build.yaml build

# Launch the service
docker compose up -d

# Watch it fly
docker compose logs --follow

Direct access to mdns-repeater

mdns-repeater accepts the following additional command-line arguments:

-b  blacklist subnet (eg. 192.168.1.1/24)
-w  whitelist subnet (eg. 192.168.1.1/24)

These can be used by passing them as commands through Docker. With Docker Compose, this is accomplished using the command key. For the Docker CLI, just append the args to the end of the docker run command.

Acknowledgements

• geekman/mdns-repeater: mdnsd is a simple wrapper around this powerful utility.

• monstrenyatko/docker-mdns-repeater - A similar project and the inspiration for mdnsd.