host-docker-internal

Container to add to Docker Compose to get host.docker.internal on Linux.

Intro

This is the source repo for docker.io/benizi/host.docker.internal. Its only purpose is to provide a Docker container (for use in Docker Compose) that forwards all its network traffic to a single IP (the host running Docker Compose). It uses the same iptables + capabilities approach as qoomon/docker-host, but its use case is intentionally much more limited in scope.

Usage

Add to Docker Compose docker-compose.yml (or docker-compose.override.yml):

version: "3.8"
services:
  host:
    image: benizi/host.docker.internal
    cap_add: [NET_ADMIN, NET_RAW]
    networks: { default: { aliases: [host.docker.internal] } }
    restart: on-failure

• The capabilities in cap_add are required for iptables
• The networks setting is optional, but that's the main reason this exists
• The restart setting is probably overly cautious (shouldn't be any reason it would fail, but it doesn't hurt)

Configuration

Configuration can be done through environment variables, e.g., in the docker-compose.yml service:

# ...
services:
  host:
    ## From above:
    image: benizi/host.docker.internal
    cap_add: [NET_ADMIN, NET_RAW]
    networks: { default: { aliases: [host.docker.internal] } }
    restart: on-failure

    ## Configuration
    environment:
      ## Set the destination ...
      # ... by IP
      IP: '10.1.2.1'
      # ... or hostname (resolved in the container)
      HOST: 'service.network.internal'
      # Forward these port ranges: (default: `0:65535`)
      PORTS: 80:100,8800:8900
      # Optionally, route all traffic to a single port:
      # TARGET_PORT: 8080 (or `DPORT` (D = destination) or `TPORT` (T = target))

      ## Print `iptables` stats this frequently (default: `1h`)
      # Should accept anything Alpine Linux `sleep` accepts
      STATS: 1m
      # Don't print any stats:
      # STATS: '0'

      ## Debugging options
      # Enable debugging (any non-empty value)
      DEBUG: 'yup'
      # ... or just for printing environment variables beforehand:
      DEBUG_ENV: 'yup'
      # ... or just for the `/entrypoint` script (mainly for my own use):
      DEBUG_SCRIPT: 'yup'

Features

• Determine IP for forwarding:
  • Specify forwarding IP via $IP (directly)
  • Specify forwarding IP via $HOST (resolved in the container)
  • Use default gateway IP otherwise
• Reroute all traffic to a single port:
  • Specify target port via $TARGET_PORT

License

Copyright © 2020–2021 Benjamin R. Haskell

Distributed under the MIT License (included in file: LICENSE).