docker-host

Docker Image Tags:

• latest
• 3
• 3.x.x

Docker image to forward TCP and UDP traffic to the docker host. This also works for (rootless) podman. This README.md uses the term docker, but you can read that as both docker and podman (it works for both), unless otherwise specified.

This container will determine the docker host address in the following order

• If set use environment variable DOCKER_HOST. Can be an IP address or a DNS name.
  • This allows you to use this image to forward traffic to arbitrary destinations, not only the docker host.
• Try to resolve host address from DNS names.
  • docker hostname host.docker.internal
  • podman hostname host.containers.internal
• Defaults to default gateway (ip -4 route show default)

Ports

By default all ports (1-65535) are forwarded to docker host.

• You may restrict ports by setting environment variable PORTS to a space and/or comma separated list of ports and/or port ranges e.g
  • docker run -e PORTS='443, 8000-9000' ....
• You may also configure port mapping e.g. 443:8443, 8000-9000:5000-6000 (CONTAINER_PORT:HOST_PORT).

---

Important

On Linux systems

• You have to bind your host applications to 0.0.0.0 or bridge network gateway in addition to 127.0.0.1.

  Use following docker command to get the bridge network gateway IP address

  docker network inspect bridge --format='{{( index .IPAM.Config 0).Gateway}}'

  For (rootless) podman, it's sufficient to bind to localhost, assuming default podman installation.

• You might need to configure your firewall of the host system to allow the docker-host container to communicate with the host on your relevant port, see #21.

On MacOS systems

Podman Only

• You probably need to add nf_nat kernal module to podman machine by running following commands

  podman machine ssh
  
  sudo modprobe nf_nat

---

Examples

These examples will send messages from docker container to docker host with netcat

Preparation

Start netcat server TCP on port 2323 to receive and display messages

nc -lk 2323

Start netcat server UDP on port 5353 to receive and display messages

nc -lk 5353 -u

Docker Link

Run the dockerhost container.

docker run \
  --name 'docker-host' \
  --cap-add=NET_ADMIN --cap-add=NET_RAW \
  --restart on-failure \
  -d qoomon/docker-host

Run your application container and link the dockerhost container. The dockerhost will be reachable through the domain/link dockerhost of the dockerhost container

This example will let you send messages to TCP netcat server on docker host.

docker run --rm \
  --link 'docker-host' \
  -it alpine nc 'docker-host' 2323 -v

This example will let you send messages to UDP netcat server on docker host.

docker run --rm \
  --link 'docker-host' \
  -it alpine nc 'docker-host' 5353 -u -v

Docker Network

Create the dockerhost network.

network_name="Network-$RANDOM"
docker network create "$network_name"

Run the dockerhost container within the dockerhost network.

docker run --name "${network_name}-docker-host" \
  --cap-add=NET_ADMIN --cap-add=NET_RAW \
  --restart on-failure \
  --net=${network_name} --network-alias 'docker-host' \
  qoomon/docker-host

Run your application container within the dockerhost network. The dockerhost will be reachable through the domain/link docker-host of the dockerhost container

This example will let you send messages to TCP netcat server on docker host.

docker run --rm \
  --link 'docker-host' \
  -it alpine nc 'docker-host' 2323 -v

This example will let you send messages to UDP netcat server on docker host.

docker run --rm \
  --link 'docker-host' \
  -it alpine nc 'docker-host' 5353 -u -v

Docker Compose

version: '2'

services:
    docker-host:
        image: qoomon/docker-host
        cap_add: [ 'NET_ADMIN', 'NET_RAW' ]
        mem_limit: 8M
        restart: on-failure
    tcp_message_emitter:
        depends_on: [ docker-host ]
        image: alpine
        command: [ "sh", "-c", "while :; do date; sleep 1; done | nc 'docker-host' 2323 -v"]
    udp_message_emitter:
        depends_on: [ docker-host ]
        image: alpine
        command: [ "sh", "-c", "while :; do date; sleep 1; done | nc 'docker-host' 5353 -u -v"]

---

External References

• https://medium.com/@sam_ngu/connecting-to-docker-host-mysql-from-docker-container-linux-ubuntu-766e526542fd