How to get the IP address of the docker host from inside a docker container

Question

As the title says. I need to be able to retrieve the IP address the docker hosts and the portmaps from the host to the container, and doing that inside of the container.

Answer 1

If you enabled the docker remote API (via -Htcp://0.0.0.0:4243 for instance) and know the host machine's hostname or IP address this can be done with a lot of bash.

Within my container's user's bashrc:

export hostIP=$(ip r | awk '/default/{print $3}')
export containerID=$(awk -F/ '/docker/{print $NF;exit;}' /proc/self/cgroup)
export proxyPort=$(
  curl -s http://$hostIP:4243/containers/$containerID/json |
  node -pe 'JSON.parse(require("fs").readFileSync("/dev/stdin").toString()).NetworkSettings.Ports["DESIRED_PORT/tcp"][0].HostPort'
)

The second line grabs the container ID from your local /proc/self/cgroup file.

Third line curls out to the host machine (assuming you're using 4243 as docker's port) then uses node to parse the returned JSON for the DESIRED_PORT.