Once in a while, you may need your Docker host's IP address. For instance, you need to be able to connect to the host network from inside a Docker container to access your app or database running locally on the host. Debugging or reverse proxies running on your host are two additional example use-cases.

I'll show you how to easily make this work simultaneously for macOS, Windows, and Linux - because their docker networking settings differ.

Docker Networking on macOS and Windows vs. Linux

For macOS and Windows the following special DNS name can be used:

The host has a changing IP address (or none if you have no network access). From 18.03 onwards our recommendation is to connect to the special DNS name host.docker.internal, which resolves to the internal IP address used by the host. This is for development purpose and will not work in a production environment outside of Docker Desktop for Mac/Windows.

The gateway is also reachable as gateway.docker.internal.

Source: https://docs.docker.com/docker-for-mac/networking/#use-cases-and-workarounds
Source: https://docs.docker.com/docker-for-windows/networking/#use-cases-and-workarounds

On Docker for Linux, the IP address of the gateway between the Docker host and the bridge network is 172.17.0.1 if you are using default networking.

Do you see the problem already? They are different, so you cannot simply run docker-compose up -d and all operating systems behave the same. But I got you covered, there's an easy approach to make this work.

Setup docker-compose

I've seen some suggestions, like creating a Linux-specific config file docker-compose.override.yml (docs), but the solution a co-worker of mine came up with seems more elegant and less complex to me.

# docker-compose.yml
version: '3.7'

services:
  app:
    image: your-app:latest
    ports:
      - "8080:8080"
    environment:
      DB_UPSTREAM: http://${DOCKER_GATEWAY_HOST:-host.docker.internal}:3000

So, what is happening here? The DB_UPSTREAM should point to the host's IP and port 3000. ${DOCKER_GATEWAY_HOST:-host.docker.internal} is the critical piece here. This expression tells docker-compose to either us the environment variable DOCKER_GATEWAY_HOST or use the fallback host.docker.internal when resolving this value.

On both macOS and Windows it works out of the box now without anything left to do. 🚀

If you are running this stack on Linux you need to have the DOCKER_GATEWAY_HOST environment variable set for the Docker gateway host. Simply put this line into your .bashrc (.bash_profile or .zshrc):

export DOCKER_GATEWAY_HOST=172.17.0.1

Now you can start the stack from macOS, Windows, and Linux without further configuration or overwrites. If you stick to this pattern - as we do - this works for every project of your company.

Great, isn't it? I hope this saves you some time!

References

• compose-file#extra_hosts: an entry with the ip address and hostname is created in /etc/hosts inside containers at build-time.

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