Running a GUI program in Docker can be a useful technique when you’re evaluating a new piece of software. You can install the software in a clean container, instead of having to pollute your host with new packages.

This approach also helps you avoid any incompatibilities with other packages in your environment. If you need to temporarily run two versions of a program, you can use Docker to avoid having to remove and reinstall the software on your host.

Forwarding An X Socket to A Docker Container

Providing a Docker container with access to your host’s X socket is a straightforward procedure. The X socket can be found in /tmp/.X11-unix on your host. The contents of this directory should be mounted into a Docker volume assigned to the container. You’ll need to use the host networking mode for this to work.

You must also provide the container with a DISPLAY environment variable. This instructs X clients – your graphical programs – which X server to connect to. Set DISPLAY in the container to the value of $DISPLAY on your host.

You can encapsulate all this configuration in one docker-compose.yml file:

version: "3"

services:
  app:
    image: my-app:latest
    build: .
    environment:
      - DISPLAY=${DISPLAY}
    volumes:
      - /tmp/.X11-unix:/tmp/.X11-unix
    network_mode: host

Next, you need to create a Dockerfile for your application. Here’s an example that runs the Firefox web browser:

FROM ubuntu:latest
RUN apt-get update && apt-get install -y firefox
CMD ["/usr/bin/firefox"]

Now build and run the image:

docker-compose build
docker-compose up

A new Firefox window should appear on your desktop! The Firefox instance will run within the container, independently of any other open Firefox windows. The container will share your host’s X socket, so the containerised Firefox still shows up on your desktop.

This approach should only be used when you trust your Docker container. Exposing the host’s display server is a security risk if you’re not completely sure what lies inside the container.

Handling X Authentication

You might need to authenticate the container to access the X Server. First get an X authentication token from your host machine. Run xauth list and note down one of the listed cookies. You’ll need to copy the entire line.

Inside the Docker container, install the xauth package. Then run xauth add, passing the token you copied in the previous step.

apt install -y xauth
xauth add <token>

Your container should now successfully authenticate to the X Server.

Another Approach – Running a VNC Server

If you’re unable to use X socket forwarding, you could setup a VNC server inside your container. This approach lets you view graphical apps in the container by connecting from a VNC client running on the host.

Add the VNC server software to your container:

FROM ubuntu:latest
RUN apt-get update && apt-get install -y firefox x11vnc xvfb
RUN echo "exec firefox" > ~/.xinitrc && chmod +x ~/.xinitrc
CMD ["v11vnc", "-create", "-forever"]

When you run this container, a VNC server will be created automatically. You must bind a host port to the container’s port 5900 – this is the port the VNC server will be exposed on.

Firefox gets launched on startup as it’s added to .xinitrc. This file will be executed when the VNC server launches and initialises a new display.

To connect to the server, you’ll need a VNC client on your host. Find the IP address of your container by running docker ps, noting down the container ID and passing it to docker inspect <container>. You’ll find the IP address near the bottom of the output, within the Network node.

Use the container’s IP address with your VNC client. Connect on port 5900 without authentication. You should now be able to interact with the graphical programs running within your Docker container.