How do I assign a port mapping to an existing Docker container?

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I’m not sure if I’ve misunderstood something here, but it seems like it’s only possible to set port mappings by creating a new container from an image. Is there a way to assign a port mapping to an existing Docker container?

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You can change the port mapping by directly editing the hostconfig.json file at
/var/lib/docker/containers/[hash_of_the_container]/hostconfig.json or /var/snap/docker/common/var-lib-docker/containers/[hash_of_the_container]/hostconfig.json, I believe, if You installed Docker as a snap.

You can determine the [hash_of_the_container] via the docker inspect <container_name> command and the value of the “Id” field is the hash.

  1. Stop the container (docker stop <container_name>).
  2. Stop docker service (per Tacsiazuma’s comment)
  3. Change the file.
  4. Restart your docker engine (to flush/clear config caches).
  5. Start the container (docker start <container_name>).

So you don’t need to create an image with this approach. You can also change the restart flag here.

P.S. You may visit https://docs.docker.com/engine/admin/ to learn how to correctly restart your docker engine as per your host machine. I used sudo systemctl restart docker to restart my docker engine that is running on Ubuntu 16.04.

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