Fix Docker Error: Failed Port Allocation

When attempting to run a Docker container with docker container run ..., you may see this error:

Bind for 0.0.0.0:8000 failed: port is already allocated

Whenever you see this error, it means that the container is unable to bind the container's exposed port to the port on the host machine.

Solution

This is an easy fix with two steps:

1. run lsof -i :8000 (replace 8000 with the port that's allocated in your error).

   lsof -i :8000
   COMMAND    PID         USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
   com.docke 1596 chaseadamsio   20u  IPv4 0x677313baaeffcfd7      0t0  TCP *:irdmi (LISTEN)
   com.docke 1596 chaseadamsio   21u  IPv6 0x677313ba9b203357      0t0  TCP localhost:irdmi (LISTEN)
   

   If the output has COMMAND of com.docker (as above), proceed to step 2.

   lsof -i :8000
   COMMAND   PID         USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
   node    46205 chaseadamsio   19u  IPv4 0x677313ba9d523357      0t0  TCP localhost:irdmi (LISTEN)
   

   If the output has COMMAND that's anything else, make a note of the PID for the process serving on that port and proceed to step 3.

2. run docker ps to get a list of running containers.

    docker ps
    CONTAINER ID        IMAGE                  COMMAND                   CREATED             STATUS                            PORTS                    NAMES
    20c6837d446e        crccheck/hello-world   "/bin/sh -c 'trap \"e…"   4 seconds ago       Up 3 seconds (health: starting)   0.0.0.0:8000->8000/tcp   web-test
   

   Under PORT you'll see a mapping of the HOST PORT (0.0.0.0:8000) -> CONTAINER PORT (8000/tcp). The port from your failure will be on one of the containers. Make a note of the CONTAINER ID and run docker stop <CONTAINER ID>. If you no longer need the container, run docker rm <CONTAINER ID> to clean it up.

3. With the PID from step 1 in hand, run ps <PID> to make sure the process serving on that port is okay to kill.

   If it is, first, check any open Terminal windows to make sure the process isn't running as a part of some other script. If it is, ctrl-c to send a SIGINT. If it isn't running in a Terminal window and you're confident that it's okay to kill, run kill <PID>. If you're not sure what it is, restart your host machine and see if it restarts. If the process restarts on machine booting, consider running your container on a different port.

After you've gone through the steps above, when you attempt to run docker container run... you should make it a little further through starting up your container.