Docker - Have new container communicate with pre-running container - docker

Question

I'm trying to setup some very simple networking between a pair of Docker containers and so far all the documentation I've seen is far more complex than for what I am trying to do.
My use case is simple:
Container 1 is already running and is listening on port 28016
Container 2 will start after container 1 and needs to connect to container 1 on port 28016.
I am aware I can set this up via Docker-Compose with ease, however Container 1 is long-lived and for this use case, I do not want to shut it down. Container 2 needs to start and automatically connect to container 1 via port 28016. Also, both containers are running on the same machine. I cannot figure out how to do this.
I've exposed 28016 in Container 1's dockerfile, and I'm running it with -p 28016:28016. What do I need to do for Container 2 to connect to Container 1?

Answer 1

There are a few ways of solving this. Most don't require you to publish the ports.
Using a user defined network
If you start your long-running container in a user-defined network, because then docker will handle
docker network create service-network
docker run --net=service-network --name Container1 service-image
If you then start your ephemeral container in the same network, it will be able to refer to the long-running container by name. E.g:
docker run --name Container2 --net=service-network ephemeral-image
Using the existing container network namespace
You can just run the ephemeral container inside the network namespace of the long running container:
docker run --name Container2 --net=container:Container1 ephemeral-image
In this case, the service would be available via localhost:28016.
Accessing the service on the host
Since you've published the service on the host with -p 28016:28016, you can refer to that access using the address of the host, which from inside the container is going to be the default gateway. You can get that with something like:
address=$(ip route | awk '$1 == "default" {print $3}')
And your service would be available on ${address}:28016.

Answer 2

Here are the steps to perform:
Create a network: docker network create my-net
Attach the network to the already running container: docker container attach <container-name> my-net
Start the new container with the --network my-net or with docker-compose add a network property:
...
networks:
- my-net
networks:
my-net:
external: true
The container should now be able to communicate using the container-name as a DNS host name