Host name for a standalone docker container

Question

I have a docker setup made up of a compose file with 2 services and one app running in a standalone container.

My compose file has this structure

version: '3'

services:

  synapse:
    image: docker.io/matrixdotorg/synapse:latest
    depends_on:
      - db
    ports:
      - 8008:8008
  db:
    image: docker.io/postgres:12-alpine
    # other commandds

When I do docker-compose up a default network called synapse_default. And the synapse service is able to connect to the db using db as host. That is my postgres URL looks like this postgres://synapse:synapse@db/postgres. You'll notice the host name is db and this works because they're been run in the same docker network.

Now I have a standalone application in another container I run with docker run --network=synapse_default and this app is able to connect to the synapse and db services using the service names as host addresses.

My problem now is to have the synapse service talk to standalone application. Since this standalone application was not part of the compose file, it does not have a service name hence I'm not sure what to specify as the host for the synapse service to be able to reach the standalone application.

Answer 1

You can find the name of your 'standalone application' with docker ps:

~$ docker ps
CONTAINER ID     CREATED          STATUS          NAMES
205f1583e0a0     42 seconds ago   Up 40 seconds   foo

foo in the example above is the name you can use. You can also use 205f1583e0a0 (CONTAINER ID) if you find it convenient. If you wish to use a specific name (like foo in the example), you have to run the standalone application with --name argument:

docker run --name=foo ...

Answer 2

I got this to work by specifying --hostname when running my standalone container.

I'm only keep this question open because I already have one response but this was a weak question as I failed to do proper research. A quick docker run --help provided me with the answer I needed.