Docker Compose hostname command not working

Question

I'm unable to get the Docker Compose hostname command to work.

I'm running a simple docker-compose.yml:

version: '3'
services:
  redis1:
    image: "redis:alpine"
    hostname: redis1host
  redis2:
    image: "redis:alpine"
    hostname: redis2host

Once I run this with docker-compose up, I should be able to run docker-compose exec redis1 /bin/ash and then ping redis2host to talk to the other Redis container, but the ping just doesn't reach its destination. I can ping the other Redis container with ping redis2.

ping redishost2 should work, no?

Answer 1

The hostname directive simply sets the hostname inside the container (that is, the name you get back in response to the hostname or uname -n commands). It does not result in a DNS alias for the service. For that, you want the aliases directive. Since that directive is per-network, you need to be explicit about networks rather than using the compose default, for example:

version: '3'
services:
  redis1:
    image: "redis:alpine"
    hostname: redis1host
    networks:
      redis:
        aliases:
          - redis1host
  redis2:
    image: "redis:alpine"
    hostname: redis2host
    networks:
      redis:
        aliases:
          - redis2host

networks:
  redis: