Links between containers not working with docker-compose version 2

Question

Linking between containers in a version 2 docker-compose file is not working.

Only when using the 'old' version 1 format, I do see the link in /etc/hosts of the container.

I have the following basic version 2 docker-compose.yml file.

version: '2'

services:
  my-app:
    image: tomcat:8.0
    container_name: my-app1
    links:
      - my-redis
  my-redis:
    image: redis
    container_name: my-redis1

When I run the following command:

docker-compose up -d

I see that two containers are started, but no link is created in the /etc/hosts file:

docker exec -it my-app1 cat /etc/hosts

    127.0.0.1       localhost
    ::1     localhost ip6-localhost ip6-loopback
    fe00::0 ip6-localnet
    ff00::0 ip6-mcastprefix
    ff02::1 ip6-allnodes
    ff02::2 ip6-allrouters
    172.18.0.3      2abb84ccada9

From 'my-app1' I can ping the other container using the IP address of 'my-redis1', but I cannot 'ping my-redis1' (based on a name).

What could be the problem here?

Additional information:

• Docker version 1.10.0, build 590d5108
• docker-compose version 1.6.0, build d99cad6
• Linux kernel: 4.3.5-300.fc23.x86_64

Answer 1

With version 2 of docker-compose the 'services' (containers) that are in the same network are linked between them by default.

Using the below docker-compose.yml file

version: '2'

services:
  my-app:
    image: tomcat:8.0
    container_name: my-app1
    links:
      - my-redis
  my-redis:
    image: redis
    container_name: my-redis1

You just can execute ping my-app from your my-redis container and ping my-redis from your my-app container to check that they are linked.

For instance:

$ docker-compose up -d
$ docker exec -it my-app1 bash
# ping my-redis

You can get more information about that in the links below: https://blog.docker.com/2016/02/compose-1-6/ https://github.com/docker/compose/blob/master/docs/networking.md

Answer 2

The problem is the firewalld of my Fedora host.

With the firewall temporarily disabled ('systemctl stop firewalld', followed by 'systemctl restart docker') everything works according to the docker documentation.

There seems to be a major problem with firewalld when used with docker, see: https://github.com/docker/docker/issues/16137.

Note that RHEL/Centos 7 also use firewalld.

-Arjen