docker - How to create a bidirectional link between containers?

Question

I have to link two containers so they can see each other. Of course the following...

docker run -i -t --name container1 --link container2:container2 ubuntu:trusty /bin/bash
docker run -i -t --name container2 --link container1:container1 ubuntu:trusty /bin/bash

...fails at line 1 because a container needs to be up and running in order to be a link target:

2014/08/15 03:20:27 Error response from daemon: Could not find entity for container2

What is the simplest way to create a bidirectional link?

Answer 1

Docker 1.10 addresses this very nicely by introducing advanced container networking. (Details: https://docs.docker.com/engine/userguide/networking/dockernetworks/ )

First, create a network. The example below creates a basic "bridge" network, which works on one host only. You can check out docker's more complete documentation to do this across hosts using an overlay network.

docker network create my-fancy-network

Docker networks in 1.10 now create a special DNS resolution inside of containers that can resolve the names in a special way. First, you can continue to use --link, but as you've pointed out your example doesn't work. What I recommend is using --net-alias= in your docker run commands:

docker run -i -t --name container1 --net=my-fancy-network --net-alias=container1 ubuntu:trusty /bin/bash
docker run -i -t --name container2 --net=my-fancy-network --net-alias=container2 ubuntu:trusty /bin/bash

Note that having --name container2 is setting the container name, which also creates a DNS entry and --net-alias=container2 just creates a DNS entry on the network, so in this particular example you could omit --net-alias but I left it there in case you wanted to rename your containers and still have a DNS alias that does not match your container name.

(Details here: https://docs.docker.com/engine/userguide/networking/configure-dns/ )

And here you go:

root@4dff6c762785:/# ping container1
PING container1 (172.19.0.2) 56(84) bytes of data.
64 bytes from container1.my-fancy-network (172.19.0.2): icmp_seq=1 ttl=64 time=0.101 ms
64 bytes from container1.my-fancy-network (172.19.0.2): icmp_seq=2 ttl=64 time=0.074 ms
64 bytes from container1.my-fancy-network (172.19.0.2): icmp_seq=3 ttl=64 time=0.072 ms

And from container1

root@4f16381fca06:/# ping container2
PING container2 (172.19.0.3) 56(84) bytes of data.
64 bytes from container2.my-fancy-network (172.19.0.3): icmp_seq=1 ttl=64 time=0.060 ms
64 bytes from container2.my-fancy-network (172.19.0.3): icmp_seq=2 ttl=64 time=0.069 ms
64 bytes from container2.my-fancy-network (172.19.0.3): icmp_seq=3 ttl=64 time=0.062 ms