In a previous article, we discussed Docker Machine, a tool to create Docker hosts in the cloud. Docker Machine can be extremely handy for local testing if you are on Windows and OS X, but it also adds another dimension when you use it to start Docker hosts in your favorite cloud provider and/or create a cluster.

We then used Machine to go straight into an advanced subject and create a Docker Swarm, which is a cluster of Docker hosts. A cluster of Docker hosts is needed to run a truly distributed application in production. In this article, we will look at setting up networking for a Swarm to allow containers to communicate with each other across hosts. Indeed, a Swarm cluster allows us to use the native single host networking of Docker, but it also allows us to create a network overlay backed by VXLAN. Containers started in this overlay can communicate out of the box with each other. This article will show you how to create, use, and test an overlay network using Docker Swarm.

Creating an Overlay Network

After you’ve set up your Swarm, you could start using it right away and start containers in the way that you are accustomed to. Docker will automatically use what is called a bridge network. Although this is good and you can expose service on your hosts, it complicates networking between containers that are started on multiple hosts as you would have to bind services to the hosts and let the containers know where to find each other.

Libnetwork provides a network overlay that can be used by your containers so that they appear on the same subnet. The huge bonus is that they can reach each other and resolve each other’s DNS names, making service discovery a breeze.

With your Swarm master or worker nodes as your active Docker Machine, you can create an overlay network with the `docker network create` command, like so:

$ docker network create foobar 165e9c2bafab44513da2f26426216217dc69ca2cd021f966ccc64e7c6bf898d9

You can list the networks available to you. You will see multiple networks. Each host will have a `bridge`, a `host`, and a `none` network. These three network types are also available on a single Docker Engine setup, with the bridge network being the default setup. Our `foobar` overlay created above appears in this list and is global to all the hosts in our Swarm.

$ docker network ls NETWORK ID NAME DRIVER 2c48d476867e swarm-master/bridge bridge 0b6ae86378f3 swarm-master/none null 967c471c311c swarm-master/host host 01f3d280bc68 swarm-node-1/bridge bridge d0f929b000bc swarm-node-1/none null 71550dff8c32 swarm-node-1/host host 165e9c2bafab foobar overlay

Using the Overlay Network

To use our overlay, we can start containers in the Swarm, giving them a name and specifying a `foobar` as our network like so:

$ docker run -d --name=foo --net=foobar nginx $ docker run -d --name=bar --net=foobar nginx

When listing our containers, we will see which host they have been started on. You might have to dive a bit into Swarm scheduling strategies to understand how Swarm picks a host to run a container. It could be that Swarm schedules your two test containers on the same host. In the test below, we had two worker nodes, and Swarm scheduled our containers on both of them, spreading the containers in the cluster.

$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 21587d81505d nginx "nginx -g 'daemon off" 2 seconds ago Up 2 seconds 80/tcp, 443/tcp swarm-node-1/bar 6d66dc56af4f nginx "nginx -g 'daemon off" 9 seconds ago Up 8 seconds 80/tcp, 443/tcp swarm-node-2/foo

Testing the Overlay Network

This approach allows us to test our overlay networking. If all went well, they should be on the same overlay network even though they are on separate hosts. This means we should be able to `ping` each container using its name — which has been used in an embedded DNS registration. Let’s try it, using the `docker exec` command:

$ docker exec -ti swarm-node-1/bar ping -c 1 foo PING bar (10.0.0.3): 56 data bytes 64 bytes from 10.0.0.3: icmp_seq=0 ttl=64 time=1.433 ms --- bar ping statistics --- 1 packets transmitted, 1 packets received, 0% packet loss round-trip min/avg/max/stddev = 1.433/1.433/1.433/0.000 ms

Indeed, we can ping the container named `foo` from the container named `bar` and we can also do the opposite:

$ docker exec -ti swarm-node-2/foo ping -c 1 bar PING foo (10.0.0.2): 56 data bytes 64 bytes from 10.0.0.2: icmp_seq=0 ttl=64 time=0.984 ms --- foo ping statistics --- 1 packets transmitted, 1 packets received, 0% packet loss round-trip min/avg/max/stddev = 0.984/0.984/0.984/0.000 ms

And just like that you have multi-host networking in Docker. Containers started on different hosts by the Swarm scheduler can reach each other on their private IP thanks to an overlay network.

In a future post, we will get back to Docker Compose and see how we can take advantage of a Swarm and its overlay networks to create a truly distributed application where containers can be started on different networks for isolation and where we can scale each service independently.