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Get started with multi-host networking

Get started with multi-host networking

This article uses an example to explain the basics of creating a multi-host network. Docker Engine supports multi-host networking out-of-the-box through the overlay network driver. Unlike bridge networks, overlay networks require some pre-existing conditions before you can create one. These conditions are:

  • Access to a key-value store. Docker supports Consul, Etcd, and ZooKeeper (Distributed store) key-value stores.
  • A cluster of hosts with connectivity to the key-value store.
  • A properly configured Engine daemon on each host in the cluster.
  • Hosts within the cluster must have unique hostnames because the key-value store uses the hostnames to identify cluster members.

Though Docker Machine and Docker Swarm are not mandatory to experience Docker multi-host networking, this example uses them to illustrate how they are integrated. You’ll use Machine to create both the key-value store server and the host cluster. This example creates a Swarm cluster.

Prerequisites

Before you begin, make sure you have a system on your network with the latest version of Docker Engine and Docker Machine installed. The example also relies on VirtualBox. If you installed on a Mac or Windows with Docker Toolbox, you have all of these installed already.

If you have not already done so, make sure you upgrade Docker Engine and Docker Machine to the latest versions.

Step 1: Set up a key-value store

An overlay network requires a key-value store. The key-value store holds information about the network state which includes discovery, networks, endpoints, IP addresses, and more. Docker supports Consul, Etcd, and ZooKeeper key-value stores. This example uses Consul.

  1. Log into a system prepared with the prerequisite Docker Engine, Docker Machine, and VirtualBox software.

  2. Provision a VirtualBox machine called mh-keystore.

    $ docker-machine create -d virtualbox mh-keystore

    When you provision a new machine, the process adds Docker Engine to the host. This means rather than installing Consul manually, you can create an instance using the consul image from Docker Hub. You’ll do this in the next step.

  3. Set your local environment to the mh-keystore machine.

    $  eval "$(docker-machine env mh-keystore)"

  4. Start a progrium/consul container running on the mh-keystore machine.

    $  docker run -d \
    -p "8500:8500" \
    -h "consul" \
    progrium/consul -server -bootstrap

    The client starts a progrium/consul image running in the mh-keystore machine. The server is called consul and is listening on port 8500.

  5. Run the docker ps command to see the consul container.

    $ docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                                                                            NAMES
4d51392253b3        progrium/consul     "/bin/start -server -"   25 minutes ago      Up 25 minutes       53/tcp, 53/udp, 8300-8302/tcp, 0.0.0.0:8500->8500/tcp, 8400/tcp, 8301-8302/udp   admiring_panini

Keep your terminal open and move onto the next step.

Step 2: Create a Swarm cluster

In this step, you use docker-machine to provision the hosts for your network. At this point, you won’t actually create the network. You’ll create several machines in VirtualBox. One of the machines will act as the Swarm master; you’ll create that first. As you create each host, you’ll pass the Engine on that machine options that are needed by the overlay network driver.

  1. Create a Swarm master.

    $ docker-machine create \
-d virtualbox \
--swarm --swarm-master \
--swarm-discovery="consul://$(docker-machine ip mh-keystore):8500" \
--engine-opt="cluster-store=consul://$(docker-machine ip mh-keystore):8500" \
--engine-opt="cluster-advertise=eth1:2376" \
mhs-demo0

    At creation time, you supply the Engine daemon with the --cluster-store option. This option tells the Engine the location of the key-value store for the overlay network. The bash expansion $(docker-machine ip mh-keystore) resolves to the IP address of the Consul server you created in “STEP 1”. The --cluster-advertise option advertises the machine on the network.

  2. Create another host and add it to the Swarm cluster.

    $ docker-machine create -d virtualbox \
    --swarm \
    --swarm-discovery="consul://$(docker-machine ip mh-keystore):8500" \
    --engine-opt="cluster-store=consul://$(docker-machine ip mh-keystore):8500" \
    --engine-opt="cluster-advertise=eth1:2376" \
  mhs-demo1

  3. List your machines to confirm they are all up and running.

    $ docker-machine ls
NAME         ACTIVE   DRIVER       STATE     URL                         SWARM
default      -        virtualbox   Running   tcp://192.168.99.100:2376
mh-keystore  *        virtualbox   Running   tcp://192.168.99.103:2376
mhs-demo0    -        virtualbox   Running   tcp://192.168.99.104:2376   mhs-demo0 (master)
mhs-demo1    -        virtualbox   Running   tcp://192.168.99.105:2376   mhs-demo0

At this point you have a set of hosts running on your network. You are ready to create a multi-host network for containers using these hosts.

Leave your terminal open and go onto the next step.

Step 3: Create the overlay Network

To create an overlay network

  1. Set your docker environment to the Swarm master.

    $ eval $(docker-machine env --swarm mhs-demo0)

    Using the --swarm flag with docker-machine restricts the docker commands to Swarm information alone.

  2. Use the docker info command to view the Swarm.

    $ docker info
Containers: 3
Images: 2
Role: primary
Strategy: spread
Filters: affinity, health, constraint, port, dependency
Nodes: 2
mhs-demo0: 192.168.99.104:2376
└ Containers: 2
└ Reserved CPUs: 0 / 1
└ Reserved Memory: 0 B / 1.021 GiB
└ Labels: executiondriver=native-0.2, kernelversion=4.1.10-boot2docker, operatingsystem=Boot2Docker 1.9.0-rc1 (TCL 6.4); master : 4187d2c - Wed Oct 14 14:00:28 UTC 2015, provider=virtualbox, storagedriver=aufs
mhs-demo1: 192.168.99.105:2376
└ Containers: 1
└ Reserved CPUs: 0 / 1
└ Reserved Memory: 0 B / 1.021 GiB
└ Labels: executiondriver=native-0.2, kernelversion=4.1.10-boot2docker, operatingsystem=Boot2Docker 1.9.0-rc1 (TCL 6.4); master : 4187d2c - Wed Oct 14 14:00:28 UTC 2015, provider=virtualbox, storagedriver=aufs
CPUs: 2
Total Memory: 2.043 GiB
Name: 30438ece0915

    From this information, you can see that you are running three containers and two images on the Master.

  3. Create your overlay network.

    $ docker network create --driver overlay --subnet=10.0.9.0/24 my-net

    You only need to create the network on a single host in the cluster. In this case, you used the Swarm master but you could easily have run it on any host in the cluster.

Note : It is highly recommended to use the --subnet option when creating a network. If the --subnet is not specified, the docker daemon automatically chooses and assigns a subnet for the network and it could overlap with another subnet in your infrastructure that is not managed by docker. Such overlaps can cause connectivity issues or failures when containers are connected to that network.

  1. Check that the network is running:

    $ docker network ls
NETWORK ID          NAME                DRIVER
412c2496d0eb        mhs-demo1/host      host
dd51763e6dd2        mhs-demo0/bridge    bridge
6b07d0be843f        my-net              overlay
b4234109bd9b        mhs-demo0/none      null
1aeead6dd890        mhs-demo0/host      host
d0bb78cbe7bd        mhs-demo1/bridge    bridge
1c0eb8f69ebb        mhs-demo1/none      null

    As you are in the Swarm master environment, you see all the networks on all the Swarm agents: the default networks on each engine and the single overlay network. Notice that each NETWORK ID is unique.

  2. Switch to each Swarm agent in turn and list the networks.

    $ eval $(docker-machine env mhs-demo0)
$ docker network ls
NETWORK ID          NAME                DRIVER
6b07d0be843f        my-net              overlay
dd51763e6dd2        bridge              bridge
b4234109bd9b        none                null
1aeead6dd890        host                host
$ eval $(docker-machine env mhs-demo1)
$ docker network ls
NETWORK ID          NAME                DRIVER
d0bb78cbe7bd        bridge              bridge
1c0eb8f69ebb        none                null
412c2496d0eb        host                host
6b07d0be843f        my-net              overlay

Both agents report they have the my-net network with the 6b07d0be843f ID. You now have a multi-host container network running!

Step 4: Run an application on your Network

Once your network is created, you can start a container on any of the hosts and it automatically is part of the network.

  1. Point your environment to the Swarm master.

    $ eval $(docker-machine env --swarm mhs-demo0)

  2. Start an Nginx web server on the mhs-demo0 instance.

    $ docker run -itd --name=web --net=my-net --env="constraint:node==mhs-demo0" nginx

  3. Run a BusyBox instance on the mhs-demo1 instance and get the contents of the Nginx server’s home page.

    $ docker run -it --rm --net=my-net --env="constraint:node==mhs-demo1" busybox wget -O- http://web
Unable to find image 'busybox:latest' locally
latest: Pulling from library/busybox
ab2b8a86ca6c: Pull complete
2c5ac3f849df: Pull complete
Digest: sha256:5551dbdfc48d66734d0f01cafee0952cb6e8eeecd1e2492240bf2fd9640c2279
Status: Downloaded newer image for busybox:latest
Connecting to web (10.0.0.2:80)
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>
body {
        width: 35em;
        margin: 0 auto;
        font-family: Tahoma, Verdana, Arial, sans-serif;
}
</style>
</head>
<body>
<h1>Welcome to nginx!</h1>
<p>If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.</p>

<p>For online documentation and support please refer to
<a href="http://nginx.org/">nginx.org</a>.<br/>
Commercial support is available at
<a href="http://nginx.com/">nginx.com</a>.</p>

<p><em>Thank you for using nginx.</em></p>
</body>
</html>
-                    100% |*******************************|   612   0:00:00 ETA

Step 5: Check external connectivity

As you’ve seen, Docker’s built-in overlay network driver provides out-of-the-box connectivity between the containers on multiple hosts within the same network. Additionally, containers connected to the multi-host network are automatically connected to the docker_gwbridge network. This network allows the containers to have external connectivity outside of their cluster.

  1. Change your environment to the Swarm agent.

    $ eval $(docker-machine env mhs-demo1)

  2. View the docker_gwbridge network, by listing the networks.

    $ docker network ls
NETWORK ID          NAME                DRIVER
6b07d0be843f        my-net              overlay
dd51763e6dd2        bridge              bridge
b4234109bd9b        none                null
1aeead6dd890        host                host
e1dbd5dff8be        docker_gwbridge     bridge

  3. Repeat steps 1 and 2 on the Swarm master.

    $ eval $(docker-machine env mhs-demo0)
$ docker network ls
NETWORK ID          NAME                DRIVER
6b07d0be843f        my-net              overlay
d0bb78cbe7bd        bridge              bridge
1c0eb8f69ebb        none                null
412c2496d0eb        host                host
97102a22e8d2        docker_gwbridge     bridge

  4. Check the Nginx container’s network interfaces.

    $ docker exec web ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
    valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
    valid_lft forever preferred_lft forever
22: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue state UP group default
link/ether 02:42:0a:00:09:03 brd ff:ff:ff:ff:ff:ff
inet 10.0.9.3/24 scope global eth0
    valid_lft forever preferred_lft forever
inet6 fe80::42:aff:fe00:903/64 scope link
    valid_lft forever preferred_lft forever
24: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
link/ether 02:42:ac:12:00:02 brd ff:ff:ff:ff:ff:ff
inet 172.18.0.2/16 scope global eth1
    valid_lft forever preferred_lft forever
inet6 fe80::42:acff:fe12:2/64 scope link
    valid_lft forever preferred_lft forever

    The eth0 interface represents the container interface that is connected to the my-net overlay network. While the eth1 interface represents the container interface that is connected to the docker_gwbridge network.

Step 6: Extra Credit with Docker Compose

Please refer to the Networking feature introduced in Compose V2 format and execute the multi-host networking scenario in the Swarm cluster used above.

doc_docker
doc_docker
2017-02-04 08:23:36
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