Following up from "Adventures with Weave and Docker" I want to line out how to setup Docker in a way that every container gets its own cluster-wide unique IP, without any overlay network layer. This for sure is neither rocket science, nor anything new. But as I see so many people using Docker in a cluster environment, still fighting with port management, I want to write down how this can be implemented in a few steps:

0. The Goal and What We Need

We aim at

  • a cluster-wide, unique, automatically assigned IP address for every container
  • which is routable (switchable) within a private network
  • without any DHCP or other complicated infrastructure
  • without an overlay network
  • without OpenVSwitch.

We need

  • a large enough free IP range ( in this example)
  • a private network interface (eth1 in this example)
  • bridge-tools installed on two Docker hosts (Ubuntu 14.04 in this example).

1. Basic Idea

We setup our own custom bridge br0 with eth1 inside on each node. We use this as the Docker bridge – instead of the usual docker0.

We assign a subnet of the private IP range for each node, without overlaps. We pass this range as --fixed-cidr to Docker. Docker will select container IPs inside this range by itself, without any help of external DHCP services.

2. Setting up the Bridge

We setup the bridge in /etc/network/interfaces:

auto br0
iface br0 inet static
    bridge_ports eth1
    bridge_stp off
    bridge_fd 0

on each node. We choose IPs of the shape 10.1.0.x as host addresses.

Then we start the bridge with

$ ifup br0

If there was an address on eth1 before, make sure to remove it before this with

$ ifconfig eth1

3. Changing the Docker Daemon Network Settings

We tell Docker about our bridge and the network IP range to use in /etc/default/docker:

DOCKER_OPTS="--bridge=br0 --fixed-cidr="

Here, the network range must be chosen according to the host, i.e. the host with 10.1.0.x gets 10.1.x.0/24 as its IP range for the containers.

Now restart Docker and check that Docker assigns the right addresses:

$ restart docker
$ docker run -it ubuntu /bin/bash
root@508b511dab3e:/# ip addr show dev eth0
7: eth0: <BROADCAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default 
    link/ether 02:42:0a:01:01:02 brd ff:ff:ff:ff:ff:ff
    inet scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::42:aff:fe01:102/64 scope link 
       valid_lft forever preferred_lft forever

UPDATE: Note that Docker assigns, not to the inteface inside the container. Hence, not further routing setup is necessary.

4. Cross Host Connectivity

Make sure IPv4 forwarding is active:

$ cat /proc/sys/net/ipv4/ip_forward

If you use VirtualBox to test this (e.g. in Vagrant), make sure to allow promiscuous mode in the VM network settings.

Test that you can access container ports on another host:

node2 $ docker run -itd sttts/python-ubuntu:latest python -m SimpleHTTPServer 80

node2 $ docker inspect --format '{{ .NetworkSettings.IPAddress }}' 2935

node1 $ curl -I
HTTP/1.0 200 OK
Server: SimpleHTTP/0.6 Python/2.7.6
Date: Sat, 31 Jan 2015 17:12:02 GMT
Content-type: text/html; charset=ANSI_X3.4-1968
Content-Length: 810

Your network works. Congratulations.