Customize the docker0 bridge

Estimated reading time: 3 minutes

The information in this section explains how to customize the Docker default bridge. This is a bridge network named bridge created automatically when you install Docker.

Note: The Docker networks feature allows you to create user-defined networks in addition to the default bridge network.

By default, the Docker server creates and configures the host system’s docker0 interface as an Ethernet bridge inside the Linux kernel that can pass packets back and forth between other physical or virtual network interfaces so that they behave as a single Ethernet network.

Docker configures docker0 with an IP address, netmask and IP allocation range. The host machine can both receive and send packets to containers connected to the bridge, and gives it an MTU – the maximum transmission unit or largest packet length that the interface will allow – of 1,500 bytes. These options are configurable at server startup:

  • --bip=CIDR: supply a specific IP address and netmask for the docker0 bridge, using standard CIDR notation. For example: 192.168.1.5/24.

  • --fixed-cidr=CIDR: restrict the IP range from the docker0 subnet, using standard CIDR notation. For example: 172.16.1.0/28. This range must be an IPv4 range for fixed IPs, such as 10.20.0.0/16, and must be a subset of the bridge IP range (docker0 or set using --bridge). For example, with --fixed-cidr=192.168.1.0/25, IPs for your containers will be chosen from the first half of addresses included in the192.168.1.0/24 subnet.

  • --mtu=BYTES: override the maximum packet length on docker0.

  • --default-gateway=Container default Gateway IPV4 address: designates the default gateway for containers connected to the docker0 bridge, which controls where they route traffic by default. Applicable for addresses set with --bip and --fixed-cidr flags. For instance, you can configure --fixed-cidr=172.17.2.0/24 and default-gateway=172.17.1.1.

  • --dns=[]: The DNS servers to use. For example: --dns=172.17.2.10. You can also specify DNS servers when starting the Docker daemon, by adding the values to /etc/docker/daemon.json (recommended) or using the --dns flag when starting dockerd manually.

Once you have one or more containers up and running, you can confirm that Docker has properly connected them to the docker0 bridge by running the brctl command on the host machine and looking at the interfaces column of the output. This example shows a docker0 bridge with two containers connected:

$ sudo brctl show

bridge name     bridge id               STP enabled     interfaces
docker0         8000.3a1d7362b4ee       no              veth65f9
                                                        vethdda6

If the brctl command is not installed on your Docker host, then on Ubuntu you should be able to run sudo apt-get install bridge-utils to install it.

Finally, the docker0 Ethernet bridge settings are used every time you create a new container. Docker selects a free IP address from the range available on the bridge each time you docker run a new container, and configures the container’s eth0 interface with that IP address and the bridge’s netmask. The Docker host’s own IP address on the bridge is used as the default gateway by which each container reaches the rest of the Internet.

# The network, as seen from a container

$ docker run --rm -it alpine /bin/ash

root@f38c87f2a42d:/# ip addr show eth0

24: eth0: <BROADCAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 32:6f:e0:35:57:91 brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.3/16 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::306f:e0ff:fe35:5791/64 scope link
       valid_lft forever preferred_lft forever

root@f38c87f2a42d:/# ip route

default via 172.17.42.1 dev eth0
172.17.0.0/16 dev eth0  proto kernel  scope link  src 172.17.0.3

root@f38c87f2a42d:/# exit

Remember that the Docker host will not be willing to forward container packets out on to the Internet unless its ip_forward system setting is 1 – see the section on Communicating to the outside world for details.

docker, bridge, docker0, network