Last Updated: April 20, 2021

Docker - Create a Bridge and Shared Network

Sometimes I want to use Docker containers like regular VMs, creating a bridge on a Docker host, having containers on the same subnet, and then logging into them via port 22. (No port forwarding, please.) So here's how to do it.

On this example, I use Vagrant and VirtualBox on my MacBook and create containers with IP addresses shown on the table below. Once you go through these steps, you should be able to extend the idea into your on-premises network.

|Host                | IP Address     |
|:-------------------|:---------------|
| MacBook            | 192.168.33.1   |
| host1(VirtualBox)  | 192.168.33.10  |
| container1         | 192.168.33.11  |
| container2         | 192.168.33.12  |
| container3         | 192.168.33.13  |

Prerequisites

This example is done with these:

Creating a `Vagrantfile`

You may want to go with docker-machine create -d virtualbox <hostname>, but it is important to change the network Adapter Type in order to make the network bridge work. It is easy on Vagrantfile to do that, which looks like this:

Vagrant.configure(2) do |config|
  config.vm.box = "ubuntu/trusty64"
  config.vm.hostname = "host1"
  config.vm.network "private_network", ip: "192.168.33.10"

  config.vm.provider "virtualbox" do |v|
    v.memory = "2048"
    # Change the network adapter type and promiscuous mode
    v.customize ['modifyvm', :id, '--nictype1', 'Am79C973']
    v.customize ['modifyvm', :id, '--nicpromisc1', 'allow-all']
    v.customize ['modifyvm', :id, '--nictype2', 'Am79C973']
    v.customize ['modifyvm', :id, '--nicpromisc2', 'allow-all']
  end

  # Install bridge-utils
  config.vm.provision "shell", inline: <<-SHELL
    apt-get update
    apt-get -y install bridge-utils
  SHELL
end

Creating a Docker machine

# Create a VM
$ vagrant up

# Setup the VM as your Docker machine
$ docker-machine create \
      --driver "generic" \
      --generic-ip-address 192.168.33.10 \
      --generic-ssh-user vagrant \
      --generic-ssh-key .vagrant/machines/default/virtualbox/private_key \
      --generic-ssh-port 22 \
      host1

Creating a bridge `docker1` and shared network `shared_nw`

# Log in to the VM
$ vagrant ssh

# Delete the IP address from eth1
$ sudo ip addr del 192.168.33.10/24 dev eth1

# Create "shared_nw" with a bridge name "docker1"
$ sudo docker network create \
    --driver bridge \
    --subnet=192.168.33.0/24 \
    --gateway=192.168.33.10 \
    --opt "com.docker.network.bridge.name"="docker1" \
    shared_nw

# Add docker1 to eth1
$ sudo brctl addif docker1 eth1

Note: You may want to use docker-machine ssh host1 to log in to host1, but you need to use vagrant ssh instead, because 192.168.33.10 will be once deleted in the process.

Running Containers

Go back to your MacBook, and launch containers:

$ eval $(docker-machine env host1) # Setup the environment
$ docker run --name container1 --net shared_nw --ip 192.168.33.11 -dt ubuntu
$ docker run --name container2 --net shared_nw --ip 192.168.33.12 -dt ubuntu
$ docker run --name container3 --net shared_nw --ip 192.168.33.13 -dt ubuntu

And now you should be able to reach the container's IP addresses:

$ ping -c 3 192.168.33.11
$ ping -c 3 192.168.33.12
$ ping -c 3 192.168.33.13

Making the network setting permanent

At this point, the network bridge will be detatched from eth1 if you
reboot the machine. So here are two more things to make the setting
permanent.

Update /etc/network/interfaces like this:

auto eth1
iface eth1 inet manual
   pre-up ifconfig $IFACE up
   post-down ifconfig $IFACE down

And add the following command on /etc/rc.local(before exit 0):

brctl addif docker1 eth1

Cleaning up

After finishing your test, you can remove the containers and VM:

$ docker stop $(docker ps -aq) # Stop all containers
$ docker rm $(docker ps -aq) # Remove all containers
$ docker-machine rm host1 # Remove host1 from your machine list
$ vagrant destroy # Remove the VM