I recently wanted to debug something on my linux server and listed all bridges. The output was disappointing. All bridges were named with random strings like this:

br-2b6b3cc75104
br-41da10d6d383

I create all my bridges with ansible and it seems that the ansible docker_network module is not very good at naming things in a human readable way. But there is a way you can influence the system bridge names when using docker through ansible.

The following applies to Linux systems.

Where do ansible managed docker bridge names come from?

When you create a new docker network with ansible, docker creates a new system bridge for you. The name consists of a prefix br- and a hex string with the first 12 characters of the id of the docker network that uses the bridge.

How to name ansible managed docker bridges

To assign a name to the system bridge when creating a docker network with ansible you can pass the driver option com.docker.network.bridge.name. The bridge name has a 15 character limit. Consider the following example of an ansible task that creates a network and sets the bridge name:

- name: Ensure plausible network
  docker_network:
    name: plausible
    appends: yes
    driver_options:
      com.docker.network.bridge.name: docker-plausibl

How to find out which bridge belongs to which docker network

When you already have several bridges with random names and you need to find out which docker network they belong to, you can inspect the docker network and look for the network id. The bridge name will use the first 12 characters of the docker network id as an identifier: 

benjamin@tnglab:~$ docker network inspect wekan
[
    {
        "Name": "wekan",
        "Id": "56055d95f648f6aed2002a78eee3530ff2ef92be7410f50bac87f5f75dbbcf62",
...

In this example the bridge name would be br-56055d95f648.

How does it look in practice?

Setting the bridge names, I went from this:

benjamin@tnglab:~$ brctl show
bridge name		bridge id		STP enabled	interfaces
br-896b3cc75144		8000.0242928bd08e	no		veth1643759
								veth46fee36
								veth7fb5ff4
br-2b6b3cc75104		8000.02420819e6ce	no		veth063b6c1
								veth08d3401
								veth1cd4190
								veth84f2ab0
								veth9de5feb
								veth9f85418
								vethe055863
br-41da10d6d383		8000.02427fa5b26d	no		veth14d2523
								vethad48648
								vethdfafa4d
								vethfd41177
br-8b204b99335b		8000.0242ae393b8e	no		veth2dc900f
								veth552856f
								veth563643b
								vetha73d54a
br-e04d746feb69		8000.02420fb1c153	no		veth25c1e98
								veth88d1e12
								vethdc13c5e
br-a7508dea30bc 	8000.0242b3bf66ba	no		veth3d4b503
								veth52b2011
								vethcd0ffb1
docker0			8000.024253a238a5	no		veth1fa3e89
								vetha16c945

To this:

benjamin@tnglab:~$ brctl show
bridge name		bridge id		STP enabled	interfaces
docker-atlassia		8000.0242fc4f8c5a	no		veth1d300c7
								veth3b8730f
								vethc7e61fa
								vethd1b6d61
docker-mail		8000.0242bfd1009e	no		veth583dc56
								veth9e2b1a7
								vethfb00e5b
docker-plausibl		8000.0242fb8c86ec	no		veth68cc751
								veth7d3879b
								vethddaac1e
docker-seafile		8000.02429be7e8d6	no		veth1f7a2da
								veth4ddc220
								veth7c9163a
								veth7e826c6
docker-traefik		8000.0242ab033b99	no		veth40176ba
								veth544d840
								veth5d26b1e
								veth8021706
								vetha2830f3
								vethe0aadff
								vethff3bcfe
docker-wekan		8000.0242b3bf66ba	no		veth3d4b503
								veth52b2011
								vethcd0ffb1
docker0			8000.024253a238a5	no		veth1fa3e89
								vetha16c945

Much more helpful. And I even forgot what I wanted to debug in the first place. Problem solved…