How to setup dolphin broadband adapter for local area multiplayer

There is an offical wiki page which describes the setup that is necessary for the broadband adapter to work. But at least I had some trouble figuring out, what one is exactly supposed to do.

My goal was to connect two (or more) dolphin instances running on different computers on my local network and eventually be able to play the LAN Mode of Mario Kart: Double Dash‼

If you want to know how I did it, continue reading.

Setting up the TAP device and network bridge

The official wiki page instructs you to install OpenVPN, but this is actually not required. What is required though, is a TAP device with a specific name that is then bridged to your real network adapter.

In order to create the required network devices the ip tool, that is usually already installed (if not the package is usually named iproute2), can be used. The following commands require root access, so either switch to root or prepend sudo to every command.

First create a TAP device named Dolphin0. This name is important, since dolphin tries to access devices labeled Dolphin0 to Dolphin31. Also make sure to substitute user and group accordingly.

      $ ip tuntap add dev Dolphin0 mode tap user user group group
$ ip link set dev Dolphin0 up
    

Next create a network bridge. The name of this device is not important to dolphin, so choose as you like.
For this example I used br0:

      $ ip link add name br0 type bridge
$ ip link set br0 up
    

The last step is to create a bridge between the TAP device and your actual network adapter. This will forward all traffic to and from your network adapter to the dolphin instance. Make sure to substitute the name of your network adapter accordingly (it's with certainty not named eth0):

      $ ip link set eth0 master br0
$ ip link set Dolphin0 master br0
    

This completes the required network setup. Repeat this on any computer in your network that is running a dolphin instance you want to connect.

Note that this setup does not persist between reboots.

Configure dolphin

As stated in the official wiki page, enable the broadband adapter. This is done within the dolphin configuration dialog. Under GameCube > SP1 select Broadband Adapter (TAP).

"Now you may play any games which use the broadband adapter."