Bridging in Linux

For many reasons, mostly experimental, I wanted to set up a bridging interface in one of my Linux boxes. (The biggest reason was that I wanted to set up a transparent Squid proxy on a few of my machines on the internal network, and this seemed to be the best way if I didn't want to put the proxy on the firewall box, which I didn't.)

Setting up a bridge is pretty easy, on the command line:

ifconfig eth0 0.0.0.0 promisc up

ifconfig eth1 0.0.0.0 promisc up

brctl addbr br0
brctl addif br0 eth0
brctl addif br0 eth1

The trick is doing this in the /etc/network/interfaces file (in Debian/Ubuntu). I was pretty sure the syntax was the same as doing a physical interface, and the documentation and Google searching I did confirmed that it should look like this:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet manual

auto br0
iface br0 inet static
        address 192.168.0.10
        network 192.168.0.0
        netmask 255.255.255.0
        broadcast 192.168.0.255
        gateway 192.168.0.1
        bridge_ports eth0
        bridge_fd 9
        bridge_hello 2
        bridge_maxage 12
        bridge_stp off

But it was those last few lines, the bridge_fd through bridge_stp that I didn't understand, and I went on a quest to discover what the extra bridging parameters meant.

Maybe I missed it, but the documentation on bridging parameters in the 'interfaces' file in Debian Linux is hard to find, or at least, hard to search for. 

I came across a great reference here , though, and it told me:

bridge_fd is the bridge forward delay time, in seconds, default 15.

bridge_hello is the bridge hello time, in seconds, default 2.

bridge_maxage is the bridge's maximum message time, in seconds, default is 20.

bridge_stp controls the spanning tree protocol, on or off. Default is off, and is recommended to stay that way.

I hope this helps someone else looking to set up a Linux bridge. Thanks to this reference for getting the whole thing started, and to this reference for telling me how the bridge is configured in the interfaces file.