Bridging Network Connections

Introduction

Bridging you network connection is a handy method for sharing your internet connection between two (or more) computers. It's useful if you can't buy a router with more than one ethernet port, or if you're a college student in a dorm room with limited ethernet jacks and no router.

Basically, bridging is plugging one computer into another computer that already has a connection to a larger network (like the internet) and letting the bridged computer use the networked computer's connection. To do so though, the networked computer needs to have two ethernet ports, one for the big network, and one for the bridged computer. Make sure that before starting that the computer you're gonna bridge through has two ethernet ports, and that the hardware is capable of bridging ethernet connections (it probably should be).

Installing the software

The program you're going to need is called bridge-utils. Find it in Synaptic, or install it using this command:

 # aptitude install bridge-utils 

This program will allow us to set up and use the bridge interface. The bridge interface appears as a new interface in ifconfig, much like eth0 or eth1. It doesn't physically exist on your computer, but instead it is a virtual interface that jsut takes the packets from one physical interface, and transparently routes them to the other.

Setting up your Bridge

Note: All these commands are to be issued on the computer with the existing network connection. To set up the computer that's going to be bridged, just set it up normally, as you would any other computer. You CAN use DHCP, or you can use a static address. It doesn't matter. The bridged computer won't even know it's being bridged.

First step to creating the bridge network is actually creating it. Issue this command to get the ball rolling and create the new interface.

 # brctl addbr br0

The name br0 is totally up to you, this is just an example name that I've chosen for the wiki article. Anyway, now that you have your bridge device, you have to add the interfaces that are gonna be bridged. Add both the interface with the second computer, and the interface that leads to the existing network. Do it with this command:

 # brctl addif br0 eth0 eth1

This will add the two interfaces eth0 and eth1 to bridge br0. Simple enough. There's no distinction with how you add the bridges, or what order you do it, or any special commands you have to add to distinguish them. So don't worry about that.

Well, now we have our bridges, so bring all the interfaces up, and you'll be set!

/etc/network/interfaces and bridging

To make your bridge a little more permanent, you're gonna need to edit /etc/network/interfaces. Using our example names, make it look like this and you're set (if you want to use DHCP):

 # This file describes the network interfaces available on your system
 # and how to activate them. For more information, see interfaces(5).
  
 # The loopback network interface
 auto lo br0
 iface lo inet loopback
 
 # Bridge setup
iface br0 inet dhcp
        bridge_ports eth0 eth1

To bring up your bridge, you just have to issue  # ifup br0  and it'll bring up the other necessary interfaces without anything in your  interfaces  file about the bridged interfaces.

If you like static ip's, then you can just add the static IP options under the br0 interface setup. Kinda like this:

 # This file describes the network interfaces available on your system
 # and how to activate them. For more information, see interfaces(5).
  
 # The loopback network interface
 auto lo br0
 iface lo inet loopback
 
 # Bridge setup
 iface br0 inet static
        bridge_ports eth0 eth1
        address 192.168.1.2
        broadcast 192.168.1.255
        netmask 255.255.255.0
        gateway 192.168.1.1

More information