Bridge

From : http://linux-net.osdl.org/index.php/Bridge

A bridge is a way to connect two Ethernet segments together in a protocol independent way. Packets are forwarded based on Ethernet address, rather than IP address (like a router). Since forwarding is done at Layer 2, all protocols can go transparently through a bridge.
The Linux bridge code implements a subset of the ANSI/IEEE 802.1d standard. [1]. The original Linux bridging was first done in Linux 2.2, then rewritten by Lennert Buytenhek. The code for bridging has been integrated into 2.4 and 2.6 kernel series.

Bridging and Firewalling

A Linux bridge is more powerful than a pure hardware bridge because it can also filter and shape traffic. The combination of bridging and firewalling is done with the companion project ebtables.

Status

The code is updated as part of the 2.4 and 2.6 kernels available at kernel.org.
Possible future enhancements are:
Document STP filtering
Netlink interface to control bridges (prototype in 2.6.18)
STP should be in user space
Support RSTP and other 802.1d STP extensions

Downloading

Bridging is supported in the current 2.4 (and 2.6) kernels from all the major distributors. The required administration utilities are in the bridge-utils package in most distributions. Package releases are maintained on the Download page.
You can also build your own up to date version by getting the latest kernel from kernel.org and build the utilities based from the source code in bridge-utils GIT repository.

$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/shemminger/bridge-utils.git
$ cd bridge-utils
$ autoconf
$ ./configure

Kernel Configuration

You need to enable bridging in the kernel. Set “networking -> 802.1d Ethernet Bridging” to either yes or module

Creating a bridge device
The command

brctl addbr “bridgename”

creates a logical bridge instance with the name bridgename. You will need at least one logical instance to do any bridging at all. You can interpret the logical bridge as a container for the interfaces taking part in the bridging. Each bridging instance is represented by a new network interface.
The corresponding shutdown command is:

brctl delbr bridgename

Adding devices to a bridge
The command

brctl addif bridgename device

adds the network device device to take part in the bridging of “bridgename.” All the devices contained in a bridge act as one big network. It is not possible to add a device to multiple bridges or bridge a bridge device, because it just wouldn’t make any sense! The bridge will take a short amount of time when a device is added to learn the Ethernet addresses on the segment before starting to forward.
The corresponding command to take an interface out of the bridge is:

brctl delif bridgename device

Showing devices in a bridge
The brctl show command gives you a summary about the overall bridge status, and the instances running as shown below:

# brctl addbr br549
# brctl addif br549 eth0
# brctl addif br549 eth1
# brctl show
bridge name bridge id STP enabled interfaces
br549 8000.00004c9f0bd2 no eth0
eth1

Once a bridge is running the brctl showmacs will show information about network addresses of traffic being forwarded (and the bridge itself).

# brctl showmacs br549
port no mac addr is local? ageing timer
1 00:00:4c:9f:0b:ae no 17.84
1 00:00:4c:9f:0b:d2 yes 0.00
2 00:00:4c:9f:0b:d3 yes 0.00
1 00:02:55:1a:35:09 no 53.84
1 00:02:55:1a:82:87 no 11.53
…

The aging time is the number of seconds a MAC address will be kept in the forwarding database after having received a packet from this MAC address. The entries in the forwarding database are periodically timed out to ensure they won’t stay around forever. Normally there should be no need to modify this parameter, but it can be changed with (time is in seconds).

# brctl setageing bridgename time

Setting ageing time to zero makes all entries permanent.

Spanning Tree Protocol
If you are running multiple or redundant bridges, then you need to enable the Spanning Tree Protocol (STP) to handle multiple hops and avoid cyclic routes.

# brctl stp br549 on

Sample setup
The basic setup of a bridge is done like:

# ifconfig eth0 0.0.0.0
# ifconfig eth1 0.0.0.0
# brctl addbr mybridge
# brctl addif mybridge eth0
# brctl addif mybridge eth1
# ifconfig mybridge up

This will set the host up as a pure bridge, it will not have an IP address for itself, so it can not be remotely accessed (or hacked) via TCP/IP.
Optionally you can configure the virtual interface mybridge to take part in your network. It behaves like one interface (like a normal network card). Exactly that way you configure it, replacing the previous command with something like:

# ifconfig mybridge 192.168.100.5 netmask 255.255.255.0

If you want your bridge to automatically get its IP address from the ADSL modem via DHCP (or a similar configuration), do this:

# ifconfig eth0 0.0.0.0
# ifconfig eth1 0.0.0.0
# brctl addbr mybridge
# brctl addif mybridge eth0
# brctl addif mybridge eth1
# dhclient mybridge

If you do this many times, you may end up with lots of dhclient processes. Either kill them impolitely or learn about omshell(1).