Using a linux box as a bridge

A bridge is a way to connect more than one Ethernet segments together in a protocol independent way. Packets are forwarded based on Ethernet address, rather than IP address (like a router does). Since forwarding is done at Layer 2, all protocols can go transparently through a bridge.

Now imagine you want to build a bridge with you own PC equipped with three different interfaces: a wlan two Gb Ethernet 1000 Base-T and a Gb Ethernet 1000 Base-SX (Fiber Optic connection).

Linux gives you the ability to do that.

You only need to install the brctl command (yet installed on most of Linux dIstributions).

 

A simple bridge configuration

Referring to the above picture we configure a bridge bitween the four interfaces, remeber that you need to be root to issue the following commands:

# ifconfig wlan0 0.0.0.0
# ifconfig eth0 0.0.0.0
# ifconfig eth1 0.0.0.0
# ifconfig eth2 0.0.0.0
# brctl addbr bridge0
# brctl addif bridge0 wlan0
# brctl addif bridge0 eth0
# brctl addif bridge0 eth1
# brctl addif bridge0 eth2
# ifconfig bridge0 up

If you issue the ifconfig command you should obtain an output like the following:

 

~# ifconfig

bridge0   Link encap:Ethernet  HWaddr yy:yy:yy:yy:yy:yy

          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

          RX packets:138 errors:0 dropped:0 overruns:0 frame:0

          TX packets:111 errors:0 dropped:0 overruns:0 carrier:0

          collisions:0 txqueuelen:0

          RX bytes:11501 (11.2 KiB)  TX bytes:9714 (9.4 KiB)




eth0      Link encap:Ethernet  HWaddr yy:yy:yy:yy:yy:yy

          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

          RX packets:17 errors:0 dropped:0 overruns:0 frame:0

          TX packets:20 errors:0 dropped:0 overruns:0 carrier:0

          collisions:0 txqueuelen:100

          RX bytes:3023 (2.9 KiB)  TX bytes:3475 (3.3 KiB)




eth1      Link encap:Ethernet  HWaddr xx:xx:xx:xx:xx:xx

          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

          RX packets:17 errors:0 dropped:0 overruns:0 frame:0

          TX packets:20 errors:0 dropped:0 overruns:0 carrier:0

          collisions:0 txqueuelen:100

          RX bytes:3023 (2.9 KiB)  TX bytes:3475 (3.3 KiB)




eth2      Link encap:Ethernet  HWaddr xx:xx:xx:xx:xx:xx

          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

          RX packets:17 errors:0 dropped:0 overruns:0 frame:0

          TX packets:20 errors:0 dropped:0 overruns:0 carrier:0

          collisions:0 txqueuelen:100

          RX bytes:3023 (2.9 KiB)  TX bytes:3475 (3.3 KiB)




lo        Link encap:Local Loopback

          inet addr:127.0.0.1  Mask:255.0.0.0

          UP LOOPBACK RUNNING  MTU:16436  Metric:1

          RX packets:8 errors:0 dropped:0 overruns:0 frame:0

          TX packets:8 errors:0 dropped:0 overruns:0 carrier:0

          collisions:0 txqueuelen:0

          RX bytes:636 (636.0 B)  TX bytes:636 (636.0 B)

 

This configuration is a configuration for a transparent bridge, it looks like a bridge you can buy in a Store. But your Linux box is a more intelligent system, so you can add services. If you want to add some services like snmp (to check the status of your Linux system) or a NTP service or a telnetd daemon or whatever you want, you can add an IP address to your bridge interface.

Simply issue the following commands:

 

# ifconfig bridge0 192.2.2.5 netmask 255.255.255.0

 

# ifconfig bridge0

 

br0       Link encap:Ethernet  HWaddr yy:yy:yy:yy:yy:yy

          inet addr:192.2.2.5  Bcast:192.2.2.255  Mask:255.255.255.0

          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

          RX packets:213 errors:0 dropped:0 overruns:0 frame:0

          TX packets:134 errors:0 dropped:0 overruns:0 carrier:0

          collisions:0 txqueuelen:0

          RX bytes:19714 (19.2 KiB)  TX bytes:12640 (12.3 KiB)