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Bridging mini−Howto

Bridging mini−Howto

Table of Contents

Bridging mini−Howto.........................................................................................................................................1

Christopher Cole, cole@coledd.com......................................................................................................1

1. Setup....................................................................................................................................................1

2. Common problems...............................................................................................................................1

1. Setup....................................................................................................................................................1

2. Common problems...............................................................................................................................2

Bridging mini−Howto

Christopher Cole,

cole@coledd.com

v1.21, March 7, 2001

This document describes how to setup an ethernet bridge. What is an ethernet bridge? An ethernet bridge is a

device that controls data packets within a subnet in an attempt to cut down the amount of traffic. A bridge is

usually placed between two separate groups of computers that talk within themselves, but not so much with

the computers in the other group. A good example of this is to consider a cluster of Macintoshes and a cluster

of unix machines. Both of these groups of machines tend to be quite chatty amongst themselves, and the

traffic they produce on the network causes collisions for the other machines who are trying to speak to one

another. A bridge would be placed between these groups of computers. The job of the bridge is then to

examine the destination of the data packets one at a time and decide whether or not to pass the packets to the

other side of the ethernet segment. The result is a faster, quieter network with less collisions.

Setup

Common problems

Setup

1. Get ``Bridge Config'':

ftp://ftp.tux.org/people/alan−cox/BRCFG.tgz

BRCFG may also be found at:

http://coledd.com/networking/bridge/

2. Enable multiple ethernet devices on your machine by adding this to your

/etc/lilo.conf,

and

re−run

lilo:

append = "ether=0,0,eth1"

If you have three interfaces on your bridge, use this line instead:

append = "ether=0,0,eth1 ether=0,0,eth2"

More interfaces can be found by adding more ether statements. By default a stock Linux kernel

probes for a single ethercard, and once one is found the probe ceases. The above append statement

tells the kernel to keep probing for more ethernet devices after the first one is found. Alternatively,

the boot parameter can be used instead:

linux ether=0,0,eth1

Bridging mini−Howto

Bridging mini−Howto

Or, with 3 interfaces, use:

linux ether=0,0,eth1 ether=0,0,eth2

3. Recompile the kernel with

BRIDGING

enabled.

4. A bridge should not have an IP address. It CAN, but a plain bridge doesn't need one. To remove the

IP address from your bridge, go to

/etc/sysconfig/network−scripts/

(for a RedHat

system) and copy

ifcfg−lo0

ifcfg−eth0

ifcfg−eth1.

In these 2 eth files, change the

line containing ``DEVICE=lo'' to ``DEVICE=eth0'' and ``DEVICE=eth1''. Other distributions

may deviate from this, do what you need to do! If there are more than 2 interfaces to this bridge, be

sure to make the corresponding configurations to those, as well.

5. Reboot, so you are running the new kernel with bridging in it, and also to make sure that an IP

addresses are not bound to the network interfaces.

6. Once the system is back up, put the ethernet cards into promiscuous mode, so they will look at every

packet that passes by its interface:

ifconfig eth0 promisc ; ifconfig eth1 promisc

All interfaces which are connected to network segments to be bridged are to be put into promiscuous

mode.

7. Turn bridging ON using the

brcfg

program:

brcfg −ena

8. Verify that there is different traffic on each interface:

tcpdump −i eth0

tcpdump −i eth1

(in one window)

(in another window)

9. Run a sniffer or

tcpdump

on another machine to verify the bridge is separating the segment

correctly.

Common problems

Question

I get the message

ioctl(SIOCGIFBR) failed: Package not installed

What does this mean?

Answer

You don't have bridging capability in your kernel. Get a 2.0 or greater kernel, and recompile

with the

BRIDGING

option enabled.

Question

Machines on one side cannot ping the other side!

Answer

2. Common problems

Bridging mini−Howto

◊

Did you enable bridging using ``brcfg

−ena''?

(brcfg should say ``bridging

is ENABLED'')

◊

Did you put the interfaces into promiscuous mode? (issue the ``ifconfig''

command. The ``PROMISC'' flag should be on for both interfaces.)

◊

If using multiple−media interface adapters, make sure that the correct one is enabled.

You may need to use the config/setup program that came with the network interface

card.

Question

I cannot

telnet/ftp

from the bridge! Why?

Answer

This is because there is no IP address bound to any of bridge interfaces. A bridge is to be a

transparent part of a network.

Question

What do I need to set up in the way of routing?

Answer

Nothing! All routing intelligence is handled by the bridging code in the kernel. To see the

ethernet addresses as they are learned by the bridge, use the

brcfg

program in debug mode:

brcfg −deb

Question

The bridge appears to work, but why doesn't ``traceroute'' show the bridge as a part of the

path?

Answer

Due to the nature of a bridge, a ``traceroute'' should NOT show the bridge as a part of the

path. A bridge is to be a transparent component of the network.

Question

Is it necessary to compile

IP_FORWARD

into the kernel?

Answer

No. The bridging code in the kernel takes care of the packet transport.

IP_FORWARD

is for a

gateway which has IP addresses bound to its interfaces.

Question

Why are the physical ethernet addresses for port 1 and port 2 the same according to the

``brcfg'' program? Shouldn't they be different?

Answer

2. Common problems

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