Bridging mini-HOWTO, v1.03 by Chris Cole Aug. 23, 1996
Any suggestions/corrections welcome at:
[email protected]
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1) Get 'Bridge Config':
shadow.cabi.net/pub/Linux/BRCFG.tgz
2) Obtain and read the 'Multiple ethernet' HOWTO:
sunsite.unc.edu:/pub/Linux/docs/HOWTO/mini/Multiple-Ethernet
3) Enable multiple ethernet devices on your box 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. This 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
Or, with 3 interfaces, use:
linux ether=0,0,eth1 ether=0,0,eth2
4) Recompile the kernel with BRIDGING enabled.
5) 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 to 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.
6) Reboot, so you are running the new kernel with bridging in it, and
also so the network interfaces are not bound to an IP.
7) Once the system is back up, put the ethernet cards into promiscuous
mode, so they will look at every packet that passes by the interface:
ifconfig promisc eth0 ; ifconfig promisc eth1
All interfaces which are connected to network segments to be bridged
are to be put into promiscuous mode.
8) Turn bridging ON using the brcfg program:
brcfg -ena
9) Verify that there is different traffic on each interface:
tcpdump -i eth0 (in one window)
tcpdump -i eth1 (in another window)
10) Run a sniffer or tcpdump on another machine to verify the bridge is
separating the segment correctly.
Common problems:
Q1) I get the message "ioctl(SIOCGIFBR) failed: Package not installed".
What does this mean?
A1) You don't have bridging capability in your kernel. Get a 2.0 or
greater kernel, and recompile with the BRIDGING option enabled.
Q2) Machines on one side cannot ping the other side!
A2) - 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.
Q3) I cannot telnet/ftp from the bridge! Why?
A3) 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.
Q4) What do I need to set up in the way of routing?
A4) Nothing! All routing intelligence is handled by the bridging code
in the kernel. To see the ethernet addresses as they are found out
by the bridge, use the brcfg program in debug mode:
brcfg -deb
Q5) The bridge appears to work, but why doesn't 'traceroute' show the
bridge as a part of the path?
A5) 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.
Q6) Is it necessary to compile IP_FORWARD into the kernel?
A6) 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.
Q7) Why are the physical ethernet addresses for port 1 and port 2 the
same according to the 'brcfg' program? Shouldn't they be different?
A7) No. Every port on a bridge intentionally is assigned the same
physical ethernet address by the bridging code.
