Bridging mini-HOWTO
Christopher Cole
            <cole@coledd.com>
         Â
March 2001
Revision History
Revision 1.22 2002-05-20 Revised by: tab
Converted to Docbook 4.1 SGML and added GFDL per Christopher Cole
Revision 1.21 2001-03-07 Revised by: cc
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.
-------------------------------------------------------------------------------
Table of Contents
1. Setup
2. Common_Problems
3. Copyright
3.1. GNU_Free_Documentation_License
3.2. PREAMBLE
3.3. APPLICABILITY_AND_DEFINITIONS
3.4. VERBATIM_COPYING
3.5. COPYING_IN_QUANTITY
3.6. MODIFICATIONS
3.7. COMBINING_DOCUMENTS
3.8. COLLECTIONS_OF_DOCUMENTS
3.9. AGGREGATION_WITH_INDEPENDENT_WORKS
3.10. TRANSLATION
3.11. TERMINATION
3.12. FUTURE_REVISIONS_OF_THIS_LICENSE
3.13. How_to_use_this_License_for_your_documents
1. Setup
* Get Bridge Config: BRCFG.tgz
* BRCFG may also be found at: http://coledd.com/networking/bridge
* Enable multiple ethernet devices on your machine by adding this line 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|
|______________________|
Or, with 3 interfaces, use:
_____________________________________
| |
| linux ether=0,0,eth1 ether=0,0,eth2|
|_____________________________________|
* Recompile the kernel with BRIDGING enabled.
* 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 two new files, change the line containing DEVICE=lo to
DEVICE=eth0 and DEVICE=eth1. Since other distributions may deviate from this,
you may need to refer to additional documentation. If there are more than 2
interfaces to this bridge, be sure to make the corresponding configurations
to those, as well.
* 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.
* Once the system is backed 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.
* Turn bridging ON using the brcfg program:
____________
| |
| brcfg -ena|
|____________|
* Verify that there is different traffic on each interface:
_________________________________________
| |
| tcpdump -i eth0 (in one window) |
| tcpdump -i eth1 (in another window)|
|_________________________________________|
* Run a sniffer or tcpdump on another machine to verify the bridge is
separating the segment correctly.
-------------------------------------------------------------------------------
2. Common Problems
Q: I_get_the_message_ioctl(SIOCGIFBR)_failed:_Package_not_installed_._What
does_this_mean?
Q: Machines_on_one_side_cannot_ping_the_other_side!
Q: I_cannot_telnet/ftp_from_the_bridge!_Why?
Q: What_do_I_need_to_set_up_in_the_way_of_routing?
Q: The_bridge_appears_to_work,_but_why_doesn't_traceroute_show_the_bridge_as
a_part_of_the_path?
Q: Is_it_necessary_to_compile_IP_FORWARD_into_the_kernel?
Q: Why_are_the_physical_ethernet_addresses_for_port_1_and_port_2_the_same
according_to_the_brcfg_program?_Shouldn't_they_be_different?
Q: Bridging_does_not_appear_to_be_an_option_when_performing_a_make_config_on
the_kernel._How_does_one_enable_it?
Q: Too_many_hubs_(4_or_more)_are_chained_one_after_another_in_series,_cause
timing_problems_on_an_ethernet._What_effect_does_a_bridge_have_in_a_subnet
that_is_layered_with_hubs?
Q: Can_a_bridge_interface_to_both_10Mb_and_100Mb_ethernet_segments?_Will_such
a_configuration_slow_down_the_rest_of_the_traffic_on_the_high_speed_side?
Q:I get the message ioctl(SIOCGIFBR) failed: Package not installed . What does
this mean?
A:You don't have bridging capability in your kernel. Get a 2.0 or greater
kernel, and recompile with the BRIDGING option enabled.
Q:Machines on one side cannot ping the other side!
A:
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.
Q:I cannot telnet/ftp from the bridge! Why?
A: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.
Q:What do I need to set up in the way of routing?
A: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|
|____________|
Q:The bridge appears to work, but why doesn't traceroute show the bridge as a
part of the path?
A: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.
Q:Is it necessary to compile IP_FORWARD into the kernel?
A:No. The bridging code in the kernel takes care of the packet transport.
IP_FORWARD is for a gateway that has IP addresses bound to its interfaces.
Q:Why are the physical ethernet addresses for port 1 and port 2 the same
according to the brcfg program? Shouldn't they be different?
A:No. Every port on a bridge intentionally is assigned the same physical
ethernet address by the bridging code.
Q:Bridging does not appear to be an option when performing a make config on the
kernel. How does one enable it?
A:During the kernel config, answer "Y" to the question, Prompt for development
and/or incomplete code/drivers (CONFIG_EXPERIMENTAL) [Y/n/?].
Q:Too many hubs (4 or more) are chained one after another in series, cause
timing problems on an ethernet. What effect does a bridge have in a subnet that
is layered with hubs?
A:A bridge resets the 3/4/5 hubs rule. A bridge does not deal with packets the
way a hub does, and is therefore not a contributor to timing problems on a
network.
Q:Can a bridge interface to both 10Mb and 100Mb ethernet segments? Will such a
configuration slow down the rest of the traffic on the high speed side?
A:Yes, a bridge can tie together a 10Mb segment with a 100Mb segment. As long
as the network card on the fast network is 100Mb capable, TCP takes care of the
rest. While it's true that the packets from a host in the 100Mb network
communicating to a host in the 10Mb network are moving at only 10Mb/s, the rest
of the traffic on the fast ethernet is not slowed down.
-------------------------------------------------------------------------------
3. Copyright
Copyright © 2002 Christopher Cole
Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.1 or any later version
published by the Free Software Foundation; with no Invariant Sections, no
Front-Cover Texts and no Back-Cover Texts. A copy of the license is included in
the section entitled "GNU Free Documentation License".
-------------------------------------------------------------------------------
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Version 1.1, March 2000
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