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Bridging mini-HOWTO

Christopher Cole

            cole@coledd.com
          

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: [ftp://ftp.tux.org/people/alan-cox/BRCFG.tgz]

BRCFG.tgz

  * BRCFG may also be found at: [http://coledd.com/networking/bridge/] 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?

  1. You don't have bridging capability in your kernel. Get a 2.0 or greater kernel, and recompile with the BRIDGING option enabled.
  2. 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?

  1. 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.
  2. What do I need to set up in the way of routing?
  3. 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 | | | +---------------------------------------------------------------------------+
  4. The bridge appears to work, but why doesn't traceroute show the bridge as a part of the path?
  5. 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.
  6. Is it necessary to compile IP_FORWARD into the kernel?
  7. 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.
  8. Why are the physical ethernet addresses for port 1 and port 2 the same according to the brcfg program? Shouldn't they be different?
  9. No. Every port on a bridge intentionally is assigned the same physical ethernet address by the bridging code.
  10. Bridging does not appear to be an option when performing a make config on the kernel. How does one enable it?
  11. During the kernel config, answer "Y" to the question, Prompt for development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL) [Y/n/?].
  12. 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?
  13. 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.
  14. 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?
  15. 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.
  16. 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".


3.1. GNU Free Documentation License

Version 1.1, March 2000

Copyright (C) 2000 Free Software Foundation, Inc. 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.


3.2. PREAMBLE

The purpose of this License is to make a manual, textbook, or other written document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.


3.3. APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you".

A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

A "Secondary Section" is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (For example, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License.

The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License.

A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, whose contents can be viewed and edited directly and straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup has been designed to thwart or discourage subsequent modification by readers is not Transparent. A copy that is not "Transparent" is called "Opaque".

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3.4. VERBATIM COPYING

You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and you may publicly display copies.


3.5. COPYING IN QUANTITY

If you publish printed copies of the Document numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a publicly-accessible computer-network location containing a complete Transparent copy of the Document, free of added material, which the general network-using public has access to download anonymously at no charge using public-standard network protocols. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.


3.6. MODIFICATIONS

You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

  1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
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  10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
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  12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
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  14. Do not retitle any existing section as "Endorsements" or to conflict in title with any Invariant Section.

If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

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The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.


3.7. COMBINING DOCUMENTS

You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice.

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In the combination, you must combine any sections entitled "History" in the various original documents, forming one section entitled "History"; likewise combine any sections entitled "Acknowledgements", and any sections entitled "Dedications". You must delete all sections entitled "Endorsements."


3.8. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.


3.9. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, does not as a whole count as a Modified Version of the Document, provided no compilation copyright is claimed for the compilation. Such a compilation is called an "aggregate", and this License does not apply to the other self-contained works thus compiled with the Document, on account of their being thus compiled, if they are not themselves derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one quarter of the entire aggregate, the Document's Cover Texts may be placed on covers that surround only the Document within the aggregate. Otherwise they must appear on covers around the whole aggregate.


3.10. TRANSLATION

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3.11. TERMINATION

You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.


3.12. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See [http://www.gnu.org/copyleft/] http:// www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.


3.13. How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

Copyright (c) YEAR YOUR NAME. 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 the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST. A copy of the license is included in the section entitled "GNU Free Documentation License".

If you have no Invariant Sections, write "with no Invariant Sections" instead of saying which ones are invariant. If you have no Front-Cover Texts, write "no Front-Cover Texts" instead of "Front-Cover Texts being LIST"; likewise for Back-Cover Texts.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.


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