<title>Bridging mini-HOWTO</title>
<!-- Title information -->
        <firstname>Christopher </firstname>
<pubdate>March 2001</pubdate>
        <revremark>Converted to Docbook 4.1 SGML and added GFDL per Christopher Cole</revremark>
<para>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 -->

    <para>Get Bridge Config:
    <ulink url="ftp://ftp.tux.org/people/alan-cox/BRCFG.tgz"> BRCFG.tgz</ulink>

    <para>BRCFG may also be found at:
    <ulink url="http://coledd.com/networking/bridge/"> http://coledd.com/networking/bridge</ulink>

    <para>Enable multiple ethernet devices on your machine by
        adding this line to your <filename>/etc/lilo.conf</filename>,
        and re-run <application>lilo</application>:
        <screen>append = "ether=0,0,eth1"</screen>
    <para>If you have three interfaces on your bridge, use this line instead:
        <screen>append = "ether=0,0,eth1 ether=0,0,eth2"</screen>
    <para>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:
        <screen>linux ether=0,0,eth1</screen>
    <para>Or, with 3 interfaces, use:
        <screen>linux ether=0,0,eth1 ether=0,0,eth2</screen>

    <para>Recompile the kernel with <envar>BRIDGING</envar> enabled.

    <para>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
        <filename>/etc/sysconfig/network-scripts/</filename> (for a RedHat system)
        and copy <filename>ifcfg-lo0</filename> to <filename>ifcfg-eth0</filename> &amp;
        In these two new files, change the line containing
        <userinput>DEVICE=lo</userinput> to <userinput>DEVICE=eth0</userinput>
        and <userinput>DEVICE=eth1</userinput>.
        Since other distributions may deviate from this, you may need to refer to additional
        If there are more than 2 interfaces to this bridge,
        be sure to make the corresponding configurations to those, as well.

    <para>Reboot so you are running the new kernel with <envar>BRIDGING</envar> in it,
        and also to make sure that an IP addresses are not bound to the
        network interfaces.

    <para>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:
        <screen>ifconfig eth0 promisc ; ifconfig eth1 promisc</screen>
        All interfaces which are connected to network segments to be bridged
        are to be put into promiscuous mode.

    <para>Turn bridging ON using the <application>brcfg</application> program:
        <screen>brcfg -ena</screen>

    <para>Verify that there is different traffic on each interface:
tcpdump -i eth0     (in one window)
tcpdump -i eth1     (in another window)

    <para>Run a sniffer or <application>tcpdump</application> on another machine
        to verify the bridge is separating the segment correctly.

<title>Common Problems</title>

    <qandaset defaultlabel='qanda'>
        <para>I get the message <errorname>ioctl(SIOCGIFBR) failed: Package not installed
        </errorname>.  What does this mean?</para>
        <para>You don't have bridging capability in your kernel.
        Get a 2.0 or greater kernel,
        and recompile with the <envar>BRIDGING</envar> option enabled.</para>
        <para>Machines on one side cannot ping the other side!</para>
        <itemizedlist spacing=compact>
          <listitem><simpara>Did you enable bridging using <application>brcfg -ena</application>? (<application>brcfg</application> should say <returnvalue>bridging is ENABLED</returnvalue>)</simpara>
          <listitem><simpara>Did you put the interfaces into promiscuous mode?
                (issue the <application>ifconfig</application> command.
                 The <property>PROMISC</property> flag should be on for
                 both interfaces.)</simpara>
          <listitem><simpara>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.</simpara>

        <para>I cannot <application>telnet</application>/<application>ftp</application> from the bridge! Why?</para>
        <para>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.</para>

        <para>What do I need to set up in the way of routing?</para>
        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 <application>brcfg</application> program in debug mode:</para>
brcfg -deb

        <para>The bridge appears to work, but why doesn't <application>traceroute</application> show the bridge as a part of the path?</para>
        <para>Due to the nature of a bridge, a <application>traceroute</application> should NOT show the bridge as a part of the path. A bridge is to be a transparent component of the network.</para>

        <para>Is it necessary to compile <envar>IP_FORWARD</envar> into the kernel?</para>
        <para>No. The bridging code in the kernel takes care of the packet
        <envar>IP_FORWARD</envar> is for a gateway that has IP addresses
        bound to its interfaces.</para>

        <para>Why are the physical ethernet addresses for port 1 and port 2 the
        same according to the <application>brcfg</application> program?
        Shouldn't they be different?</para>
        <para>No. Every port on a bridge intentionally is assigned the same
        physical ethernet address by the bridging code.</para>

        <para>Bridging does not appear to be an option when performing a make
        config on the kernel.  How does one enable it?</para>
        <para>During the kernel config, answer <quote>Y</quote> to the question, <prompt>Prompt for
        development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL) [Y/n/?]</prompt>.</para>

        <para>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?</para>
        <para>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.</para>

        <para>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?</para>
        <para>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.</para>


<para>Copyright &copy; 2002 Christopher Cole</para>
<para>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".</para>

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