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746 lines
31 KiB
Plaintext
<!DOCTYPE ARTICLE PUBLIC "-//OASIS//DTD DocBook V4.1//EN"[]>
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<article>
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<title>Bridging mini-HOWTO</title>
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<!-- Title information -->
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<articleinfo>
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<author>
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<firstname>Christopher </firstname>
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<surname>Cole</surname>
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<affiliation>
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<address>
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<email>cole@coledd.com</email>
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</address>
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</affiliation>
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</author>
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<pubdate>March 2001</pubdate>
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<revhistory>
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<revision>
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<revnumber>1.22</revnumber>
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<date>2002-05-20</date>
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<authorinitials>tab</authorinitials>
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<revremark>Converted to Docbook 4.1 SGML and added GFDL per Christopher Cole</revremark>
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</revision>
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<revision>
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<revnumber>1.21</revnumber>
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<date>2001-03-07</date>
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<authorinitials>cc</authorinitials>
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</revision>
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</revhistory>
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<abstract>
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<para>This document describes how to setup an ethernet bridge. What
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is an ethernet bridge? An ethernet bridge is a device that controls data
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packets within a subnet in an attempt to cut down the amount of traffic.
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A bridge is usually placed between two separate groups of computers that
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talk within themselves, but not so much with the computers in the other
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group. A good example of this is to consider a cluster of Macintoshes and
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a cluster of unix machines. Both of these groups of machines tend to be
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quite chatty amongst themselves, and the traffic they produce on the
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network causes collisions for the other machines who are trying to speak
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to one another. A bridge would be placed between these groups of
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computers. The job of the bridge is then to examine the destination of
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the data packets one at a time and decide whether or not to pass the
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packets to the other side of the ethernet segment. The result is a
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faster, quieter network with less collisions.
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</para>
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</abstract>
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</articleinfo>
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<!-- Table of contents -->
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<sect1>
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<title>Setup</title>
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<para>
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<itemizedlist>
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<listitem>
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<para>Get Bridge Config:
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<ulink url="ftp://ftp.tux.org/people/alan-cox/BRCFG.tgz"> BRCFG.tgz</ulink>
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</para>
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</listitem>
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<listitem>
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<para>BRCFG may also be found at:
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<ulink url="http://coledd.com/networking/bridge/"> http://coledd.com/networking/bridge</ulink>
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</para>
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</listitem>
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<listitem>
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<para>Enable multiple ethernet devices on your machine by
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adding this line to your <filename>/etc/lilo.conf</filename>,
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and re-run <application>lilo</application>:
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<screen>append = "ether=0,0,eth1"</screen>
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</para>
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<para>If you have three interfaces on your bridge, use this line instead:
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<screen>append = "ether=0,0,eth1 ether=0,0,eth2"</screen>
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</para>
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<para>More interfaces can be found by adding more ether statements.
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By default a stock Linux kernel probes for a single ethercard,
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and once one is found the probe ceases. The above append statement
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tells the kernel to keep probing for more ethernet devices after the
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first one is found.
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Alternatively, the boot parameter can be used instead:
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<screen>linux ether=0,0,eth1</screen>
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</para>
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<para>Or, with 3 interfaces, use:
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<screen>linux ether=0,0,eth1 ether=0,0,eth2</screen>
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</para>
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</listitem>
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<listitem>
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<para>Recompile the kernel with <envar>BRIDGING</envar> enabled.
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</para>
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</listitem>
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<listitem>
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<para>A bridge should not have an IP address.
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It CAN, but a plain bridge doesn't need one.
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To remove the IP address from your bridge, go to
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<filename>/etc/sysconfig/network-scripts/</filename> (for a RedHat system)
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and copy <filename>ifcfg-lo0</filename> to <filename>ifcfg-eth0</filename> &
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<filename>ifcfg-eth1</filename>.
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In these two new files, change the line containing
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<userinput>DEVICE=lo</userinput> to <userinput>DEVICE=eth0</userinput>
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and <userinput>DEVICE=eth1</userinput>.
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Since other distributions may deviate from this, you may need to refer to additional
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documentation.
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If there are more than 2 interfaces to this bridge,
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be sure to make the corresponding configurations to those, as well.
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</para>
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</listitem>
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<listitem>
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<para>Reboot so you are running the new kernel with <envar>BRIDGING</envar> in it,
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and also to make sure that an IP addresses are not bound to the
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network interfaces.
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</para>
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</listitem>
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<listitem>
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<para>Once the system is backed up, put the ethernet cards into promiscuous mode,
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so they will look at every packet that passes by its interface:
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<screen>ifconfig eth0 promisc ; ifconfig eth1 promisc</screen>
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All interfaces which are connected to network segments to be bridged
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are to be put into promiscuous mode.
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</para>
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</listitem>
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<listitem>
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<para>Turn bridging ON using the <application>brcfg</application> program:
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<screen>brcfg -ena</screen>
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</para>
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</listitem>
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<listitem>
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<para>Verify that there is different traffic on each interface:
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<screen>
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tcpdump -i eth0 (in one window)
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tcpdump -i eth1 (in another window)
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</screen>
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</para>
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</listitem>
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<listitem>
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<para>Run a sniffer or <application>tcpdump</application> on another machine
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to verify the bridge is separating the segment correctly.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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</sect1>
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<sect1>
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<title>Common Problems</title>
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<qandaset defaultlabel='qanda'>
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<qandaentry>
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<question>
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<para>I get the message <errorname>ioctl(SIOCGIFBR) failed: Package not installed
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</errorname>. What does this mean?</para>
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</question>
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<answer>
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<para>You don't have bridging capability in your kernel.
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Get a 2.0 or greater kernel,
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and recompile with the <envar>BRIDGING</envar> option enabled.</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>Machines on one side cannot ping the other side!</para>
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</question>
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<answer>
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<itemizedlist spacing=compact>
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<listitem><simpara>Did you enable bridging using <application>brcfg -ena</application>? (<application>brcfg</application> should say <returnvalue>bridging is ENABLED</returnvalue>)</simpara>
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</listitem>
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<listitem><simpara>Did you put the interfaces into promiscuous mode?
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(issue the <application>ifconfig</application> command.
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The <property>PROMISC</property> flag should be on for
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both interfaces.)</simpara>
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</listitem>
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<listitem><simpara>If using multiple-media interface adapters,
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make sure that the correct one is enabled.
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You may need to use the config/setup program that
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came with the network interface card.</simpara>
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</listitem>
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</itemizedlist>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>I cannot <application>telnet</application>/<application>ftp</application> from the bridge! Why?</para>
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</question>
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<answer>
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<para>This is because there is no IP address bound to any of bridge
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interfaces. A bridge is to be a transparent part of a network.</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>What do I need to set up in the way of routing?</para>
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</question>
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<answer>
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<para>Nothing!
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All routing intelligence is handled by
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the bridging code in the kernel.
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To see the ethernet addresses as they are learned by the bridge,
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use the <application>brcfg</application> program in debug mode:</para>
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<screen>
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brcfg -deb
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</screen>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>The bridge appears to work, but why doesn't <application>traceroute</application> show the bridge as a part of the path?</para>
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</question>
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<answer>
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<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>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>Is it necessary to compile <envar>IP_FORWARD</envar> into the kernel?</para>
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</question>
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<answer>
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<para>No. The bridging code in the kernel takes care of the packet
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transport.
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<envar>IP_FORWARD</envar> is for a gateway that has IP addresses
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bound to its interfaces.</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>Why are the physical ethernet addresses for port 1 and port 2 the
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same according to the <application>brcfg</application> program?
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Shouldn't they be different?</para>
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</question>
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<answer>
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<para>No. Every port on a bridge intentionally is assigned the same
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physical ethernet address by the bridging code.</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>Bridging does not appear to be an option when performing a make
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config on the kernel. How does one enable it?</para>
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</question>
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<answer>
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<para>During the kernel config, answer <quote>Y</quote> to the question, <prompt>Prompt for
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development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL) [Y/n/?]</prompt>.</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>Too many hubs (4 or more) are chained one after another in series,
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cause timing problems on an ethernet. What effect does a bridge
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have in a subnet that is layered with hubs?</para>
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</question>
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<answer>
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<para>A bridge resets the 3/4/5 hubs rule. A bridge does not deal with
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packets the way a hub does, and is therefore not a contributor to
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timing problems on a network.</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>Can a bridge interface to both 10Mb and 100Mb ethernet segments?
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Will such a configuration slow down the rest of the traffic on the
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high speed side?</para>
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</question>
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<answer>
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<para>Yes, a bridge can tie together a 10Mb segment with a 100Mb segment.
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As long as the network card on the fast network is 100Mb capable,
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TCP takes care of the rest. While it's true that the
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packets from a host in the 100Mb network communicating to a host
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in the 10Mb network are moving at only 10Mb/s, the rest of the
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traffic on the fast ethernet is not slowed down.</para>
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</answer>
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</qandaentry>
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</qandaset>
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</sect1>
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<sect1>
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<title>Copyright</title>
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<para>Copyright © 2002 Christopher Cole</para>
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<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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<sect2 id="gfdl">
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<title>GNU Free Documentation License</title>
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<!-- - GNU Project - Free Software Foundation (FSF) -->
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<!-- LINK REV="made" HREF="mailto:webmasters@gnu.org" -->
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<!-- sect2>
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<title>GNU Free Documentation License</title -->
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<para>Version 1.1, March 2000</para>
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<blockquote>
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<para>Copyright (C) 2000 Free Software Foundation, Inc.
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59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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Everyone is permitted to copy and distribute verbatim copies
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of this license document, but changing it is not allowed.</para>
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</blockquote>
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</sect2>
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<sect2 id="gfdl-0">
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<title>PREAMBLE</title>
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<para>The purpose of this License is to make a manual, textbook,
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or other written document "free" in the sense of freedom: to
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assure everyone the effective freedom to copy and redistribute it,
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with or without modifying it, either commercially or
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author and publisher a way to get credit for their work, while not
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being considered responsible for modifications made by
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others.</para>
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<para>This License is a kind of "copyleft", which means that
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derivative works of the document must themselves be free in the
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same sense. It complements the GNU General Public License, which
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is a copyleft license designed for free software.</para>
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<para>We have designed this License in order to use it for manuals
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a free program should come with manuals providing the same
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freedoms that the software does. But this License is not limited
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<title>APPLICABILITY AND DEFINITIONS</title>
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<para>This License applies to any manual or other work that
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<title>MODIFICATIONS</title>
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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:</para>
|
|
|
|
<orderedlist numeration="upperalpha">
|
|
<listitem><para>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.</para>
|
|
</listitem>
|
|
|
|
<listitem><para>List on the Title Page,
|
|
as authors, one or more persons or entities responsible for
|
|
authorship of the modifications in the Modified Version,
|
|
together with at least five of the principal authors of the
|
|
Document (all of its principal authors, if it has less than
|
|
five).</para>
|
|
</listitem>
|
|
|
|
<listitem><para>State on the Title page
|
|
the name of the publisher of the Modified Version, as the
|
|
publisher.</para>
|
|
</listitem>
|
|
|
|
<listitem><para>Preserve all the
|
|
copyright notices of the Document.</para>
|
|
</listitem>
|
|
|
|
<listitem><para>Add an appropriate
|
|
copyright notice for your modifications adjacent to the other
|
|
copyright notices.</para>
|
|
</listitem>
|
|
|
|
<listitem><para>Include, immediately
|
|
after the copyright notices, a license notice giving the public
|
|
permission to use the Modified Version under the terms of this
|
|
License, in the form shown in the Addendum below.</para>
|
|
</listitem>
|
|
|
|
<listitem><para>Preserve in that license
|
|
notice the full lists of Invariant Sections and required Cover
|
|
Texts given in the Document's license notice.</para>
|
|
</listitem>
|
|
|
|
<listitem><para>Include an unaltered
|
|
copy of this License.</para>
|
|
</listitem>
|
|
|
|
<listitem><para>Preserve the section
|
|
entitled "History", and its title, and add to it an item stating
|
|
at least the title, year, new authors, and publisher of the
|
|
Modified Version as given on the Title Page. If there is no
|
|
section entitled "History" in the Document, create one stating
|
|
the title, year, authors, and publisher of the Document as given
|
|
on its Title Page, then add an item describing the Modified
|
|
Version as stated in the previous sentence.</para>
|
|
</listitem>
|
|
|
|
<listitem><para>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.</para>
|
|
</listitem>
|
|
|
|
<listitem><para>In any section entitled
|
|
"Acknowledgements" or "Dedications", preserve the section's
|
|
title, and preserve in the section all the substance and tone of
|
|
each of the contributor acknowledgements and/or dedications
|
|
given therein.</para>
|
|
</listitem>
|
|
|
|
<listitem><para>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.</para>
|
|
</listitem>
|
|
|
|
<listitem><para>Delete any section
|
|
entitled "Endorsements". Such a section may not be included in
|
|
the Modified Version.</para>
|
|
</listitem>
|
|
|
|
<listitem><para>Do not retitle any
|
|
existing section as "Endorsements" or to conflict in title with
|
|
any Invariant Section.</para>
|
|
</listitem>
|
|
</orderedlist>
|
|
<para>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.</para>
|
|
|
|
<para>You may add a section entitled "Endorsements", provided it
|
|
contains nothing but endorsements of your Modified Version by
|
|
various parties--for example, statements of peer review or that
|
|
the text has been approved by an organization as the authoritative
|
|
definition of a standard.</para>
|
|
|
|
<para>You may add a passage of up to five words as a Front-Cover
|
|
Text, and a passage of up to 25 words as a Back-Cover Text, to the
|
|
end of the list of Cover Texts in the Modified Version. Only one
|
|
passage of Front-Cover Text and one of Back-Cover Text may be
|
|
added by (or through arrangements made by) any one entity. If the
|
|
Document already includes a cover text for the same cover,
|
|
previously added by you or by arrangement made by the same entity
|
|
you are acting on behalf of, you may not add another; but you may
|
|
replace the old one, on explicit permission from the previous
|
|
publisher that added the old one.</para>
|
|
|
|
<para>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.</para>
|
|
</sect2>
|
|
|
|
<sect2 id="gfdl-5">
|
|
<title>COMBINING DOCUMENTS</title>
|
|
|
|
<para>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.</para>
|
|
|
|
<para>The combined work need only contain one copy of this
|
|
License, and multiple identical Invariant Sections may be replaced
|
|
with a single copy. If there are multiple Invariant Sections with
|
|
the same name but different contents, make the title of each such
|
|
section unique by adding at the end of it, in parentheses, the
|
|
name of the original author or publisher of that section if known,
|
|
or else a unique number. Make the same adjustment to the section
|
|
titles in the list of Invariant Sections in the license notice of
|
|
the combined work.</para>
|
|
|
|
<para>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."</para>
|
|
</sect2>
|
|
|
|
<sect2 id="gfdl-6">
|
|
<title>COLLECTIONS OF DOCUMENTS</title>
|
|
|
|
<para>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.</para>
|
|
|
|
<para>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.</para>
|
|
</sect2>
|
|
|
|
<sect2 id="gfdl-7">
|
|
<title>AGGREGATION WITH INDEPENDENT WORKS</title>
|
|
<para>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.</para>
|
|
|
|
<para>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.</para>
|
|
</sect2>
|
|
|
|
<sect2 id="gfdl-8">
|
|
<title>TRANSLATION</title>
|
|
|
|
<para>Translation is considered a kind of modification, so you may
|
|
distribute translations of the Document under the terms of section
|
|
4. Replacing Invariant Sections with translations requires
|
|
special permission from their copyright holders, but you may
|
|
include translations of some or all Invariant Sections in addition
|
|
to the original versions of these Invariant Sections. You may
|
|
include a translation of this License provided that you also
|
|
include the original English version of this License. In case of
|
|
a disagreement between the translation and the original English
|
|
version of this License, the original English version will
|
|
prevail.</para>
|
|
</sect2>
|
|
|
|
<sect2 id="gfdl-9">
|
|
<title>TERMINATION</title>
|
|
<para>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.</para>
|
|
</sect2>
|
|
|
|
<sect2 id="gfdl-10">
|
|
<title>FUTURE REVISIONS OF THIS LICENSE</title>
|
|
|
|
<para>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 <ulink
|
|
url="http://www.gnu.org/copyleft/">http://www.gnu.org/copyleft/</ulink>.</para>
|
|
|
|
<para>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.</para>
|
|
</sect2>
|
|
|
|
<sect2 id="gfdl-11">
|
|
<title>How to use this License for your documents</title>
|
|
|
|
<para>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:</para>
|
|
|
|
<blockquote><para>
|
|
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".
|
|
</para></blockquote>
|
|
|
|
<para>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.</para>
|
|
|
|
<para>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.</para>
|
|
</sect2>
|
|
</sect1>
|
|
</article>
|