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shorewall_code/docs/SimpleBridge.xml
2009-09-05 08:23:35 -07:00

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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
<article>
<!--$Id$-->
<articleinfo>
<title>Shorewall and a Simple Bridge</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
<copyright>
<year>2004</year>
<year>2005</year>
<year>2006</year>
<holder>Thomas M. Eastep</holder>
</copyright>
<legalnotice>
<para>Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, with no Front-Cover, and with no Back-Cover
Texts. A copy of the license is included in the section entitled
<quote><ulink url="GnuCopyright.htm">GNU Free Documentation
License</ulink></quote>.</para>
</legalnotice>
</articleinfo>
<section id="Background">
<title>Background</title>
<para>Systems where Shorewall runs normally function as
<firstterm>routers</firstterm>. In the context of the Open System
Interconnect (OSI) reference model, a router operates at layer 3.
Shorewall may also be deployed on a GNU Linux System that acts as a
<firstterm>bridge</firstterm>. Bridges are layer-2 devices in the OSI
model (think of a bridge as an Ethernet switch).</para>
<para>Some differences between routers and bridges are:</para>
<orderedlist>
<listitem>
<para>Routers determine packet destination based on the destination IP
address while bridges route traffic based on the destination MAC
address in the Ethernet frame.</para>
</listitem>
<listitem>
<para>As a consequence of the first difference, routers can be
connected to more than one IP network while a bridge may be part of
only a single network.</para>
</listitem>
<listitem>
<para>A router cannot forward broadcast packets while a bridge
can.</para>
</listitem>
</orderedlist>
</section>
<section id="Application">
<title>Application</title>
<para>There are cases where you want to create a bridge to join two or
more LAN segments and you don't need to restrict the traffic between those
segments. This is the environment that is described in this
article.</para>
<para>If you do need to restrict traffic through the bridge, please refer
to the <ulink url="bridge-Shorewall-perl.html">Shorewall Bridge/Firewall
documentation</ulink>. Also please refer to that documentation for
information about how to create a bridge.</para>
<para>The following diagram shows a firewall for two bridged LAN
segments.</para>
<graphic align="center" fileref="images/SimpleBridge.png" valign="middle" />
<para>This is fundamentally the Two-interface Firewall described in the
<ulink url="two-interface.htm">Two-interface Quickstart Guide</ulink>. The
bridge-specific changes are restricted to the
<filename>/etc/shorewall/interfaces</filename> file.</para>
<note>
<para>Older configurations that specify an interface name in the SOURCE
column of <filename>/etc/shorewall/masq</filename> will also need to
change that file.</para>
</note>
<para>This example illustrates the bridging of two Ethernet devices but
the types of the devices really isn't important. What is shown here would
apply equally to bridging an Ethernet device to an <ulink
url="OPENVPN.html">OpenVPN</ulink> tap device (e.g.,
<filename>tap0</filename>) or to a wireless device
(<filename>ath0</filename> or <filename>wlan0</filename>).</para>
<para><filename>/etc/shorewall/interfaces</filename>:</para>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
net eth0 detect ...
loc <emphasis role="bold">br0</emphasis> 10.0.1.255 <emphasis
role="bold">routeback</emphasis>,...</programlisting>
<para>So the key points here are:</para>
<itemizedlist>
<listitem>
<para>The <emphasis role="bold">loc</emphasis> interface is <filename
class="devicefile">br0</filename>.</para>
</listitem>
<listitem>
<para>Neither <filename class="devicefile">eth1</filename> nor
<filename class="devicefile">eth2</filename> have IP addresses and
neither are mentioned in the Shorewall configuration.</para>
</listitem>
<listitem>
<para>The <emphasis role="bold">routeback</emphasis> option is
specified for <filename class="devicefile">br0</filename>.</para>
</listitem>
<listitem>
<para>The default gateway for hosts in the local segments will be
10.0.1.254 — the IP address of the bridge itself.</para>
</listitem>
</itemizedlist>
<para><emphasis role="bold">Note to Shorewall-perl users</emphasis>: You
should also specify the <emphasis role="bold">bridge</emphasis>
option:<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
net eth0 detect ...
loc <emphasis role="bold">br0</emphasis> 10.0.1.255 <emphasis
role="bold">routeback,bridge</emphasis>,...</programlisting></para>
<para>Your entry in <filename>/etc/shorewall/masq</filename> should be
unchanged:</para>
<programlisting>#INTERFACE SOURCE ADDRESS
eth0 10.0.1.0/24 ... # 10.0.1.0/24 is the local network on LAN A and LAN B</programlisting>
</section>
</article>