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126 lines
4.3 KiB
XML
126 lines
4.3 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<article>
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<!--$Id$-->
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<articleinfo>
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<title>Shorewall and a Simple Bridge</title>
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<authorgroup>
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<author>
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<firstname>Tom</firstname>
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<surname>Eastep</surname>
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</author>
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</authorgroup>
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<pubdate>2005-10-12</pubdate>
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<copyright>
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<year>2004</year>
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<year>2005</year>
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<holder>Thomas M. Eastep</holder>
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</copyright>
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<legalnotice>
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<para>Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License, Version
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1.2 or any later version published by the Free Software Foundation; with
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no Invariant Sections, with no Front-Cover, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled
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<quote><ulink url="GnuCopyright.htm">GNU Free Documentation
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License</ulink></quote>.</para>
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</legalnotice>
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</articleinfo>
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<section>
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<title>Background</title>
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<para>Systems where Shorewall runs normally function as
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<firstterm>routers</firstterm>. In the context of the Open System
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Interconnect (OSI) reference model, a router operates at layer 3.
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Shorewall may also be deployed on a GNU Linux System that acts as a
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<firstterm>bridge</firstterm>. Bridges are layer-2 devices in the OSI
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model (think of a bridge as an ethernet switch).</para>
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<para>Some differences between routers and bridges are:</para>
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<orderedlist>
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<listitem>
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<para>Routers determine packet destination based on the destination IP
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address while bridges route traffic based on the destination MAC
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address in the ethernet frame.</para>
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</listitem>
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<listitem>
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<para>As a consequence of the first difference, routers can be
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connected to more than one IP network while a bridge may be part of
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only a single network.</para>
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</listitem>
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<listitem>
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<para>A router cannot forward broadcast packets while a bridge
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can.</para>
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</listitem>
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</orderedlist>
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</section>
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<section>
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<title>Application</title>
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<para>There are cases where you want to create a bridge to join two or
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more LAN segments and you don't need to restrict the traffic between those
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segments. This is the environment that is described in this
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article.</para>
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<para>If you do need to restrict traffic through the bridge, please refer
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to the <ulink url="bridge.html">Shorewall Bridge/Firewall
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documentation</ulink>. Also please refer to that documentation for
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information about how to create a bridge.</para>
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<para>The following diagram shows a firewall for two bridged LAN
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segments.</para>
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<graphic align="center" fileref="images/SimpleBridge.png" valign="middle" />
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<para>This is fundimentally the Two-interface Firewall described in the
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<ulink url="two-interface.htm">Two-interface Quickstart Guide</ulink>. The
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bridge-specific changes are restricted to the
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<filename>/etc/shorewall/interfaces</filename> file.</para>
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<para><filename>/etc/shorewall/interfaces</filename>:</para>
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<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
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net eth0 detect ...
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loc <emphasis role="bold">br0</emphasis> 10.0.1.255 <emphasis
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role="bold">routeback</emphasis>,...</programlisting>
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<para>So the key points here are:</para>
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<itemizedlist>
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<listitem>
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<para>The <emphasis role="bold">loc</emphasis> interface is <filename
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class="devicefile">br0</filename>.</para>
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</listitem>
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<listitem>
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<para>Neither <filename class="devicefile">eth1</filename> nor
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<filename class="devicefile">eth2</filename> have IP addresses and
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neither are mentioned in the Shorewall configuration.</para>
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</listitem>
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<listitem>
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<para>The <emphasis role="bold">routeback</emphasis> option is
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specified for <filename class="devicefile">br0</filename>.</para>
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</listitem>
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<listitem>
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<para>The default gateway for hosts in the local segments will be
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10.0.1.254 — the IP address of the bridge itself.</para>
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</listitem>
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</itemizedlist>
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</section>
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</article> |