shorewall_code/docs/SimpleBridge.xml

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  <articleinfo>
    <title>Shorewall and a Simple Bridge</title>

    <authorgroup>
      <author>
        <firstname>Tom</firstname>

        <surname>Eastep</surname>
      </author>
    </authorgroup>

    <pubdate>2006-03-07</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>
    <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>
    <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.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>

    <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>
  </section>
</article>