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><?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>?FORMAT 2
#ZONE          INTERFACE       OPTIONS
net            eth0            ...
loc            <emphasis role="bold">br0</emphasis>             <emphasis
        role="bold">routeback,bridge</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> and <emphasis
        role="bold">bridge</emphasis> options 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>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>

    <para>When running Shorewall 5.0.14 or later, the equivalent
    <filename>/etc/shorewall/snat</filename> is:</para>

    <programlisting>#ACTION                SOURCE          DEST                PROTO   PORT
MASQUERADE             10.0.1.0/24     eth0</programlisting>
  </section>
</article>