shorewall_code/docs/Shorewall_and_Routing.xml

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<article>
  <!--$Id$-->

  <articleinfo>
    <title>Shorewall and Routing</title>

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

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

    <pubdate><?dbtimestamp format="Y/m/d"?></pubdate>

    <copyright>
      <year>2005</year>

      <year>2006</year>

      <year>2007</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="Routing">
    <title>Routing vs. Firewalling.</title>

    <para>One of the most misunderstood aspects of Shorewall is its
    relationship with routing. This article attempts to clear some of the fog
    that surrounds this issue.</para>

    <para>As a general principle:</para>

    <orderedlist>
      <listitem>
        <para>Routing determines where packets are to be sent.</para>
      </listitem>

      <listitem>
        <para>Once routing determines where the packet is to go, the firewall
        (Shorewall) determines if the packet is allowed to go there.</para>
      </listitem>
    </orderedlist>

    <para>There are ways that Shorewall can affect routing which are described
    in the following sections.</para>
  </section>

  <section id="Netfilter">
    <title>Routing and Netfilter</title>

    <para>The following diagram shows the relationship between routing
    decisions and Netfilter.</para>

    <graphic align="center" fileref="images/Netfilter.png" />

    <para>The light blue boxes indicate where routing decisions are made. Upon
    exit from one of these boxes, if the packet is being sent to another
    system then the interface and the next hop have been uniquely
    determined.</para>

    <para>The green boxes show where Netfilter processing takes place (as
    directed by Shorewall). You will notice that there are two different paths
    through this maze, depending on where the packet originates. We will look
    at each of these separately.</para>

    <section id="Ingress">
      <title>Packets Entering the Firewall from Outside</title>

      <para>When a packet arrives from outside, it first undergoes Netfilter
      PREROUTING processing. In Shorewall terms:</para>

      <orderedlist>
        <listitem>
          <para>Packets may be marked using entries in the <ulink
          url="???">/etc/shorewall/tcrules</ulink> file. Entries in that file
          containing ":P" in the mark column are applied here as are rules
          that default to the MARK_IN_FORWARD_CHAIN=No setting in
          <filename>/etc/shorewall/shorewall.conf</filename>. These marks may
          be used to specify that the packet should be routed using an
          <firstterm>alternate routing table</firstterm>; see the <ulink
          url="Shorewall_Squid_Usage.html">Shorewall Squid
          documentation</ulink> for examples.</para>

          <caution>
            <para>Marking packets then using the <emphasis>fwmark</emphasis>
            selector in your "<emphasis role="bold">ip rule add</emphasis>"
            commands should NOT be your first choice. In most cases, you can
            use the <emphasis>from</emphasis> or <emphasis>dev</emphasis>
            selector instead.</para>
          </caution>
        </listitem>

        <listitem>
          <para>The destination IP address may be rewritten as a consequence
          of:</para>

          <itemizedlist>
            <listitem>
              <para>DNAT[-] rules.</para>
            </listitem>

            <listitem>
              <para>REDIRECT[-] rules.</para>
            </listitem>

            <listitem>
              <para>Entries in <filename>/etc/shorewall/nat</filename>.</para>
            </listitem>
          </itemizedlist>
        </listitem>
      </orderedlist>

      <para>So the only influence that Shorewall has over where these packets
      go is via NAT or by marking them so that they may be routed using an
      alternate routing table.</para>
    </section>

    <section id="Local">
      <title>Packets Originating on the Firewall</title>

      <para>Processing of packets that originate on the firewall itself are
      initially routed using the default routing table then passed through the
      OUTPUT chains. Shorewall can influence what happens here:</para>

      <orderedlist>
        <listitem>
          <para>Packets may be marked using entries in the <ulink
          url="???">/etc/shorewall/tcrules</ulink> file (rules with "$FW" in
          the SOURCE column). These marks may be used to specify that the
          packet should be re-routed using an alternate routing table.</para>
        </listitem>

        <listitem>
          <para>The destination IP address may be rewritten as a consequence
          of:</para>

          <itemizedlist>
            <listitem>
              <para>DNAT[-] rules that specify $FW as the SOURCE.</para>
            </listitem>

            <listitem>
              <para>Entries in <filename>/etc/shorewall/nat</filename> that
              have "Yes" in LOCAL column.</para>
            </listitem>
          </itemizedlist>
        </listitem>
      </orderedlist>

      <para>So again in this case, the only influence that Shorewall has over
      the packet destination is NAT or marking.</para>
    </section>
  </section>

  <section id="RoutingTables">
    <title>Alternate Routing Table Configuration</title>

    <para>The Shorewall 2.x <ulink
    url="http://www.shorewall.net/2.0/Shorewall_Squid_Usage.html#Local">Shorewall
    Squid documentation</ulink> shows how alternate routing tables can be
    created and used. That documentation shows how you can use logic in
    <filename>/etc/shorewall/init</filename> to create and populate an
    alternate table and to add a routing rule for its use. It is fine to use
    that technique so long as you understand that you are basically just using
    the Shorewall init script (<filename>/etc/init.d/shorewall</filename>) to
    configure your alternate routing table at boot time and that <emphasis
    role="bold">other than as described in the previous section, there is no
    connection between Shorewall and routing when using Shorewall versions
    prior to 2.3.2.</emphasis></para>
  </section>

  <section id="ProxyArp">
    <title>Routing and Proxy ARP</title>

    <para>There is one instance where Shorewall creates main routing table
    entries. When an entry in <filename>/etc/shorewall/proxyarp</filename>
    contains "No" in the HAVEROUTE column then Shorewall will create a host
    route to the IP address listed in the ADDRESS column through the interface
    named in the INTERFACE column. <emphasis role="bold">This is the only case
    where Shorewall directly manipulates the main routing
    table</emphasis>.</para>

    <para>Example:</para>

    <para><filename>/etc/shorewall/proxyarp</filename>:</para>

    <programlisting>#ADDRESS        INTERFACE       EXTERNAL        HAVEROUTE       PERSISTENT
206.124.146.177 eth1            eth0            No
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE</programlisting>

    <para>The above entry will cause Shorewall to execute the following
    command:</para>

    <programlisting><emphasis role="bold">ip route add 206.124.146.177 dev eth1</emphasis></programlisting>
  </section>

  <section id="MultiISP">
    <title>Multiple Internet Connection Support in Shorewall 2.4.2 and
    Later</title>

    <para>Beginning with Shorewall 2.3.2, support is included for multiple
    internet connections. If you wish to use this feature, we recommend
    strongly that you upgrade to version 2.4.2 or later.</para>

    <para>Shorewall multi-ISP support is now covered in a <ulink
    url="MultiISP.html">separate article</ulink>.</para>
  </section>
</article>