shorewall_code/Shorewall-docs2/Shorewall_and_Routing.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<article>
  <!--$Id$-->

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

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

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

    <pubdate>2005-03-03</pubdate>

    <copyright>
      <year>2005</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>Routing vs. Firewalling.</title>

    <para>One of the most misunderstood aspects of Shorewall is its
    releationship 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>
    <title>Routing and Netfilter</title>

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

    <graphic 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>
      <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>
      <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>
    <title>Alternate Routing Table Configuration</title>

    <para>The <ulink url="Shorewall_Squid_Usage.html">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</emphasis>.</para>
  </section>
</article>