shorewall_code/docs/Multiple_Zones.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
<article id="Multiple_Zones">
  <!--$Id$-->

  <articleinfo>
    <title>Routing on One Interface</title>

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

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

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

    <copyright>
      <year>2003-2005</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>

  <caution>
    <para><emphasis role="bold">This article applies to Shorewall 4.3 and
    later. If you are running a version of Shorewall earlier than Shorewall
    4.3.5 then please see the documentation for that
    release.</emphasis></para>
  </caution>

  <section id="Intro">
    <title>Introduction</title>

    <para>While most configurations can be handled with each of the firewall's
    network interfaces assigned to a single zone, there are cases where you
    will want to divide the hosts accessed through an interface between two or
    more zones.</para>

    <itemizedlist>
      <listitem>
        <para>The interface has multiple addresses on multiple subnetworks.
        This case is covered in the <ulink
        url="Shorewall_and_Aliased_Interfaces.html">Aliased Interface
        documentation</ulink>.</para>
      </listitem>

      <listitem>
        <para>You are using some form of NAT and want to access a server by
        its external IP address from the same LAN segment. This is covered in
        <ulink url="FAQ.htm#faq2">FAQs 2 and 2a</ulink>.</para>
      </listitem>

      <listitem>
        <para>There are routers accessible through the interface and you want
        to treat the networks accessed through that router as a separate
        zone.</para>
      </listitem>

      <listitem>
        <para>Some of the hosts accessed through an interface have
        significantly different firewalling requirements from the others so
        you want to assign them to a different zone.</para>
      </listitem>
    </itemizedlist>

    <para>The key points to keep in mind when setting up multiple zones per
    interface are:</para>

    <itemizedlist>
      <listitem>
        <para>Shorewall generates rules for zones in the order that the zone
        declarations appear in /etc/shorewall/zones unless you modify the
        processing order using the explicit
        <emphasis>child-zone</emphasis>:<emphasis>parent-zone</emphasis>
        syntax, in which case the child zone rules are generated first.</para>
      </listitem>

      <listitem>
        <para>The order of entries in /etc/shorewall/hosts is immaterial as
        far as the generated rule set is concerned.</para>
      </listitem>
    </itemizedlist>

    <para><emphasis role="bold">These examples use the local zone but the same
    technique works for any zone.</emphasis> Remember that Shorewall doesn't
    have any conceptual knowledge of <quote>Internet</quote>,
    <quote>Local</quote>, or <quote>DMZ</quote> so all zones except the
    firewall itself ($FW) are the same as far as Shorewall is concerned. Also,
    the examples use private (RFC 1918) addresses but public IP addresses can
    be used in exactly the same way.</para>
  </section>

  <section id="Router">
    <title>Router in the Local Zone</title>

    <para>Here is an example of a router in the local zone.</para>

    <note>
      <para>the <emphasis role="bold">box called <quote>Router</quote> could
      be a VPN server</emphasis> or other such device; from the point of view
      of this discussion, it makes no difference.</para>
    </note>

    <graphic fileref="images/MultiZone1.png" />

    <section id="Standard">
      <title>Can You Use the Standard Configuration?</title>

      <para>In many cases, the <ulink url="two-interface.htm">standard
      two-interface Shorewall setup</ulink> will work fine in this
      configuration. It will work if:</para>

      <itemizedlist>
        <listitem>
          <para>The firewall requirements to/from the Internet are the same
          for 192.168.1.0/24 and 192.168.2.0/24.</para>
        </listitem>

        <listitem>
          <para>The hosts in 192.168.1.0/24 know that the route to
          192.168.2.0/24 is through the <emphasis
          role="bold">router</emphasis>.</para>
        </listitem>
      </itemizedlist>

      <para>All you have to do on the firewall is add a route to
      192.168.2.0/24 through the <emphasis role="bold">router</emphasis> and
      restart Shorewall.</para>
    </section>

    <section id="Enough">
      <title>Will One Zone be Enough?</title>

      <para>If the firewalling requirements for the two local networks is the
      same but the hosts in 192.168.1.0/24 don't know how to route to
      192.168.2.0/24 then you need to configure the firewall slightly
      differently. This type of configuration is rather stupid from an IP
      networking point of view but it is sometimes necessary because you
      simply don't want to have to reconfigure all of the hosts in
      192.168.1.0/24 to add a persistent route to 192.168.2.0/24. On the
      firewall:</para>

      <orderedlist>
        <listitem>
          <para>Add a route to 192.168.2.0/24 through the <emphasis
          role="bold">Router</emphasis>.</para>
        </listitem>

        <listitem>
          <para>Set the <quote>routeback</quote> option for eth1 (the local
          firewall interface) in /etc/shorewall/interfaces.</para>
        </listitem>

        <listitem>
          <para>Restart Shorewall.</para>
        </listitem>
      </orderedlist>
    </section>

    <section id="Separate">
      <title>I Need Separate Zones</title>

      <para>If you need to make 192.168.2.0/24 into its own zone, you can do
      it one of two ways; Nested Zones or Parallel Zones.</para>

      <section id="Nested">
        <title>Nested Zones</title>

        <para>You can define one zone (called it <quote>loc</quote>) as being
        all hosts connected to eth1 and a second zone <quote>loc1</quote>
        (192.168.2.0/24) as a sub-zone.</para>

        <graphic fileref="images/MultiZone1A.png" />

        <para><note>
            <para>The Router in the above diagram is assumed to NOT be doing
            SNAT for the hosts in the 192.168.2.0/24 network.</para>
          </note>The advantage of this approach is that the zone
        <quote>loc1</quote> can use CONTINUE policies such that if a
        connection request doesn't match a <quote>loc1</quote> rule, it will
        be matched against the <quote>loc</quote> rules. For example, if your
        loc1-&gt;net policy is CONTINUE then if a connection request from loc1
        to the Internet doesn't match any rules for loc1-&gt;net then it will
        be checked against the loc-&gt;net rules.</para>

        <para><filename>/etc/shorewall/zones</filename></para>

        <programlisting>#ZONE       TYPE      OPTIONS
loc         ipv4
loc1:loc    ipv4</programlisting>

        <note>
          <para>the Parent zone (loc) is defined first then the sub-zone
          (loc1) is defined using the special syntax shown.</para>
        </note>

        <para><filename>/etc/shorewall/interfaces</filename></para>

        <programlisting>#ZONE               INTERFACE           BROADCAST               OPTIONS
loc                 eth1                -</programlisting>

        <para><filename>/etc/shorewall/hosts</filename></para>

        <programlisting>#ZONE               HOSTS                        OPTIONS
loc1                eth1:192.168.2.0/24          -</programlisting>

        <para>If you don't need Shorewall to set up infrastructure to route
        traffic between <quote>loc</quote> and <quote>loc1</quote>, add these
        two policies.</para>

        <para>/etc/shorewall/policy</para>

        <programlisting>#SOURCE             DEST                 POLICY
loc                 loc1                 NONE
loc1                loc                  NONE</programlisting>
      </section>

      <section id="Parallel">
        <title>Parallel Zones</title>

        <para>You define both zones in the /etc/shorewall/hosts file to create
        two disjoint zones.</para>

        <graphic fileref="images/MultiZone1B.png" />

        <para><note>
            <para>The Router in the above diagram is assumed to NOT be doing
            SNAT for the hosts in the 192.168.2.0/24 network.</para>
          </note><filename>/etc/shorewall/zones</filename></para>

        <programlisting>#ZONE       TYPE      OPTIONS
loc1        ipv4
loc2        ipv4</programlisting>

        <para><filename>/etc/shorewall/interfaces</filename></para>

        <programlisting>#ZONE               INTERFACE           BROADCAST
-                   eth1                192.168.1.255
</programlisting>

        <para><filename>/etc/shorewall/hosts</filename></para>

        <programlisting>#ZONE               HOSTS                    OPTIONS
loc1                eth1:192.168.1.0/24      -
loc2                eth1:192.168.2.0/24      -</programlisting>

        <para>You don't need Shorewall to set up infrastructure to route
        traffic between <quote>loc</quote> and <quote>loc1</quote>, so add
        these two policies:</para>

        <programlisting>#SOURCE             DEST                 POLICY
loc1                loc2                 NONE
loc2                loc1                 NONE</programlisting>
      </section>
    </section>
  </section>

  <section id="Special">
    <title>Some Hosts have Special Firewalling Requirements</title>

    <para>There are cases where a subset of the addresses associated with an
    interface need special handling. Here's an example.</para>

    <graphic fileref="images/MultiZone2.png" />

    <para>In this example, addresses 192.168.1.8 - 192.168.1.15
    (192.168.1.8/29) are to be treated as their own zone (loc1).</para>

    <para><filename>/etc/shorewall/zones</filename></para>

    <programlisting>#ZONE       TYPE      OPTIONS
loc         ipv4
loc1:loc    ipv4</programlisting>

    <para><filename>/etc/shorewall/interfaces</filename></para>

    <programlisting>#ZONE               INTERFACE           BROADCAST
loc                 eth1                -</programlisting>

    <para><filename>/etc/shorewall/hosts</filename><programlisting>#ZONE               HOSTS                  OPTIONS
loc1                eth1:192.168.1.8/29    broadcast</programlisting></para>

    <para>The <option>broadcast</option> option causes limited broadcasts
    (destination IP address 255.255.255.255) to be checked against the loc1
    zone. If that zone allows the packet, then of course it will be seen by
    the mempers of loc that are outside of loc1 as well. If the broadcast is
    not specifically DROPped by the fw-&gt;loc1 rules then it will also be
    checked against the fw-&gt;loc rules, even if there is a DROP or REJECT
    fw-&gt;loc1 policy.</para>

    <para>You probably don't want Shorewall to set up infrastructure to route
    traffic between <quote>loc</quote> and <quote>loc1</quote> so you should
    add these two policies.</para>

    <para><filename>/etc/shorewall/policy</filename></para>

    <programlisting>#SOURCE             DEST                 POLICY
loc                 loc1                 NONE
loc1                loc                  NONE</programlisting>
  </section>

  <section id="OneArmed">
    <title>One-armed Router</title>

    <para>Nested zones may also be used to configure a
    <quote>one-armed</quote> router (I don't call it a <quote>firewall</quote>
    because it is very insecure. For example, if you connect to the Internet
    via cable modem, your next door neighbor has full access to your local
    systems as does everyone else connected to the same cable modem head-end
    controller). Here eth0 is configured with both a public IP address and an
    RFC 1918 address (More on that topic may be found <ulink
    url="Shorewall_and_Aliased_Interfaces.html">here</ulink>). Hosts in the
    <quote>loc</quote> zone are configured with their default gateway set to
    the Shorewall router's RFC1918 address.</para>

    <para><graphic fileref="images/MultiZone3.png" /></para>

    <para><filename>/etc/shorewall/zones</filename></para>

    <programlisting>#ZONE       TYPE      OPTIONS
net         ipv4
loc:net     ipv4</programlisting>

    <para><filename>/etc/shorewall/interfaces</filename></para>

    <programlisting>#ZONE               INTERFACE           BROADCAST
net                 eth0                detect</programlisting>

    <para><filename>/etc/shorewall/hosts</filename></para>

    <programlisting>#ZONE               HOSTS                              OPTIONS
loc                 eth0:192.168.1.0/24                maclist</programlisting>

    <para><filename><filename>/etc/shorewall/masq</filename></filename></para>

    <programlisting>#INTERFACE              SOURCE          ADDRESS
eth0:!192.168.1.0/24    192.168.1.0/24</programlisting>

    <para>Note that the <emphasis role="bold">maclist</emphasis> option is
    specified in <filename>/etc/shorewall/interfaces</filename>. This is to
    help protect your router from unauthorized access by your friends and
    neighbors. Start without <emphasis role="bold">maclist</emphasis> then add
    it and configure your <ulink
    url="MAC_Validation.html"><filename>/etc/shorewall/maclist</filename></ulink>
    file when everything else is working.</para>
  </section>
</article>