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<article id="IPIP">
  <!--$Id$-->

  <articleinfo>
    <title>GRE and IPIP Tunnels</title>

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

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

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

    <copyright>
      <year>2001</year>

      <year>2002</year>

      <year>2003</year>

      <year>2004</year>

      <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>

  <warning>
    <para>GRE and IPIP Tunnels are insecure when used over the Internet; use
    them at your own risk</para>
  </warning>

  <para>GRE and IPIP tunneling with Shorewall can be used to bridge two
  masqueraded networks.</para>

  <para>The simple scripts described in the <citetitle><ulink
  url="http://ds9a.nl/lartc">Linux Advanced Routing and Shaping
  HOWTO</ulink></citetitle> work fine with Shorewall. Shorewall also includes
  a tunnel script for automating tunnel configuration. If you have installed
  the RPM, the tunnel script may be found in the Shorewall documentation
  directory (usually /usr/share/doc/shorewall-&lt;version&gt;/).</para>

  <section id="Bridged">
    <title>Bridging two Masqueraded Networks</title>

    <para>Suppose that we have the following situation:</para>

    <graphic fileref="images/TwoNets1.png" />

    <para>We want systems in the 192.168.1.0/24 subnetwork to be able to
    communicate with the systems in the 10.0.0.0/8 network. This is
    accomplished through use of the /etc/shorewall/tunnels file, the
    /etc/shorewall/policy file and the /etc/shorewall/tunnel script that is
    included with Shorewall.</para>

    <para>The <quote>tunnel</quote> script is not installed in /etc/shorewall
    by default -- If you install using the tarball, the script is included in
    the tarball; if you install using the RPM, the file is in your Shorewall
    documentation directory (normally
    /usr/share/doc/shorewall-&lt;version&gt;).</para>

    <para>In the /etc/shorewall/tunnel script, set the
    <quote>tunnel_type</quote> parameter to the type of tunnel that you want
    to create.</para>

    <example id="Tunnel">
      <title>/etc/shorewall/tunnel</title>

      <programlisting>tunnel_type=gre</programlisting>
    </example>

    <warning>
      <para>If you use the PPTP connection tracking modules from Netfilter
      Patch-O-Matic (ip_conntrack_proto_gre ip_conntrack_pptp,
      ip_nat_proto_gre and ip_nat_pptp) then you cannot use GRE
      tunnels.</para>
    </warning>

    <para>On each firewall, you will need to declare a zone to represent the
    remote subnet. We'll assume that this zone is called <quote>vpn</quote>
    and declare it in /etc/shorewall/zones on both systems as follows.</para>

    <programlisting>#ZONE        TYPE           OPTIONS
vpn          ipv4</programlisting>

    <para>On system A, the 10.0.0.0/8 will comprise the <emphasis
    role="bold">vpn</emphasis> zone. In /etc/shorewall/interfaces:</para>

    <programlisting>#ZONE        INTERFACE      BROADCAST        OPTIONS
vpn          tosysb         10.255.255.255</programlisting>

    <para>In /etc/shorewall/tunnels on system A, we need the following:</para>

    <programlisting>#TYPE         ZONE          GATEWAY          GATEWAY ZONE
ipip          net           134.28.54.2</programlisting>

    <para>This entry in /etc/shorewall/tunnels, opens the firewall so that the
    IP encapsulation protocol (4) will be accepted to/from the remote
    gateway.</para>

    <para>In the tunnel script on system A:</para>

    <example id="TunnelA">
      <title>tunnel script on system A</title>

      <programlisting>tunnel=tosysb
myrealip=206.161.148.9 (for GRE tunnel only)
myip=192.168.1.1
hisip=10.0.0.1
gateway=134.28.54.2
subnet=10.0.0.0/8
</programlisting>
    </example>

    <para>Similarly, On system B the 192.168.1.0/24 subnet will comprise the
    <emphasis role="bold">vpn</emphasis> zone. In
    /etc/shorewall/interfaces:</para>

    <programlisting>#ZONE        INTERFACE          BROADCAST
vpn          tosysa             192.168.1.255</programlisting>

    <para>In /etc/shorewall/tunnels on system B, we have:</para>

    <programlisting>#TYPE        ZONE           GATEWAY           GATEWAY ZONE
ipip         net            206.191.148.9</programlisting>

    <para>And in the tunnel script on system B:</para>

    <example id="TunnelB">
      <title>tunnel script on system B</title>

      <programlisting>tunnel=tosysa
myrealip=134.28.54.2 (for GRE tunnel only)
myip=10.0.0.1
hisip=192.168.1.1
gateway=206.191.148.9
subnet=192.168.1.0/24</programlisting>
    </example>

    <para>You can rename the modified tunnel scripts if you like; be sure that
    they are secured so that root can execute them.</para>

    <para>You will need to allow traffic between the <quote>vpn</quote> zone
    and the <quote>loc</quote> zone on both systems -- if you simply want to
    admit all traffic in both directions, you can use the policy file:</para>

    <programlisting>#SOURCE          DEST          POLICY         LOG LEVEL
loc              vpn           ACCEPT
vpn              loc           ACCEPT</programlisting>

    <para>On both systems, restart Shorewall and run the modified tunnel
    script with the <quote>start</quote> argument on each system. The systems
    in the two masqueraded subnetworks can now talk to each other</para>
  </section>
</article>
