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<?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="IPIP">
<articleinfo>
<title>GRE and IPIP Tunnels</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<copyright>
<year>2001</year>
<year>2002</year>
<year>2003</year>
<holder>Thomas M. Eastep</holder>
</copyright>
<pubdate>2003-02-22</pubdate>
</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 <ulink url="http://ds9a.nl/lartc">Linux
Advanced Routing and Shaping HOWTO</ulink> 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-&#60;version&#62;/).</para>
<section>
<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 &#39;tunnel&#39; 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-&#60;version&#62;).</para>
<para>In the /etc/shorewall/tunnel script, set the &#39;tunnel_type&#39;
parameter to the type of tunnel that you want to create.</para>
<example>
<title>/etc/shorewall/tunnel</title>
<programlisting>tunnel_type=gre</programlisting>
</example>
<para>On each firewall, you will need to declare a zone to represent the
remote subnet. We&#39;ll assume that this zone is called &#39;vpn&#39; and
declare it in /etc/shorewall/zones on both systems as follows.</para>
<table>
<title>/etc/shorewall/zones system A &#38; B</title>
<tgroup cols="3">
<thead>
<row>
<entry align="center">ZONE</entry>
<entry align="center">DISPLAY</entry>
<entry align="center">COMMENTS</entry>
</row>
</thead>
<tbody>
<row>
<entry>vpn</entry>
<entry>VPN</entry>
<entry>Remote Subnet</entry>
</row>
</tbody>
</tgroup>
</table>
<para>On system A, the 10.0.0.0/8 will comprise the <emphasis role="bold">vpn</emphasis>
zone. In /etc/shorewall/interfaces:</para>
<table>
<title>/etc/shorewall/interfaces system A</title>
<tgroup cols="4">
<thead>
<row>
<entry align="center">ZONE</entry>
<entry align="center">INTERFACE</entry>
<entry align="center">BROADCAST</entry>
<entry align="center">OPTIONS</entry>
</row>
</thead>
<tbody>
<row>
<entry>vpn</entry>
<entry>tosysb</entry>
<entry>10.255.255.255</entry>
<entry></entry>
</row>
</tbody>
</tgroup>
</table>
<para>In /etc/shorewall/tunnels on system A, we need the following:</para>
<table>
<title>/etc/shorewall/tunnels system A</title>
<tgroup cols="4">
<thead>
<row>
<entry align="center">TYPE</entry>
<entry align="center">ZONE</entry>
<entry align="center">GATEWAY</entry>
<entry align="center">GATEWAY ZONE</entry>
</row>
</thead>
<tbody>
<row>
<entry>ipip</entry>
<entry>net</entry>
<entry>134.28.54.2</entry>
<entry></entry>
</row>
</tbody>
</tgroup>
</table>
<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>
<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>
<table>
<title>/etc/shorewall/interfaces system B</title>
<tgroup cols="4">
<thead>
<row>
<entry align="center">ZONE</entry>
<entry align="center">INTERFACE</entry>
<entry align="center">BROADCAST</entry>
<entry align="center">OPTIONS</entry>
</row>
</thead>
<tbody>
<row>
<entry>vpn</entry>
<entry>tosysa</entry>
<entry>192.168.1.255</entry>
<entry></entry>
</row>
</tbody>
</tgroup>
</table>
<para>In /etc/shorewall/tunnels on system B, we have:</para>
<table>
<title>/etc/shorewall/tunnels system B</title>
<tgroup cols="4">
<thead>
<row>
<entry align="center">TYPE</entry>
<entry align="center">ZONE</entry>
<entry align="center">GATEWAY</entry>
<entry align="center">GATEWAY ZONE</entry>
</row>
</thead>
<tbody>
<row>
<entry>ipip</entry>
<entry>net</entry>
<entry>206.191.148.9</entry>
<entry></entry>
</row>
</tbody>
</tgroup>
</table>
<para>And in the tunnel script on system B:</para>
<example>
<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 &#34;vpn&#34; zone and
the &#34;loc&#34; zone on both systems -- if you simply want to admit all
traffic in both directions, you can use the policy file:</para>
<table>
<title>/etc/shorewall/policy system A &#38; B</title>
<tgroup cols="4">
<thead>
<row>
<entry align="center">SOURCE</entry>
<entry align="center">DEST</entry>
<entry align="center">POLICY</entry>
<entry align="center">LOG LEVEL</entry>
</row>
</thead>
<tbody>
<row>
<entry>loc</entry>
<entry>vpn</entry>
<entry>ACCEPT</entry>
<entry></entry>
</row>
<row>
<entry>vpn</entry>
<entry>loc</entry>
<entry>ACCEPT</entry>
<entry></entry>
</row>
</tbody>
</tgroup>
</table>
<para>On both systems, restart Shorewall and run the modified tunnel
script with the &#34;start&#34; argument on each system. The systems in
the two masqueraded subnetworks can now talk to each other</para>
</section>
</article>