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  <articleinfo>
    <title>VPN, Netfilter and Shorewall — The Basics</title>

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

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

    <pubdate>2004-12-23</pubdate>

    <copyright>
      <year>2004</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>Gateway-to-gateway traffic vs. Host-to-host traffic.</title>

    <para>The purpose of a <firstterm>Virtual Private Network</firstterm>
    (VPN) is to provide for secure communication between a set of hosts.
    Communication between a pair of hosts connected by a VPN occurs in
    stages:</para>

    <orderedlist>
      <listitem>
        <para><emphasis role="bold">Local-host-to-local-gateway</emphasis>.
        This communication is not encrypted; in the case where the traffic
        originates on the gateway itself, the communication is local to that
        system.</para>
      </listitem>

      <listitem>
        <para><emphasis
        role="bold">Local-gateway-to-remote-gateway</emphasis>. This
        communication is encrypted and can use a tunneling protocol such as
        GRE, AH or ESP or a standard protocol such as UDP or TCP. Some VPNs
        use multiple protocols; for example PPTP uses TCP port 1723 and GRE
        while IPSEC uses UDP port 500 together with ESP or AH.</para>
      </listitem>

      <listitem>
        <para><emphasis role="bold">Remote-gateway-to-remote-host</emphasis>.
        This is just the unencrypted traffic described in the first item as it
        is delivered to its destination.</para>
      </listitem>
    </orderedlist>

    <para>Of course, one-way communication generally isn't useful so we need
    traffic in the other direction as well.</para>

    <orderedlist>
      <listitem>
        <para><emphasis
        role="bold">Remote-host-to-remote-gateway</emphasis>.</para>
      </listitem>

      <listitem>
        <para><emphasis
        role="bold">Remote-gateway-to-local-gateway</emphasis>.</para>
      </listitem>

      <listitem>
        <para><emphasis
        role="bold">Local-gateway-to-local-host</emphasis>.</para>
      </listitem>
    </orderedlist>
  </section>

  <section>
    <title>Relationship to Netfilter</title>

    <para>When Netfilter is configured on a VPN gateway, each VPN packet goes
    through Netfilter twice! Let's first consider outbound traffic:</para>

    <orderedlist>
      <listitem>
        <para><emphasis role="bold">Local-host-to-local-gateway</emphasis>.
        This traffic has a source address in the local network or on the
        gateway itself. The destination IP address is that of a remote host;
        either the remote gateway itself or a host behind that gaeway.</para>
      </listitem>

      <listitem>
        <para><emphasis
        role="bold">Local-gateway-to-remote-gateway.</emphasis> This
        (encrypted) traffic has a source IP address on the gateway and is
        addressed to the remote gateway.</para>
      </listitem>
    </orderedlist>

    <para>Incoming traffic is similar.</para>
  </section>

  <section>
    <title>What does this mean with Shorewall?</title>

    <para>When Shorewall is installed on a VPN gateway system, it catagorizes
    the VPN-related traffic slightly differently:</para>

    <orderedlist>
      <listitem>
        <para><emphasis role="bold">Local-host-to-remote-host</emphasis> —
        same as <emphasis role="bold">Local-host-to-local-gateway</emphasis>
        above.</para>
      </listitem>

      <listitem>
        <para><emphasis
        role="bold">Local-gateway-to-remote-gateway</emphasis>.</para>
      </listitem>

      <listitem>
        <para><emphasis
        role="bold">Remote-gateway-to-local-gateway</emphasis>.</para>
      </listitem>

      <listitem>
        <para><emphasis role="bold">Remote-host-to-local-host</emphasis> —
        same as <emphasis role="bold">Local-gateway-to-local-host</emphasis>
        above.</para>
      </listitem>
    </orderedlist>

    <para>Shorewall implements a set of features for dealing with VPN.</para>

    <orderedlist>
      <listitem>
        <para>The <filename>/etc/shorewall/tunnels</filename> file. This file
        is used to define remote gateways and the type of encrypted traffic
        that will be passed between the Shorewall system and those remote
        gateways. In other words, the tunnels file deals with <emphasis
        role="bold">Local-gateway-to-remote-gateway</emphasis> and <emphasis
        role="bold">Remote-gateway-to-local-gateway</emphasis> traffic.</para>
      </listitem>

      <listitem>
        <para>The <filename>/etc/shorewall/zones</filename> file. An entry in
        this file allows you to associated a name with the set of hosts behind
        the remote gateway (or to the remote gateway itself if it is a
        standalone system).</para>
      </listitem>

      <listitem>
        <para>The <filename>/etc/shorewall/interfaces</filename> and
        <filename>/etc/shorewall/hosts</filename> files. These files are used
        to associate a set of remote hosts with the zone name defined in
        <filename>/etc/shorewall/zones</filename>.</para>
      </listitem>

      <listitem>
        <para>The <filename>/etc/shorewall/policy </filename>and
        <filename>/etc/shorewall/rules files</filename>. These files are used
        to define the connections that are permitted between the remote and
        local hosts -- in other words, the <emphasis
        role="bold">Local-host-to-remote-host</emphasis> and <emphasis
        role="bold">Remote-host-to-local-host</emphasis> traffic.</para>
      </listitem>
    </orderedlist>
  </section>

  <section>
    <title>Defining Remote Zones</title>

    <para>Most VPN types are implemented using a virtual network device such
    as pppN (e.g., ppp0), tunN (e.g., tun0), etc. This means that in most
    cases, remote zone definition is similar to zones that you have already
    defined.</para>

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

    <programlisting>#ZONE           DISPLAY              COMMENT
net             Internet             The big bad net
loc             Local                Local LAN
<emphasis role="bold">rem           Remote             Remote LAN</emphasis></programlisting>

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

    <programlisting>#ZONE           INTERFACE          BROADCAST          OPTION
net             eth0               detect             norft1918,routefilter
loc             eth1               detect
<emphasis role="bold">rem           tun0             192.168.10.0/24</emphasis></programlisting>

    <para>The /etc/shorewall/hosts file comes into play when:</para>

    <orderedlist>
      <listitem>
        <para>You have a number of remote networks.</para>
      </listitem>

      <listitem>
        <para>The remote networks have different firewall requirements and you
        want to divide them into multiple zones.</para>
      </listitem>

      <listitem>
        <para>There is no fixed relationship between the remote networks and
        virtual network devices (for example, the VPN uses PPTP and remote
        gateways connect on demand).</para>
      </listitem>
    </orderedlist>

    <para>In this case, your configuration takes the following
    approach:</para>

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

    <programlisting>#ZONE           DISPLAY              COMMENT
net             Internet             The big bad net
loc             Local                Local LAN
<emphasis role="bold">rem1          Remote1            Remote LAN 1
rem2          Remote2            Remote LAN 2</emphasis></programlisting>

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

    <programlisting>#ZONE           INTERFACE          BROADCAST          OPTION
net             eth0               detect             norft1918,routefilter
loc             eth1               detect
<emphasis role="bold">-             tun+             -</emphasis></programlisting>

    <para>/etc/shorewall/hosts:</para>

    <programlisting>#ZONE           HOST(S)            OPTIONS
<emphasis role="bold">rem1          tun+:10.0.0.0/24
rem2          tun+:10.0.1.0/24</emphasis></programlisting>

    <para>The <filename>/etc/shorewall/hosts</filename> file is also used with
    <ulink url="IPSEC-2.6.html">kernel 2.6 native IPSEC</ulink>.</para>
  </section>

  <section>
    <title>Eliminating the /etc/shorewall/tunnels file</title>

    <para>The <filename>/etc/shorewall/tunnels</filename> file provides no
    functionality that could not be implemented using entries in
    <filename>/etc/shorewall/rules</filename> and I have elimination of the
    <filename>/etc/shorewall/tunnels</filename> file as a long-term goal. The
    following sections show how entries in
    <filename>/etc/shorewall/tunnels</filename> can be replaced by rules for
    some common tunnel types.</para>

    <section>
      <title>IPSEC</title>

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

      <blockquote>
        <programlisting>#TYPE           ZONE          GATEWAY          GATEWAY ZONE
ipsec           Z1            1.2.3.4          Z2</programlisting>
      </blockquote>

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

      <blockquote>
        <programlisting>#ACTION  SOURCE         DEST            PROTO   DEST    SOURCE 
#                                               PORT    PORT(S)
ACCEPT   $FW            Z1:1.2.3.4      udp     500
ACCEPT   Z1:1.2.3.4     $FW             udp     500
ACCEPT   $FW            Z1:1.2.3.4      50
ACCEPT   Z1:1.2.3.4     $FW             50
ACCEPT   $FW            Z1:1.2.3.4      51
ACCEPT   Z1:1.2.3.4     $FW             51
ACCEPT   $FW            Z2:1.2.3.4      udp     500
ACCEPT   Z2:1.2.3.4     $FW             udp     500</programlisting>
      </blockquote>

      <para>The "noah" option causes the rules for protocol 51 to be
      eliminated. The "ipsecnat" causes UDP port 4500 to be accepted in both
      directions. If no GATEWAY ZONE is given then the last two rules above
      are omitted.</para>
    </section>

    <section>
      <title>PPTP</title>

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

      <blockquote>
        <programlisting>#TYPE           ZONE          GATEWAY          GATEWAY ZONE
pptpserver      Z1            1.2.3.4</programlisting>
      </blockquote>

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

      <blockquote>
        <programlisting>#ACTION  SOURCE         DEST            PROTO   DEST    SOURCE 
#                                               PORT    PORT(S)

ACCEPT   Z1:1.2.3.4     $FW             tcp     1723
ACCEPT   $FW            Z1:1.2.3.4      47
ACCEPT   Z1:1.2.3.4     $FW             47</programlisting>
      </blockquote>

      <para>Tunnel type "pptpclient" simply reverses the direction of the tcp
      port 1723 rule.</para>
    </section>

    <section>
      <title>OpenVPN</title>

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

      <blockquote>
        <programlisting>#TYPE           ZONE          GATEWAY          GATEWAY ZONE
openvpn:P       Z1            1.2.3.4</programlisting>
      </blockquote>

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

      <blockquote>
        <programlisting>#ACTION  SOURCE         DEST            PROTO   DEST    SOURCE 
#                                               PORT    PORT(S)

ACCEPT   Z1:1.2.3.4     $FW             udp     P
ACCEPT   $FW            Z1:1.2.3.4      udp     P</programlisting>
      </blockquote>
    </section>
  </section>
</article>