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

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

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

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

    <copyright>
      <year>2004</year>

      <year>2005</year>

      <year>2006</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 id="Taxonomy">
    <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 id="Netfilter">
    <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 gateway.</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>

    <graphic align="center" fileref="images/VPNBasics.png" />
  </section>

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

    <para>When Shorewall is installed on a VPN gateway system, it categorizes
    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 id="Zones">
    <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           TYPE
fw              firewall
net             ipv4
loc             ipv4
<emphasis role="bold">rem             ipv4</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             ppp0               192.168.10.0/24</emphasis></programlisting>
  </section>

  <section id="Traffic">
    <title>Allowing Traffic</title>

    <para>Normally, you will just allow all traffic between your remote
    client(s) and the local zone. You can do that with a couple of
    policies:</para>

    <programlisting>#SOURCE       DESTINATION         POLICY         LEVEL          BURST/LIMIT
rem           loc                 ACCEPT
loc           rem                 ACCEPT</programlisting>

    <para>Similar policies using $FW rather than 'loc' can permit traffic from
    the remote clients to/from the firewall.</para>
  </section>

  <section id="Policies">
    <title>Different Firewall Policies for Different Remote Systems</title>

    <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 id="tunnels">
    <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 id="IPSEC">
      <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 id="PPTP">
      <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 id="OpenVPN">
      <title>OpenVPN</title>

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

      <blockquote>
        <programlisting>#TYPE           ZONE          GATEWAY          GATEWAY ZONE
openvpn:<emphasis>port</emphasis>    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     <emphasis>port</emphasis>
ACCEPT   $FW            Z1:1.2.3.4      udp     <emphasis>port</emphasis></programlisting>
      </blockquote>

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

      <blockquote>
        <programlisting>#TYPE              ZONE          GATEWAY          GATEWAY ZONE
openvpnclient:<emphasis>port</emphasis> 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     -       <emphasis>port</emphasis>
ACCEPT   $FW            Z1:1.2.3.4      udp     <emphasis>port</emphasis></programlisting>
      </blockquote>

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

      <blockquote>
        <programlisting>#TYPE              ZONE          GATEWAY          GATEWAY ZONE
openvpnserver:<emphasis>port</emphasis> 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     <emphasis>port</emphasis>
ACCEPT   $FW            Z1:1.2.3.4      udp     -       <emphasis>port</emphasis></programlisting>
      </blockquote>
    </section>
  </section>

  <section>
    <title>Links to Other VPN Articles at shorewall.net</title>

    <itemizedlist>
      <listitem>
        <para><ulink url="OPENVPN.html">OpenVPN</ulink></para>
      </listitem>

      <listitem>
        <para><ulink url="IPSEC-2.6.html">IPSEC</ulink></para>
      </listitem>

      <listitem>
        <para><ulink url="PPTP.htm">PPTP</ulink></para>
      </listitem>
    </itemizedlist>
  </section>
</article>