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

  <articleinfo>
    <title>IPSEC using Linux Kernel 2.6</title>

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

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

      <author>
        <firstname>Roberto</firstname>

        <surname>Sanchez</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>

    <copyright>
      <year>2007</year>

      <holder>Roberto C. Sanchez</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 3.0 and
    later. If you are running a version of Shorewall earlier than Shorewall
    3.0.0 then please see the documentation for that
    release.</emphasis></para>
  </caution>

  <important>
    <para>The information in this article is only applicable if you plan to
    have IPSEC end-points on the same system where Shorewall is used.</para>
  </important>

  <warning>
    <para>To use the features described in this article, your kernel must be
    2.6.16 or later <emphasis role="bold">or</emphasis> your kernel and
    iptables must include the Netfilter+ipsec patches and policy match
    support. The Netfilter patches are available from Netfilter
    Patch-O-Matic-NG and are also included in some commercial distributions
    (most notably <trademark>SUSE</trademark> 9.1 through 10.0).</para>
  </warning>

  <warning>
    <para>As of this writing, the Netfilter+ipsec and policy match support are
    broken when used with a bridge device. The problem has been reported to
    the responsible Netfilter developer who has confirmed the problem. The
    problem was presumably corrected in Kernel 2.6.20 as a result of the
    removal of defered FORWARD/OUTPUT processing of traffic destined for a
    bridge. See the <ulink
    url="bridge-Shorewall-perl.html">"<emphasis>Shorewall-perl and Bridged
    Firewalls</emphasis>"</ulink> article.</para>
  </warning>

  <section id="Overview">
    <title>Shorewall 3.0 and Kernel 2.6 IPSEC</title>

    <para>This is <emphasis role="bold">not</emphasis> a HOWTO for Kernel 2.6
    IPSEC -- for that, please see <ulink
    url="http://www.ipsec-howto.org/">http://www.ipsec-howto.org/</ulink>.</para>

    <para>The 2.6 Linux Kernel introduces new facilities for defining
    encrypted communication between hosts in a network. The network
    administrator defines a set of <firstterm>Security Policies</firstterm>
    which are stored in the kernel as a <firstterm>Security Policy
    Database</firstterm> (SPD). Security policies determine which traffic is
    subject to encryption. <firstterm>Security Associations</firstterm> are
    created between pairs of hosts in the network (one SA for traffic in each
    direction); these SAs define how traffic is to be encrypted. Outgoing
    traffic that is to be encrypted according to the contents of the SPD
    requires an appropriate SA to exist. SAs may be created manually using
    <command>setkey</command>(8) but most often, they are created by a
    cooperative process involving the ISAKMP protocol and daemons such
    as<command> racoon</command> or <command>isakmpd</command>. Incoming
    traffic is verified against the SPD to ensure that no unencrypted traffic
    is accepted in violation of the administrator's policies.</para>

    <para>There are three ways in which IPSEC traffic can interact with
    Shorewall policies and rules:</para>

    <orderedlist>
      <listitem>
        <para>Traffic that is encrypted on the firewall system. The traffic
        passes through Netfilter twice -- first as unencrypted then
        encrypted.</para>
      </listitem>

      <listitem>
        <para>Traffic that is decrypted on the firewall system. The traffic
        passes through Netfilter twice -- first as encrypted then as
        unencrypted.</para>
      </listitem>

      <listitem>
        <para>Encrypted traffic that is passed through the firewall system.
        The traffic passes through Netfilter once.</para>
      </listitem>
    </orderedlist>

    <para>In cases 1 and 2, the encrypted traffic is handled by entries in
    <filename>/etc/shorewall/tunnels</filename> (don't be mislead by the name
    of the file -- <emphasis>transport mode</emphasis> encrypted traffic is
    also handled by entries in that file). The unencrypted traffic is handled
    by normal rules and policies.</para>

    <para>Under the 2.4 Linux Kernel, the association of unencrypted traffic
    and zones was made easy by the presense of IPSEC pseudo-interfaces with
    names of the form <filename class="devicefile">ipsecn</filename> (e.g.
    <filename class="devicefile">ipsec0</filename>). Outgoing unencrypted
    traffic (case 1.) was send through an <filename
    class="devicefile">ipsecn</filename> device while incoming unencrypted
    traffic (case 2) arrived from an <filename
    class="devicefile">ipsecn</filename> device. The 2.6 kernel-based
    implementation does away with these pseudo-interfaces. Outgoing traffic
    that is going to be encrypted and incoming traffic that has been decrypted
    must be matched against policies in the SPD and/or the appropriate
    SA.</para>

    <para>Shorewall provides support for policy matching in three ways:</para>

    <orderedlist>
      <listitem>
        <para>In <filename>/etc/shorewall/masq</filename>, traffic that will
        later be encrypted is exempted from MASQUERADE/SNAT using existing
        entries. If you want to MASQUERADE/SNAT outgoing traffic that will
        later be encrypted, you must include the appropriate indication in the
        new IPSEC column in that file.</para>
      </listitem>

      <listitem>
        <para>The<filename> </filename><ulink
        url="manpages/shorewall-zones.html"><filename>/etc/shorewall/zones</filename></ulink>
        file allows you to associate zones with traffic that will be encrypted
        or that has been decrypted.</para>
      </listitem>

      <listitem>
        <para>A new option (<emphasis role="bold">ipsec</emphasis>) has been
        provided for entries in <filename>/etc/shorewall/hosts</filename>.
        When an entry has this option specified, traffic to/from the hosts
        described in the entry is assumed to be encrypted.</para>
      </listitem>
    </orderedlist>

    <para>In summary, Shorewall provides the facilities to replace the use of
    ipsec pseudo-interfaces in zone and MASQUERADE/SNAT definition.</para>

    <para>There are two cases to consider:</para>

    <orderedlist>
      <listitem>
        <para>Encrypted communication is used to/from all hosts in a
        zone.</para>

        <para>The value <emphasis role="bold">ipsec</emphasis> is placed in
        the TYPE column of the <filename>/etc/shorewall/zones</filename> entry
        for the zone.</para>
      </listitem>

      <listitem>
        <para>By default, encrypted communication is not used to communicate
        with the hosts in a zone.</para>

        <para>The value <emphasis role="bold">ipv4</emphasis> is placed in the
        TYPE column of the <filename>/etc/shorewall/zones</filename> entry for
        the zone and the new <emphasis role="bold">ipsec</emphasis> option is
        specified in <filename>/etc/shorewall/hosts</filename> for any hosts
        requiring secure communication.</para>
      </listitem>
    </orderedlist>

    <note>
      <para>For simple zones such as are shown in the following examples, the
      two techniques are equivalent and are used interchangably.</para>
    </note>

    <note>
      <para>It is redundent to have <emphasis role="bold">ipsec</emphasis> in
      the TYPE column of the <filename>/etc/shorewall/zones</filename> entry
      for a zone and to also have the <emphasis role="bold">ipsec</emphasis>
      option in <filename>/etc/shorewall/hosts</filename> entries for that
      zone.</para>
    </note>

    <para>Finally, the OPTIONS, IN OPTIONS and OUT OPTIONS columns in
    /etc/shorewall/zones can be used to match the zone to a particular (set
    of) SA(s) used to encrypt and decrypt traffic to/from the zone and the
    security policies that select which traffic to encrypt/decrypt.</para>

    <para>This article assumes the use of ipsec-tools (<ulink
    url="http://ipsec-tools.sourceforge.net">http://ipsec-tools.sourceforge.net</ulink>).
    As of this writing, I recommend that you run at least version 0.5.2.
    Debian users, please note that there are separate Debian packages for
    ipsec-tools and racoon although the ipsec-tools project releases them as a
    single package.</para>

    <para>For more information on IPSEC, Kernel 2.6 and Shorewall see <ulink
    url="LinuxFest.pdf">my presentation on the subject given at LinuxFest NW
    2005</ulink>. Be warned though that the presentation is based on Shorewall
    2.2 and there are some differences in the details of how IPSEC is
    configured.</para>
  </section>

  <section id="GwFw">
    <title>IPSec Gateway on the Firewall System</title>

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

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

    <para>We want systems in the 192.168.1.0/24 sub-network to be able to
    communicate with systems in the 10.0.0.0/8 network. We assume that on both
    systems A and B, eth0 is the internet interface.</para>

    <para>To make this work, we need to do two things:</para>

    <orderedlist numeration="loweralpha">
      <listitem>
        <para>Open the firewall so that the IPSEC tunnel can be established
        (allow the ESP and AH protocols and UDP Port 500).</para>
      </listitem>

      <listitem>
        <para>Allow traffic through the tunnel.</para>
      </listitem>
    </orderedlist>

    <para>Opening the firewall for the IPSEC tunnel is accomplished by adding
    an entry to the <filename>/etc/shorewall/tunnels</filename> file.</para>

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

    <blockquote>
      <para><filename><filename>/etc/shorewall/tunnels</filename></filename> —
      System A:</para>

      <programlisting>#TYPE         ZONE        GATEWAY             GATEWAY ZONE
ipsec         net         134.28.54.2
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>

      <para><filename><filename>/etc/shorewall/tunnels</filename></filename> —
      System B:</para>

      <programlisting>#TYPE         ZONE        GATEWAY             GATEWAY ZONE
ipsec         net         206.162.148.9
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>
    </blockquote>

    <note>
      <para>If either of the endpoints is behind a NAT gateway then the
      tunnels file entry on the <emphasis role="bold">other</emphasis>
      endpoint should specify a tunnel type of ipsecnat rather than ipsec and
      the GATEWAY address should specify the external address of the NAT
      gateway.</para>
    </note>

    <para>You need to define a zone for the remote subnet or include it in
    your local zone. In this example, we'll assume that you have created a
    zone called <quote>vpn</quote> to represent the remote subnet.</para>

    <blockquote>
      <para><filename><filename>/etc/shorewall/zones</filename></filename> —
      Systems A and B:</para>

      <programlisting>#ZONE          TYPE             OPTIONS             IN           OUT
#                                                   OPTIONS      OPTIONS
vpn            ipv4
net            ipv4
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>
    </blockquote>

    <para>Remember the assumption that both systems A and B have eth0 as their
    internet interface.</para>

    <para>You must define the vpn zone using the
    <filename>/etc/shorewall/hosts</filename> file. The hosts file entries
    below assume that you want the remote gateway to be part of the vpn zone —
    If you don't wish the remote gateway included, simply omit its IP address
    from the HOSTS column.</para>

    <blockquote>
      <para><filename>/etc/shorewall/hosts</filename> — System A</para>

      <programlisting>#ZONE             HOSTS                                OPTIONS
vpn               eth0:10.0.0.0/8,134.28.54.2        <emphasis role="bold">  ipsec</emphasis>
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>

      <para><filename>/etc/shorewall/hosts</filename> — System B</para>

      <programlisting>#ZONE             HOSTS                                OPTIONS
vpn               eth0:192.168.1.0/24,206.162.148.9    <emphasis role="bold">ipsec</emphasis>
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>
    </blockquote>

    <para>Assuming that you want to give each local network free access to the
    remote network and vice versa, you would need the following
    <filename>/etc/shorewall/policy</filename> entries on each system:</para>

    <blockquote>
      <programlisting>#SOURCE          DESTINATION            POLICY          LEVEL       BURST:LIMIT
loc              vpn                    ACCEPT
vpn              loc                    ACCEPT</programlisting>
    </blockquote>

    <para>If you need access from each firewall to hosts in the other network,
    then you could add:</para>

    <blockquote>
      <programlisting>#SOURCE          DESTINATION            POLICY          LEVEL       BURST:LIMIT
$FW              vpn                    ACCEPT</programlisting>
    </blockquote>

    <para>If you need access between the firewall's, you should describe the
    access in your /etc/shorewall/rules file. For example, to allow SSH access
    from System B, add this rule on system A:</para>

    <blockquote>
      <programlisting>#ACTION    SOURCE           DESTINATION      PROTO        POLICY
ACCEPT     vpn:134.28.54.2  $FW</programlisting>
    </blockquote>

    <para>Note that your Security Policies must also be set up to send traffic
    between 134.28.54.2 and 206.162.148.9 through the tunnel (see
    below).</para>

    <para>Once you have these entries in place, restart Shorewall (type
    shorewall restart); you are now ready to configure IPSEC.</para>

    <para>For full encrypted connectivity in this configuration (between the
    subnets, between each subnet and the opposite gateway, and between the
    gateways), you will need eight policies in
    <filename>/etc/racoon/setkey.conf</filename>. For example, on gateway
    A:</para>

    <blockquote>
      <programlisting># First of all flush the SPD and SAD databases
spdflush;
flush;

# Add some SPD rules

spdadd 192.168.1.0/24   10.0.0.0/8       any -P out ipsec esp/tunnel/206.162.148.9-134.28.54.2/require;
spdadd 192.168.1.0/24   134.28.54.2/32   any -P out ipsec esp/tunnel/206.162.148.9-134.28.54.2/require;
spdadd 206.162.148.9/32 134.28.54.2/32   any -P out ipsec esp/tunnel/206.162.148.9-134.28.54.2/require;
spdadd 206.162.148.9/32 10.0.0.0/8       any -P out ipsec esp/tunnel/206.162.148.9-134.28.54.2/require;
spdadd 10.0.0.0/8       192.168.1.0/24   any -P in  ipsec esp/tunnel/134.28.54.2-206.162.148.9/require;
spdadd 10.0.0.0/8       206.162.148.9/32 any -P in  ipsec esp/tunnel/134.28.54.2-206.162.148.9/require;
spdadd 134.28.54.2/32   192.168.1.0/24   any -P in  ipsec esp/tunnel/134.28.54.2-206.162.148.9/require;
spdadd 134.28.54.2/32   206.162.148.9/32 any -P in  ipsec esp/tunnel/134.28.54.2-206.162.148.9/require;</programlisting>
    </blockquote>

    <para>The <filename>setkey.conf</filename> file on gateway B would be
    similar.</para>

    <para>A sample <filename>/etc/racoon/racoon.conf</filename> file using
    X.509 certificates might look like:</para>

    <blockquote>
      <programlisting>path certificates "/etc/certs" ;

listen 
{
        isakmp 206.162.148.9;
}

remote 134.28.54.2
{
        exchange_mode main ;
        certificate_type x509 "GatewayA.pem" "GatewayA_key.pem" ;
        verify_cert on;
        my_identifier asn1dn ;
        peers_identifier asn1dn ;
        verify_identifier on ;
        lifetime time 24 hour ;
        proposal {
                encryption_algorithm blowfish;
                hash_algorithm sha1;
                authentication_method rsasig ;
                dh_group 2 ;
        }
}

sainfo address 192.168.1.0/24 any address 10.0.0.0/8 any
{
        pfs_group 2;
        lifetime time 12 hour ;
        encryption_algorithm blowfish ;
        authentication_algorithm hmac_sha1, hmac_md5 ;
        compression_algorithm deflate ;
}

sainfo address 206.162.148.9/32 any address 10.0.0.0/8 any
{
        pfs_group 2;
        lifetime time 12 hour ;
        encryption_algorithm blowfish ;
        authentication_algorithm hmac_sha1, hmac_md5 ;
        compression_algorithm deflate ;
}

sainfo address 206.162.148.9/32 any address 134.28.54.2/32 any
{
        pfs_group 2;
        lifetime time 12 hour ;
        encryption_algorithm blowfish ;
        authentication_algorithm hmac_sha1, hmac_md5 ;
        compression_algorithm deflate ;
}

sainfo address 192.168.1.0/24 any address 134.28.54.2/32 any
{
        pfs_group 2;
        lifetime time 12 hour ;
        encryption_algorithm blowfish ;
        authentication_algorithm hmac_sha1, hmac_md5 ;
        compression_algorithm deflate ;
}</programlisting>

      <warning>
        <para>If you have hosts that access the internet through an IPSEC
        tunnel, then it is a good idea to set the MSS value for traffic from
        those hosts explicitly in the
        <filename>/etc/shorewall/zones</filename> file. For example, if hosts
        in the <emphasis role="bold">sec</emphasis> zone access the internet
        through an ESP tunnel then the following entry would be
        appropriate:</para>

        <programlisting>#ZONE   TYPE    OPTIONS                 IN                      OUT
#                                       OPTIONS                 OPTIONS
sec     ipsec   mode=tunnel             <emphasis role="bold">mss=1400</emphasis></programlisting>

        <para>You should also set FASTACCEPT=No in shorewall.conf to ensure
        that both the SYN and SYN,ACK packets have their MSS field
        adjusted.</para>

        <para>Note that CLAMPMSS=Yes in <filename>shorewall.conf</filename>
        isn't effective with the 2.6 native IPSEC implementation because there
        is no separate ipsec device with a lower mtu as there was under the
        2.4 and earlier kernels.</para>
      </warning>
    </blockquote>
  </section>

  <section id="RoadWarrior">
    <title>Mobile System (Road Warrior)</title>

    <para>Suppose that you have a laptop system (B) that you take with you
    when you travel and you want to be able to establish a secure connection
    back to your local network.</para>

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

    <example id="roadWarrior">
      <title>Road Warrior VPN</title>

      <para>You need to define a zone for the laptop or include it in your
      local zone. In this example, we'll assume that you have created a zone
      called <quote>vpn</quote> to represent the remote host.</para>

      <blockquote>
        <para><filename>/etc/shorewall/zones</filename> — System A</para>

        <programlisting>#ZONE          TYPE             OPTIONS             IN           OUT
#                                                   OPTIONS      OPTIONS
vpn            ipsec
net            ipv4
loc            ipv4
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>
      </blockquote>

      <para>In this instance, the mobile system (B) has IP address 134.28.54.2
      but that cannot be determined in advance. In the
      <filename>/etc/shorewall/tunnels</filename> file on system A, the
      following entry should be made:<blockquote>
          <programlisting>#TYPE         ZONE        GATEWAY             GATEWAY ZONE
ipsec         net         0.0.0.0/0           vpn
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>
        </blockquote></para>

      <para><note>
          <para>the GATEWAY ZONE column contains the name of the zone
          corresponding to peer subnetworks. This indicates that the gateway
          system itself comprises the peer subnetwork; in other words, the
          remote gateway is a standalone system.</para>
        </note></para>

      <para>The VPN zone is defined using the /etc/shorewall/hosts
      file:</para>

      <blockquote>
        <para><filename>/etc/shorewall/hosts</filename> — System A:</para>

        <programlisting>#ZONE             HOSTS                  OPTIONS
vpn               eth0:0.0.0.0/0
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>
      </blockquote>

      <para>You will need to configure your <quote>through the tunnel</quote>
      policy as shown under the first example above.</para>

      <para>On the laptop:</para>

      <blockquote>
        <para><filename>/etc/shorewall/zones</filename> - System B:</para>

        <programlisting>#ZONE          TYPE             OPTIONS             IN           OUT
#                                                   OPTIONS      OPTIONS
vpn            ipsec
net            ipv4
loc            ipv4
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>

        <para><filename>/etc/shorewall/tunnels</filename> - System B:</para>

        <programlisting>#TYPE         ZONE        GATEWAY             GATEWAY ZONE
ipsec         net         206.162.148.9       vpn
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>

        <para><filename>/etc/shorewall/hosts</filename> - System B:</para>

        <programlisting>#ZONE             HOSTS                  OPTIONS
vpn               eth0:0.0.0.0/0
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>
      </blockquote>

      <para>On system A, here are the IPSEC files:</para>

      <blockquote>
        <para><filename>/etc/racoon/racoon.conf</filename> - System A:</para>

        <programlisting>path certificate "/etc/certs" ;
 
listen
{
        isakmp 206.162.148.9;
}
 
remote <emphasis role="bold">anonymous</emphasis>
{
        exchange_mode main ;
        <emphasis role="bold">generate_policy on</emphasis> ;
        <emphasis role="bold">passive on</emphasis> ;
        certificate_type x509 "GatewayA.pem" "GatewayA_key.pem" ;
        verify_cert on;
        my_identifier asn1dn ;
        peers_identifier asn1dn ;
        verify_identifier on ;
        lifetime time 24 hour ;
        proposal {
                encryption_algorithm blowfish ;
                hash_algorithm sha1;
                authentication_method rsasig ;
                dh_group 2 ;
        }
}
 
sainfo <emphasis role="bold">anonymous</emphasis>
{
        pfs_group 2;
        lifetime time 12 hour ;
        encryption_algorithm blowfish ;
        authentication_algorithm hmac_sha1, hmac_md5 ;
        compression_algorithm deflate ;
}</programlisting>

        <para><filename>/etc/racoon/setkey.conf</filename> - System A:</para>

        <programlisting>flush;
spdflush;</programlisting>
      </blockquote>

      <para>If system A is running kernel 2.6.10 or later then it must also be
      running ipsec-tools (racoon) 0.5rc1 or later.</para>

      <para>On the mobile system (system B), it is not possible to create a
      static IPSEC configuration because the IP address of the laptop's
      internet connection isn't static. I have created an 'ipsecvpn' script
      and included in the tarball and in the RPM's documentation directory;
      this script can be used to start and stop the connection.</para>

      <para>The ipsecvpn script has some variable assignments at the top -- in
      the above case, these would be as follows:</para>

      <blockquote>
        <programlisting>#
# External Interface
#
INTERFACE=eth0
#
# Remote IPSEC Gateway
#
GATEWAY=206.162.148.9
#
# Networks behind the remote gateway
#
NETWORKS="192.168.1.0/24"
#
# Directory where X.509 certificates are stored.
#
CERTS=/etc/certs
#
# Certificate to be used for this connection. The cert
# directory must contain:
#
#     ${CERT}.pem     - the certificate
#     ${CERT}_key.pem - the certificates's key
#
CERT=roadwarrior
#
#     The setkey binary
#
SETKEY=/usr/sbin/setkey
#
#     The racoon binary
#
RACOON=/usr/sbin/racoon</programlisting>
      </blockquote>

      <para>The ipsecvpn script can be installed in /etc/init.d/ but it is
      probably best installed in /usr/local/sbin and run manually:</para>

      <blockquote>
        <para><command>ipsecvpn start </command># Starts the tunnel</para>

        <para><command>ipsecvpn stop</command> # Stops the tunnel</para>
      </blockquote>
    </example>

    <warning>
      <para>Although the ipsecvpn script allows you to specify multiple remote
      NETWORKS as a space-separated list, SAs are created on the gateway only
      during ISAKMP negotiation. So in practice, only the first remote network
      accessed will be accessible from the roadwarrior.</para>
    </warning>
  </section>

  <section id="RW-L2TP">
    <title>Mobile System (Road Warrior) with Layer 2 Tunneling Protocol
    (L2TP)</title>

    <para>This section is based on the previous section. Please make sure that
    you read it thoroughly and understand it. The setup described in this
    section is more complex because you are including an additional layer of
    tunneling. Again, make sure that you have read the previous section and it
    is highly recommended to have the IPSEC-only configuration working
    first.</para>

    <para>Additionally, this section assumes that you are running IPSEC,
    xl2tpd and pppd on the same system that is running shorewall. However,
    configuration of these additional services is beyond the scope of this
    document.</para>

    <para>Getting layer 2 tunneling to work is an endeavour unto itself.
    However, if you succeed it can be very convenient. Reasons why you might
    want configure layer 2 tunneling protocol (L2TP):</para>

    <orderedlist>
      <listitem>
        <para>You want to give your road warrior an address that is in the
        same segment as the other hosts on your network.</para>
      </listitem>

      <listitem>
        <para>Your road warriors are using a legacy operating system (such as
        MS Windows or Mac OS X) and you do not want them to have to install
        third party software in order to connect to the VPN (both MS Windows
        and Mac OS X include VPN clients which natively support L2TP over
        IPSEC, but not plain IPSEC).</para>
      </listitem>

      <listitem>
        <para>You like a challenge.</para>
      </listitem>
    </orderedlist>

    <para>Since the target for a VPN including L2TP will (almost) never be a
    road warrior running Linux, I will not include the client side of the
    configuration.</para>

    <para>The first thing that needs to be done is to create a new zone called
    <quote>l2tp</quote> to represent the tunneled layer 2 traffic.</para>

    <blockquote>
      <para><filename>/etc/shorewall/zones</filename> — System A</para>

      <programlisting>#ZONE          TYPE             OPTIONS             IN           OUT
#                                                   OPTIONS      OPTIONS
vpn            ipsec
l2tp           ipv4
net            ipv4
loc            ipv4
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>
    </blockquote>

    <para>Since the L2TP will require the use of pppd, you will end up with
    one or more ppp interfaces (each representing an individual road warrior
    connection) for which you will need to account. This can be done by
    modifying the inerfaces file. (Modify with additional options as
    needed.)</para>

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

      <programlisting>#ZONE   INTERFACE       BROADCAST       OPTIONS
net     eth0            detect          routefilter
loc     eth1            192.168.1.255
l2tp    ppp+            -
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE</programlisting>
    </blockquote>

    <para>The next thing that must be done is to adjust the policy so that the
    traffic can go where it needs to go.</para>

    <para>First, you need to decide if you want for hosts in your local zone
    to be able to connect to your road warriors. You may or may not want to
    allow this. For example, one reason you might want to allow this is so
    that your support personnel can use ssh, VNC or remote desktop to fix a
    problem on the road warrior's laptop.</para>

    <para>Second, you need to decide if you want the road warrior to have
    access to hosts on the local network. You generally want to allow this.
    For example, if you have DNS servers on your local network that you want
    the road warrior to use. Or perhaps the road warrior needs to mount NFS
    shares or needs to access intranet sites which are not visible from the
    public Internet.</para>

    <para>Finally, you need to decide if you want the road warriors to be able
    to access the public Internet. You probably want to do this, unless you
    are trying to create a situation where when the road warrior connects to
    the VPN, it is no longer possible to send traffic from the road warrior's
    machine to the public Internet. Please note that this not really a strong
    security measure. The road warrior could trivially modify the routing
    table on the remote machine to have only traffic destined for systems on
    the VPN local network go through the secure channel. The rest of the
    traffic would simply travel over an Ethernet or wireless interface
    directly to the public Internet. In fact, this latter situation is
    dangerous, as a simple mistake could easily create a situation where the
    road warrior's machine is acting as a router between your local network
    and the public Internet, which you certainly do not want to happen. In
    short, it is best to allow the road warrior to connect to the public
    Internet by default.</para>

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

      <programlisting>#SOURCE         DEST            POLICY          LOG LEVEL       LIMIT:BURST
$FW             all             ACCEPT
loc             net             ACCEPT
loc             l2tp            ACCEPT # Allows local machines to connect to road warriors
l2tp            loc             ACCEPT # Allows road warriors to connect to local machines
l2tp            net             ACCEPT # Allows road warriors to connect to the Internet
net             all             DROP            info
# The FOLLOWING POLICY MUST BE LAST
all             all             REJECT          info
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>
    </blockquote>

    <para>The final step is to modify your rules file. There are three
    important components. First, you must allow the l2tp traffic to reach the
    xl2tpd process running on the firewall machine. Second, you must add rules
    to open up ports on the firewall to the road warrior for services which
    are running on the firewall. For example, if you are running a webserver
    on the firewall that must be accessible to road warriors. The reason for
    the second step is that the policy does not by default allow unrestricted
    access to the firewall itself. Finally, you should protect an exploit
    where an attacker can exploit your LT2P server do to a hole in the way
    that L2TP interacts with UDP connection tracking.</para>

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

      <programlisting>#ACTION         SOURCE  DEST    PROTO   DEST    SOURCE
#                                       PORT(S) PORT(S)
SECTION ESTABLISHED
# Prevent IPSEC bypass by hosts behind a NAT gateway
L2TP/REJECT     net     $FW
REJECT          $FW     net     udp     -       1701
# l2tp over the IPsec VPN
ACCEPT          vpn     $FW     udp     1701
# webserver that can only be accessed internally
HTTP/ACCEPT     loc     $FW
HTTP/ACCEPT     l2tp    $FW
HTTPS/ACCEPT    loc     $FW
HTTPS/ACCEPT    l2tp    $FW
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>
    </blockquote>
  </section>

  <section id="Transport">
    <title>Transport Mode</title>

    <para>In today's wireless world, it is often the case that individual
    hosts in a network need to establish secure connections with the other
    hosts in that network. In that case, IPSEC transport mode is an
    appropriate solution.</para>

    <para><graphic fileref="images/TransportMode.png" />Here's an example
    using the ipsec-tools package. The files shown are from host
    192.168.20.10; the configuration of the other nodes is similar.</para>

    <blockquote>
      <para><filename>/etc/racoon/racoon.conf</filename>:</para>

      <programlisting>path pre_shared_key "/etc/racoon/psk.txt" ;

remote anonymous
{
        exchange_mode main ;
        my_identifier address ;
        lifetime time 24 hour ;
        proposal {
                encryption_algorithm blowfish ;
                hash_algorithm sha1;
                authentication_method pre_shared_key ;
                dh_group 2 ;
        }
}

sainfo anonymous
{
        pfs_group 2;
        lifetime time 12 hour ;
        encryption_algorithm blowfish ;
        authentication_algorithm hmac_sha1, hmac_md5 ;
        compression_algorithm deflate ;
}
</programlisting>

      <para><filename>/etc/racoon/setkey.conf</filename>:</para>

      <programlisting># First of all flush the SPD database
spdflush;

# Add some SPD rules

spdadd 192.168.20.10/32 192.168.20.20/32 any -P out ipsec esp/transport/192.168.20.10-192.168.20.20/require;
spdadd 192.168.20.20/32 192.168.20.10/32 any -P in  ipsec esp/transport/192.168.20.20-192.168.20.10/require;
spdadd 192.168.20.10/32 192.168.20.30/32 any -P out ipsec esp/transport/192.168.20.10-192.168.20.30/require;
spdadd 192.168.20.30/32 192.168.20.10/32 any -P in  ipsec esp/transport/192.168.20.30-192.168.20.10/require;
spdadd 192.168.20.10/32 192.168.20.40/32 any -P out ipsec esp/transport/192.168.20.10-192.168.20.40/require;
spdadd 192.168.20.40/32 192.168.20.10/32 any -P in  ipsec esp/transport/192.168.20.40-192.168.20.10/require;
</programlisting>

      <para><filename>/etc/racoon/psk.txt</filename>:</para>

      <programlisting>192.168.20.20             &lt;key for 192.168.20.10&lt;-&gt;192.168.20.20&gt;
192.168.20.30             &lt;key for 192.168.20.10&lt;-&gt;192.168.20.30&gt;
192.168.20.40             &lt;key for 192.168.20.10&lt;-&gt;192.168.20.40&gt;</programlisting>

      <para>Note that the <emphasis role="bold">same key</emphasis>must be
      used in both directions.</para>
    </blockquote>

    <para>Shorewall configuration goes as follows:</para>

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

      <programlisting>#ZONE   INTERFACE       BROADCAST       OPTIONS
net     eth0            detect          routefilter,dhcp,tcpflags
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE</programlisting>

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

      <programlisting>#TYPE          ZONE             GATEWAY         GATEWAY
#                                               ZONE
ipsec:noah     net              192.168.20.0/24 loc</programlisting>

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

      <programlisting>#ZONE          TYPE             OPTIONS             IN           OUT
#                                                   OPTIONS      OPTIONS
loc            ipsec            mode=transport
net            ipv4</programlisting>

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

      <programlisting>#ZONE           HOST(S)                         OPTIONS
loc             eth0:192.168.20.0/24  
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS LINE -- DO NOT REMOVE</programlisting>

      <para>It is worth noting that although <emphasis>loc</emphasis> is a
      sub-zone of <emphasis>net</emphasis>, because <emphasis>loc</emphasis>
      is an IPSEC-only zone it does not need to be defined before
      <emphasis>net</emphasis> in
      <emphasis>/etc/shorewall/zones</emphasis>.</para>

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

      <programlisting>#SOURCE         DEST            POLICY          LOG LEVEL       LIMIT:BURST
$FW             all             ACCEPT
loc             $FW             ACCEPT
net             loc             NONE
loc             net             NONE
net             all             DROP            info
# The FOLLOWING POLICY MUST BE LAST
all             all             REJECT          info
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>

      <para>Since there are no cases where net&lt;-&gt;loc traffic should
      occur, NONE policies are used.</para>
    </blockquote>
  </section>

  <section id="ipcomp">
    <title>IPCOMP</title>

    <para>If your IPSEC tunnel or transport mode connection fails to work with
    Shorewall started and you see log messages like the following when you try
    to use the connection, the problem is that ip compression is being
    used.<programlisting>Feb 18 23:43:52 vpngw kernel: Shorewall:<emphasis
          role="bold">vpn2fw</emphasis>:REJECT:IN=eth2 OUT= MAC=00:e0:81:32:b3:5e:00:18:de:12:e5:15:08:00
                              SRC=172.29.59.58 DST=172.29.59.254 LEN=85 TOS=0x00 PREC=0x00 TTL=64 ID=25600 DF <emphasis
          role="bold">PROTO=4</emphasis></programlisting>The solution is to
    add an IPCOMP tunnel to /etc/shorewall/tunnels as follows:<programlisting>#TYPE                   ZONE    GATEWAY         GATEWAY
#                                               ZONE
ipip                    <emphasis role="bold">vpn</emphasis>     0.0.0.0/0</programlisting>The
    above assumes that the name of your IPSEC vpn zone is
    <emphasis>vpn</emphasis>.</para>
  </section>

  <section id="XP">
    <title>IPSEC and <trademark>Windows</trademark> XP</title>

    <para>I have successfully configured my work laptop to use IPSEC with
    X.509 certificates for wireless IP communication when it is undocked at
    home. I looked at dozens of sites and the one I found most helpful was
    <ulink
    url="http://ipsec.math.ucla.edu/services/ipsec-windows.html">http://ipsec.math.ucla.edu/services/ipsec-windows.html</ulink>.
    The instructions on that site are directed to students at UCLA but they
    worked fine for me (once I followed them very carefully).</para>

    <warning>
      <para>The instructions found on the UCLA site are complex and do not
      include any information on the generation of X.509 certificates. There
      are lots of sites however that can tell you how to generate
      certificates, including <ulink
      url="http://www.ipsec-howto.org/">http://www.ipsec-howto.org/</ulink>.</para>

      <para>One piece of information that may not be so easy to find is "How
      do I generate a PKCS#12 certificate to import into Windows?". Here's the
      openssl command that I used:</para>

      <programlisting><command>openssl pkcs12 -export -in eastepnc6000.pem -inkey eastepnc6000_key.pem -out eastepnc6000.pfx -name "IPSEC Cert for Home Wireless"</command> </programlisting>

      <para>I was prompted for a password to associate with the certificate.
      This password is entered on the Windows system during import.</para>

      <para>In the above command:</para>

      <itemizedlist>
        <listitem>
          <para><filename>eastepnc6000.pem</filename> was the laptop's
          certificate in PEM format.</para>
        </listitem>

        <listitem>
          <para><filename>eastepnc6000_key.pem</filename> was the laptop's
          private key (actually, it's the original signing request which
          includes the private key).</para>
        </listitem>

        <listitem>
          <para><filename>eastepnc6000.pfx</filename> is the PKCS#12 output
          file.</para>
        </listitem>

        <listitem>
          <para>"IPSEC Cert for Home Wireless" is the friendly name for the
          certificate.</para>
        </listitem>
      </itemizedlist>

      <para>I started to write an article about how to do this, complete with
      graphics captured from my laptop. I gave up. I had captured 12 images
      and hadn't really started yet. The Windows interface for configuring
      IPSEC is the worst GUI that I have ever used. What can be displayed on
      one split Emacs screen (racoon.conf plus setkey.conf) takes 20+
      different dialog boxes on Windows XP!!!</para>
    </warning>
  </section>

  <section id="More">
    <title>Source of Additional Samples</title>

    <para>Be sure to check out the <filename
    class="directory">src/racoon/samples</filename> subdirectory in the
    ipsec-tools source tree. It has a wide variety of sample racoon
    configuration files.</para>
  </section>
</article>
