shorewall_code/Shorewall-docs2/IPSEC-2.6.xml

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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>
    </authorgroup>

    <pubdate>2004-08-21</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>

  <warning>
    <para>To use this support, your kernel and iptables must include the
    Netfilter+ipsec patches and policy match support and you must be running
    Shorewall 2.1.5 or later.</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.</para>
  </warning>

  <section>
    <title>Shorewall 2.1 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 Security Policies which are stored in the
    kernel as a Security Policy Database (SPD). Outgoing traffic is encrypted
    according to the contents of the SPD and 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.</para>

    <para>Shorewall provides support for policy matching in two 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 an entry in the new IPSEC column
        in that file.</para>
      </listitem>

      <listitem>
        <para>A <filename>new </filename><ulink
        url="Documentation.htm#Ipsec"><filename>/etc/shorewall/ipsec</filename></ulink>
        file allows you to associate zones with traffic that will be encrypted
        or that has been decrypted.</para>
      </listitem>
    </orderedlist>

    <para>In summary, Shorewall 2.1.5 and later versions provide 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">Yes</emphasis> is placed in the
        IPSEC column of the <filename>/etc/shorewall/ipsec</filename> entry
        for the zone.</para>
      </listitem>

      <listitem>
        <para>Encrypted communication is used to/from only part of the hosts
        in a zone.</para>

        <para>The value <emphasis role="bold">No</emphasis> is placed in the
        IPSEC column of the <filename>/etc/shorewall/ipsec</filename> entry
        for the zone and the new <emphasis role="bold">ipsec</emphasis> option
        is specified in <filename>/etc/shorewall/hosts</filename> for those
        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>

    <para>Finally, the OPTIONS, IN OPTIONS and OUT OPTIONS columns in
    /etc/shorewall/ipsec 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>
  </section>

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

      <programlisting>#TYPE         ZONE        GATEWAY             GATEWAY ZONE
ipsec         net         206.161.148.9
#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>/etc/shorewall/zones</filename> — Systems A and
      B:</para>

      <programlisting>#ZONE           DISPLAY         COMMENTS
net             Internet        The big bad internet
vpn             VPN             Virtual Private Network
#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.</para>

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

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

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

      <programlisting>#ZONE             HOSTS                  OPTIONS
vpn               eth0:192.168.1.0/24    <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>Once you have these entries in place, restart Shorewall (type
    shorewall restart); you are now ready to configure IPSEC.</para>
  </section>

  <section>
    <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>
      <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>/etc/shorewall/zones — System A</para>

        <programlisting>#ZONE           DISPLAY         COMMENTS
net             Internet        The big bad internet
vpn             VPN             Road Warriors
loc             local           Local Network (192.168.1.0/24)
#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>/etc/shorewall/hosts — System A:</para>

        <programlisting>#ZONE             HOSTS                  OPTIONS
vpn               eth0:0.0.0.0/0        <emphasis role="bold"> ipsec</emphasis>
#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>
    </example>
  </section>

  <section>
    <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 3des;
                hash_algorithm sha1;
                authentication_method pre_shared_key ;
                dh_group 2 ;
        }
}

sainfo anonymous
{
        pfs_group 2;
        lifetime time 12 hour ;
        encryption_algorithm 3des, blowfish, des, rijndael ;
        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>/etc/racoon/psk.txt:</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>
    </blockquote>

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

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

      <programlisting>#ZONE   DISPLAY         COMMENTS
net     Net             Internet
loc     Local           Local Network
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE</programlisting>

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

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

      <para>/etc/shorewall/ipsec:</para>

      <programlisting>#ZONE          IPSEC            OPTIONS             IN           OUT
#              ONLY                                 OPTIONS      OPTIONS
loc            Yes              mode=transport</programlisting>

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