shorewall_code/Shorewall-docs2/NAT.xml

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

  <articleinfo>
    <title>One-to-one NAT</title>

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

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

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

    <copyright>
      <year>2001-2004</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>One-to-one NAT</title>

    <important>
      <para><emphasis role="bold">If all you want to do is forward ports to
      servers behind your firewall, you do NOT want to use one-to-one NAT.
      Port forwarding can be accomplished with simple entries in the <ulink
      url="Documentation.htm#Rules">rules file</ulink>.</emphasis></para>
    </important>

    <para>One-to-one NAT is a way to make systems behind a firewall and
    configured with private IP addresses (those reserved for private use in
    RFC 1918) appear to have public IP addresses. Before you try to use this
    technique, I strongly recommend that you read the <ulink
    url="shorewall_setup_guide.htm">Shorewall Setup Guide</ulink>.</para>

    <para>The following figure represents a one-to-one NAT environment.</para>

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

    <para>One-to-one NAT can be used to make the systems with the 10.1.1.*
    addresses appear to be on the upper (130.252.100.*) subnet. If we assume
    that the interface to the upper subnet is eth0, then the following
    <filename>/etc/shorewall/nat</filename> file would make the lower
    left-hand system appear to have IP address 130.252.100.18 and the
    right-hand one to have IP address 130.252.100.19. It should be stressed
    that these entries in the <filename>/etc/shorewall/nat</filename> file do
    not automatically enable traffic between the external network and the
    internal host(s) — such traffic is still subject to your policies and
    rules.</para>

    <para><filename>/etc/shorewall/nat</filename><programlisting>#EXTERNAL       INTERFACE         INTERNAL      ALL INTERFACES     LOCAL
130.252.100.18  eth0              10.1.1.2      no                 no
130.252.100.19  eth0              10.1.1.3      no                 no</programlisting></para>

    <para>Be sure that the internal system(s) (10.1.1.2 and 10.1.1.3 in the
    above example) is (are) not included in any specification in
    <filename>/etc/shorewall/masq</filename> or
    <filename>/etc/shorewall/proxyarp</filename>.</para>

    <note>
      <para>The <quote>ALL INTERFACES</quote> column is used to specify
      whether access to the external IP from all firewall interfaces should
      undergo NAT (Yes or yes) or if only access from the interface in the
      INTERFACE column should undergo NAT. If you leave this column empty,
      <quote>No</quote> is assumed (Shorewall 2.0.0 and later -- prior to
      this, <quote>Yes</quote> was assumed). <emphasis role="bold">Specifying
      <quote>Yes</quote> in this column will not by itself allow systems on
      the lower LAN to access each other using their public IP
      addresses.</emphasis> For example, the lower left-hand system (10.1.1.2)
      cannot connect to 130.252.100.19 and expect to be connected to the lower
      right-hand system. <ulink url="FAQ.htm#faq2a">See FAQ 2a</ulink>.</para>
    </note>

    <note>
      <para>Shorewall will automatically add the external address to the
      specified interface unless you specify <ulink
      url="Documentation.htm#Aliases">ADD_IP_ALIASES</ulink>=<quote>no</quote>
      (or <quote>No</quote>) in
      <filename>/etc/shorewall/shorewall.conf</filename>; If you do not set
      ADD_IP_ALIASES or if you set it to <quote>Yes</quote> or
      <quote>yes</quote> then you must NOT configure your own
      alias(es).</para>

      <para><important>
          <para>Shorewall versions earlier than 1.4.6 can only add external
          addresses to an interface that is configured with a single
          subnetwork -- if your external interface has addresses in more than
          one subnetwork, Shorewall 1.4.5 and earlier can only add addresses
          to the first one.</para>
        </important></para>
    </note>

    <note>
      <para>The contents of the <quote>LOCAL</quote> column determine whether
      packets originating on the firewall itself and destined for the EXTERNAL
      address are redirected to the internal ADDRESS. If this column contains
      <quote>yes</quote> or <quote>Yes</quote> (and the ALL INTERFACES COLUMN
      also contains <quote>Yes</quote> or <quote>yes</quote>) then such
      packets are redirected; otherwise, such packets are not redirected. This
      feature requires kernel 2.4.19 or later and iptables 1.2.6a or later and
      you must have enabled CONFIG_IP_NF_NAT_LOCAL in your kernel.</para>
    </note>
  </section>

  <section>
    <title>ARP cache</title>

    <para>A word of warning is in order here. ISPs typically configure their
    routers with a long ARP cache timeout. If you move a system from parallel
    to your firewall to behind your firewall with one-to-one NAT, it will
    probably be HOURS before that system can communicate with the internet.
    There are a couple of things that you can try:</para>

    <orderedlist>
      <listitem>
        <para>A reading of <citetitle>TCP/IP Illustrated, Vol 1</citetitle> by
        Stevens reveals<footnote>
            <para>Courtesy of Bradey Honsinger</para>
          </footnote> that a <quote>gratuitous</quote> ARP packet should cause
        the ISP's router to refresh their ARP cache (section 4.7). A
        gratuitous ARP is simply a host requesting the MAC address for its own
        IP; in addition to ensuring that the IP address isn't a
        duplicate...</para>

        <blockquote>
          <para>if the host sending the gratuitous ARP has just changed its
          hardware address..., this packet causes any other host...that has an
          entry in its cache for the old hardware address to update its ARP
          cache entry accordingly.</para>
        </blockquote>

        <para>Which is, of course, exactly what you want to do when you switch
        a host from being exposed to the Internet to behind Shorewall using
        one-to-one NAT (or Proxy ARP for that matter). Happily enough, recent
        versions of Redhat's iputils package include <quote>arping</quote>,
        whose <quote>-U</quote> flag does just that:</para>

        <programlisting>arping -U -I &lt;<emphasis>net if</emphasis>&gt; &lt;<emphasis>newly proxied IP</emphasis>&gt;
arping -U -I eth0 66.58.99.83             # for example</programlisting>

        <para>Stevens goes on to mention that not all systems respond
        correctly to gratuitous ARPs, but googling for <quote>arping
        -U</quote> seems to support the idea that it works most of the
        time.</para>

        <para>To use arping with one-to-one NAT in the above example, you
        would have to:</para>

        <programlisting>shorewall clear
ip addr add 130.252.100.18 dev eth0     # You need to add the addresses only if Shorewall clear
ip addr add 130.252.100.19 dev eth0     # deletes them
arping -U -c 10 -I eth0 130.252.100.18
arping -U -c 10 -I eth0 130.252.100.19
ip addr del 130.252.100.18 dev eth0     # You need to delete the addresses only if you added
ip addr del 130.252.100.19 dev eth0     # them above
shorewall start</programlisting>
      </listitem>

      <listitem>
        <para>You can call your ISP and ask them to purge the stale ARP cache
        entry but many either can't or won't purge individual entries.</para>
      </listitem>
    </orderedlist>

    <warning>
      <para>There are two distinct versions of <command>arping</command>
      available:</para>

      <orderedlist>
        <listitem>
          <para><command>arping</command> by Thomas Habets (Debian package
          <emphasis>arping</emphasis>).</para>
        </listitem>

        <listitem>
          <para><command>arping</command> as part of the iputils package by
          Alexey Kuznetsov (Debian package
          <emphasis>iputils-arping</emphasis>).</para>
        </listitem>
      </orderedlist>

      <para>You want the second one by Alexey Kuznetsov.</para>
    </warning>

    <para>You can determine if your ISP's gateway ARP cache is stale using
    ping and tcpdump. Suppose that we suspect that the gateway router has a
    stale ARP cache entry for 130.252.100.19. On the firewall, run tcpdump as
    follows:</para>

    <programlisting>tcpdump -nei eth0 icmp</programlisting>

    <para>Now from 10.1.1.3, ping the ISP's gateway (which we will assume is
    130.252.100.254):</para>

    <programlisting>ping 130.252.100.254</programlisting>

    <para>We can now observe the tcpdump output:</para>

    <programlisting>13:35:12.159321 0:4:e2:20:20:33 0:0:77:95:dd:19 ip 98: 130.252.100.19 &gt; 130.252.100.254: icmp: echo request (DF)
13:35:12.207615 0:0:77:95:dd:19 0:c0:a8:50:b2:57 ip 98: 130.252.100.254 &gt; 130.252.100.177 : icmp: echo reply</programlisting>

    <para>Notice that the source MAC address in the echo request is different
    from the destination MAC address in the echo reply!! In this case
    0:4:e2:20:20:33 was the MAC of the firewall's eth0 NIC while
    0:c0:a8:50:b2:57 was the MAC address of the system on the lower right. In
    other words, the gateway's ARP cache still associates 130.252.100.19 with
    the NIC in that system rather than with the firewall's eth0.</para>
  </section>
</article>