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shorewall_code/docs/NAT.xml
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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
<article id="NAT">
<!--$Id$-->
<articleinfo>
<title>One-to-one NAT</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate><?dbtimestamp format="Y/m/d"?></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>
<caution>
<para><emphasis role="bold">This article applies to Shorewall 4.3 and
later. If you are running a version of Shorewall earlier than Shorewall
4.3.5 then please see the documentation for that
release.</emphasis></para>
</caution>
<section id="One-to-one">
<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="manpages/shorewall-rules.html">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 . <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="manpages/shorewall.conf.html">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></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 that you enabled CONFIG_IP_NF_NAT_LOCAL in your
kernel.</para>
</note>
<para>Entries in <filename>/etc/shorewall/nat</filename> only arrange for
address translation; they do not allow traffic to pass through the
firewall in violation of your policies. In the above example, suppose that
you wish to run a web server on 10.1.1.2 (a.k.a. 130.252.100.18). You
would need the following entry in
<filename>/etc/shorewall/rules</filename>:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST SOURCE ORIG
# PORT(S) PORT(S) DEST
ACCEPT net loc:10.1.1.2 tcp 80 - 130.252.100.18</programlisting>
</section>
<section id="ARP">
<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.</para>
<para>If you sniff traffic on the firewall's external interface, you can
see incoming traffic for the internal system(s) but the traffic is never
sent out the internal interface.</para>
<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>
<para>If you have this problem, 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>
</section>
</article>