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<h1 align="center"><font color="#ffffff">Proxy ARP</font></h1>
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<p>Proxy ARP allows you to insert a firewall in front of a set of servers
without changing their IP addresses and without having to re-subnet.
Before you try to use this technique, I strongly recommend that you read the
<a href="shorewall_setup_guide.htm">Shorewall Setup Guide.</a></p>
<p>The following figure represents a Proxy ARP environment.</p>
<blockquote>
<p align="center"><strong> <img src="images/proxyarp.png"
width="519" height="397">
</strong></p>
<blockquote> </blockquote>
</blockquote>
<p align="left">Proxy ARP can be used to make the systems with addresses
130.252.100.18 and 130.252.100.19 appear to be on the upper (130.252.100.*)
subnet.� Assuming that the upper firewall interface is eth0 and the
lower interface is eth1, this is accomplished using the following entries
in /etc/shorewall/proxyarp:</p>
<blockquote>
<table border="2" cellpadding="2" style="border-collapse: collapse;">
<tbody>
<tr>
<td><b>ADDRESS</b></td>
<td><b>INTERFACE</b></td>
<td><b>EXTERNAL</b></td>
<td><b>HAVEROUTE</b></td>
</tr>
<tr>
<td>130.252.100.18</td>
<td>eth1</td>
<td>eth0</td>
<td>no</td>
</tr>
<tr>
<td>130.252.100.19</td>
<td>eth1</td>
<td>eth0</td>
<td>no</td>
</tr>
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</table>
</blockquote>
<p>Be sure that the internal systems (130.242.100.18 and 130.252.100.19�
in the above example) are not included in any specification in /etc/shorewall/masq
or /etc/shorewall/nat.</p>
<p>Note that I've used an RFC1918 IP address for eth1 - that IP address is
irrelevant. </p>
<p>The lower systems (130.252.100.18 and 130.252.100.19) should have their
subnet mask and default gateway configured exactly the same way that
the Firewall system's eth0 is configured.</p>
<div align="left">
<p align="left">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 Proxy ARP, it will
probably be HOURS before that system can communicate with the internet.
There are a couple of things that you can try:<br>
</p>
<ol>
<li>(Courtesy of Bradey Honsinger) A reading of Stevens' <i>TCP/IP Illustrated,
Vol 1</i> reveals that a <br>
<br>
"gratuitous" 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...<br>
<br>
"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."<br>
<br>
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 proxy ARP (or
static NAT for that matter). Happily enough, recent versions of Redhat's
iputils package include "arping", whose "-U" flag does just that:<br>
<br>
��� <font color="#009900"><b>arping -U -I <i><net if> <newly proxied
IP></i></b></font><br>
��� <font color="#009900"><b>arping -U -I eth0 66.58.99.83 # for example</b></font><br>
<br>
Stevens goes on to mention that not all systems respond correctly to gratuitous
ARPs, but googling for "arping -U" seems to support the idea that it works
most of the time.<br>
<br>
To use arping with Proxy ARP in the above example, you would have to:<br>
<br>
<font color="#009900"><b>� � shorewall clear<br>
</b></font>� � <font color="#009900"><b>ip addr add 130.252.100.18 dev
eth0<br>
� � ip addr add 130.252.100.19 dev eth0</b></font><br>
��� <font color="#009900"><b>arping -U -I eth0 130.252.100.18</b></font><br>
� � <font color="#009900"><b>arping -U -I eth0 130.252.100.19</b></font><br>
� � <b><font color="#009900">ip addr del 130.252.100.18 dev eth0<br>
� � ip addr del 130.252.100.19 dev eth0<br>
� � shorewall start</font></b><br>
<br>
</li>
<li>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.</li>
</ol>
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:</div>
<div align="left">
<pre> <font color="#009900"><b>tcpdump -nei eth0 icmp</b></font></pre>
</div>
<div align="left">
<p align="left">Now from 130.252.100.19, ping the ISP's gateway (which we
will assume is 130.252.100.254):</p>
</div>
<div align="left">
<pre> <b><font color="#009900">ping 130.252.100.254</font></b></pre>
</div>
<div align="left">
<p align="left">We can now observe the tcpdump output:</p>
</div>
<div align="left">
<pre> 13:35:12.159321 <u>0:4:e2:20:20:33</u> 0:0:77:95:dd:19 ip 98: 130.252.100.19 > 130.252.100.254: icmp: echo request (DF)<br> 13:35:12.207615 0:0:77:95:dd:19 <u>0:c0:a8:50:b2:57</u> ip 98: 130.252.100.254 > 130.252.100.177 : icmp: echo reply</pre>
</div>
<div align="left">
<p align="left">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 left. 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.</p>
</div>
<p><font size="2">Last updated 1/26/2003 - </font><font size="2"> <a
href="support.htm">Tom Eastep</a></font> </p>
<a href="copyright.htm"><font size="2">Copyright</font> � <font
size="2">2001, 2002, 2003 Thomas M. Eastep.</font></a><br>
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