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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="Content-Type"
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content="text/html; charset=windows-1252">
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<title>Shorewall Proxy ARP</title>
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<meta name="GENERATOR" content="Microsoft FrontPage 5.0">
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<meta name="ProgId" content="FrontPage.Editor.Document">
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<meta name="Microsoft Theme" content="none">
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</head>
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<body>
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<h1 style="text-align: center;">Proxy ARP<br>
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</h1>
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<p>Proxy ARP allows you to insert a firewall in front of a set of
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servers without changing their IP addresses and without having to
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re-subnet. Before you try to use this technique, I strongly recommend
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that you read the <a href="shorewall_setup_guide.htm">Shorewall Setup
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Guide.</a></p>
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<p>The following figure represents a Proxy ARP environment.</p>
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<blockquote>
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<p align="center"><strong> <img src="images/proxyarp.png" width="519"
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height="397"> </strong></p>
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<blockquote> </blockquote>
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</blockquote>
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<p align="left">Proxy ARP can be used to make the systems with
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addresses 130.252.100.18 and 130.252.100.19 appear to be on the upper
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(130.252.100.*) subnet. Assuming that the upper firewall
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interface is eth0 and the lower interface is eth1, this is accomplished
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using the following entries in /etc/shorewall/proxyarp:</p>
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<blockquote>
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<table border="2" cellpadding="2" style="border-collapse: collapse;">
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<tbody>
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<tr>
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<td><b>ADDRESS</b></td>
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<td><b>INTERFACE</b></td>
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<td><b>EXTERNAL</b></td>
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<td><b>HAVEROUTE</b></td>
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</tr>
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<tr>
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<td>130.252.100.18</td>
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<td>eth1</td>
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<td>eth0</td>
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<td>no</td>
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</tr>
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<tr>
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<td>130.252.100.19</td>
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<td>eth1</td>
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<td>eth0</td>
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<td>no</td>
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</tr>
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</tbody>
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</table>
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</blockquote>
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<p>Be sure that the internal systems (130.242.100.18 and
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130.252.100.19 in the above example) are not included in any
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specification in /etc/shorewall/masq or /etc/shorewall/nat.</p>
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<p>Note that I've used an RFC1918 IP address for eth1 - that IP address
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is irrelevant. </p>
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<p>The lower systems (130.252.100.18 and 130.252.100.19) should have
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their subnet mask and default gateway configured exactly the same way
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that the Firewall system's eth0 is configured. In other words, they
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should be configured just like they would be if they were parallel to
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the firewall rather than behind it.<br>
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</p>
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<p><font color="#ff0000"><b>NOTE: Do not add the Proxy ARP'ed
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address(es) (130.252.100.18 and 130.252.100.19 in the above
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example) to the external interface (eth0 in this example) of the
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firewall.</b></font><br>
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</p>
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<div align="left"> </div>
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<div align="left">
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<p align="left">A word of warning is in order here. ISPs typically
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configure their routers with a long ARP cache timeout. If you move a
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system from parallel to your firewall to behind your firewall with
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Proxy ARP, it
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will probably be HOURS before that system can communicate with the
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internet. There are a couple of things that you can try:<br>
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</p>
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<ol>
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<li>(Courtesy of Bradey Honsinger) A reading of Stevens' <i>TCP/IP
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Illustrated, Vol 1</i> reveals that a <br>
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<br>
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"gratuitous" ARP packet should cause the ISP's router to refresh their
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ARP cache (section 4.7). A gratuitous ARP is simply a host requesting
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the MAC address for its own IP; in addition to ensuring that the IP
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address
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isn't a duplicate...<br>
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<br>
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"if the host sending the gratuitous ARP has just changed its hardware
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address..., this packet causes any other host...that has an entry in
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its cache for the old hardware address to update its ARP cache entry
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accordingly."<br>
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<br>
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Which is, of course, exactly what you want to do when you switch a host
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from being exposed to the Internet to behind Shorewall using proxy ARP
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(or one-to-one NAT for that matter). Happily enough, recent versions of
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Redhat's iputils package include "arping", whose "-U" flag does just
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that:<br>
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<br>
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<font color="#009900"><b>arping -U -I <i><net
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if> <newly proxied IP></i></b></font><br>
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<font color="#009900"><b>arping -U -I eth0
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66.58.99.83 # for example</b></font><br>
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<br>
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Stevens goes on to mention that not all systems respond correctly to
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gratuitous ARPs, but googling for "arping -U" seems to support the idea
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that it works most of the time.<br>
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<br>
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To use arping with Proxy ARP in the above example, you would have to:<br>
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<br>
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<font color="#009900"><b> shorewall clear<br>
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</b></font> <font color="#009900"><b>ip addr add
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130.252.100.18 dev eth0<br>
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ip addr add 130.252.100.19 dev eth0</b></font><br>
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<font color="#009900"><b>arping -U -I eth0
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130.252.100.18</b></font><br>
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<font color="#009900"><b>arping -U -I eth0 130.252.100.19</b></font><br>
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<b><font color="#009900">ip addr del 130.252.100.18 dev
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eth0<br>
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ip addr del 130.252.100.19 dev eth0<br>
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shorewall start</font></b><br>
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<br>
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</li>
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<li>You can call your ISP and ask them to purge the stale ARP cache
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entry but many either can't or won't purge individual entries.</li>
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</ol>
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You can determine if your ISP's gateway ARP cache is stale using ping
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and tcpdump. Suppose that we suspect that the gateway router has a
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stale ARP cache entry for 130.252.100.19. On the firewall, run tcpdump
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as follows:</div>
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<div align="left">
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<pre> <font color="#009900"><b>tcpdump -nei eth0 icmp</b></font></pre>
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</div>
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<div align="left">
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<p align="left">Now from 130.252.100.19, ping the ISP's gateway (which
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we will assume is 130.252.100.254):</p>
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</div>
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<div align="left">
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<pre> <b><font color="#009900">ping 130.252.100.254</font></b></pre>
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</div>
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<div align="left">
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<p align="left">We can now observe the tcpdump output:</p>
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</div>
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<div align="left">
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<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>
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</div>
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<div align="left">
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<p align="left">Notice that the source MAC address in the echo request
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is different from the destination MAC address in the echo reply!! In
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this case 0:4:e2:20:20:33 was the MAC of the firewall's eth0 NIC while
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0:c0:a8:50:b2:57 was the MAC address of the system on the lower left.
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In other words,
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the gateway's ARP cache still associates 130.252.100.19 with the NIC
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in that system rather than with the firewall's eth0.</p>
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</div>
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<p><font size="2">Last updated 11/13/2003 - </font><font size="2"> <a
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href="support.htm">Tom Eastep</a></font> </p>
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<a href="copyright.htm"><font size="2">Copyright</font> © <font
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size="2">2001, 2002, 2003 Thomas M. Eastep.</font></a><br>
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<br>
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<br>
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<br>
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<br>
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</body>
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</html>
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