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95 lines
4.1 KiB
HTML
95 lines
4.1 KiB
HTML
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<html>
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<head>
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<meta http-equiv="Content-Type" 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="boldstri 011, default">
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</head>
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<body>
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<blockquote>
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<h1 align="center">Proxy ARP</h1>
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<p> </p>
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<p>Proxy ARP allows you to insert a firewall in front of a set of servers
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without changing their IP addresses and without having to re-subnet.</p>
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<p>The following figure represents a Proxy ARP
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environment.</p>
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<p align="center"><strong>
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<img src="images/proxyarp.png" width="444" height="397"></strong></p>
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<blockquote>
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</blockquote>
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<p align="left">Proxy ARP can be used to make the systems with addresses
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130.252.100.18 and 130.252.100.19 appear to be on the upper (130.252.100.*)
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subnet. Assuming that the upper firewall interface is eth0 and the
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lower interface is eth1, this is accomplished using the following entries in
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/etc/shorewall/proxyarp:</p>
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<table border="2" cellpadding="2" style="border-collapse: collapse">
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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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</table>
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<p>Be sure that the internal systems (130.242.100.18 and 130.252.100.19
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in the above example) are not included in any specification in
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/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 is
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irrelevant. </p>
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<p>The lower systems (130.252.100.18 and 130.252.100.19) should have their
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subnet mask and default gateway configured exactly the same way that the
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Firewall system's eth0 is configured.</p>
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<div align="left">
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<p align="left">A word of warning is in order here. ISPs typically configure
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there routers with a long ARP cache timeout. If you move a system from
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parallel to your firewall to behind your firewall with Proxy ARP, it will
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probably be HOURS before that system can communicate with the internet. You
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can call your ISP and ask them to purge the stale ARP cache entry but many
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either can't or won't purge individual entries. You can determine if your
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ISP's gateway ARP cache is stale using ping and tcpdump. Suppose that we
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suspect that the gateway router has a stale ARP cache entry for 130.252.100.19.
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On the firewall, run tcpdump as follows:</div>
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<div align="left">
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<pre> tcpdump -nei eth0 icmp</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 we will
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assume is 130.252.100.254):</div>
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<div align="left">
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<pre> ping 130.252.100.254</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:</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)
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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 is
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different from the destination MAC address in the echo reply!! In this case
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0:4:e2:20:20:33 was the MAC of the firewall's eth0 NIC while 0:c0:a8:50:b2:57
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was the MAC address of the system on the lower left. In other words, the gateway's ARP cache still
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associates 130.252.100.19 with the NIC in that system rather than with the firewall's
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eth0.</div>
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</blockquote>
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<p><font size="2">Last updated 8/11/2002 - </font><font size="2">
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<a href="support.htm">Tom
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Eastep</a></font> </p>
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<font face="Trebuchet MS"><a href="copyright.htm"><font size="2">Copyright</font>
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<EFBFBD> <font size="2">2001, 2002 Thomas M. Eastep.</font></a></font></body></html>
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