mirror of
https://gitlab.com/shorewall/code.git
synced 2024-12-12 17:30:44 +01:00
f158c11a41
git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@208 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb
106 lines
4.3 KiB
HTML
106 lines
4.3 KiB
HTML
<html>
|
||
|
||
<head>
|
||
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
|
||
<title>Shorewall Proxy ARP</title>
|
||
<meta name="GENERATOR" content="Microsoft FrontPage 5.0">
|
||
<meta name="ProgId" content="FrontPage.Editor.Document">
|
||
<meta name="Microsoft Theme" content="none">
|
||
</head>
|
||
|
||
<body>
|
||
|
||
<table border="0" cellpadding="0" cellspacing="0" style="border-collapse: collapse" width="100%" id="AutoNumber1" bgcolor="#400169" height="90">
|
||
<tr>
|
||
<td width="100%">
|
||
<h1 align="center"><font color="#FFFFFF">Proxy ARP</font></h1>
|
||
</td>
|
||
</tr>
|
||
</table>
|
||
<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.</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">
|
||
<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>
|
||
</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. 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. 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> tcpdump -nei eth0 icmp</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):</div>
|
||
<div align="left">
|
||
<pre> ping 130.252.100.254</pre>
|
||
</div>
|
||
<div align="left">
|
||
<p align="left">We can now observe the tcpdump output:</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)
|
||
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.</div>
|
||
|
||
<p><font size="2">Last updated 8/17/2002 - </font><font size="2">
|
||
<a href="support.htm">Tom
|
||
Eastep</a></font> </p>
|
||
<font face="Trebuchet MS"><a href="copyright.htm"><font size="2">Copyright</font>
|
||
<EFBFBD> <font size="2">2001, 2002 Thomas M. Eastep.</font></a></font></body></html> |