mirror of
https://gitlab.com/shorewall/code.git
synced 2024-12-16 19:30:44 +01:00
4b5254bed5
git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@476 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb
180 lines
6.8 KiB
HTML
180 lines
6.8 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
|
||
<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">
|
||
<tbody>
|
||
<tr>
|
||
<td width="100%">
|
||
<h1 align="center"><font color="#ffffff">Proxy ARP</font></h1>
|
||
</td>
|
||
</tr>
|
||
|
||
</tbody>
|
||
</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.
|
||
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>
|
||
|
||
</tbody>
|
||
</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>
|
||
<br>
|
||
</body>
|
||
</html>
|