mirror of
https://gitlab.com/shorewall/code.git
synced 2024-11-30 19:43:45 +01:00
291dc4df9d
git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@1533 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb
235 lines
11 KiB
XML
235 lines
11 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
|
|
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
|
|
<article id="ProxyARP">
|
|
<!--$Id$-->
|
|
|
|
<articleinfo>
|
|
<title>Proxy ARP</title>
|
|
|
|
<authorgroup>
|
|
<author>
|
|
<firstname>Tom</firstname>
|
|
|
|
<surname>Eastep</surname>
|
|
</author>
|
|
</authorgroup>
|
|
|
|
<pubdate>2004-08-10</pubdate>
|
|
|
|
<copyright>
|
|
<year>2001-2004</year>
|
|
|
|
<holder>Thomas M. Eastep</holder>
|
|
</copyright>
|
|
|
|
<legalnotice>
|
|
<para>Permission is granted to copy, distribute and/or modify this
|
|
document under the terms of the GNU Free Documentation License, Version
|
|
1.2 or any later version published by the Free Software Foundation; with
|
|
no Invariant Sections, with no Front-Cover, and with no Back-Cover
|
|
Texts. A copy of the license is included in the section entitled
|
|
<quote><ulink url="GnuCopyright.htm">GNU Free Documentation License</ulink></quote>.</para>
|
|
</legalnotice>
|
|
</articleinfo>
|
|
|
|
<para>Proxy ARP (RFC 1027) is a way to make a machine physically located on
|
|
one network appear to be logically part of a different physical network
|
|
connected to the same router/firewall. Typically it allows us to hide a
|
|
machine with a public IP address on a private network behind a router, and
|
|
still have the machine appear to be on the public network "in front
|
|
of" the router. The router "proxys" ARP requests and all network
|
|
traffic to and from the hidden machine to make this fiction possible.</para>
|
|
|
|
<para>Consider a router with two interface cards, one connected to a public
|
|
network PUBNET and one connected to a private network PRIVNET. We want to
|
|
hide a server machine on the PRIVNET network but have it accessible from the
|
|
PUBNET network. The IP address of the server machine lies in the PUBNET
|
|
network, even though we are placing the machine on the PRIVNET network
|
|
behind the router.</para>
|
|
|
|
<para>By enabling proxy ARP on the router, any machine on the PUBNET network
|
|
that issues an ARP "who has" request for the server's MAC
|
|
address will get a proxy ARP reply from the router containing the
|
|
router's MAC address. This tells machines on the PUBNET network that
|
|
they should be sending packets destined for the server via the router. The
|
|
router forwards the packets from the machines on the PUBNET network to the
|
|
server on the PRIVNET network.</para>
|
|
|
|
<para>Similarly, when the server on the PRIVNET network issues a "who
|
|
has" request for any machines on the PUBNET network, the router provides
|
|
its own MAC address via proxy ARP. This tells the server to send packets for
|
|
machines on the PUBNET network via the router. The router forwards the
|
|
packets from the server on the PRIVNET network to the machines on the PUBNET
|
|
network.</para>
|
|
|
|
<para>The proxy ARP provided by the router allows the server on the
|
|
PRIVNETnetwork to appear to be on the PUBNET network. It lets the router
|
|
pass ARP requests and other network packets in both directions between the
|
|
server machine and the PUBNET network, making the server machine appear to
|
|
be connected to the PUBNET network even though it is on the PRIVNET network
|
|
hidden behind the router.</para>
|
|
|
|
<para>Before you try to use this technique, I strongly recommend that you
|
|
read the <ulink url="shorewall_setup_guide.htm">Shorewall Setup Guide</ulink>.</para>
|
|
|
|
<section>
|
|
<title>Example</title>
|
|
|
|
<para>The following figure represents a Proxy ARP environment.</para>
|
|
|
|
<graphic fileref="images/proxyarp.png" />
|
|
|
|
<para>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 <filename>/etc/shorewall/proxyarp</filename>:</para>
|
|
|
|
<programlisting>#ADDRESS INTERFACE EXTERNAL HAVEROUTE PERSISTENT
|
|
130.252.100.18 eth1 eth0 no yes
|
|
130.252.100.19 eth1 eth0 no yes </programlisting>
|
|
|
|
<para>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
|
|
<filename>/etc/shorewall/masq</filename> or <filename>/etc/shorewall/nat</filename>.
|
|
</para>
|
|
|
|
<note>
|
|
<para>I've used an RFC1918 IP address for eth1 - that IP address is
|
|
largely irrelevant (see below).</para>
|
|
</note>
|
|
|
|
<para>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. In other words, they should
|
|
be configured just like they would be if they were parallel to the
|
|
firewall rather than behind it. </para>
|
|
|
|
<warning>
|
|
<para>Do not add the Proxy ARP'ed address(es) (130.252.100.18 and
|
|
130.252.100.19 in the above example) to the external interface (eth0 in
|
|
this example) of the firewall.</para>
|
|
</warning>
|
|
|
|
<note>
|
|
<para>It should be stressed that entries in the proxyarp file do not
|
|
automatically enable traffic between the external network and the
|
|
internal host(s) — such traffic is still subject to your policies and
|
|
rules.</para>
|
|
</note>
|
|
|
|
<para>While the address given to the firewall interface is largely
|
|
irrelevant, one approach you can take is to make that address the same as
|
|
the address of your external interface!</para>
|
|
|
|
<graphic align="center" fileref="images/proxyarp1.png" />
|
|
|
|
<para>It the diagram above, <filename class="devicefile">eth1</filename>
|
|
has been given the address 130.252.100.17, the same as
|
|
<filename>eth0</filename>. Note though that the VLSM is 32 so there is no
|
|
network associated with this address. This is the approach <ulink
|
|
url="myfiles.htm">that I take with my DMZ</ulink>.</para>
|
|
|
|
<warning>
|
|
<para>Your distribution's network configuration GUI may not be
|
|
capable of configuring a device in this way. It may complain about the
|
|
duplicate address or it may configure the address incorrectly. Here is
|
|
what the above configuration should look like when viewed using
|
|
<command>ip</command> (the part of the output that is in <emphasis
|
|
role="bold">bold text</emphasis> is relevant):</para>
|
|
|
|
<programlisting>gateway:~# <command>ip addr ls eth1</command>
|
|
3: eth1: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 1000
|
|
link/ether 00:a0:cc:d1:db:12 brd ff:ff:ff:ff:ff:ff
|
|
<emphasis role="bold">inet 130.252.100.17/32 scope global eth1</emphasis>
|
|
gateway:~#</programlisting>
|
|
|
|
<para>Note in particular that there is no broadcast address. <ulink
|
|
url="myfiles.htm#Interfaces">Here is how I configure a device in this
|
|
way under Debian</ulink>.</para>
|
|
</warning>
|
|
</section>
|
|
|
|
<section>
|
|
<title>ARP cache</title>
|
|
|
|
<para>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:</para>
|
|
|
|
<orderedlist>
|
|
<listitem>
|
|
<para>A reading of <citetitle>TCP/IP Illustrated, Vol 1</citetitle> by
|
|
Stevens reveals<footnote><para>Courtesy of Bradey Honsinger</para></footnote>
|
|
that a <quote>gratuitous</quote> 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...</para>
|
|
|
|
<blockquote>
|
|
<para>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.</para>
|
|
</blockquote>
|
|
|
|
<para>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 one-to-one NAT for that matter). Happily enough, recent
|
|
versions of Redhat's iputils package include <quote>arping</quote>,
|
|
whose <quote>-U</quote> flag does just that:</para>
|
|
|
|
<programlisting>arping -U -I <<emphasis>net if</emphasis>> <<emphasis>newly proxied IP</emphasis>>
|
|
arping -U -I eth0 66.58.99.83 # for example</programlisting>
|
|
|
|
<para>Stevens goes on to mention that not all systems respond
|
|
correctly to gratuitous ARPs, but googling for <quote>arping -U</quote>
|
|
seems to support the idea that it works most of the time.</para>
|
|
|
|
<para>To use arping with Proxy ARP in the above example, you would
|
|
have to:</para>
|
|
|
|
<programlisting>shorewall clear
|
|
ip addr add 130.252.100.18 dev eth0
|
|
ip addr add 130.252.100.19 dev eth0
|
|
arping -U -I eth0 130.252.100.18
|
|
arping -U -I eth0 130.252.100.19
|
|
ip addr del 130.252.100.18 dev eth0
|
|
ip addr del 130.252.100.19 dev eth0
|
|
shorewall start</programlisting>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>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.</para>
|
|
</listitem>
|
|
</orderedlist>
|
|
|
|
<para>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:</para>
|
|
|
|
<programlisting>tcpdump -nei eth0 icmp</programlisting>
|
|
|
|
<para>Now from 130.252.100.19, ping the ISP's gateway (which we will
|
|
assume is 130.252.100.254):</para>
|
|
|
|
<programlisting>ping 130.252.100.254</programlisting>
|
|
|
|
<para>We can now observe the tcpdump output:</para>
|
|
|
|
<programlisting>13:35:12.159321 0:4:e2:20:20:33 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 0:c0:a8:50:b2:57 ip 98: 130.252.100.254 > 130.252.100.177 : icmp: echo reply</programlisting>
|
|
|
|
<para>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.</para>
|
|
</section>
|
|
</article> |