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300 lines
13 KiB
XML
300 lines
13 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
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"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
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<article id="ProxyARP">
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<!--$Id$-->
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<articleinfo>
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<title>Proxy ARP</title>
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<authorgroup>
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<author>
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<firstname>Tom</firstname>
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<surname>Eastep</surname>
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</author>
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</authorgroup>
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<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
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<copyright>
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<year>2001-2006</year>
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<holder>Thomas M. Eastep</holder>
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</copyright>
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<legalnotice>
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<para>Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License, Version
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1.2 or any later version published by the Free Software Foundation; with
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no Invariant Sections, with no Front-Cover, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled
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<quote><ulink url="GnuCopyright.htm">GNU Free Documentation
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License</ulink></quote>.</para>
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</legalnotice>
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</articleinfo>
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<para>Proxy ARP (RFC 1027) is a way to make a machine physically located on
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one network appear to be logically part of a different physical network
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connected to the same router/firewall. Typically it allows us to hide a
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machine with a public IP address on a private network behind a router, and
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still have the machine appear to be on the public network "in front of" the
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router. The router "proxys" ARP requests and all network traffic to and from
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the hidden machine to make this fiction possible.</para>
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<para>Consider a router with two interface cards, one connected to a public
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network PUBNET and one connected to a private network PRIVNET. We want to
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hide a server machine on the PRIVNET network but have it accessible from the
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PUBNET network. The IP address of the server machine lies in the PUBNET
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network, even though we are placing the machine on the PRIVNET network
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behind the router.</para>
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<para>By enabling proxy ARP on the router, any machine on the PUBNET network
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that issues an ARP "who has" request for the server's MAC address will get a
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proxy ARP reply from the router containing the router's MAC address. This
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tells machines on the PUBNET network that they should be sending packets
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destined for the server via the router. The router forwards the packets from
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the machines on the PUBNET network to the server on the PRIVNET
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network.</para>
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<para>Similarly, when the server on the PRIVNET network issues a "who has"
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request for any machines on the PUBNET network, the router provides its own
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MAC address via proxy ARP. This tells the server to send packets for
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machines on the PUBNET network via the router. The router forwards the
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packets from the server on the PRIVNET network to the machines on the PUBNET
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network.</para>
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<para>The proxy ARP provided by the router allows the server on the
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PRIVNETnetwork to appear to be on the PUBNET network. It lets the router
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pass ARP requests and other network packets in both directions between the
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server machine and the PUBNET network, making the server machine appear to
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be connected to the PUBNET network even though it is on the PRIVNET network
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hidden behind the router.</para>
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<para>Before you try to use this technique, I strongly recommend that you
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read the <ulink url="shorewall_setup_guide.htm">Shorewall Setup
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Guide</ulink>.</para>
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<section id="Example">
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<title>Example</title>
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<para>The following figure represents a Proxy ARP environment.</para>
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<graphic align="center" fileref="images/proxyarp.png" />
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<para>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
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(130.252.100.*) subnet. Assuming that the upper firewall interface is eth0
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and the lower interface is eth1, this is accomplished using the following
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entries in <filename>/etc/shorewall/proxyarp</filename>:</para>
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<programlisting>#ADDRESS INTERFACE EXTERNAL HAVEROUTE PERSISTENT
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130.252.100.18 eth1 eth0 no yes
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130.252.100.19 eth1 eth0 no yes </programlisting>
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<para><emphasis role="bold">Be sure that the internal systems
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(130.242.100.18 and 130.252.100.19 in the above example) are not included
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in any specification in <filename>/etc/shorewall/masq</filename> or
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<filename>/etc/shorewall/nat</filename>.</emphasis></para>
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<note>
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<para>I've used an RFC1918 IP address for eth1 - that IP address is
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largely irrelevant (see below).</para>
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</note>
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<para>The lower systems (130.252.100.18 and 130.252.100.19) <emphasis
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role="bold">should have their subnet mask and default gateway configured
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exactly the same way that the Firewall system's eth0 is configured. In
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other words, they should be configured just like they would be if they
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were parallel to the firewall rather than behind it.</emphasis></para>
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<warning>
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<para>Do not add the Proxy ARP'ed address(es) (130.252.100.18 and
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130.252.100.19 in the above example) to the external interface (eth0 in
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this example) of the firewall.</para>
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</warning>
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<note>
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<para>It should be stressed that entries in the proxyarp file do not
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automatically enable traffic between the external network and the
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internal host(s) — such traffic is still subject to your policies and
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rules.</para>
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</note>
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<para>While the address given to the firewall interface is largely
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irrelevant, one approach you can take is to make that address the same as
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the address of your external interface!</para>
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<graphic align="center" fileref="images/proxyarp1.png" />
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<para>In the diagram above, <filename class="devicefile">eth1</filename>
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has been given the address 130.252.100.17, the same as
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<filename>eth0</filename>. Note though that the VLSM is 32 so there is no
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network associated with this address. This is the approach <ulink
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url="XenMyWay.html">that I take with my DMZ</ulink>.</para>
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<para>To permit internet hosts to connect to the local systems, you use
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ACCEPT rules. For example, if you run a web server on 130.252.100.19 which
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you have configured to be in the <emphasis role="bold">loc</emphasis> zone
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then you would need this entry in /etc/shorewall/rules:</para>
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<programlisting>#ACTION SOURCE DEST PROTO DEST
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# PORT
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ACCEPT net loc:130.252.100.19 tcp 80</programlisting>
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<warning>
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<para>Your distribution's network configuration GUI may not be capable
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of configuring a device in this way. It may complain about the duplicate
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address or it may configure the address incorrectly. Here is what the
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above configuration should look like when viewed using
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<command>ip</command> (the line "inet 130.252.100.17/32 scope global
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eth1" is the most important):</para>
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<programlisting>gateway:~# <command>ip addr ls eth1</command>
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3: eth1: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 1000
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link/ether 00:a0:cc:d1:db:12 brd ff:ff:ff:ff:ff:ff
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<emphasis role="bold">inet 130.252.100.17/32 scope global eth1</emphasis>
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gateway:~#</programlisting>
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<para>Note in particular that there is no broadcast address. Here is an
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<filename>ifcfg-eth-id-00:a0:cc:d1:db:12</filename> file from SUSE that
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produces this result (Note: SUSE ties the configuration file to the card
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by embedding the card's MAC address in the file name):</para>
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<programlisting>BOOTPROTO='static'
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BROADCAST='130.252.100.17'
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IPADDR='130.252.100.17'
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MTU=''
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NETMASK='255.255.255.255'
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NETWORK='130.252.100.17'
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REMOTE_IPADDR=''
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STARTMODE='onboot'
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UNIQUE='8otl.IPwRm6bNMRD'
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_nm_name='bus-pci-0000:00:04.0'</programlisting>
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<para>Here is an excerpt from a Debian /etc/network/interfaces file that
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does the same thing:</para>
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<programlisting>...
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auto eth1
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iface eth1 inet static
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address 130.252.100.17
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netmask 255.255.255.255
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broadcast 0.0.0.0
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...</programlisting>
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</warning>
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</section>
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<section id="ARP">
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<title>ARP cache</title>
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<para>A word of warning is in order here. ISPs typically configure their
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routers with a long ARP cache timeout. If you move a system from parallel
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to your firewall to behind your firewall with Proxy ARP, it will probably
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be <emphasis role="bold">HOURS</emphasis> before that system can
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communicate with the internet.</para>
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<para>If you sniff traffic on the firewall's external interface, you can
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see incoming traffic for the internal system(s) but the traffic is never
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sent out the internal interface.</para>
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<para>You can determine if your ISP's gateway ARP cache is stale using
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ping 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 as
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follows:</para>
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<programlisting>tcpdump -nei eth0 icmp</programlisting>
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<para>Now from 130.252.100.19, ping the ISP's gateway (which we will
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assume is 130.252.100.254):</para>
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<programlisting>ping 130.252.100.254</programlisting>
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<para>We can now observe the tcpdump output:</para>
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<programlisting>13:35:12.159321 <emphasis role="bold">0:4:e2:20:20:33</emphasis> 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 <emphasis role="bold">0:c0:a8:50:b2:57 </emphasis>ip 98: 130.252.100.254 > 130.252.100.19 : icmp: echo reply</programlisting>
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<para>Notice that the source MAC address in the echo request is different
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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
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0:c0:a8:50:b2:57 was the MAC address of the system on the lower left. In
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other words, the gateway's ARP cache still associates 130.252.100.19 with
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the NIC in that system rather than with the firewall's eth0.</para>
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<para>If you have this problem, there are a couple of things that you can
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try:</para>
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<orderedlist>
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<listitem>
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<para>A reading of <citetitle>TCP/IP Illustrated, Vol 1</citetitle> by
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Stevens reveals<footnote>
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<para>Courtesy of Bradey Honsinger</para>
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</footnote> that a <quote>gratuitous</quote> ARP packet should cause
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the ISP's router to refresh their ARP cache (section 4.7). A
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gratuitous ARP is simply a host requesting the MAC address for its own
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IP; in addition to ensuring that the IP address isn't a
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duplicate...</para>
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<blockquote>
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<para>if the host sending the gratuitous ARP has just changed its
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hardware address..., this packet causes any other host...that has an
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entry in its cache for the old hardware address to update its ARP
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cache entry accordingly.</para>
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</blockquote>
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<para>Which is, of course, exactly what you want to do when you switch
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a host from being exposed to the Internet to behind Shorewall using
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proxy ARP (or one-to-one NAT for that matter). Happily enough, recent
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versions of Redhat's iputils package include <quote>arping</quote>,
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whose <quote>-U</quote> flag does just that:</para>
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<programlisting>arping -U -I <<emphasis>net if</emphasis>> <<emphasis>newly proxied IP</emphasis>>
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arping -U -I eth0 66.58.99.83 # for example</programlisting>
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<para>Stevens goes on to mention that not all systems respond
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correctly to gratuitous ARPs, but googling for <quote>arping
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-U</quote> seems to support the idea that it works most of the
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time.</para>
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<para>To use arping with Proxy ARP in the above example, you would
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have to:</para>
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<programlisting>shorewall clear
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ip addr add 130.252.100.18 dev eth0
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ip addr add 130.252.100.19 dev eth0
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arping -U -c 10 -I eth0 130.252.100.18
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arping -U -c 10 -I eth0 130.252.100.19
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ip addr del 130.252.100.18 dev eth0
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ip addr del 130.252.100.19 dev eth0
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shorewall start</programlisting>
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</listitem>
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<listitem>
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<para>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.</para>
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</listitem>
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</orderedlist>
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<warning>
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<para>There are two distinct versions of <command>arping</command>
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available:</para>
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<orderedlist>
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<listitem>
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<para><command>arping</command> by Thomas Habets (Debian package
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<emphasis>arping</emphasis>).</para>
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</listitem>
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<listitem>
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<para><command>arping</command> as part of the iputils package by
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Alexey Kuznetsov (Debian package
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<emphasis>iputils-arping</emphasis>).</para>
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</listitem>
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</orderedlist>
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<para>You want the second one by Alexey Kuznetsov.</para>
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</warning>
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</section>
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</article>
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