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88 lines
2.9 KiB
XML
88 lines
2.9 KiB
XML
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<?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>
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<!--$Id$-->
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<articleinfo>
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<title>The Fool's Firewall</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>2009</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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<section>
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<title>Definition</title>
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<para>Occasionally, we hear from someone who has cabled his firewall's
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external and internal firewall interfaces to the same switch. I call this
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configuration <firstterm>The Fool's Firewall</firstterm>. </para>
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<para>When the external interface supports broadcast, this configuration
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has two very bad drawbacks:</para>
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<orderedlist>
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<listitem>
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<para>It is very insecure</para>
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</listitem>
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<listitem>
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<para>The up-stream router can send incoming packets to the wrong
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interface.</para>
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</listitem>
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</orderedlist>
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</section>
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<section>
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<title>Security Issue</title>
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<para>Because Fool's firewall is not physically located between the net
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and the local systems, the local systems are exposed to all of the systems
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in the same broadcast domain. Because the local systems (expecially those
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running Windows) send broadcasts, those systems can be easily detected by
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using a packet sniffer. Once the systems have been spotted, it is child's
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play to add an IP address in Fool's internal IP network and bypass his
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"Firewall". </para>
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<graphic align="center" fileref="images/Fools.png" />
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</section>
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<section>
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<title>ARP Roulette</title>
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<para>The Linux IP stack exhibits some unexpected behavior with respect to
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ARP. It will respond to ARP 'who-has' requests received on
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<emphasis>any</emphasis> interface and not just on the interface owning
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the address. So when the upstream router sends a 'who-has' request for
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Fool's external IP address, the response may come from his
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<emphasis>internal</emphasis> interface (and reflect the MAC address of
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that interface). When that happens, packets from the net start entering
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the firewall's internal interface.</para>
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<graphic fileref="images/Foolsa.png" />
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</section>
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</article>
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