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128 lines
4.6 KiB
XML
128 lines
4.6 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>
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<!--$Id$-->
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<articleinfo>
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<title>Countering Spoofing Attempts</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>2012</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>Introduction</title>
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<para><firstterm>Spoofing</firstterm> is the practice of sending packets
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with a forged source address in an attempt to circumvent security
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measures. Shorewall supports a variety of measures to counter spoofing
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attacks.</para>
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</section>
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<section>
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<title>The <emphasis>routefilter</emphasis> Interface Option</title>
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<para>This <ulink url="???">shorewall-interfaces</ulink> (5) option was
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the first measure implemented and uses
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<filename>/proc/sys/net/ipv4/conf/*/rp_filter</filename>. Many
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distributions set this option by default for all ip interfaces. The option
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works by determining the reverse path (the route from the packets
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destination to its source); it that route does not go out through the
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interface that received the packet, then the packet is declared to be a
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martian and is dropped. A kernel log message is generated if the
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interface's <option>logmartians</option> option is set
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(<filename>/proc/sys/net/ipv4/conf/*/log_martians</filename>).</para>
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<para>While this option is simple to configure, it has a couple of
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disadvantages:</para>
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<itemizedlist>
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<listitem>
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<para>It is not supported by IPv6.</para>
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</listitem>
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<listitem>
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<para>It does not use packet marks so it doesn't work with some <ulink
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url="MultiISP.html">Multi-ISP</ulink> configurations.</para>
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</listitem>
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<listitem>
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<para>The log messages produces are obscure and confusing.</para>
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</listitem>
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</itemizedlist>
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</section>
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<section>
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<title>Hairpin Filtering</title>
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<para>Spoofing can be used to exploit Netfilter's connection tracking to
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open arbitrary firewall ports. Attacks of this type establish a connection
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to a server that uses separate control and data connections such as an FTP
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server. It then sends a packet addressed to itself and from the server.
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Such packets are sent back out the same interface that received them
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(<firstterm>hairpin</firstterm>). In cases where the
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<option>routerfilter</option> option can't be used, Shorewall 4.4.20 and
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later will set up hairpinning traps (see the SFILTER_DISPOSITION and
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SFILTER_LOG_LEVEL options in <ulink
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url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5)).</para>
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<para>This automatic hairpin trapping is disabled on interfaces with the
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<option>routeback</option> option.</para>
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</section>
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<section>
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<title>The <emphasis>rpfilter</emphasis> Interface Option</title>
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<para>A new iptables/ip6tables match (rpfilter) was added in kernel 3.4.4.
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This match performs reverse path evaluation similar to
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<option>routefilter</option> but without the disadvantages:</para>
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<itemizedlist>
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<listitem>
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<para>It is supported by both IPv4 and IPv6.</para>
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</listitem>
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<listitem>
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<para>It uses packet marks so it works with all <ulink
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url="MultiISP.html">Multi-ISP</ulink> configurations.</para>
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</listitem>
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<listitem>
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<para>It produces standard Shorewall/Netfilter log messages controlled
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by the RPFILTER_LOG_LEVEL option in <ulink
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url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5)).</para>
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</listitem>
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<listitem>
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<para>Both the disposition and auditing can be controlled using the
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RPFILTER_DISPOSITION option in <ulink
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url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5)).</para>
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</listitem>
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</itemizedlist>
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</section>
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</article>
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