mirror of
https://gitlab.com/shorewall/code.git
synced 2024-11-21 23:23:13 +01:00
c93817f30b
The invariant sections clause doesn't quite match the official text. It should read: with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts not: with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
128 lines
4.6 KiB
XML
128 lines
4.6 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
|
|
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
|
|
<article>
|
|
<!--$Id$-->
|
|
|
|
<articleinfo>
|
|
<title>Countering Spoofing Attempts</title>
|
|
|
|
<authorgroup>
|
|
<author>
|
|
<firstname>Tom</firstname>
|
|
|
|
<surname>Eastep</surname>
|
|
</author>
|
|
</authorgroup>
|
|
|
|
<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
|
|
|
|
<copyright>
|
|
<year>2012</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, no Front-Cover Texts, and 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>
|
|
|
|
<section>
|
|
<title>Introduction</title>
|
|
|
|
<para><firstterm>Spoofing</firstterm> is the practice of sending packets
|
|
with a forged source address in an attempt to circumvent security
|
|
measures. Shorewall supports a variety of measures to counter spoofing
|
|
attacks.</para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>The <emphasis>routefilter</emphasis> Interface Option</title>
|
|
|
|
<para>This <ulink url="???">shorewall-interfaces</ulink> (5) option was
|
|
the first measure implemented and uses
|
|
<filename>/proc/sys/net/ipv4/conf/*/rp_filter</filename>. Many
|
|
distributions set this option by default for all ip interfaces. The option
|
|
works by determining the reverse path (the route from the packets
|
|
destination to its source); it that route does not go out through the
|
|
interface that received the packet, then the packet is declared to be a
|
|
martian and is dropped. A kernel log message is generated if the
|
|
interface's <option>logmartians</option> option is set
|
|
(<filename>/proc/sys/net/ipv4/conf/*/log_martians</filename>).</para>
|
|
|
|
<para>While this option is simple to configure, it has a couple of
|
|
disadvantages:</para>
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>It is not supported by IPv6.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>It does not use packet marks so it doesn't work with some <ulink
|
|
url="MultiISP.html">Multi-ISP</ulink> configurations.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>The log messages produces are obscure and confusing.</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Hairpin Filtering</title>
|
|
|
|
<para>Spoofing can be used to exploit Netfilter's connection tracking to
|
|
open arbitrary firewall ports. Attacks of this type establish a connection
|
|
to a server that uses separate control and data connections such as an FTP
|
|
server. It then sends a packet addressed to itself and from the server.
|
|
Such packets are sent back out the same interface that received them
|
|
(<firstterm>hairpin</firstterm>). In cases where the
|
|
<option>routefilter</option> option can't be used, Shorewall 4.4.20 and
|
|
later will set up hairpinning traps (see the SFILTER_DISPOSITION and
|
|
SFILTER_LOG_LEVEL options in <ulink
|
|
url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5)).</para>
|
|
|
|
<para>This automatic hairpin trapping is disabled on interfaces with the
|
|
<option>routeback</option> option.</para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>The <emphasis>rpfilter</emphasis> Interface Option</title>
|
|
|
|
<para>A new iptables/ip6tables match (rpfilter) was added in kernel 3.4.4.
|
|
This match performs reverse path evaluation similar to
|
|
<option>routefilter</option> but without the disadvantages:</para>
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>It is supported by both IPv4 and IPv6.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>It uses packet marks so it works with all <ulink
|
|
url="MultiISP.html">Multi-ISP</ulink> configurations.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>It produces standard Shorewall/Netfilter log messages controlled
|
|
by the RPFILTER_LOG_LEVEL option in <ulink
|
|
url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5)).</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Both the disposition and auditing can be controlled using the
|
|
RPFILTER_DISPOSITION option in <ulink
|
|
url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5)).</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
</article>
|