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Add Helpers Document
Signed-off-by: Tom Eastep <teastep@shorewall.net>
This commit is contained in:
Tom Eastep
parent
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commit
6fbb578ce6
@ -322,7 +322,7 @@
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<entry><ulink url="PortKnocking.html">Port Knocking and Other Uses
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of the 'Recent Match'</ulink></entry>
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<entry/>
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<entry></entry>
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</row>
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<row>
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@ -331,16 +331,16 @@
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<entry><ulink url="PPTP.htm">PPTP</ulink></entry>
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<entry/>
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<entry></entry>
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</row>
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<row>
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<entry><ulink url="support.htm">Getting help or answers to
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questions</ulink></entry>
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<entry><ulink url="Helpers.html">Helpers/Helper
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Modules</ulink></entry>
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<entry><ulink url="ProxyARP.htm">Proxy ARP</ulink></entry>
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<entry/>
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<entry></entry>
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</row>
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<row>
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@ -350,7 +350,7 @@
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<entry><ulink url="shorewall_quickstart_guide.htm">QuickStart
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Guides</ulink></entry>
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<entry/>
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<entry></entry>
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</row>
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<row>
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@ -358,7 +358,7 @@
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<entry><ulink url="NewRelease.html">Release Model</ulink></entry>
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<entry/>
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<entry></entry>
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</row>
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<row>
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@ -367,7 +367,7 @@
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<entry><ulink
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url="shorewall_prerequisites.htm">Requirements</ulink></entry>
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<entry/>
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<entry></entry>
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</row>
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<row>
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@ -376,7 +376,7 @@
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<entry><ulink url="Shorewall_and_Routing.html">Routing and
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Shorewall</ulink></entry>
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<entry/>
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<entry></entry>
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</row>
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<row>
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@ -385,7 +385,7 @@
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<entry><ulink url="Multiple_Zones.html">Routing on One
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Interface</ulink></entry>
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<entry/>
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<entry></entry>
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</row>
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<row>
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@ -394,7 +394,7 @@
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<entry><ulink url="samba.htm">Samba</ulink></entry>
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<entry/>
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<entry></entry>
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</row>
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<row>
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@ -404,7 +404,7 @@
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<entry><ulink url="Shorewall-init.html">Shorewall
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Init</ulink></entry>
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<entry/>
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<entry></entry>
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</row>
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<row>
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@ -414,16 +414,16 @@
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<entry><ulink url="Shorewall-Lite.html">Shorewall
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Lite</ulink></entry>
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<entry/>
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<entry></entry>
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</row>
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<row>
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<entry><ulink url="KVM.html">KVM (Kernel-mode Virtual
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Machine)</ulink></entry>
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<entry/>
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<entry></entry>
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<entry/>
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<entry></entry>
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</row>
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</tbody>
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</tgroup>
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329
docs/Helpers.xml
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329
docs/Helpers.xml
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@ -0,0 +1,329 @@
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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>Netfilter Helpers</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>Helpers - Introduction</title>
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<para>There are a number of applications that create connections
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dynamically between a client and server. These connections use temporary
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TCP or UDP ports, so static configuration of firewall rules to allow those
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connections would require a very lax firewall configuration. To deal with
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these problem applications, Netfilter supports the concept of a
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<firstterm>helper</firstterm>. Each helper monitors traffic to/from the
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default primary port used by the application and opens the firewall to
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accept temporary connections created by the primary session.</para>
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<para>There are helpers for the following applications; default ports
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monitored by each helper are listed in parentheses:</para>
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<itemizedlist>
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<listitem>
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<para>Amanda (UDP 10080)</para>
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</listitem>
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<listitem>
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<para>FTP (TCP 21)</para>
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</listitem>
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<listitem>
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<para>H323 (UDP 1719, TCP 1720)</para>
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</listitem>
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<listitem>
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<para>IRC (TCP 6667)</para>
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</listitem>
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<listitem>
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<para>Netbios-NS (UDP 137)</para>
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</listitem>
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<listitem>
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<para>PPTP (TCP 1729)</para>
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</listitem>
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<listitem>
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<para>SANE (TCP 6566)</para>
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</listitem>
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<listitem>
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<para>SIP (UDP 5060)</para>
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</listitem>
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<listitem>
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<para>SNMP (UDP 161)</para>
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</listitem>
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<listitem>
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<para>TFTP (UDP 69)</para>
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</listitem>
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</itemizedlist>
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<section>
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<title>Helper Module Loading</title>
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<para>In a modular kernel, each helper is typically packaged as two
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kernel modules. One module handles connection tracking where NAT isn't
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involved and the other module handles NAT. For example, the FTP helper
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consists of these two modules (kernels 2.6.20 and later):</para>
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<itemizedlist>
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<listitem>
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<para>nf_conntrack_ftp</para>
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</listitem>
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<listitem>
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<para>nf_nat_ftp</para>
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</listitem>
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</itemizedlist>
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<para>Note that the naming convention is
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nf_conntrack_<replaceable>application</replaceable> and
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nf_nat_<replaceable>application</replaceable>; more about that
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below.</para>
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<para>The modules are not auto-loaded and must be loaded explicitly
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using the <command>modprob</command> or <command>insmod</command>
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utilities.</para>
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<para>Many of the modules allow parameters to be specified when the
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module is loaded. Among the common parameters is the ports parameter
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that lists one or more ports that the module is to monitor. This allows
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running the application on a non-standard port.</para>
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</section>
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<section>
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<title>Iptables and Helpers</title>
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<para>Iptables supports two ways of interacting with modules:</para>
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<variablelist>
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<varlistentry>
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<term>Helper Match</term>
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<listitem>
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<para>This match (-m helper --helper
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<replaceable>name</replaceable>) allows selection of packets from
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connections monitored or created by the named helper.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>CT Target</term>
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<listitem>
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<para>This target (-j CT --helper <replaceable>name</replaceable>
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...) , introduced in the 3.4 kernels, allows for explicit
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association of a helper with a connection.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>It is important to note that the name used in iptables is not
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always the same as the name in the kernel module. Names used in iptables
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are shown in the following table:</para>
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<informaltable>
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<tgroup cols="2">
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<tbody>
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<row>
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<entry>Name of kernel module</entry>
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<entry>Name recognized by iptables</entry>
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</row>
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<row>
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<entry>nf_conntrack_amanda</entry>
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<entry>amanda</entry>
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</row>
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<row>
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<entry>nf_conntrack_ftp</entry>
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<entry>ftp</entry>
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</row>
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<row>
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<entry>nf_conntrack_h323</entry>
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<entry><emphasis role="bold">RAS (udp 1719), Q.931 (tcp
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1720)</emphasis></entry>
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</row>
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<row>
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<entry>nf_conntrack_irc</entry>
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<entry>irc</entry>
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</row>
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<row>
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<entry>nf_conntrack_netbios_ns</entry>
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<entry><emphasis role="bold">netbios-ns</emphasis></entry>
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</row>
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<row>
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<entry>nf_conntrack_pptp</entry>
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<entry>pptp</entry>
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</row>
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<row>
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<entry>nf_conntrack_sane</entry>
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<entry>sane</entry>
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</row>
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<row>
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<entry>nf_conntrack_sip</entry>
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<entry>sip</entry>
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</row>
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<row>
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<entry>nf_conntrack_snmp</entry>
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<entry>snmp</entry>
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</row>
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<row>
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<entry>nf_conntrack_tftp</entry>
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<entry>tftp</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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<para>Netfilter helpers present an opportunity for attackers to attempt
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to breach your firewall by IP address spoofing; See <ulink
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url="https://home.regit.org/netfilter-en/secure-use-of-helpers/">https://home.regit.org/netfilter-en/secure-use-of-helpers/</ulink>
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for a description of the Netfilter facilities available to meet these
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attacks.</para>
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</section>
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</section>
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<section>
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<title>Shorewall Support for Helpers</title>
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<para>Shorewall includes support for helpers is several areas. These areas
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are covered in the sections below. </para>
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<section>
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<title>Module Loading</title>
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<para>Shorewall includes support for loading the helper modules as part
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of its support for loading kernel modules in general. There are several
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options in shorewall.conf (5) that deal with kernel module
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loading:</para>
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<variablelist>
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<varlistentry>
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<term>MODULESDIR</term>
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<listitem>
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<para>This option specifies a comma-separated list of directories
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where Shorewall will look for kernel modules to load.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>MODULE_SUFFIX</term>
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<listitem>
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<para>Lists the possible suffixes for module names.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>LOAD_HELPERS_ONLY</term>
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<listitem>
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<para>Controls whether Shorewall should load only the helpers and
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leave the other modules to the auto-loader. This option
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dramatically reduces the time to process a <command>shorewall
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start</command> or <command>shorewall restart</command>
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command.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>HELPERS</term>
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<listitem>
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<para>This option was added in Shorewall 4.5.7 and lists the
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modules to be enabled for association with connections. This
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option is fully functional only on systems running kernel 3.5 or
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later. On systems running earlier kernels, the only way to totally
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disable a module is to not load it. The module names allowed in
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this list are <emphasis role="bold">amanda</emphasis>, <emphasis
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role="bold">ftp</emphasis>, <emphasis role="bold">h323</emphasis>,
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<emphasis role="bold">irc</emphasis>, <emphasis
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role="bold">netbios-ns</emphasis>, <emphasis
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role="bold">pptp</emphasis>, <emphasis
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role="bold">sane</emphasis>, <emphasis role="bold">sip</emphasis>,
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<emphasis role="bold">snmp</emphasis> and <emphasis
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role="bold">tftp</emphasis>.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>The helper modules to be loaded are listed in the file
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<filename>/usr/share/shorewall/helpers</filename>. If you wish to
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customize that file to load only a subset of the helpers or to specify
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module parameters, then copy the file to <filename>/etc/shorewall/
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</filename>and modify the copy. That way, your changes won't be
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overwritten the next time that Shorewall is updated on your
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system.</para>
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</section>
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<section>
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<title>Iptables</title>
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<para>The iptables helper match is supported by Shorewall in the form of
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the HELPER column in <ulink
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url="manpages/shorewall-tcrules.html">shorewall-tcrules</ulink>
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(5).</para>
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<para>The CT target is supported directly in <ulink
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url="manpages/shorewall-conntrack.html">shorewall-conntrack</ulink>
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(5).</para>
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<para>In these files, Shorewall supports the same module names as
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iptables; see the table above.</para>
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</section>
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</section>
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</article>
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Loading…
Reference in New Issue
Block a user