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304 lines
12 KiB
XML
304 lines
12 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<article id="usefull_links">
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<!--$Id$-->
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<articleinfo>
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<title>Introduction</title>
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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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<pubdate>2004-11-18</pubdate>
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<copyright>
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<year>2003-2004</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 type="" url="Copyright.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>The information in this document applies only to 2.x releases of
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Shorewall.</para>
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<section>
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<title>Glossary</title>
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<itemizedlist>
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<listitem>
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<para><ulink url="http://www.netfilter.org">Netfilter</ulink> - the
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packet filter facility built into the 2.4 and later Linux
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kernels.</para>
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</listitem>
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<listitem>
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<para>ipchains - the packet filter facility built into the 2.2 Linux
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kernels. Also the name of the utility program used to configure and
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control that facility. Netfilter can be used in ipchains
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compatibility mode.</para>
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</listitem>
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<listitem>
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<para>iptables - the utility program used to configure and control
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Netfilter. The term <quote>iptables</quote> is often used to refer
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to the combination of iptables+Netfilter (with Netfilter not in
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ipchains compatibility mode).</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>What is Shorewall?</title>
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<para>The Shoreline Firewall, more commonly known as
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<quote>Shorewall</quote>, is high-level tool for configuring Netfilter.
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You describe your firewall/gateway requirements using entries in a set
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of configuration files. Shorewall reads those configuration files and
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with the help of the iptables utility, Shorewall configures Netfilter to
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match your requirements. Shorewall can be used on a dedicated firewall
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system, a multi-function gateway/router/server or on a standalone
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GNU/Linux system. Shorewall does not use Netfilter's ipchains
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compatibility mode and can thus take advantage of Netfilter's connection
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state tracking capabilities.</para>
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<para>Shorewall is not a daemon. Once Shorewall has configured
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Netfilter, it's job is complete and there is no <quote>Shorewall
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process</quote> left running in your system. The <ulink
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url="starting_and_stopping_shorewall.htm">/sbin/shorewall program can be
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used at any time to monitor the Netfilter firewall</ulink>.</para>
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</section>
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</section>
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<section>
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<title>Shorewall Concepts</title>
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<para>The configuration files for Shorewall are contained in the directory
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<filename class="directory">/etc/shorewall</filename> -- for simple
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setups, you will only need to deal with a few of them.</para>
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<para>Shorewall views the network where it is running as being composed of
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a set of <firstterm>zones</firstterm>. In the <ulink
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url="three-interface.htm">three-interface sample configuration</ulink> for
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example, the following zone names are used: <informaltable frame="all"
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pgwide="0">
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<tgroup align="left" cols="2">
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<thead valign="middle">
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<row valign="middle">
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<entry align="left">Name</entry>
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<entry align="left">Description</entry>
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</row>
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</thead>
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<tbody valign="middle">
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<row valign="middle">
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<entry align="left"><varname>net</varname></entry>
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<entry align="left">The Internet</entry>
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</row>
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<row valign="middle">
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<entry align="left"><varname>loc</varname></entry>
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<entry align="left">Your Local Network</entry>
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</row>
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<row valign="middle">
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<entry align="left"><varname>dmz</varname></entry>
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<entry align="left">Demilitarized Zone</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>Zones are defined in the <ulink
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url="Documentation.htm#Zones"><filename
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class="directory">/etc/shorewall/</filename><filename>zones</filename></ulink>
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file.</para>
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<para>Shorewall also recognizes the firewall system as its own zone - by
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default, the firewall itself is known as <emphasis
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role="bold"><varname>fw</varname></emphasis>.</para>
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<para>Rules about what traffic to allow and what traffic to deny are
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expressed in terms of zones. <itemizedlist spacing="compact">
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<listitem>
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<para>You express your default policy for connections from one zone
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to another zone in the <ulink
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url="Documentation.htm#Policy"><filename
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class="directory">/etc/shorewall/</filename><filename>policy</filename></ulink>
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file. The basic choices for policy are:</para>
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<itemizedlist>
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<listitem>
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<para>ACCEPT - Accept the connection.</para>
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</listitem>
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<listitem>
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<para>DROP - Ignore the connection request.</para>
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</listitem>
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<listitem>
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<para>REJECT - Return an appropriate error to the connection
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request.</para>
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</listitem>
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</itemizedlist>
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<para>Connection request logging may be specified as part of a
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policy and it is conventional to log DROP and REJECT
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policies.</para>
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</listitem>
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<listitem>
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<para>You define exceptions to these default policies in the <ulink
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url="Documentation.htm#Rules"><filename
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class="directory">/etc/shorewall/</filename><filename>rules</filename></ulink>
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file.</para>
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</listitem>
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<listitem>
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<para>You only need concern yourself with connection requests. You
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don't need to define rules for how traffic that is part of an
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established connection is handled and in most cases you don't have
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to worry about how related connections are handled (ICMP error
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packets and <ulink url="FTP.html">related TCP connection requests
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such as used by FTP</ulink>).</para>
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</listitem>
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</itemizedlist>For each connection request entering the firewall, the
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request is first checked against the <filename
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class="directory">/etc/shorewall/</filename><filename>rules</filename>
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file. If no rule in that file matches the connection request then the
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first policy in <filename
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class="directory">/etc/shorewall/</filename><filename>policy</filename>
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that matches the request is applied. If there is a common action defined
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for the policy in /etc/shorewall/actions (or
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<filename>/usr/share/shorewall/actions.std</filename>) then that action is
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invoked before the policy is enforces. In the standard Shorewall
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distribution, the DROP policy has a common action called <emphasis
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role="bold">Drop</emphasis> and the REJECT policy has a common action
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called <emphasis role="bold">Reject</emphasis>. Common actions are used
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primarily to discard</para>
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<para>The <filename
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class="directory">/etc/shorewall/</filename><filename>policy</filename>
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file included with the three-interface sample has the following policies:
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<programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
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loc net ACCEPT
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net all DROP info
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all all REJECT info</programlisting>In the three-interface
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sample, the line below is included but commented out. If you want your
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firewall system to have full access to servers on the internet, uncomment
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that line. <programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
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fw net ACCEPT</programlisting> The above policy will:
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<itemizedlist>
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<listitem>
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<para>Allow all connection requests from your local network to the
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internet</para>
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</listitem>
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<listitem>
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<para>Drop (ignore) all connection requests from the internet to
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your firewall or local network; these ignored connection requests
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will be logged using the <emphasis>info</emphasis> syslog priority
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(log level).</para>
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</listitem>
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<listitem>
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<para>Optionally accept all connection requests from the firewall to
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the internet (if you uncomment the additional policy)</para>
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</listitem>
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<listitem>
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<para>reject all other connection requests; these rejected
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connection requests will be logged using the
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<emphasis>info</emphasis> syslog priority (log level).</para>
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</listitem>
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</itemizedlist></para>
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<para>The simplest way to define a zone is to associate the zone with a
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network interface using the <ulink
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url="Documentation.htm#Interfaces"><filename>/etc/shorewall/interfaces</filename></ulink>
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file. In the three-interface sample, the three zones are defined using
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that file as follows:</para>
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<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
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net eth0 detect dhcp,routefilter,norfc1918
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loc eth1 detect
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dmz eth2 detect</programlisting>
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<para>The above file defines the net zone as all hosts interfacing to the
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firewall through eth0, the loc zone as all hosts interfacing through eth1
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and the dmz as all hosts interfacing through eth2.</para>
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<para>To illustrate how rules provide exceptions to policies, suppose that
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you have the polcies listed above but you want to be able to connect to
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your firewall from the internet using Secure Shell (SSH). Recall that SSH
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connects uses TCP port 22.</para>
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<programlisting>#ACTION SOURCE DEST PROTO DEST
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# PORT(S)
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ACCEPT net fw tcp 22</programlisting>
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<para>So although you have a policy of ignoring all connection attempts
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from the net zone (from the internet), the above exception to that policy
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allows you to connect to the SSH server running on your firewall.</para>
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<para>Because Shorewall makes no assumptions about what traffic you want
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accepted, there are certain rules (exceptions) that need to be added to
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almost any configuration.</para>
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<itemizedlist>
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<listitem>
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<para>The <ulink url="shorewall_quickstart_guide.htm">QuickStart
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guildes</ulink> provide links to download pre-populated files for use
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in common setups and the <ulink
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url="shorewall_setup_guide.htm">Shorewall Setup Guide</ulink> shows
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you examples for use with other more complex setups.</para>
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</listitem>
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<listitem>
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<para>To keep your <ulink url="shorewall_logging.html">firewall
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log</ulink> from filling up with useless noise, Shorewall provides
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<ulink url="User_defined_Actions.html">common actions</ulink> that
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silently discard or reject such noise before it can be logged. As with
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everything in Shorewall, you can alter the behavior of these common
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actions (or do away with them entirely) as you see fit.</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>License</title>
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<para>This program is free software; you can redistribute it and/or modify
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it under the terms of <ulink
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url="http://www.gnu.org/licenses/gpl.html">Version 2 of the GNU General
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Public License</ulink> as published by the Free Software
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Foundation.</para>
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<para>This program is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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for more detail.</para>
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<para>You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software Foundation,
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Inc., 675 Mass Ave, Cambridge, MA 02139, USA</para>
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</section>
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</article> |