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509 lines
18 KiB
XML
509 lines
18 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="standalone">
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<!--$Id$-->
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<articleinfo>
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<title>Standalone 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>2003-11-15</pubdate>
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<copyright>
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<year>2002</year>
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<year>2003</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 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>Setting up Shorewall on a standalone Linux system is very easy if
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you understand the basics and follow the documentation.</para>
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<para>This guide doesn't attempt to acquaint you with all of the
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features of Shorewall. It rather focuses on what is required to configure
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Shorewall in one of its most common configurations:</para>
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<itemizedlist>
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<listitem>
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<para>Linux system</para>
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</listitem>
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<listitem>
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<para>Single external IP address</para>
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</listitem>
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<listitem>
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<para>Connection through Cable Modem, DSL, ISDN, Frame Relay,
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dial-up...</para>
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</listitem>
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</itemizedlist>
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<section>
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<title>Requirements</title>
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<para>Shorewall requires that you have the iproute/iproute2 package
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installed (on RedHat, the package is called <emphasis>iproute</emphasis>).
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You can tell if this package is installed by the presence of an
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<emphasis role="bold">ip</emphasis> program on your firewall system. As
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root, you can use the <quote>which</quote> command to check for this
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program:</para>
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<programlisting>[root@gateway root]# which ip
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/sbin/ip
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[root@gateway root]#</programlisting>
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</section>
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<section>
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<title>Before you start</title>
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<para>I recommend that you read through the guide first to familiarize
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yourself with what's involved then go back through it again making
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your configuration changes.</para>
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<caution>
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<para>If you edit your configuration files on a Windows system, you
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must save them as Unix files if your editor supports that option or
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you must run them through dos2unix before trying to use them.
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Similarly, if you copy a configuration file from your Windows hard
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drive to a floppy disk, you must run dos2unix against the copy before
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using it with Shorewall.</para>
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<simplelist>
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<member><ulink url="http://www.simtel.net/pub/pd/51438.html">Windows
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Version of dos2unix</ulink></member>
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<member><ulink url="http://www.megaloman.com/~hany/software/hd2u/">Linux
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Version of dos2unix</ulink></member>
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</simplelist>
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</caution>
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</section>
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<section>
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<title>Conventions</title>
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<para>Points at which configuration changes are recommended are flagged
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with <inlinegraphic fileref="images/BD21298_.gif" format="GIF" />.</para>
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</section>
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</section>
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<section>
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<title>PPTP/ADSL</title>
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<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /> If you
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have an ADSL Modem and you use PPTP to communicate with a server in that
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modem, you must make the <ulink url="PPTP.htm#PPTP_ADSL">changes
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recommended here</ulink> <emphasis role="underline">in addition to those
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described in the steps below</emphasis>. ADSL with PPTP is most commonly
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found in Europe, notably in Austria.</para>
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</section>
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<section>
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<title>Shorewall Concepts</title>
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<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /> The
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configuration files for Shorewall are contained in the directory
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/etc/shorewall -- for simple setups, you only need to deal with a few of
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these as described in this guide. After you have <ulink url="Install.htm">installed
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Shorewall</ulink>, <emphasis role="bold">download the <ulink
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url="http://www1.shorewall.net/pub/shorewall/Samples/">one-interface
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sample</ulink>, un-tar it (tar -zxvf one-interface.tgz) and and copy the
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files to /etc/shorewall (they will replace files with the same names that
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were placed in /etc/shorewall during Shorewall installation)</emphasis>.</para>
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<para>As each file is introduced, I suggest that you look through the
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actual file on your system -- each file contains detailed configuration
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instructions and default entries.</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 <emphasis>zones</emphasis>. In the one-interface sample
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configuration, only one zone is defined:</para>
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<informaltable>
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<tgroup cols="2">
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<thead>
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<row>
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<entry align="center">Name</entry>
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<entry align="center">Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>net</entry>
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<entry>The Internet</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>Shorewall zones are defined in <ulink url="Documentation.htm#Zones">/etc/shorewall/zones</ulink>.</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 role="bold">fw</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.</para>
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<itemizedlist>
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<listitem>
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<para>You express your default policy for connections from one zone to
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another zone in the <ulink url="Documentation.htm#Policy">/etc/shorewall/policy</ulink>
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file.</para>
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</listitem>
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<listitem>
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<para>You define exceptions to those default policies in the <ulink
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url="Documentation.htm#Rules">/etc/shorewall/rules</ulink> file.</para>
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</listitem>
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</itemizedlist>
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<para>For each connection request entering the firewall, the request is
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first checked against the /etc/shorewall/rules file. If no rule in that
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file matches the connection request then the first policy in
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/etc/shorewall/policy that matches the request is applied. If that policy
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is REJECT or DROP the request is first checked against the rules in
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/etc/shorewall/common if that file exists; otherwise the rules in
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/etc/shorewall/common.def are checked.</para>
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<para>The /etc/shorewall/policy file included with the one-interface
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sample has the following policies:</para>
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<table>
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<title>one-interface sample /etc/shorewall/policy</title>
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<tgroup cols="5">
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<thead>
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<row>
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<entry align="center">SOURCE ZONE</entry>
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<entry align="center">DESTINATION ZONE</entry>
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<entry align="center">POLICY</entry>
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<entry align="center">LOG LEVEL</entry>
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<entry align="center">LIMIT:BURST</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>fw</entry>
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<entry>net</entry>
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<entry>ACCEPT</entry>
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<entry></entry>
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<entry></entry>
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</row>
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<row>
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<entry>net</entry>
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<entry>all</entry>
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<entry>DROP</entry>
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<entry>info</entry>
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<entry></entry>
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</row>
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<row>
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<entry>all</entry>
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<entry>all</entry>
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<entry>REJECT</entry>
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<entry>info</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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</table>
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<para>The above policy will:</para>
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<orderedlist>
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<listitem>
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<para>allow all connection requests from the firewall to the 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 your
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firewall</para>
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</listitem>
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<listitem>
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<para>reject all other connection requests (Shorewall requires this
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catchall policy).</para>
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</listitem>
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</orderedlist>
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<para>At this point, edit your /etc/shorewall/policy and make any changes
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that you wish.</para>
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</section>
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<section>
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<title>External Interface</title>
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<para>The firewall has a single network interface. Where Internet
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connectivity is through a cable or DSL <quote>Modem</quote>, the
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<emphasis>External Interface</emphasis> will be the ethernet adapter (<emphasis
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role="bold">eth0</emphasis>) that is connected to that <quote>Modem</quote>
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<emphasis role="underline">unless</emphasis> you connect via
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<emphasis>Point-to-Point Protocol over Ethernet</emphasis> (PPPoE) or
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<emphasis>Point-to-Point Tunneling Protocol</emphasis> (PPTP) in which
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case the External Interface will be a <emphasis role="bold">ppp0</emphasis>.
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If you connect via a regular modem, your External Interface will also be
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<emphasis role="bold">ppp0</emphasis>. If you connect using ISDN, your
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external interface will be <emphasis role="bold">ippp0</emphasis>.</para>
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<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /> The
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Shorewall one-interface sample configuration assumes that the external
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interface is <emphasis role="bold">eth0</emphasis>. If your configuration
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is different, you will have to modify the sample /etc/shorewall/interfaces
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file accordingly. While you are there, you may wish to review the list of
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options that are specified for the interface. Some hints:</para>
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<tip>
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<para>If your external interface is <emphasis role="bold">ppp0</emphasis>
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or <emphasis role="bold">ippp0</emphasis>, you can replace the
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<quote>detect</quote> in the second column with <quote>-</quote>.</para>
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</tip>
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<tip>
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<para>If your external interface is <emphasis role="bold">ppp0</emphasis>
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or <emphasis role="bold">ippp0</emphasis> or if you have a static IP
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address, you can remove <quote>dhcp</quote> from the option list.</para>
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</tip>
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<tip>
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<para>If you specify <emphasis>norfc1918</emphasis> for your external
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interface, you will want to check the <ulink url="errata.htm">Shorewall
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Errata</ulink> periodically for updates to the /etc/shorewall/rfc1918
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file. Alternatively, you can <ulink url="myfiles.htm#RFC1918">strip down
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your /etc/shorewall/rfc1918 files as I do</ulink>.</para>
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</tip>
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</section>
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<section>
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<title>IP Addresses</title>
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<para>RFC 1918 reserves several <emphasis>Private</emphasis> IP address
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ranges for use in private networks:</para>
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<programlisting>10.0.0.0 - 10.255.255.255
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172.16.0.0 - 172.31.255.255
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192.168.0.0 - 192.168.255.255</programlisting>
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<para>These addresses are sometimes referred to as <emphasis>non-routable</emphasis>
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because the Internet backbone routers will not forward a packet whose
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destination address is reserved by RFC 1918. In some cases though, ISPs
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are assigning these addresses then using <emphasis>Network Address
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Translation</emphasis> to rewrite packet headers when forwarding to/from
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the internet.</para>
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<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /> Before
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starting Shorewall, you should look at the IP address of your external
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interface and if it is one of the above ranges, you should remove the
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<quote>norfc1918</quote> option from the entry in
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/etc/shorewall/interfaces.</para>
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</section>
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<section>
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<title>Enabling other Connections</title>
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<para>If you wish to enable connections from the internet to your
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firewall, the general format is:</para>
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<table>
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<title>/etc/shorewall/rules</title>
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<tgroup cols="7">
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<thead>
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<row>
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<entry align="center">ACTION</entry>
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<entry align="center">SOURCE</entry>
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<entry align="center">DESTINATION</entry>
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<entry align="center">PROTOCOL</entry>
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<entry align="center">PORT</entry>
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<entry align="center">SOURCE PORT</entry>
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<entry align="center">ORIGINAL ADDRESS</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>ACCEPT</entry>
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<entry>net</entry>
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<entry>fw</entry>
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<entry><<emphasis>protocol</emphasis>></entry>
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<entry><<emphasis>port</emphasis>></entry>
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<entry></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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</table>
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<example>
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<title>You want to run a Web Server and a POP3 Server on your firewall
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system:</title>
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<para><table><title>/etc/shorewall/rules</title><tgroup cols="7"><thead><row><entry
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align="center">ACTION</entry><entry align="center">SOURCE</entry><entry
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align="center">DESTINATION</entry><entry align="center">PROTOCOL</entry><entry
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align="center">PORT</entry><entry align="center">SOURCE PORT</entry><entry
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align="center">ORIGINAL ADDRESS</entry></row></thead><tbody><row><entry>ACCEPT</entry><entry>net</entry><entry>fw</entry><entry>tcp</entry><entry>80</entry><entry></entry><entry></entry></row><row><entry>ACCEPT</entry><entry>net</entry><entry>fw</entry><entry>tcp</entry><entry>110</entry><entry></entry><entry></entry></row></tbody></tgroup></table></para>
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</example>
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<para>If you don't know what port and protocol a particular
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application uses, see <ulink url="ports.htm">here</ulink>.</para>
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<important>
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<para>I don't recommend enabling telnet to/from the internet because
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it uses clear text (even for login!). If you want shell access to your
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firewall from the internet, use SSH:</para>
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<table>
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<title>/etc/shorewall/rules</title>
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<tgroup cols="7">
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<thead>
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<row>
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<entry align="center">ACTION</entry>
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<entry align="center">SOURCE</entry>
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<entry align="center">DESTINATION</entry>
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<entry align="center">PROTOCOL</entry>
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<entry align="center">PORT</entry>
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<entry align="center">SOURCE PORT</entry>
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<entry align="center">ORIGINAL ADDRESS</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>ACCEPT</entry>
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<entry>net</entry>
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<entry>fw</entry>
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<entry>tcp</entry>
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<entry>22</entry>
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<entry></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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</table>
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</important>
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<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /> At this
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point, edit /etc/shorewall/rules to add other connections as desired.</para>
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</section>
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<section>
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<title>Starting and Stopping Your Firewall</title>
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<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /> The
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<ulink url="Install.htm">installation procedure</ulink> configures your
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system to start Shorewall at system boot but beginning with Shorewall
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version 1.3.9 startup is disabled so that your system won't try to
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start Shorewall before configuration is complete. Once you have completed
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configuration of your firewall, you can enable Shorewall startup by
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removing the file /etc/shorewall/startup_disabled.</para>
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<important>
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<para><emphasis role="bold">Users of the .deb package must edit
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/etc/default/shorewall and set <quote>startup=1</quote>.</emphasis></para>
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</important>
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<para>The firewall is started using the <quote>shorewall start</quote>
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command and stopped using <quote>shorewall stop</quote>. When the firewall
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is stopped, routing is enabled on those hosts that have an entry in <ulink
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url="Documentation.htm#Routestopped">/etc/shorewall/routestopped</ulink>.
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A running firewall may be restarted using the <quote>shorewall restart</quote>
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command. If you want to totally remove any trace of Shorewall from your
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Netfilter configuration, use <quote>shorewall clear</quote>.</para>
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<warning>
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<para>If you are connected to your firewall from the internet, do not
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issue a <quote>shorewall stop</quote> command unless you have added an
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entry for the IP address that you are connected from to <ulink
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url="Documentation.htm#Routestopped">/etc/shorewall/routestopped</ulink>.
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Also, I don't recommend using <quote>shorewall restart</quote>; it
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is better to create an <emphasis><ulink
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url="configuration_file_basics.htm#Configs">alternate configuration</ulink></emphasis>
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and test it using the <ulink url="starting_and_stopping_shorewall.htm"><quote>shorewall
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try</quote> command</ulink>.</para>
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</warning>
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</section>
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<section>
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<title>Additional Recommended Reading</title>
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<para>I highly recommend that you review the <ulink
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url="configuration_file_basics.htm">Common Configuration File Features
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page</ulink> -- it contains helpful tips about Shorewall features than
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make administering your firewall easier.</para>
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</section>
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<appendix>
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<title>Revision History</title>
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<para><revhistory><revision><revnumber>1.2</revnumber><date>2003-12-30</date><authorinitials>TE</authorinitials><revremark>Add
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tip about /etc/shorewall/rfc1918 updates.</revremark></revision><revision><revnumber>1.1</revnumber><date>2003-11-15</date><authorinitials>TE</authorinitials><revremark>Initial
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Docbook Conversion</revremark></revision></revhistory></para>
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</appendix>
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</article> |