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870 lines
42 KiB
XML
870 lines
42 KiB
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<?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="three-interface">
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<!--$Id$-->
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<articleinfo>
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<title>Three-Interface 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>2004-06-11</pubdate>
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<copyright>
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<year>2002-2004</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 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 a Linux system as a firewall for a small network with DMZ
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is a fairly straight-forward task if you understand the basics and follow
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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 more popular configurations:</para>
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<itemizedlist>
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<listitem>
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<para>Linux system used as a firewall/router for a small local
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network.</para>
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</listitem>
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<listitem>
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<para>Single public IP address.</para>
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<note>
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<para>If you have more than one public IP address, this is not the
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guide you want -- see the <ulink url="shorewall_setup_guide.htm">Shorewall
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Setup Guide</ulink> instead.</para>
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</note>
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</listitem>
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<listitem>
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<para>DMZ connected to a separate ethernet interface.</para>
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</listitem>
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<listitem>
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<para>Connection through DSL, Cable Modem, ISDN, Frame Relay, dial-up,
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...</para>
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</listitem>
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</itemizedlist>
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<para>Here is a schematic of a typical installation.</para>
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<figure>
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<title>schematic of a typical installation</title>
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<mediaobject>
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<imageobject>
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<imagedata align="center" fileref="images/dmz1.png" format="PNG" />
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</imageobject>
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</mediaobject>
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</figure>
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<section>
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<title>Requirements</title>
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<para>Shorewall requires that you have the <command>iproute</command>/<command>iproute2</command>
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package installed (on <trademark>RedHat</trademark>, the package is
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called <command>iproute</command>). You can tell if this package is
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installed by the presence of an <command>ip</command> program on your
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firewall system. As <systemitem class="username">root</systemitem>, you
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can use the <command>which</command> command to check for this program:</para>
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<programlisting>[root@gateway root]# <command>which ip</command>
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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 first read through the guide 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
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<trademark>Windows</trademark> system, you must save them as
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<trademark>Unix</trademark> files if your editor supports that option
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or you must run them through <command>dos2unix</command> before trying
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to use them. Similarly, if you copy a configuration file from your
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<trademark>Windows</trademark> hard drive to a floppy disk, you must
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run <command>dos2unix</command> against the copy before using it with
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Shorewall.</para>
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<itemizedlist>
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<listitem>
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<para><ulink url="http://www.simtel.net/pub/pd/51438.html">Windows
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Version of dos2unix</ulink></para>
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</listitem>
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<listitem>
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<para><ulink url="http://www.megaloman.com/%7Ehany/software/hd2u/">Linux
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Version of dos2unix</ulink></para>
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</listitem>
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</itemizedlist>
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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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<para>Configuration notes that are unique to LEAF/Bering are marked with
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<inlinegraphic fileref="images/leaflogo.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" /></para>
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<para>If you have an ADSL Modem and you use PPTP to communicate with a
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server in that modem, you must make the <ulink url="PPTP.htm#PPTP_ADSL">changes
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recommended here</ulink> in addition to those detailed below. ADSL with
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PPTP is most commonly 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" /></para>
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<para>The configuration files for Shorewall are contained in the directory
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<filename>/etc/shorewall</filename> -- for simple setups, you will only
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need to deal with a few of these as described in this guide.<warning><para><emphasis
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role="bold">Note to Debian Users</emphasis></para><para>If you install
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using the .deb, you will find that your <filename class="directory">/etc/shorewall</filename>
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directory is empty. This is intentional. The released configuration file
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skeletons may be found on your system in the directory <filename
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class="directory">/usr/share/doc/shorewall/default-config</filename>.
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Simply copy the files you need from that directory to <filename
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class="directory">/etc/shorewall</filename> and modify the copies.</para><para>Note
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that you must copy <filename class="directory">/usr/share/doc/shorewall/default-config/shorewall.conf</filename>
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and /usr/share/doc/shorewall/default-config/modules to /etc/shorewall even
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if you do not modify those files.</para></warning></para>
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<para>After you have installed Shorewall, download the three-interface
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sample, un-tar it (<command>tar <option>-zxvf</option>
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<filename>three-interfaces.tgz</filename></command>) and and copy the
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files to <filename>/etc/shorewall</filename> (the files will replace files
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with the same names that were placed in <filename>/etc/shorewall</filename>
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when Shorewall was installed).</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 zones. In the three-interface sample configuration, the following
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zone names are used:</para>
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<informaltable>
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<tgroup align="left" cols="2">
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<thead valign="middle">
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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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<row>
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<entry>loc</entry>
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<entry>Your Local Network</entry>
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</row>
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<row>
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<entry>dmz</entry>
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<entry>Demilitarized Zone</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>Zone names are defined in <filename>/etc/shorewall/zones</filename>.</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 <varname>fw</varname>.</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 <filename>/etc/shorewall/policy</filename> 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
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<filename>/etc/shorewall/rules</filename> 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 <filename>/etc/shorewall/rules</filename> file.
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If no rule in that file matches the connection request then the first
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policy in <filename>/etc/shorewall/policy</filename> that matches the
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request is applied. If there is a <ulink
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url="shorewall_extension_scripts.htm">comon action</ulink> defined for the
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policy in <filename>/etc/shorewall/actions</filename> or
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<filename>/usr/share/shorewall/actions.std</filename> then that action is
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peformed before the action is applied.</para>
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<para>The <filename>/etc/shorewall/policy</filename> file included with
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the three-interface sample has the following policies:</para>
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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>
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<important>
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<para>In the three-interface sample, the line below is included but
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commented out. If you want your firewall system to have full access to
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servers on the internet, uncomment that line.</para>
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<programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
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fw net ACCEPT</programlisting>
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</important>
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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 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 your
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firewall or local network</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.</para>
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</listitem>
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</orderedlist>
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<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
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<para>At this point, edit your <filename>/etc/shorewall/policy</filename>
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file and make any changes that you wish.</para>
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</section>
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<section>
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<title>Network Interfaces</title>
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<figure>
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<title>DMZ</title>
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<mediaobject>
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<imageobject>
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<imagedata align="center" fileref="images/dmz1.png" format="PNG" />
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</imageobject>
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</mediaobject>
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</figure>
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<para>The firewall has three network interfaces. Where Internet
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connectivity is through a cable or DSL <quote>Modem</quote>, the External
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Interface will be the ethernet adapter that is connected to that
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<quote>Modem</quote> (e.g., <filename class="devicefile">eth0</filename>)
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unless you connect via <emphasis>Point-to-Point Protocol</emphasis> over
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Ethernet (PPPoE) or <emphasis>Point-to-Point Tunneling Protocol</emphasis>
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(PPTP) in which case the External Interface will be a <literal>ppp</literal>
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interface (e.g., <filename class="devicefile">ppp0</filename>). If you
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connect via a regular modem, your External Interface will also be
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<filename class="devicefile">ppp0</filename>. If you connect using ISDN,
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you external interface will be <filename class="devicefile">ippp0</filename>.</para>
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<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
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<para>If your external interface is <filename class="devicefile">ppp0</filename>
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or <filename class="devicefile">ippp0</filename> then you will want to set
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<varname>CLAMPMSS=yes</varname> in <filename>/etc/shorewall/shorewall.conf</filename>.</para>
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<para>Your Local Interface will be an ethernet adapter (<filename
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class="devicefile">eth0</filename>, <filename class="devicefile">eth1</filename>
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or <filename class="devicefile">eth2</filename>) and will be connected to
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a hub or switch. Your local computers will be connected to the same switch
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(note: If you have only a single local system, you can connect the
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firewall directly to the computer using a cross-over cable).</para>
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<para>Your DMZ Interface will also be an ethernet adapter (<filename
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class="devicefile">eth0</filename>, <filename class="devicefile">eth1</filename>
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or <filename class="devicefile">eth2</filename>) and will be connected to
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a hub or switch. Your DMZ computers will be connected to the same switch
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(note: If you have only a single DMZ system, you can connect the firewall
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directly to the computer using a cross-over cable).</para>
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<caution>
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<para>Do not connect the internal and external interface to the same hub
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or switch except for testing AND you are running Shorewall version 1.4.7
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or later. When using these recent versions, you can test using this kind
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of configuration if you specify the arp_filter option in
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<filename>/etc/shorewall/interfaces</filename> for all interfaces
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connected to the common hub/switch. Using such a setup with a production
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firewall is strongly recommended against.</para>
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</caution>
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<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
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<para>The Shorewall three-interface sample configuration assumes that the
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external interface is <filename class="devicefile">eth0</filename>, the
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local interface is <filename class="devicefile">eth1</filename> and the
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DMZ interface is <filename class="devicefile">eth2</filename>. If your
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|
configuration is different, you will have to modify the sample
|
||
|
|
<filename>/etc/shorewall/interfaces</filename> file accordingly. While you
|
||
|
|
are there, you may wish to review the list of options that are specified
|
||
|
|
for the interfaces. Some hints:</para>
|
||
|
|
|
||
|
|
<tip>
|
||
|
|
<para>If your external interface is <filename class="devicefile">ppp0</filename>
|
||
|
|
or <filename class="devicefile">ippp0</filename>, you can replace the
|
||
|
|
<quote>detect</quote> in the second column with <quote>-</quote>
|
||
|
|
(without the quotes).</para>
|
||
|
|
</tip>
|
||
|
|
|
||
|
|
<tip>
|
||
|
|
<para>If your external interface is <filename class="devicefile">ppp0</filename>
|
||
|
|
or <filename class="devicefile">ippp0</filename> or if you have a static
|
||
|
|
IP address, you can remove <quote>dhcp</quote> from the option list.</para>
|
||
|
|
</tip>
|
||
|
|
|
||
|
|
<tip>
|
||
|
|
<para>If you specify <emphasis>norfc1918</emphasis> for your external
|
||
|
|
interface, you will want to check the <ulink url="errata.htm">Shorewall
|
||
|
|
Errata</ulink> periodically for updates to the <filename>/usr/share/shorewall/rfc1918
|
||
|
|
file</filename>. Alternatively, you can copy <filename>/usr/share/shorewall/rfc1918</filename>
|
||
|
|
to <filename>/etc/shorewall/rfc1918</filename> then <ulink
|
||
|
|
url="myfiles.htm#RFC1918">strip down your <filename>/etc/shorewall/rfc1918</filename>
|
||
|
|
file as I do</ulink>.</para>
|
||
|
|
</tip>
|
||
|
|
</section>
|
||
|
|
|
||
|
|
<section>
|
||
|
|
<title>IP Addresses</title>
|
||
|
|
|
||
|
|
<para>Before going further, we should say a few words about Internet
|
||
|
|
Protocol (IP) addresses. Normally, your ISP will assign you a single
|
||
|
|
Public IP address. This address may be assigned via the Dynamic Host
|
||
|
|
Configuration Protocol (DHCP) or as part of establishing your connection
|
||
|
|
when you dial in (standard modem) or establish your PPP connection. In
|
||
|
|
rare cases, your ISP may assign you a static IP address; that means that
|
||
|
|
you configure your firewall's external interface to use that address
|
||
|
|
permanently. Regardless of how the address is assigned, it will be shared
|
||
|
|
by all of your systems when you access the Internet. You will have to
|
||
|
|
assign your own addresses for your internal network (the local and DMZ
|
||
|
|
Interfaces on your firewall plus your other computers). RFC 1918 reserves
|
||
|
|
several Private IP address ranges for this purpose:</para>
|
||
|
|
|
||
|
|
<programlisting>10.0.0.0 - 10.255.255.255
|
||
|
|
172.16.0.0 - 172.31.255.255
|
||
|
|
192.168.0.0 - 192.168.255.255</programlisting>
|
||
|
|
|
||
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
||
|
|
|
||
|
|
<para>Before starting Shorewall, you should look at the IP address of your
|
||
|
|
external interface and if it is one of the above ranges, you should remove
|
||
|
|
the <varname>norfc1918</varname> option from the external interface's
|
||
|
|
entry in <filename>/etc/shorewall/interfaces</filename>.</para>
|
||
|
|
|
||
|
|
<para>You will want to assign your local addresses from one sub-network or
|
||
|
|
subnet and your DMZ addresses from another subnet. For our purposes, we
|
||
|
|
can consider a subnet to consists of a range of addresses <systemitem
|
||
|
|
class="ipaddress">x.y.z.0</systemitem> - <systemitem class="ipaddress">x.y.z.255</systemitem>.
|
||
|
|
Such a subnet will have a Subnet Mask of <systemitem class="netmask">255.255.255.0</systemitem>.
|
||
|
|
The address <systemitem class="ipaddress">x.y.z.0</systemitem> is reserved
|
||
|
|
as the Subnet Address and <systemitem class="netmask">x.y.z.255</systemitem>
|
||
|
|
is reserved as the Subnet Broadcast Address. In Shorewall, a subnet is
|
||
|
|
described using Classless InterDomain Routing (CIDR) notation with
|
||
|
|
consists of the subnet address followed by <varname>/24</varname>. The
|
||
|
|
<varname>24</varname> refers to the number of consecutive <quote>1</quote>
|
||
|
|
bits from the left of the subnet mask.</para>
|
||
|
|
|
||
|
|
<table>
|
||
|
|
<title>Example sub-network</title>
|
||
|
|
|
||
|
|
<tgroup cols="2">
|
||
|
|
<colspec align="left" />
|
||
|
|
|
||
|
|
<tbody>
|
||
|
|
<row>
|
||
|
|
<entry>Range:</entry>
|
||
|
|
|
||
|
|
<entry><systemitem class="ipaddress">10.10.10.0</systemitem> -
|
||
|
|
<systemitem class="ipaddress">10.10.10.255</systemitem></entry>
|
||
|
|
</row>
|
||
|
|
|
||
|
|
<row>
|
||
|
|
<entry>Subnet Address:</entry>
|
||
|
|
|
||
|
|
<entry><systemitem class="ipaddress">10.10.10.0</systemitem></entry>
|
||
|
|
</row>
|
||
|
|
|
||
|
|
<row>
|
||
|
|
<entry>Broadcast Address:</entry>
|
||
|
|
|
||
|
|
<entry><systemitem class="ipaddress">10.10.10.255</systemitem></entry>
|
||
|
|
</row>
|
||
|
|
|
||
|
|
<row>
|
||
|
|
<entry>CIDR Notation:</entry>
|
||
|
|
|
||
|
|
<entry><systemitem class="ipaddress">10.10.10.0/24</systemitem></entry>
|
||
|
|
</row>
|
||
|
|
</tbody>
|
||
|
|
</tgroup>
|
||
|
|
</table>
|
||
|
|
|
||
|
|
<para>It is conventional to assign the internal interface either the first
|
||
|
|
usable address in the subnet (<systemitem class="ipaddress">10.10.10.1</systemitem>
|
||
|
|
in the above example) or the last usable address (<systemitem
|
||
|
|
class="ipaddress">10.10.10.254</systemitem>).</para>
|
||
|
|
|
||
|
|
<para>One of the purposes of subnetting is to allow all computers in the
|
||
|
|
subnet to understand which other computers can be communicated with
|
||
|
|
directly. To communicate with systems outside of the subnetwork, systems
|
||
|
|
send packets through a gateway (router).</para>
|
||
|
|
|
||
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
||
|
|
|
||
|
|
<para>Your local computers (Local Computers 1 & 2) should be
|
||
|
|
configured with their default gateway set to the IP address of the
|
||
|
|
firewall's internal interface and your DMZ computers (DMZ Computers 1
|
||
|
|
& 2) should be configured with their default gateway set to the IP
|
||
|
|
address of the firewall's DMZ interface.</para>
|
||
|
|
|
||
|
|
<para>The foregoing short discussion barely scratches the surface
|
||
|
|
regarding subnetting and routing. If you are interested in learning more
|
||
|
|
about IP addressing and routing, I highly recommend <quote>IP
|
||
|
|
Fundamentals: What Everyone Needs to Know about Addressing & Routing</quote>,
|
||
|
|
Thomas A. Maufer, Prentice-Hall, 1999, ISBN 0-13-975483-0.</para>
|
||
|
|
|
||
|
|
<para>The remainder of this quide will assume that you have configured
|
||
|
|
your network as shown here:</para>
|
||
|
|
|
||
|
|
<figure>
|
||
|
|
<title>DMZ</title>
|
||
|
|
|
||
|
|
<mediaobject>
|
||
|
|
<imageobject>
|
||
|
|
<imagedata fileref="images/dmz2.png" />
|
||
|
|
</imageobject>
|
||
|
|
|
||
|
|
<caption>
|
||
|
|
<para>The default gateway for the DMZ computers would be <systemitem
|
||
|
|
class="ipaddress">10.10.11.254</systemitem> and the default gateway
|
||
|
|
for the Local computers would be <systemitem class="ipaddress">10.10.10.254</systemitem>.</para>
|
||
|
|
|
||
|
|
<warning>
|
||
|
|
<para>Your ISP might assign your external interface an RFC 1918
|
||
|
|
address. If that address is in the <systemitem class="ipaddress">10.10.10.0/24</systemitem>
|
||
|
|
subnet then you will need to select a DIFFERENT RFC 1918 subnet
|
||
|
|
for your local network and if it is in the <systemitem
|
||
|
|
class="ipaddress">10.10.11.0/24</systemitem> subnet then you will
|
||
|
|
need to select a different RFC 1918 subnet for your DMZ.</para>
|
||
|
|
</warning>
|
||
|
|
</caption>
|
||
|
|
</mediaobject>
|
||
|
|
</figure>
|
||
|
|
</section>
|
||
|
|
|
||
|
|
<section>
|
||
|
|
<title>IP Masquerading (SNAT)</title>
|
||
|
|
|
||
|
|
<para>The addresses reserved by RFC 1918 are sometimes referred to as
|
||
|
|
non-routable because the Internet backbone routers don't forward
|
||
|
|
packets which have an RFC-1918 destination address. When one of your local
|
||
|
|
systems (let's assume local computer 1) sends a connection request to
|
||
|
|
an internet host, the firewall must perform Network Address Translation
|
||
|
|
(NAT). The firewall rewrites the source address in the packet to be the
|
||
|
|
address of the firewall's external interface; in other words, the
|
||
|
|
firewall makes it look as if the firewall itself is initiating the
|
||
|
|
connection. This is necessary so that the destination host will be able to
|
||
|
|
route return packets back to the firewall (remember that packets whose
|
||
|
|
destination address is reserved by RFC 1918 can't be routed accross
|
||
|
|
the internet). When the firewall receives a return packet, it rewrites the
|
||
|
|
destination address back to 10.10.10.1 and forwards the packet on to local
|
||
|
|
computer 1.</para>
|
||
|
|
|
||
|
|
<para>On Linux systems, the above process is often referred to as IP
|
||
|
|
Masquerading and you will also see the term Source Network Address
|
||
|
|
Translation (SNAT) used. Shorewall follows the convention used with
|
||
|
|
Netfilter: <itemizedlist><listitem><para><emphasis>Masquerade</emphasis>
|
||
|
|
describes the case where you let your firewall system automatically detect
|
||
|
|
the external interface address.</para></listitem><listitem><para><emphasis>SNAT</emphasis>
|
||
|
|
refers to the case when you explicitly specify the source address that you
|
||
|
|
want outbound packets from your local network to use.</para></listitem></itemizedlist>
|
||
|
|
In Shorewall, both Masquerading and SNAT are configured with entries in
|
||
|
|
the <filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
|
||
|
|
file.</para>
|
||
|
|
|
||
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
||
|
|
|
||
|
|
<para>If your external firewall interface is <filename class="devicefile">eth0</filename>,
|
||
|
|
your local interface <filename class="devicefile">eth1</filename> and your
|
||
|
|
DMZ interface is <filename class="devicefile">eth2</filename> then you do
|
||
|
|
not need to modify the file provided with the sample. Otherwise, edit
|
||
|
|
<filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
|
||
|
|
and change it to match your configuration.</para>
|
||
|
|
|
||
|
|
<para>If, despite all advice to the contrary, you are using this guide and
|
||
|
|
want to use one-to-one NAT or Proxy ARP for your DMZ, remove the entry for
|
||
|
|
eth2 from <filename>/etc/shorewall/masq</filename>.</para>
|
||
|
|
|
||
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
||
|
|
|
||
|
|
<para>If your external IP is static, you can enter it in the third column
|
||
|
|
in the <filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
|
||
|
|
entry if you like although your firewall will work fine if you leave that
|
||
|
|
column empty. Entering your static IP in column 3 makes processing
|
||
|
|
outgoing packets a little more efficient.</para>
|
||
|
|
|
||
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
||
|
|
|
||
|
|
<para>If you are using the Debian package, please check your
|
||
|
|
<filename>shorewall.conf</filename> file to ensure that the following are
|
||
|
|
set correctly; if they are not, change them appropriately:
|
||
|
|
<itemizedlist><listitem><para><varname>NAT_ENABLED=Yes</varname>
|
||
|
|
(Shorewall versions earlier than 1.4.6)</para></listitem><listitem><para><varname>IP_FORWARDING=On</varname></para></listitem></itemizedlist></para>
|
||
|
|
</section>
|
||
|
|
|
||
|
|
<section>
|
||
|
|
<title>Port Forwarding (DNAT)</title>
|
||
|
|
|
||
|
|
<para>One of your goals will be to run one or more servers on your DMZ
|
||
|
|
computers. Because these computers have RFC-1918 addresses, it is not
|
||
|
|
possible for clients on the Internet to connect directly to them. It is
|
||
|
|
rather necessary for those clients to address their connection requests to
|
||
|
|
your firewall who rewrites the destination address to the address of your
|
||
|
|
server and forwards the packet to that server. When your server responds,
|
||
|
|
the firewall automatically performs SNAT to rewrite the source address in
|
||
|
|
the response.</para>
|
||
|
|
|
||
|
|
<para>The above process is called <emphasis>Port Forwarding</emphasis> or
|
||
|
|
<emphasis>Destination Network Address Translation</emphasis> (DNAT). You
|
||
|
|
configure port forwarding using DNAT rules in the <filename
|
||
|
|
class="directory">/etc/shorewall/</filename><filename>rules</filename>
|
||
|
|
file.</para>
|
||
|
|
|
||
|
|
<para>The general form of a simple port forwarding rule in <filename
|
||
|
|
class="directory">/etc/shorewall/</filename><filename>rules</filename> is:
|
||
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
DNAT net dmz:<emphasis><server local IP address></emphasis>[:<emphasis><server port></emphasis>] <emphasis><protocol></emphasis> <emphasis><port></emphasis></programlisting>
|
||
|
|
If you don't specify the <emphasis><varname><server port></varname></emphasis>,
|
||
|
|
it is assumed to be the same as <emphasis><varname><port></varname></emphasis>.</para>
|
||
|
|
|
||
|
|
<example>
|
||
|
|
<title>You run a Web Server on DMZ Computer 2 and you want to forward
|
||
|
|
incoming TCP port 80 to that system</title>
|
||
|
|
|
||
|
|
<para><programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
DNAT net dmz:10.10.11.2 tcp 80
|
||
|
|
ACCEPT loc dmz:10.10.11.2 tcp 80</programlisting><itemizedlist><listitem><para>Entry
|
||
|
|
1 forwards port 80 from the Internet.</para></listitem><listitem><para>Entry
|
||
|
|
2 allows connections from the local network.</para></listitem></itemizedlist>
|
||
|
|
Several important points to keep in mind:<itemizedlist><listitem><para>When
|
||
|
|
you are connecting to your server from your local systems, you must use
|
||
|
|
the server's internal IP address (<systemitem class="ipaddress">10.10.11.2</systemitem>).</para></listitem><listitem><para>Many
|
||
|
|
ISPs block incoming connection requests to port 80. If you have problems
|
||
|
|
connecting to your web server, try the following rule and try connecting
|
||
|
|
to port 5000 (e.g., connect to <literal>http://w.x.y.z:5000 where
|
||
|
|
w.x.y.z</literal> is your external IP).<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE
|
||
|
|
# PORT(S)
|
||
|
|
DNAT net dmz:10.10.11.2:80 tcp 80 5000</programlisting></para></listitem><listitem><para>If
|
||
|
|
you want to be able to access your server from the local network using
|
||
|
|
your external address, then if you have a static external IP you can
|
||
|
|
replace the loc->dmz rule above with:<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL
|
||
|
|
# PORT(S) DEST
|
||
|
|
DNAT loc dmz:10.10.11.2 tcp 80 - <emphasis><external IP></emphasis></programlisting>If
|
||
|
|
you have a dynamic IP then you must ensure that your external interface
|
||
|
|
is up before starting Shorewall and you must take steps as follows
|
||
|
|
(assume that your external interface is <filename class="devicefile">eth0</filename>):<orderedlist><listitem><para>Include
|
||
|
|
the following in /etc/shorewall/params:</para><para><command>ETH0_IP=$(find_interface_address
|
||
|
|
eth0)</command></para></listitem><listitem><para>Make your
|
||
|
|
<literal>loc->dmz</literal> rule: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL
|
||
|
|
# PORT(S) DEST
|
||
|
|
DNAT loc dmz:10.10.11.2 tcp 80 - $ETH0_IP</programlisting></para></listitem></orderedlist></para></listitem><listitem><para>If
|
||
|
|
you want to access your server from the DMZ using your external IP
|
||
|
|
address, see <ulink url="FAQ.htm#faq2a">FAQ 2a</ulink>.</para></listitem></itemizedlist></para>
|
||
|
|
</example>
|
||
|
|
|
||
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
||
|
|
|
||
|
|
<para>At this point, add the DNAT and ACCEPT rules for your servers.</para>
|
||
|
|
</section>
|
||
|
|
|
||
|
|
<section>
|
||
|
|
<title>Domain Name Server (DNS)</title>
|
||
|
|
|
||
|
|
<para>Normally, when you connect to your ISP, as part of getting an IP
|
||
|
|
address your firewall's <emphasis>Domain Name Service</emphasis> (DNS)
|
||
|
|
resolver will be automatically configured (e.g., the <filename>/etc/resolv.conf</filename>
|
||
|
|
file will be written). Alternatively, your ISP may have given you the IP
|
||
|
|
address of a pair of DNS name servers for you to manually configure as
|
||
|
|
your primary and secondary name servers. It is your responsibility to
|
||
|
|
configure the resolver in your internal systems. You can take one of two
|
||
|
|
approaches: <itemizedlist><listitem><para>You can configure your internal
|
||
|
|
systems to use your ISP's name servers. If your ISP gave you the
|
||
|
|
addresses of their servers or if those addresses are available on their
|
||
|
|
web site, you can configure your internal systems to use those addresses.
|
||
|
|
If that information isn't available, look in <filename>/etc/resolv.conf</filename>
|
||
|
|
on your firewall system -- the name servers are given in <quote>nameserver</quote>
|
||
|
|
records in that file.</para></listitem><listitem><para><inlinegraphic
|
||
|
|
fileref="images/BD21298_.gif" format="GIF" /></para><para>You can
|
||
|
|
configure a <emphasis>Caching Name Server</emphasis> on your firewall or
|
||
|
|
in your DMZ. <trademark>Red Hat</trademark> has an RPM for a caching name
|
||
|
|
server (which also requires the '<command>bind</command>' RPM) and
|
||
|
|
for Bering users, there is <filename>dnscache.lrp</filename>. If you take
|
||
|
|
this approach, you configure your internal systems to use the caching name
|
||
|
|
server as their primary (and only) name server. You use the internal IP
|
||
|
|
address of the firewall (<systemitem class="ipaddress">10.10.10.254</systemitem>
|
||
|
|
in the example above) for the name server address if you choose to run the
|
||
|
|
name server on your firewall. To allow your local systems to talk to your
|
||
|
|
caching name server, you must open port 53 (both UDP and TCP) from the
|
||
|
|
local network to the server; you do that by adding the rules in
|
||
|
|
<filename>/etc/shorewall/rules</filename>.</para></listitem></itemizedlist>
|
||
|
|
If you run the name server on the firewall:
|
||
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
AllowDNS loc fw
|
||
|
|
AllowDNS dmz fw </programlisting> Run name server on DMZ
|
||
|
|
computer 1: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
AllowDNS loc dmz:10.10.11.1
|
||
|
|
AllowDNS fw dmz:10.10.11.1 </programlisting></para>
|
||
|
|
|
||
|
|
<para>In the rules shown above, <quote>AllowDNS</quote> is an example of a
|
||
|
|
<emphasis>defined action</emphasis>. Shorewall includes a number of
|
||
|
|
defined actions and <ulink url="User_defined_Actions.html">you can add
|
||
|
|
your own</ulink>. To see the list of actions included with your version of
|
||
|
|
Shorewall, look in the file <filename>/etc/shorewall/actions.std</filename>.
|
||
|
|
Those actions that accept connection requests have names that begin with
|
||
|
|
<quote>Allow</quote>.</para>
|
||
|
|
|
||
|
|
<para>You don't have to use defined actions when coding a rule in
|
||
|
|
<filename>/etc/shorewall/rules</filename>; the generated Netfilter ruleset
|
||
|
|
is slightly more efficient if you code your rules directly rather than
|
||
|
|
using defined actions. The first example above (name server on the
|
||
|
|
firewall) could also have been coded as follows:</para>
|
||
|
|
|
||
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
ACCEPT loc fw tcp 53
|
||
|
|
ACCEPT loc fw udp 53
|
||
|
|
ACCEPT dmz fw tcp 53
|
||
|
|
ACCEPT dmz fw udp 53 </programlisting>
|
||
|
|
|
||
|
|
<para>In cases where Shorewall doesn't include a defined action to
|
||
|
|
meet your needs, you can either define the action yourself or you can
|
||
|
|
simply code the appropriate rules directly.</para>
|
||
|
|
</section>
|
||
|
|
|
||
|
|
<section>
|
||
|
|
<title>Other Connections</title>
|
||
|
|
|
||
|
|
<para>The three-interface sample includes the following rule:
|
||
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
AllowDNS fw net </programlisting>That rule allow DNS access from
|
||
|
|
your firewall and may be removed if you commented out the line in
|
||
|
|
<filename>/etc/shorewall/policy</filename> allowing all connections from
|
||
|
|
the firewall to the Internet.</para>
|
||
|
|
|
||
|
|
<para>The sample also includes: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
AllowSSH loc fw
|
||
|
|
AllowSSH loc dmz </programlisting>Those rules allow you to run
|
||
|
|
an SSH server on your firewall and in each of your DMZ systems and to
|
||
|
|
connect to those servers from your local systems.</para>
|
||
|
|
|
||
|
|
<para>If you wish to enable other connections between your systems, the
|
||
|
|
general format for using a defined action is:
|
||
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
<<emphasis>action</emphasis>> <emphasis><source zone> <destination zone></emphasis></programlisting></para>
|
||
|
|
|
||
|
|
<para>The general format when not using a defined action is:<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
ACCEPT <emphasis><source zone> <destination zone> <protocol> <port> </emphasis></programlisting></para>
|
||
|
|
|
||
|
|
<example>
|
||
|
|
<title>You want to run a publicly-available DNS server on your firewall
|
||
|
|
system</title>
|
||
|
|
|
||
|
|
<para>Using defined actions:</para>
|
||
|
|
|
||
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
AllowDNS net fw</programlisting>
|
||
|
|
|
||
|
|
<para>Not using defined actions:</para>
|
||
|
|
|
||
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
ACCEPT net fw tcp 53
|
||
|
|
ACCEPT net fw udp 53 </programlisting>
|
||
|
|
|
||
|
|
<para>Those rules would of course be in addition to the rules listed
|
||
|
|
above under "If you run the name server on your firewall".</para>
|
||
|
|
</example>
|
||
|
|
|
||
|
|
<para>If you don't know what port and protocol a particular
|
||
|
|
application uses, <ulink url="ports.htm">look here</ulink>.</para>
|
||
|
|
|
||
|
|
<important>
|
||
|
|
<para>I don't recommend enabling telnet to/from the Internet because
|
||
|
|
it uses clear text (even for login!). If you want shell access to your
|
||
|
|
firewall from the Internet, use SSH: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
AllowSSH net fw</programlisting></para>
|
||
|
|
</important>
|
||
|
|
|
||
|
|
<para><inlinegraphic fileref="images/leaflogo.gif" format="GIF" /> Bering
|
||
|
|
users will want to add the following two rules to be compatible with
|
||
|
|
Jacques's Shorewall configuration: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
||
|
|
ACCEPT loc fw udp 53
|
||
|
|
ACCEPT net fw tcp 80 </programlisting><itemizedlist><listitem><para>Entry
|
||
|
|
1 allows the DNS Cache to be used.</para></listitem><listitem><para>Entry
|
||
|
|
2 allows the <quote>weblet</quote> to work.</para></listitem></itemizedlist><inlinegraphic
|
||
|
|
fileref="images/BD21298_.gif" format="GIF" /></para>
|
||
|
|
|
||
|
|
<para>Now modify <filename>/etc/shorewall/rules</filename> to add or
|
||
|
|
remove other connections as required.</para>
|
||
|
|
</section>
|
||
|
|
|
||
|
|
<section>
|
||
|
|
<title>Some Things to Keep in Mind</title>
|
||
|
|
|
||
|
|
<itemizedlist>
|
||
|
|
<listitem>
|
||
|
|
<para><emphasis role="bold">You cannot test your firewall from the
|
||
|
|
inside</emphasis>. Just because you send requests to your firewall
|
||
|
|
external IP address does not mean that the request will be associated
|
||
|
|
with the external interface or the <quote>net</quote> zone. Any
|
||
|
|
traffic that you generate from the local network will be associated
|
||
|
|
with your local interface and will be treated as loc->fw traffic.</para>
|
||
|
|
</listitem>
|
||
|
|
|
||
|
|
<listitem>
|
||
|
|
<para><emphasis role="bold">IP addresses are properties of systems,
|
||
|
|
not of interfaces</emphasis>. It is a mistake to believe that your
|
||
|
|
firewall is able to forward packets just because you can ping the IP
|
||
|
|
address of all of the firewall's interfaces from the local
|
||
|
|
network. The only conclusion you can draw from such pinging success is
|
||
|
|
that the link between the local system and the firewall works and that
|
||
|
|
you probably have the local system's default gateway set
|
||
|
|
correctly.</para>
|
||
|
|
</listitem>
|
||
|
|
|
||
|
|
<listitem>
|
||
|
|
<para><emphasis role="bold">All IP addresses configured on firewall
|
||
|
|
interfaces are in the $FW (fw) zone</emphasis>. If 192.168.1.254 is
|
||
|
|
the IP address of your internal interface then you can write
|
||
|
|
<quote><emphasis role="bold">$FW:192.168.1.254</emphasis></quote> in a
|
||
|
|
rule but you may not write <quote><emphasis role="bold">loc:192.168.1.254</emphasis></quote>.
|
||
|
|
Similarly, it is nonsensical to add 192.168.1.254 to the <emphasis
|
||
|
|
role="bold">loc</emphasis> zone using an entry in
|
||
|
|
<filename>/etc/shorewall/hosts</filename>.</para>
|
||
|
|
</listitem>
|
||
|
|
|
||
|
|
<listitem>
|
||
|
|
<para><emphasis role="bold">Reply packets do NOT automatically follow
|
||
|
|
the reverse path of the one taken by the original request</emphasis>.
|
||
|
|
All packets are routed according to the routing table of the host at
|
||
|
|
each step of the way. This issue commonly comes up when people install
|
||
|
|
a Shorewall firewall parallel to an existing gateway and try to use
|
||
|
|
DNAT through Shorewall without changing the default gateway of the
|
||
|
|
system receiving the forwarded requests. Requests come in through the
|
||
|
|
Shorewall firewall where the destination IP address gets rewritten but
|
||
|
|
replies go out unmodified through the old gateway.</para>
|
||
|
|
</listitem>
|
||
|
|
|
||
|
|
<listitem>
|
||
|
|
<para><emphasis role="bold">Shorewall itself has no notion of inside
|
||
|
|
or outside</emphasis>. These concepts are embodied in how Shorewall is
|
||
|
|
configured.</para>
|
||
|
|
</listitem>
|
||
|
|
</itemizedlist>
|
||
|
|
</section>
|
||
|
|
|
||
|
|
<section>
|
||
|
|
<title>Starting and Stopping Your Firewall</title>
|
||
|
|
|
||
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
||
|
|
|
||
|
|
<para>The <ulink url="Install.htm">installation procedure</ulink>
|
||
|
|
configures your system to start Shorewall at system boot but beginning
|
||
|
|
with Shorewall version 1.3.9 startup is disabled so that your system
|
||
|
|
won't try to start Shorewall before configuration is complete. Once
|
||
|
|
you have completed configuration of your firewall, you can enable
|
||
|
|
Shorewall startup by removing the file <filename>/etc/shorewall/startup_disabled</filename>.
|
||
|
|
<important><para>Users of the <filename>.deb</filename> package must edit
|
||
|
|
<filename>/etc/default/shorewall</filename> and set <varname>startup=1</varname>.</para></important>The
|
||
|
|
firewall is started using the <command>shorewall start</command> command
|
||
|
|
and stopped using <command>shorewall stop</command>. When the firewall is
|
||
|
|
stopped, routing is enabled on those hosts that have an entry in <ulink
|
||
|
|
url="Documentation.htm#Routestopped"><filename>/etc/shorewall/routestopped</filename></ulink>.
|
||
|
|
A running firewall may be restarted using the <command>shorewall restart</command>
|
||
|
|
command. If you want to totally remove any trace of Shorewall from your
|
||
|
|
Netfilter configuration, use <command>shorewall clear</command>.</para>
|
||
|
|
|
||
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
||
|
|
|
||
|
|
<para>The three-interface sample assumes that you want to enable routing
|
||
|
|
to/from <filename class="devicefile">eth1</filename> (your local network)
|
||
|
|
and <filename class="devicefile">eth2</filename> (DMZ) when Shorewall is
|
||
|
|
stopped. If these two interfaces don't connect to your local network
|
||
|
|
and DMZ or if you want to enable a different set of hosts, modify
|
||
|
|
<filename>/etc/shorewall/routestopped</filename> accordingly.
|
||
|
|
<warning><para>If you are connected to your firewall from the Internet, do
|
||
|
|
not issue a <command>shorewall stop</command> command unless you have
|
||
|
|
added an entry for the IP address that you are connected from to <ulink
|
||
|
|
url="Documentation.htm#Routestopped"><filename>/etc/shorewall/routestopped</filename></ulink>.
|
||
|
|
Also, I don't recommend using <command>shorewall restart</command>; it
|
||
|
|
is better to create an <ulink url="configuration_file_basics.htm#Levels">alternate
|
||
|
|
configuration</ulink> and test it using the <ulink
|
||
|
|
url="starting_and_stopping_shorewall.htm"><command>shorewall try</command>
|
||
|
|
command</ulink>.</para></warning></para>
|
||
|
|
</section>
|
||
|
|
|
||
|
|
<section>
|
||
|
|
<title>Additional Recommended Reading</title>
|
||
|
|
|
||
|
|
<para>I highly recommend that you review the <ulink
|
||
|
|
url="configuration_file_basics.htm">Common Configuration File Features</ulink>
|
||
|
|
page -- it contains helpful tips about Shorewall features than make
|
||
|
|
administering your firewall easier.</para>
|
||
|
|
</section>
|
||
|
|
</article>
|