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git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@1922 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb
989 lines
48 KiB
XML
989 lines
48 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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<!-- $Id$ -->
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<article id="two-interface">
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<articleinfo>
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<title>Basic Two-Interface Firewall</title>
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<author>
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<firstname>Tom</firstname>
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<surname>Eastep</surname>
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</author>
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<pubdate>2004-09-06</pubdate>
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<copyright>
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<year>2002</year>
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<year>2003</year>
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<year>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
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License</ulink></quote>.</para>
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</legalnotice>
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</articleinfo>
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<section>
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<title>Introduction</title>
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<para>Setting up a Linux system as a firewall for a small network is a
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fairly straight-forward task if you understand the basics and follow the
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documentation.</para>
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<para>This guide doesn't attempt to acquaint you with all of the features
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of Shorewall. It rather focuses on what is required to configure Shorewall
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in its most common configuration:</para>
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<itemizedlist mark="bullet" spacing="compact">
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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><emphasis role="bold">Single public IP address.</emphasis> If
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you have more than one public IP address, this is not the guide you
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want -- see the <ulink url="shorewall_setup_guide.htm">Shorewall Setup
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Guide</ulink> instead.</para>
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</listitem>
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<listitem>
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<para>Internet 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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<para>Here is a schematic of a typical installation: <figure label="1">
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<title>Common two interface firewall configuration</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="images/basics.png" format="PNG" />
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</imageobject>
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</mediaobject>
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</figure> <tip>
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<title>Shorewall and <trademark>Mandrake</trademark> 9.0+</title>
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<para>If you are running Shorewall under
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<trademark>Mandrake</trademark> 9.0 or later, you can easily configure
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the above setup using the <trademark>Mandrake</trademark>
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<quote>Internet Connection Sharing</quote> applet. From the
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<emphasis><interface>Mandrake Control Center</interface></emphasis>,
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select <quote><guimenuitem>Network</guimenuitem> &
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<guisubmenu>Internet</guisubmenu></quote> then
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<quote><interface>Connection Sharing</interface></quote>.</para>
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<para>Note however, that the Shorewall configuration produced by
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<emphasis>Mandrake Internet Connection Sharing</emphasis> is strange
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and is apt to confuse you if you use the rest of this documentation
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(it has two local zones; <varname>loc</varname> and
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<varname>masq</varname> where <varname>loc</varname> is empty; this
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conflicts with this documentation which assumes a single local zone
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<varname>loc</varname>). We therefore recommend that once you have set
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up this sharing that you uninstall the <trademark>Mandrake</trademark>
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Shorewall RPM and install the one from the <ulink
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url="download.htm">download</ulink> page then follow the instructions
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in this Guide.</para>
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</tip><note>
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<para><emphasis role="bold">The above Shorewall Issue is corrected in
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Mandrake 10.0 and later.</emphasis></para>
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</note> <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. <itemizedlist>
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<listitem>
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<para><ulink
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url="http://www.simtel.net/pub/pd/51438.html"><trademark>Windows</trademark>
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Version of <command>dos2unix</command></ulink></para>
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</listitem>
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<listitem>
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<para><ulink
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url="http://www.megaloman.com/%7Ehany/software/hd2u/">Linux
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Version of <command>dos2unix</command></ulink></para>
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</listitem>
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</itemizedlist></para>
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</caution></para>
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<section>
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<title>System Requirements</title>
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<para>Shorewall requires that you have the
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<command>iproute</command>/<command>iproute2</command> package installed
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(on <trademark>RedHat</trademark>, the package is called
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<command>iproute</command>). You can tell if this package is installed
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by the presence of an <command>ip</command> program on your firewall
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system. As <systemitem class="username">root</systemitem>, you can use
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the <command>which</command> command to check for this program:
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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> I recommend that you first read through
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the guide to familiarize yourself with what's involved then go back
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through it again making your configuration changes.</para>
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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"
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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 <acronym>ADSL</acronym> Modem and you use
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<acronym>PPTP</acronym> to communicate with a server in that modem, you
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must make the changes recommended <ulink
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url="PPTP.htm#PPTP_ADSL">here</ulink> in addition to those detailed below.
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<acronym>ADSL</acronym> with <acronym>PPTP</acronym> 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></para>
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<para>The configuration files for Shorewall are contained in the directory
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<filename class="directory">/etc/shorewall</filename> -- for simple
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setups, you will only need to deal with a few of these as described in
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this guide.<warning>
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<para><emphasis role="bold">Note to Debian Users</emphasis></para>
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<para>If you install using the .deb, you will find that your <filename
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class="directory">/etc/shorewall</filename> directory is empty. This
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is intentional. The released configuration file skeletons may be found
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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
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copies.</para>
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<para>Note that you must copy <filename
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class="directory">/usr/share/doc/shorewall/default-config/shorewall.conf</filename>
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and /usr/share/doc/shorewall/default-config/modules to <filename
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class="directory">/etc/shorewall</filename> even if you do not modify
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those files.</para>
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</warning></para>
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<para><inlinegraphic fileref="images/BD21298_.gif"
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format="GIF" /><important>
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<para>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/">two-interface
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sample</ulink>, un-tar it </emphasis>(<command>tar
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<option>-zxvf</option>
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<filename>two-interfaces.tgz</filename></command>) and and copy the
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files to <filename class="directory">/etc/shorewall</filename>
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<emphasis role="bold">(these files will replace files with the same
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name)</emphasis>.</para>
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</important> As each file is introduced, I suggest that you look through
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the actual file on your system -- each file contains detailed
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configuration 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 two-interface sample configuration, the following
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zone names are used: <informaltable frame="all" pgwide="0">
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<tgroup align="left" cols="2">
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<thead valign="middle">
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<row valign="middle">
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<entry align="left">Name</entry>
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<entry align="left">Description</entry>
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</row>
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</thead>
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<tbody valign="middle">
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<row valign="middle">
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<entry align="left"><varname>net</varname></entry>
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<entry align="left">The Internet</entry>
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</row>
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<row valign="middle">
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<entry align="left"><varname>loc</varname></entry>
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<entry align="left">Your Local Network</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable> Zones are defined in the <ulink
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url="Documentation.htm#Zones"><filename
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class="directory">/etc/shorewall/</filename><filename>zones</filename></ulink>
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file.</para>
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<para>Shorewall also recognizes the firewall system as its own zone - by
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default, the firewall itself is known as <emphasis
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role="bold"><varname>fw</varname></emphasis>.</para>
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<para>Rules about what traffic to allow and what traffic to deny are
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expressed in terms of zones. <itemizedlist spacing="compact">
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<listitem>
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<para>You express your default policy for connections from one zone
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to another zone in the <ulink
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url="Documentation.htm#Policy"><filename
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class="directory">/etc/shorewall/</filename><filename>policy</filename></ulink>
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file.</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"><filename
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class="directory">/etc/shorewall/</filename><filename>rules</filename></ulink>
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file.</para>
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</listitem>
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</itemizedlist> For each connection request entering the firewall, the
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request is first checked against the <filename
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class="directory">/etc/shorewall/</filename><filename>rules</filename>
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file. If no rule in that file matches the connection request then the
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first policy in <filename
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class="directory">/etc/shorewall/</filename><filename>policy</filename>
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that matches the request is applied. If there is a <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
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class="directory">/etc/shorewall/</filename><filename>policy</filename>
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file included with the two-interface sample has the following policies:
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<programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
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loc net ACCEPT
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net all DROP info
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all all REJECT info</programlisting> In the two-interface
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sample, the line below is included but commented out. If you want your
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firewall system to have full access to servers on the internet, uncomment
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that line. <programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
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fw net ACCEPT</programlisting> The above policy will:
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<itemizedlist>
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<listitem>
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<para>Allow all connection requests from your local network to the
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internet</para>
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</listitem>
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<listitem>
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<para>Drop (ignore) all connection requests from the internet to
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your firewall or local network</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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</itemizedlist> <inlinegraphic fileref="images/BD21298_.gif"
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format="GIF" /></para>
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<para>At this point, edit your <filename
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class="directory">/etc/shorewall/</filename><filename>policy</filename>
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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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<mediaobject>
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<imageobject>
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<imagedata fileref="images/basics.png" format="PNG" />
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</imageobject>
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</mediaobject>
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<para>The firewall has two network interfaces. Where Internet connectivity
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is through a cable or <acronym>DSL</acronym> <quote>Modem</quote>, the
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<emphasis>External Interface</emphasis> will be the ethernet adapter that
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is connected to that <quote>Modem</quote> (e.g., <filename
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class="devicefile">eth0</filename>) unless you connect via
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<emphasis>Point-to-Point Protocol</emphasis> over Ethernet
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(<acronym>PPPoE</acronym>) or <emphasis>Point-to-Point Tunneling
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Protocol</emphasis> (<acronym>PPTP</acronym>) in which case the External
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Interface will be a <literal>ppp</literal> interface (e.g., <filename
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class="devicefile">ppp0</filename>). If you connect via a regular modem,
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your External Interface will also be <filename
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class="devicefile">ppp0</filename>. If you connect via
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<acronym>ISDN</acronym>, your external interface will be <filename
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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
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class="devicefile">ppp0</filename> or <filename
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class="devicefile">ippp0</filename> then you will want to set
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<varname>CLAMPMSS=yes</varname> in <filename
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class="directory">/etc/shorewall/</filename><filename>shorewall.conf</filename>.</para>
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<para>Your <emphasis>Internal Interface</emphasis> will be an ethernet
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adapter (<filename class="devicefile">eth1</filename> or <filename
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class="devicefile">eth0</filename>) and will be connected to a hub or
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switch. Your other computers will be connected to the same hub/switch
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(note: If you have only a single internal system, you can connect the
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firewall directly to the computer using a cross-over cable). <warning>
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<para>Do not connect the internal and external interface to the same
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hub or switch except for testing AND you are running Shorewall version
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1.4.7 or later. When using these recent versions, you can test using
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this kind of configuration if you specify the arp_filter option in
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<filename
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class="directory">/etc/shorewall/</filename><filename>interfaces</filename>
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for all interfaces connected to the common hub/switch. Using such a
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setup with a production firewall is strongly recommended
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against.</para>
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</warning> <inlinegraphic fileref="images/BD21298_.gif"
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format="GIF" /></para>
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<para>The Shorewall two-interface sample configuration assumes that the
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external interface is <filename class="devicefile">eth0</filename> and the
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internal interface is <filename class="devicefile">eth1</filename>. If
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your configuration is different, you will have to modify the sample
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<filename
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class="directory">/etc/shorewall/</filename><filename>interfaces</filename>
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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 interfaces. Some hints: <tip>
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<para>If your external interface is <filename
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class="devicefile">ppp0</filename> or <filename
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class="devicefile">ippp0</filename>, you can replace the
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<varname>detect</varname> in the second column with a <quote>-</quote>
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(minus the quotes).</para>
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</tip><tip>
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<para>If your external interface is <filename
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class="devicefile">ppp0</filename> or <filename
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class="devicefile">ippp0</filename> or if you have a static
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<acronym>IP</acronym> address, you can remove <varname>dhcp</varname>
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from the option list.</para>
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</tip><tip>
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<para>If your internal interface is a bridge create using the
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<command>brctl</command> utility then you must add the
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<varname>routeback</varname> option to the option list.</para>
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</tip><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
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url="errata.htm">Shorewall Errata</ulink> periodically for updates to
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the <filename>/usr/share/shorewall/rfc1918 file</filename>.
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Alternatively, you can copy
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<filename>/usr/share/shorewall/rfc1918</filename> to
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<filename>/etc/shorewall/rfc1918</filename> then <ulink
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url="myfiles.htm#RFC1918">strip down your
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<filename>/etc/shorewall/rfc1918</filename> file as I
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do</ulink>.</para>
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</tip></para>
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</section>
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<section>
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<title>IP Addresses</title>
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<para>Before going further, we should say a few words about Internet
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Protocol (<acronym>IP</acronym>) addresses. Normally, your
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<acronym>ISP</acronym> will assign you a single Public IP address. This
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address may be assigned via the Dynamic Host Configuration Protocol
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(<acronym>DHCP</acronym>) or as part of establishing your connection when
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you dial in (standard modem) or establish your <acronym>PPP</acronym>
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connection. In rare cases, your <acronym>ISP</acronym> may assign you a
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static <acronym>IP</acronym> address; that means that you configure your
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firewall's external interface to use that address permanently. However
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your external address is assigned, it will be shared by all of your
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systems when you access the Internet. You will have to assign your own
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addresses in your internal network (the Internal Interface on your
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firewall plus your other computers). <emphasis role="bold">RFC
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1918</emphasis> reserves several <emphasis>Private</emphasis>
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<acronym>IP</acronym> address ranges for this purpose: <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> <inlinegraphic
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fileref="images/BD21298_.gif" format="GIF" /></para>
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<para>Before starting Shorewall, you should look at the IP address of your
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external interface and if it is one of the above ranges, you should remove
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the 'norfc1918' option from the external interface's entry in <filename
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class="directory">/etc/shorewall/</filename><filename>interfaces</filename>.</para>
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<para>You will want to assign your addresses from the same sub-network
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(subnet). For our purposes, we can consider a subnet to consists of a
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range of addresses <varname>x.y.z.0 - x.y.z.255</varname>. Such a subnet
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will have a Subnet Mask of <systemitem
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class="netmask">255.255.255.0</systemitem>. The address
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<varname>x.y.z.0</varname> is reserved as the <emphasis>Subnet
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Address</emphasis> and <varname>x.y.z.255</varname> is reserved as the
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<emphasis>Subnet Broadcast Address</emphasis>. In Shorewall, a subnet is
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described using <ulink url="shorewall_setup_guide.htm#Subnets">Classless
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|
InterDomain Routing (CIDR) notation</ulink> with consists of the subnet
|
|
address followed by <varname>/24</varname>. The <quote>24</quote> refers
|
|
to the number of consecutive leading <quote>1</quote> bits from the left
|
|
of the subnet mask. <informaltable frame="all" label="Example sub-network"
|
|
pgwide="0">
|
|
<!--
|
|
Orientation types for tables are not supported by fop yet so we'll fake it by using boldface on left side entries.
|
|
-->
|
|
|
|
<tgroup align="left" cols="2">
|
|
<tbody valign="middle">
|
|
<row valign="middle">
|
|
<entry align="left"><emphasis
|
|
role="bold">Range:</emphasis></entry>
|
|
|
|
<entry><systemitem class="ipaddress">10.10.10.0</systemitem> -
|
|
<systemitem class="ipaddress">10.10.10.255</systemitem></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry align="left"><emphasis role="bold">Subnet
|
|
Address:</emphasis></entry>
|
|
|
|
<entry align="left"><systemitem
|
|
class="netmask">10.10.10.0</systemitem></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry align="left"><emphasis role="bold">Broadcast
|
|
Address:</emphasis></entry>
|
|
|
|
<entry align="left"><systemitem
|
|
class="ipaddress">10.10.10.255</systemitem></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry align="left"><emphasis role="bold">CIDR
|
|
Notation:</emphasis></entry>
|
|
|
|
<entry align="left"><systemitem
|
|
class="ipaddress">10.10.10.0/24</systemitem></entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</informaltable> 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 (computer 1 and computer 2 in the above
|
|
diagram) should be configured with their default gateway to be the
|
|
<acronym>IP</acronym> address of the firewall's internal interface.</para>
|
|
|
|
<para>The foregoing short discussion barely scratches the surface
|
|
regarding subnetting and routing. If you are interested in learning more
|
|
about <acronym>IP</acronym> 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
|
|
(<ulink
|
|
url="http://www.phptr.com/browse/product.asp?product_id={58D4F6D4-54C5-48BA-8EDD-86EBD7A42AF6}">link</ulink>).</para>
|
|
|
|
<para>The remainder of this quide will assume that you have configured
|
|
your network as shown here: <mediaobject>
|
|
<imageobject>
|
|
<imagedata fileref="images/basics1.png" format="PNG" />
|
|
</imageobject>
|
|
</mediaobject> The default gateway for computer's 1 & 2 would be
|
|
<systemitem class="ipaddress">10.10.10.254</systemitem>. <warning>
|
|
<para>Your <acronym>ISP</acronym> might assign your external interface
|
|
an <emphasis role="bold">RFC 1918</emphasis> 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.</para>
|
|
</warning></para>
|
|
</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 computer 1) sends a connection request to an internet host,
|
|
the firewall must perform <emphasis>Network Address Translation</emphasis>
|
|
(<acronym>NAT</acronym>). 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 across
|
|
the internet so the remote host can't address its response to computer 1).
|
|
When the firewall receives a return packet, it rewrites the destination
|
|
address back to <systemitem class="ipaddress">10.10.10.1</systemitem> and
|
|
forwards the packet on to computer 1.</para>
|
|
|
|
<para>On Linux systems, the above process is often referred to as
|
|
<emphasis>IP Masquerading</emphasis> but you will also see the term
|
|
<emphasis>Source Network Address Translation</emphasis>
|
|
(<acronym>SNAT</acronym>) 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><acronym>SNAT</acronym></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 <emphasis>Masquerading</emphasis> and
|
|
<emphasis><acronym>SNAT</acronym></emphasis> are configured with entries
|
|
in the <filename
|
|
class="directory">/etc/shorewall/</filename><filename>masq</filename>
|
|
file. You will normally use Masquerading if your external
|
|
<acronym>IP</acronym> is dynamic and <acronym>SNAT</acronym> if the
|
|
<acronym>IP</acronym> is static.</para>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>If your external firewall interface is <filename
|
|
class="devicefile">eth0</filename>, 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 the first column to the name of your external interface and the
|
|
second column to the name of your internal interface.</para>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>If your external <acronym>IP</acronym> 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 <acronym>IP</acronym> 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
|
|
spacing="compact">
|
|
<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 may be to run one or more servers on your local
|
|
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
|
|
the 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 <acronym>SNAT</acronym> 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>
|
|
(<acronym>DNAT</acronym>). You configure port forwarding using
|
|
<acronym>DNAT</acronym> 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 loc:<emphasis><server local ip address></emphasis>[:<emphasis><server port></emphasis>] <emphasis><protocol></emphasis> <emphasis><port></emphasis></programlisting>
|
|
<example label="1">
|
|
<title>Web Server</title>
|
|
|
|
<para>You run a Web Server on computer 2 and you want to forward
|
|
incoming <acronym>TCP</acronym> port 80 to that system:
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
DNAT net loc:10.10.10.2 tcp 80</programlisting></para>
|
|
</example> <example label="2">
|
|
<title>FTP Server</title>
|
|
|
|
<para>You run an <acronym>FTP</acronym> Server on computer 1 so you
|
|
want to forward incoming <acronym>TCP</acronym> port 21 to that
|
|
system: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
DNAT net loc:10.10.10.1 tcp 21</programlisting> For
|
|
<acronym>FTP</acronym>, you will also need to have
|
|
<acronym>FTP</acronym> connection tracking and <acronym>NAT</acronym>
|
|
support in your kernel. For vendor-supplied kernels, this means that
|
|
the <filename class="libraryfile">ip_conntrack_ftp</filename> and
|
|
<filename class="libraryfile">ip_nat_ftp</filename> modules must be
|
|
loaded. Shorewall will automatically load these modules if they are
|
|
available and located in the standard place under <filename
|
|
class="directory">/lib/modules/<kernel
|
|
version>/kernel/net/ipv4/netfilter</filename>.</para>
|
|
</example> A couple of important points to keep in mind: <itemizedlist>
|
|
<listitem>
|
|
<para>You must test the above rule from a client outside of your
|
|
local network (i.e., don't test from a browser running on computers
|
|
1 or 2 or on the firewall). If you want to be able to access your
|
|
web server and/or <acronym>FTP</acronym> server from inside your
|
|
firewall using the <acronym>IP</acronym> address of your external
|
|
interface, see <ulink url="FAQ.htm#faq2">Shorewall FAQ
|
|
#2</ulink>.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Many <acronym>ISP</acronym>s 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.</para>
|
|
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
DNAT net loc:10.10.10.2:80 tcp 5000</programlisting>
|
|
</listitem>
|
|
</itemizedlist> <inlinegraphic fileref="images/BD21298_.gif"
|
|
format="GIF" /></para>
|
|
|
|
<para>At this point, modify <filename
|
|
class="directory">/etc/shorewall/</filename><filename>rules</filename> to
|
|
add any <acronym>DNAT</acronym> rules that you require.</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>
|
|
(<acronym>DNS</acronym>) resolver will be automatically configured (e.g.,
|
|
the <filename
|
|
class="directory">/etc/</filename><filename>resolv.conf</filename> file
|
|
will be written). Alternatively, your ISP may have given you the
|
|
<acronym>IP</acronym> address of a pair of <acronym>DNS</acronym> name
|
|
servers for you to manually configure as your primary and secondary name
|
|
servers. Regardless of how <acronym>DNS</acronym> gets configured on your
|
|
firewall, it is your responsibility to configure the resolver in your
|
|
internal systems. You can take one of two approaches: <itemizedlist
|
|
spacing="compact">
|
|
<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 /etc/resolv.conf on your firewall system --
|
|
the name servers are given in "nameserver" records in that
|
|
file.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para><anchor id="cachingdns" /> You can configure a
|
|
<emphasis>Caching Name Server</emphasis> on your firewall.
|
|
<trademark>Red Hat</trademark> has an <acronym>RPM</acronym> for a
|
|
caching name server (the <acronym>RPM</acronym> also requires the
|
|
<command>bind</command><acronym>RPM</acronym>) and for Bering users,
|
|
there is <command>dnscache.lrp</command>. If you take this approach,
|
|
you configure your internal systems to use the firewall itself as
|
|
their primary (and only) name server. You use the internal
|
|
<acronym>IP</acronym> address of the firewall (<systemitem
|
|
class="ipaddress">10.10.10.254</systemitem> in the example above)
|
|
for the name server address. To allow your local systems to talk to
|
|
your caching name server, you must open port 53 (both
|
|
<acronym>UDP</acronym> and <acronym>TCP</acronym>) from the local
|
|
network to the firewall; you do that by adding the following rules
|
|
in <filename
|
|
class="directory">/etc/shorewall/</filename><filename>rules</filename>.
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
AllowDNS loc fw</programlisting></para>
|
|
</listitem>
|
|
</itemizedlist></para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Other Connections</title>
|
|
|
|
<para>The two-interface sample includes the following rules:
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
AllowDNS fw net</programlisting>This rule allows
|
|
<acronym>DNS</acronym> access from your firewall and may be removed if you
|
|
uncommented the line in <filename
|
|
class="directory">/etc/shorewall/</filename><filename>policy</filename>
|
|
allowing all connections from the firewall to the internet.</para>
|
|
|
|
<para>In the rule shown above, <quote>AllowDNS</quote> is an example of a
|
|
<emphasis>defined action</emphasis>. Shorewall includes a number of
|
|
defined actions and <ulink url="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>/usr/share/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 the rule shown above could also have been coded
|
|
as follows:<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
ACCEPT fw net udp 53
|
|
ACCEPT fw net tcp 53</programlisting></para>
|
|
|
|
<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>
|
|
|
|
<para>The sample also includes: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
AllowSSH loc fw</programlisting> That rule allows you to run an
|
|
<acronym>SSH</acronym> server on your firewall and connect to that server
|
|
from your local systems.</para>
|
|
|
|
<para>If you wish to enable other connections from your firewall to other
|
|
systems, the general format using an <quote>Allow</quote> action is:
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
<action> fw <emphasis><destination zone></emphasis></programlisting>The
|
|
general format when not using defined actions is:<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
ACCEPT fw <emphasis><destination zone> <protocol> <port></emphasis></programlisting><example>
|
|
<title>Web Server on Firewall</title>
|
|
|
|
<para>You want to run a Web Server on your firewall system:
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
AllowWeb net fw
|
|
AllowWeb loc fw</programlisting> Those two rules would of course be in
|
|
addition to the rules listed above under <quote><link
|
|
linkend="cachingdns">You can configure a Caching Name Server on your
|
|
firewall</link></quote>.</para>
|
|
</example> If you don't know what port and protocol a particular
|
|
application uses, look <ulink url="ports.htm">here</ulink>. <important>
|
|
<para>I don't recommend enabling <command>telnet</command> to/from the
|
|
internet because it uses clear text (even for login!). If you want
|
|
shell access to your firewall from the internet, use
|
|
<acronym>SSH</acronym>:</para>
|
|
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
AllowSSH net fw</programlisting>
|
|
</important> <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 #Allow DNS Cache to work
|
|
ACCEPT loc fw tcp 80 #Allow Weblet to work</programlisting>
|
|
<inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>Now edit your <filename
|
|
class="directory">/etc/shorewall/</filename><filename>rules</filename>
|
|
file to add or delete 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
|
|
class="directory">/etc/shorewall/</filename><filename>startup_disabled</filename>.
|
|
<important>
|
|
<para>Users of the .deb package must edit <filename
|
|
class="directory">/etc/default/</filename><filename>shorewall</filename>
|
|
and set <varname>startup=1</varname>.</para>
|
|
</important><important>
|
|
<para>Users running Shorewall 2.1.3 or later must edit
|
|
/etc/shorewall/shorewall.conf and set STARTUP_ENABLED=Yes.</para>
|
|
</important> The firewall is started using the <quote><command>shorewall
|
|
start</command></quote> command and stopped using
|
|
<quote><command>shorewall stop</command></quote>. When the firewall is
|
|
stopped, routing is enabled on those hosts that have an entry in <filename
|
|
class="directory">/etc/shorewall/</filename><filename><ulink
|
|
url="Documentation.htm#Routestopped">routestopped</ulink></filename>. A
|
|
running firewall may be restarted using the <quote><command>shorewall
|
|
restart</command></quote> command. If you want to totally remove any trace
|
|
of Shorewall from your Netfilter configuration, use
|
|
<quote><command>shorewall clear</command></quote>.</para>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>The two-interface sample assumes that you want to enable routing
|
|
to/from <filename class="devicefile">eth1</filename> (the local network)
|
|
when Shorewall is stopped. If your local network isn't connected to
|
|
<filename class="devicefile">eth1</filename> or if you wish to enable
|
|
access to/from other hosts, change <filename
|
|
class="directory">/etc/shorewall/</filename><filename>routestopped</filename>
|
|
accordingly. <warning>
|
|
<para>If you are connected to your firewall from the internet, do not
|
|
issue a <quote><command>shorewall stop</command></quote> command
|
|
unless you have added an entry for the <acronym>IP</acronym> address
|
|
that you are connected from to <filename
|
|
class="directory">/etc/shorewall/</filename><filename>routestopped</filename>.
|
|
Also, I don't recommend using <quote><command>shorewall
|
|
restart</command></quote>; it is better to create an alternate
|
|
configuration and test it using the <quote><command>shorewall
|
|
try</command></quote> command.</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
|
|
page</ulink> -- it contains helpful tips about Shorewall features than
|
|
make administering your firewall easier.</para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Adding a Wireless Segment to your Two-Interface Firewall</title>
|
|
|
|
<para>Once you have the two-interface setup working, the next logical step
|
|
is to add a Wireless Network. The first step involves adding an additional
|
|
network card to your firewall, either a Wireless card or an ethernet card
|
|
that is connected to a Wireless Access Point.<caution>
|
|
<para>When you add a network card, it won't necessarily be detected as
|
|
the next highest ethernet interface. For example, if you have two
|
|
ethernet cards in your system (<filename
|
|
class="devicefile">eth0</filename> and <filename
|
|
class="devicefile">eth1</filename>) and you add a third card that uses
|
|
the same driver as one of the other two, that third card won't
|
|
necessarily be detected as <filename
|
|
class="devicefile">eth2</filename>; it could rather be detected as
|
|
<filename class="devicefile">eth0</filename> or <filename
|
|
class="devicefile">eth1</filename>! You can either live with that or
|
|
you can shuffle the cards around in the slots until the new card is
|
|
detected as <filename class="devicefile">eth2</filename>.</para>
|
|
</caution></para>
|
|
|
|
<para>Your new network will look similar to what is shown in the following
|
|
figure.<graphic fileref="images/basics2.png" /></para>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>The first thing to note is that the computers in your wireless
|
|
network will be in a different subnet from those on your wired local LAN.
|
|
In the above example, we have chosen to use the network 10.10.11.0/24.
|
|
Computers 3 and 4 would be configured with a default gateway IP address of
|
|
10.10.11.254.</para>
|
|
|
|
<para>Second, we have chosen to include the wireless network as part of
|
|
the local zone. Since Shorewall allows intra-zone traffic by default,
|
|
traffic may flow freely between the local wired network and the wireless
|
|
network.</para>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>There are only two changes that need to be made to the Shorewall
|
|
configuration:</para>
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>An entry needs to be added to
|
|
<filename>/etc/shorewall/interfaces</filename> for the wireless
|
|
network interface. If the wireless interface is <filename
|
|
class="devicefile">wlan0</filename>, the entry might look like:</para>
|
|
|
|
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
|
|
loc wlan0 detect maclist</programlisting>
|
|
|
|
<para>As shown in the above entry, I recommend using the <ulink
|
|
url="MAC_Validation.html">maclist option</ulink> for the wireless
|
|
segment. By adding entries for computers 3 and 4 in
|
|
<filename>/etc/shorewall/maclist</filename>, you help ensure that your
|
|
neighbors aren't getting a free ride on your internet connection.
|
|
Start by omitting that option; when you have everything working, then
|
|
add the option and configure your
|
|
<filename>/etc/shorewall/maclist</filename> file.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>You need to add an entry to the
|
|
<filename>/etc/shorewall/masq</filename> file to masquerade traffic
|
|
from the wireless network to the internet. If your internet interface
|
|
is <filename class="devicefile">eth0</filename> and your wireless
|
|
interface is <filename class="devicefile">wlan0</filename>, the entry
|
|
would be:</para>
|
|
|
|
<programlisting>#INTERFACE SUBNET ADDRESS
|
|
eth0 wlan0</programlisting>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
<para>One other thing to note. To get <trademark>Microsoft</trademark>
|
|
networking working between the wireless and wired networks, you will need
|
|
either a WINS server or a PDC. I personally use Samba configured as a WINS
|
|
server running on my firewall. Running a WINS server on your firewall
|
|
requires the rules listed in the <ulink url="samba.htm">Shorewall/Samba
|
|
documentation</ulink>.</para>
|
|
</section>
|
|
</article>
|