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957 lines
36 KiB
HTML
957 lines
36 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="Content-Language" content="en-us">
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<meta name="GENERATOR" content="Microsoft FrontPage 5.0">
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<meta name="ProgId" content="FrontPage.Editor.Document">
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<meta http-equiv="Content-Type"
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content="text/html; charset=windows-1252">
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<title>Two-Interface Firewall</title>
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<meta name="Microsoft Theme" content="none">
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</head>
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<body>
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<h1 style="text-align: center;">Basic Two-Interface Firewall<br>
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</h1>
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<p align="left">Setting up a Linux system as a firewall for a small
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network is a fairly straight-forward task if you understand the basics
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and follow the documentation.</p>
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<p>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
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Shorewall in its most common configuration:</p>
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<ul>
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<li>Linux system used as a firewall/router for a small local network.</li>
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<li style="font-weight: bold;">Single public IP address. If you have
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more than one public IP address, this is not the guide you want -- see
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the <a href="shorewall_setup_guide.htm">Shorewall Setup Guide</a>
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instead.</li>
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<li>Internet connection through cable modem, DSL, ISDN, Frame Relay,
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dial-up ...</li>
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</ul>
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<p align="left">Here is a schematic of a typical installation.</p>
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<p align="center"> <img border="0" src="images/basics.png" width="444"
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height="635"> </p>
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<p><b>If you are running Shorewall under Mandrake 9.0 or later, you can
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easily configure the above setup using the Mandrake "Internet
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Connection
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Sharing" applet. From the Mandrake Control Center, select "Network
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& Internet" then "Connection Sharing".<br>
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</b></p>
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<p><b>Note however, that the Shorewall configuration produced by
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Mandrake Internet Connection Sharing is strange and is apt to confuse
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you if you use the rest of this documentation (it has two local zones;
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"loc" and "masq" where "loc" is empty; this conflicts with this
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documentation which assumes a single local zone "loc"). We therefore
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recommend that once you have set up this sharing that you uninstall the
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Mandrake Shorewall RPM and install the one from the <a
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href="download.htm">download page</a> then follow the instructions in
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this Guide.</b><br>
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</p>
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<p>Shorewall requires that you have the iproute/iproute2 package
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installed (on RedHat, the package is called <i>iproute</i>)<i>. </i>You
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can tell if this package is installed by the presence of an <b>ip</b>
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program on your firewall system. As root, you can use the 'which'
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command to check for this program:</p>
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<pre> [root@gateway root]# which ip<br> /sbin/ip<br> [root@gateway root]#</pre>
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<p>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 your
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configuration changes. Points at which configuration changes are
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recommended are flagged with <img border="0" src="images/BD21298_.gif"
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width="13" height="13"> . Configuration notes that are unique to
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LEAF/Bering are marked with <img src="images/leaflogo.gif"
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alt="(LEAF Logo)" width="49" height="36"> </p>
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<p><img border="0" src="images/j0213519.gif" width="60" height="60">
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If you edit your configuration files on a Windows
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system, you must save them as Unix files if your editor supports that
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option or you must run them through dos2unix before trying
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to use them. Similarly, if you copy a configuration file from your
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Windows hard drive to a floppy disk, you must run dos2unix against the
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copy before using it with Shorewall.</p>
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<ul>
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<li><a href="http://www.simtel.net/pub/pd/51438.html">Windows Version
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of dos2unix</a></li>
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<li><a href="http://www.megaloman.com/%7Ehany/software/hd2u/">Linux
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Version
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of dos2unix</a></li>
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</ul>
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<h2 align="left">PPTP/ADSL</h2>
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<img style="border: 0px solid ; width: 13px; height: 13px;"
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src="images/BD21298_3.gif" title="" alt=""> If you
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have an ADSL Modem and you use PPTP to communicate with a server in
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that modem, you must make the <a href="PPTP.htm#PPTP_ADSL">changes
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recommended here</a> in addition to those detailed below. ADSL with
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PPTP is most commonly found in Europe, notably in Austria.<br>
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<h2 align="left">Shorewall Concepts</h2>
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<p> <img border="0" src="images/BD21298_.gif" width="13" height="13"
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alt=""> The configuration files for Shorewall are
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contained in the directory /etc/shorewall -- for simple setups, you
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will only need to deal with a few of these as described in this guide.
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After you have <a href="Install.htm">installed Shorewall</a>, <b>download
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the <a href="http://www1.shorewall.net/pub/shorewall/Samples/">two-interface
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sample</a>, un-tar it (tar -zxvf two-interfaces.tgz) and and copy the
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files
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to /etc/shorewall (these files will replace files with the same
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name).</b></p>
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<p>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.</p>
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<p>Shorewall views the network where it is running as being composed of
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a set of <i>zones.</i> In the two-interface sample configuration, the
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following zone names are used:</p>
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<table border="0" style="border-collapse: collapse;" cellpadding="3"
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cellspacing="0" id="AutoNumber2">
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<tbody>
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<tr>
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<td><u><b>Name</b></u></td>
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<td><u><b>Description</b></u></td>
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</tr>
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<tr>
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<td><b>net</b></td>
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<td><b>The Internet</b></td>
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</tr>
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<tr>
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<td><b>loc</b></td>
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<td><b>Your Local Network</b></td>
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</tr>
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</tbody>
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</table>
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<p>Zones are defined in the <a href="Documentation.htm#Zones">
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/etc/shorewall/zones</a> file.</p>
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<p>Shorewall also recognizes the firewall system as its own zone - by
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default, the firewall itself is known as <b>fw.</b></p>
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<p>Rules about what traffic to allow and what traffic to deny are
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expressed in terms of zones.</p>
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<ul>
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<li>You express your default policy for connections from one zone to
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another zone in the<a href="Documentation.htm#Policy">
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/etc/shorewall/policy </a>file.</li>
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<li>You define exceptions to those default policies in the <a
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href="Documentation.htm#Rules">/etc/shorewall/rules </a>file.</li>
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</ul>
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<p>For each connection request entering the firewall, the request is
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first checked against the /etc/shorewall/rules file. If no rule in
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that file matches the connection request then the first policy
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in /etc/shorewall/policy that matches the request is applied.
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If that policy is REJECT or DROP the request is first checked
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against
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the rules in /etc/shorewall/common if that file exists; otherwise the
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rules in /etc/shorewall/common.def are checked.</p>
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<p>The /etc/shorewall/policy file included with the two-interface
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sample
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has the following policies:</p>
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<blockquote>
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<table border="1" cellpadding="2" style="border-collapse: collapse;"
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id="AutoNumber3">
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<tbody>
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<tr>
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<td><u><b>Source Zone</b></u></td>
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<td><u><b>Destination Zone</b></u></td>
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<td><u><b>Policy</b></u></td>
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<td><u><b>Log Level</b></u></td>
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<td><u><b>Limit:Burst</b></u></td>
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</tr>
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<tr>
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<td>loc</td>
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<td>net</td>
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<td>ACCEPT</td>
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<td> </td>
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<td> </td>
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</tr>
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<tr>
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<td>net</td>
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<td>all</td>
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<td>DROP</td>
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<td>info</td>
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<td> </td>
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</tr>
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<tr>
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<td>all</td>
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<td>all</td>
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<td>REJECT</td>
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<td>info</td>
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<td> </td>
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</tr>
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</tbody>
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</table>
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</blockquote>
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<blockquote>
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<p>In the two-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.</p>
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<table border="1" cellpadding="2" style="border-collapse: collapse;"
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id="AutoNumber3">
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<tbody>
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<tr>
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<td><u><b>Source Zone</b></u></td>
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<td><u><b>Destination Zone</b></u></td>
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<td><u><b>Policy</b></u></td>
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<td><u><b>Log Level</b></u></td>
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<td><u><b>Limit:Burst</b></u></td>
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</tr>
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<tr>
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<td>fw</td>
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<td>net</td>
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<td>ACCEPT</td>
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<td> </td>
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<td> </td>
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</tr>
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</tbody>
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</table>
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</blockquote>
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<p>The above policy will:</p>
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<ol>
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<li>allow all connection requests from your local network to the
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internet</li>
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<li>drop (ignore) all connection requests from the internet to your
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firewall or local network</li>
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<li>optionally accept all connection requests from the firewall to
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the internet (if you uncomment the additional policy)</li>
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<li>reject all other connection requests.</li>
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</ol>
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<p><img border="0" src="images/BD21298_.gif" width="13" height="13">
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At this point, edit your /etc/shorewall/policy
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and make any changes that you wish.</p>
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<h2 align="left">Network Interfaces</h2>
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<p align="center"> <img border="0" src="images/basics.png" width="444"
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height="635"> </p>
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<p align="left">The firewall has two network interfaces. Where Internet
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connectivity
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is through a cable or DSL "Modem", the <i>External Interface</i> will
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be
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the ethernet adapter that is connected to that "Modem" (e.g., <b>eth0</b>)
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<u>unless</u> you connect via <i><u>P</u>oint-to-<u>P</u>oint
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<u>P</u>rotocol over <u>E</u>thernet</i> (PPPoE) or <i><u>P</u>oint-to-<u>P</u>oint
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<u>T</u>unneling <u>P</u>rotocol </i>(PPTP) in which case the
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External Interface will be a ppp interface (e.g., <b>ppp0</b>). If you
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connect via a regular modem, your External Interface will also be <b>ppp0</b>.
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If you connect via ISDN, your external interface will be <b>ippp0.</b></p>
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<p align="left"><img border="0" src="images/BD21298_1.gif" width="13"
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height="13"> If your external interface is <b>ppp0</b>
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or<b> ippp0</b> then you will want to set CLAMPMSS=yes in <a
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href="Documentation.htm#Conf"> /etc/shorewall/shorewall.conf.</a></p>
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<p align="left">Your <i>Internal Interface</i> will be an ethernet
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adapter (eth1 or eth0) and will be connected to a hub or switch. Your
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other computers will be connected to the same hub/switch (note:
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If you have only a single internal system, you can connect the firewall
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directly to the computer using a <i>cross-over </i> cable).</p>
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<p align="left"><u><b> <img border="0" src="images/j0213519.gif"
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width="60" height="60"> </b></u>Do not connect the internal and
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external interface to the same hub or switch except for testing AND you
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are running Shorewall version 1.4.7 or later. When using these
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recent versions, you can test using this kind of configuration if you
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specify the <span style="font-weight: bold;">arp_filter</span> option
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in /etc/shorewall/interfaces for all interfaces connected to the common
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hub/switch. Using such a setup with a production firewall is strongly
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recommended against.<br>
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</p>
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<p align="left"><img border="0" src="images/BD21298_.gif" align="left"
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width="13" height="13"> The Shorewall two-interface
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sample configuration assumes that the external interface is <b>eth0</b>
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and the internal interface is <b>eth1</b>. If your configuration is
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different, you will have to modify the sample <a
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href="Documentation.htm#Interfaces">/etc/shorewall/interfaces</a> file
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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:</p>
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<ul>
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<li>
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<p align="left">If your external interface is <b>ppp0</b> or <b>ippp0</b>,
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you can replace the "detect" in the second column with "-". </p>
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</li>
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<li>
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<p align="left">If your external interface is <b>ppp0</b> or <b>ippp0</b>
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or if you have a static IP address, you can remove "dhcp" from the
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option list. </p>
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</li>
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</ul>
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<h2 align="left">IP Addresses</h2>
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<p align="left">Before going further, we should say a few words about
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Internet Protocol (IP) <i>addresses</i>. Normally, your ISP will
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assign you a single <i> Public</i> IP address. This address may be
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assigned via the<i> Dynamic Host Configuration Protocol</i> (DHCP) or
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as part of establishing your connection when you dial in (standard
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modem) or establish your PPP connection. In rare cases, your ISP may
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assign you a<i> static</i> IP address; that means that you configure
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your firewall's external interface to use that address permanently.<i> </i>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). RFC 1918 reserves several <i>Private
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</i>IP address ranges for this purpose:</p>
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<div align="left">
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<pre> 10.0.0.0 - 10.255.255.255<br> 172.16.0.0 - 172.31.255.255<br> 192.168.0.0 - 192.168.255.255</pre>
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</div>
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<div align="left">
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<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
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height="13"> Before starting Shorewall, you should
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look at the IP address of your external interface and if it is one of
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the above ranges, you should remove the 'norfc1918' option from the
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external interface's entry in /etc/shorewall/interfaces.</p>
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</div>
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<div align="left">
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<p align="left">You will want to assign your addresses from the same <i>
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sub-network </i>(<i>subnet)</i>. For our purposes, we can
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consider a subnet to consists of a range of addresses x.y.z.0 -
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x.y.z.255. Such a subnet will have a <i>Subnet Mask </i>of
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255.255.255.0. The
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address x.y.z.0 is reserved as the <i>Subnet Address</i> and x.y.z.255
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is reserved as the <i>Subnet Broadcast</i> <i>Address</i>. In
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Shorewall, a subnet is described using <a
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href="shorewall_setup_guide.htm#Subnets"><i>Classless InterDomain
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Routing </i>(CIDR) notation</a> with consists of the subnet address
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followed by "/24". The "24" refers to the number of consecutive leading
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"1" bits from the left of the subnet mask. </p>
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</div>
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<div align="left">
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<p align="left">Example sub-network:</p>
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</div>
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<div align="left">
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<blockquote>
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<table border="1" style="border-collapse: collapse;" id="AutoNumber1"
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cellpadding="2">
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<tbody>
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<tr>
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<td><b>Range:</b></td>
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<td>10.10.10.0 - 10.10.10.255</td>
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</tr>
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<tr>
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<td><b>Subnet Address:</b></td>
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<td>10.10.10.0</td>
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</tr>
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<tr>
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<td><b>Broadcast Address:</b></td>
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<td>10.10.10.255</td>
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</tr>
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<tr>
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<td><b>CIDR Notation:</b></td>
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<td>10.10.10.0/24</td>
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</tr>
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</tbody>
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</table>
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</blockquote>
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</div>
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<div align="left">
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<p align="left">It is conventional to assign the internal interface
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either the first usable address in the subnet (10.10.10.1 in the above
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example) or the last usable address (10.10.10.254).</p>
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</div>
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<div align="left">
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<p align="left">One of the purposes of subnetting is to allow all
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computers in the subnet to understand which other computers can be
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communicated with directly. To communicate with systems outside of the
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subnetwork, systems send packets through a<i> gateway</i>
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(router).</p>
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</div>
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<div align="left">
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<p align="left"><img border="0" src="images/BD21298_1.gif" width="13"
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height="13"> Your local computers (computer 1 and
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computer 2 in the above diagram) should be configured with their<i>
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default gateway</i> to be the IP address of the firewall's internal
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interface.<i> </i> </p>
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</div>
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<p align="left">The foregoing short discussion barely scratches the
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surface regarding subnetting and routing. If you are interested in
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learning more about IP addressing and routing, I highly recommend <i>"IP
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Fundamentals: What Everyone Needs to Know about Addressing &
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Routing",</i> Thomas A. Maufer, Prentice-Hall, 1999, ISBN 0-13-975483-0.</p>
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<p align="left">The remainder of this quide will assume that you have
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configured your network as shown here:</p>
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<p align="center"> <img border="0" src="images/basics1.png" width="444"
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height="635"> </p>
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<p align="left">The default gateway for computer's 1 & 2 would be
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10.10.10.254.<br>
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</p>
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<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
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height="13" alt=""> <font color="#ff0000"><b>WARNING:
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</b></font><b>Your ISP might assign your external interface an RFC 1918
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address. If that address
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is in the 10.10.10.0/24 subnet then you will need to select a DIFFERENT
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RFC 1918 subnet for your local network.</b><br>
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</p>
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<h2 align="left">IP Masquerading (SNAT)</h2>
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<p align="left">The addresses reserved by RFC 1918 are sometimes
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referred to as <i>non-routable</i> because the Internet backbone
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routers don't forward packets which have an RFC-1918 destination
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address. When one of your local systems (let's assume computer 1) sends
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a
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connection request to an internet host, the firewall must perform
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<i>Network Address Translation </i>(NAT). The firewall rewrites
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the source address in the packet to be the address of the firewall's
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external interface; in other words, the firewall makes it look as
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if the firewall itself is initiating the connection. This is
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necessary
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so that the destination host will be able to route return packets back
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to the firewall (remember that packets whose destination address
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is reserved by RFC 1918 can't be routed across the internet so the
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remote host can't address its response to computer 1). When the
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firewall
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receives a return packet, it rewrites the destination address back to
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10.10.10.1 and forwards the packet on to computer 1. </p>
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<p align="left">On Linux systems, the above process is often referred
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to
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as<i> IP Masquerading</i> but you will also see the term <i>Source
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Network
|
|
Address Translation </i>(SNAT) used. Shorewall follows the convention
|
|
used
|
|
with Netfilter:</p>
|
|
<ul>
|
|
<li>
|
|
<p align="left"><i>Masquerade</i> describes the case where you let
|
|
your firewall system automatically detect the external interface
|
|
address. </p>
|
|
</li>
|
|
<li>
|
|
<p align="left"><i>SNAT</i> refers to the case when you explicitly
|
|
specify the source address that you want outbound packets from your
|
|
local network to use. </p>
|
|
</li>
|
|
</ul>
|
|
<p align="left">In Shorewall, both Masquerading and SNAT are configured
|
|
with entries in the /etc/shorewall/masq file. You will normally use
|
|
Masquerading if your external IP is dynamic and SNAT if the IP
|
|
is static.</p>
|
|
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
|
|
height="13"> If your external firewall interface is
|
|
<b>eth0</b>, you do not need to modify the file provided with the
|
|
sample. Otherwise, edit /etc/shorewall/masq and change the first column
|
|
to the name of your external interface and the second column to the
|
|
name of your internal interface.</p>
|
|
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
|
|
height="13"> If your external IP is static, you can
|
|
enter it in the third column in the /etc/shorewall/masq 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.<br>
|
|
<br>
|
|
<img border="0" src="images/BD21298_.gif" width="13" height="13" alt="">
|
|
If you are using the Debian package, please check
|
|
your
|
|
shorewall.conf file to ensure that the following are set correctly;
|
|
if they are not, change them appropriately:<br>
|
|
</p>
|
|
<ul>
|
|
<li>NAT_ENABLED=Yes (Shorewall versions earlier than 1.4.6)</li>
|
|
<li>IP_FORWARDING=On<br>
|
|
</li>
|
|
</ul>
|
|
<h2 align="left">Port Forwarding (DNAT)</h2>
|
|
<p align="left">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 SNAT to rewrite the source address in the response.</p>
|
|
<p align="left">The above process is called<i> Port Forwarding</i> or <i>
|
|
Destination Network Address Translation</i> (DNAT). You configure port
|
|
forwarding using DNAT rules in the /etc/shorewall/rules file.</p>
|
|
<p>The general form of a simple port forwarding rule in
|
|
/etc/shorewall/rules is:</p>
|
|
<blockquote>
|
|
<table border="1" cellpadding="2" style="border-collapse: collapse;"
|
|
id="AutoNumber4">
|
|
<tbody>
|
|
<tr>
|
|
<td><u><b>ACTION</b></u></td>
|
|
<td><u><b>SOURCE</b></u></td>
|
|
<td><u><b>DESTINATION</b></u></td>
|
|
<td><u><b>PROTOCOL</b></u></td>
|
|
<td><u><b>PORT</b></u></td>
|
|
<td><u><b>SOURCE PORT</b></u></td>
|
|
<td><u><b>ORIGINAL ADDRESS</b></u></td>
|
|
</tr>
|
|
<tr>
|
|
<td>DNAT</td>
|
|
<td>net</td>
|
|
<td>loc:<i><server local ip address> </i>[:<i><server
|
|
port></i>]</td>
|
|
<td><i><protocol></i></td>
|
|
<td><i><port></i></td>
|
|
<td> </td>
|
|
<td> </td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</blockquote>
|
|
<p>Example 1 - you run a Web Server on computer 2 and you want to
|
|
forward
|
|
incoming TCP port 80 to that system:</p>
|
|
<blockquote>
|
|
<table border="1" cellpadding="2" style="border-collapse: collapse;"
|
|
id="AutoNumber4">
|
|
<tbody>
|
|
<tr>
|
|
<td><u><b>ACTION</b></u></td>
|
|
<td><u><b>SOURCE</b></u></td>
|
|
<td><u><b>DESTINATION</b></u></td>
|
|
<td><u><b>PROTOCOL</b></u></td>
|
|
<td><u><b>PORT</b></u></td>
|
|
<td><u><b>SOURCE PORT</b></u></td>
|
|
<td><u><b>ORIGINAL ADDRESS</b></u></td>
|
|
</tr>
|
|
<tr>
|
|
<td>DNAT</td>
|
|
<td>net</td>
|
|
<td>loc:10.10.10.2</td>
|
|
<td>tcp</td>
|
|
<td>80</td>
|
|
<td> </td>
|
|
<td> </td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</blockquote>
|
|
<p>Example 2 - you run an FTP Server on computer 1 so you want to
|
|
forward
|
|
incoming TCP port 21 to that system:</p>
|
|
<blockquote>
|
|
<table border="1" cellpadding="2" style="border-collapse: collapse;"
|
|
id="AutoNumber4">
|
|
<tbody>
|
|
<tr>
|
|
<td><u><b>ACTION</b></u></td>
|
|
<td><u><b>SOURCE</b></u></td>
|
|
<td><u><b>DESTINATION</b></u></td>
|
|
<td><u><b>PROTOCOL</b></u></td>
|
|
<td><u><b>PORT</b></u></td>
|
|
<td><u><b>SOURCE PORT</b></u></td>
|
|
<td><u><b>ORIGINAL ADDRESS</b></u></td>
|
|
</tr>
|
|
<tr>
|
|
<td>DNAT</td>
|
|
<td>net</td>
|
|
<td>loc:10.10.10.1</td>
|
|
<td>tcp</td>
|
|
<td>21<br>
|
|
</td>
|
|
<td> </td>
|
|
<td> </td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</blockquote>
|
|
<p>For FTP, you will also need to have FTP connection tracking and NAT
|
|
support
|
|
in your kernel. For vendor-supplied kernels, this means that the
|
|
ip_conntrack_ftp
|
|
and ip_nat_ftp modules must be loaded. Shorewall will automatically
|
|
load
|
|
these modules if they are available and located in the standard place
|
|
under
|
|
/lib/modules/<<i>kernel version</i>>/kernel/net/ipv4/netfilter.<br>
|
|
</p>
|
|
<p>A couple of important points to keep in mind:</p>
|
|
<ul>
|
|
<li>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 FTP server from inside your firewall using the IP address of
|
|
your external interface, see <a href="FAQ.htm#faq2">Shorewall FAQ #2</a>.</li>
|
|
<li>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.</li>
|
|
</ul>
|
|
<blockquote>
|
|
<table border="1" cellpadding="2" style="border-collapse: collapse;"
|
|
id="AutoNumber4">
|
|
<tbody>
|
|
<tr>
|
|
<td><u><b>ACTION</b></u></td>
|
|
<td><u><b>SOURCE</b></u></td>
|
|
<td><u><b>DESTINATION</b></u></td>
|
|
<td><u><b>PROTOCOL</b></u></td>
|
|
<td><u><b>PORT</b></u></td>
|
|
<td><u><b>SOURCE PORT</b></u></td>
|
|
<td><u><b>ORIGINAL ADDRESS</b></u></td>
|
|
</tr>
|
|
<tr>
|
|
<td>DNAT</td>
|
|
<td>net</td>
|
|
<td>loc:10.10.10.2:80</td>
|
|
<td>tcp</td>
|
|
<td>5000</td>
|
|
<td> </td>
|
|
<td> </td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</blockquote>
|
|
<p> <img border="0" src="images/BD21298_.gif" width="13" height="13">
|
|
At this point, modify /etc/shorewall/rules to
|
|
add any DNAT rules that you require.</p>
|
|
<h2 align="left">Domain Name Server (DNS)</h2>
|
|
<p align="left">Normally, when you connect to your ISP, as part of
|
|
getting an IP address your firewall's <i>Domain Name Service </i>(DNS)
|
|
resolver will be automatically configured (e.g., the /etc/resolv.conf
|
|
file will be written). Alternatively, your ISP may have given you
|
|
the IP address of a pair of DNS <i> name servers</i> for you to
|
|
manually configure as your primary and secondary name servers.
|
|
Regardless
|
|
of how DNS gets configured on your firewall, it is <u>your</u>
|
|
responsibility to configure the resolver in your internal systems. You
|
|
can take
|
|
one of two approaches:</p>
|
|
<ul>
|
|
<li>
|
|
<p align="left">You can configure your internal systems to use your
|
|
ISP's name servers. If you 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. </p>
|
|
</li>
|
|
<li>
|
|
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
|
|
height="13"> You can configure a<i> Caching Name
|
|
Server </i>on your firewall.<i> </i>Red Hat has an RPM for a caching
|
|
name server (the RPM also requires the 'bind' RPM) and for Bering
|
|
users, there is dnscache.lrp. 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 IP address of the firewall
|
|
(10.10.10.254 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 UDP and TCP) from the local network to the
|
|
firewall; you do that by adding the following rules in
|
|
/etc/shorewall/rules. </p>
|
|
</li>
|
|
</ul>
|
|
<blockquote>
|
|
<table border="1" cellpadding="2" style="border-collapse: collapse;"
|
|
id="AutoNumber4">
|
|
<tbody>
|
|
<tr>
|
|
<td><u><b>ACTION</b></u></td>
|
|
<td><u><b>SOURCE</b></u></td>
|
|
<td><u><b>DESTINATION</b></u></td>
|
|
<td><u><b>PROTOCOL</b></u></td>
|
|
<td><u><b>PORT</b></u></td>
|
|
<td><u><b>SOURCE PORT</b></u></td>
|
|
<td><u><b>ORIGINAL ADDRESS</b></u></td>
|
|
</tr>
|
|
<tr>
|
|
<td>ACCEPT</td>
|
|
<td>loc</td>
|
|
<td>fw</td>
|
|
<td>tcp</td>
|
|
<td>53</td>
|
|
<td> </td>
|
|
<td> </td>
|
|
</tr>
|
|
<tr>
|
|
<td>ACCEPT</td>
|
|
<td>loc</td>
|
|
<td>fw</td>
|
|
<td>udp</td>
|
|
<td>53</td>
|
|
<td> </td>
|
|
<td> </td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</blockquote>
|
|
<div align="left">
|
|
<h2 align="left">Other Connections</h2>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left">The two-interface sample includes the following rules:</p>
|
|
</div>
|
|
<div align="left">
|
|
<blockquote>
|
|
<table border="1" cellpadding="2" style="border-collapse: collapse;"
|
|
id="AutoNumber4">
|
|
<tbody>
|
|
<tr>
|
|
<td><u><b>ACTION</b></u></td>
|
|
<td><u><b>SOURCE</b></u></td>
|
|
<td><u><b>DESTINATION</b></u></td>
|
|
<td><u><b>PROTOCOL</b></u></td>
|
|
<td><u><b>PORT</b></u></td>
|
|
<td><u><b>SOURCE PORT</b></u></td>
|
|
<td><u><b>ORIGINAL ADDRESS</b></u></td>
|
|
</tr>
|
|
<tr>
|
|
<td>ACCEPT</td>
|
|
<td>fw</td>
|
|
<td>net</td>
|
|
<td>tcp</td>
|
|
<td>53</td>
|
|
<td> </td>
|
|
<td> </td>
|
|
</tr>
|
|
<tr>
|
|
<td>ACCEPT</td>
|
|
<td>fw</td>
|
|
<td>net</td>
|
|
<td>udp</td>
|
|
<td>53</td>
|
|
<td> </td>
|
|
<td> </td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</blockquote>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left">Those rules allow DNS access from your firewall and may
|
|
be removed if you uncommented the line in /etc/shorewall/policy
|
|
allowing all connections from the firewall to the internet.</p>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left">The sample also includes:</p>
|
|
</div>
|
|
<div align="left">
|
|
<blockquote>
|
|
<table border="1" cellpadding="2" style="border-collapse: collapse;"
|
|
id="AutoNumber4">
|
|
<tbody>
|
|
<tr>
|
|
<td><u><b>ACTION</b></u></td>
|
|
<td><u><b>SOURCE</b></u></td>
|
|
<td><u><b>DESTINATION</b></u></td>
|
|
<td><u><b>PROTOCOL</b></u></td>
|
|
<td><u><b>PORT</b></u></td>
|
|
<td><u><b>SOURCE PORT</b></u></td>
|
|
<td><u><b>ORIGINAL ADDRESS</b></u></td>
|
|
</tr>
|
|
<tr>
|
|
<td>ACCEPT</td>
|
|
<td>loc</td>
|
|
<td>fw</td>
|
|
<td>tcp</td>
|
|
<td>22</td>
|
|
<td> </td>
|
|
<td> </td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</blockquote>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left">That rule allows you to run an SSH server on your
|
|
firewall and connect to that server from your local systems.</p>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left">If you wish to enable other connections between your
|
|
firewall and other systems, the general format is:</p>
|
|
</div>
|
|
<div align="left">
|
|
<blockquote>
|
|
<table border="1" cellpadding="2" style="border-collapse: collapse;"
|
|
id="AutoNumber4">
|
|
<tbody>
|
|
<tr>
|
|
<td><u><b>ACTION</b></u></td>
|
|
<td><u><b>SOURCE</b></u></td>
|
|
<td><u><b>DESTINATION</b></u></td>
|
|
<td><u><b>PROTOCOL</b></u></td>
|
|
<td><u><b>PORT</b></u></td>
|
|
<td><u><b>SOURCE PORT</b></u></td>
|
|
<td><u><b>ORIGINAL ADDRESS</b></u></td>
|
|
</tr>
|
|
<tr>
|
|
<td>ACCEPT</td>
|
|
<td><i><source zone></i></td>
|
|
<td><i><destination zone></i></td>
|
|
<td><i><protocol></i></td>
|
|
<td><i><port></i></td>
|
|
<td> </td>
|
|
<td> </td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</blockquote>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left">Example - You want to run a Web Server on your firewall
|
|
system:</p>
|
|
</div>
|
|
<div align="left">
|
|
<blockquote>
|
|
<table border="1" cellpadding="2" style="border-collapse: collapse;"
|
|
id="AutoNumber4">
|
|
<tbody>
|
|
<tr>
|
|
<td><u><b>ACTION</b></u></td>
|
|
<td><u><b>SOURCE</b></u></td>
|
|
<td><u><b>DESTINATION</b></u></td>
|
|
<td><u><b>PROTOCOL</b></u></td>
|
|
<td><u><b>PORT</b></u></td>
|
|
<td><u><b>SOURCE PORT</b></u></td>
|
|
<td><u><b>ORIGINAL ADDRESS</b></u></td>
|
|
</tr>
|
|
<tr>
|
|
<td>ACCEPT</td>
|
|
<td>net</td>
|
|
<td>fw</td>
|
|
<td>tcp</td>
|
|
<td>80</td>
|
|
<td>#Allow web access</td>
|
|
<td>from the internet</td>
|
|
</tr>
|
|
<tr>
|
|
<td>ACCEPT</td>
|
|
<td>loc</td>
|
|
<td>fw</td>
|
|
<td>tcp</td>
|
|
<td>80</td>
|
|
<td>#Allow web access</td>
|
|
<td>from the local network</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</blockquote>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left">Those two rules would of course be in addition to the
|
|
rules listed above under "You can configure a Caching Name Server
|
|
on your firewall"</p>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left">If you don't know what port and protocol a particular
|
|
application uses, look <a href="ports.htm">here</a>.</p>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left"><b>Important: </b>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:</p>
|
|
</div>
|
|
<div align="left">
|
|
<blockquote>
|
|
<table border="1" cellpadding="2" style="border-collapse: collapse;"
|
|
id="AutoNumber4">
|
|
<tbody>
|
|
<tr>
|
|
<td><u><b>ACTION</b></u></td>
|
|
<td><u><b>SOURCE</b></u></td>
|
|
<td><u><b>DESTINATION</b></u></td>
|
|
<td><u><b>PROTOCOL</b></u></td>
|
|
<td><u><b>PORT</b></u></td>
|
|
<td><u><b>SOURCE PORT</b></u></td>
|
|
<td><u><b>ORIGINAL ADDRESS</b></u></td>
|
|
</tr>
|
|
<tr>
|
|
<td>ACCEPT</td>
|
|
<td>net</td>
|
|
<td>fw</td>
|
|
<td>tcp</td>
|
|
<td>22</td>
|
|
<td> </td>
|
|
<td> </td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</blockquote>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left"><img src="images/leaflogo.gif" alt="(LEAF Logo)"
|
|
width="49" height="36"> Bering users will want to
|
|
add the following two rules to be
|
|
compatible with Jacques's Shorewall configuration.</p>
|
|
<div align="left">
|
|
<blockquote>
|
|
<table border="1" cellpadding="2" style="border-collapse: collapse;"
|
|
id="AutoNumber4">
|
|
<tbody>
|
|
<tr>
|
|
<td><u><b>ACTION</b></u></td>
|
|
<td><u><b>SOURCE</b></u></td>
|
|
<td><u><b>DESTINATION</b></u></td>
|
|
<td><u><b>PROTOCOL</b></u></td>
|
|
<td><u><b>PORT</b></u></td>
|
|
<td><u><b>SOURCE PORT</b></u></td>
|
|
<td><u><b>ORIGINAL ADDRESS</b></u></td>
|
|
</tr>
|
|
<tr>
|
|
<td>ACCEPT</td>
|
|
<td>loc<br>
|
|
</td>
|
|
<td>fw</td>
|
|
<td>udp<br>
|
|
</td>
|
|
<td>53<br>
|
|
</td>
|
|
<td>#Allow DNS Cache to</td>
|
|
<td>work<br>
|
|
</td>
|
|
</tr>
|
|
<tr>
|
|
<td>ACCEPT</td>
|
|
<td>loc</td>
|
|
<td>fw</td>
|
|
<td>tcp</td>
|
|
<td>80</td>
|
|
<td>#Allow weblet to work</td>
|
|
<td><br>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</blockquote>
|
|
</div>
|
|
<p align="left"><br>
|
|
<img border="0" src="images/BD21298_.gif" width="13" height="13">
|
|
Now edit your /etc/shorewall/rules file to add or
|
|
delete other connections as required.</p>
|
|
</div>
|
|
<div align="left">
|
|
<h2 align="left">Starting and Stopping Your Firewall</h2>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left"> <img border="0" src="images/BD21298_2.gif" width="13"
|
|
height="13" alt="Arrow"> The <a href="Install.htm">installation
|
|
procedure </a> 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 /etc/shorewall/startup_disabled.<br>
|
|
</p>
|
|
<p align="left"><font color="#ff0000"><b>IMPORTANT</b>: </font><font
|
|
color="#ff0000">Users of the .deb package must edit
|
|
/etc/default/shorewall and set 'startup=1'.</font><br>
|
|
</p>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left">The firewall is started using the "shorewall start"
|
|
command and stopped using "shorewall stop". When the firewall is
|
|
stopped, routing is enabled on those hosts that have an entry in <a
|
|
href="Documentation.htm#Routestopped">/etc/shorewall/routestopped</a>.
|
|
A running firewall may be restarted using the "shorewall restart"
|
|
command. If you want to totally remove any trace of Shorewall from your
|
|
Netfilter configuration, use "shorewall clear".</p>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
|
|
height="13"> The two-interface sample assumes that
|
|
you want
|
|
to enable routing to/from <b>eth1 </b>(the local network) when
|
|
Shorewall is stopped. If your local network isn't connected to <b>eth1</b>
|
|
or if you wish to enable access to/from other hosts, change
|
|
/etc/shorewall/routestopped accordingly.</p>
|
|
</div>
|
|
<div align="left">
|
|
<p align="left"><b>WARNING: </b>If you are connected to your firewall
|
|
from the internet, do not issue a "shorewall stop" command unless you
|
|
have added an entry for the IP address that you are connected from to <a
|
|
href="Documentation.htm#Routestopped">/etc/shorewall/routestopped</a>.
|
|
Also, I don't recommend using "shorewall restart"; it is better
|
|
to create an <i><a href="configuration_file_basics.htm#Configs">alternate
|
|
configuration</a></i> and test it using the <a
|
|
href="starting_and_stopping_shorewall.htm">"shorewall try" command</a>.<br>
|
|
</p>
|
|
<h2>Additional Recommended Reading</h2>
|
|
I highly recommend that you review the <a
|
|
href="configuration_file_basics.htm">Common Configuration File
|
|
Features page</a> -- it contains helpful tips about Shorewall features
|
|
than make administering your firewall easier.
|
|
</div>
|
|
<p align="left"><font size="2">Last updated 11/15/2003 - <a
|
|
href="support.htm">Tom Eastep</a></font></p>
|
|
<p align="left"><a href="copyright.htm"><font size="2">Copyright 2002,
|
|
2003 Thomas M. Eastep</font></a><br>
|
|
</p>
|
|
</body>
|
|
</html>
|