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899 lines
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HTML
899 lines
30 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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<table border="0" cellpadding="0" cellspacing="0"
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style="border-collapse: collapse;" width="100%" id="AutoNumber5"
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bgcolor="#400169" height="90">
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<tbody>
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<tr>
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<td width="100%">
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<h1 align="center"><font color="#ffffff">Basic Two-Interface Firewall</font></h1>
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</td>
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</tr>
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</tbody>
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</table>
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<p align="left">Setting up a Linux system as a firewall for a small network
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is a fairly straight-forward task if you understand the basics and follow
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the documentation.</p>
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<p>This guide doesn't attempt to acquaint you with all of the features of
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||
Shorewall. It rather focuses on what is required to configure Shorewall
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||
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>Single public IP address.</li>
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<li>Internet connection through cable modem, DSL, ISDN, Frame Relay, dial-up
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...</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">
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</p>
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<p>This guide assumes that you have the iproute/iproute2 package installed
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||
(on RedHat, the package is called <i>iproute</i>)<i>. </i>You can tell if
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this package is installed by the presence of an <b>ip</b> program on your
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||
firewall system. As root, you can use the 'which' command to check for this
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||
program:</p>
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||
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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 yourself
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||
with what's involved then go back through it again making your configuration
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||
changes. Points at which configuration changes are recommended are flagged
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||
with <img border="0" src="images/BD21298_.gif" width="13" height="13">
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||
.</p>
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||
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<p><img border="0" src="images/j0213519.gif" width="60" height="60">
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<EFBFBD><EFBFBD><EFBFBD> If you edit your configuration files on a Windows system, you must save
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||
them as Unix files if your editor supports that option or you must run them
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||
through dos2unix before trying to use them. Similarly, if you copy a configuration
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||
file from your Windows hard drive to a floppy disk, you must run dos2unix
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||
against the copy before using it with Shorewall.</p>
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||
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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 Version
|
||
of dos2unix</a></li>
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||
|
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</ul>
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<h2 align="left">Shorewall Concepts</h2>
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<p>The configuration files for Shorewall are contained in the directory /etc/shorewall
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-- for simple setups, you will only need to deal with a few of these as
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described in this guide. After you have <a href="Install.htm">installed
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Shorewall</a>, download the <a
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||
href="/pub/shorewall/LATEST.samples/two-interfaces.tgz">two-interface sample</a>,
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un-tar it (tar -zxvf two-interfaces.tgz) and and copy the files to /etc/shorewall
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||
(these files will replace files with the same name).</p>
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||
|
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<p>As each file is introduced, I suggest that you look through the actual
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||
file on your system -- each file contains detailed configuration instructions
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||
and default entries.</p>
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||
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<p>Shorewall views the network where it is running as being composed of a
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set of <i>zones.</i> In the two-interface sample configuration, the following
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||
zone names are used:</p>
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||
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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"> /etc/shorewall/zones</a>
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file.</p>
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<p>Shorewall also recognizes the firewall system as its own zone - by default,
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||
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 expressed
|
||
in terms of zones.</p>
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||
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<ul>
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||
<li>You express your default policy for connections from one zone to another
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||
zone in the<a href="Documentation.htm#Policy"> /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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||
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||
<p>For each connection request entering the firewall, the request is first
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||
checked against the /etc/shorewall/rules file. If no rule in that file matches
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the connection request then the first policy in /etc/shorewall/policy that
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||
matches the request is applied. If that policy is REJECT or DROP<4F> the
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||
request is first checked against the rules in /etc/shorewall/common (the
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||
samples provide that file for you).</p>
|
||
|
||
<p>The /etc/shorewall/policy file included with the two-interface sample
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has the following policies:</p>
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||
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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><EFBFBD></td>
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<td><EFBFBD></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><EFBFBD></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><EFBFBD></td>
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</tr>
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||
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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 commented
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out. If you want your firewall system to have full access to servers on
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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><EFBFBD></td>
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||
<td><EFBFBD></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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||
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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 internet</li>
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||
<li>drop (ignore) all connection requests from the internet to your firewall
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||
or local network</li>
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||
<li>optionally accept all connection requests from the firewall to the
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internet (if you uncomment the additional policy)</li>
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||
<li>reject all other connection requests.</li>
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||
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||
</ol>
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||
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<p><img border="0" src="images/BD21298_.gif" width="13" height="13">
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<EFBFBD><EFBFBD><EFBFBD> At this point, edit your /etc/shorewall/policy and make any changes that
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you wish.</p>
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<h2 align="left">Network Interfaces</h2>
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||
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<p align="center"> <img border="0" src="images/basics.png" width="444"
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height="635">
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</p>
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<p align="left">The firewall has two network interfaces. Where Internet connectivity
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is through a cable or DSL "Modem", the <i>External Interface</i> will be
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||
the ethernet adapter that is connected to that "Modem" (e.g., <b>eth0</b>)<29>
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||
<u>unless</u> you connect via <i><u>P</u>oint-to-<u>P</u>oint <u>P</u>rotocol
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||
over <u>E</u>thernet</i> (PPPoE) or <i><u>P</u>oint-to-<u>P</u>oint <u>T</u>unneling
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||
<u>P</u>rotocol </i>(PPTP) in which case the External Interface will be a
|
||
ppp interface (e.g., <b>ppp0</b>). If you connect via a regular modem, your
|
||
External Interface will also be <b>ppp0</b>. If you connect via ISDN, your
|
||
external interface will be <b>ippp0.</b></p>
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||
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||
<p align="left"><img border="0" src="images/BD21298_1.gif" width="13"
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||
height="13">
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||
<EFBFBD><EFBFBD><EFBFBD> If your external interface is <b>ppp0</b> or<b> ippp0</b><EFBFBD> then you
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||
will want to set CLAMPMSS=yes in <a href="Documentation.htm#Conf"> /etc/shorewall/shorewall.conf.</a></p>
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||
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||
<p align="left">Your <i>Internal Interface</i> will be an ethernet adapter
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||
(eth1 or eth0) and will be connected to a hub or switch. Your other computers
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||
will be connected to the same hub/switch (note: If you have only a single
|
||
internal system, you can connect the firewall directly to the computer using
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||
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">
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||
</b></u>Do not connect the internal and external interface to the same hub
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||
or switch (even for testing). It won't work the way that you think that it
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||
will and you will end up confused and believing that Shorewall doesn't work
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||
at all.</p>
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||
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||
<p align="left"><img border="0" src="images/BD21298_.gif" align="left"
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||
width="13" height="13">
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||
<EFBFBD><EFBFBD><EFBFBD> The Shorewall two-interface sample configuration assumes that the external
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||
interface is <b>eth0</b> and the internal interface is <b>eth1</b>. If your
|
||
configuration is different, you will have to modify the sample <a
|
||
href="Documentation.htm#Interfaces">/etc/shorewall/interfaces</a> file accordingly.
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||
While you are there, you may wish to review the list of options that are
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||
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>,
|
||
you can replace the "detect" in the second column with "-". </p>
|
||
</li>
|
||
<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 option
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||
list. </p>
|
||
</li>
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||
</ul>
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||
|
||
<h2 align="left">IP Addresses</h2>
|
||
|
||
<p align="left">Before going further, we should say a few words about Internet
|
||
Protocol (IP) <i>addresses</i>. Normally, your ISP will assign you a single
|
||
<i> Public</i> IP address. This address may be assigned via the<i> Dynamic
|
||
Host Configuration Protocol</i> (DHCP) or as part of establishing your connection
|
||
when you dial in (standard modem) or establish your PPP connection. In rare
|
||
cases, your ISP may assign you a<i> static</i> IP address; that means that
|
||
you configure your firewall's external interface to use that address permanently.<i>
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||
</i>However your external address is assigned, it will be shared by all of
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||
your systems when you access the Internet. You will have to assign your
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||
own addresses in your internal network (the Internal Interface on your firewall
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||
plus your other computers). RFC 1918 reserves several <i>Private </i>IP
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||
address ranges for this purpose:</p>
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||
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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>
|
||
</div>
|
||
|
||
<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">
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||
<EFBFBD><EFBFBD><EFBFBD> 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 /etc/shorewall/interfaces.</p>
|
||
</div>
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||
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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>.<2E> For our purposes, we can consider a subnet
|
||
to consists of a range of addresses x.y.z.0 - x.y.z.255. Such a subnet
|
||
will have a <i>Subnet Mask </i>of 255.255.255.0. The address x.y.z.0 is
|
||
reserved as the <i>Subnet Address</i> and x.y.z.255 is reserved as the
|
||
<i>Subnet Broadcast</i> <i>Address</i>. In Shorewall, a subnet is described
|
||
using <a href="subnet_masks.htm"> <i>Variable-Length Subnet Mask </i>(VLSM)
|
||
notation</a> with consists of the subnet address followed by "/24". The
|
||
"24" refers to the number of consecutive leading "1" bits from the left
|
||
of the subnet mask. </p>
|
||
</div>
|
||
|
||
<div align="left">
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||
<p align="left">Example sub-network:</p>
|
||
</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>
|
||
<tr>
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||
<td><b>Broadcast Address:</b></td>
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||
<td>10.10.10.255</td>
|
||
</tr>
|
||
<tr>
|
||
<td><b>VLSM Notation:</b></td>
|
||
<td>10.10.10.0/24</td>
|
||
</tr>
|
||
|
||
</tbody>
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||
</table>
|
||
</blockquote>
|
||
</div>
|
||
|
||
<div align="left">
|
||
<p align="left">It is conventional to assign the internal interface either
|
||
the first usable address in the subnet (10.10.10.1 in the above example)
|
||
or the last usable address (10.10.10.254).</p>
|
||
</div>
|
||
|
||
<div align="left">
|
||
<p align="left">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<i><EFBFBD> gateway</i><EFBFBD> (router).</p>
|
||
</div>
|
||
|
||
<div align="left">
|
||
<p align="left"><img border="0" src="images/BD21298_1.gif" width="13"
|
||
height="13">
|
||
<EFBFBD><EFBFBD><EFBFBD> Your local computers (computer 1 and computer 2 in the above diagram)
|
||
should be configured with their<i> default gateway</i> to be the IP address
|
||
of the firewall's internal interface.<i><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> </i> </p>
|
||
</div>
|
||
|
||
<p align="left">The foregoing short discussion barely scratches the surface
|
||
regarding subnetting and routing. If you are interested in learning more
|
||
about IP addressing and routing, I highly recommend <i>"IP Fundamentals:
|
||
What Everyone Needs to Know about Addressing & Routing",</i> Thomas
|
||
A. Maufer, Prentice-Hall, 1999, ISBN 0-13-975483-0.</p>
|
||
|
||
<p align="left">The remainder of this quide will assume that you have configured
|
||
your network as shown here:</p>
|
||
|
||
<p align="center"> <img border="0" src="images/basics1.png" width="444"
|
||
height="635">
|
||
</p>
|
||
|
||
<p align="left">The default gateway for computer's 1 & 2 would be 10.10.10.254.</p>
|
||
|
||
<h2 align="left">IP Masquerading (SNAT)</h2>
|
||
|
||
<p align="left">The addresses reserved by RFC 1918 are sometimes referred
|
||
to as <i>non-routable</i> 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 <i>Network Address Translation </i>(NAT).
|
||
The firewall rewrites the source address in the packet to be the address
|
||
of the firewall's external interface; in other words, the firewall makes
|
||
it look as if the firewall itself is initiating the connection.<2E> 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 10.10.10.1 and
|
||
forwards the packet on to computer 1. </p>
|
||
|
||
<p align="left">On Linux systems, the above process is often referred to
|
||
as<i> IP Masquerading</i> but you will also see the term <i>Source 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">
|
||
<EFBFBD><EFBFBD><EFBFBD> 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">
|
||
<EFBFBD><EFBFBD><EFBFBD> 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. </p>
|
||
|
||
<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><EFBFBD></td>
|
||
<td><EFBFBD></td>
|
||
</tr>
|
||
|
||
</tbody>
|
||
</table>
|
||
</blockquote>
|
||
|
||
<p>Example - 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><EFBFBD></td>
|
||
<td><EFBFBD></td>
|
||
</tr>
|
||
|
||
</tbody>
|
||
</table>
|
||
</blockquote>
|
||
|
||
<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 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><EFBFBD></td>
|
||
<td><EFBFBD></td>
|
||
</tr>
|
||
|
||
</tbody>
|
||
</table>
|
||
</blockquote>
|
||
|
||
<p> <img border="0" src="images/BD21298_.gif" width="13" height="13">
|
||
<EFBFBD><EFBFBD><EFBFBD> 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">
|
||
<EFBFBD><EFBFBD><EFBFBD> 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><EFBFBD></td>
|
||
<td><EFBFBD></td>
|
||
</tr>
|
||
<tr>
|
||
<td>ACCEPT</td>
|
||
<td>loc</td>
|
||
<td>fw</td>
|
||
<td>udp</td>
|
||
<td>53</td>
|
||
<td><EFBFBD></td>
|
||
<td><EFBFBD></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><EFBFBD></td>
|
||
<td><EFBFBD></td>
|
||
</tr>
|
||
<tr>
|
||
<td>ACCEPT</td>
|
||
<td>fw</td>
|
||
<td>net</td>
|
||
<td>udp</td>
|
||
<td>53</td>
|
||
<td><EFBFBD></td>
|
||
<td><EFBFBD></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 commented out 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><EFBFBD></td>
|
||
<td><EFBFBD></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><EFBFBD></td>
|
||
<td><EFBFBD></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><EFBFBD></td>
|
||
<td><EFBFBD></td>
|
||
</tr>
|
||
|
||
</tbody>
|
||
</table>
|
||
</blockquote>
|
||
</div>
|
||
|
||
<div align="left">
|
||
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
|
||
height="13">
|
||
<EFBFBD><EFBFBD><EFBFBD> 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">The <a href="Install.htm">installation procedure </a> configures
|
||
your system to start Shorewall at system boot.</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">
|
||
<EFBFBD><EFBFBD><EFBFBD> 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="Documentation.htm#Configs">alternate configuration</a></i>
|
||
and test it using the <a href="Documentation.htm#Starting">"shorewall
|
||
try" command</a>.</p>
|
||
</div>
|
||
|
||
<p align="left"><font size="2">Last updated 9/16/2002 - <a
|
||
href="support.htm">Tom Eastep</a></font></p>
|
||
|
||
<p align="left"><a href="copyright.htm"><font size="2">Copyright 2002 Thomas
|
||
M. Eastep</font></a></p>
|
||
<br>
|
||
</body>
|
||
</html>
|