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<meta name="ProgId" content="FrontPage.Editor.Document">
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<title>Three-Interface Firewall</title>
</head>
<body>
<table border="0" cellpadding="0" cellspacing="0"
style="border-collapse: collapse;" bordercolor="#111111" width="100%"
id="AutoNumber5" bgcolor="#400169" height="90">
<tbody>
<tr>
<td width="100%">
<h1 align="center"><font color="#ffffff">Three-Interface Firewall</font></h1>
</td>
</tr>
</tbody>
</table>
<h2 align="center">Version 2.0.1</h2>
<p align="left">Setting up a Linux system as a firewall for a small network
with DMZ is a fairly straight-forward task if you understand the
basics and follow the documentation.</p>
<p>This guide doesn't attempt to acquaint you with all of the features of
Shorewall. It rather focuses on what is required to configure Shorewall
in one of its more popular configurations:</p>
<ul>
<li>Linux system used as a firewall/router for a small local
network.</li>
<li>Single public IP address.</li>
<li>DMZ connected to a separate ethernet interface.</li>
<li>Connection through DSL, Cable Modem, ISDN, Frame Relay,
dial-up, ...</li>
</ul>
<p align="left">Here is a schematic of a typical installation.</p>
<p align="center"> <img border="0" src="images/dmz1.png" width="692"
height="635">
</p>
<p>This guide assumes that you have the iproute/iproute2 package installed
(on RedHat, the package is called <i>iproute</i>)<i>. </i>You can
tell if this package is installed by the presence of an <b>ip</b> program
on your firewall system. As root, you can use the 'which' command to
check for this program:</p>
<pre> [root@gateway root]# which ip<br> /sbin/ip<br> [root@gateway root]#</pre>
<p>I recommend that you first read through the guide to familiarize yourself
with what's involved then go back through it again making your configuration
changes. Points at which configuration changes are recommended are
flagged with <img border="0" src="images/BD21298_.gif" width="13"
height="13">
</p>
<p><img border="0" src="images/j0213519.gif" width="60" height="60">
<20><><EFBFBD> If you edit your configuration files on a Windows system,
you must save them as Unix files if your editor supports that option
or you must run them through dos2unix before trying to use them. Similarly,
if you copy a configuration file from your Windows hard drive to a floppy
disk, you must run dos2unix against the copy before using it with Shorewall.</p>
<ul>
<li><a href="http://www.simtel.net/pub/pd/51438.html">Windows
Version of dos2unix</a></li>
<li><a
href="http://www.megaloman.com/%7Ehany/software/hd2u/">Linux Version
of dos2unix</a></li>
</ul>
<h2 align="left">Shorewall Concepts</h2>
<p> <img border="0" src="images/BD21298_.gif" width="13" height="13"
alt="">
<20><><EFBFBD> The configuration files for Shorewall are contained in the directory
/etc/shorewall -- for simple setups, you will only need to deal with a
few of these as described in this guide. After you have <a
href="Install.htm">installed Shorewall</a>, <b>download the <a
href="/pub/shorewall/LATEST.samples/three-interfaces.tgz">three-interface
sample</a>, un-tar it (tar -zxvf three-interfaces.tgz) and and copy
the files to /etc/shorewall (the files will replace files with the same
names that were placed in /etc/shorewall when Shorewall was installed)</b>.</p>
<p>As each file is introduced, I suggest that you look through the actual
file on your system -- each file contains detailed configuration instructions
and default entries.</p>
<p>Shorewall views the network where it is running as being composed of a
set of <i>zones.</i> In the three-interface sample configuration,
the following zone names are used:</p>
<table border="0" style="border-collapse: collapse;" cellpadding="3"
cellspacing="0" id="AutoNumber2">
<tbody>
<tr>
<td><u><b>Name</b></u></td>
<td><u><b>Description</b></u></td>
</tr>
<tr>
<td><b>net</b></td>
<td><b>The Internet</b></td>
</tr>
<tr>
<td><b>loc</b></td>
<td><b>Your Local Network</b></td>
</tr>
<tr>
<td><b>dmz</b></td>
<td><b>Demilitarized Zone</b></td>
</tr>
</tbody>
</table>
<p>Zone names are defined in <a href="Documentation.htm#Zones"> /etc/shorewall/zones</a>.</p>
<p>Shorewall also recognizes the firewall system as its own zone - by default,
the firewall itself is known as <b>fw</b>.</p>
<p>Rules about what traffic to allow and what traffic to deny are expressed
in terms of zones.</p>
<ul>
<li>You express your default policy for connections from one
zone to another zone in the<a href="Documentation.htm#Policy"> /etc/shorewall/policy
</a>file.</li>
<li>You define exceptions to those default policies in the
<a href="Documentation.htm#Rules">/etc/shorewall/rules </a>file.</li>
</ul>
<p>For each connection request entering the firewall, the request is first
checked against the /etc/shorewall/rules file. If no rule in that
file matches the connection request then the first policy in /etc/shorewall/policy
that matches the request is applied. If that policy is REJECT or
DROP<EFBFBD> the request is first checked against the rules in /etc/shorewall/common
(the samples provide that file for you).</p>
<p>The /etc/shorewall/policy file included with the three-interface sample
has the following policies:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber3">
<tbody>
<tr>
<td><u><b>Source Zone</b></u></td>
<td><u><b>Destination Zone</b></u></td>
<td><u><b>Policy</b></u></td>
<td><u><b>Log Level</b></u></td>
<td><u><b>Limit:Burst</b></u></td>
</tr>
<tr>
<td>loc</td>
<td>net</td>
<td>ACCEPT</td>
<td><EFBFBD></td>
<td><EFBFBD></td>
</tr>
<tr>
<td>net</td>
<td>all</td>
<td>DROP</td>
<td>info</td>
<td><EFBFBD></td>
</tr>
<tr>
<td>all</td>
<td>all</td>
<td>REJECT</td>
<td>info</td>
<td><EFBFBD></td>
</tr>
</tbody>
</table>
</blockquote>
<blockquote>
<p>In the three-interface sample, the line below is included but commented
out. If you want your firewall system to have full access to servers
on the internet, uncomment that line.</p>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber3">
<tbody>
<tr>
<td><u><b>Source Zone</b></u></td>
<td><u><b>Destination Zone</b></u></td>
<td><u><b>Policy</b></u></td>
<td><u><b>Log Level</b></u></td>
<td><u><b>Limit:Burst</b></u></td>
</tr>
<tr>
<td>fw</td>
<td>net</td>
<td>ACCEPT</td>
<td><EFBFBD></td>
<td><EFBFBD></td>
</tr>
</tbody>
</table>
</blockquote>
<p>The above policy will:</p>
<ol>
<li>allow all connection requests from your local network
to the internet</li>
<li>drop (ignore) all connection requests from the internet
to your firewall or local network</li>
<li>optionally accept all connection requests from the firewall
to the internet (if you uncomment the additional policy)</li>
<li>reject all other connection requests.</li>
</ol>
<p><img border="0" src="images/BD21298_1.gif" width="13" height="13">
<20><><EFBFBD> At this point, edit your /etc/shorewall/policy file and
make any changes that you wish.</p>
<h2 align="left">Network Interfaces</h2>
<p align="center"> <img border="0" src="images/dmz1.png" width="692"
height="635">
</p>
<p align="left">The firewall has three network interfaces. Where Internet
connectivity is through a cable or DSL "Modem", the <i>External Interface</i>
will be the ethernet adapter that is connected to that "Modem" (e.g.,
<b>eth0</b>)<29> <u>unless</u> you connect via <i><u>P</u>oint-to-<u>P</u>oint
<u>P</u>rotocol over <u>E</u>thernet</i> (PPPoE) or <i><u>P</u>oint-to-<u>P</u>oint
<u>T</u>unneling <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 using ISDN, you external interface will be <b>ippp0.</b></p>
<p align="left"><img border="0" src="images/BD21298_1.gif" width="13"
height="13">
<20><><EFBFBD> If your external interface is <b>ppp0</b> or <b>ippp0 </b>then
you will want to set CLAMPMSS=yes in <a
href="Documentation.htm#Conf"> /etc/shorewall/shorewall.conf.</a></p>
<p align="left">Your <i>Local Interface</i> will be an ethernet adapter (eth0,
eth1 or eth2) and will be connected to a hub or switch. Your local
computers will be connected to the same switch (note: If you have only
a single local system, you can connect the firewall directly to the
computer using a <i>cross-over </i> cable).</p>
<p align="left">Your <i>DMZ Interface</i> will also be an ethernet adapter
(eth0, eth1 or eth2) and will be connected to a hub or switch. Your
DMZ computers will be connected to the same switch (note: If you have
only a single DMZ system, you can connect the firewall directly to the
computer using a <i>cross-over </i> cable).</p>
<p align="left"><u><b> <img border="0" src="images/j0213519.gif"
width="60" height="60">
</b></u>Do not connect more than one interface to the same hub
or switch (even for testing). It won't work the way that you expect
it to and you will end up confused and believing that Shorewall doesn't
work at all.</p>
<p align="left"><img border="0" src="images/BD21298_2.gif" width="13"
height="13">
<20><><EFBFBD> The Shorewall three-interface sample configuration assumes
that the external interface is <b>eth0, </b>the local interface is
<b>eth1 </b>and the DMZ interface is <b> eth2</b>. If your configuration
is different, you will have to modify the sample /etc/shorewall/interfaces
file accordingly. While you are there, you may wish to review the list
of options that are specified for the interfaces. Some hints:</p>
<ul>
<li>
<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>
<p align="left">If your external interface is <b>ppp0</b> or <b>ippp0</b>
or if you have a static IP address, you can remove "dhcp" from the
option list. </p>
</li>
</ul>
<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> </i>Regardless of how the address
is assigned, it will be shared by all of your systems when you access
the Internet. You will have to assign your own addresses for your internal
network (the local and DMZ Interfaces on your firewall plus your other
computers). RFC 1918 reserves several <i>Private </i>IP address ranges for
this purpose:</p>
<div align="left">
<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">
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
height="13">
<20><><EFBFBD> Before starting Shorewall, you should look at the IP address
of your external interface and if it is one of the above ranges, you
should remove the 'norfc1918' option from the external interface's
entry in /etc/shorewall/interfaces.</p>
</div>
<div align="left">
<p align="left">You will want to assign your local addresses from one <i>
sub-network </i>or <i>subnet</i> and your DMZ addresses from another
subnet. 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<6E><a
href="shorewall_setup_guide.htm#Subnets"><i>Classless InterDomain Routing
</i>(CIDR)</a> notation with consists of the subnet address followed
by "/24". The "24" refers to the number of consecutive "1" bits from
the left of the subnet mask. </p>
</div>
<div align="left">
<p align="left">Example sub-network:</p>
</div>
<div align="left">
<blockquote>
<table border="1" style="border-collapse: collapse;" id="AutoNumber1"
cellpadding="2">
<tbody>
<tr>
<td><b>Range:</b></td>
<td>10.10.10.0 - 10.10.10.255</td>
</tr>
<tr>
<td><b>Subnet Address:</b></td>
<td>10.10.10.0</td>
</tr>
<tr>
<td><b>Broadcast Address:</b></td>
<td>10.10.10.255</td>
</tr>
<tr>
<td><b>CIDR<EFBFBD>Notation:</b></td>
<td>10.10.10.0/24</td>
</tr>
</tbody>
</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">
<20><><EFBFBD> Your local computers (Local Computers 1 &amp; 2) should
be configured with their<i> default gateway</i> set to the IP address
of the firewall's internal interface and your DMZ computers ( DMZ
Computers 1 &amp; 2) should be configured with their default gateway
set to the IP address of the firewall's DMZ interface.<2E><> </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 &amp; 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/dmz2.png" width="721"
height="635">
</p>
<p align="left">The default gateway for the DMZ computers would be 10.10.11.254
and the default gateway for the Local computers would be 10.10.10.254.<br>
</p>
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
height="13" alt="">
<EFBFBD><EFBFBD><EFBFBD> <font color="#ff0000"><b>WARNING: </b></font><b>Your ISP<53> might assign
your external interface an RFC 1918 address. If that address is in the 10.10.10.0/24
subnet then you will need to select a DIFFERENT RFC 1918 subnet for your
local network and if it is in the 10.10.11.0/24 subnet then you will need
to select a different RFC 1918 subnet for your DMZ.</b><br>
</p>
<p align="left">IP Masquerading (SNAT)</p>
<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 local 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
accross the internet). When the firewall receives a return packet, it
rewrites the destination address back to 10.10.10.1 and forwards the
packet on to local computer 1. </p>
<p align="left">On Linux systems, the above process is often referred to
as<i> IP Masquerading</i> and 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.</p>
<p align="left"><img border="0" src="images/BD21298_2.gif" width="13"
height="13">
<20><><EFBFBD> If your external firewall interface is <b>eth0</b>, your
local interface <b>eth1 </b>and your DMZ interface is <b>eth2</b> then
you do not need to modify the file provided with the sample. Otherwise,
edit /etc/shorewall/masq and change it to match your configuration.</p>
<p align="left"><img border="0" src="images/BD21298_2.gif" width="13"
height="13">
<20><><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 <br>
processing outgoing packets a little more efficient.<br>
</p>
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
height="13" alt="">
<20><><EFBFBD> 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</li>
<li>IP_FORWARDING=On<br>
</li>
</ul>
<h2 align="left">Port Forwarding (DNAT)</h2>
<p align="left">One of your goals will be to run one or more servers on your
DMZ computers. Because these computers have RFC-1918 addresses, it
is not possible for clients on the internet to connect directly to
them. It is rather necessary for those clients to address their connection
requests to your firewall who rewrites the destination address to the
address of your server and forwards the packet to that server. When your
server responds, the firewall automatically performs SNAT to rewrite
the source address in the response.</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>dmz:<i>&lt;server local ip address&gt; </i>[:<i>&lt;server
port&gt;</i>]</td>
<td><i>&lt;protocol&gt;</i></td>
<td><i>&lt;port&gt;</i></td>
<td><EFBFBD></td>
<td><EFBFBD></td>
</tr>
</tbody>
</table>
</blockquote>
<p>If you don't specify the <i>&lt;server port&gt;</i>, it is assumed to
be the same as <i>&lt;port&gt;</i>.</p>
<p>Example - you run a Web Server on DMZ 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>dmz:10.10.11.2</td>
<td>tcp</td>
<td>80</td>
<td># Forward port 80</td>
<td>from the internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz:10.10.11.2</td>
<td>tcp</td>
<td>80</td>
<td>#Allow connections </td>
<td>from the local network</td>
</tr>
</tbody>
</table>
</blockquote>
<p>A couple of important points to keep in mind:</p>
<ul>
<li>When you are connecting to your server from your local
systems, you must use the server's internal IP address (10.10.11.2).</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 (e.g., connect to <a
href="http://w.x.y.z:5000"> http://w.x.y.z:5000</a> where w.x.y.z is your
external IP).</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>dmz:10.10.11.2:80</td>
<td>tcp</td>
<td>5000</td>
<td><EFBFBD></td>
<td><EFBFBD></td>
</tr>
</tbody>
</table>
</blockquote>
<p>If you want to be able to access your server from the local network using
your external address, then if you have a static external IP you can
replace the loc-&gt;dmz rule above with:</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>dmz:10.10.11.2:80</td>
<td>tcp</td>
<td>80</td>
<td>-</td>
<td><i>&lt;external IP&gt;</i></td>
</tr>
</tbody>
</table>
</blockquote>
<p>If you have a dynamic ip then you must ensure that your external interface
is up before starting Shorewall and you must take steps as follows
(assume that your external interface is <b>eth0</b>):</p>
<ol>
<li>Include the following in /etc/shorewall/params:<br>
<br>
ETH0_IP=`find_interface_address eth0`<br>
<20></li>
<li>Make your loc-&gt;dmz rule:</li>
</ol>
<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>loc<br>
</td>
<td>dmz:10.10.11.2:80</td>
<td>tcp</td>
<td>80</td>
<td>-</td>
<td>$ETH0_IP</td>
</tr>
</tbody>
</table>
</blockquote>
<p>If you want to access your server from the DMZ using your external IP
address, see <a href="FAQ.htm#faq2a">FAQ 2a</a>.</p>
<p><img border="0" src="images/BD21298_2.gif" width="13" height="13">
<20><><EFBFBD> At this point, add the DNAT and ACCEPT rules for your servers.
</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. 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_2.gif"
width="13" height="13">
<20><><EFBFBD> You can configure a<i> Caching Name Server </i>on your
firewall or in your DMZ.<i> </i>Red Hat has an RPM for a caching name
server (which 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 caching name server 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 if you choose to
run the name server on your firewall. To allow your local systems to
talk to your caching name server, you must open port 53 (both UDP
and TCP) from the local network to the server; you do that by adding
the rules in /etc/shorewall/rules. </p>
</li>
</ul>
<blockquote>
<p align="left">If you run the name server on the firewall:
<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>
<tr>
<td>ACCEPT</td>
<td>dmz</td>
<td>fw</td>
<td>tcp</td>
<td>53</td>
<td><EFBFBD></td>
<td><EFBFBD></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>dmz</td>
<td>fw</td>
<td>udp</td>
<td>53</td>
<td><EFBFBD></td>
<td><EFBFBD></td>
</tr>
</tbody>
</table>
</p>
</blockquote>
<div align="left">
<blockquote>
<p>Run name server on DMZ computer 1</p>
<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>dmz:10.10.11.1</td>
<td>tcp</td>
<td>53</td>
<td><EFBFBD></td>
<td><EFBFBD></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz:10.10.11.1</td>
<td>udp</td>
<td>53</td>
<td><EFBFBD></td>
<td><EFBFBD></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>fw</td>
<td>dmz:10.10.10.1</td>
<td>tcp</td>
<td>53</td>
<td><EFBFBD></td>
<td><EFBFBD></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>fw</td>
<td>dmz:10.10.10.1</td>
<td>udp</td>
<td>53</td>
<td><EFBFBD></td>
<td><EFBFBD></td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<h2 align="left">Other Connections</h2>
</div>
<div align="left">
<p align="left">The three-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>udp</td>
<td>53</td>
<td><EFBFBD></td>
<td><EFBFBD></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>
</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>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz</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 in each of your DMZ systems and to connect to those servers
from your local systems.</p>
</div>
<div align="left">
<p align="left">If you wish to enable other connections between your 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>&lt;source zone&gt;</i></td>
<td><i>&lt;destination zone&gt;</i></td>
<td><i>&lt;protocol&gt;</i></td>
<td><i>&lt;port&gt;</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 publicly-available DNS 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>53</td>
<td>#Allow DNS access</td>
<td>from the internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>fw</td>
<td>tcp</td>
<td>53</td>
<td>#Allow DNS access</td>
<td>from the internet</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 "If you run the 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_2.gif" width="13"
height="13">
<20><><EFBFBD> Now modify /etc/shorewall/rules to add or remove 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">
<20><><EFBFBD> The <a href="Install.htm">installation procedure </a> configures
your system to start Shorewall at system boot<6F> 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_2.gif" width="13"
height="13">
<20><><EFBFBD> The three-interface sample assumes that you want to enable
routing to/from <b>eth1 (</b>your local network) and<b> eth2 </b>(DMZ)
when Shorewall is stopped. If these two interfaces don't connect to
your local network and DMZ or if you want to enable a different set
of hosts, modify /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>.</p>
</div>
<p align="left"><font size="2">Last updated 1/21/2003 - <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></p>
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