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<meta name="ProgId" content="FrontPage.Editor.Document">
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<title>Two-Interface Firewall</title>
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<table border="0" cellpadding="0" cellspacing="0"
style="border-collapse: collapse;" width="100%" id="AutoNumber5"
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<h1 align="center"><font color="#ffffff">Basic Two-Interface Firewall</font></h1>
</td>
</tr>
</tbody>
</table>
<p align="left">Setting up a Linux system as a firewall for a small network
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 its most common configuration:</p>
<ul>
<li>Linux system used as a firewall/router for a small local
network.</li>
<li>Single public IP address.</li>
<li>Internet connection through cable modem, DSL, 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/basics.png" width="444"
height="635">
</p>
<p><b>If you are running Shorewall under Mandrake 9.0 or later, you can easily
configure the above setup using the Mandrake "Internet Connection Sharing"
applet. From the Mandrake Control Center, select "Network &amp; Internet"
then "Connection Sharing". You should not need to refer to this guide.</b><br>
</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/two-interfaces.tgz">two-interface sample</a>,
un-tar it (tar -zxvf two-interfaces.tgz) and and copy the files to /etc/shorewall
(these files will replace files with the same name).</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 two-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>
</tbody>
</table>
<p>Zones are defined in the <a href="Documentation.htm#Zones"> /etc/shorewall/zones</a>
file.</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<4F>
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 two-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 two-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_.gif" width="13" height="13">
<20><><EFBFBD> At this point, edit your /etc/shorewall/policy and make any
changes that you wish.</p>
<h2 align="left">Network Interfaces</h2>
<p align="center"> <img border="0" src="images/basics.png" width="444"
height="635">
</p>
<p align="left">The firewall has two 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 via ISDN,
your 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><EFBFBD>
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>Internal Interface</i> will be an ethernet adapter
(eth1 or eth0) and will be connected to a hub or switch. Your other computers
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 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 the internal and external interface to the
same hub or switch (even for testing). It won't work the way that you think
that it will 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_.gif" align="left"
width="13" height="13">
<20><><EFBFBD> The Shorewall two-interface sample configuration assumes that
the external 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. 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>However your external 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 in your internal network (the
Internal Interface 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 addresses from the same <i>
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<6E><a href="shorewall_setup_guide.htm#Subnets"><i>Classless
InterDomain Routing </i>(CIDR) 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">
<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 (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 &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/basics1.png" width="444"
height="635">
</p>
<p align="left">The default gateway for computer's 1 &amp; 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">
<20><><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">
<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 processing outgoing packets a little more efficient.<br>
<br>
<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 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>&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>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">
<20><><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">
<20><><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 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><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>&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 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">
<20><><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"> <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_.gif" width="13"
height="13">
<20><><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="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 12/20/2002 - <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>
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