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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Three-Interface Firewall</title><meta name="generator" content="DocBook XSL Stylesheets V1.62.4" /></head><body><div class="article" lang="en" xml:lang="en"><div class="titlepage"><div><div><h1 class="title"><a id="three-interface"></a>Three-Interface Firewall</h1></div><div><div class="authorgroup"><div class="author"><h3 class="author"><span class="firstname">Tom</span> <span class="surname">Eastep</span></h3></div></div></div><div><p class="copyright">Copyright © 2002-2004 Thomas M. Eastep</p></div><div><div class="legalnotice"><p>Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, with no Front-Cover, and with no Back-Cover
Texts. A copy of the license is included in the section entitled
“<span class="quote"><a href="GnuCopyright.htm" target="_self">GNU Free Documentation License</a></span>”.</p></div></div><div><p class="pubdate">2004-02-12</p></div></div><div></div><hr /></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="section"><a href="#id2803947">Introduction</a></span></dt><dd><dl><dt><span class="section"><a href="#id2807729">Requirements</a></span></dt><dt><span class="section"><a href="#id2807804">Before you start</a></span></dt><dt><span class="section"><a href="#id2810224">Conventions</a></span></dt></dl></dd><dt><span class="section"><a href="#id2810263">PPTP/ADSL</a></span></dt><dt><span class="section"><a href="#id2810295">Shorewall Concepts</a></span></dt><dt><span class="section"><a href="#id2859594">Network Interfaces</a></span></dt><dt><span class="section"><a href="#id2859977">IP Addresses</a></span></dt><dt><span class="section"><a href="#id2806632">IP Masquerading (SNAT)</a></span></dt><dt><span class="section"><a href="#id2806881">Port Forwarding (DNAT)</a></span></dt><dt><span class="section"><a href="#id2865302">Domain Name Server (DNS)</a></span></dt><dt><span class="section"><a href="#id2865536">Other Connections</a></span></dt><dt><span class="section"><a href="#id2865767">Starting and Stopping Your Firewall</a></span></dt><dt><span class="section"><a href="#id2865997">Additional Recommended Reading</a></span></dt></dl></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2803947"></a>Introduction</h2></div></div><div></div></div><p>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><div class="itemizedlist"><ul type="disc"><li><p>Linux system used as a firewall/router for a small local
network.</p></li><li><p>Single public IP address.</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>If you have more than one public IP address, this is not the
guide you want -- see the <a href="shorewall_setup_guide.htm" target="_self">Shorewall
Setup Guide</a> instead.</p></div></li><li><p>DMZ connected to a separate ethernet interface.</p></li><li><p>Connection through DSL, Cable Modem, ISDN, Frame Relay, dial-up,
...</p></li></ul></div><p>Here is a schematic of a typical installation.</p><div class="figure"><a id="id2807700"></a><p class="title"><b>Figure 1. schematic of a typical installation</b></p><div class="mediaobject" align="center"><img src="images/dmz1.png" align="middle" alt="schematic of a typical installation" /></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2807729"></a>Requirements</h3></div></div><div></div></div><p>Shorewall requires that you have the <span><b class="command">iproute</b></span>/<span><b class="command">iproute2</b></span>
package installed (on <span class="trademark">RedHat</span>™, the package is
called <span><b class="command">iproute</b></span>). You can tell if this package is
installed by the presence of an <span><b class="command">ip</b></span> program on your
firewall system. As <tt class="systemitem">root</tt>, you
can use the <span><b class="command">which</b></span> command to check for this program:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">[root@gateway root]# <span><b class="command">which ip</b></span>
/sbin/ip
[root@gateway root]#</pre></td></tr></table></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2807804"></a>Before you start</h3></div></div><div></div></div><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.</p><div class="caution" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Caution</h3><p>If you edit your configuration files on a
<span class="trademark">Windows</span>™ system, you must save them as
<span class="trademark">Unix</span>™ files if your editor supports that option
or you must run them through <span><b class="command">dos2unix</b></span> before trying
to use them. Similarly, if you copy a configuration file from your
<span class="trademark">Windows</span>™ hard drive to a floppy disk, you must
run <span><b class="command">dos2unix</b></span> against the copy before using it with
Shorewall.</p><div class="itemizedlist"><ul type="disc"><li><p><a href="http://www.simtel.net/pub/pd/51438.html" target="_self">Windows
Version of dos2unix</a></p></li><li><p><a href="http://www.megaloman.com/%7Ehany/software/hd2u/" target="_self">Linux
Version of dos2unix</a></p></li></ul></div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2810224"></a>Conventions</h3></div></div><div></div></div><p>Points at which configuration changes are recommended are flagged
with <img src="images/BD21298_.gif" />.</p><p>Configuration notes that are unique to LEAF/Bering are marked with
<img src="images/leaflogo.gif" />.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2810263"></a>PPTP/ADSL</h2></div></div><div></div></div><p><img src="images/BD21298_.gif" /></p><p>If you have an ADSL Modem and you use PPTP to communicate with a
server in that modem, you must make the <a href="PPTP.htm#PPTP_ADSL" target="_self">changes
recommended here</a> in addition to those detailed below. ADSL with
PPTP is most commonly found in Europe, notably in Austria.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2810295"></a>Shorewall Concepts</h2></div></div><div></div></div><p><img src="images/BD21298_.gif" /></p><p>The configuration files for Shorewall are contained in the directory
<tt class="filename">/etc/shorewall</tt> -- for simple setups, you will only
need to deal with a few of these as described in this guide. After you
have installed Shorewall, download the three-interface sample, un-tar it (<span><b class="command">tar
<tt class="option">-zxvf</tt> <tt class="filename">three-interfaces.tgz</tt></b></span>)
and and copy the files to <tt class="filename">/etc/shorewall</tt> (the files
will replace files with the same names that were placed in
<tt class="filename">/etc/shorewall</tt> when Shorewall was installed).</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 zones. In the three-interface sample configuration, the following
zone names are used:</p><div class="informaltable"><table border="1"><colgroup><col /><col /></colgroup><thead valign="middle"><tr><th align="center">Name</th><th align="center">Description</th></tr></thead><tbody><tr><td align="left">net</td><td align="left">The Internet</td></tr><tr><td align="left">loc</td><td align="left">Your Local Network</td></tr><tr><td align="left">dmz</td><td align="left">Demilitarized Zone</td></tr></tbody></table></div><p>Zone names are defined in <tt class="filename">/etc/shorewall/zones</tt>.</p><p>Shorewall also recognizes the firewall system as its own zone - by
default, the firewall itself is known as <tt class="varname">fw</tt>.</p><p>Rules about what traffic to allow and what traffic to deny are
expressed in terms of zones.</p><div class="itemizedlist"><ul type="disc"><li><p>You express your default policy for connections from one zone to
another zone in the <tt class="filename">/etc/shorewall/policy</tt> file.</p></li><li><p>You define exceptions to those default policies in the
<tt class="filename">/etc/shorewall/rules</tt> file.</p></li></ul></div><p>For each connection request entering the firewall, the request is
first checked against the <tt class="filename">/etc/shorewall/rules</tt> file.
If no rule in that file matches the connection request then the first
policy in <tt class="filename">/etc/shorewall/policy</tt> that matches the
request is applied. If that policy is REJECT or DROP the request is first
checked against the rules in <tt class="filename">/etc/shorewall/common</tt> if
that file exists; otherwise the file <tt class="filename">/etc/shorewall/common.def</tt>
is checked</p><p>The <tt class="filename">/etc/shorewall/policy</tt> file included with
the three-interface sample has the following policies:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
loc net ACCEPT
net all DROP info
all all REJECT info</pre></td></tr></table><div class="important" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Important</h3><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="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
fw net ACCEPT</pre></td></tr></table></div><p>The above policy will:</p><div class="orderedlist"><ol type="1"><li><p>allow all connection requests from your local network to the
internet</p></li><li><p>drop (ignore) all connection requests from the internet to your
firewall or local network</p></li><li><p>optionally accept all connection requests from the firewall to
the internet (if you uncomment the additional policy)</p></li><li><p>reject all other connection requests.</p></li></ol></div><p><img src="images/BD21298_.gif" /></p><p>At this point, edit your <tt class="filename">/etc/shorewall/policy</tt>
file and make any changes that you wish.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2859594"></a>Network Interfaces</h2></div></div><div></div></div><div class="figure"><a id="id2859601"></a><p class="title"><b>Figure 2. DMZ</b></p><div class="mediaobject" align="center"><img src="images/dmz1.png" align="middle" alt="DMZ" /></div></div><p>The firewall has three network interfaces. Where Internet
connectivity is through a cable or DSL “<span class="quote">Modem</span>”, the External
Interface will be the ethernet adapter that is connected to that
“<span class="quote">Modem</span>” (e.g., <tt class="filename">eth0</tt>)
unless you connect via <span class="emphasis"><em>Point-to-Point Protocol</em></span> over
Ethernet (PPPoE) or <span class="emphasis"><em>Point-to-Point Tunneling Protocol</em></span>
(PPTP) in which case the External Interface will be a <tt class="literal">ppp</tt>
interface (e.g., <tt class="filename">ppp0</tt>). If you
connect via a regular modem, your External Interface will also be
<tt class="filename">ppp0</tt>. If you connect using ISDN,
you external interface will be <tt class="filename">ippp0</tt>.</p><p><img src="images/BD21298_.gif" /></p><p>If your external interface is <tt class="filename">ppp0</tt>
or <tt class="filename">ippp0</tt> then you will want to set
<tt class="varname">CLAMPMSS=yes</tt> in <tt class="filename">/etc/shorewall/shorewall.conf</tt>.</p><p>Your Local Interface will be an ethernet adapter (<tt class="filename">eth0</tt>, <tt class="filename">eth1</tt>
or <tt class="filename">eth2</tt>) 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 cross-over cable).</p><p>Your DMZ Interface will also be an ethernet adapter (<tt class="filename">eth0</tt>, <tt class="filename">eth1</tt>
or <tt class="filename">eth2</tt>) 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 cross-over cable).</p><div class="caution" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Caution</h3><p>Do not connect the internal and external interface to the same hub
or switch except for testing AND you are running Shorewall version 1.4.7
or later. When using these recent versions, you can test using this kind
of configuration if you specify the arp_filter option in
<tt class="filename">/etc/shorewall/interfaces</tt> for all interfaces
connected to the common hub/switch. Using such a setup with a production
firewall is strongly recommended against.</p></div><p><img src="images/BD21298_.gif" /></p><p>The Shorewall three-interface sample configuration assumes that the
external interface is <tt class="filename">eth0</tt>, the
local interface is <tt class="filename">eth1</tt> and the
DMZ interface is <tt class="filename">eth2</tt>. If your
configuration is different, you will have to modify the sample
<tt class="filename">/etc/shorewall/interfaces</tt> file accordingly. While you
are there, you may wish to review the list of options that are specified
for the interfaces. Some hints:</p><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Tip</h3><p>If your external interface is <tt class="filename">ppp0</tt>
or <tt class="filename">ippp0</tt>, you can replace the
“<span class="quote">detect</span>” in the second column with “<span class="quote">-</span>”
(without the quotes).</p></div><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Tip</h3><p>If your external interface is <tt class="filename">ppp0</tt>
or <tt class="filename">ippp0</tt> or if you have a static
IP address, you can remove “<span class="quote">dhcp</span>” from the option list.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2859977"></a>IP Addresses</h2></div></div><div></div></div><p>Before going further, we should say a few words about Internet
Protocol (IP) addresses. Normally, your ISP will assign you a single
Public IP address. This address may be assigned via the Dynamic Host
Configuration Protocol (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 static IP address; that means that
you configure your firewall's external interface to use that address
permanently. 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 Private IP address ranges for this purpose:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255</pre></td></tr></table><p><img src="images/BD21298_.gif" /></p><p>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 <tt class="varname">norfc1918</tt> option from the external interface's
entry in <tt class="filename">/etc/shorewall/interfaces</tt>.</p><p>You will want to assign your local addresses from one sub-network or
subnet and your DMZ addresses from another subnet. For our purposes, we
can consider a subnet to consists of a range of addresses <tt class="systemitem">x.y.z.0</tt> - <tt class="systemitem">x.y.z.255</tt>.
Such a subnet will have a Subnet Mask of <tt class="systemitem">255.255.255.0</tt>.
The address <tt class="systemitem">x.y.z.0</tt> is reserved
as the Subnet Address and <tt class="systemitem">x.y.z.255</tt>
is reserved as the Subnet Broadcast Address. In Shorewall, a subnet is
described using Classless InterDomain Routing (CIDR) notation with
consists of the subnet address followed by <tt class="varname">/24</tt>. The
<tt class="varname">24</tt> refers to the number of consecutive “<span class="quote">1</span>”
bits from the left of the subnet mask.</p><div class="table"><a id="id2809952"></a><p class="title"><b>Table 1. Example sub-network</b></p><table summary="Example sub-network" border="1"><colgroup><col align="left" /><col /></colgroup><tbody><tr><td align="left">Range:</td><td><tt class="systemitem">10.10.10.0</tt> -
<tt class="systemitem">10.10.10.255</tt></td></tr><tr><td align="left">Subnet Address:</td><td><tt class="systemitem">10.10.10.0</tt></td></tr><tr><td align="left">Broadcast Address:</td><td><tt class="systemitem">10.10.10.255</tt></td></tr><tr><td align="left">CIDR Notation:</td><td><tt class="systemitem">10.10.10.0/24</tt></td></tr></tbody></table></div><p>It is conventional to assign the internal interface either the first
usable address in the subnet (<tt class="systemitem">10.10.10.1</tt>
in the above example) or the last usable address (<tt class="systemitem">10.10.10.254</tt>).</p><p>One of the purposes of subnetting is to allow all computers in the
subnet to understand which other computers can be communicated with
directly. To communicate with systems outside of the subnetwork, systems
send packets through a gateway (router).</p><p><img src="images/BD21298_.gif" /></p><p>Your local computers (Local Computers 1 &amp; 2) should be
configured with their default gateway 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.</p><p>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 “<span class="quote">IP
Fundamentals: What Everyone Needs to Know about Addressing &amp; Routing</span>”,
Thomas A. Maufer, Prentice-Hall, 1999, ISBN 0-13-975483-0.</p><p>The remainder of this quide will assume that you have configured
your network as shown here:</p><div class="figure"><a id="id2810143"></a><p class="title"><b>Figure 3. DMZ</b></p><div class="mediaobject"><img src="images/dmz2.png" alt="DMZ" /><div class="caption"><p>The default gateway for the DMZ computers would be <tt class="systemitem">10.10.11.254</tt> and the default gateway
for the Local computers would be <tt class="systemitem">10.10.10.254</tt>.</p><div class="warning" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Warning</h3><p>Your ISP might assign your external interface an RFC 1918
address. If that address is in the <tt class="systemitem">10.10.10.0/24</tt>
subnet then you will need to select a DIFFERENT RFC 1918 subnet
for your local network and if it is in the <tt class="systemitem">10.10.11.0/24</tt> subnet then you will
need to select a different RFC 1918 subnet for your DMZ.</p></div></div></div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2806632"></a>IP Masquerading (SNAT)</h2></div></div><div></div></div><p>The addresses reserved by RFC 1918 are sometimes referred to as
non-routable because the Internet backbone routers don't forward
packets which have an RFC-1918 destination address. When one of your local
systems (let's assume local computer 1) sends a connection request to
an internet host, the firewall must perform Network Address Translation
(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. 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>On Linux systems, the above process is often referred to as IP
Masquerading and you will also see the term Source Network Address
Translation (SNAT) used. Shorewall follows the convention used with
Netfilter: </p><div class="itemizedlist"><ul type="disc"><li><p><span class="emphasis"><em>Masquerade</em></span>
describes the case where you let your firewall system automatically detect
the external interface address.</p></li><li><p><span class="emphasis"><em>SNAT</em></span>
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></div><p>
In Shorewall, both Masquerading and SNAT are configured with entries in
the <tt class="filename">/etc/shorewall/</tt><tt class="filename">masq</tt>
file.</p><p><img src="images/BD21298_.gif" /></p><p>If your external firewall interface is <tt class="filename">eth0</tt>,
your local interface <tt class="filename">eth1</tt> and your
DMZ interface is <tt class="filename">eth2</tt> then you do
not need to modify the file provided with the sample. Otherwise, edit
<tt class="filename">/etc/shorewall/</tt><tt class="filename">masq</tt>
and change it to match your configuration.</p><p>If, despite all advice to the contrary, you are using this guide and
want to use one-to-one NAT or Proxy ARP for your DMZ, remove the entry for
eth2 from <tt class="filename">/etc/shorewall/masq</tt>.</p><p><img src="images/BD21298_.gif" /></p><p>If your external IP is static, you can enter it in the third column
in the <tt class="filename">/etc/shorewall/</tt><tt class="filename">masq</tt>
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><p><img src="images/BD21298_.gif" /></p><p>If you are using the Debian package, please check your
<tt class="filename">shorewall.conf</tt> file to ensure that the following are
set correctly; if they are not, change them appropriately:
</p><div class="itemizedlist"><ul type="disc"><li><p><tt class="varname">NAT_ENABLED=Yes</tt>
(Shorewall versions earlier than 1.4.6)</p></li><li><p><tt class="varname">IP_FORWARDING=On</tt></p></li></ul></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2806881"></a>Port Forwarding (DNAT)</h2></div></div><div></div></div><p>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>The above process is called <span class="emphasis"><em>Port Forwarding</em></span> or
<span class="emphasis"><em>Destination Network Address Translation</em></span> (DNAT). You
configure port forwarding using DNAT rules in the <tt class="filename">/etc/shorewall/</tt><tt class="filename">rules</tt>
file.</p><p>The general form of a simple port forwarding rule in <tt class="filename">/etc/shorewall/</tt><tt class="filename">rules</tt> is:
</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
DNAT net dmz:<span class="emphasis"><em>&lt;server local ip address&gt;</em></span>[:<span class="emphasis"><em>&lt;server port&gt;</em></span>] <span class="emphasis"><em>&lt;protocol&gt;</em></span> <span class="emphasis"><em>&lt;port&gt;</em></span></pre></td></tr></table><p>
If you don't specify the <span class="emphasis"><em><tt class="varname">&lt;server port&gt;</tt></em></span>,
it is assumed to be the same as <span class="emphasis"><em><tt class="varname">&lt;port&gt;</tt></em></span>.</p><div class="example"><a id="id2807001"></a><p class="title"><b>Example 1. You run a Web Server on DMZ Computer 2 and you want to forward
incoming TCP port 80 to that system</b></p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
DNAT net dmz:10.10.11.2 tcp 80
ACCEPT loc dmz:10.10.11.2 tcp 80</pre></td></tr></table><div class="itemizedlist"><ul type="disc"><li><p>Entry
1 forwards port 80 from the Internet.</p></li><li><p>Entry
2 allows connections from the local network.</p></li></ul></div><p>
Several important points to keep in mind:</p><div class="itemizedlist"><ul type="disc"><li><p>When
you are connecting to your server from your local systems, you must use
the server's internal IP address (<tt class="systemitem">10.10.11.2</tt>).</p></li><li><p>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 <tt class="literal">http://w.x.y.z:5000 where
w.x.y.z</tt> is your external IP).</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE
# PORT(S)
DNAT net dmz:10.10.11.2:80 tcp 80 5000</pre></td></tr></table></li><li><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><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL
# PORT(S) DEST
DNAT loc dmz:10.10.11.2 tcp 80 - <span class="emphasis"><em>&lt;external ip&gt;</em></span></pre></td></tr></table><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 <tt class="filename">eth0</tt>):</p><div class="orderedlist"><ol type="1"><li><p>Include
the following in /etc/shorewall/params:</p><p><span><b class="command">ETH0_IP=$(find_interface_address
eth0)</b></span></p></li><li><p>Make your
<tt class="literal">loc-&gt;dmz</tt> rule: </p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL
# PORT(S) DEST
DNAT loc dmz:10.10.11.2 tcp 80 - $ETH0_IP</pre></td></tr></table></li></ol></div></li><li><p>If
you want to access your server from the DMZ using your external IP
address, see <a href="FAQ.htm#faq2a" target="_self">FAQ 2a</a>.</p></li></ul></div></div><p><img src="images/BD21298_.gif" /></p><p>At this point, add the DNAT and ACCEPT rules for your servers.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2865302"></a>Domain Name Server (DNS)</h2></div></div><div></div></div><p>Normally, when you connect to your ISP, as part of getting an IP
address your firewall's <span class="emphasis"><em>Domain Name Service</em></span> (DNS)
resolver will be automatically configured (e.g., the <tt class="filename">/etc/resolv.conf</tt>
file will be written). Alternatively, your ISP may have given you the IP
address of a pair of DNS name servers for you to manually configure as
your primary and secondary name servers. It is your responsibility to
configure the resolver in your internal systems. You can take one of two
approaches: </p><div class="itemizedlist"><ul type="disc"><li><p>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 <tt class="filename">/etc/resolv.conf</tt>
on your firewall system -- the name servers are given in “<span class="quote">nameserver</span>”
records in that file.</p></li><li><p><img src="images/BD21298_.gif" /></p><p>You can
configure a <span class="emphasis"><em>Caching Name Server</em></span> on your firewall or
in your DMZ. <span class="trademark">Red Hat</span>™ has an RPM for a caching name
server (which also requires the '<span><b class="command">bind</b></span>' RPM) and
for Bering users, there is <tt class="filename">dnscache.lrp</tt>. 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 (<tt class="systemitem">10.10.10.254</tt>
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
<tt class="filename">/etc/shorewall/rules</tt>.</p></li></ul></div><p>
If you run the name server on the firewall:
</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowDNS loc fw
AllowDNS dmz fw </pre></td></tr></table><p> Run name server on DMZ
computer 1: </p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowDNS loc dmz:10.10.11.1
AllowDNS fw dmz:10.10.11.1 </pre></td></tr></table><p>In the rules shown above, “<span class="quote">AllowDNS</span>” is an example of a
<span class="emphasis"><em>defined action</em></span>. Shorewall includes a number of
defined actions and <a href="User_defined_Actions.html" target="_self">you can add
your own</a>. To see the list of actions included with your version of
Shorewall, look in the file <tt class="filename">/etc/shorewall/actions.std</tt>.
Those actions that accept connection requests have names that begin with
“<span class="quote">Allow</span>”.</p><p>You don't have to use defined actions when coding a rule in
<tt class="filename">/etc/shorewall/rules</tt>; the generated Netfilter ruleset
is slightly more efficient if you code your rules directly rather than
using defined actions. The first example above (name server on the
firewall) could also have been coded as follows:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT loc fw tcp 53
ACCEPT loc fw udp 53
ACCEPT dmz fw tcp 53
ACCEPT dmz fw udp 53 </pre></td></tr></table><p>In cases where Shorewall doesn't include a defined action to
meet your needs, you can either define the action yourself or you can
simply code the appropriate rules directly.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2865536"></a>Other Connections</h2></div></div><div></div></div><p>The three-interface sample includes the following rule:
</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowDNS fw net </pre></td></tr></table><p>That rule allow DNS access from
your firewall and may be removed if you commented out the line in
<tt class="filename">/etc/shorewall/policy</tt> allowing all connections from
the firewall to the internet.</p><p>The sample also includes: </p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowSSH loc fw
AllowSSH loc dmz </pre></td></tr></table><p>Those rules allow 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><p>If you wish to enable other connections between your systems, the
general format for using a defined action is:
</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
&lt;<span class="emphasis"><em>action</em></span>&gt; <span class="emphasis"><em>&lt;source zone&gt; &lt;destination zone&gt;</em></span></pre></td></tr></table><p>The general format when not using a defined action is:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT <span class="emphasis"><em>&lt;source zone&gt; &lt;destination zone&gt; &lt;protocol&gt; &lt;port&gt; </em></span></pre></td></tr></table><div class="example"><a id="id2865625"></a><p class="title"><b>Example 2. You want to run a publicly-available DNS server on your firewall
system</b></p><p>Using defined actions:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowDNS net fw</pre></td></tr></table><p>Not using defined actions:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT net fw tcp 53
ACCEPT net fw udp 53 </pre></td></tr></table><p>Those rules would of course be in addition to the rules listed
above under &quot;If you run the name server on your firewall&quot;.</p></div><p>If you don't know what port and protocol a particular
application uses, <a href="ports.htm" target="_self">look here</a>.</p><div class="important" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Important</h3><p>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><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowSSH net fw</pre></td></tr></table></div><p><img src="images/leaflogo.gif" /> Bering
users will want to add the following two rules to be compatible with
Jacques's Shorewall configuration: </p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT loc fw udp 53
ACCEPT net fw tcp 80 </pre></td></tr></table><div class="itemizedlist"><ul type="disc"><li><p>Entry
1 allows the DNS Cache to be used.</p></li><li><p>Entry
2 allows the “<span class="quote">weblet</span>” to work.</p></li></ul></div><p><img src="images/BD21298_.gif" /></p><p>Now modify <tt class="filename">/etc/shorewall/rules</tt> to add or
remove other connections as required.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2865767"></a>Starting and Stopping Your Firewall</h2></div></div><div></div></div><p><img src="images/BD21298_.gif" /></p><p>The <a href="Install.htm" target="_self">installation procedure</a>
configures your system to start Shorewall at system boot but beginning
with Shorewall version 1.3.9 startup is disabled so that your system
won't try to start Shorewall before configuration is complete. Once
you have completed configuration of your firewall, you can enable
Shorewall startup by removing the file <tt class="filename">/etc/shorewall/startup_disabled</tt>.
</p><div class="important" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Important</h3><p>Users of the <tt class="filename">.deb</tt> package must edit
<tt class="filename">/etc/default/shorewall</tt> and set <tt class="varname">startup=1</tt>.</p></div><p>
The firewall is started using the <span><b class="command">shorewall start</b></span>
command and stopped using <span><b class="command">shorewall stop</b></span>. When the
firewall is stopped, routing is enabled on those hosts that have an entry
in <a href="Documentation.htm#Routestopped" target="_self"><tt class="filename">/etc/shorewall/routestopped</tt></a>.
A running firewall may be restarted using the <span><b class="command">shorewall restart</b></span>
command. If you want to totally remove any trace of Shorewall from your
Netfilter configuration, use <span><b class="command">shorewall clear</b></span>.</p><p><img src="images/BD21298_.gif" /></p><p>The three-interface sample assumes that you want to enable routing
to/from <tt class="filename">eth1</tt> (your local network)
and <tt class="filename">eth2</tt> (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
<tt class="filename">/etc/shorewall/routestopped</tt> accordingly.
</p><div class="warning" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Warning</h3><p>If you are connected to your firewall from the internet, do
not issue a <span><b class="command">shorewall stop</b></span> command unless you have
added an entry for the IP address that you are connected from to <a href="Documentation.htm#Routestopped" target="_self"><tt class="filename">/etc/shorewall/routestopped</tt></a>.
Also, I don't recommend using <span><b class="command">shorewall restart</b></span>; it
is better to create an <a href="configuration_file_basics.htm#Levels" target="_self">alternate
configuration</a> and test it using the <a href="starting_and_stopping_shorewall.htm" target="_self"><span><b class="command">shorewall try</b></span>
command</a>.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2865997"></a>Additional Recommended Reading</h2></div></div><div></div></div><p>I highly recommend that you review the <a href="configuration_file_basics.htm" target="_self">Common Configuration File Features</a>
page -- it contains helpful tips about Shorewall features than make
administering your firewall easier.</p></div></div></body></html>
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