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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><link rel="stylesheet" href="html.css" type="text/css" /><meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /></head><body><div class="article" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="three-interface"></a>Three-Interface Firewall</h2></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 © 2002-2007 Thomas M. Eastep</p></div><div><div class="legalnotice"><a id="id273856"></a><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 class="ulink" href="GnuCopyright.htm" target="_self">GNU Free Documentation
License</a></span>”.</p></div></div><div><p class="pubdate">2008/12/15</p></div></div><hr /></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="section"><a href="#Intro">Introduction</a></span></dt><dd><dl><dt><span class="section"><a href="#Reqs">Requirements</a></span></dt><dt><span class="section"><a href="#Before">Before you start</a></span></dt><dt><span class="section"><a href="#Conventions">Conventions</a></span></dt></dl></dd><dt><span class="section"><a href="#PPTP">PPTP/ADSL</a></span></dt><dt><span class="section"><a href="#Concepts">Shorewall Concepts</a></span></dt><dt><span class="section"><a href="#Interfaces">Network Interfaces</a></span></dt><dt><span class="section"><a href="#Addresses">IP Addresses</a></span></dt><dt><span class="section"><a href="#SNAT">IP Masquerading (SNAT)</a></span></dt><dt><span class="section"><a href="#DNAT">Port Forwarding (DNAT)</a></span></dt><dt><span class="section"><a href="#DNS">Domain Name Server (DNS)</a></span></dt><dt><span class="section"><a href="#Open">Other Connections</a></span></dt><dt><span class="section"><a href="#Logging">Logging</a></span></dt><dt><span class="section"><a href="#Other">Some Things to Keep in Mind</a></span></dt><dt><span class="section"><a href="#Starting">Starting and Stopping Your Firewall</a></span></dt><dt><span class="section"><a href="#Trouble">If it Doesn't Work</a></span></dt><dt><span class="section"><a href="#Reading">Additional Recommended Reading</a></span></dt></dl></div><div class="caution" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Caution</h3><p><span class="bold"><strong>This article applies to Shorewall 4.0 and
later. If you are running a version of Shorewall earlier than Shorewall
4.0.0 then please see the documentation for that
release.</strong></span></p></div><div class="caution" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Caution</h3><p><span class="bold"><strong>Do not attempt to install Shorewall on a
remote system. You are virtually assured to lock yourself out of that
system.</strong></span></p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="Intro"></a>Introduction</h2></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 class="ulink" href="shorewall_setup_guide.htm" target="_self">Shorewall Setup Guide</a>
instead.</p></div></li><li><p>DMZ connected to a separate Ethernet interface. The purpose of a
DMZ is to isolate those servers that are exposed to the Internet from
your local systems so that if one of those servers is compromised
there is still a firewall between the hacked server and your local
systems.</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="Figure1"></a><p class="title"><b>Figure 1. schematic of a typical installation</b></p><div class="figure-contents"><div class="mediaobject" align="center"><img src="images/dmz1.png" align="middle" alt="schematic of a typical installation" /></div></div></div><br class="figure-break" /><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="Reqs"></a>Requirements</h3></div></div></div><p>Shorewall requires that you have the
<span class="command"><strong>iproute</strong></span>/<span class="command"><strong>iproute2</strong></span> package installed
(on <span class="trademark">RedHat</span>™, the package is called
<span class="command"><strong>iproute</strong></span>). You can tell if this package is installed
by the presence of an <span class="command"><strong>ip</strong></span> program on your firewall
system. As <code class="systemitem">root</code>, you can use
the <span class="command"><strong>which</strong></span> command to check for this program:</p><pre class="programlisting">[root@gateway root]# <span class="command"><strong>which ip</strong></span>
/sbin/ip
[root@gateway root]#</pre></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="Before"></a>Before you start</h3></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 class="command"><strong>dos2unix</strong></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 class="command"><strong>dos2unix</strong></span> against the copy before using it with
Shorewall.</p><div class="itemizedlist"><ul type="disc"><li><p><a class="ulink" href="http://www.simtel.net/pub/pd/51438.html" target="_self">Windows
Version of dos2unix</a></p></li><li><p><a class="ulink" 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="Conventions"></a>Conventions</h3></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="PPTP"></a>PPTP/ADSL</h2></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 class="ulink" 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="Concepts"></a>Shorewall Concepts</h2></div></div></div><p>The configuration files for Shorewall are contained in the directory
<code class="filename">/etc/shorewall</code> -- for simple setups, you will only
need to deal with a few of these as described in this guide.</p><div class="warning" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Warning</h3><p><span class="bold"><strong>Note to Debian Users</strong></span></p><p>If you install using the .deb, you will find that your <code class="filename">/etc/shorewall</code> directory is empty. This
is intentional. The released configuration file skeletons may be found
on your system in the directory <code class="filename">/usr/share/doc/shorewall-common/default-config</code>.
Simply copy the files you need from that directory to <code class="filename">/etc/shorewall</code> and modify the
copies.</p></div><p><img src="images/BD21298_.gif" /></p><p>After you have installed Shorewall, locate the three-interface
Sample configuration:</p><div class="orderedlist"><ol type="1"><li><p>If you installed using an RPM, the samples will be in the
Samples/three-interfaces/ subdirectory of the Shorewall documentation
directory. If you don't know where the Shorewall documentation
directory is, you can find the samples using this command:</p><pre class="programlisting">~# rpm -ql shorewall-common | fgrep three-interfaces
/usr/share/doc/packages/shorewall/Samples/three-interfaces
/usr/share/doc/packages/shorewall/Samples/three-interfaces/interfaces
/usr/share/doc/packages/shorewall/Samples/three-interfaces/masq
/usr/share/doc/packages/shorewall/Samples/three-interfaces/policy
/usr/share/doc/packages/shorewall/Samples/three-interfaces/routestopped
/usr/share/doc/packages/shorewall/Samples/three-interfaces/rules
/usr/share/doc/packages/shorewall/Samples/three-interfaces/zones
~#</pre></li><li><p>If you installed using the tarball, the samples are in the
Samples/three-interfaces directory in the tarball.</p></li><li><p>If you installed using a Shorewall 3.x .deb, the samples are in
/usr/share/doc/shorewall/examples/three-interfaces. You must install
the shorewall-doc package.</p></li><li><p>If you installed using a Shorewall 4.x .deb, the samples are in
<code class="filename">/usr/share/doc/shorewall-common/examples/three-interfaces</code>.
You do not need the shorewall-doc package to have access to the
samples.</p></li></ol></div><p>If you are installing Shorewall version 3.4.0 or later then as each
file is introduced, I suggest that you look at the actual file on your
system and that you look at the <a class="ulink" href="configuration_file_basics.htm#Manpages" target="_self">man page</a> for that
file. For example, to look at the man page for the
<code class="filename">/etc/shorewall/zones</code> file, type <span class="command"><strong>man
shorewall-zones</strong></span> at a shell prompt.</p><p>If you are installing a Shorewall version earlier than 3.4.0, then
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><pre class="programlisting">#ZONE TYPE OPTIONS IN OUT
# OPTIONS OPTIONS
fw firewall
net ipv4
loc ipv4
dmz ipv4</pre><p>Zone names are defined in
<code class="filename">/etc/shorewall/zones</code>.</p><p>Note that Shorewall recognizes the firewall system as its own zone.
When the /etc/shorewall/zones file is processed, he name of the firewall
zone is stored in the shell variable <em class="firstterm">$FW</em> which may
be used throughout the Shorewall configuration to refer to the firewall
zone.</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 <code class="filename">/etc/shorewall/policy</code>
file.</p></li><li><p>You define exceptions to those default policies in the
<code class="filename">/etc/shorewall/rules</code> file.</p></li></ul></div><p>For each connection request entering the firewall, the request is
first checked against the <code class="filename">/etc/shorewall/rules</code> file.
If no rule in that file matches the connection request then the first
policy in <code class="filename">/etc/shorewall/policy</code> that matches the
request is applied. If there is a <a class="ulink" href="shorewall_extension_scripts.htm" target="_self">common action</a> defined for the
policy in <code class="filename">/etc/shorewall/actions</code> or
<code class="filename">/usr/share/shorewall/actions.std</code> then that action is
performed before the action is applied. The purpose of the common action is
two-fold:</p><div class="itemizedlist"><ul type="disc"><li><p>It silently drops or rejects harmless common traffic that would
otherwise clutter up your log — Broadcasts for example.</p></li><li><p>If ensures that traffic critical to correct operation is allowed
through the firewall — ICMP <span class="emphasis"><em>fragmentation-needed</em></span>
for example.</p></li></ul></div><p>The <code class="filename">/etc/shorewall/policy</code> file included with
the three-interface sample has the following policies:</p><pre class="programlisting">#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
loc net ACCEPT
net all DROP info
all all REJECT info</pre><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><pre class="programlisting">#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
$FW net ACCEPT</pre></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>The word <em class="firstterm">info</em> in the LOG LEVEL column for the
DROP and REJECT policies indicates that packets dropped or rejected under
those policies should be <a class="ulink" href="shorewall_logging.html" target="_self">logged at
that level</a>.</p><p>It is important to note that Shorewall policies (and rules) refer to
<span class="bold"><strong>connections</strong></span> and not packet flow. With the
policies defined in the <code class="filename">/etc/shorewall/policy</code> file shown above,
connections are allowed from the <span class="emphasis"><em>loc</em></span> zone to the
<span class="emphasis"><em>net</em></span> zone even though connections are not allowed from
the <span class="emphasis"><em>loc</em></span> zone to the firewall itself.</p><p><img src="images/BD21298_.gif" /></p><p>At this point, edit your <code class="filename">/etc/shorewall/policy</code>
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="Interfaces"></a>Network Interfaces</h2></div></div></div><div class="figure"><a id="Figure2"></a><p class="title"><b>Figure 2. DMZ</b></p><div class="figure-contents"><div class="mediaobject" align="center"><img src="images/dmz1.png" align="middle" alt="DMZ" /></div></div></div><br class="figure-break" /><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., <code class="filename">eth0</code>)
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
<code class="literal">ppp</code> interface (e.g., <code class="filename">ppp0</code>). If you connect via a regular modem,
your External Interface will also be <code class="filename">ppp0</code>. If you connect using ISDN, you
external interface will be <code class="filename">ippp0</code>.</p><div class="caution" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Caution</h3><p>Be sure you know which interface is your external interface. Many
hours have been spent floundering by users who have configured the wrong
interface. If you are unsure, then as root type <span class="command"><strong>ip route
ls</strong></span> at the command line. The device listed in the last
(default) route should be your external interface.</p><p>Example:</p><pre class="programlisting">root@lists:~# ip route ls
192.168.1.1 dev eth0 scope link
192.168.2.2 dev tun0 proto kernel scope link src 192.168.2.1
192.168.3.0/24 dev br0 proto kernel scope link src 192.168.3.254
10.13.10.0/24 dev tun1 scope link
192.168.2.0/24 via 192.168.2.2 dev tun0
192.168.1.0/24 dev br0 proto kernel scope link src 192.168.1.254
206.124.146.0/24 dev eth0 proto kernel scope link src 206.124.146.176
10.10.10.0/24 dev tun1 scope link
default via 206.124.146.254 dev <span class="bold"><strong>eth0</strong></span>
root@lists:~# </pre><p>In that example, <code class="filename">eth0</code> is
the external interface.</p></div><p><img src="images/BD21298_.gif" /></p><p>I<span class="bold"><strong>f your external interface is <code class="filename">ppp0</code> or <code class="filename">ippp0</code> then you will want to set
<code class="varname">CLAMPMSS=yes</code> in
<code class="filename">/etc/shorewall/shorewall.conf</code>.</strong></span></p><p>Your Local Interface will be an Ethernet adapter (<code class="filename">eth0</code>, <code class="filename">eth1</code> or <code class="filename">eth2</code>) 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 (<code class="filename">eth0</code>, <code class="filename">eth1</code> or <code class="filename">eth2</code>) 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><span class="bold"><strong>Do NOT connect multiple interfaces to the
same hub or switch except for testing</strong></span>. You can test using
this kind of configuration if you specify the <span class="bold"><strong>arp_filter</strong></span> option or the <span class="bold"><strong>arp_ignore</strong></span> option in
<code class="filename">/etc/shorewall/interfaces</code> for all interfaces
connected to the common hub/switch. <span class="bold"><strong>Using such a
setup with a production firewall is strongly recommended
against</strong></span>.</p></div><p><img src="images/BD21298_.gif" /></p><p>The Shorewall three-interface sample configuration assumes that the
external interface is <code class="filename">eth0</code>, the
local interface is <code class="filename">eth1</code> and the
DMZ interface is <code class="filename">eth2</code>. If your
configuration is different, you will have to modify the sample
<code class="filename">/etc/shorewall/interfaces</code> 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 <code class="filename">ppp0</code> or <code class="filename">ippp0</code>, 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 <code class="filename">ppp0</code> or <code class="filename">ippp0</code> 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="Addresses"></a>IP Addresses</h2></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><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><p><img src="images/BD21298_.gif" /></p><p>Before starting Shorewall, <span class="bold"><strong>you should look at
the IP address of your external interface and if it is one of the above
ranges, you should remove the <code class="varname">norfc1918</code> option from the
external interface's entry in
<code class="filename">/etc/shorewall/interfaces</code>.</strong></span></p><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Tip</h3><p>To determine the IP address of your external interface, as root
type <span class="command"><strong>ip addr ls dev <interface></strong></span> at the
command line where <<span class="emphasis"><em>interface</em></span>> is your
external interface. The line beginning with <span class="emphasis"><em>inet</em></span>
identifies your IP address.</p><p>Example:</p><pre class="programlisting">root@lists:~# ip addr ls dev eth0
2: eth0: <BROADCAST,MULTICAST,UP,10000> mtu 1500 qdisc htb qlen 1000
link/ether 00:02:e3:08:48:4c brd ff:ff:ff:ff:ff:ff
inet <span class="bold"><strong>206.124.146.176</strong></span>/24 brd 206.124.146.255 scope global eth0
inet6 fe80::202:e3ff:fe08:484c/64 scope link
valid_lft forever preferred_lft forever
root@lists:~# </pre><p>In this example, the IP address of the external interface is
206.124.146.176</p></div><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 <code class="systemitem">x.y.z.0</code> - <code class="systemitem">x.y.z.255</code>. Such a subnet will have a Subnet
Mask of <code class="systemitem">255.255.255.0</code>. The
address <code class="systemitem">x.y.z.0</code> is reserved as
the Subnet Address and <code class="systemitem">x.y.z.255</code>
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 <code class="varname">/24</code>. The
<code class="varname">24</code> 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="Table1"></a><p class="title"><b>Table 1. Example sub-network</b></p><div class="table-contents"><table summary="Example sub-network" border="1"><colgroup><col align="left" /><col /></colgroup><tbody><tr><td align="left">Range:</td><td><code class="systemitem">10.10.10.0</code> -
<code class="systemitem">10.10.10.255</code></td></tr><tr><td align="left">Subnet Address:</td><td><code class="systemitem">10.10.10.0</code></td></tr><tr><td align="left">Broadcast Address:</td><td><code class="systemitem">10.10.10.255</code></td></tr><tr><td align="left">CIDR Notation:</td><td><code class="systemitem">10.10.10.0/24</code></td></tr></tbody></table></div></div><br class="table-break" /><p>It is conventional to assign the internal interface either the first
usable address in the subnet (<code class="systemitem">10.10.10.1</code> in the above example) or the
last usable address (<code class="systemitem">10.10.10.254</code>).</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 & 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
& 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 &
Routing</span>”, Thomas A. Maufer, Prentice-Hall, 1999, ISBN
0-13-975483-0.</p><p>The remainder of this guide will assume that you have configured
your network as shown here:</p><div class="figure"><a id="Figure3"></a><p class="title"><b>Figure 3. DMZ</b></p><div class="figure-contents"><div class="mediaobject"><img src="images/dmz2.png" alt="DMZ" /><div class="caption"><p>The default gateway for the DMZ computers would be <code class="systemitem">10.10.11.254</code> and the default gateway
for the Local computers would be <code class="systemitem">10.10.10.254</code>.</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 <code class="systemitem">10.10.10.0/24</code> subnet then you will
need to select a DIFFERENT RFC 1918 subnet for your local network
and if it is in the <code class="systemitem">10.10.11.0/24</code> subnet then you will
need to select a different RFC 1918 subnet for your DMZ.</p></div></div></div></div></div><br class="figure-break" /></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="SNAT"></a>IP Masquerading (SNAT)</h2></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 across 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 <code class="filename">/etc/shorewall/</code><code class="filename">masq</code>
file.</p><p><img src="images/BD21298_.gif" /></p><p>If your external firewall interface is <code class="filename">eth0</code>, your local interface <code class="filename">eth1</code> and your DMZ interface is <code class="filename">eth2</code> then you do not need to modify the file
provided with the sample. Otherwise, edit <code class="filename">/etc/shorewall/</code><code class="filename">masq</code> and
change it to match your configuration.</p><p>If, in spite of 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 <code class="filename">/etc/shorewall/masq</code>.</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 <code class="filename">/etc/shorewall/</code><code class="filename">masq</code>
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><span class="bold"><strong>If you are using the Debian package, please
check your <code class="filename">shorewall.conf</code> file to ensure that the
following is set correctly; if it is not, change it appropriately:
</strong></span></p><div class="itemizedlist"><ul type="disc"><li><p><code class="varname">IP_FORWARDING=On</code></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="DNAT"></a>Port Forwarding (DNAT)</h2></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 <code class="filename">/etc/shorewall/</code><code class="filename">rules</code>
file.</p><p>The general form of a simple port forwarding rule in <code class="filename">/etc/shorewall/</code><code class="filename">rules</code> is:
</p><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
DNAT net dmz:<span class="emphasis"><em><server local IP address></em></span>[:<span class="emphasis"><em><server port></em></span>] <span class="emphasis"><em><protocol></em></span> <span class="emphasis"><em><port></em></span></pre><p>
If you don't specify the <span class="emphasis"><em><code class="varname"><server
port></code></em></span>, it is assumed to be the same as
<span class="emphasis"><em><code class="varname"><port></code></em></span>.</p><div class="important" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Important</h3><p>Be sure to add your rules after the line that reads <span class="bold"><strong>SECTION NEW.</strong></span></p></div><div class="example"><a id="Example1"></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><div class="example-contents"><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
Web/DNAT net dmz:10.10.11.2
Web/ACCEPT loc dmz:10.10.11.2</pre><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
(<code class="systemitem">10.10.11.2</code>).</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
<code class="literal">http://w.x.y.z:5000 where w.x.y.z</code> is your
external IP).</p><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></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->dmz rule above
with:</p><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><external IP></em></span></pre><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 <code class="filename">eth0</code>):</p><div class="orderedlist"><ol type="1"><li><p>Include the following in /etc/shorewall/params:</p><p><span class="command"><strong>ETH0_IP=$(find_interface_address
eth0)</strong></span></p></li><li><p>Make your <code class="literal">loc->dmz</code> rule:
</p><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></li></ol></div></li><li><p>If you want to access your server from the DMZ using your
external IP address, see <a class="ulink" href="FAQ.htm#faq2a" target="_self">FAQ
2a</a>.</p></li></ul></div></div></div><br class="example-break" /><p><img src="images/BD21298_.gif" /></p><p>At this point, add the DNAT and ACCEPT rules for your
servers.</p><div class="important" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Important</h3><p>When testing DNAT rules like those shown above, you must test from
a client OUTSIDE YOUR FIREWALL (in the 'net' zone). You cannot test
these rules from inside the firewall!</p><p>For DNAT troubleshooting tips, <a class="ulink" href="FAQ.htm#faq1a" target="_self">see FAQs
1a and 1b</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="DNS"></a>Domain Name Server (DNS)</h2></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
<code class="filename">/etc/resolv.conf</code> 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 your 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 <code class="filename">/etc/resolv.conf</code> 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 class="command"><strong>bind</strong></span>' RPM) and for Bering users, there is
<code class="filename">dnscache.lrp</code>. 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 (<code class="systemitem">10.10.10.254</code> 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
<code class="filename">/etc/shorewall/rules</code>.</p></li></ul></div><p> If you run the name server on the firewall:
</p><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
DNS/ACCEPT loc $FW
DNS/ACCEPT dmz $FW </pre><p> Run name server on DMZ
computer 1: </p><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
DNS/ACCEPT loc dmz:10.10.11.1
DNS/ACCEPT $FW dmz:10.10.11.1 </pre><p>In the rules shown above, “<span class="quote">DNS/ACCEPT</span>” is an example of
a <span class="emphasis"><em>defined macro</em></span>. Shorewall includes a number of
defined macros and <a class="ulink" href="Macros.html" target="_self">you can add your own</a>.
To see the list of macros included with your version of Shorewall, run the
command <span class="command"><strong>ls
<code class="filename">/usr/share/shorewall/macro.*</code></strong></span>.</p><p>You don't have to use defined macros when coding a rule in
<code class="filename">/etc/shorewall/rules</code>. The first example above (name
server on the firewall) could also have been coded as follows:</p><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><p>In cases where Shorewall doesn't include a defined macro to meet
your needs, you can either define the macro yourself or you can simply
code the appropriate rules directly. <a class="ulink" href="ports.html" target="_self">This
page</a> can be of help if you don't know the protocol and port
involved.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="Open"></a>Other Connections</h2></div></div></div><p>The three-interface sample includes the following rule:
</p><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
DNS/ACCEPT $FW net </pre><p>That rule allow DNS access
from your firewall and may be removed if you commented out the line in
<code class="filename">/etc/shorewall/policy</code> allowing all connections from
the firewall to the Internet.</p><p>The sample also includes: </p><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
SSH/ACCEPT loc $FW
SSH/ACCEPT loc dmz </pre><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 macro is: </p><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
<<span class="emphasis"><em>macro</em></span>>/ACCEPT <span class="emphasis"><em><source zone> <destination zone></em></span></pre><p>The general format when not using a defined action
is:</p><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT <span class="emphasis"><em><source zone> <destination zone> <protocol> <port> </em></span></pre><div class="example"><a id="Example2"></a><p class="title"><b>Example 2. You want to run a publicly-available DNS server on your firewall
system</b></p><div class="example-contents"><p>Using defined macros:</p><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
DNS/ACCEPT net $FW</pre><p>Not using defined macros:</p><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT net $FW tcp 53
ACCEPT net $FW udp 53 </pre><p>Those 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><br class="example-break" /><p>If you don't know what port and protocol a particular application
uses, <a class="ulink" 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><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
SSH/ACCEPT net $FW</pre></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><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT loc $FW udp 53
ACCEPT net $FW tcp 80 </pre><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 <code class="filename">/etc/shorewall/rules</code> 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="Logging"></a>Logging</h2></div></div></div><p>Shorewall does not maintain a log itself but rather relies on your
<a class="ulink" href="shorewall_logging.htm" target="_self">system's logging configuration</a>.
The following <a class="ulink" href="manpages/shorewall.html" target="_self">commands</a> rely
on knowing where Netfilter messages are logged:</p><div class="itemizedlist"><ul type="disc"><li><p><span class="command"><strong>shorewall show log</strong></span> (Displays the last 20
Netfilter log messages)</p></li><li><p><span class="command"><strong>shorewall logwatch</strong></span> (Polls the log at a
settable interval</p></li><li><p><span class="command"><strong>shorewall dump</strong></span> (Produces an extensive report
for inclusion in Shorewall problem reports)</p></li></ul></div><p>It is important that these commands work properly because when you
encounter connection problems when Shorewall is running, the first thing
that you should do is to look at the Netfilter log; with the help of
<a class="ulink" href="FAQ.htm#faq17" target="_self">Shorewall FAQ 17</a>, you can usually
resolve the problem quickly.</p><p>Most commonly, Netfilter messages are logged to
<code class="filename">/var/log/messages</code>. Recent
<span class="trademark">SuSE/OpenSuSE</span>™ releases come preconfigured with
syslog-ng and log netfilter messages to
<code class="filename">/var/log/firewall</code>.</p><p><img src="images/BD21298_.gif" /></p><p>If you are running a distribution that logs netfilter messages to a
log other than <code class="filename">/var/log/messages</code>, then modify the
LOGFILE setting in <code class="filename">/etc/shorewall/shorewall.conf</code> to
specify the name of your log.</p><div class="important" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Important</h3><p>The LOGFILE setting does not control where the Netfilter log is
maintained -- it simply tells the /sbin/<code class="filename">shorewall</code>
utility where to find the log.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="Other"></a>Some Things to Keep in Mind</h2></div></div></div><div class="itemizedlist"><ul type="disc"><li><p><span class="bold"><strong>You cannot test your firewall from the
inside</strong></span>. Just because you send requests to your firewall
external IP address does not mean that the request will be associated
with the external interface or the “<span class="quote">net</span>” zone. Any
traffic that you generate from the local network will be associated
with your local interface and will be treated as loc->fw
traffic.</p></li><li><p><span class="bold"><strong>IP addresses are properties of systems,
not of interfaces</strong></span>. It is a mistake to believe that your
firewall is able to forward packets just because you can ping the IP
address of all of the firewall's interfaces from the local network.
The only conclusion you can draw from such pinging success is that the
link between the local system and the firewall works and that you
probably have the local system's default gateway set correctly.</p></li><li><p><span class="bold"><strong>All IP addresses configured on firewall
interfaces are in the $FW (fw) zone</strong></span>. If 192.168.1.254 is
the IP address of your internal interface then you can write
“<span class="quote"><span class="bold"><strong>$FW:192.168.1.254</strong></span></span>” in a
rule but you may not write “<span class="quote"><span class="bold"><strong>loc:192.168.1.254</strong></span></span>”. Similarly, it is
nonsensical to add 192.168.1.254 to the <span class="bold"><strong>loc</strong></span> zone using an entry in
<code class="filename">/etc/shorewall/hosts</code>.</p></li><li><p><span class="bold"><strong>Reply packets do NOT automatically follow
the reverse path of the one taken by the original request</strong></span>.
All packets are routed according to the routing table of the host at
each step of the way. This issue commonly comes up when people install
a Shorewall firewall parallel to an existing gateway and try to use
DNAT through Shorewall without changing the default gateway of the
system receiving the forwarded requests. Requests come in through the
Shorewall firewall where the destination IP address gets rewritten but
replies go out unmodified through the old gateway.</p></li><li><p><span class="bold"><strong>Shorewall itself has no notion of inside
or outside</strong></span>. These concepts are embodied in how Shorewall is
configured.</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="Starting"></a>Starting and Stopping Your Firewall</h2></div></div></div><p><img src="images/BD21298_.gif" /></p><p>The <a class="ulink" href="Install.htm" target="_self">installation procedure</a>
configures your system to start Shorewall at system boot but 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 editing
<code class="filename">/etc/shorewall/shorewall.conf</code> and setting
STARTUP_ENABLED=Yes.</p><div class="important" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Important</h3><p>Users of the <code class="filename">.deb</code> package must edit
<code class="filename">/etc/default/shorewall</code> and set
<code class="varname">startup=1</code>.</p></div><p>The firewall is started using the <span class="command"><strong>shorewall
start</strong></span> command and stopped using <span class="command"><strong>shorewall
stop</strong></span>. When the firewall is stopped, routing is enabled on those
hosts that have an entry in <a class="ulink" href="manpages/shorewall-routestopped.html" target="_self"><code class="filename">/etc/shorewall/routestopped</code></a>.
A running firewall may be restarted using the <span class="command"><strong>shorewall
restart</strong></span> command. If you want to totally remove any trace of
Shorewall from your Netfilter configuration, use <span class="command"><strong>shorewall
clear</strong></span>.</p><p><img src="images/BD21298_.gif" /></p><p>The three-interface sample assumes that you want to enable routing
to/from <code class="filename">eth1</code> (your local network)
and <code class="filename">eth2</code> (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
<code class="filename">/etc/shorewall/routestopped</code> 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 class="command"><strong>shorewall stop</strong></span> command unless you have
added an entry for the IP address that you are connected from to
<a class="ulink" href="manpages/shorewall-routestopped.html" target="_self"><code class="filename">/etc/shorewall/routestopped</code></a>.
Also, I don't recommend using <span class="command"><strong>shorewall restart</strong></span>; it
is better to create an <a class="ulink" href="configuration_file_basics.htm#Levels" target="_self">alternate
configuration</a> and test it using the <a class="ulink" href="starting_and_stopping_shorewall.htm" target="_self"><span class="command"><strong>shorewall
try</strong></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="Trouble"></a>If it Doesn't Work</h2></div></div></div><div class="itemizedlist"><ul type="disc"><li><p>Re-check each of the items flagged with a red arrow
above.</p></li><li><p>Check your <a class="ulink" href="shorewall_logging.html" target="_self">log</a>.</p></li><li><p>Check the <a class="ulink" href="troubleshoot.htm" target="_self">Troubleshooting
Guide</a>.</p></li><li><p>Check the <a class="ulink" href="FAQ.htm" target="_self">FAQ</a>.</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="Reading"></a>Additional Recommended Reading</h2></div></div></div><p>I highly recommend that you review the <a class="ulink" 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>
