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<title>Shorewall Setup Guide</title>
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<body>
<h1 style="text-align: center;">Shorewall Setup Guide<br>
</h1>
<p><a href="#Introduction">1.0 Introduction</a><br>
<a href="#Concepts">2.0 Shorewall Concepts</a><br>
<a href="#Interfaces">3.0 Network Interfaces</a><br>
<a href="#Addressing">4.0 Addressing, Subnets and Routing</a></p>
<blockquote>
<p><a href="#Addresses">4.1 IP Addresses</a><br>
<a href="#Subnets">4.2 Subnets</a><br>
<a href="#Routing">4.3 Routing</a><br>
<a href="#ARP">4.4 Address Resolution Protocol (ARP)</a><br>
<a href="#RFC1918">4.5 RFC 1918</a></p>
</blockquote>
<p><a href="#Options">5.0 Setting up your Network</a></p>
<blockquote>
<p><a href="#Routed">5.1 Routed</a><br>
<a href="#NonRouted">5.2 Non-routed</a></p>
<blockquote>
<p><a href="#SNAT">5.2.1 SNAT</a><br>
<a href="#DNAT">5.2.2 DNAT</a><br>
<a href="#ProxyARP">5.2.3 Proxy ARP</a><br>
<a href="#NAT">5.2.4 Static NAT</a></p>
</blockquote>
<p><a href="#Rules">5.3 Rules</a><br>
<a href="#OddsAndEnds">5.4 Odds and Ends</a></p>
</blockquote>
<p><a href="#DNS">6.0 DNS</a><br>
<a href="#StartingAndStopping">7.0 Starting and Stopping
the Firewall</a></p>
<h2><a name="Introduction"></a>1.0 Introduction</h2>
<p>This guide is intended for users who are setting up Shorewall in an
environment where a set of public IP addresses must be managed or who
want to know more about Shorewall than is contained in the <a
href="shorewall_quickstart_guide.htm">single-address guides</a>.
Because the range of possible applications is so broad, the Guide will
give you general guidelines and will point you to other resources as
necessary.</p>
<p><img border="0" src="images/j0213519.gif" width="60" height="60">
&nbsp;&nbsp;&nbsp; If you run LEAF Bering, your Shorewall configuration
is NOT what I release -- I suggest that you consider installing a stock
Shorewall lrp from the shorewall.net site before you proceed.</p>
<p>Shorewall requires that the iproute/iproute2 package be 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">
&nbsp;&nbsp;&nbsp; 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
with Shorewall. 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"><a name="Concepts"></a>2.0 Shorewall Concepts</h2>
<p>The configuration files for Shorewall are contained in the directory
/etc/shorewall -- for most setups, you will only need to deal with a
few of these as described in this guide. Skeleton files are created
during the <a href="Install.htm">Shorewall Installation Process</a>.</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 some contain default entries.</p>
<p>Shorewall views the network where it is running as being composed of
a set of <i>zones.</i> In the default installation, the following zone
names are used:</p>
<table border="0" style="border-collapse: collapse;" cellpadding="3"
cellspacing="0" id="AutoNumber2">
<tbody>
<tr>
<td><u><b>Name</b></u></td>
<td><u><b>Description</b></u></td>
</tr>
<tr>
<td><b>net</b></td>
<td><b>The Internet</b></td>
</tr>
<tr>
<td><b>loc</b></td>
<td><b>Your Local Network</b></td>
</tr>
<tr>
<td><b>dmz</b></td>
<td><b>Demilitarized Zone</b></td>
</tr>
</tbody>
</table>
<p>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> but that may be
changed
in the <a href="Documentation.htm#Configs">/etc/shorewall/shorewall.conf</a>
file. In this guide, the default name (<b>fw</b>) will be used.</p>
<p>With the exception of <b>fw</b>, Shorewall attaches absolutely no
meaning to zone names. Zones are entirely what YOU make of them. That
means that you should not expect Shorewall to do something special
"because this is the internet zone" or "because that is the DMZ".</p>
<p><img border="0" src="images/BD21298_.gif" width="13" height="13">
&nbsp;&nbsp;&nbsp; Edit the /etc/shorewall/zones file and make any
changes necessary.</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> Shorewall is built on top of the <a href="http://www.netfilter.org">Netfilter</a>
kernel facility. Netfilter implements a <a
href="http://www.cs.princeton.edu/%7Ejns/security/iptables/iptables_conntrack.html">connection
tracking function</a> that allows what is often referred to as <i>stateful
inspection</i> of packets. This stateful property allows firewall rules
to be defined in terms of <i>connections</i> rather than in
terms of packets. With Shorewall, you:</p>
<ol>
<li> Identify the source zone.</li>
<li> Identify the destination zone.</li>
<li> If the POLICY from the client's zone to the server's zone is
what you want for this client/server pair, you need do nothing further.</li>
<li> If the POLICY is not what you want, then you must add a rule.
That rule is expressed in terms of the client's zone and the server's
zone.</li>
</ol>
<p> Just because connections of a particular type are allowed from zone
A
to the firewall and are also allowed from the firewall to zone B <font
color="#ff6633"><b><u> DOES NOT mean that these connections are
allowed from zone A to zone B</u></b></font>. It rather means that you
can have a proxy running on the firewall that accepts a connection
from zone A and then establishes its own separate connection from the
firewall to zone B.</p>
<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&nbsp; the request is first checked against the
rules in /etc/shorewall/common.def.</p>
<p>The default /etc/shorewall/policy file 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>&nbsp;</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>net</td>
<td>all</td>
<td>DROP</td>
<td>info</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>all</td>
<td>all</td>
<td>REJECT</td>
<td>info</td>
<td>&nbsp;</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 and log a message at the <i>info</i> level (<a
href="shorewall_logging.html">here</a> is a description of
log levels).</li>
<li>reject all other connection requests and log a message at the <i>info</i>
level. When a request is rejected, the firewall will return an RST (if
the protocol is TCP) or an ICMP port-unreachable packet for other
protocols.</li>
</ol>
<p><img border="0" src="images/BD21298_.gif" width="13" height="13">
&nbsp;&nbsp;&nbsp; At this point, edit your /etc/shorewall/policy and
make any changes that you wish.</p>
<h2 align="left"><a name="Interfaces"></a>3.0 Network Interfaces</h2>
<p align="left">For the remainder of this guide, we'll refer to the
following diagram. While it may not look like your own network, it can
be used to illustrate the important aspects of Shorewall configuration.</p>
<p align="left">In this diagram:</p>
<ul>
<li>The DMZ Zone consists of systems DMZ 1 and DMZ 2. A DMZ is used
to isolate your internet-accessible servers from your local systems so
that if one of those servers is compromised, you still have the
firewall between the compromised system and your local systems. </li>
<li>The Local Zone consists of systems Local 1, Local 2 and Local 3. </li>
<li>All systems from the ISP outward comprise the Internet Zone. </li>
</ul>
<p align="center"> <img border="0" src="images/dmz3.png" width="692"
height="635"> </p>
<p align="left">The simplest way to define zones is to simply associate
the zone name (previously defined in /etc/shorewall/zones) with a
network interface. This is done in the <a
href="Documentation.htm#Interfaces">/etc/shorewall/interfaces</a> file.</p>
<p align="left">The firewall illustrated above has three network
interfaces. Where Internet connectivity is through a cable or DSL
"Modem", the <i>External Interface</i> will be the Ethernet adapter
that is connected to that "Modem" (e.g., <b>eth0</b>)&nbsp; <u>unless</u>
you connect via
<i><u>P</u>oint-to-<u>P</u>oint <u>P</u>rotocol over <u>E</u>thernet</i>
(PPPoE) or <i><u>P</u>oint-to-<u>P</u>oint <u>T</u>unneling <u>P</u>rotocol
</i>(PPTP) in which case the External Interface will be a ppp interface
(e.g., <b>ppp0</b>). If you connect via a regular modem, your External
Interface will also be <b>ppp0</b>. If you connect using ISDN, you
external interface will be <b>ippp0.</b></p>
<p align="left"><img border="0" src="images/BD21298_1.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; If your external interface is <b>ppp0</b>
or <b>ippp0 </b>then you will want to set CLAMPMSS=yes in <a
href="Documentation.htm#Conf"> /etc/shorewall/shorewall.conf.</a></p>
<p align="left">Your <i>Local Interface</i> will be an Ethernet
adapter (eth0, eth1 or eth2) and will be connected to a hub or switch.
Your local computers will be connected to the same switch (note: If you
have
only a single local system, you can connect the firewall directly to
the computer using a <i>cross-over </i> cable).</p>
<p align="left">Your <i>DMZ Interface</i> will also be an Ethernet
adapter (eth0, eth1 or eth2) and will be connected to a hub or switch.
Your DMZ computers will be connected to the same switch (note: If you
have only a single DMZ system, you can connect the firewall directly to
the computer using a <i>cross-over </i> cable).</p>
<p align="left"><u><b> <img border="0" src="images/j0213519.gif"
width="60" height="60"> </b></u>Do not connect 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.&nbsp; When using these
recent
versions, you can test using this kind of configuration if you specify
the <span style="font-weight: bold;">arp_filter</span>
option in /etc/shorewall/interfaces for all interfaces connected to the
common hub/switch. Using such a setup with a production firewall is
strongly recommended against.</p>
<p align="left">For the remainder of this Guide, we will assume that:</p>
<ul>
<li>
<p align="left">The external interface is <b>eth0</b>.</p>
</li>
<li>
<p align="left">The Local interface is <b>eth1</b>.</p>
</li>
<li>
<p align="left">The DMZ interface is <b>eth2</b>.</p>
</li>
</ul>
<p align="left">The Shorewall default configuration does not define the
contents of any zone. To define the above configuration using the
/etc/shorewall/interfaces file, that file would might contain:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>Zone</b></u></td>
<td><u><b>Interface</b></u></td>
<td><u><b>Broadcast</b></u></td>
<td><u><b>Options</b></u></td>
</tr>
<tr>
<td>net</td>
<td>eth0</td>
<td>detect</td>
<td>norfc1918</td>
</tr>
<tr>
<td>loc</td>
<td>eth1</td>
<td>detect</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>dmz</td>
<td>eth2</td>
<td>detect</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
<p align="left"><img border="0" src="images/BD21298_2.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; Edit the /etc/shorewall/interfaces
file and define the network interfaces on your firewall and associate
each interface
with a zone. If you have a zone that is interfaced through more than
one interface, simply include one entry for each interface and repeat
the zone name as many times as necessary.</p>
<p align="left">Example:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>Zone</b></u></td>
<td><u><b>Interface</b></u></td>
<td><u><b>Broadcast</b></u></td>
<td><u><b>Options</b></u></td>
</tr>
<tr>
<td>net</td>
<td>eth0</td>
<td>detect</td>
<td>norfc1918</td>
</tr>
<tr>
<td>loc</td>
<td>eth1</td>
<td>detect</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>loc</td>
<td>eth2</td>
<td>detect</td>
<td>dhcp</td>
</tr>
</tbody>
</table>
</blockquote>
<div align="left">
<p align="left">When you have more than one interface to a zone, you
will usually want a policy that permits intra-zone traffic:</p>
</div>
<div align="left">
<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>loc</td>
<td>ACCEPT</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<p align="left"><img border="0" src="images/BD21298_2.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; You may define more complicated zones
using the <a href="Documentation.htm#Hosts">/etc/shorewall/hosts</a>
file but in most cases, that isn't necessary.</p>
<h2 align="left"><a name="Addressing"></a>4.0 Addressing, Subnets and
Routing</h2>
<p align="left">Normally, your ISP will assign you a set of <i> Public</i>
IP addresses. You will configure your firewall's external interface to
use one of those addresses permanently and you will then have to
decide how you are going to use the rest of your addresses. Before we
tackle that question though, some background is in order.</p>
<p align="left">If you are thoroughly familiar with IP addressing and
routing, you may <a href="#Options">go to the next section</a>.</p>
<p align="left">The following discussion barely scratches the surface
of addressing
and routing. If you are interested in learning more about this subject,
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>
<h3 align="left"><a name="Addresses"></a>4.1 IP Addresses</h3>
<p align="left">IP version 4 (<i>IPv4) </i>addresses are 32-bit
numbers. The notation w.x.y.z refers to an address where the high-order
byte has value "w", the next byte has value "x", etc. If we take the
address 192.0.2.14 and express it in hexadecimal, we get:</p>
<blockquote>
<p align="left">C0.00.02.0E</p>
</blockquote>
<p align="left">or looking at it as a 32-bit integer</p>
<blockquote>
<p align="left">C000020E</p>
</blockquote>
<h3 align="left"><a name="Subnets"></a>4.2 Subnets</h3>
<p align="left">You will still hear the terms "Class A network", "Class
B network" and "Class C network". In the early days of IP, networks
only came in three sizes (there were also Class D networks but they
were used differently):</p>
<blockquote>
<p align="left">Class A - netmask 255.0.0.0, size = 2 ** 24</p>
<p align="left">Class B - netmask 255.255.0.0, size = 2 ** 16</p>
<p align="left">Class C - netmask 255.255.255.0, size = 256</p>
</blockquote>
<p align="left">The class of a network was uniquely determined by the
value of the high order byte of its address so you could look at an IP
address and immediately determine the associated <i>netmask</i>.
The netmask is a number that when logically ANDed with an address
isolates
the <i>network number</i>; the remainder of the address is the <i>host
number</i>. For example, in the Class C address 192.0.2.14, the network
number is hex C00002 and the host number is hex 0E.</p>
<p align="left">As the internet grew, it became clear that such a gross
partitioning of the 32-bit address space was going to be very limiting
(early on, large corporations and universities were assigned their own
class A network!). After some false starts, the current technique of <i>subnetting</i>
these networks into smaller <i>subnetworks</i> evolved; that technique
is referred to as&nbsp;<i>Classless InterDomain Routing</i> (CIDR).
Today, any system that you are likely to work with will understand CIDR
and Class-based networking is largely a thing of the past.</p>
<p align="left">A <i>subnetwork</i> (often referred to as a <i>subnet)
</i>is a contiguous set of IP addresses such that:</p>
<ol>
<li>
<p align="left">The number of addresses in the set is a power of 2;
and</p>
</li>
<li>
<p align="left">The first address in the set is a multiple of the
set size.</p>
</li>
<li>
<p align="left">The first address in the subnet is reserved and is
referred to as the <i> subnet address.</i></p>
</li>
<li>
<p align="left">The last address in the subnet is reserved as the <i>subnet's
broadcast address.</i></p>
</li>
</ol>
<p align="left">As you can see by this definition, in each subnet of
size <b>n</b> there are (<b>n</b> - 2) usable addresses (addresses
that can be assigned to hosts). The first and last address in the
subnet are used for the subnet address and subnet broadcast address
respectively. Consequently, small subnetworks are more wasteful of IP
addresses than are large ones. </p>
<p align="left">Since <b>n</b> is a power of two, we can easily
calculate the <i>Natural Logarithm</i> (<b>log2</b>) of <b>n</b>. For
the
more common subnet sizes, the size and its natural logarithm are given
in the following table:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber5">
<tbody>
<tr>
<td><u><b>n</b></u></td>
<td><u><b>log2 n</b></u></td>
<td><u><b>(32 - log2 n)</b></u></td>
</tr>
<tr>
<td>8</td>
<td>3</td>
<td>29</td>
</tr>
<tr>
<td>16</td>
<td>4</td>
<td>28</td>
</tr>
<tr>
<td>32</td>
<td>5</td>
<td>27</td>
</tr>
<tr>
<td>64</td>
<td>6</td>
<td>26</td>
</tr>
<tr>
<td>128</td>
<td>7</td>
<td>25</td>
</tr>
<tr>
<td>256</td>
<td>8</td>
<td>24</td>
</tr>
<tr>
<td>512</td>
<td>9</td>
<td>23</td>
</tr>
<tr>
<td>1024</td>
<td>10</td>
<td>22</td>
</tr>
<tr>
<td>2048</td>
<td>11</td>
<td>21</td>
</tr>
<tr>
<td>4096</td>
<td>12</td>
<td>20</td>
</tr>
<tr>
<td>8192</td>
<td>13</td>
<td>19</td>
</tr>
<tr>
<td>16384</td>
<td>14</td>
<td>18</td>
</tr>
<tr>
<td>32768</td>
<td>15</td>
<td>17</td>
</tr>
<tr>
<td>65536</td>
<td>16</td>
<td>16</td>
</tr>
</tbody>
</table>
</blockquote>
<p align="left">You will notice that the above table also contains a
column for (32 - log2 <b>n</b>). That number is the <i>Variable
Length
Subnet Mask</i> for a network of size <b>n</b>. From the above table,
we can derive the following one which is a little easier to use.</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber5">
<tbody>
<tr>
<td><u><b>Size of Subnet</b></u></td>
<td><u><b>VLSM</b></u></td>
<td><u><b>Subnet Mask</b></u></td>
</tr>
<tr>
<td>8</td>
<td>/29</td>
<td>255.255.255.248</td>
</tr>
<tr>
<td>16</td>
<td>/28</td>
<td>255.255.255.240</td>
</tr>
<tr>
<td>32</td>
<td>/27</td>
<td>255.255.255.224</td>
</tr>
<tr>
<td>64</td>
<td>/26</td>
<td>255.255.255.192</td>
</tr>
<tr>
<td>128</td>
<td>/25</td>
<td>255.255.255.128</td>
</tr>
<tr>
<td>256</td>
<td>/24</td>
<td>255.255.255.0</td>
</tr>
<tr>
<td>512</td>
<td>/23</td>
<td>255.255.254.0</td>
</tr>
<tr>
<td>1024</td>
<td>/22</td>
<td>255.255.252.0</td>
</tr>
<tr>
<td>2048</td>
<td>/21</td>
<td>255.255.248.0</td>
</tr>
<tr>
<td>4096</td>
<td>/20</td>
<td>255.255.240.0</td>
</tr>
<tr>
<td>8192</td>
<td>/19</td>
<td>255.255.224.0</td>
</tr>
<tr>
<td>16384</td>
<td>/18</td>
<td>255.255.192.0</td>
</tr>
<tr>
<td>32768</td>
<td>/17</td>
<td>255.255.128.0</td>
</tr>
<tr>
<td>65536</td>
<td>/16</td>
<td>255.255.0.0</td>
</tr>
<tr>
<td>2 ** 24</td>
<td>/8</td>
<td>255.0.0.0</td>
</tr>
</tbody>
</table>
</blockquote>
<p align="left">Notice that the VLSM is written with a slash ("/") --
you will often hear a subnet of size 64 referred to as a "slash 26"
subnet and one of size 8 referred to as a "slash 29".</p>
<p align="left">The subnet's mask (also referred to as its <i>netmask)
</i>is simply a 32-bit number with the first "VLSM" bits set to one and
the remaining bits set to zero. For example, for a subnet of size
64, the subnet mask has 26 leading one bits:</p>
<blockquote>
<p align="left">11111111111111111111111111000000 = FFFFFFC0 =
FF.FF.FF.C0 = 255.255.255.192</p>
</blockquote>
<p align="left">The subnet mask has the property that if you logically
AND the subnet mask with an address in the subnet, the result is the
subnet address. Just as important, if you logically AND the subnet mask
with an address outside the subnet, the result is NOT the subnet
address. As we will see below, this property of subnet masks is very
useful in routing.</p>
<p align="left">For a subnetwork whose address is <b>a.b.c.d</b> and
whose Variable Length Subnet Mask is <b>/v</b>, we denote the
subnetwork as "<b>a.b.c.d/v</b>" using <i>CIDR</i> <i>Notation</i>.&nbsp;
</p>
<p align="left">Example:</p>
<blockquote>
<table border="2" style="border-collapse: collapse;" id="AutoNumber1"
cellpadding="2">
<tbody>
<tr>
<td><b>Subnet:</b></td>
<td>10.10.10.0 - 10.10.10.127</td>
</tr>
<tr>
<td><b>Subnet Size:</b></td>
<td>128</td>
</tr>
<tr>
<td><b>Subnet Address:</b></td>
<td>10.10.10.0</td>
</tr>
<tr>
<td><b>Broadcast Address:</b></td>
<td>10.10.10.127</td>
</tr>
<tr>
<td><b>CIDR&nbsp;Notation:</b></td>
<td>10.10.10.0/25</td>
</tr>
</tbody>
</table>
</blockquote>
<p align="left">There are two degenerate subnets that need mentioning;
namely, the subnet with one member and the subnet with 2 ** 32 members.</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber5">
<tbody>
<tr>
<td><u><b>Size of Subnetwork</b></u></td>
<td><u><b>VLSM Length</b></u></td>
<td><u><b>Subnet Mask</b></u></td>
<td><u><b>CIDR&nbsp;Notation</b></u></td>
</tr>
<tr>
<td>1</td>
<td>32</td>
<td>255.255.255.255</td>
<td>a.b.c.d/32</td>
</tr>
<tr>
<td>2 ** 32</td>
<td>0</td>
<td>0.0.0.0</td>
<td>0.0.0.0/0</td>
</tr>
</tbody>
</table>
</blockquote>
<p align="left">So any address <b>a.b.c.d </b>may also be written <b>
a.b.c.d/32</b> and the set of all possible IP addresses is written <b>0.0.0.0/0</b>.</p>
<p align="left">Later in this guide, you will see the notation <b>a.b.c.d/v</b>
used to describe the ip configuration of a network interface (the
'ip' utility also uses this syntax). This simply means that the
interface is configured with ip address <b>a.b.c.d</b> and with the
netmask that corresponds to VLSM <b>/v</b>.</p>
<p align="left">Example: 192.0.2.65/29</p>
<p align="left">&nbsp;&nbsp;&nbsp; The interface is configured with IP
address 192.0.2.65 and netmask 255.255.255.248.<br>
</p>
<p align="left">Beginning with Shorewall 1.4.6, /sbin/shorewall
supports an
<b>ipcalc</b> command that automatically calculates information about a
[sub]network.<br>
</p>
<p align="left">Example 1:<br>
</p>
<blockquote>
<pre><b><font color="#009900">ipcalc 10.10.10.0/25<br></font></b> CIDR=10.10.10.0/25<br> NETMASK=255.255.255.128<br> NETWORK=10.10.10.0<br> BROADCAST=10.10.10.127<br></pre>
</blockquote>
Example 2
<blockquote>
<pre><b><font color="#009900">ipcalc 10.10.10.0 255.255.255.128</font></b><br> CIDR=10.10.10.0/25<br> NETMASK=255.255.255.128<br> NETWORK=10.10.10.0<br> BROADCAST=10.10.10.127<br></pre>
</blockquote>
<h3 align="left"><a name="Routing"></a>4.3 Routing</h3>
<p align="left">One of the purposes of subnetting is that it forms the
basis for routing. Here's the routing table on my firewall:</p>
<div align="left">
<blockquote>
<pre>[root@gateway root]# netstat -nr<br>Kernel IP routing table<br>Destination Gateway Genmask Flags MSS Window irtt Iface<br>192.168.9.1 0.0.0.0 255.255.255.255 UH 40 0 0 texas<br>206.124.146.177 0.0.0.0 255.255.255.255 UH 40 0 0 eth1<br>206.124.146.180 0.0.0.0 255.255.255.255 UH 40 0 0 eth3<br>192.168.3.0 0.0.0.0 255.255.255.0 U 40 0 0 eth3<br>192.168.2.0 0.0.0.0 255.255.255.0 U 40 0 0 eth1<br>192.168.1.0 0.0.0.0 255.255.255.0 U 40 0 0 eth2<br>206.124.146.0 0.0.0.0 255.255.255.0 U 40 0 0 eth0<br>192.168.9.0 192.0.2.223 255.255.255.0 UG 40 0 0 texas<br>127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo<br>0.0.0.0 206.124.146.254 0.0.0.0 UG 40 0 0 eth0<br>[root@gateway root]#</pre>
</blockquote>
</div>
<p align="left">The device <i>texas</i> is a GRE tunnel to a peer site
in the Dallas, Texas area.<br>
<br>
The first three routes are <i>host routes</i> since they
indicate how to get to a single host. In the 'netstat' output this
can be seen by the "Genmask" (Subnet Mask) of 255.255.255.255 and
the "H" in the Flags column. The remainder are 'net' routes since they
tell the kernel how to route packets to a subnetwork. The last route
is the <i>default route</i> and the gateway mentioned in that route is
called the <i>default gateway</i>.</p>
<p align="left">When the kernel is trying to send a packet to IP
address <b>A</b>, it starts at the top of the routing table and:</p>
<ul>
<li>
<p align="left"><b>A</b> is logically ANDed with the 'Genmask'
value in
the table entry.</p>
</li>
<li>
<p align="left">The result is compared with the 'Destination' value
in the table entry.</p>
</li>
<li>
<p align="left">If the result and the 'Destination' value are the
same, then:</p>
<ul>
<li>
<p align="left">If the 'Gateway' column is non-zero, the packet
is sent to the gateway over the interface named in the 'Iface' column.</p>
</li>
<li>
<p align="left">Otherwise, the packet is sent directly to <b>A
</b>over the interface named in the 'iface' column.</p>
</li>
</ul>
</li>
<li>
<p align="left">Otherwise, the above steps are repeated on the next
entry in the table.</p>
</li>
</ul>
<p align="left">Since the default route matches any IP address (<b>A</b>
land 0.0.0.0 = 0.0.0.0), packets that don't match any of the other
routing table entries are sent to the <i>default gateway</i> which is
usually a router
at your ISP.</p>
<p align="left">Lets take an example. Suppose that we want to route a
packet to 192.168.1.5. That address clearly doesn't match any of the
host routes in the table but if we logically and that address with
255.255.255.0, the result is 192.168.1.0 which matches this routing
table entry:</p>
<div align="left">
<blockquote>
<pre>192.168.1.0 0.0.0.0 255.255.255.0 U 40 0 0 eth2</pre>
</blockquote>
<p>So to route a packet to 192.168.1.5, the packet is sent directly
over eth2.</p>
</div>
<p align="left">One more thing needs to be emphasized -- all outgoing
packet are sent using the routing table and reply packets are not a
special case. There seems to be a common mis-conception whereby people
think that request packets are like salmon and contain a genetic code
that is magically transferred to reply packets so that the replies
follow the reverse route taken by the request. That isn't the case; the
replies may take a totally different route back to the client than was
taken by the requests -- they are totally independent.</p>
<h3 align="left"><a name="ARP"></a>4.4 Address Resolution Protocol (ARP)</h3>
<p align="left">When sending packets over Ethernet, IP addresses aren't
used. Rather Ethernet addressing is based on <i>Media Access Control</i>
(MAC) addresses. Each Ethernet device has it's own unique&nbsp; MAC
address which is burned into a PROM on the device during manufacture.
You can obtain the MAC of an Ethernet device using the 'ip' utility:</p>
<blockquote>
<div align="left">
<pre>[root@gateway root]# ip addr show eth0<br>2: eth0: &lt;BROADCAST,MULTICAST,UP&gt; mtu 1500 qdisc htb qlen 100<br>link/ether <u>02:00:08:e3:fa:55</u> brd ff:ff:ff:ff:ff:ff<br>inet 206.124.146.176/24 brd 206.124.146.255 scope global eth0<br>inet 206.124.146.178/24 brd 206.124.146.255 scope global secondary eth0<br>inet 206.124.146.179/24 brd 206.124.146.255 scope global secondary eth0<br>[root@gateway root]#</pre>
</div>
</blockquote>
<div align="left">
<p align="left">As you can see from the above output, the MAC is 6
bytes (48 bits) wide. A card's MAC is usually also printed on a label
attached to
the card itself. </p>
</div>
<div align="left">
<p align="left">Because IP uses IP addresses and Ethernet uses MAC
addresses, a mechanism is required to translate an IP address into a
MAC address; that is the purpose of the <i>Address Resolution Protocol</i>
(ARP). Here is ARP in action:</p>
</div>
<div align="left">
<blockquote>
<div align="left">
<pre>[root@gateway root]# tcpdump -nei eth2 arp<br>tcpdump: listening on eth2<br>09:56:49.766757 2:0:8:e3:4c:48 0:6:25:aa:8a:f0 arp 42: arp who-has 192.168.1.19 tell 192.168.1.254<br>09:56:49.769372 0:6:25:aa:8a:f0 2:0:8:e3:4c:48 arp 60: arp reply 192.168.1.19 is-at 0:6:25:aa:8a:f0<br><br>2 packets received by filter<br>0 packets dropped by kernel<br>[root@gateway root]#<br></pre>
</div>
</blockquote>
</div>
<p align="left">In this exchange, 192.168.1.254 (MAC 2:0:8:e3:4c:48)
wants to know the MAC of the device with IP address 192.168.1.19. The
system having that IP address is responding that the MAC address of the
device with IP address 192.168.1.19 is 0:6:25:aa:8a:f0.</p>
<p align="left">In order to avoid having to exchange ARP information
each time that an IP packet is to be sent, systems maintain an <i>ARP
cache</i> of IP&lt;-&gt;MAC correspondences. You can see the ARP
cache on your system (including your Windows system) using the 'arp'
command:</p>
<blockquote>
<div align="left">
<pre>[root@gateway root]# arp -na<br>? (206.124.146.177) at 00:A0:C9:15:39:78 [ether] on eth1<br>? (192.168.1.3) at 00:A0:CC:63:66:89 [ether] on eth2<br>? (192.168.1.5) at 00:A0:CC:DB:31:C4 [ether] on eth2<br>? (206.124.146.254) at 00:03:6C:8A:18:38 [ether] on eth0<br>? (192.168.1.19) at 00:06:25:AA:8A:F0 [ether] on eth2</pre>
</div>
</blockquote>
<p align="left">The leading question marks are a result of my having
specified the 'n' option (Windows 'arp' doesn't allow that option)
which causes the 'arp' program to forego IP-&gt;DNS name translation.
Had I not
given that option, the question marks would have been replaced with
the FQDN corresponding to each IP address. Notice that the last entry
in the table records the information we saw using tcpdump above.</p>
<h3 align="left"><a name="RFC1918"></a>4.5 RFC 1918</h3>
<p align="left">IP addresses are allocated by the <i> <a
href="http://www.iana.org">Internet Assigned Number Authority</a> </i>(IANA)
who delegates allocations on a geographic basis to <i>Regional
Internet Registries</i> (RIRs). For example, allocation for the
Americas and for sub-Sahara Africa is delegated to the <i><a
href="http://www.arin.net">American Registry for Internet Numbers</a> </i>(ARIN).
These RIRs may in turn delegate to national registries. Most of us
don't deal with these registrars but rather get our IP addresses from
our ISP.</p>
<p align="left">It's a fact of life that most of us can't afford as
many Public IP addresses as we have devices to assign them to so we end
up making use
of <i> Private </i>IP addresses. RFC 1918 reserves several 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">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.
This is understandable given that anyone can select any of these
addresses for their private use.</p>
</div>
<div align="left">
<p align="left">When selecting addresses from these ranges, there's a
couple of things to keep in mind:</p>
</div>
<div align="left">
<ul>
<li>
<p align="left">As the IPv4 address space becomes depleted, more
and more organizations (including ISPs) are beginning to use RFC 1918
addresses in their infrastructure. </p>
</li>
<li>
<p align="left">You don't want to use addresses that are being used
by your ISP or by another organization with whom you want to establish
a VPN relationship. </p>
</li>
</ul>
</div>
<div align="left">
<p align="left">So it's a good idea to check with your ISP to see if
they are using (or are planning to use) private addresses before you
decide the addresses that you are going to use.</p>
</div>
<div align="left">
<h2 align="left"><a name="Options"></a>5.0 Setting up your Network</h2>
</div>
<div align="left">
<p align="left">The choice of how to set up your network depends
primarily on how many Public IP addresses you have vs. how many
addressable entities you have in your network. Regardless of how many
addresses you have, your ISP will handle that set of addresses in one
of two ways:</p>
</div>
<div align="left">
<ol>
<li>
<p align="left"><b>Routed - </b>Traffic to any of your addresses
will be routed through a single <i>gateway address</i>. This will
generally only be done if your ISP has assigned you a complete subnet
(/29 or larger). In this case, you will assign the gateway address as
the IP address of your firewall/router's external interface. </p>
</li>
<li>
<p align="left"><b>Non-routed - </b>Your ISP will send traffic to
each of your addresses directly. </p>
</li>
</ol>
</div>
<div align="left">
<p align="left">In the subsections that follow, we'll look at each of
these separately.<br>
</p>
<p align="left">Before we begin, there is one thing for you to check:</p>
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
height="13" alt=""> &nbsp;&nbsp;&nbsp; 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 (Shorewall versions earlier than 1.4.6)</li>
<li>IP_FORWARDING=On<br>
</li>
</ul>
</div>
<div align="left">
<h3 align="left"><a name="Routed"></a>5.1 Routed</h3>
</div>
<div align="left">
<p align="left">Let's assume that your ISP has assigned you the subnet
192.0.2.64/28 routed through 192.0.2.65. That means that you have IP
addresses 192.0.2.64 - 192.0.2.79 and that your firewall's external IP
address is 192.0.2.65. Your ISP has also told you that you should use a
netmask of 255.255.255.0 (so your /28 is part of a larger /24). With
this many IP addresses, you are able to subnet your /28 into two /29's
and set up your network as shown in the following diagram.</p>
</div>
<div align="left">
<p align="center"> <img border="0" src="images/dmz4.png" width="726"
height="635"> </p>
</div>
<div align="left">
<p align="left">Here, the DMZ comprises the subnet 192.0.2.64/29 and
the Local network is 192.0.2.72/29. The default gateway for hosts in
the DMZ would
be configured to 192.0.2.66 and the default gateway for hosts in the
local network would be 192.0.2.73.</p>
</div>
<div align="left">
<p align="left">Notice that this arrangement is rather wasteful of
public IP addresses since it is using 192.0.2.64 and 192.0.2.72 for
subnet addresses, 192.0.2.71 and 192.0.2.79 for subnet broadcast
addresses and 192.0.2.66 and 168.0.2.73 for internal addresses on the
firewall/router. Nevertheless, it shows how subnetting can work and if
we were dealing with a /24 rather than a /28 network, the use of 6 IP
addresses
out of 256 would be justified because of the simplicity of the setup.</p>
</div>
<div align="left">
<p align="left">The astute reader may have noticed that the
Firewall/Router's external interface is actually part of the DMZ subnet
(192.0.2.64/29). What if DMZ 1 (192.0.2.67) tries to communicate with
192.0.2.65? The routing table on DMZ 1 will look like this:</p>
</div>
<div align="left">
<blockquote>
<pre>Kernel IP routing table<br>Destination Gateway Genmask Flags MSS Window irtt Iface<br>192.0.2.64 0.0.0.0 255.255.255.248 U 40 0 0 eth0<br>0.0.0.0 192.0.2.66 0.0.0.0 UG 40 0 0 eth0</pre>
</blockquote>
</div>
<div align="left">
<p align="left">This means that DMZ 1 will send an ARP "who-has
192.0.2.65" request and no device on the DMZ Ethernet segment has that
IP address. Oddly enough, the firewall will respond to the request with
the
MAC address of its <u>DMZ Interface!!</u> DMZ 1 can then send Ethernet
frames addressed to that MAC address and the frames will be received
(correctly) by the firewall/router.</p>
</div>
<div align="left">
<p align="left">It is this rather unexpected ARP behavior on the part
of the Linux Kernel that prompts the warning earlier in this guide
regarding the connecting of multiple firewall/router interfaces to the
same hub or switch. When an ARP request for one of the
firewall/router's IP addresses is sent
by another system connected to the hub/switch, all of the firewall's
interfaces that connect to the hub/switch can respond! It is then a
race as to which "here-is" response reaches the sender first.</p>
</div>
<div align="left">
<h3 align="left"><a name="NonRouted"></a>5.2 Non-routed</h3>
</div>
<div align="left">
<p align="left">If you have the above situation but it is non-routed,
you
can configure your network exactly as described above with one
additional twist; simply specify the "proxyarp" option on all three
firewall interfaces in the /etc/shorewall/interfaces file.</p>
</div>
<div align="left">
<p align="left">Most of us don't have the luxury of having enough
public IP addresses to set up our networks as shown in the preceding
example (even
if the setup is routed). </p>
</div>
<div align="left">
<p align="left"><b>For the remainder of this section, assume that your
ISP has assigned you IP addresses 192.0.2.176-180 and has told you
to use netmask 255.255.255.0 and default gateway 192.0.2.254.</b></p>
</div>
<div align="left">
<p align="left">Clearly, that set of addresses doesn't comprise a
subnetwork and there aren't enough addresses for all of the network
interfaces. There are four different techniques that can be used to
work around this problem.</p>
</div>
<div align="left">
<ul>
<li>
<p align="left"><i>Source Network Address Translation </i>(SNAT). </p>
</li>
<li>
<p align="left"><i>Destination Network Address Translation </i>(DNAT)
also known as <i>Port Forwarding.</i> </p>
</li>
<li>
<p align="left"><i>Proxy ARP.</i> </p>
</li>
<li>
<p align="left"><i>Network Address Translation</i> (NAT) also
referred to as <i>Static NAT</i>. </p>
</li>
</ul>
</div>
<div align="left">
<p align="left">Often a combination of these techniques is used. Each
of these will be discussed in the sections that follow.</p>
</div>
<div align="left">
<h4 align="left"><a name="SNAT"></a>&nbsp;5.2.1 SNAT</h4>
</div>
<div align="left">
<p align="left">With SNAT, an internal LAN segment is configured using
RFC 1918 addresses. When a host <b>A </b>on this internal segment
initiates a connection to host <b>B</b> on the internet, the
firewall/router rewrites the IP header in the request to use one of
your public
IP addresses as the source address. When <b>B</b> responds and the
response is received by the firewall, the firewall changes the
destination address back to the RFC 1918 address of <b>A</b> and
forwards the response back to <b>A.</b></p>
</div>
<div align="left">
<p align="left">Let's suppose that you decide to use SNAT on your local
zone and use public address 192.0.2.176 as both your firewall's
external IP address and the source IP address of internet requests sent
from that zone.</p>
</div>
<div align="left">
<h2 align="center"> <img border="0" src="images/dmz5.png" width="745"
height="635"> </h2>
</div>
<div align="left"> The local zone has been subnetted as
192.168.201.0/29 (netmask 255.255.255.248).</div>
<div align="left"> &nbsp;</div>
<div align="left"> <img border="0" src="images/BD21298_2.gif"
width="13" height="13"> &nbsp;&nbsp;&nbsp; The systems in the local
zone would be configured with a default gateway of 192.168.201.1 (the
IP address of the firewall's local interface).</div>
<div align="left"> &nbsp;</div>
<div align="left"> <img border="0" src="images/BD21298_2.gif"
width="13" height="13"> &nbsp;&nbsp;&nbsp; SNAT is configured in
Shorewall using the <a href="Documentation.htm#Masq">
/etc/shorewall/masq file</a>.</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber6">
<tbody>
<tr>
<td width="33%"><u><b>INTERFACE</b></u></td>
<td width="33%"><u><b>SUBNET</b></u></td>
<td width="34%"><u><b>ADDRESS</b></u></td>
</tr>
<tr>
<td width="33%">eth0</td>
<td width="33%">192.168.201.0/29</td>
<td width="34%">192.0.2.176</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">This example used the normal technique of assigning the
same public IP address for the firewall external interface and for
SNAT. If you wanted to use a different IP address, you would either
have to use your distributions network configuration tools to add that
IP address to the external interface or you could set
ADD_SNAT_ALIASES=Yes in /etc/shorewall/shorewall.conf and Shorewall
will add the address for you.</p>
</div>
<div align="left">
<h4 align="left"><a name="DNAT"></a>5.2.2 DNAT</h4>
</div>
<div align="left">
<p align="left">When SNAT is used, it is impossible for hosts on the
internet to initiate a connection to one of the internal systems since
those systems do not have a public IP address. DNAT provides a way to
allow selected connections from the internet.</p>
</div>
<div align="left">
<p align="left"><img border="0" src="images/BD21298_2.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp;&nbsp; Suppose that your daughter wants
to run a web server on her system "Local 3". You could allow
connections to the internet to her server by adding the following entry
in <a href="Documentation.htm#Rules">/etc/shorewall/rules</a>:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber7">
<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 DESTINATION</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:192.168.201.4</td>
<td>tcp</td>
<td>www</td>
<td>-</td>
<td>192.0.2.176</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">If one of your daughter's friends at address <b>A</b>
wants to access your daughter's server, she can connect to <a
href="http://192.0.2.176"> http://192.0.2.176</a> (the firewall's
external IP address) and the firewall will rewrite the destination IP
address to 192.168.201.4 (your daughter's system) and forward the
request. When your daughter's server responds, the firewall will
rewrite the source address back to 192.0.2.176 and send the response
back to <b>A.</b></p>
</div>
<div align="left">
<p align="left">This example used the firewall's external IP address
for DNAT. You can use another of your public IP addresses but Shorewall
will not
add that address to the firewall's external interface for you.</p>
</div>
<div align="left">
<h4 align="left"><a name="ProxyARP"></a>5.2.3 Proxy ARP</h4>
</div>
<div align="left">
<p align="left">The idea behind proxy ARP is that:</p>
</div>
<div align="left">
<ul>
<li>
<p align="left">A host <b>H </b>behind your firewall is assigned
one of
your public IP addresses (<b>A)</b> and is assigned the same netmask <b>(M)
</b>as the firewall's external interface. </p>
</li>
<li>
<p align="left">The firewall responds to ARP "who has" requests for
<b>A.</b> </p>
</li>
<li>
<p align="left">When <b>H</b> issues an ARP "who has" request for
an address in the subnetwork defined by <b>A</b> and <b>M</b>, the
firewall will
respond (with the MAC if the firewall interface) to <b>H</b>. </p>
</li>
</ul>
</div>
<div align="left">
<p align="left">Let us suppose that we decide to use Proxy ARP on the
DMZ
in our example network.</p>
</div>
<div align="left">
<p align="center"> <img border="0" src="images/dmz6.png" width="745"
height="635"> </p>
</div>
<div align="left"> Here, we've assigned the IP addresses 192.0.2.177 to
system DMZ 1 and 192.0.2.178 to DMZ 2. Notice that we've just assigned
an arbitrary RFC 1918 IP address and subnet mask to the DMZ interface
on the firewall. That address and netmask isn't relevant - just
be sure it doesn't overlap another subnet that you've defined.</div>
<div align="left"> &nbsp;</div>
<div align="left"> <img border="0" src="images/BD21298_2.gif"
width="13" height="13"> &nbsp;&nbsp;&nbsp; The Shorewall configuration
of Proxy ARP is done using the <a href="Documentation.htm#ProxyArp">/etc/shorewall/proxyarp</a>
file.</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" id="AutoNumber8"
style="border-collapse: collapse;">
<tbody>
<tr>
<td><u><b>ADDRESS</b></u></td>
<td><u><b>INTERFACE</b></u></td>
<td><u><b>EXTERNAL</b></u></td>
<td><u><b>HAVE ROUTE</b></u></td>
</tr>
<tr>
<td>192.0.2.177</td>
<td>eth2</td>
<td>eth0</td>
<td>No</td>
</tr>
<tr>
<td>192.0.2.178</td>
<td>eth2</td>
<td>eth0</td>
<td>No</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">Because the HAVE ROUTE column contains No, Shorewall
will add host routes thru eth2 to 192.0.2.177 and 192.0.2.178.<br>
</p>
<p align="left">The ethernet interfaces on DMZ 1 and DMZ 2 should be
configured to have the IP addresses shown but should have the same
default gateway as the firewall itself -- namely 192.0.2.254. In other
words, they should be configured just like they would be if they were
parallel to the firewall rather than behind it.<br>
</p>
<p><font color="#ff0000"><b>NOTE: Do not add the Proxy ARP'ed
address(es) (192.0.2.177 and 192.0.2.178 in the above example)&nbsp; to
the external interface (eth0 in this example) of the firewall.</b></font><br>
</p>
<div align="left"> </div>
</div>
<div align="left">
<p align="left"></p>
</div>
<div align="left">
<div align="left">
<p align="left">A word of warning is in order here. ISPs typically
configure their routers with a long ARP cache timeout. If you move a
system from parallel to your firewall to behind your firewall with
Proxy
ARP, it will probably be HOURS before that system can communicate with
the internet. There are a couple of things that you can try:<br>
</p>
<ol>
<li>(Courtesy of Bradey Honsinger) A reading of Stevens' <i>TCP/IP
Illustrated, Vol 1</i> reveals that a <br>
<br>
"gratuitous" ARP packet should cause the ISP's router to refresh their
ARP cache (section 4.7). A gratuitous ARP is simply a host requesting
the MAC address for its own IP; in addition to ensuring that the IP
address isn't a duplicate,...<br>
<br>
"if the host sending the gratuitous ARP has just changed its hardware
address..., this packet causes any other host...that has an entry in
its cache for the old hardware address to update its ARP cache entry
accordingly."<br>
<br>
Which is, of course, exactly what you want to do when you switch a host
from being exposed to the Internet to behind Shorewall using proxy ARP
(or static NAT for that matter). Happily enough, recent versions
of Redhat's iputils package include "arping", whose "-U" flag does just
that:<br>
<br>
&nbsp;&nbsp;&nbsp; <font color="#009900"><b>arping -U -I &lt;net
if&gt; &lt;newly proxied IP&gt;</b></font><br>
&nbsp;&nbsp;&nbsp; <font color="#009900"><b>arping -U -I eth0
66.58.99.83 # for example</b></font><br>
<br>
Stevens goes on to mention that not all systems respond correctly to
gratuitous ARPs, but googling for "arping -U" seems to support the idea
that it works most of the time.<br>
<br>
</li>
<li>You can call your ISP and ask them to purge the stale ARP cache
entry but many either can't or won't purge individual
entries.</li>
</ol>
You can determine if your ISP's gateway ARP cache is stale using ping
and tcpdump. Suppose that we suspect that the gateway
router has a stale ARP cache entry for 192.0.2.177. On the firewall,
run tcpdump as follows:</div>
<div align="left">
<pre> <font color="#009900"><b>tcpdump -nei eth0 icmp</b></font></pre>
</div>
<div align="left">
<p align="left">Now from 192.0.2.177, ping the ISP's gateway (which we
will assume is 192.0.2.254):</p>
</div>
<div align="left">
<pre> <b><font color="#009900">ping 192.0.2.254</font></b></pre>
</div>
</div>
<div align="left">
<p align="left">We can now observe the tcpdump output:</p>
</div>
<div align="left">
<pre> 13:35:12.159321 <u>0:4:e2:20:20:33</u> 0:0:77:95:dd:19 ip 98: 192.0.2.177 &gt; 192.0.2.254: icmp: echo request (DF)<br> 13:35:12.207615 0:0:77:95:dd:19 <u>0:c0:a8:50:b2:57</u> ip 98: 192.0.2.254 &gt; 192.0.2.177 : icmp: echo reply</pre>
</div>
<div align="left">
<p align="left">Notice that the source MAC address in the echo request
is different from the destination MAC address in the echo reply!!
In this case 0:4:e2:20:20:33 was the MAC of the firewall's eth0
NIC while 0:c0:a8:50:b2:57 was the MAC address of DMZ 1. In other
words, the gateway's ARP cache still associates 192.0.2.177 with
the NIC in DMZ 1 rather than with the firewall's eth0.</p>
</div>
<div align="left">
<h4 align="left"><a name="NAT"></a>5.2.4 Static NAT</h4>
</div>
<div align="left">
<p align="left">With static NAT, you assign local systems RFC 1918
addresses then establish a one-to-one mapping between those addresses
and
public IP addresses. For outgoing connections SNAT (Source Network
Address Translation) occurs and on incoming connections DNAT
(Destination Network Address Translation) occurs. Let's go back to our
earlier example involving your daughter's web server running on system
Local 3.</p>
</div>
<div align="left">
<p align="center"><img border="0" src="images/dmz5.png" width="745"
height="635"> </p>
</div>
<div align="left">
<p align="left">Recall that in this setup, the local network is using
SNAT and is sharing the firewall external IP (192.0.2.176) for outbound
connections. This is done with the following entry in
/etc/shorewall/masq:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber6">
<tbody>
<tr>
<td width="33%"><u><b>INTERFACE</b></u></td>
<td width="33%"><u><b>SUBNET</b></u></td>
<td width="34%"><u><b>ADDRESS</b></u></td>
</tr>
<tr>
<td width="33%">eth0</td>
<td width="33%">192.168.201.0/29</td>
<td width="34%">192.0.2.176</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left"><img border="0" src="images/BD21298_1.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; Suppose now that you have decided to
give your daughter her own IP address (192.0.2.179) for both inbound
and outbound connections. You would do that by adding an entry in <a
href="Documentation.htm#NAT">/etc/shorewall/nat</a>.</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellspacing="0" cellpadding="2" id="AutoNumber9"
style="border-collapse: collapse;">
<tbody>
<tr>
<td>EXTERNAL</td>
<td>INTERFACE</td>
<td>INTERNAL</td>
<td>ALL INTERFACES </td>
<td>LOCAL</td>
</tr>
<tr>
<td>192.0.2.179</td>
<td>eth0</td>
<td>192.168.201.4</td>
<td>No</td>
<td>No</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">With this entry in place, you daughter has her own IP
address and the other two local systems share the firewall's IP address.</p>
</div>
<div align="left">
<p align="left"><img border="0" src="images/BD21298_1.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; Once the relationship between
192.0.2.179 and 192.168.201.4 is established by the nat file entry
above, it is no longer appropriate to use a DNAT rule for you
daughter's web server -- you would rather just use an ACCEPT rule:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber7">
<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 DESTINATION</b></u></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>loc:192.168.201.4</td>
<td>tcp</td>
<td>www</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<div align="left">
<div align="left">
<p align="left">A word of warning is in order here. ISPs typically
configure their routers with a long ARP cache timeout. If you move a
system from parallel to your firewall to behind your firewall with
static NAT, it will probably be HOURS before that system can
communicate
with the internet. There are a couple of things that you can try:<br>
</p>
<ol>
<li>(Courtesy of Bradey Honsinger) A reading of Stevens' <i>TCP/IP
Illustrated, Vol 1</i> reveals that a <br>
<br>
"gratuitous" ARP packet should cause the ISP's router to refresh their
ARP cache (section 4.7). A gratuitous ARP is simply a host requesting
the MAC address for its own IP; in addition to ensuring that the IP
address isn't a duplicate,...<br>
<br>
"if the host sending the gratuitous ARP has just changed its hardware
address..., this packet causes any other host...that has an entry in
its cache for the old hardware address to update its ARP cache entry
accordingly."<br>
<br>
Which is, of course, exactly what you want to do when you switch a host
from being exposed to the Internet to behind Shorewall using proxy ARP
(or static NAT for that matter). Happily enough, recent versions
of Redhat's iputils package include "arping", whose "-U" flag does just
that:<br>
<br>
&nbsp;&nbsp;&nbsp; <font color="#009900"><b>arping -U -I &lt;net
if&gt; &lt;newly proxied IP&gt;</b></font><br>
&nbsp;&nbsp;&nbsp; <font color="#009900"><b>arping -U -I eth0
66.58.99.83 # for example</b></font><br>
<br>
Stevens goes on to mention that not all systems respond correctly to
gratuitous ARPs, but googling for "arping -U" seems to support the idea
that it works most of the time.<br>
<br>
</li>
<li>You can call your ISP and ask them to purge the stale ARP cache
entry but many either can't or won't purge individual entries.</li>
</ol>
You can determine if your ISP's gateway ARP cache is stale using ping
and tcpdump. Suppose that we suspect that the gateway
router has a stale ARP cache entry for 209.0.2.179. On the firewall,
run tcpdump as follows:</div>
<div align="left">
<pre> <font color="#009900"><b>tcpdump -nei eth0 icmp</b></font></pre>
</div>
<div align="left">
<p align="left">Now from the 192.168.201.4, ping the ISP's gateway
(which we will assume is 192.0.2.254):</p>
</div>
<div align="left">
<pre> <b><font color="#009900">ping 192.0.2.254</font></b></pre>
</div>
</div>
<div align="left">
<p align="left">We can now observe the tcpdump output:</p>
</div>
<div align="left">
<pre> 13:35:12.159321 <u>0:4:e2:20:20:33</u> 0:0:77:95:dd:19 ip 98: 192.0.2.179 &gt; 192.0.2.254: icmp: echo request (DF)<br> 13:35:12.207615 0:0:77:95:dd:19 <u>0:c0:a8:50:b2:57</u> ip 98: 192.0.2.254 &gt; 192.0.2.179 : icmp: echo reply</pre>
</div>
<div align="left">
<p align="left">Notice that the source MAC address in the echo request
is different from the destination MAC address in the echo reply!!
In this case 0:4:e2:20:20:33 was the MAC of the firewall's eth0
NIC while 0:c0:a8:50:b2:57 was the MAC address of DMZ 1. In other
words, the gateway's ARP cache still associates 192.0.2.179 with
the NIC in the local zone rather than with the firewall's eth0.</p>
</div>
<h3 align="left"><a name="Rules"></a>5.3 Rules</h3>
</div>
<div align="left">
<p align="left"><img border="0" src="images/BD21298_1.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; With the default policies, your local
systems (Local 1-3) can access any servers on the internet and the DMZ
can't access any other host (including the firewall). With the
exception of
<a href="#DNAT">DNAT rules</a> which cause address translation and
allow the translated connection request to pass through the firewall,
the way to allow connection requests through your firewall is to
use ACCEPT rules.</p>
</div>
<div align="left">
<p align="left"><b>NOTE: Since the SOURCE PORT and ORIG. DEST. Columns
aren't used in this section, they won't be shown</b></p>
</div>
<div align="left">
<p align="left">You probably want to allow ping between your zones:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber7">
<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>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz</td>
<td>icmp</td>
<td>echo-request</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>loc</td>
<td>icmp</td>
<td>echo-request</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>dmz</td>
<td>loc</td>
<td>icmp</td>
<td>echo-request</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz</td>
<td>icmp</td>
<td>echo-request</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">Let's suppose that you run mail and pop3 servers on DMZ
2 and a Web Server on DMZ 1. The rules that you would need are:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber7">
<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>COMMENTS</b></u></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>smtp</td>
<td># Mail from the Internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>pop3</td>
<td># Pop3 from the Internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>smtp</td>
<td># Mail from the Local Network</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>pop3</td>
<td># Pop3 from the Local Network</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>fw</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>smtp</td>
<td># Mail from the Firewall</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>dmz:192.0.2.178</td>
<td>net</td>
<td>tcp</td>
<td>smtp</td>
<td># Mail to the Internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz:192.0.2.177</td>
<td>tcp</td>
<td>http</td>
<td># WWW from the Net</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz:192.0.2.177</td>
<td>tcp</td>
<td>https</td>
<td># Secure HTTP from the Net</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz:192.0.2.177</td>
<td>tcp</td>
<td>https</td>
<td># Secure HTTP from the Local Net</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">If you run a public DNS server on 192.0.2.177, you
would need to add the following rules:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber7">
<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>COMMENTS</b></u></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz:192.0.2.177</td>
<td>udp</td>
<td>domain</td>
<td># UDP DNS from the Internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz:192.0.2.177</td>
<td>tcp</td>
<td>domain</td>
<td># TCP DNS from the internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>fw</td>
<td>dmz:192.0.2.177</td>
<td>udp</td>
<td>domain</td>
<td># UDP DNS from firewall</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>fw</td>
<td>dmz:192.0.2.177</td>
<td>tcp</td>
<td>domain</td>
<td># TCP DNS from firewall</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz:192.0.2.177</td>
<td>udp</td>
<td>domain</td>
<td># UDP DNS from the local Net</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz:192.0.2.177</td>
<td>tcp</td>
<td>domain</td>
<td># TCP DNS from the local Net</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>dmz:192.0.2.177</td>
<td>net</td>
<td>udp</td>
<td>domain</td>
<td># UDP DNS to the Internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>dmz:192.0.2.177</td>
<td>net</td>
<td>tcp</td>
<td>domain</td>
<td># TCP DNS to the Internet</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">You probably want some way to communicate with your
firewall and DMZ systems from the local network -- I recommend SSH
which
through its scp utility can also do publishing and software update
distribution.</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber7">
<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>COMMENTS</b></u></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz</td>
<td>tcp</td>
<td>ssh</td>
<td># SSH to the DMZ</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>fw</td>
<td>tcp</td>
<td>ssh</td>
<td># SSH to the Firewall</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<h3 align="left"><a name="OddsAndEnds"></a>5.4 Odds and Ends</h3>
</div>
<div align="left">
<p align="left">The above discussion reflects my personal preference
for using Proxy ARP for my servers in my DMZ and SNAT/NAT for my local
systems. I
prefer to use NAT only in cases where a system that is part of an RFC
1918
subnet needs to have it's own public IP.&nbsp;</p>
</div>
<div align="left">
<p align="left"><img border="0" src="images/BD21298_2.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; If you haven't already, it would be a
good idea to
browse through <a href="Documentation.htm#Conf">/etc/shorewall/shorewall.conf</a>
just to see if there is anything there that might be of interest. You
might also want to look at the other configuration files that you
haven't touched yet just to get a feel for the other things that
Shorewall can do.</p>
</div>
<div align="left">
<p align="left">In case you haven't been keeping score, here's the
final set of configuration files for our sample network. Only those
that were modified from the original installation are shown.</p>
</div>
<div align="left">
<p align="left">/etc/shorewall/interfaces (The "options" will be very
site-specific).</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="0" cellspacing="0"
style="border-collapse: collapse;" id="AutoNumber4">
<tbody>
<tr>
<td><u><b>Zone</b></u></td>
<td><u><b>Interface</b></u></td>
<td><u><b>Broadcast</b></u></td>
<td><u><b>Options</b></u></td>
</tr>
<tr>
<td>net</td>
<td>eth0</td>
<td>detect</td>
<td>norfc1918,routefilter </td>
</tr>
<tr>
<td>loc</td>
<td>eth1</td>
<td>detect</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>dmz</td>
<td>eth2</td>
<td>detect</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">The setup described here requires that your network
interfaces be brought up before Shorewall can start. This opens a short
window during which you have no firewall protection. If you replace
'detect' with the actual broadcast addresses in the entries above, you
can bring up Shorewall before you bring up your network interfaces.</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="0" cellspacing="0"
style="border-collapse: collapse;" id="AutoNumber4">
<tbody>
<tr>
<td><u><b>Zone</b></u></td>
<td><u><b>Interface</b></u></td>
<td><u><b>Broadcast</b></u></td>
<td><u><b>Options</b></u></td>
</tr>
<tr>
<td>net</td>
<td>eth0</td>
<td>192.0.2.255</td>
<td>norfc1918,routefilter </td>
</tr>
<tr>
<td>loc</td>
<td>eth1</td>
<td>192.168.201.7</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>dmz</td>
<td>eth2</td>
<td>192.168.202.7</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">/etc/shorewall/masq - Local subnet</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber6">
<tbody>
<tr>
<td width="33%"><u><b>INTERFACE</b></u></td>
<td width="33%"><u><b>SUBNET</b></u></td>
<td width="34%"><u><b>ADDRESS</b></u></td>
</tr>
<tr>
<td width="33%">eth0</td>
<td width="33%">192.168.201.0/29</td>
<td width="34%">192.0.2.176</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">/etc/shorewall/proxyarp - DMZ</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" id="AutoNumber8"
style="border-collapse: collapse;">
<tbody>
<tr>
<td><u><b>ADDRESS</b></u></td>
<td><u><b>INTERFACE</b></u></td>
<td><u><b>EXTERNAL</b></u></td>
<td><u><b>HAVE ROUTE</b></u></td>
</tr>
<tr>
<td>192.0.2.177</td>
<td>eth2</td>
<td>eth0</td>
<td>No</td>
</tr>
<tr>
<td>192.0.2.178</td>
<td>eth2</td>
<td>eth0</td>
<td>No</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">/etc/shorewall/nat- Daughter's System</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" id="AutoNumber9"
style="border-collapse: collapse;">
<tbody>
<tr>
<td>EXTERNAL</td>
<td>INTERFACE</td>
<td>INTERNAL</td>
<td>ALL INTERFACES </td>
<td>LOCAL</td>
</tr>
<tr>
<td>192.0.2.179</td>
<td>eth0</td>
<td>192.168.201.4</td>
<td>No</td>
<td>No</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">/etc/shorewall/rules</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber7">
<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>COMMENTS</b></u></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>smtp</td>
<td># Mail from the Internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>pop3</td>
<td># Pop3 from the Internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>smtp</td>
<td># Mail from the Local Network</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>pop3</td>
<td># Pop3 from the Local Network</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>fw</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>smtp</td>
<td># Mail from the Firewall</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>dmz:192.0.2.178</td>
<td>net</td>
<td>tcp</td>
<td>smtp</td>
<td># Mail to the Internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>http</td>
<td># WWW from the Net</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>https</td>
<td># Secure HTTP from the Net</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz:192.0.2.178</td>
<td>tcp</td>
<td>https</td>
<td># Secure HTTP from the Local Net</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz:192.0.2.177</td>
<td>udp</td>
<td>domain</td>
<td># UDP DNS from the Internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz:192.0.2.177</td>
<td>tcp</td>
<td>domain</td>
<td># TCP DNS from the internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>fw</td>
<td>dmz:192.0.2.177</td>
<td>udp</td>
<td>domain</td>
<td># UDP DNS from firewall</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>fw</td>
<td>dmz:192.0.2.177</td>
<td>tcp</td>
<td>domain</td>
<td># TCP DNS from firewall</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz:192.0.2.177</td>
<td>udp</td>
<td>domain</td>
<td># UDP DNS from the local Net</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz:192.0.2.177</td>
<td>tcp</td>
<td>domain</td>
<td># TCP DNS from the local Net</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>dmz:192.0.2.177</td>
<td>net</td>
<td>udp</td>
<td>domain</td>
<td># UDP DNS to the Internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>dmz:192.0.2.177</td>
<td>net</td>
<td>tcp</td>
<td>domain</td>
<td># TCP DNS to the Internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>dmz</td>
<td>icmp</td>
<td>echo-request</td>
<td># Ping</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>loc</td>
<td>icmp</td>
<td>echo-request</td>
<td>#&nbsp; "</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>dmz</td>
<td>loc</td>
<td>icmp</td>
<td>echo-request</td>
<td># "</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz</td>
<td>icmp</td>
<td>echo-request</td>
<td># "</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>dmz</td>
<td>tcp</td>
<td>ssh</td>
<td># SSH to the DMZ</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>fw</td>
<td>tcp</td>
<td>ssh</td>
<td># SSH to the Firewall</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<h2 align="left"><a name="DNS"></a>6.0 DNS</h2>
</div>
<div align="left">
<p align="left">Given the collection of RFC 1918 and public addresses
in this setup, it only makes sense to have separate internal and
external DNS
servers. You can combine the two into a single BIND 9 server using <i>Views.
</i> If you are not interested in Bind 9 views, you can <a
href="#StartingAndStopping">go to the next section</a>.</p>
</div>
<div align="left">
<p align="left">Suppose that your domain is foobar.net and you want the
two DMZ systems named www.foobar.net and mail.foobar.net and you want
the three local systems named "winken.foobar.net, blinken.foobar.net
and nod.foobar.net. You want your firewall to be known as
firewall.foobar.net externally and it's interface to the local network
to be know as
gateway.foobar.net and its interface to the dmz as dmz.foobar.net.
Let's have the DNS server on 192.0.2.177 which will also be known
by the name ns1.foobar.net.</p>
</div>
<div align="left">
<p align="left">The /etc/named.conf file would look like this:</p>
</div>
<div align="left">
<blockquote>
<div align="left">
<pre>options {<br> directory "/var/named";<br> listen-on { 127.0.0.1 ; 192.0.2.177; };<br>};<br><br>logging {<br> channel xfer-log {<br> file "/var/log/named/bind-xfer.log";<br> print-category yes;<br> print-severity yes;<br> print-time yes;<br> severity info;<br> };<br> category xfer-in { xfer-log; };<br> category xfer-out { xfer-log; };<br> category notify { xfer-log; };<br>};</pre>
</div>
<div align="left">
<pre>#<br># This is the view presented to our internal systems<br>#<br><br>view "internal" {<br> #<br> # These are the clients that see this view<br> #<br> match-clients { 192.168.201.0/29;<br> 192.168.202.0/29;<br> 127.0.0.0/8;<br> 192.0.2.176/32; <br> 192.0.2.178/32;<br> 192.0.2.179/32;<br> 192.0.2.180/32; };<br> #<br> # If this server can't complete the request, it should use outside<br> # servers to do so<br> #<br> recursion yes;<br><br> zone "." in {<br> type hint;<br> file "int/root.cache";<br> };<br><br> zone "foobar.net" in {<br> type master;<br> notify no;<br> allow-update { none; };<br> file "int/db.foobar";<br> };<br><br> zone "0.0.127.in-addr.arpa" in {<br> type master;<br> notify no;<br> allow-update { none; };<br> file "int/db.127.0.0"; <br> };<br><br> zone "201.168.192.in-addr.arpa" in {<br> type master;<br> notify no;<br> allow-update { none; };<br> file "int/db.192.168.201";<br> };<br><br> zone "202.168.192.in-addr.arpa" in {<br> type master;<br> notify no;<br> allow-update { none; };<br> file "int/db.192.168.202";<br> };<br><br> zone "176.2.0.192.in-addr.arpa" in {<br> type master;<br> notify no;<br> allow-update { none; };<br> file "db.192.0.2.176";<br> };<br> (or status NAT for that matter)<br> zone "177.2.0.192.in-addr.arpa" in {<br> type master;<br> notify no;<br> allow-update { none; };<br> file "db.192.0.2.177";<br> };<br><br> zone "178.2.0.192.in-addr.arpa" in {<br> type master;<br> notify no;<br> allow-update { none; };<br> file "db.192.0.2.178";<br> };<br><br> zone "179.2.0.192.in-addr.arpa" in {<br> type master;<br> notify no;<br> allow-update { none; };<br> file "db.206.124.146.179";<br> };<br><br>};<br>#<br># This is the view that we present to the outside world<br>#<br>view "external" {<br> match-clients { any; };<br> #<br> # If we can't answer the query, we tell the client so<br> #<br> recursion no;<br><br> zone "foobar.net" in {<br> type master;<br> notify yes;<br> allow-update {none; };<br> allow-transfer { <i>&lt;secondary NS IP&gt;</i>; };<br> file "ext/db.foobar";<br> };<br><br> zone "176.2.0.192.in-addr.arpa" in {<br> type master;<br> notify yes;<br> allow-update { none; };<br> allow-transfer { <i>&lt;secondary NS IP&gt;</i>; };<br> file "db.192.0.2.176";<br> };<br><br> zone "177.2.0.192.in-addr.arpa" in {<br> type master;<br> notify yes;<br> allow-update { none; };<br> allow-transfer { <i>&lt;secondary NS IP&gt;</i>; };<br> file "db.192.0.2.177";<br> };<br><br> zone "178.2.0.192.in-addr.arpa" in {<br> type master;<br> notify yes;<br> allow-update { none; };<br> allow-transfer { <i>&lt;secondary NS IP&gt;</i>; };<br> file "db.192.0.2.178";<br> };<br><br> zone "179.2.0.192.in-addr.arpa" in {<br> type master;<br> notify yes;<br> allow-update { none; };<br> allow-transfer { <i>&lt;secondary NS IP&gt;</i>; };<br> file "db.192.0.2.179";<br> };<br>};</pre>
</div>
</blockquote>
</div>
<div align="left">
<p align="left">Here are the files in /var/named (those not shown are
usually included in your bind disbribution).</p>
<p align="left">db.192.0.2.176 - This is the reverse zone for the
firewall's external interface</p>
<blockquote>
<pre>; ############################################################<br>; Start of Authority (Inverse Address Arpa) for 192.0.2.176/32<br>; Filename: db.192.0.2.176<br>; ############################################################<br>@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (<br> 2001102303 ; serial<br> 10800 ; refresh (3 hour)<br> 3600 ; retry (1 hour)<br> 604800 ; expire (7 days)<br> 86400 ) ; minimum (1 day)<br>;<br>; ############################################################<br>; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)<br>; ############################################################<br>@ 604800 IN NS ns1.foobar.net.<br>@ 604800 IN NS <i>&lt;name of secondary ns&gt;</i>.<br>;<br>; ############################################################<br>; Iverse Address Arpa Records (PTR's) <br>; ############################################################<br>176.2.0.192.in-addr.arpa. 86400 IN PTR firewall.foobar.net.<br></pre>
</blockquote>
</div>
<div align="left">
<div align="left"> db.192.0.2.177 - This is the reverse zone for the
www/DNS server
<blockquote>
<pre>; ############################################################<br>; Start of Authority (Inverse Address Arpa) for 192.0.2.177/32<br>; Filename: db.192.0.2.177<br>; ############################################################<br>@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (<br> 2001102303 ; serial<br> 10800 ; refresh (3 hour)<br> 3600 ; retry (1 hour)<br> 604800 ; expire (7 days)<br> 86400 ) ; minimum (1 day)<br>;<br>; ############################################################<br>; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)<br>; ############################################################<br>@ 604800 IN NS ns1.foobar.net.<br>@ 604800 IN NS <i>&lt;name of secondary ns&gt;</i>.<br>;<br>; ############################################################<br>; Iverse Address Arpa Records (PTR's) <br>; ############################################################<br>177.2.0.192.in-addr.arpa. 86400 IN PTR www.foobar.net.<br></pre>
</blockquote>
</div>
</div>
<div align="left">
<div align="left"> db.192.0.2.178 - This is the reverse zone for the
mail server
<blockquote>
<pre>; ############################################################<br>; Start of Authority (Inverse Address Arpa) for 192.0.2.178/32<br>; Filename: db.192.0.2.178<br>; ############################################################<br>@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (<br> 2001102303 ; serial<br> 10800 ; refresh (3 hour)<br> 3600 ; retry (1 hour)<br> 604800 ; expire (7 days)<br> 86400 ) ; minimum (1 day)<br>;<br>; ############################################################<br>; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)<br>; ############################################################<br>@ 604800 IN NS ns1.foobar.net.<br>@ 604800 IN NS <i>&lt;name of secondary ns&gt;</i>.<br>;<br>; ############################################################<br>; Iverse Address Arpa Records (PTR's) <br>; ############################################################<br>178.2.0.192.in-addr.arpa. 86400 IN PTR mail.foobar.net.<br></pre>
</blockquote>
</div>
</div>
<div align="left">
<div align="left"> db.192.0.2.179 - This is the reverse zone for
daughter's web server's public IP
<blockquote>
<pre>; ############################################################<br>; Start of Authority (Inverse Address Arpa) for 192.0.2.179/32<br>; Filename: db.192.0.2.179<br>; ############################################################<br>@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (<br> 2001102303 ; serial<br> 10800 ; refresh (3 hour)<br> 3600 ; retry (1 hour)<br> 604800 ; expire (7 days)<br> 86400 ) ; minimum (1 day)<br>;<br>; ############################################################<br>; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)<br>; ############################################################<br>@ 604800 IN NS ns1.foobar.net.<br>@ 604800 IN NS <i>&lt;name of secondary ns&gt;</i>.<br>;<br>; ############################################################<br>; Iverse Address Arpa Records (PTR's) <br>; ############################################################<br>179.2.0.192.in-addr.arpa. 86400 IN PTR nod.foobar.net.<br></pre>
</blockquote>
</div>
</div>
<div align="left">
<p align="left">int/db.127.0.0 - The reverse zone for localhost</p>
</div>
<div align="left">
<blockquote>
<pre>; ############################################################<br>; Start of Authority (Inverse Address Arpa) for 127.0.0.0/8<br>; Filename: db.127.0.0<br>; ############################################################<br>@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (<br> 2001092901 ; serial<br> 10800 ; refresh (3 hour)<br> 3600 ; retry (1 hour)<br> 604800 ; expire (7 days)<br> 86400 ) ; minimum (1 day)<br>; ############################################################<br>; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)<br>; ############################################################<br>@ 604800 IN NS ns1.foobar.net.<br><br>; ############################################################<br>; Iverse Address Arpa Records (PTR's)<br>; ############################################################<br>1 86400 IN PTR localhost.foobar.net.</pre>
</blockquote>
</div>
<div align="left">
<p align="left">int/db.192.168.201 - Reverse zone for the local net.
This is only shown to internal clients</p>
</div>
<div align="left">
<blockquote>
<pre>; ############################################################<br>; Start of Authority (Inverse Address Arpa) for 192.168.201.0/29<br>; Filename: db.192.168.201<br>; ############################################################<br>@ 604800 IN SOA ns1.foobar.net netadmin.foobar.net. (<br> 2002032501 ; serial<br> 10800 ; refresh (3 hour)<br> 3600 ; retry (1 hour)<br> 604800 ; expire (7 days)<br> 86400 ) ; minimum (1 day)<br><br>; ############################################################<br>; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)<br>; ############################################################<br>@ 604800 IN NS ns1.foobar.net.<br><br>; ############################################################<br>; Iverse Address Arpa Records (PTR's)<br>; ############################################################<br>1 86400 IN PTR gateway.foobar.net.<br>2 86400 IN PTR winken.foobar.net.<br>3 86400 IN PTR blinken.foobar.net.<br>4 86400 IN PTR nod.foobar.net.</pre>
</blockquote>
</div>
<div align="left">
<p align="left">int/db.192.168.202 - Reverse zone for the firewall's
DMZ interface</p>
</div>
<div align="left">
<blockquote>
<div align="left">
<pre>; ############################################################<br>; Start of Authority (Inverse Address Arpa) for 192.168.202.0/29<br>; Filename: db.192.168.202<br>; ############################################################<br>@ 604800 IN SOA ns1.foobar.net netadmin.foobar.net. (<br> 2002032501 ; serial<br> 10800 ; refresh (3 hour)<br> 3600 ; retry (1 hour)<br> 604800 ; expire (7 days)<br> 86400 ) ; minimum (1 day)<br><br>; ############################################################<br>; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)<br>; ############################################################<br>@ 604800 IN NS ns1.foobar.net.<br><br>; ############################################################<br>; Iverse Address Arpa Records (PTR's)<br>; ############################################################<br>1 86400 IN PTR dmz.foobar.net.</pre>
</div>
</blockquote>
</div>
<div align="left">
<p align="left">int/db.foobar - Forward zone for use by internal
clients.</p>
</div>
<div align="left">
<blockquote>
<pre>;##############################################################<br>; Start of Authority for foobar.net.<br>; Filename: db.foobar<br>;##############################################################<br>@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (<br> 2002071501 ; serial<br> 10800 ; refresh (3 hour)<br> 3600 ; retry (1 hour)<br> 604800 ; expire (7 days)<br> 86400 ); minimum (1 day)<br>;############################################################<br>; foobar.net Nameserver Records (NS)<br>;############################################################<br>@ 604800 IN NS ns1.foobar.net.<br><br>;############################################################<br>; Foobar.net Office Records (ADDRESS)<br>;############################################################<br>localhost 86400 IN A 127.0.0.1<br><br>firewall 86400 IN A 192.0.2.176<br>www 86400 IN A 192.0.2.177<br>ns1 86400 IN A 192.0.2.177<br>www 86400 IN A 192.0.2.177<br><br>gateway 86400 IN A 192.168.201.1<br>winken 86400 IN A 192.168.201.2<br>blinken 86400 IN A 192.168.201.3<br>nod 86400 IN A 192.168.201.4</pre>
</blockquote>
</div>
<div align="left">
<p align="left">ext/db.foobar - Forward zone for external clients</p>
</div>
<div align="left">
<blockquote>
<div align="left">
<pre>;##############################################################<br>; Start of Authority for foobar.net.<br>; Filename: db.foobar<br>;##############################################################<br>@ 86400 IN SOA ns1.foobar.net. netadmin.foobar.net. (<br> 2002052901 ; serial<br> 10800 ; refresh (3 hour)<br> 3600 ; retry (1 hour)<br> 604800 ; expire (7 days)<br> 86400 ); minimum (1 day)<br>;############################################################<br>; Foobar.net Nameserver Records (NS)<br>;############################################################<br>@ 86400 IN NS ns1.foobar.net.<br>@ 86400 IN NS <i>&lt;secondary NS&gt;</i>.<br>;############################################################<br>; Foobar.net Foobar Wa Office Records (ADDRESS)<br>;############################################################<br>localhost 86400 IN A 127.0.0.1<br>;<br>; The firewall itself<br>;<br>firewall 86400 IN A 192.0.2.176<br>;<br>; The DMZ<br>;<br>ns1 86400 IN A 192.0.2.177<br>www 86400 IN A 192.0.2.177<br>mail 86400 IN A 192.0.2.178<br>;<br>; The Local Network<br>;<br>nod 86400 IN A 192.0.2.179<br><br>;############################################################<br>; Current Aliases for foobar.net (CNAME)<br>;############################################################<br><br>;############################################################<br>; foobar.net MX Records (MAIL EXCHANGER)<br>;############################################################<br>foobar.net. 86400 IN A 192.0.2.177<br> 86400 IN MX 0 mail.foobar.net.<br> 86400 IN MX 1 <i>&lt;backup MX&gt;</i>.</pre>
</div>
</blockquote>
</div>
<div align="left">
<h2 align="left"><a name="StartingAndStopping"></a>7.0 Starting and
Stopping Your Firewall</h2>
</div>
<div align="left">
<p align="left">The <a href="Install.htm">installation procedure </a>
configures your system to start Shorewall at system boot.</p>
</div>
<div align="left">
<p align="left">The firewall is started using the "shorewall start"
command and stopped using "shorewall stop". When the firewall is
stopped, routing is enabled on those hosts that have an entry in <a
href="Documentation.htm#Routestopped">/etc/shorewall/routestopped</a>.
A running firewall may be restarted using the "shorewall restart"
command. If you want to totally remove any trace of Shorewall from
your Netfilter configuration, use "shorewall clear".</p>
</div>
<div align="left">
<p align="left"><img border="0" src="images/BD21298_2.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; Edit the /etc/shorewall/routestopped
file and configure those systems that you want to be able to access the
firewall when it is stopped.</p>
</div>
<div align="left">
<p align="left"><b>WARNING: </b>If you are connected to your firewall
from the internet, do not issue a "shorewall stop" command unless you
have added an entry for the IP address that you are connected from to <a
href="Documentation.htm#Routestopped">/etc/shorewall/routestopped</a>.
Also, I don't recommend using "shorewall restart"; it is better to
create an <i><a href="Documentation.htm#Configs">alternate
configuration</a></i> and test it using the <a
href="Documentation.htm#Starting">"shorewall try" command</a>.</p>
</div>
<p align="left"><font size="2">Last updated 7/6/2003 - <a
href="support.htm">Tom Eastep</a></font></p>
<p align="left"><a href="copyright.htm"><font size="2">Copyright 2002,
2003 Thomas M. Eastep</font></a><br>
</p>
<br>
<br>
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