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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<article id="IPIP">
<!--$Id$-->
<articleinfo>
<title>Shorewall Setup Guide</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate>2004-06-11</pubdate>
<copyright>
<year>2001-2004</year>
<holder>Thomas M. Eastep</holder>
</copyright>
<legalnotice>
<para>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
<quote><ulink url="GnuCopyright.htm">GNU Free Documentation License</ulink></quote>.</para>
</legalnotice>
</articleinfo>
<section id="Introduction">
<title>Introduction</title>
<para>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 <ulink
url="shorewall_quickstart_guide.htm">single-address guides</ulink>.
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.</para>
<caution>
<para>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. Shorewall requires that
the iproute/iproute2 package be installed (on RedHat, the package is
called iproute). You can tell if this package is installed by the
presence of an <emphasis role="bold">ip</emphasis> program on your
firewall system. As root, you can use the <quote>which</quote> command
to check for this program:</para>
<programlisting>[root@gateway root]# <command>which ip</command>
/sbin/ip
[root@gateway root]#
</programlisting>
<para>I recommend that you first read through the guide to familiarize
yourself with what&#39;s involved then go back through it again making
your configuration changes. Points at which configuration changes are
recommended are flagged with <inlinegraphic
fileref="images/BD21298_.gif" />.</para>
</caution>
<caution>
<para>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.</para>
<itemizedlist>
<listitem>
<para><ulink url="http://www.simtel.net/pub/pd/51438.html">Windows
Version of dos2unix</ulink></para>
</listitem>
<listitem>
<para><ulink url="http://www.megaloman.com/%7Ehany/software/hd2u/">Linux
Version of dos2unix</ulink></para>
</listitem>
</itemizedlist>
</caution>
</section>
<section id="Concepts">
<title>Shorewall Concepts</title>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>The configuration files for Shorewall are contained in the directory
<filename class="directory">/etc/shorewall</filename> -- 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 Shorewall <ulink url="Install.htm">Installation
Process</ulink>.<warning><para><emphasis role="bold">Note to Debian Users</emphasis></para><para>If
you install using the .deb, you will find that your <filename
class="directory">/etc/shorewall</filename> directory is empty. This is
intentional. The released configuration file skeletons may be found on
your system in the directory <filename class="directory">/usr/share/doc/shorewall/default-config</filename>.
Simply copy the files you need from that directory to <filename
class="directory">/etc/shorewall</filename> and modify the copies.</para><para>Note
that you must copy <filename class="directory">/usr/share/doc/shorewall/default-config/shorewall.conf</filename>
and /usr/share/doc/shorewall/default-config/modules to <filename
class="directory">/etc/shorewall</filename> even if you do not modify
those files.</para></warning></para>
<para>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.</para>
<para>Shorewall views the network where it is running as being composed of
a set of zones. In the default installation, the following zone names are
used:</para>
<table>
<title>Zones</title>
<tgroup cols="2">
<tbody>
<row>
<entry align="left"><emphasis role="bold">Name</emphasis></entry>
<entry align="left" role="underline"><emphasis role="bold">Description</emphasis></entry>
</row>
<row>
<entry>net</entry>
<entry>The Internet</entry>
</row>
<row>
<entry>loc</entry>
<entry>Your Local Network</entry>
</row>
<row>
<entry>dmz</entry>
<entry>Demilitarized Zone</entry>
</row>
</tbody>
</tgroup>
</table>
<para>Zones are defined in the file <filename><ulink
url="Documentation.htm#Zones"><filename>/etc/shorewall/zones</filename></ulink></filename>.</para>
<para>Shorewall also recognizes the firewall system as its own zone - by
default, the firewall itself is known as <emphasis role="bold">fw</emphasis>
but that may be changed in the <ulink url="Documentation.htm#Config"><filename>/etc/shorewall/shorewall.conf</filename></ulink>
file. In this guide, the default name (<emphasis role="bold">fw</emphasis>)
will be used. With the exception of <emphasis role="bold">fw</emphasis>,
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 <quote>because this is the internet zone</quote> or
<quote>because that is the DMZ</quote>.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>Edit the /etc/shorewall/zones file and make any changes necessary.</para>
<para>Rules about what traffic to allow and what traffic to deny are
expressed in terms of zones.</para>
<itemizedlist>
<listitem>
<para>You express your default policy for connections from one zone to
another zone in the <filename><ulink url="Documentation.htm#Policy">/etc/shorewall/policy</ulink></filename>
file.</para>
</listitem>
<listitem>
<para>You define exceptions to those default policies in the
<filename><ulink url="Documentation.htm#Rules">/etc/shorewall/rules</ulink></filename>.</para>
</listitem>
</itemizedlist>
<para>Shorewall is built on top of the <ulink
url="http://www.netfilter.org">Netfilter</ulink> kernel facility.
Netfilter implements a <ulink
url="http://www.cs.princeton.edu/~jns/security/iptables/iptables_conntrack.html">connection
tracking function</ulink> that allows what is often referred to as
stateful inspection of packets. This stateful property allows firewall
rules to be defined in terms of connections rather than in terms of
packets. With Shorewall, you:</para>
<orderedlist>
<listitem>
<para>Identify the source zone.</para>
</listitem>
<listitem>
<para>Identify destination zone.</para>
</listitem>
<listitem>
<para>If the POLICY from the client&#39;s zone to the server&#39;s
zone is what you want for this client/server pair, you need do nothing
further.</para>
</listitem>
<listitem>
<para>If the POLICY is not what you want, then you must add a rule.
That rule is expressed in terms of the client&#39;s zone and the
server&#39;s zone.</para>
</listitem>
</orderedlist>
<para>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
<emphasis role="bold">DOES NOT mean that these connections are allowed
from zone A to zone B</emphasis>. 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.</para>
<para>For each connection request entering the firewall, the request is
first checked against the <filename>/etc/shorewall/rules</filename> file.
If no rule in that file matches the connection request then the first
policy in <filename>/etc/shorewall/policy</filename> that matches the
request is applied. If that policy is REJECT or DROP the request is first
checked against the rules in <filename>/etc/shorewall/common.def</filename>.</para>
<para>The default <filename>/etc/shorewall/policy</filename> file has the
following policies:</para>
<programlisting>#SOURCE ZONE DESTINATION ZONE POLICY LOG LIMIT:BURST
# LEVEL
fw net ACCEPT
net all DROP info
all all REJECT info</programlisting>
<para>The above policy will:</para>
<orderedlist>
<listitem>
<para>allow all connection requests from your local network to the
internet</para>
</listitem>
<listitem>
<para>drop (ignore) all connection requests from the internet to your
firewall or local network and log a message at the info level (here is
a description of log levels).</para>
</listitem>
<listitem>
<para>reject all other connection requests and log a message at the
info 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.</para>
</listitem>
</orderedlist>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>At this point, edit your <filename>/etc/shorewall/policy </filename>and
make any changes that you wish.</para>
</section>
<section id="Interfaces">
<title>Network Interfaces</title>
<para>For the remainder of this guide, we&#39;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.</para>
<para>In this diagram:</para>
<itemizedlist>
<listitem>
<para>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.</para>
</listitem>
<listitem>
<para>The Local Zone consists of systems Local 1, Local 2 and Local 3.</para>
</listitem>
<listitem>
<para>All systems from the ISP outward comprise the Internet Zone.</para>
</listitem>
</itemizedlist>
<graphic align="center" fileref="images/dmz3.png" />
<para>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 <ulink url="Documentation.htm#Interfaces">/etc/shorewall/interfaces</ulink>
file. The firewall illustrated above has three network interfaces. Where
Internet connectivity is through a cable or DSL <quote>Modem</quote>, the
<emphasis>External Interface</emphasis> will be the Ethernet adapter that
is connected to that <quote>Modem</quote> (e.g., <filename
class="devicefile">eth0</filename>) unless you connect via Point-to-Point
Protocol over Ethernet (PPPoE) or Point-to-Point Tunneling Protocol (PPTP)
in which case the External Interface will be a ppp interface (e.g.,
<filename class="devicefile">ppp0</filename>). If you connect via a
regular modem, your External Interface will also be <filename
class="devicefile">ppp0</filename>. If you connect using ISDN, you
external interface will be <filename class="devicefile">ippp0</filename>.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>If your external interface is <filename class="devicefile">ppp0</filename>
or <filename class="devicefile">ippp0</filename> then you will want to set
CLAMPMSS=yes in <filename><ulink url="Documentation.htm#Config">/etc/shorewall/shorewall.conf</ulink></filename>.</para>
<para>Your <emphasis>Local Interface</emphasis> will be an Ethernet
adapter (<filename class="devicefile">eth0</filename>,
<filename>eth1</filename> or <filename class="devicefile">eth2</filename>)
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).</para>
<para>Your <emphasis>DMZ Interface</emphasis> will also be an Ethernet
adapter (<filename class="devicefile">eth0</filename>, <filename
class="devicefile">eth1</filename> or <filename class="devicefile">eth2</filename>)
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).</para>
<caution>
<para>Do not connect the internal and external interface to the same hub
or switch except for testing AND you are running Shorewall version 1.4.7
or later. When using these recent versions, you can test using this kind
of configuration if you specify the <emphasis role="bold">arp_filter</emphasis>
option in <filename>/etc/shorewall/interfaces</filename> for all
interfaces connected to the common hub/switch. Using such a setup with a
production firewall is strongly recommended against.</para>
</caution>
<para>For the remainder of this Guide, we will assume that:</para>
<itemizedlist>
<listitem>
<para>The External Interface is <filename class="devicefile">eth0</filename>.</para>
</listitem>
<listitem>
<para>The Local Interface <filename class="devicefile">eth1</filename>.</para>
</listitem>
<listitem>
<para>The DMZ Interface <filename class="devicefile">eth2</filename>.</para>
</listitem>
</itemizedlist>
<para>The Shorewall default configuration does not define the contents of
any zone. To define the above configuration using the <ulink
url="Documentation.htm#Interfaces">/etc/shorewall/interfaces </ulink>file,
that file would might contain:</para>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
net eth0 detect rfc1918
loc eth1 detect
dmz eth2 detect</programlisting>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>Edit the <filename>/etc/shorewall/interfaces</filename> 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.</para>
<example>
<title>Multiple Interfaces to a Zone</title>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
net eth0 detect rfc1918
loc eth1 detect
loc eth2 detect</programlisting>
</example>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>You may define more complicated zones using the<filename> <ulink
url="Documentation.htm#Hosts">/etc/shorewall/hosts</ulink></filename> file
but in most cases, that isn&#39;t necessary.</para>
</section>
<section id="Addressing">
<title>Addressing, Subnets and Routing</title>
<para>Normally, your ISP will assign you a set of Public IP addresses. You
will configure your firewall&#39;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.</para>
<para>If you are thoroughly familiar with IP addressing and routing, you
may go to the next section.</para>
<para>The following discussion barely scratches the surface of addressing
and routing. If you are interested in learning more about this subject, I
highly recommend <quote><emphasis>IP Fundamentals: What Everyone Needs to
Know about Addressing &#38; Routing</emphasis></quote>, Thomas A. Maufer,
Prentice-Hall, 1999, ISBN 0-13-975483-0.</para>
<section id="Addresses">
<title>IP Addresses</title>
<para>IP version 4 (IPv4) addresses are 32-bit numbers. The notation
w.x.y.z refers to an address where the high-order byte has value
<quote>w</quote>, the next byte has value <quote>x</quote>, etc. If we
take the address 192.0.2.14 and express it in hexadecimal, we get:</para>
<para><programlisting>C0.00.02.0E</programlisting>or looking at it as a
32-bit integer</para>
<para><programlisting>C000020E</programlisting></para>
</section>
<section id="Subnets">
<title>Subnets</title>
<para>You will still hear the terms <quote>Class A network</quote>,
<quote>Class B network</quote> and <quote>Class C network</quote>. In
the early days of IP, networks only came in three sizes (there were also
Class D networks but they were used differently):</para>
<simplelist>
<member>Class A - netmask 255.0.0.0, size = 2 ** 24</member>
<member>Class B - netmask 255.255.0.0, size = 2 ** 16</member>
<member>Class C - netmask 255.255.255.0, size = 256</member>
</simplelist>
<para>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 netmask. The netmask is a number
that when logically ANDed with an address isolates the network number;
the remainder of the address is the host number. For example, in the
Class C address 192.0.2.14, the network number is hex C00002 and the
host number is hex 0E.</para>
<para>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
subnetting these networks into smaller subnetworks evolved; that
technique is referred to as <emphasis>Classless InterDomain Routing</emphasis>
(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.</para>
<para>A <emphasis>subnetwork</emphasis> (often referred to as a
<emphasis>subnet</emphasis>) is a contiguous set of IP addresses such
that:</para>
<orderedlist>
<listitem>
<para>The number of addresses in the set is a power of 2; and</para>
</listitem>
<listitem>
<para>The first address in the set is a multiple of the set size.</para>
</listitem>
<listitem>
<para>The first address in the subnet is reserved and is referred to
as the <emphasis>subnet address</emphasis>.</para>
</listitem>
<listitem>
<para>The last address in the subnet is reserved as the subnet&#39;s
<emphasis>broadcast address</emphasis>.</para>
</listitem>
</orderedlist>
<para>As you can see by this definition, in each subnet of size n there
are (n - 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.</para>
<para>Since n is a power of two, we can easily calculate the
<emphasis>Natural Logarithm</emphasis> (log2) of n. For the more common
subnet sizes, the size and its natural logarithm are given in the
following table:</para>
<table>
<title>Natural Logarithms</title>
<tgroup cols="3">
<tbody>
<row>
<entry><emphasis role="bold">n</emphasis></entry>
<entry><emphasis role="bold">log2 n</emphasis></entry>
<entry><emphasis role="bold">(32 - log2 n)</emphasis></entry>
</row>
<row>
<entry>8</entry>
<entry>3</entry>
<entry>29</entry>
</row>
<row>
<entry>16</entry>
<entry>4</entry>
<entry>28</entry>
</row>
<row>
<entry>32</entry>
<entry>5</entry>
<entry>27</entry>
</row>
<row>
<entry>64</entry>
<entry>6</entry>
<entry>26</entry>
</row>
<row>
<entry>128</entry>
<entry>7</entry>
<entry>25</entry>
</row>
<row>
<entry>256</entry>
<entry>8</entry>
<entry>24</entry>
</row>
<row>
<entry>512</entry>
<entry>9</entry>
<entry>23</entry>
</row>
<row>
<entry>1024</entry>
<entry>10</entry>
<entry>22</entry>
</row>
<row>
<entry>2048</entry>
<entry>11</entry>
<entry>21</entry>
</row>
<row>
<entry>4096</entry>
<entry>12</entry>
<entry>20</entry>
</row>
<row>
<entry>8192</entry>
<entry>13</entry>
<entry>19</entry>
</row>
<row>
<entry>16384</entry>
<entry>14</entry>
<entry>18</entry>
</row>
<row>
<entry>32768</entry>
<entry>15</entry>
<entry>17</entry>
</row>
<row>
<entry>65536</entry>
<entry>16</entry>
<entry>16</entry>
</row>
</tbody>
</tgroup>
</table>
<para>You will notice that the above table also contains a column for
(32 - log2 <emphasis role="bold">n)</emphasis>. That number is the
<emphasis>Variable Length Subnet Mask</emphasis> (VLSM) for a network of
size n. From the above table, we can derive the following one which is a
little easier to use.</para>
<table>
<title>VLSM</title>
<tgroup cols="3">
<tbody>
<row>
<entry><emphasis role="bold">Subnet Size</emphasis></entry>
<entry><emphasis role="bold">VLSM</emphasis></entry>
<entry><emphasis role="bold">Subnet Mask</emphasis></entry>
</row>
<row>
<entry>8</entry>
<entry>/29</entry>
<entry>255.255.255.248</entry>
</row>
<row>
<entry>16</entry>
<entry>/28</entry>
<entry>255.255.255.240</entry>
</row>
<row>
<entry>32</entry>
<entry>/27</entry>
<entry>255.255.255.224</entry>
</row>
<row>
<entry>64</entry>
<entry>/26</entry>
<entry>255.255.255.192</entry>
</row>
<row>
<entry>128</entry>
<entry>/25</entry>
<entry>255.255.255.128</entry>
</row>
<row>
<entry>256</entry>
<entry>/24</entry>
<entry>255.255.255.0</entry>
</row>
<row>
<entry>512</entry>
<entry>/23</entry>
<entry>255.255.254.0</entry>
</row>
<row>
<entry>1024</entry>
<entry>/22</entry>
<entry>255.255.252.0</entry>
</row>
<row>
<entry>2048</entry>
<entry>/21</entry>
<entry>255.255.248.0</entry>
</row>
<row>
<entry>4096</entry>
<entry>/20</entry>
<entry>255.255.240.0</entry>
</row>
<row>
<entry>8192</entry>
<entry>/19</entry>
<entry>255.255.224.0</entry>
</row>
<row>
<entry>16384</entry>
<entry>/18</entry>
<entry>255.255.192.0</entry>
</row>
<row>
<entry>32768</entry>
<entry>/17</entry>
<entry>255.255.128.0</entry>
</row>
<row>
<entry>65536</entry>
<entry>/16</entry>
<entry>255.255.0.0</entry>
</row>
<row>
<entry>2 ** 24</entry>
<entry>/8</entry>
<entry>255.0.0.0</entry>
</row>
</tbody>
</tgroup>
</table>
<para>Notice that the VLSM is written with a slash (<quote>/</quote>) --
you will often hear a subnet of size 64 referred to as a <quote>slash 26</quote>
subnet and one of size 8 referred to as a <quote>slash 29</quote>.</para>
<para>The subnet&#39;s mask (also referred to as its
<emphasis>netmask</emphasis>) is simply a 32-bit number with the first
<quote>VLSM</quote> 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:</para>
<para><programlisting>11111111111111111111111111000000 = FFFFFFC0 = FF.FF.FF.C0 = 255.255.255.192</programlisting>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.</para>
<para>For a subnetwork whose address is <emphasis role="bold">a.b.c.d</emphasis>
and whose Variable Length Subnet Mask is <emphasis role="bold">/v</emphasis>,
we denote the subnetwork as <quote><emphasis role="bold">a.b.c.d/v</emphasis></quote>
using <emphasis>CIDR Notation</emphasis>. Example:</para>
<table>
<title>Subnet</title>
<tgroup cols="2">
<tbody>
<row>
<entry><emphasis role="bold">Subnet:</emphasis></entry>
<entry>10.10.10.0 - 10.10.10.127</entry>
</row>
<row>
<entry><emphasis role="bold">Subnet Size:</emphasis></entry>
<entry>128</entry>
</row>
<row>
<entry><emphasis role="bold">Subnet Address:</emphasis></entry>
<entry>10.10.10.0</entry>
</row>
<row>
<entry><emphasis role="bold">Broadcast Address:</emphasis></entry>
<entry>10.10.10.127</entry>
</row>
<row>
<entry><emphasis role="bold">CIDR Notation:</emphasis></entry>
<entry>10.10.10.0/25</entry>
</row>
</tbody>
</tgroup>
</table>
<para>There are two degenerate subnets that need mentioning; namely, the
subnet with one member and the subnet with 2 ** 32 members.</para>
<table>
<title>/32 and /0</title>
<tgroup cols="4">
<tbody>
<row>
<entry><emphasis role="bold">Subnet Size</emphasis></entry>
<entry><emphasis role="bold">VLSM Length</emphasis></entry>
<entry><emphasis role="bold">Subnet Mask</emphasis></entry>
<entry><emphasis role="bold">CIDR Notation</emphasis></entry>
</row>
<row>
<entry>1</entry>
<entry>32</entry>
<entry>255.255.255.255</entry>
<entry>a.b.c.d/32</entry>
</row>
<row>
<entry>32</entry>
<entry>0</entry>
<entry>0.0.0.0</entry>
<entry>0.0.0.0/0</entry>
</row>
</tbody>
</tgroup>
</table>
<para>So any address <emphasis role="bold">a.b.c.d</emphasis> may also
be written <emphasis role="bold">a.b.c.d/32</emphasis> and the set of
all possible IP addresses is written <emphasis role="bold">0.0.0.0/0</emphasis>.</para>
<para role="bold">Later in this guide, you will see the notation
<emphasis role="bold">a.b.c.d/v</emphasis> used to describe the ip
configuration of a network interface (the <quote>ip</quote> utility also
uses this syntax). This simply means that the interface is configured
with ip address <emphasis role="bold">a.b.c.d</emphasis> and with the
netmask that corresponds to VLSM /<emphasis role="bold">v</emphasis>.</para>
<example>
<title>192.0.2.65/29</title>
<para>The interface is configured with IP address 192.0.2.65 and
netmask 255.255.255.248.</para>
</example>
<para>Beginning with Shorewall 1.4.6, /sbin/shorewall supports an ipcalc
command that automatically calculates information about a [sub]network.</para>
<example>
<title>Using the <command>ipcalc </command>command</title>
<programlisting>shorewall ipcalc 10.10.10.0/25
CIDR=10.10.10.0/25
NETMASK=255.255.255.128
NETWORK=10.10.10.0
BROADCAST=10.10.10.127
</programlisting>
</example>
<example>
<title>Using the <command>ipcalc</command> command</title>
<programlisting>shorewall ipcalc 10.10.10.0 255.255.255.128
CIDR=10.10.10.0/25
NETMASK=255.255.255.128
NETWORK=10.10.10.0
BROADCAST=10.10.10.127</programlisting>
</example>
</section>
<section id="Routing">
<title>Routing</title>
<para>One of the purposes of subnetting is that it forms the basis for
routing. Here&#39;s the routing table on my firewall (compressed for
PDF):</para>
<programlisting>[root@gateway root]# netstat -nr
Kernel IP routing table
Destination Gateway Genmask Flgs MSS Win irtt Iface
192.168.9.1 0.0.0.0 255.255.255.255 UH 40 0 0 texas
206.124.146.177 0.0.0.0 255.255.255.255 UH 40 0 0 eth1
206.124.146.180 0.0.0.0 255.255.255.255 UH 40 0 0 eth3
192.168.3.0 0.0.0.0 255.255.255.0 U 40 0 0 eth3
192.168.2.0 0.0.0.0 255.255.255.0 U 40 0 0 eth1
192.168.1.0 0.0.0.0 255.255.255.0 U 40 0 0 eth2
206.124.146.0 0.0.0.0 255.255.255.0 U 40 0 0 eth0
192.168.9.0 192.0.2.223 255.255.255.0 UG 40 0 0 texas
127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo
0.0.0.0 206.124.146.254 0.0.0.0 UG 40 0 0 eth0
[root@gateway root]#</programlisting>
<para>The device <emphasis>texas</emphasis> is a GRE tunnel to a peer
site in the Dallas, Texas area.</para>
<para>The first three routes are <emphasis>host routes</emphasis> since
they indicate how to get to a single host. In the <quote>netstat</quote>
output this can be seen by the <quote>Genmask</quote> (Subnet Mask) of
255.255.255.255 and the <quote>H</quote> in the Flags column. The
remainder are <emphasis><quote>net</quote> routes</emphasis> since they
tell the kernel how to route packets to a subnetwork. The last route is
the <emphasis>default route </emphasis>and the gateway mentioned in that
route is called the <emphasis>default gateway</emphasis>.</para>
<para>When the kernel is trying to send a packet to IP address <emphasis
role="bold">A</emphasis>, it starts at the top of the routing table and:</para>
<itemizedlist>
<listitem>
<para><emphasis role="bold">A</emphasis> is logically ANDed with the
<quote>Genmask</quote> value in the table entry.</para>
</listitem>
<listitem>
<para>The result is compared with the <quote>Destination</quote>
value in the table entry.</para>
</listitem>
<listitem>
<para>If the result and the <quote>Destination</quote> value are the
same, then:</para>
<itemizedlist>
<listitem>
<para>If the <quote>Gateway</quote> column is non-zero, the
packet is sent to the gateway over the interface named in the
<quote>Iface</quote> column.</para>
</listitem>
<listitem>
<para>Otherwise, the packet is sent directly to <emphasis
role="bold">A</emphasis> over the interface named in the
<quote>iface</quote> column.</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>Otherwise, the above steps are repeated on the next entry in
the table.</para>
</listitem>
</itemizedlist>
<para>Since the default route matches any IP address (<emphasis
role="bold">A </emphasis>LAND 0.0.0.0 = 0.0.0.0), packets that don&#39;t
match any of the other routing table entries are sent to the default
gateway which is usually a router at your ISP. Lets take an example.
Suppose that we want to route a packet to 192.168.1.5. That address
clearly doesn&#39;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:</para>
<para><programlisting>192.168.1.0 0.0.0.0 255.255.255.0 U 40 0 0 eth2</programlisting></para>
<para>So to route a packet to 192.168.1.5, the packet is sent directly
over eth2.</para>
<para>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&#39;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.</para>
</section>
<section>
<title id="ARP">Address Resolution Protocol (ARP)</title>
<para>When sending packets over Ethernet, IP addresses aren&#39;t used.
Rather Ethernet addressing is based on <emphasis>Media Access Control</emphasis>
(MAC) addresses. Each Ethernet device has it&#39;s own unique MAC
address which is burned into a PROM on the device during manufacture.
You can obtain the MAC of an Ethernet device using the <quote>ip</quote>
utility:</para>
<programlisting>[root@gateway root]# <command>ip addr show eth0</command>
2: eth0: &#60;BROADCAST,MULTICAST,UP&#62; mtu 1500 qdisc htb qlen 100
link/ether 02:00:08:e3:fa:55 brd ff:ff:ff:ff:ff:ff
inet 206.124.146.176/24 brd 206.124.146.255 scope global eth0
inet 206.124.146.178/24 brd 206.124.146.255 scope global secondary eth0
inet 206.124.146.179/24 brd 206.124.146.255 scope global secondary eth0
[root@gateway root]#
</programlisting>
<para>As you can see from the above output, the MAC is 6 bytes (48 bits)
wide. A card&#39;s MAC is usually also printed on a label attached to
the card itself. 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 <emphasis>Address Resolution
Protocol </emphasis>(ARP). Here is ARP in action:</para>
<programlisting>[root@gateway root]# <command>tcpdump -nei eth2 arp</command>
tcpdump: listening on eth2
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
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
2 packets received by filter
0 packets dropped by kernel
[root@gateway root]#</programlisting>
<para>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.</para>
<para>In order to avoid having to exchange ARP information each time
that an IP packet is to be sent, systems maintain an
<emphasis>ARP cache</emphasis> of IP&#60;-&#62;MAC correspondences. You
can see the ARP cache on your system (including your Windows system)
using the <quote>arp</quote> command:</para>
<programlisting>[root@gateway root]# <command>arp -na</command>
? (206.124.146.177) at 00:A0:C9:15:39:78 [ether] on eth1
? (192.168.1.3) at 00:A0:CC:63:66:89 [ether] on eth2
? (192.168.1.5) at 00:A0:CC:DB:31:C4 [ether] on eth2
? (206.124.146.254) at 00:03:6C:8A:18:38 [ether] on eth0
? (192.168.1.19) at 00:06:25:AA:8A:F0 [ether] on eth2
</programlisting>
<para>The leading question marks are a result of my having specified the
<quote>n</quote> option (Windows <quote>arp</quote> doesn&#39;t allow
that option) which causes the <quote>arp</quote> program to forego
IP-&#62;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.</para>
</section>
<section id="RFC1918">
<title>RFC 1918</title>
<para>IP addresses are allocated by the <ulink
url="http://www.iana.org/">Internet Assigned Number Authority</ulink>
(IANA) who delegates allocations on a geographic basis to Regional
Internet Registries (RIRs). For example, allocation for the Americas and
for sub-Sahara Africa is delegated to the <ulink
url="http://www.arin.net/">American Registry for Internet Numbers</ulink>
(ARIN). These RIRs may in turn delegate to national registries. Most of
us don&#39;t deal with these registrars but rather get our IP addresses
from our ISP. It&#39;s a fact of life that most of us can&#39;t afford
as many Public IP addresses as we have devices to assign them to so we
end up making use of Private IP addresses. RFC 1918 reserves several IP
address ranges for this purpose:</para>
<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</programlisting>
<para>The addresses reserved by RFC 1918 are sometimes referred to as
non-routable because the Internet backbone routers don&#39;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.</para>
<para>When selecting addresses from these ranges, there&#39;s a couple
of things to keep in mind:</para>
<itemizedlist>
<listitem>
<para>As the IPv4 address space becomes depleted, more and more
organizations (including ISPs) are beginning to use RFC 1918
addresses in their infrastructure.</para>
</listitem>
<listitem>
<para>You don&#39;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.</para>
</listitem>
</itemizedlist>
<para>So it&#39;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.</para>
<note>
<para><emphasis role="bold">In this document, external <quote>real</quote>
IP addresses are of the form 192.0.2.x. 192.0.2.0/24 is reserved by
RFC 3330 for use as public IP addresses in printed examples. These
addresses are not to be confused with addresses in 192.168.0.0/16; as
described above, these addresses are reserved by RFC 1918 for private
use.</emphasis></para>
</note>
</section>
</section>
<section id="Options">
<title>Setting Up Your Network</title>
<para>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:</para>
<itemizedlist>
<listitem>
<para><emphasis role="bold">Routed</emphasis> - Traffic to any of your
addresses will be routed through a single gateway address. 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&#39;s external interface.</para>
</listitem>
<listitem>
<para><emphasis role="bold">Non-routed</emphasis> - Your ISP will send
traffic to each of your addresses directly.</para>
</listitem>
</itemizedlist>
<para>In the subsections that follow, we&#39;ll look at each of these
separately.</para>
<para>Before we begin, there is one thing for you to check:</para>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>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:</para>
<itemizedlist>
<listitem>
<para>NAT_ENABLED=Yes (Shorewall versions earlier than 1.4.6)</para>
</listitem>
<listitem>
<para>IP_FORWARDING=On</para>
</listitem>
</itemizedlist>
<section id="Routed">
<title>Routed</title>
<para>Let&#39;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&#39;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&#39;s and set up your network as shown in the following diagram.</para>
<graphic align="center" fileref="images/dmz4.png" />
<para>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.</para>
<para>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.</para>
<para>The astute reader may have noticed that the Firewall/Router&#39;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:</para>
<programlisting format="linespecific">Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
192.0.2.64 0.0.0.0 255.255.255.248 U 40 0 0 eth0
0.0.0.0 192.0.2.66 0.0.0.0 UG 40 0 0 eth0</programlisting>
<para>This means that DMZ 1 will send an ARP <quote>who-has 192.0.2.65</quote>
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 <emphasis role="underline">DMZ Interface</emphasis>!! DMZ
1 can then send Ethernet frames addressed to that MAC address and the
frames will be received (correctly) by the firewall/router.</para>
<para>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&#39;s IP
addresses is sent by another system connected to the hub/switch, all of
the firewall&#39;s interfaces that connect to the hub/switch can
respond! It is then a race as to which <quote>here-is</quote> response
reaches the sender first.</para>
</section>
<section id="NonRouted">
<title>Non-routed</title>
<para>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 <quote>proxyarp</quote> option on all three
firewall interfaces in the /etc/shorewall/interfaces file.</para>
<para>Most of us don&#39;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).</para>
<para><emphasis role="bold">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.</emphasis></para>
<para>Clearly, that set of addresses doesn&#39;t comprise a subnetwork
and there aren&#39;t enough addresses for all of the network interfaces.
There are four different techniques that can be used to work around this
problem.</para>
<itemizedlist>
<listitem>
<para><emphasis>Source Network Address Translation</emphasis>
(SNAT).</para>
</listitem>
<listitem>
<para><emphasis>Destination Network Address Translation</emphasis>
(DNAT) also known as <emphasis>Port Forwarding</emphasis>.</para>
</listitem>
<listitem>
<para><emphasis>Proxy ARP</emphasis>.</para>
</listitem>
<listitem>
<para><emphasis>Network Address Translation</emphasis> (NAT) also
referred to as <emphasis>One-to-one NA</emphasis>T.</para>
</listitem>
</itemizedlist>
<para>Often a combination of these techniques is used. Each of these
will be discussed in the sections that follow.</para>
<section id="SNAT">
<title>SNAT</title>
<para>With SNAT, an internal LAN segment is configured using RFC 1918
addresses. When a host <emphasis role="bold">A</emphasis> on this
internal segment initiates a connection to host <emphasis role="bold">B</emphasis>
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 <emphasis role="bold">B</emphasis> responds and the response is
received by the firewall, the firewall changes the destination address
back to the RFC 1918 address of <emphasis role="bold">A</emphasis> and
forwards the response back to <emphasis role="bold">A.</emphasis></para>
<para>Let&#39;s suppose that you decide to use SNAT on your local zone
and use public address 192.0.2.176 as both your firewall&#39;s
external IP address and the source IP address of internet requests
sent from that zone.</para>
<graphic align="center" fileref="images/dmz5.png" />
<para>The local zone has been subnetted as 192.168.201.0/29 (netmask
255.255.255.248).</para>
<simplelist>
<member><inlinegraphic fileref="images/BD21298_.gif" /></member>
<member>The systems in the local zone would be configured with a
default gateway of 192.168.201.1 (the IP address of the
firewall&#39;s local interface).</member>
<member><inlinegraphic fileref="images/BD21298_.gif" /></member>
<member>SNAT is configured in Shorewall using the
<filename><ulink url="Documentation.htm#Masq">/etc/shorewall/masq</ulink></filename>
file.</member>
</simplelist>
<programlisting>#INTERFACE SUBNET ADDRESS
eth0 192.168.201.0/29 192.0.2.176</programlisting>
<para>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.</para>
</section>
<section id="dnat">
<title>DNAT</title>
<para>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.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>Suppose that your daughter wants to run a web server on her
system <quote>Local 3</quote>. You could allow connections to the
internet to her server by adding the following entry in
<filename><ulink url="Documentation.htm#Rules">/etc/shorewall/rules</ulink></filename>:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST SOURCE ORIGINAL
# PORT(S) PORT(S) DEST
DNAT net loc:192.168.201.4 tcp www</programlisting>
<para>If one of your daughter&#39;s friends at address <emphasis
role="bold">A</emphasis> wants to access your daughter&#39;s server,
she can connect to http://192.0.2.176 (the firewall&#39;s external IP
address) and the firewall will rewrite the destination IP address to
192.168.201.4 (your daughter&#39;s system) and forward the request.
When your daughter&#39;s server responds, the firewall will rewrite
the source address back to 192.0.2.176 and send the response back to
<emphasis role="bold">A</emphasis>.</para>
<para>This example used the firewall&#39;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&#39;s external interface for
you.</para>
</section>
<section id="ProxyARP">
<title>Proxy ARP</title>
<para>The idea behind Proxy ARP is that:</para>
<itemizedlist>
<listitem>
<para>A host <emphasis role="bold">H</emphasis> behind your
firewall is assigned one of your public IP addresses (<emphasis
role="bold">A</emphasis>), and is assigned the same netmask (<emphasis
role="bold">M</emphasis>) as the firewall&#39;s external
interface.</para>
</listitem>
<listitem>
<para>The firewall responds to ARP <quote>who has</quote> requests
for <emphasis role="bold">A</emphasis>.</para>
</listitem>
<listitem>
<para>When <emphasis role="bold">H</emphasis> <emphasis
role="bold">A </emphasis>andissues an ARP <quote>who has</quote>
request for an address in the subnetwork defined by <emphasis
role="bold">M</emphasis>, the firewall will respond (with the MAC
if the firewall interface) to <emphasis role="bold">H</emphasis>.</para>
</listitem>
</itemizedlist>
<para>Let us suppose that we decide to use Proxy ARP on the DMZ in our
example network.</para>
<graphic align="center" fileref="images/dmz6.png" />
<para>Here, we&#39;ve assigned the IP addresses 192.0.2.177 to system
DMZ 1 and 192.0.2.178 to DMZ 2. Notice that we&#39;ve just assigned an
arbitrary RFC 1918 IP address and subnet mask to the DMZ interface on
the firewall. That address and netmask isn&#39;t relevant - just be
sure it doesn&#39;t overlap another subnet that you&#39;ve defined.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>The Shorewall configuration of Proxy ARP is done using the<ulink
url="ProxyARP.htm"><filename>/etc/shorewall/proxyarp</filename></ulink>
file.</para>
<programlisting>#ADDRESS EXTERNAL INTERFACE HAVE ROUTE
192.0.2.177 eth2 eth0 No
192.0.2.178 eth2 eth0 No</programlisting>
<para>Because the HAVE ROUTE column contains No, Shorewall will add
host routes thru eth2 to 192.0.2.177 and 192.0.2.178. 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.</para>
<caution>
<para><emphasis role="bold">Do not add the Proxy ARP&#39;ed
address(es) (192.0.2.177 and 192.0.2.178 in the above example) to
the external interface (eth0 in this example) of the firewall.</emphasis></para>
</caution>
<para>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:</para>
<orderedlist>
<listitem>
<para>(Courtesy of Bradey Honsinger) A reading of Stevens&#39;
TCP/IP Illustrated, Vol 1 reveals that a</para>
<blockquote>
<para><quote>gratuitous</quote> ARP packet should cause the
ISP&#39;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&#39;t a duplicate,...</para>
<para><quote>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.</quote></para>
</blockquote>
<para>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 one-to-one NAT for that matter).
Happily enough, recent versions of Redhat&#39;s iputils package
include <quote>arping</quote>, whose <quote>-U</quote> flag does
just that:</para>
<para><programlisting><command>arping -U -I &#60;net if&#62; &#60;newly proxied IP&#62;</command>
<command>arping -U -I eth0 66.58.99.83</command> # for example</programlisting>Stevens
goes on to mention that not all systems respond correctly to
gratuitous ARPs, but googling for <quote>arping -U</quote> seems
to support the idea that it works most of the time.</para>
</listitem>
<listitem>
<para>You can call your ISP and ask them to purge the stale ARP
cache entry but many either can&#39;t or won&#39;t purge
individual entries.</para>
</listitem>
</orderedlist>
<para>You can determine if your ISP&#39;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:</para>
<para><programlisting><command>tcpdump -nei eth0 icmp</command></programlisting></para>
<para>Now from 192.0.2.177, ping the ISP&#39;s gateway (which we will
assume is 192.0.2.254):</para>
<para><programlisting><command>ping 192.0.2.254</command></programlisting></para>
<para>We can now observe the tcpdump output:</para>
<para><programlisting>13:35:12.159321 <emphasis role="bold">0:4:e2:20:20:33</emphasis> 0:0:77:95:dd:19 ip 98:
192.0.2.177 &#62; 192.0.2.254: icmp: echo request (DF)
13:35:12.207615 0:0:77:95:dd:19 <emphasis role="bold">0:c0:a8:50:b2:57</emphasis> ip 98:
192.0.2.254 &#62; 192.0.2.177 : icmp: echo reply</programlisting>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&#39;s eth0 NIC while
0:c0:a8:50:b2:57 was the MAC address of DMZ 1. In other words, the
gateway&#39;s ARP cache still associates 192.0.2.177 with the NIC in
DMZ 1 rather than with the firewall&#39;s eth0.</para>
</section>
<section>
<title id="NAT">One-to-one NAT</title>
<para>With one-to-one 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&#39;s go back to our earlier
example involving your daughter&#39;s web server running on system
Local 3.</para>
<graphic align="center" fileref="images/dmz6.png" />
<para>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
<filename>/etc/shorewall/masq</filename>:</para>
<programlisting>#INTERFACE SUBNET ADDRESS
eth0 192.168.201.0/29 192.0.2.176</programlisting>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>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 <filename><ulink url="NAT.htm">/etc/shorewall/nat</ulink></filename>.</para>
<programlisting>#EXTERNAL INTERFACE INTERNAL ALL INTERFACES LOCAL
192.0.2.179 eth0 192.168.201.4 No No</programlisting>
<para>With this entry in place, you daughter has her own IP address
and the other two local systems share the firewall&#39;s IP address.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>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&#39;s web server -- you would
rather just use an ACCEPT rule:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST SOURCE ORIGINAL
# PORT(S) PORT(S) DEST
ACCEPT net loc:192.168.201.4 tcp www</programlisting>
<para>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 one-to-one NAT,
it will probably be HOURS before that system can communicate with the
internet. There are a couple of things that you can try:</para>
<orderedlist>
<listitem>
<para>(Courtesy of Bradey Honsinger) A reading of Stevens&#39;
TCP/IP Illustrated, Vol 1 reveals that a</para>
<blockquote>
<para><quote>gratuitous</quote> ARP packet should cause the
ISP&#39;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&#39;t a duplicate,...</para>
<para><quote>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.</quote></para>
</blockquote>
<para>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 one-to-one NAT. Happily enough, recent versions of
Redhat&#39;s iputils package include <quote>arping</quote>, whose
<quote>-U</quote> flag does just that:</para>
<para><programlisting><command>arping -U -I &#60;net if&#62; &#60;newly proxied IP&#62;
</command>
<command>arping -U -I eth0 66.58.99.83</command> # for example</programlisting>Stevens
goes on to mention that not all systems respond correctly to
gratuitous ARPs, but googling for <quote>arping -U</quote> seems
to support the idea that it works most of the time.</para>
</listitem>
<listitem>
<para>You can call your ISP and ask them to purge the stale ARP
cache entry but many either can&#39;t or won&#39;t purge
individual entries.</para>
</listitem>
</orderedlist>
<para>You can determine if your ISP&#39;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:</para>
<para><programlisting><command>tcpdump -nei eth0 icmp</command></programlisting></para>
<para>Now from 192.0.2.177, ping the ISP&#39;s gateway (which we will
assume is 192.0.2.254):</para>
<para><programlisting><command>ping 192.0.2.254</command></programlisting></para>
<para>We can now observe the tcpdump output:</para>
<para><programlisting>13:35:12.159321 <emphasis role="bold">0:4:e2:20:20:33</emphasis> 0:0:77:95:dd:19 ip 98:
192.0.2.177 &#62; 192.0.2.254: icmp: echo request (DF)
13:35:12.207615 0:0:77:95:dd:19 <emphasis role="bold">0:c0:a8:50:b2:57</emphasis> ip 98:
192.0.2.254 &#62; 192.0.2.177 : icmp: echo reply</programlisting>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&#39;s eth0 NIC while
0:c0:a8:50:b2:57 was the MAC address of DMZ 1. In other words, the
gateway&#39;s ARP cache still associates 192.0.2.177 with the NIC in
DMZ 1 rather than with the firewall&#39;s eth0.</para>
</section>
</section>
<section id="Rules">
<title>Rules</title>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>With the default policies, your local systems (Local 1-3) can
access any servers on the internet and the DMZ can&#39;t access any
other host (including the firewall). With the exception of DNAT rules
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.</para>
<note>
<para>Since the SOURCE PORT(S) and ORIG. DEST. Columns aren&#39;t used
in this section, they won&#39;t be shown</para>
</note>
<para>You probably want to allow ping between your zones:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST
# PORT(S)
ACCEPT net dmz icmp echo-request
ACCEPT net loc icmp echo-request
ACCEPT dmz loc icmp echo-request
ACCEPT loc dmz icmp echo-request</programlisting>
<para>Let&#39;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:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST COMMENTS
# PORT(S)
ACCEPT net dmz:192.0.2.178 tcp smtp #Mail from
#Internet
ACCEPT net dmz:192.0.2.178 tcp pop3 #Pop3 from
#Internet
ACCEPT loc dmz:192.0.2.178 tcp smtp #Mail from local
#Network
ACCEPT loc dmz:192.0.2.178 tcp pop3 #Pop3 from local
#Network
ACCEPT fw dmz:192.0.2.178 tcp smtp #Mail from the
#Firewall
ACCEPT dmz:192.0.2.178 net tcp smtp #Mail to the
#Internet
ACCEPT net dmz:192.0.2.177 tcp http #WWW from
#Internet
ACCEPT net dmz:192.0.2.177 tcp https #Secure WWW
#from Internet
ACCEPT loc dmz:192.0.2.177 tcp https #Secure WWW
#from local
#Network</programlisting>
<para>If you run a public DNS server on 192.0.2.177, you would need to
add the following rules:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST COMMENTS
# PORT(S)
ACCEPT net dmz:192.0.2.177 udp domain #UDP DNS from
#Internet
ACCEPT net dmz:192.0.2.177 tcp domain #TCP DNS from
#Internet
ACCEPT loc dmz:192.0.2.177 udp domain #UDP DNS from
#Local Network
ACCEPT loc dmz:192.0.2.177 tcp domain #TCP DNS from
#Local Network
ACCEPT fw dmz:192.0.2.177 udp domain #UDP DNS from
#the Firewall
ACCEPT fw dmz:192.0.2.177 tcp domain #TCP DNS from
#the Firewall
ACCEPT dmz:192.0.2.177 net udp domain #UDP DNS to
#the Internet
ACCEPT dmz:192.0.2.177 net tcp domain #TCPP DNS to
#the Internet</programlisting>
<para>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.</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST COMMENTS
# PORT(S)
ACCEPT loc dmz tcp ssh #SSH to the DMZ
ACCEPT net fw tcp ssh #SSH to the
#Firewall</programlisting>
</section>
<section id="OddsAndEnds">
<title>Odds and Ends</title>
<para>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&#39;s own public IP.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>If you haven&#39;t already, it would be a good idea to browse
through <ulink url="Documentation.htm#Config"><filename>/etc/shorewall/shorewall.conf</filename></ulink>
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&#39;t touched yet just to get a feel for the other things that
Shorewall can do.</para>
<para>In case you haven&#39;t been keeping score, here&#39;s the final
set of configuration files for our sample network. Only those that were
modified from the original installation are shown.</para>
<para><filename>/etc/shorewall/interfaces</filename> (The
<quote>options</quote> will be very site-specific).</para>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
net eth0 detect rfc1918,routefilter
loc eth1 detect
dmz eth2 detect</programlisting>
<para>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 <quote>detect</quote>
with the actual broadcast addresses in the entries above, you can bring
up Shorewall before you bring up your network interfaces.</para>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
net eth0 192.0.2.255 rfc1918
loc eth1 192.168.201.7
dmz eth2 192.168.202.7</programlisting>
<para><filename>/etc/shorewall/masq</filename> - Local Subnet</para>
<programlisting>#INTERFACE SUBNET ADDRESS
eth0 192.168.201.0/29 192.0.2.176</programlisting>
<para><filename>/etc/shorewall/proxyarp</filename> - DMZ</para>
<programlisting>#ADDRESS EXTERNAL INTERFACE HAVE ROUTE
192.0.2.177 eth2 eth0 No
192.0.2.178 eth2 eth0 No</programlisting>
<para><filename>/etc/shorewall/nat</filename>- Daughter&#39;s System</para>
<programlisting>#EXTERNAL INTERFACE INTERNAL ALL INTERFACES LOCAL
192.0.2.179 eth0 192.168.201.4 No No</programlisting>
<para><filename>/etc/shorewall/rules</filename></para>
<programlisting>#ACTION SOURCE DEST PROTO DEST COMMENTS
# PORT(S)
ACCEPT net dmz icmp echo-request
ACCEPT net loc icmp echo-request
ACCEPT dmz loc icmp echo-request
ACCEPT loc dmz icmp echo-request
ACCEPT net loc:192.168.201.4 tcp www #Daughter&#39;s
#Server
ACCEPT net dmz:192.0.2.178 tcp smtp #Mail from
#Internet
ACCEPT net dmz:192.0.2.178 tcp pop3 #Pop3 from
#Internet
ACCEPT loc dmz:192.0.2.178 tcp smtp #Mail from local
#Network
ACCEPT loc dmz:192.0.2.178 tcp pop3 #Pop3 from local
#Network
ACCEPT fw dmz:192.0.2.178 tcp smtp #Mail from the
#Firewall
ACCEPT dmz:192.0.2.178 net tcp smtp #Mail to the
#Internet
ACCEPT net dmz:192.0.2.177 tcp http #WWW from
#Internet
ACCEPT net dmz:192.0.2.177 tcp https #Secure WWW
#from Internet
ACCEPT loc dmz:192.0.2.177 tcp https #Secure WWW
#from local
#Network
ACCEPT net dmz:192.0.2.177 udp domain #UDP DNS from
#Internet
ACCEPT net dmz:192.0.2.177 tcp domain #TCP DNS from
#Internet
ACCEPT loc dmz:192.0.2.177 udp domain #UDP DNS from
#Local Network
ACCEPT loc dmz:192.0.2.177 tcp domain #TCP DNS from
#Local Network
ACCEPT fw dmz:192.0.2.177 udp domain #UDP DNS from
#the Firewall
ACCEPT fw dmz:192.0.2.177 tcp domain #TCP DNS from
#the Firewall
ACCEPT dmz:192.0.2.177 net udp domain #UDP DNS to
#the Internet
ACCEPT dmz:192.0.2.177 net tcp domain #TCPP DNS to
#the Internet
ACCEPT loc dmz tcp ssh #SSH to the DMZ
ACCEPT net fw tcp ssh #SSH to the
#Firewall</programlisting>
</section>
</section>
<section id="DNS">
<title>DNS</title>
<para>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 Views. If you
are not interested in Bind 9 views, you can go to the next section.</para>
<para>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 &#34;winken.foobar.net, blinken.foobar.net and
nod.foobar.net. You want your firewall to be known as firewall.foobar.net
externally and it&#39;s interface to the local network to be know as
gateway.foobar.net and its interface to the dmz as dmz.foobar.net.
Let&#39;s have the DNS server on 192.0.2.177 which will also be known by
the name ns1.foobar.net.</para>
<para>The <filename>/etc/named.conf </filename>file would look like this:</para>
<programlisting>
options {
directory &#34;/var/named&#34;;
listen-on { 127.0.0.1 ; 192.0.2.177; };
};
logging {
channel xfer-log {
file &#34;/var/log/named/bind-xfer.log&#34;;
print-category yes;
print-severity yes;
print-time yes;
severity info;
};
category xfer-in { xfer-log; };
category xfer-out { xfer-log; };
category notify { xfer-log; };
};
#
# This is the view presented to our internal systems
#
view &#34;internal&#34; {
#
# These are the clients that see this view
#
match-clients { 192.168.201.0/29;
192.168.202.0/29;
127.0.0.0/8;
192.0.2.176/32;
192.0.2.178/32;
192.0.2.179/32;
192.0.2.180/32; };
#
# If this server can&#39;t complete the request, it should use
# outside servers to do so
#
recursion yes;
zone &#34;.&#34; in {
type hint;
file &#34;int/root.cache&#34;;
};
zone &#34;foobar.net&#34; in {
type master;
notify no;
allow-update { none; };
file &#34;int/db.foobar&#34;;
};
zone &#34;0.0.127.in-addr.arpa&#34; in {
type master;
notify no;
allow-update { none; };
file &#34;int/db.127.0.0&#34;;
};
zone &#34;201.168.192.in-addr.arpa&#34; in {
type master;
notify no;
allow-update { none; };
file &#34;int/db.192.168.201&#34;;
};
zone &#34;202.168.192.in-addr.arpa&#34; in {
type master;
notify no;
allow-update { none; };
file &#34;int/db.192.168.202&#34;;
};
zone &#34;176.2.0.192.in-addr.arpa&#34; in {
type master;
notify no;
allow-update { none; };
file &#34;db.192.0.2.176&#34;;
};
zone &#34;177.2.0.192.in-addr.arpa&#34; in {
type master;
notify no;
allow-update { none; };
file &#34;db.192.0.2.177&#34;;
};
zone &#34;178.2.0.192.in-addr.arpa&#34; in {
type master;
notify no;
allow-update { none; };
file &#34;db.192.0.2.178&#34;;
};
zone &#34;179.2.0.192.in-addr.arpa&#34; in {
type master;
notify no;
allow-update { none; };
file &#34;db.206.124.146.179&#34;;
};
};
#
# This is the view that we present to the outside world
#
view &#34;external&#34; {
match-clients { any; };
#
# If we can&#39;t answer the query, we tell the client so
#
recursion no;
zone &#34;foobar.net&#34; in {
type master;
notify yes;
allow-update {none; };
allow-transfer { &#60;secondary NS IP&#62;; };
file &#34;ext/db.foobar&#34;;
};
zone &#34;176.2.0.192.in-addr.arpa&#34; in {
type master;
notify yes;
allow-update { none; };
allow-transfer { &#60;secondary NS IP&#62;; };
file &#34;db.192.0.2.176&#34;;
};
zone &#34;177.2.0.192.in-addr.arpa&#34; in {
type master;
notify yes;
allow-update { none; };
allow-transfer { &#60;secondary NS IP&#62;; };
file &#34;db.192.0.2.177&#34;;
};
zone &#34;178.2.0.192.in-addr.arpa&#34; in {
type master;
notify yes;
allow-update { none; };
allow-transfer { &#60;secondary NS IP&#62;; };
file &#34;db.192.0.2.178&#34;;
};
zone &#34;179.2.0.192.in-addr.arpa&#34; in {
type master;
notify yes;
allow-update { none; };
allow-transfer { &#60;secondary NS IP&#62;; };
file &#34;db.192.0.2.179&#34;;
};
};</programlisting>
<para>Here are the files in <filename class="directory">/var/named</filename>
(those not shown are usually included in your bind disbribution).</para>
<para><filename>db.192.0.2.176</filename> - This is the reverse zone for
the firewall&#39;s external interface</para>
<programlisting>; ############################################################
; Start of Authority (Inverse Address Arpa) for 192.0.2.176/32
; Filename: db.192.0.2.176
; ############################################################
@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2001102303 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
;
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
@ 604800 IN NS &#60;name of secondary ns&#62;.
;
; ############################################################
; Iverse Address Arpa Records (PTR&#39;s)
; ############################################################
176.2.0.192.in-addr.arpa. 86400 IN PTR firewall.foobar.net.</programlisting>
<para><filename>db.192.0.2.177</filename> - Reverse zone www server</para>
<programlisting>; ############################################################
; Start of Authority (Inverse Address Arpa) for 192.0.2.177/32
; Filename: db.192.0.2.177
; ############################################################
@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2001102303 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
;
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
@ 604800 IN NS &#60;name of secondary ns&#62;.
;
; ############################################################
; Iverse Address Arpa Records (PTR&#39;s)
; ############################################################
177.2.0.192.in-addr.arpa. 86400 IN PTR www.foobar.net.</programlisting>
<para><filename>db.192.0.2.178</filename> - Reverse zone for the mail
server</para>
<programlisting>; ############################################################
; Start of Authority (Inverse Address Arpa) for 192.0.2.178/32
; Filename: db.192.0.2.178
; ############################################################
@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2001102303 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
;
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
@ 604800 IN NS &#60;name of secondary ns&#62;.
;
; ############################################################
; Iverse Address Arpa Records (PTR&#39;s)
; ############################################################
178.2.0.192.in-addr.arpa. 86400 IN PTR mail.foobar.net.</programlisting>
<para><filename>db.192.0.2.179</filename> - Reverse zone for
Daughter&#39;s public web server</para>
<programlisting>; ############################################################
; Start of Authority (Inverse Address Arpa) for 192.0.2.179/32
; Filename: db.192.0.2.179
; ############################################################
@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2001102303 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
;
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
@ 604800 IN NS &#60;name of secondary ns&#62;.
;
; ############################################################
; Iverse Address Arpa Records (PTR&#39;s)
; ############################################################
179.2.0.192.in-addr.arpa. 86400 IN PTR nod.foobar.net.</programlisting>
<para><filename>int/db.127.0.0</filename> - Reverse zone for localhost</para>
<programlisting>; ############################################################
; Start of Authority (Inverse Address Arpa) for 127.0.0.0/8
; Filename: db.127.0.0
; ############################################################
@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2001092901 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
; ############################################################
; Iverse Address Arpa Records (PTR&#39;s)
; ############################################################
1 86400 IN PTR localhost.foobar.net.</programlisting>
<para><filename>int/db.192.168.201</filename> - Reverse zone for the local
network. This is only shown to internal clients.</para>
<programlisting>; ############################################################
; Start of Authority (Inverse Address Arpa) for 192.168.201.0/29
; Filename: db.192.168.201
; ############################################################
@ 604800 IN SOA ns1.foobar.net netadmin.foobar.net. (
2002032501 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
; ############################################################
; Iverse Address Arpa Records (PTR&#39;s)
; ############################################################
1 86400 IN PTR gateway.foobar.net.
2 86400 IN PTR winken.foobar.net.
3 86400 IN PTR blinken.foobar.net.
4 86400 IN PTR nod.foobar.net.</programlisting>
<para><filename>int/db.192.168.202</filename> - Reverse zone for the
firewall&#39;s DMZ Interface</para>
<programlisting>; ############################################################
; Start of Authority (Inverse Address Arpa) for 192.168.202.0/29
; Filename: db.192.168.202
; ############################################################
@ 604800 IN SOA ns1.foobar.net netadmin.foobar.net. (
2002032501 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
; ############################################################
; Iverse Address Arpa Records (PTR&#39;s)
; ############################################################
1 86400 IN PTR dmz.foobar.net.</programlisting>
<para><filename>int/db.foobar </filename>- Forward zone for internal
clients.</para>
<programlisting>;##############################################################
; Start of Authority for foobar.net.
; Filename: db.foobar
;##############################################################
@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2002071501 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ); minimum (1 day)
;############################################################
; foobar.net Nameserver Records (NS)
;############################################################
@ 604800 IN NS ns1.foobar.net.
;############################################################
; Foobar.net Office Records (ADDRESS)
;############################################################
localhost 86400 IN A 127.0.0.1
firewall 86400 IN A 192.0.2.176
www 86400 IN A 192.0.2.177
ns1 86400 IN A 192.0.2.177
www 86400 IN A 192.0.2.177
gateway 86400 IN A 192.168.201.1
winken 86400 IN A 192.168.201.2
blinken 86400 IN A 192.168.201.3
nod 86400 IN A 192.168.201.4</programlisting>
<para><filename>ext/db.foobar </filename>- Forward zone for external
clients.</para>
<programlisting>;##############################################################
; Start of Authority for foobar.net.
; Filename: db.foobar
;##############################################################
@ 86400 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2002052901 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ); minimum (1 day)
;############################################################
; Foobar.net Nameserver Records (NS)
;############################################################
@ 86400 IN NS ns1.foobar.net.
@ 86400 IN NS &#60;secondary NS&#62;.
;############################################################
; Foobar.net Foobar Wa Office Records (ADDRESS)
;############################################################
localhost 86400 IN A 127.0.0.1
;
; The firewall itself
;
firewall 86400 IN A 192.0.2.176
;
; The DMZ
;
ns1 86400 IN A 192.0.2.177
www 86400 IN A 192.0.2.177
mail 86400 IN A 192.0.2.178
;
; The Local Network
;
nod 86400 IN A 192.0.2.179
;############################################################
; Current Aliases for foobar.net (CNAME)
;############################################################
;############################################################
; foobar.net MX Records (MAIL EXCHANGER)
;############################################################
foobar.net. 86400 IN A 192.0.2.177
86400 IN MX 0 mail.foobar.net.
86400 IN MX 1 &#60;backup MX&#62;.</programlisting>
</section>
<section>
<title>Some Things to Keep in Mind</title>
<itemizedlist>
<listitem>
<para><emphasis role="bold">You cannot test your firewall from the
inside</emphasis>. 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 <quote>net</quote> zone. Any
traffic that you generate from the local network will be associated
with your local interface and will be treated as loc-&#62;fw traffic.</para>
</listitem>
<listitem>
<para><emphasis role="bold">IP addresses are properties of systems,
not of interfaces</emphasis>. 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&#39;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&#39;s default gateway set
correctly.</para>
</listitem>
<listitem>
<para><emphasis role="bold">All IP addresses configured on firewall
interfaces are in the $FW (fw) zone</emphasis>. If 192.168.1.254 is
the IP address of your internal interface then you can write
<quote><emphasis role="bold">$FW:192.168.1.254</emphasis></quote> in a
rule but you may not write <quote><emphasis role="bold">loc:192.168.1.254</emphasis></quote>.
Similarly, it is nonsensical to add 192.168.1.254 to the <emphasis
role="bold">loc</emphasis> zone using an entry in
<filename>/etc/shorewall/hosts</filename>.</para>
</listitem>
<listitem>
<para><emphasis role="bold">Reply packets do NOT automatically follow
the reverse path of the one taken by the original request</emphasis>.
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.</para>
</listitem>
<listitem>
<para><emphasis role="bold">Shorewall itself has no notion of inside
or outside</emphasis>. These concepts are embodied in how Shorewall is
configured.</para>
</listitem>
</itemizedlist>
</section>
<section id="StartingAndStopping">
<title>Starting and Stopping the Firewall</title>
<para>The <ulink url="Install.htm">Installation procedure</ulink>
configures your system to start Shorewall at system boot.</para>
<para>The firewall is started using the <quote>shorewall start</quote>
command and stopped using <quote>shorewall stop</quote>. When the firewall
is stopped, routing is enabled on those hosts that have an entry in
<filename><ulink url="Documentation.htm#Routestopped">/etc/shorewall/routestopped</ulink></filename>.
A running firewall may be restarted using the <quote>shorewall restart</quote>
command. If you want to totally remove any trace of Shorewall from your
Netfilter configuration, use <quote>shorewall clear</quote>.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" /></para>
<para>Edit the <filename><ulink url="Documentation.htm#Routestopped">/etc/shorewall/routestopped</ulink></filename>
file and configure those systems that you want to be able to access the
firewall when it is stopped.</para>
<caution>
<para>If you are connected to your firewall from the internet, do not
issue a <quote>shorewall stop</quote> command unless you have added an
entry for the IP address that you are connected from to
<filename><ulink url="Documentation.htm#Routestopped">/etc/shorewall/routestopped</ulink></filename>.
Also, I don&#39;t recommend using <quote>shorewall restart</quote>; it
is better to create an <ulink url="starting_and_stopping_shorewall.htm"><emphasis>an
alternate configuration</emphasis></ulink>&#x00A0; and test it using the
<quote><ulink url="starting_and_stopping_shorewall.htm"><command>shorewall
try</command></ulink></quote> command.</para>
</caution>
</section>
</article>
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