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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="three-interface">
<!--$Id$-->
<articleinfo>
<title>Three-Interface Firewall</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate>2004-06-11</pubdate>
<copyright>
<year>2002-2004</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>
<title>Introduction</title>
<para>Setting up a Linux system as a firewall for a small network with DMZ
is a fairly straight-forward task if you understand the basics and follow
the documentation.</para>
<para>This guide doesn&#39;t attempt to acquaint you with all of the
features of Shorewall. It rather focuses on what is required to configure
Shorewall in one of its more popular configurations:</para>
<itemizedlist>
<listitem>
<para>Linux system used as a firewall/router for a small local
network.</para>
</listitem>
<listitem>
<para>Single public IP address.</para>
<note>
<para>If you have more than one public IP address, this is not the
guide you want -- see the <ulink url="shorewall_setup_guide.htm">Shorewall
Setup Guide</ulink> instead.</para>
</note>
</listitem>
<listitem>
<para>DMZ connected to a separate ethernet interface.</para>
</listitem>
<listitem>
<para>Connection through DSL, Cable Modem, ISDN, Frame Relay, dial-up,
...</para>
</listitem>
</itemizedlist>
<para>Here is a schematic of a typical installation.</para>
<figure>
<title>schematic of a typical installation</title>
<mediaobject>
<imageobject>
<imagedata align="center" fileref="images/dmz1.png" format="PNG" />
</imageobject>
</mediaobject>
</figure>
<section>
<title>Requirements</title>
<para>Shorewall requires that you have the <command>iproute</command>/<command>iproute2</command>
package installed (on <trademark>RedHat</trademark>, the package is
called <command>iproute</command>). You can tell if this package is
installed by the presence of an <command>ip</command> program on your
firewall system. As <systemitem class="username">root</systemitem>, you
can use the <command>which</command> command to check for this program:</para>
<programlisting>[root@gateway root]# <command>which ip</command>
/sbin/ip
[root@gateway root]#</programlisting>
</section>
<section>
<title>Before you start</title>
<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.</para>
<caution>
<para>If you edit your configuration files on a
<trademark>Windows</trademark> system, you must save them as
<trademark>Unix</trademark> files if your editor supports that option
or you must run them through <command>dos2unix</command> before trying
to use them. Similarly, if you copy a configuration file from your
<trademark>Windows</trademark> hard drive to a floppy disk, you must
run <command>dos2unix</command> 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>
<title>Conventions</title>
<para>Points at which configuration changes are recommended are flagged
with <inlinegraphic fileref="images/BD21298_.gif" format="GIF" />.</para>
<para>Configuration notes that are unique to LEAF/Bering are marked with
<inlinegraphic fileref="images/leaflogo.gif" format="GIF" />.</para>
</section>
</section>
<section>
<title>PPTP/ADSL</title>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>If you have an ADSL Modem and you use PPTP to communicate with a
server in that modem, you must make the <ulink url="PPTP.htm#PPTP_ADSL">changes
recommended here</ulink> in addition to those detailed below. ADSL with
PPTP is most commonly found in Europe, notably in Austria.</para>
</section>
<section>
<title>Shorewall Concepts</title>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>The configuration files for Shorewall are contained in the directory
<filename>/etc/shorewall</filename> -- for simple setups, you will only
need to deal with a few of these as described in this guide.<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 /etc/shorewall even
if you do not modify those files.</para></warning></para>
<para>After you have installed Shorewall, download the three-interface
sample, un-tar it (<command>tar <option>-zxvf</option>
<filename>three-interfaces.tgz</filename></command>) and and copy the
files to <filename>/etc/shorewall</filename> (the files will replace files
with the same names that were placed in <filename>/etc/shorewall</filename>
when Shorewall was installed).</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 default entries.</para>
<para>Shorewall views the network where it is running as being composed of
a set of zones. In the three-interface sample configuration, the following
zone names are used:</para>
<informaltable>
<tgroup align="left" cols="2">
<thead valign="middle">
<row>
<entry align="center">Name</entry>
<entry align="center">Description</entry>
</row>
</thead>
<tbody>
<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>
</informaltable>
<para>Zone names are defined in <filename>/etc/shorewall/zones</filename>.</para>
<para>Shorewall also recognizes the firewall system as its own zone - by
default, the firewall itself is known as <varname>fw</varname>.</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>/etc/shorewall/policy</filename> file.</para>
</listitem>
<listitem>
<para>You define exceptions to those default policies in the
<filename>/etc/shorewall/rules</filename> file.</para>
</listitem>
</itemizedlist>
<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 there is a <ulink
url="shorewall_extension_scripts.htm">comon action</ulink> defined for the
policy in <filename>/etc/shorewall/actions</filename> or
<filename>/usr/share/shorewall/actions.std</filename> then that action is
peformed before the action is applied.</para>
<para>The <filename>/etc/shorewall/policy</filename> file included with
the three-interface sample has the following policies:</para>
<programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
loc net ACCEPT
net all DROP info
all all REJECT info</programlisting>
<important>
<para>In the three-interface sample, the line below is included but
commented out. If you want your firewall system to have full access to
servers on the internet, uncomment that line.</para>
<programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
fw net ACCEPT</programlisting>
</important>
<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</para>
</listitem>
<listitem>
<para>optionally accept all connection requests from the firewall to
the internet (if you uncomment the additional policy)</para>
</listitem>
<listitem>
<para>reject all other connection requests.</para>
</listitem>
</orderedlist>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>At this point, edit your <filename>/etc/shorewall/policy</filename>
file and make any changes that you wish.</para>
</section>
<section>
<title>Network Interfaces</title>
<figure>
<title>DMZ</title>
<mediaobject>
<imageobject>
<imagedata align="center" fileref="images/dmz1.png" format="PNG" />
</imageobject>
</mediaobject>
</figure>
<para>The firewall has three network interfaces. Where Internet
connectivity is through a cable or DSL <quote>Modem</quote>, the External
Interface will be the ethernet adapter that is connected to that
<quote>Modem</quote> (e.g., <filename class="devicefile">eth0</filename>)
unless you connect via <emphasis>Point-to-Point Protocol</emphasis> over
Ethernet (PPPoE) or <emphasis>Point-to-Point Tunneling Protocol</emphasis>
(PPTP) in which case the External Interface will be a <literal>ppp</literal>
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" format="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
<varname>CLAMPMSS=yes</varname> in <filename>/etc/shorewall/shorewall.conf</filename>.</para>
<para>Your Local Interface will 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 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 DMZ Interface 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 arp_filter 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><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>The Shorewall three-interface sample configuration assumes that the
external interface is <filename class="devicefile">eth0</filename>, the
local interface is <filename class="devicefile">eth1</filename> and the
DMZ interface is <filename class="devicefile">eth2</filename>. If your
configuration is different, you will have to modify the sample
<filename>/etc/shorewall/interfaces</filename> file accordingly. While you
are there, you may wish to review the list of options that are specified
for the interfaces. Some hints:</para>
<tip>
<para>If your external interface is <filename class="devicefile">ppp0</filename>
or <filename class="devicefile">ippp0</filename>, you can replace the
<quote>detect</quote> in the second column with <quote>-</quote>
(without the quotes).</para>
</tip>
<tip>
<para>If your external interface is <filename class="devicefile">ppp0</filename>
or <filename class="devicefile">ippp0</filename> or if you have a static
IP address, you can remove <quote>dhcp</quote> from the option list.</para>
</tip>
<tip>
<para>If you specify <emphasis>norfc1918</emphasis> for your external
interface, you will want to check the <ulink url="errata.htm">Shorewall
Errata</ulink> periodically for updates to the <filename>/usr/share/shorewall/rfc1918
file</filename>. Alternatively, you can copy <filename>/usr/share/shorewall/rfc1918</filename>
to <filename>/etc/shorewall/rfc1918</filename> then <ulink
url="myfiles.htm#RFC1918">strip down your <filename>/etc/shorewall/rfc1918</filename>
file as I do</ulink>.</para>
</tip>
</section>
<section>
<title>IP Addresses</title>
<para>Before going further, we should say a few words about Internet
Protocol (IP) addresses. Normally, your ISP will assign you a single
Public IP address. This address may be assigned via the Dynamic Host
Configuration Protocol (DHCP) or as part of establishing your connection
when you dial in (standard modem) or establish your PPP connection. In
rare cases, your ISP may assign you a static IP address; that means that
you configure your firewall&#39;s external interface to use that address
permanently. Regardless of how the address is assigned, it will be shared
by all of your systems when you access the Internet. You will have to
assign your own addresses for your internal network (the local and DMZ
Interfaces on your firewall plus your other computers). RFC 1918 reserves
several Private IP address ranges for this purpose:</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><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>Before starting Shorewall, you should look at the IP address of your
external interface and if it is one of the above ranges, you should remove
the <varname>norfc1918</varname> option from the external interface&#39;s
entry in <filename>/etc/shorewall/interfaces</filename>.</para>
<para>You will want to assign your local addresses from one sub-network or
subnet and your DMZ addresses from another subnet. For our purposes, we
can consider a subnet to consists of a range of addresses <systemitem
class="ipaddress">x.y.z.0</systemitem> - <systemitem class="ipaddress">x.y.z.255</systemitem>.
Such a subnet will have a Subnet Mask of <systemitem class="netmask">255.255.255.0</systemitem>.
The address <systemitem class="ipaddress">x.y.z.0</systemitem> is reserved
as the Subnet Address and <systemitem class="netmask">x.y.z.255</systemitem>
is reserved as the Subnet Broadcast Address. In Shorewall, a subnet is
described using Classless InterDomain Routing (CIDR) notation with
consists of the subnet address followed by <varname>/24</varname>. The
<varname>24</varname> refers to the number of consecutive <quote>1</quote>
bits from the left of the subnet mask.</para>
<table>
<title>Example sub-network</title>
<tgroup cols="2">
<colspec align="left" />
<tbody>
<row>
<entry>Range:</entry>
<entry><systemitem class="ipaddress">10.10.10.0</systemitem> -
<systemitem class="ipaddress">10.10.10.255</systemitem></entry>
</row>
<row>
<entry>Subnet Address:</entry>
<entry><systemitem class="ipaddress">10.10.10.0</systemitem></entry>
</row>
<row>
<entry>Broadcast Address:</entry>
<entry><systemitem class="ipaddress">10.10.10.255</systemitem></entry>
</row>
<row>
<entry>CIDR Notation:</entry>
<entry><systemitem class="ipaddress">10.10.10.0/24</systemitem></entry>
</row>
</tbody>
</tgroup>
</table>
<para>It is conventional to assign the internal interface either the first
usable address in the subnet (<systemitem class="ipaddress">10.10.10.1</systemitem>
in the above example) or the last usable address (<systemitem
class="ipaddress">10.10.10.254</systemitem>).</para>
<para>One of the purposes of subnetting is to allow all computers in the
subnet to understand which other computers can be communicated with
directly. To communicate with systems outside of the subnetwork, systems
send packets through a gateway (router).</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>Your local computers (Local Computers 1 &#38; 2) should be
configured with their default gateway set to the IP address of the
firewall&#39;s internal interface and your DMZ computers (DMZ Computers 1
&#38; 2) should be configured with their default gateway set to the IP
address of the firewall&#39;s DMZ interface.</para>
<para>The foregoing short discussion barely scratches the surface
regarding subnetting and routing. If you are interested in learning more
about IP addressing and routing, I highly recommend <quote>IP
Fundamentals: What Everyone Needs to Know about Addressing &#38; Routing</quote>,
Thomas A. Maufer, Prentice-Hall, 1999, ISBN 0-13-975483-0.</para>
<para>The remainder of this quide will assume that you have configured
your network as shown here:</para>
<figure>
<title>DMZ</title>
<mediaobject>
<imageobject>
<imagedata fileref="images/dmz2.png" />
</imageobject>
<caption>
<para>The default gateway for the DMZ computers would be <systemitem
class="ipaddress">10.10.11.254</systemitem> and the default gateway
for the Local computers would be <systemitem class="ipaddress">10.10.10.254</systemitem>.</para>
<warning>
<para>Your ISP might assign your external interface an RFC 1918
address. If that address is in the <systemitem class="ipaddress">10.10.10.0/24</systemitem>
subnet then you will need to select a DIFFERENT RFC 1918 subnet
for your local network and if it is in the <systemitem
class="ipaddress">10.10.11.0/24</systemitem> subnet then you will
need to select a different RFC 1918 subnet for your DMZ.</para>
</warning>
</caption>
</mediaobject>
</figure>
</section>
<section>
<title>IP Masquerading (SNAT)</title>
<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. When one of your local
systems (let&#39;s assume local computer 1) sends a connection request to
an internet host, the firewall must perform Network Address Translation
(NAT). The firewall rewrites the source address in the packet to be the
address of the firewall&#39;s external interface; in other words, the
firewall makes it look as if the firewall itself is initiating the
connection. This is necessary so that the destination host will be able to
route return packets back to the firewall (remember that packets whose
destination address is reserved by RFC 1918 can&#39;t be routed accross
the internet). When the firewall receives a return packet, it rewrites the
destination address back to 10.10.10.1 and forwards the packet on to local
computer 1.</para>
<para>On Linux systems, the above process is often referred to as IP
Masquerading and you will also see the term Source Network Address
Translation (SNAT) used. Shorewall follows the convention used with
Netfilter: <itemizedlist><listitem><para><emphasis>Masquerade</emphasis>
describes the case where you let your firewall system automatically detect
the external interface address.</para></listitem><listitem><para><emphasis>SNAT</emphasis>
refers to the case when you explicitly specify the source address that you
want outbound packets from your local network to use.</para></listitem></itemizedlist>
In Shorewall, both Masquerading and SNAT are configured with entries in
the <filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
file.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>If your external firewall interface is <filename class="devicefile">eth0</filename>,
your local interface <filename class="devicefile">eth1</filename> and your
DMZ interface is <filename class="devicefile">eth2</filename> then you do
not need to modify the file provided with the sample. Otherwise, edit
<filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
and change it to match your configuration.</para>
<para>If, despite all advice to the contrary, you are using this guide and
want to use one-to-one NAT or Proxy ARP for your DMZ, remove the entry for
eth2 from <filename>/etc/shorewall/masq</filename>.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>If your external IP is static, you can enter it in the third column
in the <filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
entry if you like although your firewall will work fine if you leave that
column empty. Entering your static IP in column 3 makes processing
outgoing packets a little more efficient.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>If you are using the Debian package, please check your
<filename>shorewall.conf</filename> file to ensure that the following are
set correctly; if they are not, change them appropriately:
<itemizedlist><listitem><para><varname>NAT_ENABLED=Yes</varname>
(Shorewall versions earlier than 1.4.6)</para></listitem><listitem><para><varname>IP_FORWARDING=On</varname></para></listitem></itemizedlist></para>
</section>
<section>
<title>Port Forwarding (DNAT)</title>
<para>One of your goals will be to run one or more servers on your DMZ
computers. Because these computers have RFC-1918 addresses, it is not
possible for clients on the Internet to connect directly to them. It is
rather necessary for those clients to address their connection requests to
your firewall who rewrites the destination address to the address of your
server and forwards the packet to that server. When your server responds,
the firewall automatically performs SNAT to rewrite the source address in
the response.</para>
<para>The above process is called <emphasis>Port Forwarding</emphasis> or
<emphasis>Destination Network Address Translation</emphasis> (DNAT). You
configure port forwarding using DNAT rules in the <filename
class="directory">/etc/shorewall/</filename><filename>rules</filename>
file.</para>
<para>The general form of a simple port forwarding rule in <filename
class="directory">/etc/shorewall/</filename><filename>rules</filename> is:
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
DNAT net dmz:<emphasis>&#60;server local IP address&#62;</emphasis>[:<emphasis>&#60;server port&#62;</emphasis>] <emphasis>&#60;protocol&#62;</emphasis> <emphasis>&#60;port&#62;</emphasis></programlisting>
If you don&#39;t specify the <emphasis><varname>&#60;server port&#62;</varname></emphasis>,
it is assumed to be the same as <emphasis><varname>&#60;port&#62;</varname></emphasis>.</para>
<example>
<title>You run a Web Server on DMZ Computer 2 and you want to forward
incoming TCP port 80 to that system</title>
<para><programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
DNAT net dmz:10.10.11.2 tcp 80
ACCEPT loc dmz:10.10.11.2 tcp 80</programlisting><itemizedlist><listitem><para>Entry
1 forwards port 80 from the Internet.</para></listitem><listitem><para>Entry
2 allows connections from the local network.</para></listitem></itemizedlist>
Several important points to keep in mind:<itemizedlist><listitem><para>When
you are connecting to your server from your local systems, you must use
the server&#39;s internal IP address (<systemitem class="ipaddress">10.10.11.2</systemitem>).</para></listitem><listitem><para>Many
ISPs block incoming connection requests to port 80. If you have problems
connecting to your web server, try the following rule and try connecting
to port 5000 (e.g., connect to <literal>http://w.x.y.z:5000 where
w.x.y.z</literal> is your external IP).<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE
# PORT(S)
DNAT net dmz:10.10.11.2:80 tcp 80 5000</programlisting></para></listitem><listitem><para>If
you want to be able to access your server from the local network using
your external address, then if you have a static external IP you can
replace the loc-&#62;dmz rule above with:<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL
# PORT(S) DEST
DNAT loc dmz:10.10.11.2 tcp 80 - <emphasis>&#60;external IP&#62;</emphasis></programlisting>If
you have a dynamic IP then you must ensure that your external interface
is up before starting Shorewall and you must take steps as follows
(assume that your external interface is <filename class="devicefile">eth0</filename>):<orderedlist><listitem><para>Include
the following in /etc/shorewall/params:</para><para><command>ETH0_IP=$(find_interface_address
eth0)</command></para></listitem><listitem><para>Make your
<literal>loc-&#62;dmz</literal> rule: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL
# PORT(S) DEST
DNAT loc dmz:10.10.11.2 tcp 80 - $ETH0_IP</programlisting></para></listitem></orderedlist></para></listitem><listitem><para>If
you want to access your server from the DMZ using your external IP
address, see <ulink url="FAQ.htm#faq2a">FAQ 2a</ulink>.</para></listitem></itemizedlist></para>
</example>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>At this point, add the DNAT and ACCEPT rules for your servers.</para>
</section>
<section>
<title>Domain Name Server (DNS)</title>
<para>Normally, when you connect to your ISP, as part of getting an IP
address your firewall&#39;s <emphasis>Domain Name Service</emphasis> (DNS)
resolver will be automatically configured (e.g., the <filename>/etc/resolv.conf</filename>
file will be written). Alternatively, your ISP may have given you the IP
address of a pair of DNS name servers for you to manually configure as
your primary and secondary name servers. It is your responsibility to
configure the resolver in your internal systems. You can take one of two
approaches: <itemizedlist><listitem><para>You can configure your internal
systems to use your ISP&#39;s name servers. If your ISP gave you the
addresses of their servers or if those addresses are available on their
web site, you can configure your internal systems to use those addresses.
If that information isn&#39;t available, look in <filename>/etc/resolv.conf</filename>
on your firewall system -- the name servers are given in <quote>nameserver</quote>
records in that file.</para></listitem><listitem><para><inlinegraphic
fileref="images/BD21298_.gif" format="GIF" /></para><para>You can
configure a <emphasis>Caching Name Server</emphasis> on your firewall or
in your DMZ. <trademark>Red Hat</trademark> has an RPM for a caching name
server (which also requires the &#39;<command>bind</command>&#39; RPM) and
for Bering users, there is <filename>dnscache.lrp</filename>. If you take
this approach, you configure your internal systems to use the caching name
server as their primary (and only) name server. You use the internal IP
address of the firewall (<systemitem class="ipaddress">10.10.10.254</systemitem>
in the example above) for the name server address if you choose to run the
name server on your firewall. To allow your local systems to talk to your
caching name server, you must open port 53 (both UDP and TCP) from the
local network to the server; you do that by adding the rules in
<filename>/etc/shorewall/rules</filename>.</para></listitem></itemizedlist>
If you run the name server on the firewall:
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowDNS loc fw
AllowDNS dmz fw </programlisting> Run name server on DMZ
computer 1: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowDNS loc dmz:10.10.11.1
AllowDNS fw dmz:10.10.11.1 </programlisting></para>
<para>In the rules shown above, <quote>AllowDNS</quote> is an example of a
<emphasis>defined action</emphasis>. Shorewall includes a number of
defined actions and <ulink url="User_defined_Actions.html">you can add
your own</ulink>. To see the list of actions included with your version of
Shorewall, look in the file <filename>/etc/shorewall/actions.std</filename>.
Those actions that accept connection requests have names that begin with
<quote>Allow</quote>.</para>
<para>You don&#39;t have to use defined actions when coding a rule in
<filename>/etc/shorewall/rules</filename>; the generated Netfilter ruleset
is slightly more efficient if you code your rules directly rather than
using defined actions. The first example above (name server on the
firewall) could also have been coded as follows:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT loc fw tcp 53
ACCEPT loc fw udp 53
ACCEPT dmz fw tcp 53
ACCEPT dmz fw udp 53 </programlisting>
<para>In cases where Shorewall doesn&#39;t include a defined action to
meet your needs, you can either define the action yourself or you can
simply code the appropriate rules directly.</para>
</section>
<section>
<title>Other Connections</title>
<para>The three-interface sample includes the following rule:
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowDNS fw net </programlisting>That rule allow DNS access from
your firewall and may be removed if you commented out the line in
<filename>/etc/shorewall/policy</filename> allowing all connections from
the firewall to the Internet.</para>
<para>The sample also includes: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowSSH loc fw
AllowSSH loc dmz </programlisting>Those rules allow you to run
an SSH server on your firewall and in each of your DMZ systems and to
connect to those servers from your local systems.</para>
<para>If you wish to enable other connections between your systems, the
general format for using a defined action is:
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
&#60;<emphasis>action</emphasis>&#62; <emphasis>&#60;source zone&#62; &#60;destination zone&#62;</emphasis></programlisting></para>
<para>The general format when not using a defined action is:<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT <emphasis>&#60;source zone&#62; &#60;destination zone&#62; &#60;protocol&#62; &#60;port&#62; </emphasis></programlisting></para>
<example>
<title>You want to run a publicly-available DNS server on your firewall
system</title>
<para>Using defined actions:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowDNS net fw</programlisting>
<para>Not using defined actions:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT net fw tcp 53
ACCEPT net fw udp 53 </programlisting>
<para>Those rules would of course be in addition to the rules listed
above under &#34;If you run the name server on your firewall&#34;.</para>
</example>
<para>If you don&#39;t know what port and protocol a particular
application uses, <ulink url="ports.htm">look here</ulink>.</para>
<important>
<para>I don&#39;t recommend enabling telnet to/from the Internet because
it uses clear text (even for login!). If you want shell access to your
firewall from the Internet, use SSH: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowSSH net fw</programlisting></para>
</important>
<para><inlinegraphic fileref="images/leaflogo.gif" format="GIF" /> Bering
users will want to add the following two rules to be compatible with
Jacques&#39;s Shorewall configuration: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT loc fw udp 53
ACCEPT net fw tcp 80 </programlisting><itemizedlist><listitem><para>Entry
1 allows the DNS Cache to be used.</para></listitem><listitem><para>Entry
2 allows the <quote>weblet</quote> to work.</para></listitem></itemizedlist><inlinegraphic
fileref="images/BD21298_.gif" format="GIF" /></para>
<para>Now modify <filename>/etc/shorewall/rules</filename> to add or
remove other connections as required.</para>
</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>
<title>Starting and Stopping Your Firewall</title>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>The <ulink url="Install.htm">installation procedure</ulink>
configures your system to start Shorewall at system boot but beginning
with Shorewall version 1.3.9 startup is disabled so that your system
won&#39;t try to start Shorewall before configuration is complete. Once
you have completed configuration of your firewall, you can enable
Shorewall startup by removing the file <filename>/etc/shorewall/startup_disabled</filename>.
<important><para>Users of the <filename>.deb</filename> package must edit
<filename>/etc/default/shorewall</filename> and set <varname>startup=1</varname>.</para></important>The
firewall is started using the <command>shorewall start</command> command
and stopped using <command>shorewall stop</command>. When the firewall is
stopped, routing is enabled on those hosts that have an entry in <ulink
url="Documentation.htm#Routestopped"><filename>/etc/shorewall/routestopped</filename></ulink>.
A running firewall may be restarted using the <command>shorewall restart</command>
command. If you want to totally remove any trace of Shorewall from your
Netfilter configuration, use <command>shorewall clear</command>.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>The three-interface sample assumes that you want to enable routing
to/from <filename class="devicefile">eth1</filename> (your local network)
and <filename class="devicefile">eth2</filename> (DMZ) when Shorewall is
stopped. If these two interfaces don&#39;t connect to your local network
and DMZ or if you want to enable a different set of hosts, modify
<filename>/etc/shorewall/routestopped</filename> accordingly.
<warning><para>If you are connected to your firewall from the Internet, do
not issue a <command>shorewall stop</command> command unless you have
added an entry for the IP address that you are connected from to <ulink
url="Documentation.htm#Routestopped"><filename>/etc/shorewall/routestopped</filename></ulink>.
Also, I don&#39;t recommend using <command>shorewall restart</command>; it
is better to create an <ulink url="configuration_file_basics.htm#Levels">alternate
configuration</ulink> and test it using the <ulink
url="starting_and_stopping_shorewall.htm"><command>shorewall try</command>
command</ulink>.</para></warning></para>
</section>
<section>
<title>Additional Recommended Reading</title>
<para>I highly recommend that you review the <ulink
url="configuration_file_basics.htm">Common Configuration File Features</ulink>
page -- it contains helpful tips about Shorewall features than make
administering your firewall easier.</para>
</section>
</article>
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