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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">
<!-- $Id$ -->
<article id="two-interface">
<articleinfo>
<title>Basic Two-Interface Firewall</title>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
<pubdate>2004-09-06</pubdate>
<copyright>
<year>2002</year>
<year>2003</year>
<year>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 is a
fairly straight-forward task if you understand the basics and follow the
documentation.</para>
<para>This guide doesn't attempt to acquaint you with all of the features
of Shorewall. It rather focuses on what is required to configure Shorewall
in its most common configuration:</para>
<itemizedlist mark="bullet" spacing="compact">
<listitem>
<para>Linux system used as a firewall/router for a small local
network.</para>
</listitem>
<listitem>
<para><emphasis role="bold">Single public IP address.</emphasis> 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>
</listitem>
<listitem>
<para>Internet connection through cable modem, DSL, ISDN, Frame Relay,
dial-up ...</para>
</listitem>
</itemizedlist>
<para>Here is a schematic of a typical installation: <figure label="1">
<title>Common two interface firewall configuration</title>
<mediaobject>
<imageobject>
<imagedata fileref="images/basics.png" format="PNG" />
</imageobject>
</mediaobject>
</figure> <tip>
<title>Shorewall and <trademark>Mandrake</trademark> 9.0+</title>
<para>If you are running Shorewall under
<trademark>Mandrake</trademark> 9.0 or later, you can easily configure
the above setup using the <trademark>Mandrake</trademark>
<quote>Internet Connection Sharing</quote> applet. From the
<emphasis><interface>Mandrake Control Center</interface></emphasis>,
select <quote><guimenuitem>Network</guimenuitem> &amp;
<guisubmenu>Internet</guisubmenu></quote> then
<quote><interface>Connection Sharing</interface></quote>.</para>
<para>Note however, that the Shorewall configuration produced by
<emphasis>Mandrake Internet Connection Sharing</emphasis> is strange
and is apt to confuse you if you use the rest of this documentation
(it has two local zones; <varname>loc</varname> and
<varname>masq</varname> where <varname>loc</varname> is empty; this
conflicts with this documentation which assumes a single local zone
<varname>loc</varname>). We therefore recommend that once you have set
up this sharing that you uninstall the <trademark>Mandrake</trademark>
Shorewall RPM and install the one from the <ulink
url="download.htm">download</ulink> page then follow the instructions
in this Guide.</para>
</tip><note>
<para><emphasis role="bold">The above Shorewall Issue is corrected in
Mandrake 10.0 and later.</emphasis></para>
</note> <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. <itemizedlist>
<listitem>
<para><ulink
url="http://www.simtel.net/pub/pd/51438.html"><trademark>Windows</trademark>
Version of <command>dos2unix</command></ulink></para>
</listitem>
<listitem>
<para><ulink
url="http://www.megaloman.com/%7Ehany/software/hd2u/">Linux
Version of <command>dos2unix</command></ulink></para>
</listitem>
</itemizedlist></para>
</caution></para>
<section>
<title>System 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:
<programlisting>[root@gateway root]# <command>which ip</command>
/sbin/ip
[root@gateway root]#</programlisting> I recommend that you first read through
the guide to familiarize yourself with what's involved then go back
through it again making your configuration changes.</para>
</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 <acronym>ADSL</acronym> Modem and you use
<acronym>PPTP</acronym> to communicate with a server in that modem, you
must make the changes recommended <ulink
url="PPTP.htm#PPTP_ADSL">here</ulink> in addition to those detailed below.
<acronym>ADSL</acronym> with <acronym>PPTP</acronym> is most commonly
found in Europe, notably in Austria.</para>
</section>
<section>
<title>Shorewall Concepts</title>
<para></para>
<para>The configuration files for Shorewall are contained in the directory
<filename class="directory">/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 <filename
class="directory">/etc/shorewall</filename> even if you do not modify
those files.</para>
</warning></para>
<para><inlinegraphic fileref="images/BD21298_.gif"
format="GIF" /><important>
<para>After you have <ulink url="Install.htm">installed
Shorewall</ulink>, <emphasis role="bold">download the <ulink
url="http://www1.shorewall.net/pub/shorewall/Samples/">two-interface
sample</ulink>, un-tar it </emphasis>(<command>tar
<option>-zxvf</option>
<filename>two-interfaces.tgz</filename></command>) and and copy the
files to <filename class="directory">/etc/shorewall</filename>
<emphasis role="bold">(these files will replace files with the same
name)</emphasis>.</para>
</important> 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 two-interface sample configuration, the following
zone names are used: <informaltable frame="all" pgwide="0">
<tgroup align="left" cols="2">
<thead valign="middle">
<row valign="middle">
<entry align="left">Name</entry>
<entry align="left">Description</entry>
</row>
</thead>
<tbody valign="middle">
<row valign="middle">
<entry align="left"><varname>net</varname></entry>
<entry align="left">The Internet</entry>
</row>
<row valign="middle">
<entry align="left"><varname>loc</varname></entry>
<entry align="left">Your Local Network</entry>
</row>
</tbody>
</tgroup>
</informaltable> Zones are defined in the <ulink
url="Documentation.htm#Zones"><filename
class="directory">/etc/shorewall/</filename><filename>zones</filename></ulink>
file.</para>
<para>Shorewall also recognizes the firewall system as its own zone - by
default, the firewall itself is known as <emphasis
role="bold"><varname>fw</varname></emphasis>.</para>
<para>Rules about what traffic to allow and what traffic to deny are
expressed in terms of zones. <itemizedlist spacing="compact">
<listitem>
<para>You express your default policy for connections from one zone
to another zone in the <ulink
url="Documentation.htm#Policy"><filename
class="directory">/etc/shorewall/</filename><filename>policy</filename></ulink>
file.</para>
</listitem>
<listitem>
<para>You define exceptions to those default policies in the <ulink
url="Documentation.htm#Rules"><filename
class="directory">/etc/shorewall/</filename><filename>rules</filename></ulink>
file.</para>
</listitem>
</itemizedlist> For each connection request entering the firewall, the
request is first checked against the <filename
class="directory">/etc/shorewall/</filename><filename>rules</filename>
file. If no rule in that file matches the connection request then the
first policy in <filename
class="directory">/etc/shorewall/</filename><filename>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
class="directory">/etc/shorewall/</filename><filename>policy</filename>
file included with the two-interface sample has the following policies:
<programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
loc net ACCEPT
net all DROP info
all all REJECT info</programlisting> In the two-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. <programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
fw net ACCEPT</programlisting> The above policy will:
<itemizedlist>
<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>
</itemizedlist> <inlinegraphic fileref="images/BD21298_.gif"
format="GIF" /></para>
<para>At this point, edit your <filename
class="directory">/etc/shorewall/</filename><filename>policy</filename>
and make any changes that you wish.</para>
</section>
<section>
<title>Network Interfaces</title>
<mediaobject>
<imageobject>
<imagedata fileref="images/basics.png" format="PNG" />
</imageobject>
</mediaobject>
<para>The firewall has two network interfaces. Where Internet connectivity
is through a cable or <acronym>DSL</acronym> <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
<emphasis>Point-to-Point Protocol</emphasis> over Ethernet
(<acronym>PPPoE</acronym>) or <emphasis>Point-to-Point Tunneling
Protocol</emphasis> (<acronym>PPTP</acronym>) 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 via
<acronym>ISDN</acronym>, your 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
class="directory">/etc/shorewall/</filename><filename>shorewall.conf</filename>.</para>
<para>Your <emphasis>Internal Interface</emphasis> will be an ethernet
adapter (<filename class="devicefile">eth1</filename> or <filename
class="devicefile">eth0</filename>) and will be connected to a hub or
switch. Your other computers will be connected to the same hub/switch
(note: If you have only a single internal system, you can connect the
firewall directly to the computer using a cross-over cable). <warning>
<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
class="directory">/etc/shorewall/</filename><filename>interfaces</filename>
for all interfaces connected to the common hub/switch. Using such a
setup with a production firewall is strongly recommended
against.</para>
</warning> <inlinegraphic fileref="images/BD21298_.gif"
format="GIF" /></para>
<para>The Shorewall two-interface sample configuration assumes that the
external interface is <filename class="devicefile">eth0</filename> and the
internal interface is <filename class="devicefile">eth1</filename>. If
your configuration is different, you will have to modify the sample
<filename
class="directory">/etc/shorewall/</filename><filename>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: <tip>
<para>If your external interface is <filename
class="devicefile">ppp0</filename> or <filename
class="devicefile">ippp0</filename>, you can replace the
<varname>detect</varname> in the second column with a <quote>-</quote>
(minus 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
<acronym>IP</acronym> address, you can remove <varname>dhcp</varname>
from the option list.</para>
</tip><tip>
<para>If your internal interface is a bridge create using the
<command>brctl</command> utility then you must add the
<varname>routeback</varname> option to 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></para>
</section>
<section>
<title>IP Addresses</title>
<para>Before going further, we should say a few words about Internet
Protocol (<acronym>IP</acronym>) addresses. Normally, your
<acronym>ISP</acronym> will assign you a single Public IP address. This
address may be assigned via the Dynamic Host Configuration Protocol
(<acronym>DHCP</acronym>) or as part of establishing your connection when
you dial in (standard modem) or establish your <acronym>PPP</acronym>
connection. In rare cases, your <acronym>ISP</acronym> may assign you a
static <acronym>IP</acronym> address; that means that you configure your
firewall's external interface to use that address permanently. However
your external 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 in your internal network (the Internal Interface on your
firewall plus your other computers). <emphasis role="bold">RFC
1918</emphasis> reserves several <emphasis>Private</emphasis>
<acronym>IP</acronym> address ranges for this purpose: <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> <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 'norfc1918' option from the external interface's entry in <filename
class="directory">/etc/shorewall/</filename><filename>interfaces</filename>.</para>
<para>You will want to assign your addresses from the same sub-network
(subnet). For our purposes, we can consider a subnet to consists of a
range of addresses <varname>x.y.z.0 - x.y.z.255</varname>. Such a subnet
will have a Subnet Mask of <systemitem
class="netmask">255.255.255.0</systemitem>. The address
<varname>x.y.z.0</varname> is reserved as the <emphasis>Subnet
Address</emphasis> and <varname>x.y.z.255</varname> is reserved as the
<emphasis>Subnet Broadcast Address</emphasis>. In Shorewall, a subnet is
described using <ulink url="shorewall_setup_guide.htm#Subnets">Classless
InterDomain Routing (CIDR) notation</ulink> with consists of the subnet
address followed by <varname>/24</varname>. The <quote>24</quote> refers
to the number of consecutive leading <quote>1</quote> bits from the left
of the subnet mask. <informaltable frame="all" label="Example sub-network"
pgwide="0">
<!--
Orientation types for tables are not supported by fop yet so we'll fake it by using boldface on left side entries.
-->
<tgroup align="left" cols="2">
<tbody valign="middle">
<row valign="middle">
<entry align="left"><emphasis
role="bold">Range:</emphasis></entry>
<entry><systemitem class="ipaddress">10.10.10.0</systemitem> -
<systemitem class="ipaddress">10.10.10.255</systemitem></entry>
</row>
<row>
<entry align="left"><emphasis role="bold">Subnet
Address:</emphasis></entry>
<entry align="left"><systemitem
class="netmask">10.10.10.0</systemitem></entry>
</row>
<row>
<entry align="left"><emphasis role="bold">Broadcast
Address:</emphasis></entry>
<entry align="left"><systemitem
class="ipaddress">10.10.10.255</systemitem></entry>
</row>
<row>
<entry align="left"><emphasis role="bold">CIDR
Notation:</emphasis></entry>
<entry align="left"><systemitem
class="ipaddress">10.10.10.0/24</systemitem></entry>
</row>
</tbody>
</tgroup>
</informaltable> 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 (computer 1 and computer 2 in the above
diagram) should be configured with their default gateway to be the
<acronym>IP</acronym> address of the firewall's internal interface.</para>
<para>The foregoing short discussion barely scratches the surface
regarding subnetting and routing. If you are interested in learning more
about <acronym>IP</acronym> addressing and routing, I highly recommend
<quote>IP Fundamentals: What Everyone Needs to Know about Addressing &amp;
Routing</quote>, Thomas A. Maufer, Prentice-Hall, 1999, ISBN 0-13-975483-0
(<ulink
url="http://www.phptr.com/browse/product.asp?product_id={58D4F6D4-54C5-48BA-8EDD-86EBD7A42AF6}">link</ulink>).</para>
<para>The remainder of this quide will assume that you have configured
your network as shown here: <mediaobject>
<imageobject>
<imagedata fileref="images/basics1.png" format="PNG" />
</imageobject>
</mediaobject> The default gateway for computer's 1 &amp; 2 would be
<systemitem class="ipaddress">10.10.10.254</systemitem>. <warning>
<para>Your <acronym>ISP</acronym> might assign your external interface
an <emphasis role="bold">RFC 1918</emphasis> 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.</para>
</warning></para>
</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't forward packets
which have an RFC-1918 destination address. When one of your local systems
(let's assume computer 1) sends a connection request to an internet host,
the firewall must perform <emphasis>Network Address Translation</emphasis>
(<acronym>NAT</acronym>). The firewall rewrites the source address in the
packet to be the address of the firewall's external interface; in other
words, the firewall makes it look as if the firewall itself is initiating
the connection. This is necessary so that the destination host will be
able to route return packets back to the firewall (remember that packets
whose destination address is reserved by RFC 1918 can't be routed across
the internet so the remote host can't address its response to computer 1).
When the firewall receives a return packet, it rewrites the destination
address back to <systemitem class="ipaddress">10.10.10.1</systemitem> and
forwards the packet on to computer 1.</para>
<para>On Linux systems, the above process is often referred to as
<emphasis>IP Masquerading</emphasis> but you will also see the term
<emphasis>Source Network Address Translation</emphasis>
(<acronym>SNAT</acronym>) 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><acronym>SNAT</acronym></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 <emphasis>Masquerading</emphasis> and
<emphasis><acronym>SNAT</acronym></emphasis> are configured with entries
in the <filename
class="directory">/etc/shorewall/</filename><filename>masq</filename>
file. You will normally use Masquerading if your external
<acronym>IP</acronym> is dynamic and <acronym>SNAT</acronym> if the
<acronym>IP</acronym> is static.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>If your external firewall interface is <filename
class="devicefile">eth0</filename>, 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 the first column to the name of your external interface and the
second column to the name of your internal interface.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>If your external <acronym>IP</acronym> 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 <acronym>IP</acronym> 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
spacing="compact">
<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 may be to run one or more servers on your local
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
the 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 <acronym>SNAT</acronym> 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>
(<acronym>DNAT</acronym>). You configure port forwarding using
<acronym>DNAT</acronym> 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 loc:<emphasis>&lt;server local ip address&gt;</emphasis>[:<emphasis>&lt;server port&gt;</emphasis>] <emphasis>&lt;protocol&gt;</emphasis> <emphasis>&lt;port&gt;</emphasis></programlisting>
<example label="1">
<title>Web Server</title>
<para>You run a Web Server on computer 2 and you want to forward
incoming <acronym>TCP</acronym> port 80 to that system:
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
DNAT net loc:10.10.10.2 tcp 80</programlisting></para>
</example> <example label="2">
<title>FTP Server</title>
<para>You run an <acronym>FTP</acronym> Server on computer 1 so you
want to forward incoming <acronym>TCP</acronym> port 21 to that
system: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
DNAT net loc:10.10.10.1 tcp 21</programlisting> For
<acronym>FTP</acronym>, you will also need to have
<acronym>FTP</acronym> connection tracking and <acronym>NAT</acronym>
support in your kernel. For vendor-supplied kernels, this means that
the <filename class="libraryfile">ip_conntrack_ftp</filename> and
<filename class="libraryfile">ip_nat_ftp</filename> modules must be
loaded. Shorewall will automatically load these modules if they are
available and located in the standard place under <filename
class="directory">/lib/modules/&lt;kernel
version&gt;/kernel/net/ipv4/netfilter</filename>.</para>
</example> A couple of important points to keep in mind: <itemizedlist>
<listitem>
<para>You must test the above rule from a client outside of your
local network (i.e., don't test from a browser running on computers
1 or 2 or on the firewall). If you want to be able to access your
web server and/or <acronym>FTP</acronym> server from inside your
firewall using the <acronym>IP</acronym> address of your external
interface, see <ulink url="FAQ.htm#faq2">Shorewall FAQ
#2</ulink>.</para>
</listitem>
<listitem>
<para>Many <acronym>ISP</acronym>s 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.</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
DNAT net loc:10.10.10.2:80 tcp 5000</programlisting>
</listitem>
</itemizedlist> <inlinegraphic fileref="images/BD21298_.gif"
format="GIF" /></para>
<para>At this point, modify <filename
class="directory">/etc/shorewall/</filename><filename>rules</filename> to
add any <acronym>DNAT</acronym> rules that you require.</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's <emphasis>Domain Name Service</emphasis>
(<acronym>DNS</acronym>) resolver will be automatically configured (e.g.,
the <filename
class="directory">/etc/</filename><filename>resolv.conf</filename> file
will be written). Alternatively, your ISP may have given you the
<acronym>IP</acronym> address of a pair of <acronym>DNS</acronym> name
servers for you to manually configure as your primary and secondary name
servers. Regardless of how <acronym>DNS</acronym> gets configured on your
firewall, it is your responsibility to configure the resolver in your
internal systems. You can take one of two approaches: <itemizedlist
spacing="compact">
<listitem>
<para>You can configure your internal systems to use your ISP's name
servers. If your ISP gave you the addresses of their servers or if
those addresses are available on their web site, you can configure
your internal systems to use those addresses. If that information
isn't available, look in /etc/resolv.conf on your firewall system --
the name servers are given in "nameserver" records in that
file.</para>
</listitem>
<listitem>
<para><anchor id="cachingdns" /> You can configure a
<emphasis>Caching Name Server</emphasis> on your firewall.
<trademark>Red Hat</trademark> has an <acronym>RPM</acronym> for a
caching name server (the <acronym>RPM</acronym> also requires the
<command>bind</command><acronym>RPM</acronym>) and for Bering users,
there is <command>dnscache.lrp</command>. If you take this approach,
you configure your internal systems to use the firewall itself as
their primary (and only) name server. You use the internal
<acronym>IP</acronym> address of the firewall (<systemitem
class="ipaddress">10.10.10.254</systemitem> in the example above)
for the name server address. To allow your local systems to talk to
your caching name server, you must open port 53 (both
<acronym>UDP</acronym> and <acronym>TCP</acronym>) from the local
network to the firewall; you do that by adding the following rules
in <filename
class="directory">/etc/shorewall/</filename><filename>rules</filename>.
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowDNS loc fw</programlisting></para>
</listitem>
</itemizedlist></para>
</section>
<section>
<title>Other Connections</title>
<para>The two-interface sample includes the following rules:
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowDNS fw net</programlisting>This rule allows
<acronym>DNS</acronym> access from your firewall and may be removed if you
uncommented the line in <filename
class="directory">/etc/shorewall/</filename><filename>policy</filename>
allowing all connections from the firewall to the internet.</para>
<para>In the rule 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>/usr/share/shorewall/actions.std</filename>. Those actions that
accept connection requests have names that begin with
<quote>Allow</quote>.</para>
<para>You don'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 the rule shown above could also have been coded
as follows:<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT fw net udp 53
ACCEPT fw net tcp 53</programlisting></para>
<para>In cases where Shorewall doesn't include a defined action to meet
your needs, you can either define the action yourself or you can simply
code the appropriate rules directly.</para>
<para>The sample also includes: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowSSH loc fw</programlisting> That rule allows you to run an
<acronym>SSH</acronym> server on your firewall and connect to that server
from your local systems.</para>
<para>If you wish to enable other connections from your firewall to other
systems, the general format using an <quote>Allow</quote> action is:
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
&lt;action&gt; fw <emphasis>&lt;destination zone&gt;</emphasis></programlisting>The
general format when not using defined actions is:<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT fw <emphasis>&lt;destination zone&gt; &lt;protocol&gt; &lt;port&gt;</emphasis></programlisting><example>
<title>Web Server on Firewall</title>
<para>You want to run a Web Server on your firewall system:
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowWeb net fw
AllowWeb loc fw</programlisting> Those two rules would of course be in
addition to the rules listed above under <quote><link
linkend="cachingdns">You can configure a Caching Name Server on your
firewall</link></quote>.</para>
</example> If you don't know what port and protocol a particular
application uses, look <ulink url="ports.htm">here</ulink>. <important>
<para>I don't recommend enabling <command>telnet</command> to/from the
internet because it uses clear text (even for login!). If you want
shell access to your firewall from the internet, use
<acronym>SSH</acronym>:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
AllowSSH net fw</programlisting>
</important> <inlinegraphic fileref="images/leaflogo.gif"
format="GIF" />Bering users will want to add the following two rules to be
compatible with Jacques's Shorewall configuration.<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT loc fw udp 53 #Allow DNS Cache to work
ACCEPT loc fw tcp 80 #Allow Weblet to work</programlisting>
<inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>Now edit your <filename
class="directory">/etc/shorewall/</filename><filename>rules</filename>
file to add or delete 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-&gt;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's interfaces from the local network.
The only conclusion you can draw from such pinging success is that the
link between the local system and the firewall works and that you
probably have the local system's default gateway set correctly.</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'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
class="directory">/etc/shorewall/</filename><filename>startup_disabled</filename>.
<important>
<para>Users of the .deb package must edit <filename
class="directory">/etc/default/</filename><filename>shorewall</filename>
and set <varname>startup=1</varname>.</para>
</important> The firewall is started using the <quote><command>shorewall
start</command></quote> command and stopped using
<quote><command>shorewall stop</command></quote>. When the firewall is
stopped, routing is enabled on those hosts that have an entry in <filename
class="directory">/etc/shorewall/</filename><filename><ulink
url="Documentation.htm#Routestopped">routestopped</ulink></filename>. A
running firewall may be restarted using the <quote><command>shorewall
restart</command></quote> command. If you want to totally remove any trace
of Shorewall from your Netfilter configuration, use
<quote><command>shorewall clear</command></quote>.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>The two-interface sample assumes that you want to enable routing
to/from <filename class="devicefile">eth1</filename> (the local network)
when Shorewall is stopped. If your local network isn't connected to
<filename class="devicefile">eth1</filename> or if you wish to enable
access to/from other hosts, change <filename
class="directory">/etc/shorewall/</filename><filename>routestopped</filename>
accordingly. <warning>
<para>If you are connected to your firewall from the internet, do not
issue a <quote><command>shorewall stop</command></quote> command
unless you have added an entry for the <acronym>IP</acronym> address
that you are connected from to <filename
class="directory">/etc/shorewall/</filename><filename>routestopped</filename>.
Also, I don't recommend using <quote><command>shorewall
restart</command></quote>; it is better to create an alternate
configuration and test it using the <quote><command>shorewall
try</command></quote> command.</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
page</ulink> -- it contains helpful tips about Shorewall features than
make administering your firewall easier.</para>
</section>
<section>
<title>Adding a Wireless Segment to your Two-Interface Firewall</title>
<para>Once you have the two-interface setup working, the next logical step
is to add a Wireless Network. The first step involves adding an additional
network card to your firewall, either a Wireless card or an ethernet card
that is connected to a Wireless Access Point.<caution>
<para>When you add a network card, it won't necessarily be detected as
the next highest ethernet interface. For example, if you have two
ethernet cards in your system (<filename
class="devicefile">eth0</filename> and <filename
class="devicefile">eth1</filename>) and you add a third card that uses
the same driver as one of the other two, that third card won't
necessarily be detected as <filename
class="devicefile">eth2</filename>; it could rather be detected as
<filename class="devicefile">eth0</filename> or <filename
class="devicefile">eth1</filename>! You can either live with that or
you can shuffle the cards around in the slots until the new card is
detected as <filename class="devicefile">eth2</filename>.</para>
</caution></para>
<para>Your new network will look similar to what is shown in the following
figure.<graphic fileref="images/basics2.png" /></para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>The first thing to note is that the computers in your wireless
network will be in a different subnet from those on your wired local LAN.
In the above example, we have chosen to use the network 10.10.11.0/24.
Computers 3 and 4 would be configured with a default gateway IP address of
10.10.11.254.</para>
<para>Second, we have chosen to include the wireless network as part of
the local zone. Since Shorewall allows intra-zone traffic by default,
traffic may flow freely between the local wired network and the wireless
network.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>There are only two changes that need to be made to the Shorewall
configuration:</para>
<itemizedlist>
<listitem>
<para>An entry needs to be added to
<filename>/etc/shorewall/interfaces</filename> for the wireless
network interface. If the wireless interface is <filename
class="devicefile">wlan0</filename>, the entry might look like:</para>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
loc wlan0 detect maclist</programlisting>
<para>As shown in the above entry, I recommend using the <ulink
url="MAC_Validation.html">maclist option</ulink> for the wireless
segment. By adding entries for computers 3 and 4 in
<filename>/etc/shorewall/maclist</filename>, you help ensure that your
neighbors aren't getting a free ride on your internet connection.
Start by omitting that option; when you have everything working, then
add the option and configure your
<filename>/etc/shorewall/maclist</filename> file.</para>
</listitem>
<listitem>
<para>You need to add an entry to the
<filename>/etc/shorewall/masq</filename> file to masquerade traffic
from the wireless network to the internet. If your internet interface
is <filename class="devicefile">eth0</filename> and your wireless
interface is <filename class="devicefile">wlan0</filename>, the entry
would be:</para>
<programlisting>#INTERFACE SUBNET ADDRESS
eth0 wlan0</programlisting>
</listitem>
</itemizedlist>
<para>One other thing to note. To get <trademark>Microsoft</trademark>
networking working between the wireless and wired networks, you will need
either a WINS server or a PDC. I personally use Samba configured as a WINS
server running on my firewall. Running a WINS server on your firewall
requires the rules listed in the <ulink url="samba.htm">Shorewall/Samba
documentation</ulink>.</para>
</section>
</article>
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