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<article id= "two-interface" >
<articleinfo >
<title > Basic Two-Interface Firewall</title>
<author >
<firstname > Tom</firstname>
<surname > Eastep</surname>
</author>
2004-07-11 01:29:06 +02:00
<pubdate > 2003-07-10</pubdate>
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<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> &
<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>
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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
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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 > <inlinegraphic fileref= "images/BD21298_.gif" format= "GIF" /> </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
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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>
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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>
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<para > <tip > <para > After you have <ulink url= "Install.htm" > installed
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Shorewall</ulink> , download the <ulink
url="http://www1.shorewall.net/pub/shorewall/Samples/">two-interface
sample</ulink> , un-tar it (<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> </tip>
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>
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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>
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<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
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options that are specified for the interfaces. Some hints: <tip > <para > If
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your external interface is <filename class= "devicefile" > ppp0</filename> or
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<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
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<acronym > IP</acronym> address, you can remove <varname > dhcp</varname> from
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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>
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</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 &
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 & 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 > < server local ip address> </emphasis> [:<emphasis > < server port> </emphasis> ] <emphasis > < protocol> </emphasis> <emphasis > < port> </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/< kernel
version> /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
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to use your ISP' s name servers. If your ISP gave you the addresses of
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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
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Shorewall, look in the file <filename > /usr/share/shorewall/actions.std</filename> .
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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)
< action> fw <emphasis > < destination zone> </emphasis> </programlisting> The
general format when not using defined actions is:<programlisting > #ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT fw <emphasis > < destination zone> < protocol> < port> </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>
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<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-> 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>
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<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>
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<para > <inlinegraphic fileref= "images/BD21298_.gif" format= "GIF" /> </para>
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<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
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documentation</ulink> .</para>
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</section>
</article>
