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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="standalone">
<!--$Id$-->
<articleinfo>
<title>Standalone Firewall</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate>2005-09-12</pubdate>
<copyright>
<year>2002-2005</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 Shorewall on a standalone Linux system is very easy 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 one of its most common configurations:</para>
<itemizedlist>
<listitem>
<para>Linux system</para>
</listitem>
<listitem>
<para>Single external IP address</para>
</listitem>
<listitem>
<para>Connection through Cable Modem, DSL, ISDN, Frame Relay,
dial-up... or connected to a LAN and you simply wish to protect your
Linux system from other systems on that LAN.</para>
</listitem>
</itemizedlist>
<section>
<title>Requirements</title>
<para>Shorewall requires that you have the iproute/iproute2 package
installed (on RedHat, the package is called
<emphasis>iproute</emphasis>). You can tell if this package is installed
by the presence of an <emphasis role="bold">ip</emphasis> program on
your firewall system. As root, you can use the <quote>which</quote>
command to check for this program:</para>
<programlisting>[root@gateway root]# <command>which ip</command>
/sbin/ip
[root@gateway root]#</programlisting>
</section>
<section>
<title>Before you start</title>
<para>I recommend that you read through the guide first to familiarize
yourself with what's involved then go back through it again making your
configuration changes.</para>
<caution>
<para>If you edit your configuration files on a Windows system, you
must save them as Unix files if your editor supports that option or
you must run them through dos2unix before trying to use them.
Similarly, if you copy a configuration file from your Windows hard
drive to a floppy disk, you must run dos2unix against the copy before
using it with Shorewall.</para>
<simplelist>
<member><ulink url="http://www.simtel.net/pub/pd/51438.html">Windows
Version of dos2unix</ulink></member>
<member><ulink
url="http://www.megaloman.com/~hany/software/hd2u/">Linux Version of
dos2unix</ulink></member>
</simplelist>
</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>
</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> <emphasis
role="underline">in addition to those described in the steps
below</emphasis>. 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 class="directory">/etc/shorewall</filename> -- for simple
setups, you only need to deal with a few of these as described in this
guide. After you have <ulink url="Install.htm">installed
Shorewall</ulink>, <emphasis role="bold">download the <ulink
url="http://www1.shorewall.net/pub/shorewall/Samples/">one-interface
sample</ulink>, un-tar it (tar -zxvf one-interface.tgz) and and copy the
files to /etc/shorewall (they will replace files with the same names that
were placed in /etc/shorewall during Shorewall
installation)</emphasis>.</para>
<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>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 <emphasis>zones</emphasis>. In the one-interface sample
configuration, only two zones are defined:</para>
<programlisting>#ZONE TYPE OPTIONS IN OUT
# OPTIONS OPTIONS
fw firewall
net plain</programlisting>
<para>Shorewall zones are defined in <ulink
url="Documentation.htm#Zones"><filename>/etc/shorewall/zones</filename></ulink>.</para>
<para>Note that Shorewall recognizes the firewall system as its own zone.
The name of the firewall zone (<emphasis role="bold">fw</emphasis> in the
above example) is stored in the shell variable <firstterm>$FW</firstterm>
which may be used throughout the rest of the Shorewall configuration to
refer to the firewall itself.</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 <ulink
url="Documentation.htm#Policy"><filename>/etc/shorewall/policy</filename></ulink>
file.</para>
</listitem>
<listitem>
<para>You define exceptions to those default policies in the <ulink
url="Documentation.htm#Rules"><filename>/etc/shorewall/rules</filename></ulink>
file.</para>
</listitem>
</itemizedlist>
<para>For each connection request entering the firewall, the request is
first checked against the
<filename><filename>/etc/shorewall/rules</filename></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 one-interface sample has the following policies:</para>
<programlisting>#SOURCE ZONE DESTINATION ZONE POLICY LOG LEVEL LIMIT:BURST
$FW net ACCEPT
net all DROP info
all all REJECT info</programlisting>
<para>The above policy will:</para>
<orderedlist>
<listitem>
<para>allow all connection requests from the firewall to the
internet</para>
</listitem>
<listitem>
<para>drop (ignore) all connection requests from the internet to your
firewall</para>
</listitem>
<listitem>
<para>reject all other connection requests (Shorewall requires this
catchall policy).</para>
</listitem>
</orderedlist>
<para>At this point, edit your <filename>/etc/shorewall/policy</filename>
and make any changes that you wish.</para>
</section>
<section>
<title>External Interface</title>
<para>The firewall has a single network interface. Where Internet
connectivity is through a cable or DSL <quote>Modem</quote>, the
<emphasis>External Interface</emphasis> will be the ethernet adapter
(<emphasis role="bold">eth0</emphasis>) that is connected to that
<quote>Modem</quote> <emphasis role="underline">unless</emphasis> you
connect via <emphasis>Point-to-Point Protocol over Ethernet</emphasis>
(PPPoE) or <emphasis>Point-to-Point Tunneling Protocol</emphasis> (PPTP)
in which case the External Interface will be a <emphasis
role="bold">ppp0</emphasis>. If you connect via a regular modem, your
External Interface will also be <emphasis role="bold">ppp0</emphasis>. If
you connect using ISDN, your external interface will be <emphasis
role="bold">ippp0</emphasis>.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>The Shorewall one-interface sample configuration assumes that the
external interface is <emphasis role="bold">eth0</emphasis>. If your
configuration is different, you will have to modify the sample
/etc/shorewall/interfaces file accordingly. While you are there, you may
wish to review the list of options that are specified for the interface.
Some hints:</para>
<tip>
<para>If your external interface is <emphasis
role="bold">ppp0</emphasis> or <emphasis role="bold">ippp0</emphasis>,
you can replace the <quote>detect</quote> in the second column with
<quote>-</quote>.</para>
</tip>
<tip>
<para>If your external interface is <emphasis
role="bold">ppp0</emphasis> or <emphasis role="bold">ippp0</emphasis> or
if you have a static IP address, you can remove <quote>dhcp</quote> from
the option list.</para>
</tip>
</section>
<section>
<title>IP Addresses</title>
<para>RFC 1918 reserves several <emphasis>Private</emphasis> IP address
ranges for use in private networks:</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>These addresses are sometimes referred to as
<emphasis>non-routable</emphasis> because the Internet backbone routers
will not forward a packet whose destination address is reserved by RFC
1918. In some cases though, ISPs are assigning these addresses then using
<emphasis>Network Address Translation</emphasis> to rewrite packet headers
when forwarding to/from the internet.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>Before starting Shorewall, <emphasis role="bold">you should look at
the IP address of your external interface and if it is one of the above
ranges, you should remove the <quote>norfc1918</quote> option from the
entry in <filename>/etc/shorewall/interfaces</filename>.</emphasis></para>
</section>
<section>
<title>Enabling other Connections</title>
<para>Shorewall includes a collection of macros that can be used to
quickly allow or deny services. You can find a list of the macros included
in your version of Shorewall using the command <command>ls
<filename>/usr/share/shorewall/macro.*</filename></command>.</para>
<para>If you wish to enable connections from the internet to your firewall
and you find an appropriate macro in
<filename>/etc/shorewall/macro.*</filename>, the general format of a rule
in <filename>/etc/shorewall/rules</filename> is:</para>
<programlisting>#ACTION SOURCE DESTINATION PROTO DEST PORT(S)
&lt;<emphasis>macro</emphasis>&gt;/ACCEPT net $FW</programlisting>
<example>
<title>You want to run a Web Server and a IMAP Server on your firewall
system:</title>
<programlisting>#ACTION SOURCE DESTINATION PROTO DEST PORT(S)
Web/ACCEPT net $FW
IMAP/ACCEPT net $FW</programlisting>
</example>
<para>You may also choose to code your rules directly without using the
pre-defined macros. This will be necessary in the event that there is not
a pre-defined macro that meets your requirements. In that case the general
format of a rule in <filename>/etc/shorewall/rules</filename> is:</para>
<programlisting>#ACTION SOURCE DESTINATION PROTO DEST PORT(S)
ACCEPT net $FW <emphasis>&lt;protocol&gt;</emphasis> <emphasis>&lt;port&gt;</emphasis></programlisting>
<example>
<title>You want to run a Web Server and a IMAP Server on your firewall
system:</title>
<para><programlisting>#ACTION SOURCE DESTINATION PROTO DEST PORT(S)
ACCEPT net $FW tcp 80
ACCEPT net $FW tcp 143</programlisting></para>
</example>
<para>If you don't know what port and protocol a particular application
uses, see <ulink url="ports.htm">here</ulink>.</para>
<important>
<para>I don'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:</para>
<programlisting>#ACTION SOURCE DESTINATION PROTO DEST PORT(S)
SSH/ACCEPT net $FW </programlisting>
</important>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>At this point, edit <filename>/etc/shorewall/rules</filename> to add
other connections as desired.</para>
</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>/etc/shorewall/startup_disabled</filename>.</para>
<important>
<para><emphasis role="bold">Users of the .deb package must edit
<filename>/etc/default/shorewall</filename> and set
<quote>startup=1</quote>.</emphasis></para>
</important>
<important>
<para><emphasis role="bold">You must enable startup by editing
/etc/shorewall/shorewall.conf and setting
STARTUP_ENABLED=Yes.</emphasis></para>
</important>
<para>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><ulink
url="Documentation.htm#Routestopped">/etc/shorewall/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>
<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 IP address that you are connected from
to <ulink
url="Documentation.htm#Routestopped"><filename>/etc/shorewall/routestopped</filename></ulink>.
Also, I don't recommend using <quote><command>shorewall
restart</command></quote>; it is better to create an <emphasis><ulink
url="configuration_file_basics.htm#Configs">alternate
configuration</ulink></emphasis> and test it using the <ulink
url="starting_and_stopping_shorewall.htm"><quote><command>shorewall
try</command></quote> command</ulink>.</para>
</warning>
</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>
<appendix>
<title>Revision History</title>
<para><revhistory>
<revision>
<revnumber>2.0</revnumber>
<date>2005-09-12</date>
<authorinitials>TE</authorinitials>
<revremark>More 3.0 Updates</revremark>
</revision>
<revision>
<revnumber>1.9</revnumber>
<date>2005-09-02</date>
<authorinitials>CR</authorinitials>
<revremark>Update for Shorewall 3.0</revremark>
</revision>
<revision>
<revnumber>1.8</revnumber>
<date>2005-07-12</date>
<authorinitials>TE</authorinitials>
<revremark>Change reference to rfc1918 to bogons.</revremark>
</revision>
<revision>
<revnumber>1.7</revnumber>
<date>2004-02-16</date>
<authorinitials>TE</authorinitials>
<revremark>Move /etc/shorewall/rfc1918 to
/usr/share/shorewall.</revremark>
</revision>
<revision>
<revnumber>1.6</revnumber>
<date>2004-02-05</date>
<authorinitials>TE</authorinitials>
<revremark>Update for Shorewall 2.0</revremark>
</revision>
<revision>
<revnumber>1.5</revnumber>
<date>2004-01-05</date>
<authorinitials>TE</authorinitials>
<revremark>Standards Changes</revremark>
</revision>
<revision>
<revnumber>1.4</revnumber>
<date>2003-12-30</date>
<authorinitials>TE</authorinitials>
<revremark>Add tip about /etc/shorewall/rfc1918 updates.</revremark>
</revision>
<revision>
<revnumber>1.3</revnumber>
<date>2003-11-15</date>
<authorinitials>TE</authorinitials>
<revremark>Initial Docbook Conversion</revremark>
</revision>
</revhistory></para>
</appendix>
</article>
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