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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>2003-11-15</pubdate>
<copyright>
<year>2002</year>
<year>2003</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&#39;t attempt to acquaint you with all of the
features of Shorewall. It rather focuses on what is required to configure
Shorewall in one of its 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...</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]# which ip
/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&#39;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="???">Windows Version of dos2unix</ulink></member>
<member><ulink url="???">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" /> If you
have an ADSL Modem and you use PPTP to communicate with a server in that
modem, you must make the <ulink url="PPTP.htm#PPTP_ADSL">changes
recommended here</ulink> in addition to those described in the steps
below. ADSL with PPTP is most commonly found in Europe, notably in
Austria.</para>
</section>
<section>
<title>Shorewall Concepts</title>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /> The
configuration files for Shorewall are contained in the directory
/etc/shorewall -- 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>
<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 one zone is defined:</para>
<informaltable>
<tgroup cols="2">
<thead>
<row>
<entry align="center">Name</entry>
<entry align="center">Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>net</entry>
<entry>The Internet</entry>
</row>
</tbody>
</tgroup>
</informaltable>
<para>Shorewall zones are defined in <ulink url="Documentation.htm#Zones">/etc/shorewall/zones</ulink>.</para>
<para>Shorewall also recognizes the firewall system as its own zone - by
default, the firewall itself is known as <emphasis role="bold">fw</emphasis>.</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">/etc/shorewall/policy</ulink>
file.</para>
</listitem>
<listitem>
<para>You define exceptions to those default policies in the <ulink
url="Documentation.htm#Rules">/etc/shorewall/rules</ulink> file.</para>
</listitem>
</itemizedlist>
<para>For each connection request entering the firewall, the request is
first checked against the /etc/shorewall/rules file. If no rule in that
file matches the connection request then the first policy in
/etc/shorewall/policy that matches the request is applied. If that policy
is REJECT or DROP the request is first checked against the rules in
/etc/shorewall/common if that file exists; otherwise the rules in
/etc/shorewall/common.def are checked.</para>
<para>The /etc/shorewall/policy file included with the one-interface
sample has the following policies:</para>
<table>
<title>one-interface sample /etc/shorewall/policy</title>
<tgroup cols="5">
<thead>
<row>
<entry align="center">SOURCE ZONE</entry>
<entry align="center">DESTINATION ZONE</entry>
<entry align="center">POLICY</entry>
<entry align="center">LOG LEVEL</entry>
<entry align="center">LIMIT:BURST</entry>
</row>
</thead>
<tbody>
<row>
<entry>fw</entry>
<entry>net</entry>
<entry>ACCEPT</entry>
<entry></entry>
<entry></entry>
</row>
<row>
<entry>net</entry>
<entry>all</entry>
<entry>DROP</entry>
<entry>info</entry>
<entry></entry>
</row>
<row>
<entry>all</entry>
<entry>all</entry>
<entry>REJECT</entry>
<entry>info</entry>
<entry></entry>
</row>
</tbody>
</tgroup>
</table>
<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 /etc/shorewall/policy 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" /> 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" /> 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
<quote>norfc1918</quote> option from the entry in
/etc/shorewall/interfaces.</para>
</section>
<section>
<title>Enabling other Connections</title>
<para>If you wish to enable connections from the internet to your
firewall, the general format is:</para>
<table>
<title>/etc/shorewall/rules</title>
<tgroup cols="7">
<thead>
<row>
<entry align="center">ACTION</entry>
<entry align="center">SOURCE</entry>
<entry align="center">DESTINATION</entry>
<entry align="center">PROTOCOL</entry>
<entry align="center">PORT</entry>
<entry align="center">SOURCE PORT</entry>
<entry align="center">ORIGINAL ADDRESS</entry>
</row>
</thead>
<tbody>
<row>
<entry>ACCEPT</entry>
<entry>net</entry>
<entry>fw</entry>
<entry>&#60;<emphasis>protocol</emphasis>&#62;</entry>
<entry>&#60;<emphasis>port</emphasis>&#62;</entry>
<entry></entry>
<entry></entry>
</row>
</tbody>
</tgroup>
</table>
<example>
<title>You want to run a Web Server and a POP3 Server on your firewall
system:</title>
<para><table><title>/etc/shorewall/rules</title><tgroup cols="7"><thead><row><entry
align="center">ACTION</entry><entry align="center">SOURCE</entry><entry
align="center">DESTINATION</entry><entry align="center">PROTOCOL</entry><entry
align="center">PORT</entry><entry align="center">SOURCE PORT</entry><entry
align="center">ORIGINAL ADDRESS</entry></row></thead><tbody><row><entry>ACCEPT</entry><entry>net</entry><entry>fw</entry><entry>tcp</entry><entry>80</entry><entry></entry><entry></entry></row><row><entry>ACCEPT</entry><entry>net</entry><entry>fw</entry><entry>tcp</entry><entry>110</entry><entry></entry><entry></entry></row></tbody></tgroup></table></para>
</example>
<para>If you don&#39;t know what port and protocol a particular
application uses, see <ulink url="ports.htm">here</ulink>.</para>
<important>
<para>I don&#39;t recommend enabling telnet to/from the internet because
it uses clear text (even for login!). If you want shell access to your
firewall from the internet, use SSH:</para>
<table>
<title>/etc/shorewall/rules</title>
<tgroup cols="7">
<thead>
<row>
<entry align="center">ACTION</entry>
<entry align="center">SOURCE</entry>
<entry align="center">DESTINATION</entry>
<entry align="center">PROTOCOL</entry>
<entry align="center">PORT</entry>
<entry align="center">SOURCE PORT</entry>
<entry align="center">ORIGINAL ADDRESS</entry>
</row>
</thead>
<tbody>
<row>
<entry>ACCEPT</entry>
<entry>net</entry>
<entry>fw</entry>
<entry>tcp</entry>
<entry>22</entry>
<entry></entry>
<entry></entry>
</row>
</tbody>
</tgroup>
</table>
</important>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /> At this
point, edit /etc/shorewall/rules to add other connections as desired.</para>
</section>
<section>
<title>Starting and Stopping Your Firewall</title>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /> The
<ulink url="Install.htm">installation procedure</ulink> configures your
system to start Shorewall at system boot but beginning with Shorewall
version 1.3.9 startup is disabled so that your system won&#39;t try to
start Shorewall before configuration is complete. Once you have completed
configuration of your firewall, you can enable Shorewall startup by
removing the file /etc/shorewall/startup_disabled.</para>
<important>
<para><emphasis role="bold">Users of the .deb package must edit
/etc/default/shorewall and set <quote>startup=1</quote>.</emphasis></para>
</important>
<para>The firewall is started using the <quote>shorewall start</quote>
command and stopped using <quote>shorewall stop</quote>. When the firewall
is stopped, routing is enabled on those hosts that have an entry in <ulink
url="Documentation.htm#Routestopped">/etc/shorewall/routestopped</ulink>.
A running firewall may be restarted using the <quote>shorewall restart</quote>
command. If you want to totally remove any trace of Shorewall from your
Netfilter configuration, use <quote>shorewall clear</quote>.</para>
<warning>
<para>If you are connected to your firewall from the internet, do not
issue a <quote>shorewall stop</quote> command unless you have added an
entry for the IP address that you are connected from to <ulink
url="Documentation.htm#Routestopped">/etc/shorewall/routestopped</ulink>.
Also, I don&#39;t recommend using <quote>shorewall restart</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>shorewall
try</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>
</article>