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teastep 5083f753c4 Explain the obvious to the oblivious 
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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>2006-01-15</pubdate>
<copyright>
<year>2002-2006</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>
<caution>
<para><emphasis role="bold">This article applies to Shorewall 3.0 and
later. If you are running a version of Shorewall earlier than Shorewall
3.0.0 then please see the documentation for that
release.</emphasis></para>
</caution>
<warning>
<para>The sample configuration files included with Shorewall 3.0.0 and
3.0.1 were broken. The first error produced was:</para>
<para><emphasis role="bold">ERROR: No Firewall Zone
Defined</emphasis></para>
<para>There are corrected 'zones' and 'interfaces' files available at
<ulink
url="http://www.shorewall.net/pub/shorewall/3.0/shorewall-3.0.1/errata/one-interface/">http://www.shorewall.net/pub/shorewall/3.0/shorewall-3.0.1/errata/one-interface/</ulink>.</para>
<para>This problem was fixed in Shorewall 3.0.2.</para>
</warning>
<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 <acronym>IP</acronym> address</para>
</listitem>
<listitem>
<para>Connection through Cable Modem, <acronym>DSL</acronym>,
<acronym>ISDN</acronym>, Frame Relay, dial-up... or connected to a
<acronym>LAN</acronym> and you simply wish to protect your Linux
system from other systems on that <acronym>LAN</acronym>.</para>
</listitem>
</itemizedlist>
<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 root, you can use the <command>which</command> command to
check for this program:</para>
<programlisting>[root@gateway root]# <command>which ip</command>
/sbin/ip
[root@gateway root]#</programlisting>
</section>
<section>
<title>Before you start</title>
<para>I recommend that you 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
<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>
</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 <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 only need to deal with a few of these as described in this
guide. After you have <ulink url="Install.htm">installed
Shorewall</ulink>, you can find the Samples as follows:</para>
<orderedlist>
<listitem>
<para>If you installed using an <acronym>RPM</acronym>, the samples
will be in the <filename
class="directory">Samples/one-interface</filename> subdirectory of the
Shorewall documentation directory. If you don't know where the
Shorewall documentation directory is, you can find the samples using
this command:</para>
<programlisting>~# rpm -ql shorewall | fgrep one-interface
/usr/share/doc/packages/shorewall/Samples/one-interface
/usr/share/doc/packages/shorewall/Samples/one-interface/interfaces
/usr/share/doc/packages/shorewall/Samples/one-interface/policy
/usr/share/doc/packages/shorewall/Samples/one-interface/rules
/usr/share/doc/packages/shorewall/Samples/one-interface/zones
~#</programlisting>
</listitem>
<listitem>
<para>If you installed using the tarball, the samples are in the
<filename class="directory">Samples/one-interface</filename> directory
in the tarball.</para>
</listitem>
<listitem>
<para>If you installed using the .deb, the samples are in <filename
class="directory">/usr/share/doc/shorewall/examples/one-interface</filename>.</para>
</listitem>
</orderedlist>
<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 <filename
class="directory">/usr/share/doc/shorewall/default-config/modules</filename>
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 ipv4</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.
When the <filename>/etc/shorewall/zones</filename> file is processed, the
name of the firewall zone (<quote>fw</quote> in the above example) is
stored in the shell variable <firstterm>$FW</firstterm> which may be used
to refer to the firewall zone throughout the Shorewall
configuration.</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 policy 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 <acronym>DSL</acronym>
<quote>Modem</quote>, the <emphasis>External Interface</emphasis> will be
the ethernet adapter (<filename class="devicefile">eth0</filename>) that
is connected to that <quote>Modem</quote> <emphasis
role="underline">unless</emphasis> you connect via
<emphasis>Point-to-Point Protocol over Ethernet</emphasis>
(<acronym>PPPoE</acronym>) or <emphasis>Point-to-Point Tunneling
Protocol</emphasis> (<acronym>PPTP</acronym>) in which case the External
Interface will be a <acronym>PPP</acronym> interface (e.g., <filename
class="devicefile">ppp0</filename>). If you connect via a regular modem,
your External Interface will also be <filename
class="devicefile">ppp0</filename>. If you connect using
<acronym>ISDN</acronym>, your external interface will be <filename
class="devicefile">ippp0</filename>.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>The Shorewall one-interface sample configuration assumes that the
external interface is <filename class="devicefile">eth0</filename>. If
your configuration is different, you will have to modify the sample
<filename>/etc/shorewall/interfaces</filename> file accordingly. While you
are there, you may wish to review the list of options that are specified
for the interface. Some hints:</para>
<tip>
<para>If your external interface is <filename
class="devicefile">ppp0</filename> or <filename
class="devicefile">ippp0</filename>, you can replace the
<quote>detect</quote> in the second column with <quote>-</quote> (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 IP address,
you can remove <quote>dhcp</quote> from the option list.</para>
</tip>
</section>
<section>
<title>IP Addresses</title>
<para>Before going further, we should say a few words about
<emphasis>Internet Protocol</emphasis> (<acronym>IP</acronym>) addresses.
Normally, your <emphasis>Internet Service Provider</emphasis>
(<acronym>ISP</acronym>) will assign you a single <acronym>IP</acronym>
address. That address can be assigned statically, by the <emphasis>Dynamic
Host Configuration Protocol</emphasis> (<acronym>DHCP</acronym>), through
the establishment of your dial-up connection, or during establishment of
your other type of <acronym>PPP</acronym> (<acronym>PPPoA</acronym>,
<acronym>PPPoE</acronym>, etc.) connection.</para>
<para><emphasis role="bold">RFC-1918</emphasis> reserves several
<emphasis>Private</emphasis> <acronym>IP</acronym> 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
<emphasis role="bold">RFC-1918</emphasis>. In some cases though,
<acronym>ISP</acronym>s are assigning these addresses then using
<emphasis>Network Address Translation</emphasis> <emphasis>-
</emphasis><acronym>NAT</acronym>) 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 in 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 <acronym>SSH</acronym>:</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 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 must edit /etc/shorewall/shorewall.conf and set
STARTUP_ENABLED=Yes.</para>
<important>
<para>Users of the .deb package must edit
<filename>/etc/default/shorewall</filename> and set
<varname>STARTUP=1.</varname></para>
</important>
<important>
<para>You must enable startup by editing
<filename>/etc/shorewall/shorewall.conf</filename> and setting
<varname>STARTUP_ENABLED=Yes.</varname></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></ulink> command.</para>
</warning>
</section>
<section>
<title>If it Doesn't Work</title>
<itemizedlist>
<listitem>
<para>Re-check each of the items flagged with a red arrow
above.</para>
</listitem>
<listitem>
<para>Check your <ulink
url="shorewall_logging.html">log</ulink>.</para>
</listitem>
<listitem>
<para>Check the <ulink url="troubleshoot.htm">Troubleshooting
Guide</ulink>.</para>
</listitem>
<listitem>
<para>Check the <ulink url="FAQ.htm">FAQ</ulink>.</para>
</listitem>
</itemizedlist>
</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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