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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><?dbtimestamp format="Y/m/d"?></pubdate>
<copyright>
<year>2002-2009</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 4.4 and
later. If you are running a version of Shorewall earlier than Shorewall
4.4.0 then please see the documentation for that
release.</emphasis></para>
</caution>
<section id="Introduction">
<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 id="System">
<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 id="Before">
<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 id="Conventions">
<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 Debian and it's derivatives
are marked with <inlinegraphic fileref="images/openlogo-nd-25.png"
format="GIF" />.</para>
</section>
</section>
<section id="PPTP">
<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 id="Concepts">
<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>
2011-04-04 04:32:39 +02:00
<para><graphic align="left" fileref="images/openlogo-nd-25.png" />If
you installed using a Shorewall 4.x .deb, the samples are in <emphasis
role="bold"><filename
class="directory">/usr/share/doc/shorewall/examples/one-interface</filename>..</emphasis>
You do not need the shorewall-doc package to have access to the
samples.</para>
</listitem>
</orderedlist>
<warning>
<para><emphasis role="bold">Note to Debian Users</emphasis></para>
2011-04-04 04:32:39 +02:00
<para>You will find that your <filename
class="directory">/etc/shorewall</filename> directory is empty. This is
2011-04-04 04:32:39 +02:00
intentional. If you need configuration files other than those found in
<emphasis role="bold"><filename
class="directory">/usr/share/doc/shorewall/examples/one-interface</filename>,
</emphasis> they 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>
</warning>
<para>As each file is introduced, I suggest that you look at the actual
file on your system and that you look at the <ulink
url="configuration_file_basics.htm#Manpages">man page</ulink> for that
file. For example, to look at the man page for the
<filename>/etc/shorewall/zones</filename> file, type <command>man
shorewall-zones</command> at a shell prompt.</para>
<para>Note: Beginning with Shorewall 4.4.20.1, there are versions of the
sample files that are annotated with the corresponding manpage contents.
These files have names ending in '.annotated'. You might choose to look at
those files instead.</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="manpages/shorewall-zones.html"><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="manpages/shorewall-policy.html"><filename>/etc/shorewall/policy</filename></ulink>
file.</para>
</listitem>
<listitem>
<para>You define exceptions to those default policies in the <ulink
url="manpages/shorewall-rules.html"><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">common
action</ulink> defined for the policy in
<filename>/etc/shorewall/actions</filename> or
<filename>/usr/share/shorewall/actions.std</filename> then that action is
performed before the policy is applied. The purpose of the common action
is two-fold:</para>
<itemizedlist>
<listitem>
<para>It silently drops or rejects harmless common traffic that would
otherwise clutter up your log — Broadcasts for example.</para>
</listitem>
<listitem>
<para>If ensures that traffic critical to correct operation is allowed
through the firewall — ICMP <emphasis>fragmentation-needed</emphasis>
for example.</para>
</listitem>
</itemizedlist>
<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>The word <firstterm>info</firstterm> in the LOG LEVEL column for the
last two policies indicates that packets dropped or rejected under those
policies should be <ulink url="shorewall_logging.html">logged at that
level</ulink>.</para>
<para>At this point, edit your <filename>/etc/shorewall/policy</filename>
and make any changes that you wish.</para>
</section>
<section id="External">
<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>
<caution>
<para>Be sure you know which interface is your external interface. Many
hours have been spent floundering by users who have configured the wrong
interface. If you are unsure, then as root type <command>ip route
ls</command> at the command line. The device listed in the last
(default) route should be your external interface.</para>
<para>Example:</para>
<programlisting>root@lists:~# ip route ls
192.168.2.2 dev tun0 proto kernel scope link src 192.168.2.1
10.13.10.0/24 dev tun1 scope link
192.168.2.0/24 via 192.168.2.2 dev tun0
206.124.146.0/24 dev eth0 proto kernel scope link src 206.124.146.176
10.10.10.0/24 dev tun1 scope link
default via 206.124.146.254 dev <emphasis role="bold">eth0</emphasis>
root@lists:~# </programlisting>
<para>In that example, <filename class="devicefile">eth0</filename> is
the external interface.</para>
</caution>
<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 id="Addresses">
<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>
</section>
<section id="Logging">
<title>Logging</title>
<para>Shorewall does not maintain a log itself but rather relies on your
<ulink url="shorewall_logging.html">system's logging
configuration</ulink>. The following <ulink
url="manpages/shorewall.html">commands</ulink> rely on knowing where
Netfilter messages are logged:</para>
<itemizedlist>
<listitem>
<para><command>shorewall show log</command> (Displays the last 20
Netfilter log messages)</para>
</listitem>
<listitem>
<para><command>shorewall logwatch</command> (Polls the log at a
settable interval</para>
</listitem>
<listitem>
<para><command>shorewall dump</command> (Produces an extensive report
for inclusion in Shorewall problem reports)</para>
</listitem>
</itemizedlist>
<para>It is important that these commands work properly because when you
encounter connection problems when Shorewall is running, the first thing
that you should do is to look at the Netfilter log; with the help of
<ulink url="FAQ.htm#faq17">Shorewall FAQ 17</ulink>, you can usually
resolve the problem quickly.</para>
<para>The Netfilter log location is distribution-dependent:</para>
<itemizedlist>
<listitem>
<para>Debian and its derivatives log Netfilter messages to
<filename>/var/log/kern.log</filename>.</para>
</listitem>
<listitem>
<para>Recent <trademark>SuSE/OpenSuSE</trademark> releases come
preconfigured with syslog-ng and log netfilter messages to
<filename>/var/log/firewall</filename>.</para>
</listitem>
<listitem>
<para>For other distributions, Netfilter messages are most commonly
logged to <filename>/var/log/messages</filename>.</para>
</listitem>
</itemizedlist>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>If you are running a distribution that logs Netfilter messages to a
log other than <filename>/var/log/messages</filename>, then modify the
LOGFILE setting in <filename>/etc/shorewall/shorewall.conf</filename> to
specify the name of your log.</para>
<important>
<para>The LOGFILE setting does not control where the Netfilter log is
maintained -- it simply tells the /sbin/<filename>shorewall</filename>
utility where to find the log.</para>
</important>
</section>
<section id="Modules">
<title>Kernel Module Loading</title>
<para>Beginning in Shorewall 4.4.7,
<filename>/etc/shorewall/shorewall.conf</filename> contains a
LOAD_HELPERS_ONLY option which is set to <option>Yes</option> in the
samples. This causes Shorewall to attempt to load the modules listed in
<filename>/usr/share/shorewall/helpers</filename>. In addition, it sets
<emphasis role="bold">sip_direct_media=0</emphasis> when loading the
nf_conntrack_sip module. That setting is somewhat less secure than
<emphasis role="bold">sip_direct_media=1</emphasis>, but it generally
makes VOIP through the firewall work much better.</para>
<para>The modules in <filename>/usr/share/shorewall/helpers</filename> are
those that are not autoloaded. If your kernel does not support module
autoloading and you want Shorewall to attempt to load all netfilter
modules that it might require, then set LOAD_HELPERS_ONLY=No. That will
cause Shorewall to try to load the modules listed in
<filename>/usr/share/shorewall/modules</filename>. That file does not set
<emphasis role="bold">sip_direct_media=0</emphasis>.</para>
<para>If you need to modify either
<filename>/usr/share/shorewall/helpers</filename> or
<filename>/usr/share/shorewall/modules</filename> then copy the file to
<filename>/etc/shorewall</filename> and modify the copy.</para>
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
<para>Modify the setting of LOAD_HELPER_ONLY as necessary.</para>
</section>
<section id="Open">
<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>
<important>
<para>Be sure to add your rules after the line that reads <emphasis
role="bold">SECTION NEW.</emphasis></para>
</important>
<example id="Example1">
<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 id="Example2">
<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>Disabling your existing Firewall</title>
<para>Before starting Shorewall for the first time, it's a good idea to
stop your existing firewall. On Redhat/CentOS/Fedora:</para>
<programlisting><command>service iptables stop</command></programlisting>
<para>If you are running SuSE, use Yast or Yast2 to stop
SuSEFirewall.</para>
<para>Once you have Shorewall running to your satisfaction, you should
totally disable your existing firewall. On /Redhat/CentOS/Fedora:</para>
<programlisting><command>chkconfig --del iptables</command></programlisting>
</section>
<section id="Starting">
<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.<graphic align="left"
fileref="images/openlogo-nd-25.png" /></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>While you are editing <filename>shorewall.conf</filename>, it is a
good idea to check the value of the SUBSYSLOCK option. You can find a
description of this option by typing 'man shorewall.conf' at a shell
prompt and searching for SUBSYSLOCK.</para>
<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="manpages/shorewall-routestopped.html">/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="manpages/shorewall-routestopped.html"><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>
<para>The firewall will start after your network interface has been
brought up. This leaves a small window between the time that the network
interface is working and when the firewall is controlling connections
through that interface. If this is a concern, you can close that window by
installing the <ulink url="Shorewall-init.html">Shorewall Init
Package</ulink>.</para>
</section>
<section id="Problems">
<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 id="Other">
<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
2009-04-21 00:31:07 +02:00
make administering your firewall easier. Also, <ulink
url="starting_and_stopping_shorewall.htm">Operating Shorewall and
Shorewall Lite</ulink> contains a lot of useful operational hints.</para>
</section>
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