shorewall_code/docs/standalone.xml

<?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>
        <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>

      <para>You will find that your <filename
      class="directory">/etc/shorewall</filename> directory is empty. This is
      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
    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>
</article>