shorewall_code/Shorewall-docs/standalone.xml

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  <articleinfo>
    <title>Standalone Firewall</title>

    <authorgroup>
      <author>
        <firstname>Tom</firstname>

        <surname>Eastep</surname>
      </author>
    </authorgroup>

    <pubdate>2004-01-06</pubdate>

    <copyright>
      <year>2002-2004</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="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>

    <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"><filename>/etc/shorewall/zones</filename></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"><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 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 <filename>/etc/shorewall/common.def</filename>
    are checked.</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>

    <tip>
      <para>If you specify <emphasis>norfc1918</emphasis> for your external
      interface, you will want to check the <ulink url="errata.htm">Shorewall
      Errata</ulink> periodically for updates to the <filename>/etc/shorewall/rfc1918
      file</filename>. Alternatively, you can <ulink url="myfiles.htm#RFC1918">strip
      down your <filename>/etc/shorewall/rfc1918</filename> file as I do</ulink>.</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, 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>.</para>
  </section>

  <section>
    <title>Enabling other Connections</title>

    <para>If you wish to enable connections from the internet to your
    firewall, 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>&#60;protocol&#62;</emphasis>  <emphasis>&#60;port&#62;</emphasis></programlisting>

    <example>
      <title>You want to run a Web Server and a POP3 Server on your firewall
      system:</title>

      <para><programlisting>#ACTION   SOURCE    DESTINATION     PROTO       DEST PORT(S)
ACCEPT    net       fw              tcp          80
ACCEPT    net       fw              tcp          110</programlisting></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>

      <programlisting>#ACTION   SOURCE    DESTINATION     PROTO       DEST PORT(S)
ACCEPT    net       fw              tcp         22</programlisting>
    </important>

    <para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>

    <para>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" /></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&#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 <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>

    <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&#39;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>1.5</revnumber><date>2003-01-05</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>