shorewall_code/Shorewall-docs2/standalone.xml

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

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

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

    <pubdate>2005-07-12</pubdate>

    <copyright>
      <year>2002-2005</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'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... or connected to a LAN and you simply wish to protect your
        Linux system from other systems on that LAN.</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]# <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 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>

    <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 /usr/share/doc/shorewall/default-config/modules 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 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 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 action 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 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>nobogons</emphasis> for your external
      interface, you will want to check the <ulink url="errata.htm">Shorewall
      Errata</ulink> periodically for updates to the
      <filename>/usr/share/shorewall/bogons file</filename>.</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>Shorewall 2.0.0 and later include a collection of actions that can
    be used to quickly allow or deny services. You can find a list of the
    actions included in your version of Shorewall in the file
    <filename>/usr/share/shorewall/actions.std</filename>.</para>

    <para>Those actions that allow a connection begin with
    <quote>Allow</quote>.</para>

    <para>If you wish to enable connections from the internet to your firewall
    and you find an appropriate <quote>Allow</quote> action in
    <filename>/etc/shorewall/actions.std</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>action</emphasis>&gt;  net       fw</programlisting>

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

      <programlisting>#ACTION   SOURCE    DESTINATION     PROTO       DEST PORT(S)
AllowWeb  net       fw
AllowPOP3 net       fw</programlisting>
    </example>

    <para>You may also choose to code your rules directly without using the
    pre-defined actions. This will be necessary in the event that there is not
    a pre-defined action 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 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'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 SSH:</para>

      <programlisting>#ACTION   SOURCE    DESTINATION     PROTO       DEST PORT(S)
AllowSSH  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 beginning
    with Shorewall version 1.3.9 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 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>

    <important>
      <para><emphasis role="bold">If you are running Shorewall 2.1.3 or later,
      you must enable startup by editing /etc/shorewall/shorewall.conf and
      setting STARTUP_ENABLED=Yes.</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'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>1.8</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>