shorewall_code/Shorewall-docs/three-interface.xml

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<article id="three-interface">
  <!--$Id$-->

  <articleinfo>
    <title>Three-Interface Firewall</title>

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

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

    <pubdate>2004-02-12</pubdate>

    <copyright>
      <year>2002</year>

      <year>2003</year>

      <year>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 a Linux system as a firewall for a small network with DMZ
    is a fairly straight-forward task 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 more popular configurations:</para>

    <itemizedlist>
      <listitem>
        <para>Linux system used as a firewall/router for a small local
        network.</para>
      </listitem>

      <listitem>
        <para>Single public IP address.</para>

        <note>
          <para>If you have more than one public IP address, this is not the
          guide you want -- see the <ulink url="shorewall_setup_guide.htm">Shorewall
          Setup Guide</ulink> instead.</para>
        </note>
      </listitem>

      <listitem>
        <para>DMZ connected to a separate ethernet interface.</para>
      </listitem>

      <listitem>
        <para>Connection through DSL, Cable Modem, ISDN, Frame Relay, dial-up,
        ...</para>
      </listitem>
    </itemizedlist>

    <para>Here is a schematic of a typical installation.</para>

    <figure>
      <title>schematic of a typical installation</title>

      <mediaobject>
        <imageobject>
          <imagedata align="center" fileref="images/dmz1.png" format="PNG" />
        </imageobject>
      </mediaobject>
    </figure>

    <section>
      <title>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 <systemitem class="username">root</systemitem>, 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 first read through the guide 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
        <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.</para>

        <itemizedlist>
          <listitem>
            <para><ulink url="http://www.simtel.net/pub/pd/51438.html">Windows
            Version of dos2unix</ulink></para>
          </listitem>

          <listitem>
            <para><ulink url="http://www.megaloman.com/%7Ehany/software/hd2u/">Linux
            Version of dos2unix</ulink></para>
          </listitem>
        </itemizedlist>
      </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>

      <para>Configuration notes that are unique to LEAF/Bering are marked with
      <inlinegraphic fileref="images/leaflogo.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> in addition to those detailed below. 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>/etc/shorewall</filename> -- for simple setups, you will only
    need to deal with a few of these as described in this guide. After you
    have installed Shorewall, download the three-interface sample, un-tar it (<command>tar
    <option>-zxvf</option> <filename>three-interfaces.tgz</filename></command>)
    and and copy the files to <filename>/etc/shorewall</filename> (the files
    will replace files with the same names that were placed in
    <filename>/etc/shorewall</filename> when Shorewall was installed).</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 zones. In the three-interface sample configuration, the following
    zone names are used:</para>

    <informaltable>
      <tgroup align="left" cols="2">
        <thead valign="middle">
          <row>
            <entry align="center">Name</entry>

            <entry align="center">Description</entry>
          </row>
        </thead>

        <tbody>
          <row>
            <entry>net</entry>

            <entry>The Internet</entry>
          </row>

          <row>
            <entry>loc</entry>

            <entry>Your Local Network</entry>
          </row>

          <row>
            <entry>dmz</entry>

            <entry>Demilitarized Zone</entry>
          </row>
        </tbody>
      </tgroup>
    </informaltable>

    <para>Zone names are defined in <filename>/etc/shorewall/zones</filename>.</para>

    <para>Shorewall also recognizes the firewall system as its own zone - by
    default, the firewall itself is known as <varname>fw</varname>.</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 <filename>/etc/shorewall/policy</filename> file.</para>
      </listitem>

      <listitem>
        <para>You define exceptions to those default policies in the
        <filename>/etc/shorewall/rules</filename> file.</para>
      </listitem>
    </itemizedlist>

    <para>For each connection request entering the firewall, the request is
    first checked against the <filename>/etc/shorewall/rules</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 <filename>/etc/shorewall/common</filename> if
    that file exists; otherwise the file <filename>/etc/shorewall/common.def</filename>
    is checked</para>

    <para>The <filename>/etc/shorewall/policy</filename> file included with
    the three-interface sample has the following policies:</para>

    <programlisting>#SOURCE    DEST        POLICY      LOG LEVEL    LIMIT:BURST
loc        net         ACCEPT
net        all         DROP        info
all        all         REJECT      info</programlisting>

    <important>
      <para>In the three-interface sample, the line below is included but
      commented out. If you want your firewall system to have full access to
      servers on the internet, uncomment that line.</para>

      <programlisting>#SOURCE    DEST        POLICY      LOG LEVEL    LIMIT:BURST
fw         net         ACCEPT</programlisting>
    </important>

    <para>The above policy will:</para>

    <orderedlist>
      <listitem>
        <para>allow all connection requests from your local network to the
        internet</para>
      </listitem>

      <listitem>
        <para>drop (ignore) all connection requests from the internet to your
        firewall or local network</para>
      </listitem>

      <listitem>
        <para>optionally accept all connection requests from the firewall to
        the internet (if you uncomment the additional policy)</para>
      </listitem>

      <listitem>
        <para>reject all other connection requests.</para>
      </listitem>
    </orderedlist>

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

    <para>At this point, edit your <filename>/etc/shorewall/policy</filename>
    file and make any changes that you wish.</para>
  </section>

  <section>
    <title>Network Interfaces</title>

    <figure>
      <title>DMZ</title>

      <mediaobject>
        <imageobject>
          <imagedata align="center" fileref="images/dmz1.png" format="PNG" />
        </imageobject>
      </mediaobject>
    </figure>

    <para>The firewall has three network interfaces. Where Internet
    connectivity is through a cable or DSL <quote>Modem</quote>, the External
    Interface will be the ethernet adapter that is connected to that
    <quote>Modem</quote> (e.g., <filename class="devicefile">eth0</filename>)
    unless you connect via <emphasis>Point-to-Point Protocol</emphasis> over
    Ethernet (PPPoE) or <emphasis>Point-to-Point Tunneling Protocol</emphasis>
    (PPTP) in which case the External Interface will be a <literal>ppp</literal>
    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 ISDN,
    you external interface will be <filename class="devicefile">ippp0</filename>.</para>

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

    <para>If your external interface is <filename class="devicefile">ppp0</filename>
    or <filename class="devicefile">ippp0</filename> then you will want to set
    <varname>CLAMPMSS=yes</varname> in <filename>/etc/shorewall/shorewall.conf</filename>.</para>

    <para>Your Local Interface will be an ethernet adapter (<filename
    class="devicefile">eth0</filename>, <filename class="devicefile">eth1</filename>
    or <filename class="devicefile">eth2</filename>) and will be connected to
    a hub or switch. Your local computers will be connected to the same switch
    (note: If you have only a single local system, you can connect the
    firewall directly to the computer using a cross-over cable).</para>

    <para>Your DMZ Interface will also be an ethernet adapter (<filename
    class="devicefile">eth0</filename>, <filename class="devicefile">eth1</filename>
    or <filename class="devicefile">eth2</filename>) and will be connected to
    a hub or switch. Your DMZ computers will be connected to the same switch
    (note: If you have only a single DMZ system, you can connect the firewall
    directly to the computer using a cross-over cable).</para>

    <caution>
      <para>Do not connect the internal and external interface to the same hub
      or switch except for testing AND you are running Shorewall version 1.4.7
      or later. When using these recent versions, you can test using this kind
      of configuration if you specify the arp_filter option in
      <filename>/etc/shorewall/interfaces</filename> for all interfaces
      connected to the common hub/switch. Using such a setup with a production
      firewall is strongly recommended against.</para>
    </caution>

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

    <para>The Shorewall three-interface sample configuration assumes that the
    external interface is <filename class="devicefile">eth0</filename>, the
    local interface is <filename class="devicefile">eth1</filename> and the
    DMZ interface is <filename class="devicefile">eth2</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 interfaces. 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>
      (without 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 Internet
    Protocol (IP) addresses. Normally, your ISP will assign you a single
    Public IP address. This address may be assigned via the Dynamic Host
    Configuration Protocol (DHCP) or as part of establishing your connection
    when you dial in (standard modem) or establish your PPP connection. In
    rare cases, your ISP may assign you a static IP address; that means that
    you configure your firewall&#39;s external interface to use that address
    permanently. Regardless of how the address is assigned, it will be shared
    by all of your systems when you access the Internet. You will have to
    assign your own addresses for your internal network (the local and DMZ
    Interfaces on your firewall plus your other computers). RFC 1918 reserves
    several Private IP address ranges for this purpose:</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><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 <varname>norfc1918</varname> option from the external interface&#39;s
    entry in <filename>/etc/shorewall/interfaces</filename>.</para>

    <para>You will want to assign your local addresses from one sub-network or
    subnet and your DMZ addresses from another subnet. For our purposes, we
    can consider a subnet to consists of a range of addresses <systemitem
    class="ipaddress">x.y.z.0</systemitem> - <systemitem class="ipaddress">x.y.z.255</systemitem>.
    Such a subnet will have a Subnet Mask of <systemitem class="netmask">255.255.255.0</systemitem>.
    The address <systemitem class="ipaddress">x.y.z.0</systemitem> is reserved
    as the Subnet Address and <systemitem class="netmask">x.y.z.255</systemitem>
    is reserved as the Subnet Broadcast Address. In Shorewall, a subnet is
    described using Classless InterDomain Routing (CIDR) notation with
    consists of the subnet address followed by <varname>/24</varname>. The
    <varname>24</varname> refers to the number of consecutive <quote>1</quote>
    bits from the left of the subnet mask.</para>

    <table>
      <title>Example sub-network</title>

      <tgroup cols="2">
        <colspec align="left" />

        <tbody>
          <row>
            <entry>Range:</entry>

            <entry><systemitem class="ipaddress">10.10.10.0</systemitem> -
            <systemitem class="ipaddress">10.10.10.255</systemitem></entry>
          </row>

          <row>
            <entry>Subnet Address:</entry>

            <entry><systemitem class="ipaddress">10.10.10.0</systemitem></entry>
          </row>

          <row>
            <entry>Broadcast Address:</entry>

            <entry><systemitem class="ipaddress">10.10.10.255</systemitem></entry>
          </row>

          <row>
            <entry>CIDR Notation:</entry>

            <entry><systemitem class="ipaddress">10.10.10.0/24</systemitem></entry>
          </row>
        </tbody>
      </tgroup>
    </table>

    <para>It is conventional to assign the internal interface either the first
    usable address in the subnet (<systemitem class="ipaddress">10.10.10.1</systemitem>
    in the above example) or the last usable address (<systemitem
    class="ipaddress">10.10.10.254</systemitem>).</para>

    <para>One of the purposes of subnetting is to allow all computers in the
    subnet to understand which other computers can be communicated with
    directly. To communicate with systems outside of the subnetwork, systems
    send packets through a gateway (router).</para>

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

    <para>Your local computers (Local Computers 1 &#38; 2) should be
    configured with their default gateway set to the IP address of the
    firewall&#39;s internal interface and your DMZ computers (DMZ Computers 1
    &#38; 2) should be configured with their default gateway set to the IP
    address of the firewall&#39;s DMZ interface.</para>

    <para>The foregoing short discussion barely scratches the surface
    regarding subnetting and routing. If you are interested in learning more
    about IP addressing and routing, I highly recommend <quote>IP
    Fundamentals: What Everyone Needs to Know about Addressing &#38; Routing</quote>,
    Thomas A. Maufer, Prentice-Hall, 1999, ISBN 0-13-975483-0.</para>

    <para>The remainder of this quide will assume that you have configured
    your network as shown here:</para>

    <figure>
      <title>DMZ</title>

      <mediaobject>
        <imageobject>
          <imagedata fileref="images/dmz2.png" />
        </imageobject>

        <caption>
          <para>The default gateway for the DMZ computers would be <systemitem
          class="ipaddress">10.10.11.254</systemitem> and the default gateway
          for the Local computers would be <systemitem class="ipaddress">10.10.10.254</systemitem>.</para>

          <warning>
            <para>Your ISP might assign your external interface an RFC 1918
            address. If that address is in the <systemitem class="ipaddress">10.10.10.0/24</systemitem>
            subnet then you will need to select a DIFFERENT RFC 1918 subnet
            for your local network and if it is in the <systemitem
            class="ipaddress">10.10.11.0/24</systemitem> subnet then you will
            need to select a different RFC 1918 subnet for your DMZ.</para>
          </warning>
        </caption>
      </mediaobject>
    </figure>
  </section>

  <section>
    <title>IP Masquerading (SNAT)</title>

    <para>The addresses reserved by RFC 1918 are sometimes referred to as
    non-routable because the Internet backbone routers don&#39;t forward
    packets which have an RFC-1918 destination address. When one of your local
    systems (let&#39;s assume local computer 1) sends a connection request to
    an internet host, the firewall must perform Network Address Translation
    (NAT). The firewall rewrites the source address in the packet to be the
    address of the firewall&#39;s external interface; in other words, the
    firewall makes it look as if the firewall itself is initiating the
    connection. This is necessary so that the destination host will be able to
    route return packets back to the firewall (remember that packets whose
    destination address is reserved by RFC 1918 can&#39;t be routed accross
    the internet). When the firewall receives a return packet, it rewrites the
    destination address back to 10.10.10.1 and forwards the packet on to local
    computer 1.</para>

    <para>On Linux systems, the above process is often referred to as IP
    Masquerading and you will also see the term Source Network Address
    Translation (SNAT) used. Shorewall follows the convention used with
    Netfilter: <itemizedlist><listitem><para><emphasis>Masquerade</emphasis>
    describes the case where you let your firewall system automatically detect
    the external interface address.</para></listitem><listitem><para><emphasis>SNAT</emphasis>
    refers to the case when you explicitly specify the source address that you
    want outbound packets from your local network to use.</para></listitem></itemizedlist>
    In Shorewall, both Masquerading and SNAT are configured with entries in
    the <filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
    file.</para>

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

    <para>If your external firewall interface is <filename class="devicefile">eth0</filename>,
    your local interface <filename class="devicefile">eth1</filename> and your
    DMZ interface is <filename class="devicefile">eth2</filename> then you do
    not need to modify the file provided with the sample. Otherwise, edit
    <filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
    and change it to match your configuration.</para>

    <para>If, despite all advice to the contrary, you are using this guide and
    want to use one-to-one NAT or Proxy ARP for your DMZ, remove the entry for
    eth2 from <filename>/etc/shorewall/masq</filename>.</para>

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

    <para>If your external IP is static, you can enter it in the third column
    in the <filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
    entry if you like although your firewall will work fine if you leave that
    column empty. Entering your static IP in column 3 makes processing
    outgoing packets a little more efficient.</para>

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

    <para>If you are using the Debian package, please check your
    <filename>shorewall.conf</filename> file to ensure that the following are
    set correctly; if they are not, change them appropriately:
    <itemizedlist><listitem><para><varname>NAT_ENABLED=Yes</varname>
    (Shorewall versions earlier than 1.4.6)</para></listitem><listitem><para><varname>IP_FORWARDING=On</varname></para></listitem></itemizedlist></para>
  </section>

  <section>
    <title>Port Forwarding (DNAT)</title>

    <para>One of your goals will be to run one or more servers on your DMZ
    computers. Because these computers have RFC-1918 addresses, it is not
    possible for clients on the internet to connect directly to them. It is
    rather necessary for those clients to address their connection requests to
    your firewall who rewrites the destination address to the address of your
    server and forwards the packet to that server. When your server responds,
    the firewall automatically performs SNAT to rewrite the source address in
    the response.</para>

    <para>The above process is called <emphasis>Port Forwarding</emphasis> or
    <emphasis>Destination Network Address Translation</emphasis> (DNAT). You
    configure port forwarding using DNAT rules in the <filename
    class="directory">/etc/shorewall/</filename><filename>rules</filename>
    file.</para>

    <para>The general form of a simple port forwarding rule in <filename
    class="directory">/etc/shorewall/</filename><filename>rules</filename> is:
    <programlisting>#ACTION   SOURCE    DEST                                          PROTO      DEST PORT(S)
DNAT      net       dmz:<emphasis>&#60;server local ip address&#62;</emphasis>[:<emphasis>&#60;server port&#62;</emphasis>] <emphasis>&#60;protocol&#62;</emphasis> <emphasis>&#60;port&#62;</emphasis></programlisting>
    If you don&#39;t specify the <emphasis><varname>&#60;server port&#62;</varname></emphasis>,
    it is assumed to be the same as <emphasis><varname>&#60;port&#62;</varname></emphasis>.</para>

    <example>
      <title>You run a Web Server on DMZ Computer 2 and you want to forward
      incoming TCP port 80 to that system</title>

      <para><programlisting>#ACTION   SOURCE    DEST                PROTO      DEST PORT(S)
DNAT      net       dmz:10.10.11.2      tcp        80
ACCEPT    loc       dmz:10.10.11.2      tcp        80</programlisting><itemizedlist><listitem><para>Entry
      1 forwards port 80 from the Internet.</para></listitem><listitem><para>Entry
      2 allows connections from the local network.</para></listitem></itemizedlist>
      Several important points to keep in mind:<itemizedlist><listitem><para>When
      you are connecting to your server from your local systems, you must use
      the server&#39;s internal IP address (<systemitem class="ipaddress">10.10.11.2</systemitem>).</para></listitem><listitem><para>Many
      ISPs block incoming connection requests to port 80. If you have problems
      connecting to your web server, try the following rule and try connecting
      to port 5000 (e.g., connect to <literal>http://w.x.y.z:5000 where
      w.x.y.z</literal> is your external IP).<programlisting>#ACTION   SOURCE    DEST                PROTO      DEST PORT(S)    SOURCE
#                                                                  PORT(S)
DNAT      net       dmz:10.10.11.2:80   tcp        80              5000</programlisting></para></listitem><listitem><para>If
      you want to be able to access your server from the local network using
      your external address, then if you have a static external IP you can
      replace the loc-&#62;dmz rule above with:<programlisting>#ACTION   SOURCE    DEST            PROTO  DEST PORT(S)  SOURCE   ORIGINAL
#                                                        PORT(S)  DEST
DNAT      loc       dmz:10.10.11.2  tcp    80            -        <emphasis>&#60;external ip&#62;</emphasis></programlisting>If
      you have a dynamic ip then you must ensure that your external interface
      is up before starting Shorewall and you must take steps as follows
      (assume that your external interface is <filename class="devicefile">eth0</filename>):<orderedlist><listitem><para>Include
      the following in /etc/shorewall/params:</para><para><command>ETH0_IP=$(find_interface_address
      eth0)</command></para></listitem><listitem><para>Make your
      <literal>loc-&#62;dmz</literal> rule: <programlisting>#ACTION   SOURCE    DEST             PROTO   DEST PORT(S)  SOURCE   ORIGINAL
#                                                          PORT(S)  DEST
DNAT      loc       dmz:10.10.11.2   tcp     80            -        $ETH0_IP</programlisting></para></listitem></orderedlist></para></listitem><listitem><para>If
      you want to access your server from the DMZ using your external IP
      address, see <ulink url="FAQ.htm#faq2a">FAQ 2a</ulink>.</para></listitem></itemizedlist></para>
    </example>

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

    <para>At this point, add the DNAT and ACCEPT rules for your servers.</para>
  </section>

  <section>
    <title>Domain Name Server (DNS)</title>

    <para>Normally, when you connect to your ISP, as part of getting an IP
    address your firewall&#39;s <emphasis>Domain Name Service</emphasis> (DNS)
    resolver will be automatically configured (e.g., the <filename>/etc/resolv.conf</filename>
    file will be written). Alternatively, your ISP may have given you the IP
    address of a pair of DNS name servers for you to manually configure as
    your primary and secondary name servers. It is your responsibility to
    configure the resolver in your internal systems. You can take one of two
    approaches: <itemizedlist><listitem><para>You can configure your internal
    systems to use your ISP&#39;s name servers. If you ISP gave you the
    addresses of their servers or if those addresses are available on their
    web site, you can configure your internal systems to use those addresses.
    If that information isn&#39;t available, look in <filename>/etc/resolv.conf</filename>
    on your firewall system -- the name servers are given in <quote>nameserver</quote>
    records in that file.</para></listitem><listitem><para><inlinegraphic
    fileref="images/BD21298_.gif" format="GIF" /></para><para>You can
    configure a <emphasis>Caching Name Server</emphasis> on your firewall or
    in your DMZ. <trademark>Red Hat</trademark> has an RPM for a caching name
    server (which also requires the &#39;<command>bind</command>&#39; RPM) and
    for Bering users, there is <filename>dnscache.lrp</filename>. If you take
    this approach, you configure your internal systems to use the caching name
    server as their primary (and only) name server. You use the internal IP
    address of the firewall (<systemitem class="ipaddress">10.10.10.254</systemitem>
    in the example above) for the name server address if you choose to run the
    name server on your firewall. To allow your local systems to talk to your
    caching name server, you must open port 53 (both UDP and TCP) from the
    local network to the server; you do that by adding the rules in
    <filename>/etc/shorewall/rules</filename>.</para></listitem></itemizedlist>
    If you run the name server on the firewall:
    <programlisting>#ACTION   SOURCE    DEST                PROTO      DEST PORT(S)                      
ACCEPT    loc       fw                  tcp        53
ACCEPT    loc       fw                  udp        53
ACCEPT    dmz       fw                  tcp        53
ACCEPT    dmz       fw                  udp        53              </programlisting>
    Run name server on DMZ computer 1: <programlisting>#ACTION   SOURCE    DEST                PROTO      DEST PORT(S)                      
ACCEPT    loc       dmz:10.10.11.1      tcp        53
ACCEPT    loc       dmz:10.10.11.1      udp        53
ACCEPT    fw        dmz:10.10.11.1      tcp        53
ACCEPT    fw        dmz:10.10.11.1      udp        53              </programlisting></para>
  </section>

  <section>
    <title>Other Connections</title>

    <para>The three-interface sample includes the following rules:
    <programlisting>#ACTION   SOURCE    DEST                PROTO      DEST PORT(S)                      
ACCEPT    fw        net                 udp        53
ACCEPT    fw        net                 tcp        53        </programlisting>Those
    rules allow DNS access from your firewall and may be removed if you
    commented out the line in <filename>/etc/shorewall/policy</filename>
    allowing all connections from the firewall to the internet.</para>

    <para>The sample also includes: <programlisting>#ACTION   SOURCE    DEST                PROTO      DEST PORT(S)                      
ACCEPT    loc       fw                  tcp        22
ACCEPT    loc       fw                  tcp        22        </programlisting>That
    rule allows you to run an SSH server on your firewall and in each of your
    DMZ systems and to connect to those servers from your local systems.</para>

    <para>If you wish to enable other connections between your systems, the
    general format is: <programlisting>#ACTION   SOURCE        DEST                PROTO      DEST PORT(S)                      
ACCEPT    <emphasis>&#60;source zone&#62; &#60;destination zone&#62;  &#60;protocol&#62; &#60;port&#62; </emphasis></programlisting></para>

    <example>
      <title>You want to run a publicly-available DNS server on your firewall
      system</title>

      <programlisting>#ACTION   SOURCE    DEST                PROTO      DEST PORT(S)                      
ACCEPT    net       fw                  tcp        53
ACCEPT    net       fw                  udp        53        </programlisting>

      <para>Those two rules would of course be in addition to the rules listed
      above under &#34;If you run the name server on your firewall&#34;.</para>
    </example>

    <para>If you don&#39;t know what port and protocol a particular
    application uses, <ulink url="ports.htm">look 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: <programlisting>#ACTION   SOURCE    DEST                PROTO      DEST PORT(S)                      
ACCEPT    net       fw                  tcp        22</programlisting></para>
    </important>

    <para><inlinegraphic fileref="images/leaflogo.gif" format="GIF" /> Bering
    users will want to add the following two rules to be compatible with
    Jacques&#39;s Shorewall configuration: <programlisting>#ACTION   SOURCE    DEST                PROTO      DEST PORT(S)                      
ACCEPT    loc       fw                  udp        53
ACCEPT    net       fw                  tcp        80       </programlisting><itemizedlist><listitem><para>Entry
    1 allows the DNS Cache to be used.</para></listitem><listitem><para>Entry
    2 allows the <quote>weblet</quote> to work.</para></listitem></itemizedlist><inlinegraphic
    fileref="images/BD21298_.gif" format="GIF" /></para>

    <para>Now modify <filename>/etc/shorewall/rules</filename> to add or
    remove other connections as required.</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>.
    <important><para>Users of the <filename>.deb</filename> package must edit
    <filename>/etc/default/shorewall</filename> and set <varname>startup=1</varname>.</para></important>
    The firewall is started using the <command>shorewall start</command>
    command and stopped using <command>shorewall stop</command>. When the
    firewall is stopped, routing is enabled on those hosts that have an entry
    in <ulink url="Documentation.htm#Routestopped"><filename>/etc/shorewall/routestopped</filename></ulink>.
    A running firewall may be restarted using the <command>shorewall restart</command>
    command. If you want to totally remove any trace of Shorewall from your
    Netfilter configuration, use <command>shorewall clear</command>.</para>

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

    <para>The three-interface sample assumes that you want to enable routing
    to/from <filename class="devicefile">eth1</filename> (your local network)
    and <filename class="devicefile">eth2</filename> (DMZ) when Shorewall is
    stopped. If these two interfaces don&#39;t connect to your local network
    and DMZ or if you want to enable a different set of hosts, modify
    <filename>/etc/shorewall/routestopped</filename> accordingly.
    <warning><para>If you are connected to your firewall from the internet, do
    not issue a <command>shorewall stop</command> 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 <command>shorewall restart</command>; it
    is better to create an <ulink url="configuration_file_basics.htm#Levels">alternate
    configuration</ulink> and test it using the <ulink
    url="starting_and_stopping_shorewall.htm"><command>shorewall try</command>
    command</ulink>.</para></warning></para>
  </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</ulink>
    page -- it contains helpful tips about Shorewall features than make
    administering your firewall easier.</para>
  </section>
</article>