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<article id="two-interface">
  <articleinfo>
    <title>Basic Two-Interface Firewall</title>

    <author>
      <firstname>Tom</firstname>

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

    <pubdate>2003-02-08</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 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 its most common configuration:</para>

    <itemizedlist mark="bullet" spacing="compact">
      <listitem>
        <para>Linux system used as a firewall/router for a small local
        network.</para>
      </listitem>

      <listitem>
        <para><emphasis role="bold">Single public IP address.</emphasis> 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>
      </listitem>

      <listitem>
        <para>Internet connection through cable modem, DSL, ISDN, Frame Relay,
        dial-up ...</para>
      </listitem>
    </itemizedlist>

    <para>Here is a schematic of a typical installation: <figure label="1"><title>Common
    two interface firewall configuration</title><mediaobject><imageobject><imagedata
    fileref="images/basics.png" format="PNG" /></imageobject></mediaobject></figure>
    <tip><title>Shorewall and <trademark>Mandrake</trademark> 9.0+</title><para>If
    you are running Shorewall under <trademark>Mandrake</trademark> 9.0 or
    later, you can easily configure the above setup using the
    <trademark>Mandrake</trademark> <quote>Internet Connection Sharing</quote>
    applet. From the <emphasis><interface>Mandrake Control Center</interface></emphasis>,
    select <quote><guimenuitem>Network</guimenuitem> &#38;
    <guisubmenu>Internet</guisubmenu></quote> then <quote><interface>Connection
    Sharing</interface></quote>.</para><para>Note however, that the Shorewall
    configuration produced by <emphasis>Mandrake Internet Connection Sharing</emphasis>
    is strange and is apt to confuse you if you use the rest of this
    documentation (it has two local zones; <varname>loc</varname> and
    <varname>masq</varname> where <varname>loc</varname> is empty; this
    conflicts with this documentation which assumes a single local zone
    <varname>loc</varname>). We therefore recommend that once you have set up
    this sharing that you uninstall the <trademark>Mandrake</trademark>
    Shorewall RPM and install the one from the <ulink url="download.htm">download</ulink>
    page then follow the instructions in this Guide.</para></tip>
    <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></para>

    <section>
      <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 <systemitem class="username">root</systemitem>, you
      can use the <command>which</command> command to check for this program:
      <programlisting>[root@gateway root]# <command>which ip</command>
/sbin/ip
[root@gateway root]#</programlisting> 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>
    </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 <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>
    <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 will only need to deal with a few of these as described in
    this guide. <tip><para>After you have <ulink url="Install.htm">installed
    Shorewall</ulink>, download the <ulink
    url="http://www1.shorewall.net/pub/shorewall/Samples/">two-interface
    sample</ulink>, un-tar it (<command>tar <option>-zxvf</option>
    <filename>two-interfaces.tgz</filename></command>) and and copy the files
    to <filename class="directory">/etc/shorewall</filename> <emphasis
    role="bold">(these files will replace files with the same name)</emphasis>.</para></tip>
    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 two-interface sample configuration, the following
    zone names are used: <informaltable frame="all" pgwide="0"><tgroup
    align="left" cols="2"><thead valign="middle"><row valign="middle"><entry
    align="left">Name</entry><entry align="left">Description</entry></row></thead><tbody
    valign="middle"><row valign="middle"><entry align="left"><varname>net</varname></entry><entry
    align="left">The Internet</entry></row><row valign="middle"><entry
    align="left"><varname>loc</varname></entry><entry align="left">Your Local
    Network</entry></row></tbody></tgroup></informaltable> Zones are defined
    in the <ulink url="Documentation.htm#Zones"><filename class="directory">/etc/shorewall/</filename><filename>zones</filename></ulink>
    file.</para>

    <para>Shorewall also recognizes the firewall system as its own zone - by
    default, the firewall itself is known as <emphasis role="bold"><varname>fw</varname></emphasis>.</para>

    <para>Rules about what traffic to allow and what traffic to deny are
    expressed in terms of zones. <itemizedlist spacing="compact"><listitem><para>You
    express your default policy for connections from one zone to another zone
    in the <ulink url="Documentation.htm#Policy"><filename class="directory">/etc/shorewall/</filename><filename>policy</filename></ulink>
    file.</para></listitem><listitem><para>You define exceptions to those
    default policies in the <ulink url="Documentation.htm#Rules"><filename
    class="directory">/etc/shorewall/</filename><filename>rules</filename></ulink>
    file.</para></listitem></itemizedlist> For each connection request
    entering the firewall, the request is first checked against the <filename
    class="directory">/etc/shorewall/</filename><filename>rules</filename>
    file. If no rule in that file matches the connection request then the
    first policy in <filename class="directory">/etc/shorewall/</filename><filename>policy</filename>
    that matches the request is applied. If that policy is <varname>REJECT</varname>
    or <varname>DROP</varname> the request is first checked against the rules
    in <filename class="directory">/etc/shorewall/</filename><filename>common</filename>
    if that file exists; otherwise the rules in <filename class="directory">/etc/shorewall/</filename><filename>common.def</filename>
    are checked.</para>

    <para>The <filename class="directory">/etc/shorewall/</filename><filename>policy</filename>
    file included with the two-interface sample has the following policies:
    <programlisting>#SOURCE    DEST        POLICY      LOG LEVEL    LIMIT:BURST
loc        net         ACCEPT
net        all         DROP        info
all        all         REJECT      info</programlisting> In the two-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. <programlisting>#SOURCE    DEST        POLICY      LOG LEVEL    LIMIT:BURST
fw         net         ACCEPT</programlisting> The above policy will:
    <itemizedlist><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></itemizedlist> <inlinegraphic
    fileref="images/BD21298_.gif" format="GIF" /></para>

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

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

    <mediaobject>
      <imageobject>
        <imagedata fileref="images/basics.png" format="PNG" />
      </imageobject>
    </mediaobject>

    <para>The firewall has two network interfaces. Where Internet connectivity
    is through a cable or <acronym>DSL</acronym> <quote>Modem</quote>, the
    <emphasis>External Interface</emphasis> 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 (<acronym>PPPoE</acronym>)
    or <emphasis>Point-to-Point Tunneling Protocol</emphasis> (<acronym>PPTP</acronym>)
    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 via
    <acronym>ISDN</acronym>, your 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 class="directory">/etc/shorewall/</filename><filename>shorewall.conf</filename>.</para>

    <para>Your <emphasis>Internal Interface</emphasis> will be an ethernet
    adapter (<filename class="devicefile">eth1</filename> or <filename
    class="devicefile">eth0</filename>) and will be connected to a hub or
    switch. Your other computers will be connected to the same hub/switch
    (note: If you have only a single internal system, you can connect the
    firewall directly to the computer using a cross-over cable).
    <warning><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 class="directory">/etc/shorewall/</filename><filename>interfaces</filename>
    for all interfaces connected to the common hub/switch. Using such a setup
    with a production firewall is strongly recommended against.</para></warning>
    <inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>

    <para>The Shorewall two-interface sample configuration assumes that the
    external interface is <filename class="devicefile">eth0</filename> and the
    internal interface is <filename class="devicefile">eth1</filename>. If
    your configuration is different, you will have to modify the sample
    <filename class="directory">/etc/shorewall/</filename><filename>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: <itemizedlist
    spacing="compact"><listitem><para>If your external interface is <filename
    class="devicefile">ppp0</filename> or <filename class="devicefile">ippp0</filename>,
    you can replace the <varname>detect</varname> in the second column with a
    <quote>-</quote> (minus the quotes).</para></listitem><listitem><para>If
    your external interface is <filename class="devicefile">ppp0</filename> or
    <filename class="devicefile">ippp0</filename> or if you have a static
    <acronym>IP</acronym> address, you can remove <varname>dhcp</varname> from
    the option list.</para></listitem></itemizedlist></para>
  </section>

  <section>
    <title>IP Addresses</title>

    <para>Before going further, we should say a few words about Internet
    Protocol (<acronym>IP</acronym>) addresses. Normally, your
    <acronym>ISP</acronym> will assign you a single Public IP address. This
    address may be assigned via the Dynamic Host Configuration Protocol (<acronym>DHCP</acronym>)
    or as part of establishing your connection when you dial in (standard
    modem) or establish your <acronym>PPP</acronym> connection. In rare cases,
    your <acronym>ISP</acronym> may assign you a static <acronym>IP</acronym>
    address; that means that you configure your firewall&#39;s external
    interface to use that address permanently. However your external 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 in your internal
    network (the Internal Interface on your firewall plus your other
    computers). <emphasis role="bold">RFC 1918</emphasis> reserves several
    <emphasis>Private</emphasis> <acronym>IP</acronym> address ranges for this
    purpose: <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> <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 &#39;norfc1918&#39; option from the external interface&#39;s entry in
    <filename class="directory">/etc/shorewall/</filename><filename>interfaces</filename>.</para>

    <para>You will want to assign your addresses from the same sub-network
    (subnet). For our purposes, we can consider a subnet to consists of a
    range of addresses <varname>x.y.z.0 - x.y.z.255</varname>. Such a subnet
    will have a Subnet Mask of <systemitem class="netmask">255.255.255.0</systemitem>.
    The address <varname>x.y.z.0</varname> is reserved as the
    <emphasis>Subnet Address</emphasis> and <varname>x.y.z.255</varname> is
    reserved as the <emphasis>Subnet Broadcast Address</emphasis>. In
    Shorewall, a subnet is described using <ulink
    url="shorewall_setup_guide.htm#Subnets">Classless InterDomain Routing
    (CIDR) notation</ulink> with consists of the subnet address followed by
    <varname>/24</varname>. The <quote>24</quote> refers to the number of
    consecutive leading <quote>1</quote> bits from the left of the subnet
    mask. <informaltable frame="all" label="Example sub-network" pgwide="0"><!--
				Orientation types for tables are not supported by fop yet so we'll fake it by using boldface on left side entries.
				 --><tgroup align="left" cols="2"><tbody valign="middle"><row
    valign="middle"><entry align="left"><emphasis role="bold">Range:</emphasis></entry><entry><systemitem
    class="ipaddress">10.10.10.0</systemitem> - <systemitem class="ipaddress">10.10.10.255</systemitem></entry></row><row><entry
    align="left"><emphasis role="bold">Subnet Address:</emphasis></entry><entry
    align="left"><systemitem class="netmask">10.10.10.0</systemitem></entry></row><row><entry
    align="left"><emphasis role="bold">Broadcast Address:</emphasis></entry><entry
    align="left"><systemitem class="ipaddress">10.10.10.255</systemitem></entry></row><row><entry
    align="left"><emphasis role="bold">CIDR Notation:</emphasis></entry><entry
    align="left"><systemitem class="ipaddress">10.10.10.0/24</systemitem></entry></row></tbody></tgroup></informaltable>
    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 (computer 1 and computer 2 in the above
    diagram) should be configured with their default gateway to be the
    <acronym>IP</acronym> address of the firewall&#39;s internal interface.</para>

    <para>The foregoing short discussion barely scratches the surface
    regarding subnetting and routing. If you are interested in learning more
    about <acronym>IP</acronym> 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
    (<ulink
    url="http://www.phptr.com/browse/product.asp?product_id={58D4F6D4-54C5-48BA-8EDD-86EBD7A42AF6}">link</ulink>).</para>

    <para>The remainder of this quide will assume that you have configured
    your network as shown here: <mediaobject><imageobject><imagedata
    fileref="images/basics1.png" format="PNG" /></imageobject></mediaobject>
    The default gateway for computer&#39;s 1 &#38; 2 would be <systemitem
    class="ipaddress">10.10.10.254</systemitem>. <warning><para>Your
    <acronym>ISP</acronym> might assign your external interface an <emphasis
    role="bold">RFC 1918</emphasis> 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.</para></warning></para>
  </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 computer 1) sends a connection request to an
    internet host, the firewall must perform <emphasis>Network Address
    Translation</emphasis> (<acronym>NAT</acronym>). 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 across the internet so the remote host
    can&#39;t address its response to computer 1). When the firewall receives
    a return packet, it rewrites the destination address back to <systemitem
    class="ipaddress">10.10.10.1</systemitem> and forwards the packet on to
    computer 1.</para>

    <para>On Linux systems, the above process is often referred to as
    <emphasis>IP Masquerading</emphasis> but you will also see the term
    <emphasis>Source Network Address Translation</emphasis> (<acronym>SNAT</acronym>)
    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><acronym>SNAT</acronym></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 <emphasis>Masquerading</emphasis> and
    <emphasis><acronym>SNAT</acronym></emphasis> are configured with entries
    in the <filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
    file. You will normally use Masquerading if your external
    <acronym>IP</acronym> is dynamic and <acronym>SNAT</acronym> if the
    <acronym>IP</acronym> is static.</para>

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

    <para>If your external firewall interface is <filename class="devicefile">eth0</filename>,
    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 the first column to the name of your external interface and the
    second column to the name of your internal interface.</para>

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

    <para>If your external <acronym>IP</acronym> 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 <acronym>IP</acronym> 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
    spacing="compact"><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 may be to run one or more servers on your local
    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
    the 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 <acronym>SNAT</acronym> 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> (<acronym>DNAT</acronym>).
    You configure port forwarding using <acronym>DNAT</acronym> 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       loc:<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>
    <example label="1"><title>Web Server</title><para>You run a Web Server on
    computer 2 and you want to forward incoming <acronym>TCP</acronym> port 80
    to that system: <programlisting>#ACTION   SOURCE    DEST             PROTO     DEST PORT(S)
DNAT      net       loc:10.10.10.2   tcp       80</programlisting></para></example>
    <example label="2"><title>FTP Server</title><para>You run an
    <acronym>FTP</acronym> Server on computer 1 so you want to forward
    incoming <acronym>TCP</acronym> port 21 to that system:
    <programlisting>#ACTION    SOURCE    DEST            PROTO     DEST PORT(S)
DNAT       net       loc:10.10.10.1  tcp       21</programlisting> For
    <acronym>FTP</acronym>, you will also need to have <acronym>FTP</acronym>
    connection tracking and <acronym>NAT</acronym> support in your kernel. For
    vendor-supplied kernels, this means that the <filename class="libraryfile">ip_conntrack_ftp</filename>
    and <filename class="libraryfile">ip_nat_ftp</filename> modules must be
    loaded. Shorewall will automatically load these modules if they are
    available and located in the standard place under <filename
    class="directory">/lib/modules/&#60;kernel
    version&#62;/kernel/net/ipv4/netfilter</filename>.</para></example> A
    couple of important points to keep in mind: <itemizedlist><listitem><para>You
    must test the above rule from a client outside of your local network
    (i.e., don&#39;t test from a browser running on computers 1 or 2 or on the
    firewall). If you want to be able to access your web server and/or
    <acronym>FTP</acronym> server from inside your firewall using the
    <acronym>IP</acronym> address of your external interface, see <ulink
    url="FAQ.htm#faq2">Shorewall FAQ #2</ulink>.</para></listitem><listitem><para>Many
    <acronym>ISP</acronym>s 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.</para><programlisting>#ACTION    SOURCE    DEST               PROTO     DEST PORT(S)
DNAT       net       loc:10.10.10.2:80  tcp       5000</programlisting></listitem></itemizedlist>
    <inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>

    <para>At this point, modify <filename class="directory">/etc/shorewall/</filename><filename>rules</filename>
    to add any <acronym>DNAT</acronym> rules that you require.</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> (<acronym>DNS</acronym>)
    resolver will be automatically configured (e.g., the <filename
    class="directory">/etc/</filename><filename>resolv.conf</filename> file
    will be written). Alternatively, your ISP may have given you the
    <acronym>IP</acronym> address of a pair of <acronym>DNS</acronym> name
    servers for you to manually configure as your primary and secondary name
    servers. Regardless of how <acronym>DNS</acronym> gets configured on your
    firewall, it is your responsibility to configure the resolver in your
    internal systems. You can take one of two approaches: <itemizedlist
    spacing="compact"><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 /etc/resolv.conf on your firewall
    system -- the name servers are given in &#34;nameserver&#34; records in
    that file.</para></listitem><listitem><para><anchor id="cachingdns" /> You
    can configure a <emphasis>Caching Name Server</emphasis> on your firewall.
    <trademark>Red Hat</trademark> has an <acronym>RPM</acronym> for a caching
    name server (the <acronym>RPM</acronym> also requires the
    <command>bind</command><acronym>RPM</acronym>) and for Bering users, there
    is <command>dnscache.lrp</command>. If you take this approach, you
    configure your internal systems to use the firewall itself as their
    primary (and only) name server. You use the internal <acronym>IP</acronym>
    address of the firewall (<systemitem class="ipaddress">10.10.10.254</systemitem>
    in the example above) for the name server address. To allow your local
    systems to talk to your caching name server, you must open port 53 (both
    <acronym>UDP</acronym> and <acronym>TCP</acronym>) from the local network
    to the firewall; you do that by adding the following rules in <filename
    class="directory">/etc/shorewall/</filename><filename>rules</filename>.
    <programlisting>#ACTION    SOURCE    DEST               PROTO     DEST PORT(S)
ACCEPT     loc       fw                 tcp       53
ACCEPT     loc       fw                 udp       53</programlisting></para></listitem></itemizedlist></para>
  </section>

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

    <para>The two-interface sample includes the following rules:
    <programlisting>#ACTION    SOURCE    DEST               PROTO     DEST PORT(S)
ACCEPT     fw        net                tcp       53
ACCEPT     fw        net                udp       53</programlisting>Those
    rules allow <acronym>DNS</acronym> access from your firewall and may be
    removed if you uncommented the line in <filename class="directory">/etc/shorewall/</filename><filename>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</programlisting> That
    rule allows you to run an <acronym>SSH</acronym> server on your firewall
    and connect to that server from your local systems.</para>

    <para>If you wish to enable other connections between your firewall and
    other systems, the general format is: <programlisting>#ACTION    SOURCE    DEST               PROTO      DEST PORT(S)
ACCEPT     fw        <emphasis>&#60;destination zone&#62; &#60;protocol&#62; &#60;port&#62;</emphasis></programlisting>
    <example><title>Web Server on Firewall</title><para>You want to run a Web
    Server on your firewall system: <programlisting>#ACTION    SOURCE    DEST               PROTO     DEST PORT(S)
ACCEPT     net       fw                 tcp       80
ACCEPT     loc       fw                 tcp       80</programlisting> Those
    two rules would of course be in addition to the rules listed above under
    <quote><link linkend="cachingdns">You can configure a Caching Name Server
    on your firewall</link></quote>.</para></example> If you don&#39;t know
    what port and protocol a particular application uses, look <ulink
    url="ports.htm">here</ulink>. <important><para>I don&#39;t recommend
    enabling <command>telnet</command> 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    DEST               PROTO     DEST PORT(S)
ACCEPT     net       fw                 tcp       22</programlisting></important>
    <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          #Allow DNS Cache to work
ACCEPT     loc       fw      tcp       80          #Allow Weblet to work</programlisting>
    <inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>

    <para>Now edit your <filename class="directory">/etc/shorewall/</filename><filename>rules</filename>
    file to add or delete 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 class="directory">/etc/shorewall/</filename><filename>startup_disabled</filename>.
    <important><para>Users of the .deb package must edit <filename
    class="directory">/etc/default/</filename><filename>shorewall</filename>
    and set <varname>startup=1</varname>.</para></important> 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 class="directory">/etc/shorewall/</filename><filename><ulink
    url="Documentation.htm#Routestopped">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>

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

    <para>The two-interface sample assumes that you want to enable routing
    to/from <filename class="devicefile">eth1</filename> (the local network)
    when Shorewall is stopped. If your local network isn&#39;t connected to
    <filename class="devicefile">eth1</filename> or if you wish to enable
    access to/from other hosts, change <filename class="directory">/etc/shorewall/</filename><filename>routestopped</filename>
    accordingly. <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 <acronym>IP</acronym>
    address that you are connected from to <filename class="directory">/etc/shorewall/</filename><filename>routestopped</filename>.
    Also, I don&#39;t recommend using <quote><command>shorewall restart</command></quote>;
    it is better to create an alternate configuration and test it using the
    <quote><command>shorewall try</command></quote> command.</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
    page</ulink> -- it contains helpful tips about Shorewall features than
    make administering your firewall easier.</para>
  </section>

  <section>
    <title>Adding a Wireless Segment to your Two-Interface Firewall</title>

    <para>Once you have the two-interface setup working, the next logical step
    is to add a Wireless Network. The first step involves adding an additional
    network card to your firewall, either a Wireless card or an ethernet card
    that is connected to a Wireless Access Point.<caution><para>When you add a
    network card, it won&#39;t necessarily be detected as the next highest
    ethernet interface. For example, if you have two ethernet cards in your
    system (<filename class="devicefile">eth0</filename> and <filename
    class="devicefile">eth1</filename>) and you add a third card that uses the
    same driver as one of the other two, that third card won&#39;t necessarily
    be detected as <filename class="devicefile">eth2</filename>; it could
    rather be detected as <filename class="devicefile">eth0</filename> or
    <filename class="devicefile">eth1</filename>! You can either live with
    that or you can shuffle the cards around in the slots until the new card
    is detected as <filename class="devicefile">eth2</filename>.</para></caution></para>

    <para>Your new network will look similar to what is shown in the following
    figure.<graphic fileref="images/basics2.png" /></para>

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

    <para>The first thing to note is that the computers in your wireless
    network will be in a different subnet from those on your wired local LAN.
    In the above example, we have chosen to use the network 10.10.11.0/24.
    Computers 3 and 4 would be configured with a default gateway IP address of
    10.10.11.254.</para>

    <para>Second, we have chosen to include the wireless network as part of
    the local zone. Since Shorewall allows intra-zone traffic by default,
    traffic may flow freely between the local wired network and the wireless
    network.</para>

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

    <para>There are only two changes that need to be made to the Shorewall
    configuration:</para>

    <itemizedlist>
      <listitem>
        <para>An entry needs to be added to <filename>/etc/shorewall/interfaces</filename>
        for the wireless network interface. If the wireless interface is
        <filename class="devicefile">wlan0</filename>, the entry might look
        like:</para>

        <programlisting>#ZONE     INTERFACE       BROADCAST          OPTIONS
loc       wlan0           detect             maclist</programlisting>

        <para>As shown in the above entry, I recommend using the <ulink
        url="MAC_Validation.html">maclist option</ulink> for the wireless
        segment. By adding entries for computers 3 and 4 in
        <filename>/etc/shorewall/maclist</filename>, you help ensure that your
        neighbors aren&#39;t getting a free ride on your internet connection.
        Start by omitting that option; when you have everything working, then
        add the option and configure your <filename>/etc/shorewall/maclist</filename>
        file.</para>
      </listitem>

      <listitem>
        <para>You need to add an entry to the <filename>/etc/shorewall/masq</filename>
        file to masquerade traffic from the wireless network to the internet.
        If your internet interface is <filename class="devicefile">eth0</filename>
        and your wireless interface is <filename class="devicefile">wlan0</filename>,
        the entry would be:</para>

        <programlisting>#INTERFACE           SUBNET             ADDRESS
eth0                 wlan0</programlisting>
      </listitem>
    </itemizedlist>

    <para>One other thing to note. To get <trademark>Microsoft</trademark>
    networking working between the wireless and wired networks, you will need
    either a WINS server or a PDC. I personally use Samba configured as a WINS
    server running on my firewall. Running a WINS server on your firewall
    requires the rules listed in the <ulink url="samba.htm">Shorewall/Samba
    documentation</ulink>. </para>
  </section>
</article>