shorewall_code/docs/two-interface.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- $Id$ -->
<article id="two-interface">
  <articleinfo>
    <title>Basic Two-Interface Firewall</title>

    <author>
      <firstname>Tom</firstname>

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

    <pubdate><?dbtimestamp format="Y/m/d"?></pubdate>

    <copyright>
      <year>2002-2007</year>

      <holder>Thomas M. Eastep</holder>
    </copyright>

    <legalnotice>
      <para>Permission is granted to copy, distribute and/or modify this
      document under the terms of the GNU Free Documentation License, Version
      1.2 or any later version published by the Free Software Foundation; with
      no Invariant Sections, with no Front-Cover, and with no Back-Cover
      Texts. A copy of the license is included in the section entitled
      <quote><ulink url="GnuCopyright.htm">GNU Free Documentation
      License</ulink></quote>.</para>
    </legalnotice>
  </articleinfo>

  <caution>
    <para><emphasis role="bold">This article applies to Shorewall 4.0 and
    later. If you are running a version of Shorewall earlier than Shorewall
    4.0.0 then please see the documentation for that
    release.</emphasis></para>
  </caution>

  <caution>
    <para><emphasis role="bold">Do not attempt to install Shorewall on a
    remote system. You are virtually assured to lock yourself out of that
    system.</emphasis></para>
  </caution>

  <section id="Intro">
    <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'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 id="Figure1"
        label="1">
        <title>Common two interface firewall configuration</title>

        <mediaobject>
          <imageobject>
            <imagedata align="center" fileref="images/basics.png" format="PNG" />
          </imageobject>
        </mediaobject>
      </figure> <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 id="System">
      <title>System Requirements</title>

      <para>Shorewall requires that you have the
      <command>iproute</command>/<command>iproute2</command> package installed
      (on <trademark>RedHat</trademark>, the package is called
      <command>iproute</command>). You can tell if this package is installed
      by the presence of an <command>ip</command> program on your firewall
      system. As <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's involved then go back
      through it again making your configuration changes.</para>
    </section>

    <section id="Conventions">
      <title>Conventions</title>

      <para>Points at which configuration changes are recommended are flagged
      with <inlinegraphic fileref="images/BD21298_.gif"
      format="GIF" />.</para>

      <para>Configuration notes that are unique to LEAF/Bering are marked with
      <inlinegraphic fileref="images/leaflogo.gif" format="GIF" />.</para>
    </section>
  </section>

  <section id="PPTP">
    <title>PPTP/ADSL</title>

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

    <para>If you have an <acronym>ADSL</acronym> Modem and you use
    <acronym>PPTP</acronym> to communicate with a server in that modem, you
    must make the changes recommended <ulink
    url="PPTP.htm#PPTP_ADSL">here</ulink> in addition to those detailed below.
    <acronym>ADSL</acronym> with <acronym>PPTP</acronym> is most commonly
    found in Europe, notably in Austria.</para>
  </section>

  <section id="Concepts">
    <title>Shorewall Concepts</title>

    <para></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.<warning>
        <para><emphasis role="bold">Note to Debian and Ubuntu
        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>
      </warning></para>

    <para><inlinegraphic fileref="images/BD21298_.gif"
    format="GIF" /><important>
        <para>After you have <ulink url="Install.htm">installed
        Shorewall</ulink>, locate the two-interfaces samples:</para>

        <orderedlist>
          <listitem>
            <para>If you installed using an RPM, the samples will be in the
            Samples/two-interfaces/ subdirectory of the Shorewall
            documentation directory. If you don't know where the Shorewall
            documentation directory is, you can find the samples using this
            command:</para>

            <programlisting>~# rpm -ql shorewall-common | fgrep two-interfaces
/usr/share/doc/packages/shorewall/Samples/two-interfaces
/usr/share/doc/packages/shorewall/Samples/two-interfaces/interfaces
/usr/share/doc/packages/shorewall/Samples/two-interfaces/masq
/usr/share/doc/packages/shorewall/Samples/two-interfaces/policy
/usr/share/doc/packages/shorewall/Samples/two-interfaces/routestopped
/usr/share/doc/packages/shorewall/Samples/two-interfaces/rules
/usr/share/doc/packages/shorewall/Samples/two-interfaces/zones
~#</programlisting>
          </listitem>

          <listitem>
            <para>If you installed using the tarball, the samples are in the
            Samples/two-interfaces directory in the tarball.</para>
          </listitem>

          <listitem>
            <para>If you installed using the .deb, the samples are in
            /usr/share/doc/shorewall/examples/two-interfaces. You must install
            the shorewall-doc package.</para>
          </listitem>
        </orderedlist>
      </important></para>

    <para>If you are installing Shorewall version 3.4.0 or later then as each
    file is introduced, I suggest that you look at the actual file on your
    system and that you look at the <ulink
    url="configuration_file_basics.htm#Manpages">man page</ulink> for that
    file. For example, to look at the man page for the
    <filename>/etc/shorewall/zones</filename> file, type <command>man
    shorewall-zones</command> at a shell prompt.</para>

    <para>If you are installing a Shorewall version earlier than 3.4.0, then
    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:</para>

    <para><programlisting>#ZONE   TYPE     OPTIONS                 IN                      OUT
#                                        OPTIONS                 OPTIONS
fw      firewall
net     ipv4
loc     ipv4</programlisting>Zones are defined in the <ulink
    url="manpages/shorewall-zones.html"><filename
    class="directory">/etc/shorewall/</filename><filename>zones</filename></ulink>
    file.</para>

    <para>Note that Shorewall recognizes the firewall system as its own zone -
    when the /etc/shorewall/zones file is processed, the name of the firewall
    zone is stored in the shell variable $FW which may be used to refer to the
    firewall zone throughout the Shorewall configuration.</para>

    <para>Rules about what traffic to allow and what traffic to deny are
    expressed in terms of zones. <itemizedlist spacing="compact">
        <listitem>
          <para>You express your default policy for connections from one zone
          to another zone in the <ulink
          url="manpages/shorewall-policy.html"><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="manpages/shorewall-rules.html"><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 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. The purpose of the common action is
    two-fold:</para>

    <itemizedlist>
      <listitem>
        <para>It silently drops or rejects harmless common traffic that would
        otherwise clutter up your log — Broadcasts for example.</para>
      </listitem>

      <listitem>
        <para>If ensures that traffic critical to correct operation is allowed
        through the firewall — ICMP <emphasis>fragmentation-needed</emphasis>
        for example.</para>
      </listitem>
    </itemizedlist>

    <para>The <filename
    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>It is important to note that Shorewall policies (and rules) refer to
    <emphasis role="bold">connections</emphasis> and not packet flow. With the
    policies defined in the <filename
    class="directory">/etc/shorewall/policy</filename> file shown above,
    connections are allowed from the <emphasis>loc</emphasis> zone to the
    <emphasis>net</emphasis> zone even though connections are not allowed from
    the <emphasis>loc</emphasis> zone to the firewall itself.</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 id="Interfaces">
    <title>Network Interfaces</title>

    <mediaobject>
      <imageobject>
        <imagedata align="center" 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>

    <caution>
      <para>Be sure you know which interface is your external interface. Many
      hours have been spent floundering by users who have configured the wrong
      interface. If you are unsure, then as root type <command>ip route
      ls</command> at the command line. The device listed in the last
      (default) route should be your external interface.</para>

      <para>Example:</para>

      <programlisting>root@lists:~# ip route ls
192.168.1.1 dev eth0  scope link 
192.168.2.2 dev tun0  proto kernel  scope link  src 192.168.2.1 
192.168.3.0/24 dev br0  proto kernel  scope link  src 192.168.3.254 
10.13.10.0/24 dev tun1  scope link 
192.168.2.0/24 via 192.168.2.2 dev tun0 
192.168.1.0/24 dev br0  proto kernel  scope link  src 192.168.1.254 
206.124.146.0/24 dev eth0  proto kernel  scope link  src 206.124.146.176 
10.10.10.0/24 dev tun1  scope link 
default via 206.124.146.254 dev <emphasis role="bold">eth0</emphasis> 
root@lists:~# </programlisting>

      <para>In that example, <filename class="devicefile">eth0</filename> is
      the external interface.</para>
    </caution>

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

    <para>I<emphasis role="bold">f 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></emphasis>.</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><emphasis role="bold">Do not connect the internal and external
        interface to the same hub or switch except for testing</emphasis>.You
        can test using this kind of configuration if you specify the <emphasis
        role="bold">arp_filter</emphasis> option or the <emphasis
        role="bold">arp_ignore</emphasis> option in <filename
        class="directory">/etc/shorewall/</filename><filename>interfaces</filename>
        for all interfaces connected to the common hub/switch. <emphasis
        role="bold">Using such a setup with a production firewall is strongly
        recommended against</emphasis>.</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:<tip>
        <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>
      </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
        <acronym>IP</acronym> address, you can remove <varname>dhcp</varname>
        from the option list.</para>
      </tip><tip>
        <para>If your internal interface is a bridge create using the
        <command>brctl</command> utility then <emphasis role="bold">you must
        add the <varname>routeback</varname> option to the option
        list.</emphasis></para>
      </tip></para>
  </section>

  <section id="Addresses">
    <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'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, <emphasis role="bold">you should look at
    the IP address of your external interface and if it is one of the above
    ranges, you should remove the 'norfc1918' option from the external
    interface's entry in <filename
    class="directory">/etc/shorewall/</filename><filename>interfaces</filename>.</emphasis><tip>
        <para>To determine the IP address of your external interface, as root
        type <command>ip addr ls dev &lt;interface&gt;</command> at the
        command line where &lt;<emphasis>interface</emphasis>&gt; is your
        external interface. The line beginning with <emphasis>inet</emphasis>
        identifies your IP address.</para>

        <para>Example:</para>

        <programlisting>root@lists:~# ip addr ls dev eth0
2: eth0: &lt;BROADCAST,MULTICAST,UP,10000&gt; mtu 1500 qdisc htb qlen 1000
    link/ether 00:02:e3:08:48:4c brd ff:ff:ff:ff:ff:ff
    inet <emphasis role="bold">206.124.146.176</emphasis>/24 brd 206.124.146.255 scope global eth0
    inet6 fe80::202:e3ff:fe08:484c/64 scope link 
       valid_lft forever preferred_lft forever
root@lists:~# </programlisting>

        <para>In this example, the IP address of the external interface is
        206.124.146.176</para>
      </tip></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'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 &amp;
    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 id="Diagram">The remainder of this quide will assume that you have
    configured your network as shown here: <mediaobject>
        <imageobject>
          <imagedata align="center" fileref="images/basics1.png" format="PNG" />
        </imageobject>
      </mediaobject> The default gateway for computer's 1 &amp; 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 <emphasis role="bold">you will need to select a DIFFERENT
        RFC 1918 subnet for your local network.</emphasis></para>
      </warning></para>
  </section>

  <section id="SNAT">
    <title>IP Masquerading (SNAT)</title>

    <para>The addresses reserved by RFC 1918 are sometimes referred to as
    <firstterm>non-routable</firstterm> because the Internet backbone routers
    don't forward packets which have an RFC-1918 destination address. When one
    of your local systems (let's assume computer 1 in the <link
    linkend="Diagram">above diagram</link>) 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's external
    interface; in other words, the firewall makes it appear to the destination
    internet host 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't be routed across the internet so the
    remote host can'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 <ulink url="manpages/shorewall-masq.html"><filename
    class="directory">/etc/shorewall/</filename><filename>masq</filename></ulink>
    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 <link linkend="Concepts">the sample</link>. 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 (Masquerade). Entering your static <acronym>IP</acronym> in
    column 3 (SNAT) makes the processing of outgoing packets a little more
    efficient.</para>

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

    <para>I<emphasis role="bold">f you are using the Debian package, please
    check your <filename>shorewall.conf</filename> file to ensure that the
    following is set correctly; if it is not, change it
    appropriately:</emphasis> <itemizedlist spacing="compact">
        <listitem>
          <para><varname>IP_FORWARDING=On</varname></para>
        </listitem>
      </itemizedlist></para>
  </section>

  <section id="DNAT">
    <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><link
    linkend="SNAT">SNAT</link></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>&lt;server local ip address&gt;</emphasis>[:<emphasis>&lt;server port&gt;</emphasis>] <emphasis>&lt;protocol&gt;</emphasis> <emphasis>&lt;port&gt;</emphasis></programlisting><important>
        <para>Be sure to add your rules after the line that reads <emphasis
        role="bold">SECTON NEW.</emphasis></para>
      </important><important>
        <para>The server must have a static IP address. If you assign IP
        addresses to your local system using DHCP, you need to configure your
        DHCP server to always assign the same IP address to systems that are
        the target of a DNAT rule.</para>
      </important>Shorewall has <ulink url="Macros.html">macros</ulink> for
    many popular applications. Look at the output of <command>shorewall show
    macros</command> to see what is available in your release. Macros simplify
    creating DNAT rules by supplying the protocol and port(s) as shown in the
    following examples.</para>

    <para><example id="Example1" label="1">
        <title>Web Server</title>

        <para>You run a Web Server on computer 2 in <link
        linkend="Diagram">the above diagram</link> and you want to forward
        incoming <acronym>TCP</acronym> port 80 to that system:
        <programlisting>#ACTION   SOURCE    DEST             PROTO     DEST PORT(S)
Web/DNAT  net       loc:10.10.10.2</programlisting></para>
      </example> <example id="Example2" label="2">
        <title>FTP Server</title>

        <para>You run an <acronym>FTP</acronym> Server on <link
        linkend="Diagram">computer 1</link> so you want to forward incoming
        <acronym>TCP</acronym> port 21 to that system: <programlisting>#ACTION    SOURCE    DEST            PROTO     DEST PORT(S)
FTP/DNAT   net       loc:10.10.10.1</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
        (<filename>nf_conntrack_ftp</filename> and
        <filename>nf_nat_ftp</filename> in later 2.6 kernels) 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/&lt;kernel
        version&gt;/kernel/net/ipv4/netfilter</filename>. See the <ulink
        url="FTP.html">Shorewall FTP documentation</ulink> for more
        information.</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'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>

    <important>
      <para>When testing DNAT rules like those shown above, you must test from
      a client OUTSIDE YOUR FIREWALL (in the 'net' zone). You cannot test
      these rules from inside the firewall!</para>

      <para>For DNAT troubleshooting tips, <ulink url="FAQ.htm#faq1a">see FAQs
      1a and 1b</ulink>.</para>
    </important>

    <para>For information about DNAT when there are multiple external IP
    addresses, see the <ulink
    url="Shorewall_and_Aliased_Interfaces.html">Shorewall Aliased Interface
    documentation</ulink> and the <ulink
    url="shorewall_setup_guide.htm#dnat">Shorewall Setup Guide</ulink>.</para>
  </section>

  <section id="DNS">
    <title>Domain Name Server (DNS)</title>

    <para>Normally, when you connect to your ISP, as part of getting an IP
    address your firewall'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's name
          servers. If your 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't available, look in /etc/resolv.conf on your firewall system --
          the name servers are given in "nameserver" 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)
DNS/ACCEPT loc       $FW</programlisting></para>
        </listitem>
      </itemizedlist></para>
  </section>

  <section id="Open">
    <title>Other Connections</title>

    <para>The two-interface sample includes the following rules:
    <programlisting>#ACTION     SOURCE    DEST               PROTO     DEST PORT(S)
DNS/ACCEPT  $FW       net</programlisting>This rule allows
    <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>In the rule shown above, <quote>DNS/ACCEPT</quote> is an example of
    a <emphasis>macro invocation</emphasis>. Shorewall includes a number of
    macros (see <filename>/usr/share/shorewall/macro.*</filename>) and <ulink
    url="Macros.html">you can add your own</ulink>.</para>

    <para>You don't have to use defined macros when coding a rule in
    <filename>/etc/shorewall/rules</filename>; Shorewall will start slightly
    faster if you code your rules directly rather than using macros. The the
    rule shown above could also have been coded as follows:<programlisting>#ACTION    SOURCE    DEST               PROTO     DEST PORT(S)
ACCEPT     $FW       net                udp       53
ACCEPT     $FW       net                tcp       53</programlisting></para>

    <para>In cases where Shorewall doesn't include a defined macro to meet
    your needs, you can either define the macro yourself or you can simply
    code the appropriate rules directly.</para>

    <para>The sample also includes: <programlisting>#ACTION      SOURCE    DEST               PROTO     DEST PORT(S)
SSH/ACCEPT   loc       $FW  </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 from your firewall to other
    systems, the general format using a macro is: <programlisting>#ACTION         SOURCE    DEST               PROTO      DEST PORT(S)
&lt;macro&gt;/ACCEPT  $FW       <emphasis>&lt;destination zone&gt;</emphasis></programlisting>The
    general format when not using defined actions is:<programlisting>#ACTION    SOURCE    DEST               PROTO      DEST PORT(S)
ACCEPT     $FW       <emphasis>&lt;destination zone&gt; &lt;protocol&gt; &lt;port&gt;</emphasis></programlisting><example
        id="Example3">
        <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)
Web/ACCEPT  net       $FW
Web/ACCEPT  loc       $FW       </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't know what port and protocol a particular
    application uses, look <ulink url="ports.htm">here</ulink>. <important>
        <para>I don'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)
SSH/ACCEPT   net       $FW</programlisting>
      </important> <inlinegraphic fileref="images/leaflogo.gif"
    format="GIF" />Bering users will want to add the following two rules to be
    compatible with Jacques'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 id="Logging">
    <title>Logging</title>

    <para>Shorewall does not maintain a log itself but rather relies on your
    <ulink url="shorewall_logging.htm">system's logging configuration</ulink>.
    The following <ulink url="manpages/shorewall.html">commands</ulink> rely
    on knowing where Netfilter messages are logged:</para>

    <itemizedlist>
      <listitem>
        <para><command>shorewall show log</command> (Displays the last 20
        netfilter log messages)</para>
      </listitem>

      <listitem>
        <para><command>shorewall logwatch</command> (Polls the log at a
        settable interval</para>
      </listitem>

      <listitem>
        <para><command>shorewall dump</command> (Produces an extensive report
        for inclusion in Shorewall problem reports)</para>
      </listitem>
    </itemizedlist>

    <para>It is important that these commands work properly because when you
    encounter connection problems when Shorewall is running, the first thing
    that you should do is to look at the Netfilter log; with the help of
    <ulink url="FAQ.htm#faq17">Shorewall FAQ 17</ulink>, you can usually
    resolve the problem quickly.</para>

    <para>Most commonly, Netfilter messages are logged to
    <filename>/var/log/messages</filename>. Recent
    <trademark>SuSE/OpenSuSE</trademark> releases come preconfigured with
    syslog-ng and log netfilter messages to
    <filename>/var/log/firewall</filename>.</para>

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

    <para>If you are running a distribution that logs netfilter messages to a
    log other than <filename>/var/log/messages</filename>, then modify the
    LOGFILE setting in <filename>/etc/shorewall/shorewall.conf</filename> to
    specify the name of your log.</para>

    <important>
      <para>The LOGFILE setting does not control where the Netfilter log is
      maintained -- it simply tells the /sbin/<filename>shorewall</filename>
      utility where to find the log.</para>
    </important>
  </section>

  <section id="Other">
    <title>Some Things to Keep in Mind</title>

    <itemizedlist>
      <listitem>
        <para><emphasis role="bold">You cannot test your firewall from the
        inside</emphasis>. Just because you send requests to your firewall
        external IP address does not mean that the request will be associated
        with the external interface or the <quote>net</quote> zone. Any
        traffic that you generate from the local network will be associated
        with your local interface and will be treated as loc-&gt;fw
        traffic.</para>
      </listitem>

      <listitem>
        <para><emphasis role="bold">IP addresses are properties of systems,
        not of interfaces</emphasis>. It is a mistake to believe that your
        firewall is able to forward packets just because you can ping the IP
        address of all of the firewall's interfaces from the local network.
        The only conclusion you can draw from such pinging success is that the
        link between the local system and the firewall works and that you
        probably have the local system's default gateway set correctly.</para>
      </listitem>

      <listitem>
        <para><emphasis role="bold">All IP addresses configured on firewall
        interfaces are in the $FW (fw) zone</emphasis>. If 192.168.1.254 is
        the IP address of your internal interface then you can write
        <quote><emphasis role="bold">$FW:192.168.1.254</emphasis></quote> in a
        rule but you may not write <quote><emphasis
        role="bold">loc:192.168.1.254</emphasis></quote>. Similarly, it is
        nonsensical to add 192.168.1.254 to the <emphasis
        role="bold">loc</emphasis> zone using an entry in
        <filename>/etc/shorewall/hosts</filename>.</para>
      </listitem>

      <listitem>
        <para><emphasis role="bold">Reply packets do NOT automatically follow
        the reverse path of the one taken by the original request</emphasis>.
        All packets are routed according to the routing table of the host at
        each step of the way. This issue commonly comes up when people install
        a Shorewall firewall parallel to an existing gateway and try to use
        DNAT through Shorewall without changing the default gateway of the
        system receiving the forwarded requests. Requests come in through the
        Shorewall firewall where the destination IP address gets rewritten but
        replies go out unmodified through the old gateway.</para>
      </listitem>

      <listitem>
        <para><emphasis role="bold">Shorewall itself has no notion of inside
        or outside</emphasis>. These concepts are embodied in how Shorewall is
        configured.</para>
      </listitem>
    </itemizedlist>
  </section>

  <section id="Starting">
    <title>Starting and Stopping Your Firewall</title>

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

    <para>The <ulink url="Install.htm">installation procedure</ulink>
    configures your system to start Shorewall at system boot but startup is
    disabled so that your system won't try to start Shorewall before
    configuration is complete. Once you have completed configuration of your
    firewall, you must edit /etc/shorewall/shorewall.conf and set
    STARTUP_ENABLED=Yes.<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="manpages/shorewall-routestopped.html">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'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'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 id="Trouble">
    <title>If it Doesn't Work</title>

    <itemizedlist>
      <listitem>
        <para>Re-check each of the items flagged with a red arrow
        above.</para>
      </listitem>

      <listitem>
        <para>Check your <ulink
        url="shorewall_logging.html">log</ulink>.</para>
      </listitem>

      <listitem>
        <para>Check the <ulink url="troubleshoot.htm">Troubleshooting
        Guide</ulink>.</para>
      </listitem>

      <listitem>
        <para>Check the <ulink url="FAQ.htm">FAQ</ulink>.</para>
      </listitem>
    </itemizedlist>
  </section>

  <section id="Reading">
    <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 id="Wireless">
    <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'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'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>

        <para><emphasis role="bold">Update</emphasis>: Distributions are
        getting better about this. <trademark>SuSE</trademark> now associates
        a unique interface name with each MAC address. Other distributions
        have add-on packages to manage the relationship between MAC addresses
        and device names.</para>
      </caution></para>

    <para>Your new network will look similar to what is shown in the following
    figure.<graphic align="center" 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'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>