shorewall_code/docs/FAQ.xml

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  <articleinfo>
    <title>Shorewall FAQs</title>

    <authorgroup>
      <corpauthor>Shorewall Community</corpauthor>

      <author>
        <firstname>Tom</firstname>

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

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

    <copyright>
      <year>2001-2008</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 3.0 and
    later. If you are running a version of Shorewall earlier than Shorewall
    3.0.0 then please see the documentation for that
    release.</emphasis></para>
  </caution>

  <section id="Install">
    <title>Installing Shorewall</title>

    <section id="Howto">
      <title>Where do I find Step by Step Installation and Configuration
      Instructions?</title>

      <para><emphasis role="bold">Answer:</emphasis> Check out the <ulink
      url="shorewall_quickstart_guide.htm">QuickStart Guides</ulink>.</para>
    </section>

    <section id="faq37">
      <title>(FAQ 37) I just installed Shorewall on Debian and the
      /etc/shorewall directory is almost empty!!!</title>

      <para><emphasis role="bold">Answer:</emphasis></para>

      <important>
        <para>Once you have installed the .deb package and before you attempt
        to configure Shorewall, please heed the advice of Lorenzo Martignoni,
        the Shorewall Debian Maintainer:</para>

        <para><quote>For more information about Shorewall usage on Debian
        system please look at /usr/share/doc/shorewall-common/README.Debian
        provided by [the] shorewall-common Debian package.</quote></para>
      </important>

      <para>If you install using the .deb, you will find that your <filename
      class="directory">/etc/shorewall</filename> directory is almost 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-common/default-config</filename>.
      Simply copy the files you need from that directory to <filename
      class="directory">/etc/shorewall</filename> and modify the
      copies.</para>

      <section id="faq37a">
        <title>(FAQ 37a) I just installed Shorewall on Debian and I can't find
        the sample configurations.</title>

        <para><emphasis role="bold">Answer:</emphasis> With Shorewall 3.x, the
        samples are included in the shorewall package and are installed in
        <filename
        class="directory">/usr/share/doc/shorewall/examples/</filename>.
        Beginning with Shorewall 4.0, the samples are in the shorewall-common
        package and are installed in <filename
        class="directory">/usr/share/doc/shorewall-common/examples/</filename>.</para>
      </section>
    </section>

    <section id="faq75">
      <title>(FAQ 75) I can't find the Shorewall 4.x shorewall-common RPM.
      Where is it?</title>

      <para><emphasis role="bold">Answer:</emphasis> If you use Simon Matter's
      Redhat/Fedora/CentOS rpms, be aware that Simon calls the
      <emphasis>shorewall-common</emphasis> RPM
      <emphasis>shorewall</emphasis>. So you should download and install the
      appropriate <emphasis>shorewall-4.x.y</emphasis> RPM from his
      site.</para>
    </section>
  </section>

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

    <section id="faq66">
      <title>(FAQ 66) I'm trying to upgrade to Shorewall 4.0; where is the
      'shorewall' package?</title>

      <para><emphasis role="bold">Answer:</emphasis> Please see the <ulink
      url="upgrade_issues.htm">upgrade issues.</ulink></para>

      <section id="faq66a">
        <title>(FAQ 66a) I'm trying to upgrade to Shorewall 4.0; do I have to
        uninstall the 'shorewall' package?</title>

        <para><emphasis role="bold">Answer:</emphasis> Please see the <ulink
        url="upgrade_issues.htm">upgrade issues.</ulink></para>
      </section>

      <section id="faq66b">
        <title>(FAQ 66b) I'm trying to upgrade to Shorewall 4.0: which of
        these packages do I need to install?</title>

        <para><emphasis role="bold">Answer:</emphasis> Please see the <ulink
        url="upgrade_issues.htm">upgrade issues.</ulink></para>
      </section>
    </section>

    <section id="faq76">
      <title>(FAQ 76) I just upgraded my Debian system and now masquerading
      doesn't work? What happened?</title>

      <para><emphasis role="bold">Answer:</emphasis> This happens to people
      who ignore <ulink url="Install.htm#Upgrade_Deb">our advice</ulink> and
      allow the installer to replace their working
      <filename>/etc/shorewall/shorewall.conf</filename> with one that has
      default settings. Failure to forward traffic (such as during masqueraded
      net access from a local network) usually means that <filename><ulink
      url="manpages/shorewall.conf.html">/etc/shorewall/shorewall.conf</ulink></filename>
      contains the Debian default setting IP_FORWARDING=Keep; it should be
      IP_FORWARDING=On.</para>

      <section id="faq76a">
        <title>(FAQ 76a) I just upgraded my Ubuntu system and now masquerading
        doesn't work? What happened?</title>

        <para><emphasis role="bold">Answer:</emphasis> See <link
        linkend="faq76">above</link>.</para>
      </section>

      <section id="faq76b">
        <title>(FAQ 76b) I just upgraded my Kubuntu system and now
        masquerading doesn't work? What happened?</title>

        <para><emphasis role="bold">Answer:</emphasis> See <link
        linkend="faq76">above</link>.</para>
      </section>
    </section>
  </section>

  <section id="PortForwarding">
    <title>Port Forwarding (Port Redirection)</title>

    <section id="faq1">
      <title>(FAQ 1) I want to forward UDP port 7777 to my personal PC with IP
      address 192.168.1.5. I've looked everywhere and can't find how to do
      it.</title>

      <para><emphasis role="bold">Answer:</emphasis> The format of a
      port-forwarding rule to a local system is as follows:</para>

      <programlisting>#ACTION    SOURCE      DEST                                   PROTO        DEST PORT
DNAT       net         loc:<emphasis>local-IP-address</emphasis>[:<emphasis>local-port</emphasis>]      <emphasis>protocol</emphasis>     <emphasis>port-number</emphasis></programlisting>

      <para>So to forward UDP port 7777 to internal system 192.168.1.5, the
      rule is:</para>

      <programlisting>#ACTION    SOURCE   DEST             PROTO    DEST PORT
DNAT       net      loc:192.168.1.5  udp      7777</programlisting>

      <para>If you want to forward requests directed to a particular address (
      <emphasis>external-IP</emphasis> ) on your firewall to an internal
      system:</para>

      <programlisting>#ACTION SOURCE DEST                                   PROTO       DEST PORT     SOURCE  ORIGINAL
#                                                                               PORT    DEST.
DNAT    net    loc:<emphasis>local-IP-address</emphasis>&gt;[:<emphasis>local-port</emphasis>]     <emphasis>protocol</emphasis>    <emphasis>port-number</emphasis>   -       <emphasis>external-IP</emphasis></programlisting>

      <para>If you want to forward requests from a particular Internet address
      ( <emphasis>address</emphasis> ):</para>

      <programlisting>#ACTION SOURCE        DEST                                   PROTO       DEST PORT     SOURCE  ORIGINAL
#                                                                                      PORT    DEST.
DNAT    net:<emphasis>address</emphasis>   loc:<emphasis>local-IP-address</emphasis>[:<emphasis>local-port</emphasis>]  <emphasis>    protocol</emphasis>    <emphasis>port-number</emphasis>   -</programlisting>

      <para>Finally, if you need to forward a range of ports, in the DEST PORT
      column specify the range as
      <emphasis>low-port:high-port</emphasis>.</para>

      <section id="faq1a">
        <title>(FAQ 1a) Okay -- I followed those instructions but it doesn't
        work</title>

        <para><emphasis role="bold">Answer:</emphasis> That is usually the
        result of one of four things:</para>

        <itemizedlist>
          <listitem>
            <para>You are trying to test from inside your firewall (no, that
            won't work -- see <xref linkend="faq2" />).</para>
          </listitem>

          <listitem>
            <para>You have a more basic problem with your local system (the
            one that you are trying to forward to) such as an incorrect
            default gateway (it must be set to the IP address of your
            firewall's internal interface; if that isn't possible for some
            reason, see <link linkend="faq1f">FAQ 1f</link>).</para>
          </listitem>

          <listitem>
            <para>Your ISP is blocking that particular port inbound or, for
            TCP, your ISP is dropping the outbound SYN,ACK response.</para>
          </listitem>

          <listitem>
            <para>You are running Mandriva Linux prior to 10.0 final and have
            configured Internet Connection Sharing. In that case, the name of
            your local zone is 'masq' rather than 'loc' (change all instances
            of 'loc' to 'masq' in your rules). You may want to consider
            re-installing Shorewall in a configuration which matches the
            Shorewall documentation. See the <ulink
            url="two-interface.htm">two-interface QuickStart Guide</ulink> for
            details.</para>
          </listitem>
        </itemizedlist>
      </section>

      <section id="faq1b">
        <title>(FAQ 1b) I'm still having problems with port forwarding</title>

        <para><emphasis role="bold">Answer:</emphasis> To further diagnose
        this problem:</para>

        <itemizedlist>
          <listitem>
            <para>As root, type <quote> <command>shorewall reset</command>
            </quote> ("<command>shorewall-lite reset</command>", if you are
            running Shorewall Lite). This clears all Netfilter
            counters.</para>
          </listitem>

          <listitem>
            <para>Try to connect to the redirected port from an external
            host.</para>
          </listitem>

          <listitem>
            <para>As root type <quote> <command>shorewall show nat</command>
            </quote> ("<command>shorewall-lite show nat</command>", if you are
            running Shorewall Lite).</para>
          </listitem>

          <listitem>
            <para>Locate the appropriate DNAT rule. It will be in a chain
            called <emphasis>&lt;source zone&gt;</emphasis>_dnat
            (<quote>net_dnat</quote> in the above examples).</para>
          </listitem>

          <listitem>
            <para>Is the packet count in the first column non-zero? If so, the
            connection request is reaching the firewall and is being
            redirected to the server. In this case, the problem is usually a
            missing or incorrect default gateway setting on the local system
            (the system you are trying to forward to -- its default gateway
            should be the IP address of the firewall's interface to that
            system).</para>
          </listitem>

          <listitem>
            <para>If the packet count is zero:</para>

            <itemizedlist>
              <listitem>
                <para>the connection request is not reaching your server
                (possibly it is being blocked by your ISP); or</para>
              </listitem>

              <listitem>
                <para>you are trying to connect to a secondary IP address on
                your firewall and your rule is only redirecting the primary IP
                address (You need to specify the secondary IP address in the
                <quote>ORIG. DEST.</quote> column in your DNAT rule);
                or</para>
              </listitem>

              <listitem>
                <para>your DNAT rule doesn't match the connection request in
                some other way. In that case, you may have to use a packet
                sniffer such as tcpdump or ethereal to further diagnose the
                problem.</para>
              </listitem>
            </itemizedlist>
          </listitem>

          <listitem>
            <para>If the packet count is non-zero, check your log to see if
            the connection is being dropped or rejected. If it is, then you
            may have a zone definition problem such that the server is in a
            different zone than what is specified in the DEST column. At a
            root prompt, type "<command>shorewall show zones</command>"
            ("<command>shorewall-lite show zones</command>") then be sure that
            in the DEST column you have specified the <emphasis
            role="bold">first</emphasis> zone in the list that matches
            OUT=&lt;dev&gt; and DEST= &lt;ip&gt;from the REJECT/DROP log
            message.</para>
          </listitem>

          <listitem>
            <para>If everything seems to be correct according to these tests
            but the connection doesn't work, it may be that your ISP is
            blocking SYN,ACK responses. This technique allows your ISP to
            detect when you are running a server (usually in violation of your
            service agreement) and to stop connections to that server from
            being established.</para>
          </listitem>
        </itemizedlist>
      </section>

      <section id="faq1c">
        <title>(FAQ 1c) From the Internet, I want to connect to port 1022 on
        my firewall and have the firewall forward the connection to port 22 on
        local system 192.168.1.3. How do I do that?</title>

        <para><emphasis role="bold">Answer:</emphasis>In
        /<filename>etc/shorewall/rules</filename>:</para>

        <programlisting>#ACTION    SOURCE   DEST                PROTO    DEST PORT
DNAT       net      loc:192.168.1.3:22  tcp      1022</programlisting>
      </section>

      <section id="faq1d">
        <title>(FAQ 1d) I have a web server in my DMZ and I use port
        forwarding to make that server accessible from the Internet. That
        works fine but when my local users try to connect to the server using
        the Firewall's external IP address, it doesn't work.</title>

        <para><emphasis role="bold">Answer:</emphasis> See <link
        linkend="faq2b">FAQ 2b</link>.</para>
      </section>

      <section id="faq1e">
        <title>(FAQ 1e) In order to discourage brute force attacks I would
        like to redirect all connections on a non-standard port (4104) to port
        22 on the router/firewall. I notice that setting up a REDIRECT rule
        causes the firewall to open both ports 4104 and 22 to connections from
        the net. Is it possible to only redirect 4104 to the localhost port 22
        and have connection attempts to port 22 from the net dropped?</title>

        <para><emphasis role="bold">Answer </emphasis>courtesy of Ryan: Assume
        that the IP address of your local firewall interface is 192.168.1.1.
        If you configure SSHD to only listen on that interface and add the
        following rule then from the net, you will have 4104 listening, from
        your LAN, port 22.</para>

        <programlisting>#ACTION SOURCE  DEST                    PROTO   DEST PORT(S)
DNAT    net     fw:192.168.1.1:22       tcp     4104</programlisting>
      </section>

      <section id="faq1f">
        <title>(FAQ 1f) Why must the server that I port forward to have it's
        default gateway set to my Shorewall system's IP address?</title>

        <para><emphasis role="bold">Answer:</emphasis> Let's take an example.
        Suppose that</para>

        <itemizedlist>
          <listitem>
            <para>Your Shorewall firewall's external IP address is
            206.124.146.176 (eth0) and its internal IP address is 192.168.1.1
            (eth1).</para>
          </listitem>

          <listitem>
            <para>You have another gateway router with external IP address
            130.252.100.109 and internal IP address 192.168.1.254.</para>
          </listitem>

          <listitem>
            <para>You have an FTP server behind both routers with IP address
            192.168.1.4</para>
          </listitem>

          <listitem>
            <para>The FTP server's default gateway is through the second
            router (192.168.1.254).</para>
          </listitem>

          <listitem>
            <para>You have this rule on the Shorewall system:<programlisting>#ACTION    SOURCE        DEST               PROTO    DEST PORT   SOURCE    ORIGINAL
#                                                                PORT      DEST.
DNAT       net           loc:192.168.1.4    tcp      21          -         206.124.146.176</programlisting></para>
          </listitem>

          <listitem>
            <para>Internet host 16.105.221.4 issues the command <command>ftp
            206.124.146.176</command></para>
          </listitem>
        </itemizedlist>

        <para>This results in the following set of events:</para>

        <orderedlist>
          <listitem>
            <para>16.105.221.4 sends a TCP SYN packet to 206.124.146.176
            specifying destination port 21.</para>
          </listitem>

          <listitem>
            <para>The Shorewall box rewrites the destination IP address to
            192.168.1.4 and forwards the packet.</para>
          </listitem>

          <listitem>
            <para>The FTP server receives the packet and accepts the
            connection, generating a SYN,ACK packet back to 16.105.221.4.
            Because the server's default gateway is through the second router,
            it sends the packet to that router.</para>
          </listitem>
        </orderedlist>

        <para>At this point, one of two things can happen. Either the second
        router discards or rejects the packet; or, it rewrites the source IP
        address to 130.252.100.109 and forwards the packet back to
        16.105.221.4. Regardless of which happens, the connection is doomed.
        Clearly if the packet is rejected or dropped, the connection will not
        be successful. But even if the packet reaches 16.105.221.4, that host
        will reject it since it's SOURCE IP address (130.252.100.109) doesn't
        match the DESTINATION IP ADDRESS (206.124.146.176) of the original SYN
        packet.</para>

        <para>The best way to work around this problem is to change the
        default gateway on the FTP server to the Shorewall system's internal
        IP address (192.168.1.1). But if that isn't possible, you can work
        around the problem with the following ugly hack in
        <filename>/etc/shorewall/masq</filename>:<programlisting>#INTERFACE              SOURCE             ADDRESS            PROTO         PORT
eth1:192.168.1.4        0.0.0.0/0          192.168.1.1        tcp           21</programlisting></para>

        <para>This rule has the undesirable side effect that it makes all FTP
        connections from the net appear to the FTP server as if they
        originated on the Shorewall system. But it will force the FTP server
        to reply back through the Shorewall system who can then rewrite the
        SOURCE IP address in the responses properly.</para>
      </section>

      <section id="faq1g">
        <title>(FAQ 1g) I would like to redirect port 80 on my public IP
        address (206.124.146.176) to port 993 on Internet host
        66.249.93.111</title>

        <para><emphasis role="bold">Answer:</emphasis> This requires a vile
        hack similar to the one in <link linkend="faq2">FAQ 2</link>. Assuming
        that your Internet zone is named <emphasis>net</emphasis> and connects
        on interface <filename class="devicefile">eth0</filename>:</para>

        <para>In <filename>/etc/shorewall/rules</filename>:<programlisting>#ACTION    SOURCE        DEST                   PROTO    DEST PORT   SOURCE    ORIGINAL
#                                                                    PORT      DEST.
DNAT       net           net:66.249.93.111:993  tcp      80          -         206.124.146.176</programlisting></para>

        <para>In <filename>/etc/shorewall/interfaces</filename>, specify the
        <emphasis role="bold">routeback</emphasis> option on
        eth0:<programlisting>#ZONE      INTERFACE     BROADCAST            OPTIONS
net        eth0          detect               <emphasis role="bold">routeback</emphasis></programlisting></para>

        <para>And in <filename>/etc/shorewall/masq</filename>;<programlisting>#INTERFACE          SOURCE      ADDRESS          PROTO        PORT
eth0:66.249.93.111  0.0.0.0/0   206.124.146.176  tcp          993</programlisting></para>

        <para>Like the hack in FAQ 2, this one results in all forwarded
        connections looking to the server (66.249.93.11) as if they originated
        on your firewall (206.124.146.176).</para>
      </section>
    </section>

    <section id="faq30">
      <title>(FAQ 30) I'm confused about when to use DNAT rules and when to
      use ACCEPT rules.</title>

      <para><emphasis role="bold">Answer:</emphasis> It would be a good idea
      to review the <ulink url="shorewall_quickstart_guide.htm">QuickStart
      Guide</ulink> appropriate for your setup; the guides cover this topic in
      a tutorial fashion. DNAT rules should be used for connections that need
      to go the opposite direction from SNAT/MASQUERADE. So if you masquerade
      or use SNAT from your local network to the Internet then you will need
      to use DNAT rules to allow connections from the Internet to your local
      network. You also want to use DNAT rules when you intentionally want to
      rewrite the destination IP address or port number. In all other cases,
      you use ACCEPT unless you need to hijack connections as they go through
      your firewall and handle them on the firewall box itself; in that case,
      you use a REDIRECT rule.</para>
    </section>

    <section id="faq38">
      <title>(FAQ 38) Where can I find more information about DNAT?</title>

      <para><emphasis role="bold">Answer:</emphasis> Ian Allen has written a
      <ulink url="http://ian.idallen.ca/dnat.txt">Paper about DNAT and
      Linux</ulink>.</para>
    </section>

    <section id="faq48">
      <title>(FAQ 48) How do I Set up Transparent HTTP Proxy with
      Shorewall?</title>

      <para><emphasis role="bold">Answer:</emphasis> See <ulink
      url="Shorewall_Squid_Usage.html">Shorewall_Squid_Usage.html</ulink>.</para>
    </section>
  </section>

  <section id="DNS-DNAT">
    <title id="DNS">DNS and Port Forwarding/NAT</title>

    <section id="faq2">
      <title>(FAQ 2) I port forward www requests to www.mydomain.com (IP
      130.151.100.69) to system 192.168.1.5 in my local network. External
      clients can browse http://www.mydomain.com but internal clients
      can't.</title>

      <para><emphasis role="bold">Answer:</emphasis> I have two objections to
      this setup.</para>

      <itemizedlist>
        <listitem>
          <para>Having an Internet-accessible server in your local network is
          like raising foxes in the corner of your hen house. If the server is
          compromised, there's nothing between that server and your other
          internal systems. For the cost of another NIC and a cross-over
          cable, you can put your server in a DMZ such that it is isolated
          from your local systems - assuming that the Server can be located
          near the Firewall, of course :-)</para>
        </listitem>

        <listitem>
          <para>The accessibility problem is best solved using
          <firstterm>Split DNS</firstterm> (either <ulink
          url="SplitDNS.html">use a separate DNS server</ulink> for local
          clients or use <ulink url="shorewall_setup_guide.htm#DNS">Bind
          Version 9 <quote>views</quote></ulink> on your main name server)
          such that www.mydomain.com resolves to 130.141.100.69 externally and
          192.168.1.5 internally. That's what I do here at shorewall.net for
          my local systems that use one-to-one NAT.</para>
        </listitem>
      </itemizedlist>

      <para>So the best and most secure way to solve this problem is to move
      your Internet-accessible server(s) to a separate LAN segment with it's
      own interface to your firewall and follow <link linkend="faq2b">FAQ
      2b</link>. That way, your local systems are still safe if your server
      gets hacked and you don't have to run a split DNS configuration
      (separate server or Bind 9 views).</para>

      <para>If physical limitations make it impractical to segregate your
      servers on a separate LAN, the next best solution it to use Split DNS.
      Before you complain "It's too hard to set up split DNS!", <ulink
      url="SplitDNS.html"><emphasis role="bold">check
      here</emphasis></ulink>.</para>

      <para>But if you are the type of person who prefers quick and dirty
      hacks to "doing it right", then proceed as described below.<warning>
          <para>All traffic redirected through use of this hack will look to
          the server as if it originated on the firewall rather than on the
          original client! So the server's access logs will be useless for
          determining which local hosts are accessing the server.</para>
        </warning></para>

      <para>Assuming that your external interface is eth0 and your internal
      interface is eth1 and that eth1 has IP address 192.168.1.254 with subnet
      192.168.1.0/24, then:</para>

      <itemizedlist>
        <listitem>
          <para>In <filename>/etc/shorewall/interfaces</filename>:</para>

          <programlisting>#ZONE    INTERFACE    BROADCAST    OPTIONS
loc      eth1         detect       <emphasis role="bold">routeback</emphasis>    </programlisting>
        </listitem>

        <listitem>
          <para>In <filename>/etc/shorewall/masq</filename>:</para>

          <programlisting>#INTERFACE              SOURCE          ADDRESS         PROTO   PORT(S)
<emphasis role="bold">eth1:192.168.1.5        eth1            192.168.1.254   tcp     www</emphasis></programlisting>
        </listitem>

        <listitem>
          <para>In <filename>/etc/shorewall/rules</filename>:</para>

          <programlisting>#ACTION    SOURCE       DEST               PROTO    DEST PORT   SOURCE    ORIGINAL
#                                                               PORT      DEST.
<emphasis role="bold">DNAT       loc          loc:192.168.1.5    tcp      www         -         130.151.100.69</emphasis></programlisting>

          <para>That rule only works of course if you have a static external
          IP address. If you have a dynamic IP address then include this in
          <filename>/etc/shorewall/params</filename> (or your
          <filename>&lt;export directory&gt;/init</filename> file if you are
          using Shorewall Lite on the firewall system):</para>

          <programlisting><command>ETH0_IP=`find_first_interface_address eth0`</command>        </programlisting>

          <para>and make your DNAT rule:</para>

          <programlisting>#ACTION    SOURCE        DEST               PROTO    DEST PORT   SOURCE    ORIGINAL
#                                                                PORT      DEST.
DNAT       loc           loc:192.168.1.5    tcp      www         -         <emphasis
              role="bold">$ETH0_IP</emphasis></programlisting>

          <para>Using this technique, you will want to configure your
          DHCP/PPPoE/PPTP/… client to automatically restart Shorewall each
          time that you get a new IP address.<note>
              <para>If you are running Shorewall 3.2.6 on a Debian-based
              system, the call to
              <command>find_first_interface_address</command> in
              <filename>/etc/shorewall/params</filename> must be preceded with
              a load of the Shorewall function library:<programlisting><command>. /usr/share/shorewall/functions</command>
<command>ETH0_IP=`find_first_interface_address eth0`</command></programlisting></para>
            </note></para>
        </listitem>
      </itemizedlist>

      <section id="faq2a">
        <title>(FAQ 2a) I have a zone <quote>Z</quote> with an RFC1918 subnet
        and I use one-to-one NAT to assign non-RFC1918 addresses to hosts in
        Z. Hosts in Z cannot communicate with each other using their external
        (non-RFC1918 addresses) so they can't access each other using their
        DNS names.</title>

        <note>
          <para>If the ALL INTERFACES column in /etc/shorewall/nat is empty or
          contains <quote>Yes</quote>, you will also see log messages like the
          following when trying to access a host in Z from another host in Z
          using the destination host's public address:</para>

          <programlisting>Oct 4 10:26:40 netgw kernel:
          Shorewall:FORWARD:REJECT:IN=eth1 OUT=eth1 SRC=192.168.118.200
          DST=192.168.118.210 LEN=48 TOS=0x00 PREC=0x00 TTL=127 ID=1342 DF
          PROTO=TCP SPT=1494 DPT=1491 WINDOW=17472 RES=0x00 ACK SYN URGP=0</programlisting>
        </note>

        <para><emphasis role="bold">Answer:</emphasis> This is another problem
        that is best solved using Bind Version 9 <quote>views</quote>. It
        allows both external and internal clients to access a NATed host using
        the host's DNS name.</para>

        <para>Another good way to approach this problem is to switch from
        one-to-one NAT to Proxy ARP. That way, the hosts in Z have non-RFC1918
        addresses and can be accessed externally and internally using the same
        address.</para>

        <para>If you don't like those solutions and prefer, incredibly, to
        route all Z-&gt;Z traffic through your firewall then:</para>

        <orderedlist>
          <listitem>
            <para>Set the routeback option on the interface to Z.</para>
          </listitem>

          <listitem>
            <para>Set the ALL INTERFACES column in the nat file to
            <quote>Yes</quote>.</para>
          </listitem>
        </orderedlist>

        <example id="Example1">
          <title>Example:</title>

          <literallayout>Zone: dmz, Interface: eth2, Subnet: 192.168.2.0/24, Address: 192.168.2.254</literallayout>

          <para>In <filename>/etc/shorewall/interfaces</filename>:</para>

          <programlisting>#ZONE    INTERFACE    BROADCAST       OPTIONS
dmz      eth2         192.168.2.255   <emphasis role="bold">routeback</emphasis> </programlisting>

          <para>In <filename>/etc/shorewall/nat</filename>, be sure that you
          have <quote>Yes</quote> in the ALL INTERFACES column.</para>

          <para>In /etc/shorewall/masq:</para>

          <programlisting>#INTERFACE    SOURCE      ADDRESS
<emphasis role="bold">eth2          eth2        192.168.2.254</emphasis></programlisting>

          <para>Like the silly hack in FAQ 2 above, this will make all
          dmz-&gt;dmz traffic appear to originate on the firewall.</para>
        </example>
      </section>

      <section id="faq2b">
        <title>(FAQ 2b) I have a web server in my DMZ and I use port
        forwarding to make that server accessible from the Internet as
        www.mydomain.com. That works fine but when my local users try to
        connect to www.mydomain.com, it doesn't work.</title>

        <para><emphasis role="bold">Answer:</emphasis> Let's assume the
        following:</para>

        <itemizedlist>
          <listitem>
            <para>External IP address is 206.124.146.176 on <filename
            class="devicefile">eth0</filename> (www.mydomain.com).</para>
          </listitem>

          <listitem>
            <para>Server's IP address is 192.168.2.4</para>
          </listitem>
        </itemizedlist>

        <para>You can enable access to the server from your local network
        using the firewall's external IP address by adding this rule:</para>

        <programlisting>#ACTION    SOURCE   DEST                PROTO    DEST PORT(S)    SOURCE      ORIGINAL
#                                                                PORT        DEST                 
<emphasis role="bold">DNAT       loc      dmz:192.168.2.4     tcp      80              -           206.124.146.176</emphasis></programlisting>

        <para>If your external IP address is dynamic, then you must do the
        following:</para>

        <para>In <filename>/etc/shorewall/params</filename> (or in your
        <filename>&lt;export directory&gt;/init</filename> file if you are
        using Shorewall Lite on the firewall system):</para>

        <programlisting><command>ETH0_IP=`find_first_interface_address eth0`</command>  </programlisting>

        <para>and make your DNAT rule:</para>

        <programlisting>#ACTION    SOURCE        DEST               PROTO    DEST PORT   SOURCE    ORIGINAL
#                                                                PORT      DEST.
DNAT       loc           dmz:192.168.2.4    tcp      80          -         <emphasis
            role="bold">$ETH0_IP</emphasis></programlisting>

        <warning>
          <para>With dynamic IP addresses, you probably don't want to use
          <ulink
          url="starting_and_stopping_shorewall.htm"><command>shorewall[-lite]
          save</command> and <command>shorewall[-lite]
          restore</command></ulink>.</para>
        </warning>

        <note>
          <para>If you are running Shorewall 3.2.6 on a Debian-based system,
          the call to <command>find_first_interface_address</command> in
          <filename>/etc/shorewall/params</filename> must be preceded with a
          load of the Shorewall function library:<programlisting><command>. /usr/share/shorewall/functions</command>
<command>ETH0_IP=`find_first_interface_address eth0`</command></programlisting></para>
        </note>
      </section>

      <section id="faq2c">
        <title>(FAQ 2c) I tried to apply the answer to FAQ 2 to my external
        interface and the net zone and it didn't work. Why?</title>

        <para><emphasis role="bold">Answer:</emphasis> Did you set <emphasis
        role="bold">IP_FORWARDING=On</emphasis> in
        <filename>shorewall.conf</filename>?</para>
      </section>
    </section>
  </section>

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

    <section id="faq63">
      <title>(FAQ 63) I just blacklisted IP address 206.124.146.176 and I can
      still ping it. What did I do wrong?</title>

      <para><emphasis role="bold">Answer:</emphasis> Nothing.</para>

      <para>Blacklisting an IP address blocks incoming traffic from that IP
      address. And if you set BLACKLISTNEWONLY=Yes in
      <filename>shorewall.conf</filename>, then only new connections <emphasis
      role="bold">from</emphasis> that address are disallowed; traffic from
      that address that is part of an established connection (such as ping
      replies) is allowed.</para>
    </section>
  </section>

  <section id="MSN">
    <title>Netmeeting/MSN</title>

    <section id="faq3">
      <title>(FAQ 3) I want to use Netmeeting or MSN Instant Messenger with
      Shorewall. What do I do?</title>

      <para><emphasis role="bold">Answer:</emphasis> There is an <ulink
      url="http://www.kfki.hu/~kadlec/sw/netfilter/newnat-suite/">H.323
      connection tracking/NAT module</ulink> that helps with Netmeeting. Note
      however that one of the Netfilter developers recently posted the
      following:</para>

      <blockquote>
        <para><programlisting>&gt; I know PoM -ng is going to address this issue, but till it is ready, and
&gt; all the extras are ported to it, is there any way to use the h.323
&gt; conntrack module kernel patch with a 2.6 kernel?
&gt; Running 2.6.1 - no 2.4 kernel stuff on the system, so downgrade is not
&gt; an option... The module is not ported yet to 2.6, sorry.
&gt; Do I have any options besides a gatekeeper app (does not work in my
&gt; network) or a proxy (would prefer to avoid them)?

I suggest everyone to setup a proxy (gatekeeper) instead: the module is
really dumb and does not deserve to exist at all. It was an excellent tool
to debug/develop the newnat interface.</programlisting></para>
      </blockquote>

      <para>Look <ulink url="UPnP.html">here</ulink> for a solution for MSN IM
      but be aware that there are significant security risks involved with
      this solution. Also check the Netfilter mailing list archives at <ulink
      url="http://www.netfilter.org">http://www.netfilter.org</ulink>.</para>
    </section>
  </section>

  <section id="Openports">
    <title>Open Ports</title>

    <section id="faq51">
      <title>(FAQ 51) How do I Open Ports in Shorewall?</title>

      <para><emphasis role="bold">Answer:</emphasis> No one who has installed
      Shorewall using one of the <ulink
      url="shorewall_quickstart_guide.htm">Quick Start Guides</ulink> should
      have to ask this question.</para>

      <para>Regardless of which guide you used, all outbound communication is
      open by default. So you do not need to 'open ports' for output.</para>

      <para>For input:</para>

      <itemizedlist>
        <listitem>
          <para>If you installed using the Standalone Guide, then please
          <ulink url="standalone.htm#Open">re-read this
          section</ulink>.</para>
        </listitem>

        <listitem>
          <para>If you installed using the Two-interface Guide, then please
          re-read these sections: <ulink url="two-interface.htm#DNAT">Port
          Forwarding (DNAT)</ulink>, and <ulink
          url="two-interface.htm#Open">Other Connections</ulink></para>
        </listitem>

        <listitem>
          <para>If you installed using the Three-interface Guide, then please
          re-read these sections: <ulink url="three-interface.htm#DNAT">Port
          Forwarding (DNAT)</ulink> and <ulink
          url="three-interface.htm#Open">Other Connections</ulink></para>
        </listitem>

        <listitem>
          <para>If you installed using the <ulink
          url="shorewall_setup_guide.htm">Shorewall Setup Guide</ulink> then
          you had better read the guide again -- you clearly missed a
          lot.</para>
        </listitem>
      </itemizedlist>

      <para>Also please see the <link linkend="PortForwarding">Port Forwarding
      section of this FAQ</link>.</para>
    </section>

    <section id="faq4">
      <title>(FAQ 4) I just used an online port scanner to check my firewall
      and it shows some ports as <quote>closed</quote> rather than
      <quote>blocked</quote>. Why?</title>

      <para><emphasis role="bold">Answer:</emphasis> The default Shorewall
      setup invokes the <emphasis role="bold">Drop</emphasis> action prior to
      enforcing a DROP policy and the default policy to all zones from the
      Internet is DROP. The Drop action is defined in
      <filename>/usr/share/shorewall/action.Drop</filename> which in turn
      invokes the <emphasis role="bold">Auth</emphasis> macro (defined in
      <filename>/usr/share/shorewall/macro.Auth</filename>) specifying the
      <emphasis role="bold">REJECT</emphasis> action (i.e., <emphasis
      role="bold">Auth/REJECT</emphasis>). This is necessary to prevent
      outgoing connection problems to services that use the
      <quote>Auth</quote> mechanism for identifying requesting users. That is
      the only service which the default setup rejects.</para>

      <para>If you are seeing closed TCP ports other than 113 (auth) then
      either you have added rules to REJECT those ports or a router outside of
      your firewall is responding to connection requests on those
      ports.</para>

      <section id="faq4a">
        <title>(FAQ 4a) I just ran an nmap UDP scan of my firewall and it
        showed 100s of ports as open!!!!</title>

        <para><emphasis role="bold">Answer:</emphasis> Take a deep breath and
        read the nmap man page section about UDP scans. If nmap gets <emphasis
        role="bold">nothing</emphasis> back from your firewall then it reports
        the port as open. If you want to see which UDP ports are really open,
        temporarily change your net-&gt;all policy to REJECT, restart
        Shorewall and do the nmap UDP scan again.</para>
      </section>

      <section id="faq4b">
        <title>(FAQ 4b) I have a port that I can't close no matter how I
        change my rules.</title>

        <para>I had a rule that allowed telnet from my local network to my
        firewall; I removed that rule and restarted Shorewall but my telnet
        session still works!!!</para>

        <para><emphasis role="bold">Answer:</emphasis> Rules only govern the
        establishment of new connections. Once a connection is established
        through the firewall it will be usable until disconnected (tcp) or
        until it times out (other protocols). If you stop telnet and try to
        establish a new session your firewall will block that attempt.</para>
      </section>

      <section id="faq4c">
        <title>(FAQ 4c) How do I use Shorewall with PortSentry?</title>

        <para><ulink
        url="http://www.shorewall.net/pub/shorewall/contrib/PortsentryHOWTO.txt"><emphasis
        role="bold">Answer:</emphasis> Here's a writeup</ulink> describing a
        nice integration of Shorewall and PortSentry.</para>
      </section>
    </section>
  </section>

  <section id="Connections">
    <title>Connection Problems</title>

    <section id="pseudofaq17">
      <title>Why are these packets being Dropped/Rejected? How do I decode
      Shorewall log messages?</title>

      <para>Please see <link linkend="faq17">FAQ 17</link>.</para>
    </section>

    <section id="faq5">
      <title>(FAQ 5) I've installed Shorewall and now I can't ping through the
      firewall</title>

      <para><emphasis role="bold">Answer:</emphasis> For a complete
      description of Shorewall <quote>ping</quote> management, see <ulink
      url="ping.html">this page</ulink>.</para>
    </section>

    <section id="faq15">
      <title>(FAQ 15) My local systems can't see out to the net</title>

      <para><emphasis role="bold">Answer:</emphasis> Every time I read
      <quote>systems can't see out to the net</quote>, I wonder where the
      poster bought computers with eyes and what those computers will
      <quote>see</quote> when things are working properly. That aside, the
      most common causes of this problem are:</para>

      <orderedlist>
        <listitem>
          <para>The default gateway on each local system isn't set to the IP
          address of the local firewall interface.</para>
        </listitem>

        <listitem>
          <para>The entry for the local network in the
          <filename>/etc/shorewall/masq</filename> file is wrong or
          missing.</para>
        </listitem>

        <listitem>
          <para>The DNS settings on the local systems are wrong or the user is
          running a DNS server on the firewall and hasn't enabled UDP and TCP
          port 53 from the local net to the firewall or from the firewall to
          the Internet.</para>
        </listitem>

        <listitem>
          <para>Forwarding is not enabled (This is often the problem for
          Debian users). Enter this command:</para>

          <programlisting>cat /proc/sys/net/ipv4/ip_forward</programlisting>

          <para>If the value displayed is 0 (zero) then set <emphasis
          role="bold">IP_FORWARDING=On</emphasis> in
          <filename>/etc/shorewall/shorewall.conf</filename> and restart
          Shorewall.</para>
        </listitem>
      </orderedlist>
    </section>

    <section id="faq29">
      <title>(FAQ 29) FTP Doesn't Work</title>

      <para><emphasis role="bold">Answer:</emphasis> See the <ulink
      url="FTP.html">Shorewall and FTP page</ulink>.</para>
    </section>

    <section id="faq33">
      <title>(FAQ 33) From clients behind the firewall, connections to some
      sites fail. Connections to the same sites from the firewall itself work
      fine. What's wrong.</title>

      <para><emphasis role="bold">Answer:</emphasis> Most likely, you need to
      set CLAMPMSS=Yes in <filename><ulink
      url="manpages/shorewall.conf.html">/etc/shorewall/shorewall.conf</ulink></filename>.</para>
    </section>

    <section id="faq35">
      <title>(FAQ 35) I have two Ethernet interfaces to my local network which
      I have bridged. When Shorewall is started, I'm unable to pass traffic
      through the bridge. I have defined the bridge interface (br0) as the
      local interface in <filename>/etc/shorewall/interfaces</filename>; the
      bridged Ethernet interfaces are not defined to Shorewall. How do I tell
      Shorewall to allow traffic through the bridge?</title>

      <para><emphasis role="bold">Answer:</emphasis> Add the
      <firstterm>routeback</firstterm> option to <filename
      class="devicefile">br0</filename> in <filename><ulink
      url="manpages/shorewall-interfaces.html">/etc/shorewall/interfaces</ulink></filename>.</para>

      <para>For more information on this type of configuration, see the <ulink
      url="SimpleBridge.html">Shorewall Simple Bridge
      documentation</ulink>.</para>
    </section>

    <section id="faq64">
      <title>(FAQ 64) I just upgraded my kernel to 2.6.20 and my
      bridge/firewall stopped working. What is wrong?</title>

      <para><emphasis role="bold">Answer:</emphasis> In kernel 2.6.20, the
      Netfilter <firstterm>physdev match</firstterm> feature was changed such
      that it is no longer capable of matching the output device of
      non-bridged traffic. You will see messages such as the following in your
      log:</para>

      <programlisting>Apr 20 15:03:50 wookie kernel: [14736.560947] physdev match: using --physdev-out in the OUTPUT, FORWARD and POSTROUTING chains for
                                                             non-bridged traffic is not supported anymore.</programlisting>

      <para>This kernel change, while necessary, means that Shorewall zones
      may no longer be defined in terms of bridge ports. See <ulink
      url="bridge-Shorewall-perl.html">the new Shorewall-shell bridging
      documentation</ulink> for information about configuring a
      bridge/firewall under kernel 2.6.20 and later with Shorewall shell or
      the<ulink url="bridge-Shorewall-perl.html"> Shorewall-perl bridging
      documentation</ulink> if you use Shorewall-perl
      (highly-recommended).<note>
          <para>Following the instructions in the new bridging documentation
          will not prevent the above message from being issued.</para>
        </note></para>
    </section>
  </section>

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

    <section id="faq6">
      <title>(FAQ 6) Where are the log messages written and how do I change
      the destination?</title>

      <para><emphasis role="bold">Answer:</emphasis> NetFilter uses the
      kernel's equivalent of syslog (see <quote>man syslog</quote>) to log
      messages. It always uses the LOG_KERN (kern) facility (see <quote>man
      openlog</quote>) and you get to choose the log level (again, see
      <quote>man syslog</quote>) in your <filename><ulink
      url="manpages/shorewall-policy.html">policies</ulink></filename> and
      <filename><ulink
      url="manpages/shorewall-rules.html">rules</ulink></filename>. The
      destination for messages logged by syslog is controlled by
      <filename>/etc/syslog.conf</filename> (see <quote>man
      syslog.conf</quote>). When you have changed
      <filename>/etc/syslog.conf</filename>, be sure to restart syslogd (on a
      RedHat system, <quote>service syslog restart</quote>).</para>

      <para>By default, older versions of Shorewall rate-limited log messages
      through <ulink url="manpages/shorewall.conf.html">settings</ulink> in
      <filename>/etc/shorewall/shorewall.conf</filename> -- If you want to log
      all messages, set:</para>

      <programlisting>LOGLIMIT=""
LOGBURST=""</programlisting>

      <para>It is also possible to <ulink url="shorewall_logging.html">set up
      Shorewall to log all of its messages to a separate file</ulink>.</para>

      <section id="faq6a">
        <title>(FAQ 6a) Are there any log parsers that work with
        Shorewall?</title>

        <para><emphasis role="bold">Answer:</emphasis> Here are several links
        that may be helpful:</para>

        <literallayout>
          <ulink url="http://www.shorewall.net/pub/shorewall/parsefw/">http://www.shorewall.net/pub/shorewall/parsefw/</ulink>
          <ulink url="http://aaron.marasco.com/linux.html">http://aaron.marasco.com/linux.html</ulink>
          <ulink url="http://cert.uni-stuttgart.de/projects/fwlogwatch">http://cert.uni-stuttgart.de/projects/fwlogwatch</ulink>
          <ulink url="http://www.logwatch.org">http://www.logwatch.org</ulink>
        </literallayout>

        <para>I personally use <ulink
        url="http://www.logwatch.org">Logwatch</ulink>. It emails me a report
        each day from my various systems with each report summarizing the
        logged activity on the corresponding system. I use the brief report
        format; here's a sample:</para>

        <blockquote>
          <programlisting> --------------------- iptables firewall Begin ------------------------ 

 Dropped 111 packets on interface eth0
   From 58.20.162.142 - 5 packets to tcp(1080) 
   From 62.163.19.50 - 1 packet to udp(6348) 
   From 66.111.45.60 - 9 packets to tcp(192) 
   From 69.31.82.50 - 18 packets to tcp(3128) 
   From 72.232.183.102 - 2 packets to tcp(3128) 
   From 82.96.96.3 - 6 packets to tcp(808,1080,1978,7600,65506) 
   From 128.48.51.209 - 5 packets to tcp(143) 
   From 164.77.223.150 - 12 packets to tcp(873) 
   From 165.233.109.23 - 8 packets to tcp(22) 
   From 202.99.172.175 - 4 packets to udp(2,4081) 
   From 206.59.41.101 - 2 packets to tcp(5900) 
   From 217.91.30.224 - 24 packets to tcp(873) 
   From 218.87.47.114 - 6 packets to tcp(3128) 
   From 220.110.219.234 - 4 packets to tcp(22) 
   From 220.133.116.173 - 5 packets to tcp(3128) 
 
 ---------------------- iptables firewall End -------------------------</programlisting>
        </blockquote>
      </section>

      <section id="faq6b">
        <title>(FAQ 6b) DROP messages on port 10619 are flooding the logs with
        their connect requests. Can I exclude these error messages for this
        port temporarily from logging in Shorewall?</title>

        <para><emphasis role="bold">Answer:</emphasis> Temporarily add the
        following rule:</para>

        <programlisting>#ACTION   SOURCE    DEST    PROTO    DEST PORT(S)
DROP      net       fw      udp      10619</programlisting>

        <para>Alternatively, if you do not set BLACKLIST_LOGLEVEL and you have
        specifed the 'blacklist' option on your external interface in
        <filename>/etc/shorewall/interfaces</filename>, then you can blacklist
        the port. In <filename>/etc/shorewall/blacklist</filename>:</para>

        <programlisting>#ADDRESS/SUBNET         PROTOCOL        PORT
-                       udp             10619</programlisting>
      </section>

      <section id="faq6d">
        <title>(FAQ 6d) Why is the MAC address in Shorewall log messages so
        long? I thought MAC addresses were only 6 bytes in length.</title>

        <para><emphasis role="bold">Answer:</emphasis> What is labeled as the
        MAC address in a Netfilter (Shorewall) log message is actually the
        Ethernet frame header. It contains:</para>

        <itemizedlist>
          <listitem>
            <para>the destination MAC address (6 bytes)</para>
          </listitem>

          <listitem>
            <para>the source MAC address (6 bytes)</para>
          </listitem>

          <listitem>
            <para>the Ethernet frame type (2 bytes)</para>
          </listitem>
        </itemizedlist>

        <para><example id="Example5">
            <title id="Example2">Example</title>

            <para><programlisting>MAC=00:04:4c:dc:e2:28:00:b0:8e:cf:3c:4c:08:00</programlisting>
            <itemizedlist>
                <listitem>
                  <para>Destination MAC address = 00:04:4c:dc:e2:28</para>
                </listitem>

                <listitem>
                  <para>Source MAC address = 00:b0:8e:cf:3c:4c</para>
                </listitem>

                <listitem>
                  <para>Ethernet Frame Type = 08:00 (IP Version 4)</para>
                </listitem>
              </itemizedlist></para>
          </example></para>
      </section>
    </section>

    <section id="faq16">
      <title>(FAQ 16) Shorewall is writing log messages all over my console
      making it unusable!</title>

      <para><emphasis role="bold">Answer:</emphasis></para>

      <para>Just to be clear, it is not Shorewall that is writing all over
      your console. Shorewall issues a single log message during each
      <command>start</command>, <command>restart</command>,
      <command>stop</command>, etc. It is rather the klogd daemon that is
      writing messages to your console. Shorewall itself has no control over
      where a particular class of messages are written. See the <ulink
      url="shorewall_logging.html">Shorewall logging
      documentation</ulink>.</para>

      <para>The max log level to be sent to the console is available in
      /proc/sys/kernel/printk:<programlisting>teastep@ursa:~$ <emphasis
            role="bold">cat /proc/sys/kernel/printk</emphasis>
6      6       1       7
teastep@ursa:~$ </programlisting>The first number determines the maximum log
      level (syslog priority) sent to the console. Messages with priority
      <emphasis role="bold">less than</emphasis> this number are sent to the
      console. On the system shown in the example above, priorities 0-5 are
      sent to the console. Since Shorewall defaults to using 'info' (6), the
      Shorewall-generated Netfilter rule set will generate log messages that
      <emphasis role="bold">will not appear on the console.</emphasis></para>

      <para>The second number is the default log level for kernel printk()
      calls that do not specify a log level.</para>

      <para>The third number specifies the minimum console log level while the
      fourth gives the default console log level.</para>

      <para>If, on your system, the first number is 7 or greater, then the
      default Shorewall configurations will cause messages to be written to
      your console. The simplest solution is to add this to your
      <filename>/etc/sysctl.conf</filename> file:<programlisting>kernel.printk = 4 4 1 7</programlisting></para>

      <para>then<programlisting><command>sysctl -p /etc/sysctl.conf</command></programlisting></para>

      <section id="faq16a">
        <title>(FAQ 16a) Why can't I see any Shorewall messages in
        /var/log/messages?</title>

        <para>Some people who ask this question report that the only Shorewall
        messages that they see in <filename>/var/log/messages</filename> are
        'started', 'restarted' and 'stopped' messages.</para>

        <para><emphasis role="bold">Answer:</emphasis> First of all, it is
        important to understand that Shorewall itself does not control where
        Netfilter log messages are written. The LOGFILE setting in
        <filename>shorewall.conf</filename> simply tells the
        <filename>/sbin/shorewall[-lite]</filename> program where to look for
        the log. Also, it is important to understand that a log level of
        "debug" will generally cause Netfilter messages to be written to fewer
        files in <filename class="directory">/var/log</filename> than a log
        severity of "info". The log level does not control the number of log
        messages or the content of the messages.</para>

        <para>The actual log file where Netfilter messages are written is not
        standardized and will vary by distribution and distribution version.
        But anytime you see no logging, it's time to look outside the
        Shorewall configuration for the cause. As an example, recent
        <trademark>SUSE</trademark> releases use syslog-ng by default and
        write Shorewall messages to
        <filename>/var/log/firewall</filename>.</para>

        <para>Please see the <ulink url="shorewall_logging.html">Shorewall
        logging documentation</ulink> for further information.</para>
      </section>
    </section>

    <section id="faq17">
      <title>(FAQ 17) Why are these packets being Dropped/Rejected? How do I
      decode Shorewall log messages?</title>

      <para><emphasis role="bold">Answer:</emphasis> Logging of
      dropped/rejected packets occurs out of a number of chains (as indicated
      in the log message) in Shorewall:</para>

      <variablelist>
        <varlistentry>
          <term>man1918 or logdrop</term>

          <listitem>
            <para>The destination address is listed in
            <filename>/usr/share/shorewall/rfc1918</filename> with a <emphasis
            role="bold">logdrop</emphasis> target -- see <filename> <ulink
            url="manpages/shorewall-rfc1918.html">/usr/share/shorewall/rfc1918</ulink>
            </filename>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>rfc1918 or logdrop</term>

          <listitem>
            <para>The source or destination address is listed in
            <filename>/usr/share/shorewall/rfc1918</filename> with a <emphasis
            role="bold">logdrop</emphasis> target -- see <filename> <ulink
            url="manpages/shorewall-rfc1918.html">/usr/share/shorewall/rfc1918</ulink>
            </filename>.</para>

            <note>
              <para>If you see packets being dropped in the rfc1918 chain and
              neither the source nor the destination IP address is reserved by
              RFC 1918, that usually means that you have a old
              <filename>rfc1918</filename> file in <filename
              class="directory">/etc/shorewall</filename> (this problem most
              frequently occurs if you are running Debian or one if its
              derivatives). The <filename>rfc1918</filename> file used to
              include bogons as well as the three ranges reserved by RFC 1918
              and it resided in <filename
              class="directory">/etc/shorewall</filename>. The file now only
              includes the three RFC 1918 ranges and it resides in <filename
              class="directory">/usr/share/shorewall</filename>. Remove the
              stale <filename>rfc1918</filename> file in <filename
              class="directory">/etc/shorewall</filename>.</para>
            </note>
          </listitem>
        </varlistentry>

        <varlistentry id="all2all">
          <term>all2<emphasis>zone</emphasis>, <emphasis>zone</emphasis>2all
          or all2all</term>

          <listitem>
            <para>You have a <filename><ulink
            url="manpages/shorewall-policy.html">policy</ulink></filename>
            that specifies a log level and this packet is being logged under
            that policy. If you intend to ACCEPT this traffic then you need a
            <ulink url="manpages/shorewall-rules.html">rule</ulink> to that
            effect.</para>

            <para>Beginning with Shorewall 3.3.3, packets logged out of these
            chains may have a source and/or destination that is not in any
            defined zone (see the output of <command>shorewall[-lite] show
            zones</command>). Remember that zone membership involves both a
            firewall interface and an ip address.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><emphasis>zone</emphasis>12<emphasis>zone2</emphasis></term>

          <listitem>
            <para>Either you have a <ulink
            url="manpages/shorewall-policy.html">policy</ulink> for
            <emphasis>zone1</emphasis> to <emphasis>zone2</emphasis> that
            specifies a log level and this packet is being logged under that
            policy or this packet matches a <ulink
            url="manpages/shorewall-rules.html">rule</ulink> that includes a
            log level.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>@<emphasis>source</emphasis>2<emphasis>dest</emphasis></term>

          <listitem>
            <para>You have a policy for traffic from
            <emphasis>source</emphasis> to <emphasis>dest</emphasis> that
            specifies TCP connection rate limiting (value in the LIMIT:BURST
            column). The logged packet exceeds that limit and was dropped.
            Note that these log messages themselves are severely rate-limited
            so that a syn-flood won't generate a secondary DOS because of
            excessive log message. These log messages were added in Shorewall
            2.2.0 Beta 7.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><emphasis>interface</emphasis>_mac or
          <emphasis>interface</emphasis>_rec</term>

          <listitem>
            <para>The packet is being logged under the <emphasis
            role="bold">maclist</emphasis> <ulink
            url="manpages/shorewall-interfaces.html">interface
            option</ulink>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>blacklist</term>

          <listitem>
            <para>The packet is being logged because the source IP is
            blacklisted in the <filename> <ulink
            url="manpages/shorewall-blacklist.html">/etc/shorewall/blacklist</ulink>
            </filename> file.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>INPUT or FORWARD</term>

          <listitem>
            <para>The packet has a source IP address that isn't in any of your
            defined zones (<quote><command>shorewall[-lite] show
            zones</command></quote> and look at the printed zone definitions)
            or the chain is FORWARD and the destination IP isn't in any of
            your defined zones. If the chain is FORWARD and the IN and OUT
            interfaces are the same, then you probably need the <emphasis
            role="bold">routeback</emphasis> option on that interface in
            <filename> <ulink
            url="manpages/shorewall-interfaces.html">/etc/shorewall/interfaces</ulink>
            </filename>, you need the <emphasis
            role="bold">routeback</emphasis> option in the relevant entry in
            <filename> <ulink
            url="manpages/shorewall-hosts.html">/etc/shorewall/hosts</ulink>
            or you've done something silly like define a default route out of
            an internal interface.</filename></para>

            <para>In Shorewall 3.3.3 and later versions with OPTIMIZE=1 in
            <ulink url="manpages/shorewall.conf.html">shorewall.conf</ulink>,
            such packets may also be logged out of a &lt;zone&gt;2all chain or
            the all2all chain.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>OUTPUT</term>

          <listitem>
            <para>The packet has a destination IP address that isn't in any of
            your defined zones(<command>shorewall[-lite] show zones</command>
            and look at the printed zone definitions).</para>

            <para>In Shorewall 3.3.3 and later versions with OPTIMIZE=1 in
            <ulink url="manpages/shorewall.conf.html">shorewall.conf</ulink>,
            such packets may also be logged out of the fw2all chain or the
            all2all chain.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>logflags</term>

          <listitem>
            <para>The packet is being logged because it failed the checks
            implemented by the <emphasis role="bold">tcpflags</emphasis>
            <ulink url="manpages/shorewall-interfaces.html">interface
            option</ulink>.</para>
          </listitem>
        </varlistentry>
      </variablelist>

      <example id="Example3">
        <title>Here is an example:</title>

        <programlisting>Jun 27 15:37:56 gateway kernel:
        Shorewall:<emphasis role="bold">all2all:REJECT</emphasis>:<emphasis
            role="bold">IN=eth2</emphasis>
          <emphasis role="bold">OUT=eth1</emphasis>
          <emphasis role="bold">SRC=192.168.2.2</emphasis>
          <emphasis role="bold">DST=192.168.1.3 </emphasis>LEN=67 TOS=0x00 PREC=0x00 TTL=63 ID=5805 DF <emphasis
            role="bold">PROTO=UDP</emphasis>
        SPT=1803 <emphasis role="bold">DPT=53</emphasis> LEN=47</programlisting>

        <para>Let's look at the important parts of this message:</para>

        <variablelist>
          <varlistentry>
            <term>all2all:REJECT</term>

            <listitem>
              <para>This packet was REJECTed out of the <emphasis
              role="bold">all2all</emphasis> chain -- the packet was rejected
              under the <quote>all</quote>-&gt;<quote>all</quote> REJECT
              policy (<link linkend="all2all">all2all</link> above).</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>IN=eth2</term>

            <listitem>
              <para>the packet entered the firewall via eth2. If you see
              <quote>IN=</quote> with no interface name, the packet originated
              on the firewall itself.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>OUT=eth1</term>

            <listitem>
              <para>if accepted, the packet would be sent on eth1. If you see
              <quote>OUT=</quote> with no interface name, the packet would be
              processed by the firewall itself.</para>

              <note>
                <para>When a DNAT rule is logged, there will never be an OUT=
                shown because the packet is being logged before it is routed.
                Also, DNAT logging will show the <emphasis>original</emphasis>
                destination IP address and destination port number. When a
                REDIRECT rule is logged, the message will also show the
                original destination IP address and port number.</para>
              </note>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>SRC=192.168.2.2</term>

            <listitem>
              <para>the packet was sent by 192.168.2.2</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>DST=192.168.1.3</term>

            <listitem>
              <para>the packet is destined for 192.168.1.3</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>PROTO=UDP</term>

            <listitem>
              <para>UDP Protocol</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>DPT=53</term>

            <listitem>
              <para>The destination port is 53 (DNS)</para>
            </listitem>
          </varlistentry>
        </variablelist>

        <para>In this case, 192.168.2.2 was in the <quote>dmz</quote> zone and
        192.168.1.3 is in the <quote>loc</quote> zone. I was missing the
        rule:</para>

        <programlisting>ACCEPT dmz loc udp 53</programlisting>
      </example>
    </section>

    <section id="faq21">
      <title>(FAQ 21) I see these strange log entries occasionally; what are
      they?</title>

      <programlisting>Nov 25 18:58:52 linux kernel:
      Shorewall:net2all:DROP:IN=eth1 OUT=
      MAC=00:60:1d:f0:a6:f9:00:60:1d:f6:35:50:08:00 SRC=206.124.146.179
      DST=192.0.2.3 LEN=56 TOS=0x00 PREC=0x00 TTL=110 ID=18558 <emphasis
          role="bold">PROTO=ICMP</emphasis>
      <emphasis role="bold">TYPE=3 CODE=3</emphasis> [SRC=192.0.2.3 DST=172.16.1.10 LEN=128 TOS=0x00 PREC=0x00
      TTL=47 ID=0 DF PROTO=UDP SPT=53 DPT=2857 LEN=108 ]</programlisting>

      <para>192.0.2.3 is external on my firewall... 172.16.0.0/24 is my
      internal LAN</para>

      <para><emphasis role="bold">Answer:</emphasis> First of all, please note
      that the above is a very specific type of log message dealing with ICMP
      port unreachable packets (PROTO=ICMP TYPE=3 CODE=3). Do not read this
      answer and assume that all Shorewall log messages have something to do
      with ICMP (hint -- see <link linkend="faq17">FAQ 17</link>).</para>

      <para>While most people associate the Internet Control Message Protocol
      (ICMP) with <quote>ping</quote>, ICMP is a key piece of IP. ICMP is used
      to report problems back to the sender of a packet; this is what is
      happening here. Unfortunately, where NAT is involved (including SNAT,
      DNAT and Masquerade), there are many broken implementations. That is
      what you are seeing with these messages. When Netfilter displays these
      messages, the part before the "[" describes the ICMP packet and the part
      between the "[" and "]" describes the packet for which the ICMP is a
      response.</para>

      <para>Here is my interpretation of what is happening -- to confirm this
      analysis, one would have to have packet sniffers placed a both ends of
      the connection.</para>

      <para>Host 172.16.1.10 behind NAT gateway 206.124.146.179 sent a UDP DNS
      query to 192.0.2.3 and your DNS server tried to send a response (the
      response information is in the brackets -- note source port 53 which
      marks this as a DNS reply). When the response was returned to to
      206.124.146.179, it rewrote the destination IP TO 172.16.1.10 and
      forwarded the packet to 172.16.1.10 who no longer had a connection on
      UDP port 2857. This causes a port unreachable (type 3, code 3) to be
      generated back to 192.0.2.3. As this packet is sent back through
      206.124.146.179, that box correctly changes the source address in the
      packet to 206.124.146.179 but doesn't reset the DST IP in the original
      DNS response similarly. When the ICMP reaches your firewall (192.0.2.3),
      your firewall has no record of having sent a DNS reply to 172.16.1.10 so
      this ICMP doesn't appear to be related to anything that was sent. The
      final result is that the packet gets logged and dropped in the all2all
      chain. I have also seen cases where the source IP in the ICMP itself
      isn't set back to the external IP of the remote NAT gateway; that causes
      your firewall to log and drop the packet out of the rfc1918 chain
      because the source IP is reserved by RFC 1918.</para>
    </section>

    <section id="faq52">
      <title>(FAQ 52) When I blacklist an IP address with "shorewall[-lite]
      drop www.xxx.yyy.zzz", why does my log still show REDIRECT and DNAT
      entries from that address?</title>

      <para>I blacklisted the address 130.252.100.59 using <command>shorewall
      drop 130.252.100.59</command> but I am still seeing these log
      messages:</para>

      <programlisting>Jan 30 15:38:34 server Shorewall:net_dnat:REDIRECT:IN=eth1 OUT= MAC=00:4f:4e:14:97:8e:00:01:5c:23:24:cc:08:00
                       SRC=130.252.100.59 DST=206.124.146.176 LEN=64 TOS=0x00 PREC=0x00 TTL=43 ID=42444 DF
                       PROTO=TCP SPT=2215 DPT=139 WINDOW=53760 RES=0x00 SYN URGP=0</programlisting>

      <para><emphasis role="bold">Answer:</emphasis> Please refer to the
      <ulink url="NetfilterOverview.html">Shorewall Netfilter
      Documentation</ulink>. Logging of REDIRECT and DNAT rules occurs in the
      nat table's PREROUTING chain where the original destination IP address
      is still available. Blacklisting occurs out of the filter table's INPUT
      and FORWARD chains which aren't traversed until later.</para>
    </section>

    <section id="faq56">
      <title>(FAQ 56) When I start or restart Shorewall, I see these messages
      in my log. Are they harmful?</title>

      <blockquote>
        <programlisting>modprobe: Can't locate module ipt_physdev
modprobe: Can't locate module iptable_raw</programlisting>
      </blockquote>

      <para><emphasis role="bold">Answer:</emphasis> No. These occur when
      Shorewall probes your system to determine the features that it support.
      They are completely harmless.</para>
    </section>
  </section>

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

    <section id="faq32">
      <title>(FAQ 32) My firewall has two connections to the Internet from two
      different ISPs. How do I set this up in Shorewall?</title>

      <para><emphasis role="bold">Answer:</emphasis> See <ulink
      url="MultiISP.html">this article on Shorewall and Multiple
      ISPs</ulink>.</para>
    </section>

    <section id="faq49">
      <title>(FAQ 49) When I start Shorewall, my routing table gets blown
      away. Why does Shorewall do that?</title>

      <para><emphasis role="bold">Answer:</emphasis> This is usually the
      consequence of a one-to-one nat configuration blunder:</para>

      <orderedlist>
        <listitem>
          <para>Specifying the primary IP address for an interface in the
          EXTERNAL column of <filename>/etc/shorewall/nat</filename> even
          though the documentation (and the comments in the file) warn you not
          to do that.</para>
        </listitem>

        <listitem>
          <para>Specifying ADD_IP_ALIASES=Yes and RETAIN_ALIASES=No in
          /etc/shorewall/shorewall.conf.</para>
        </listitem>
      </orderedlist>

      <para>This combination causes Shorewall to delete the primary IP address
      from the network interface specified in the INTERFACE column which
      usually causes all routes out of that interface to be deleted. The
      solution is to <emphasis role="bold">not specify the primary IP address
      of an interface in the EXTERNAL column</emphasis>.</para>
    </section>
  </section>

  <section id="Start-Stop">
    <title>Starting and Stopping</title>

    <section id="faq7">
      <title>(FAQ 7) When I stop Shorewall using <quote>shorewall[-lite]
      stop</quote>, I can't connect to anything. Why doesn't that command
      work?</title>

      <para><emphasis role="bold">Answer:</emphasis> The <quote>
      <command>stop</command> </quote> command is intended to place your
      firewall into a safe state whereby only those hosts listed in
      <filename>/etc/shorewall/routestopped</filename> are activated. If you
      want to totally open up your firewall, you must use the <quote>
      <command>shorewall[-lite] clear</command> </quote> command.</para>
    </section>

    <section id="faq8">
      <title>(FAQ 8) When I try to start Shorewall on RedHat, I get messages
      about insmod failing -- what's wrong?</title>

      <para><emphasis role="bold">Answer:</emphasis> The output you will see
      looks something like this:</para>

      <programlisting>/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: init_module: Device or resource busy
Hint: insmod errors can be caused by incorrect module parameters, including invalid IO or IRQ parameters
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o failed
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod ip_tables failed
iptables v1.2.3: can't initialize iptables table `nat': iptables who? (do you need to insmod?)
Perhaps iptables or your kernel needs to be upgraded.</programlisting>

      <para>This problem is usually corrected through the following sequence
      of commands</para>

      <programlisting><command>service ipchains stop
chkconfig --delete ipchains
rmmod ipchains</command></programlisting>

      <section id="faq8a">
        <title>(FAQ 8a) When I try to start Shorewall on RedHat I get a
        message referring me to FAQ #8</title>

        <para><emphasis role="bold">Answer:</emphasis> This is usually cured
        by the sequence of commands shown above in <xref
        linkend="faq8" />.</para>
      </section>
    </section>

    <section id="faq9">
      <title>(FAQ 9) Why can't Shorewall detect my interfaces properly at
      startup?</title>

      <para>I just installed Shorewall and when I issue the
      <command>start</command> command, I see the following:</para>

      <programlisting>Processing /etc/shorewall/params ...
Processing /etc/shorewall/shorewall.conf ...
Starting Shorewall...
Loading Modules...
Initializing...
Determining Zones...
   Zones: net loc
Validating interfaces file...
Validating hosts file...
Determining Hosts in Zones...
    <emphasis role="bold">Net Zone: eth0:0.0.0.0/0
    </emphasis><emphasis role="bold">Local Zone: eth1:0.0.0.0/0</emphasis>
Deleting user chains...
Creating input Chains...
...</programlisting>

      <para>Why can't Shorewall detect my interfaces properly?</para>

      <para><emphasis role="bold">Answer:</emphasis> The above output is
      perfectly normal. The Net zone is defined as all hosts that are
      connected through <filename class="devicefile">eth0</filename> and the
      local zone is defined as all hosts connected through <filename
      class="devicefile">eth1</filename>. You can set the <emphasis
      role="bold">routefilter</emphasis> option on an internal interface if
      you wish to guard against '<firstterm>Martians</firstterm>' (a Martian
      is a packet with a source IP address that is not routed out of the
      interface on which the packet was received). If you do that, it is a
      good idea to also set the <emphasis role="bold">logmartians</emphasis>
      option.</para>
    </section>

    <section id="faq22">
      <title>(FAQ 22) I have some iptables commands that I want to run when
      Shorewall starts. Which file do I put them in?</title>

      <para><emphasis role="bold">Answer:</emphasis>You can place these
      commands in one of the <ulink
      url="shorewall_extension_scripts.htm">Shorewall Extension
      Scripts</ulink>. Be sure that you look at the contents of the chain(s)
      that you will be modifying with your commands so that the commands will
      do what is intended. Many iptables commands published in HOWTOs and
      other instructional material use the -A command which adds the rules to
      the end of the chain. Most chains that Shorewall constructs end with an
      unconditional DROP, ACCEPT or REJECT rule and any rules that you add
      after that will be ignored. Check <quote>man iptables</quote> and look
      at the -I (--insert) command.</para>
    </section>

    <section id="faq34">
      <title>(FAQ 34) How can I speed up Shorewall start (restart)?</title>

      <para><emphasis role="bold">Answer:</emphasis> Switch to using <ulink
      url="Shorewall-perl.html">Shorewall-perl</ulink>.</para>
    </section>

    <section id="faq69">
      <title>(FAQ 69) When I restart Shorewall, new connections are blocked
      for a long time. Is there a way to avoid that?</title>

      <para><emphasis role="bold">Answer:</emphasis> Switch to using <ulink
      url="Shorewall-perl.html">Shorewall-perl</ulink>.</para>
    </section>

    <section id="faq43">
      <title>(FAQ 43) I just installed the Shorewall RPM and Shorewall doesn't
      start at boot time.</title>

      <para><emphasis role="bold">Answer:</emphasis> When you install using
      the "rpm -U" command, Shorewall doesn't run your distribution's tool for
      configuring Shorewall startup. You will need to run that tool (insserv,
      chkconfig, run-level editor, …) to configure Shorewall to start in the
      the default run-levels of your firewall system.</para>
    </section>

    <section id="faq45">
      <title>(FAQ 45) Why does "shorewall[-lite] start" fail when trying to
      set up SNAT/Masquerading?</title>

      <para><command>shorewall start</command> produces the following
      output:</para>

      <programlisting>…
Processing /etc/shorewall/policy...
   Policy ACCEPT for fw to net using chain fw2net
   Policy ACCEPT for loc0 to net using chain loc02net
   Policy ACCEPT for loc1 to net using chain loc12net
   Policy ACCEPT for wlan to net using chain wlan2net
Masqueraded Networks and Hosts:
iptables: Invalid argument
   ERROR: Command "/sbin/iptables -t nat -A …" Failed</programlisting>

      <para><emphasis role="bold">Answer:</emphasis> 99.999% of the time, this
      error is caused by a mismatch between your iptables and kernel.</para>

      <orderedlist numeration="loweralpha">
        <listitem>
          <para>Your iptables must be compiled against a kernel source tree
          that is Netfilter-compatible with the kernel that you are
          running.</para>
        </listitem>

        <listitem>
          <para>If you rebuild iptables using the defaults and install it, it
          will be installed in /usr/local/sbin/iptables. As shown above, you
          have the IPTABLES variable in shorewall.conf set to
          "/sbin/iptables".</para>
        </listitem>
      </orderedlist>
    </section>

    <section id="faq59">
      <title>(FAQ 59) After I start Shorewall, there are lots of unused
      Netfilter modules loaded. How do I avoid that?</title>

      <para><emphasis role="bold">Answer:</emphasis> Copy
      <filename>/usr/share/shorewall[-lite]/modules</filename> to
      <filename>/etc/shorewall/modules </filename>and modify the copy to
      include only the modules that you need.</para>
    </section>

    <section id="faq61">
      <title>(FAQ 61) I just installed the latest Debian kernel and now
      "shorewall start" fails with the message "ipt_policy: matchsize 116 !=
      308". What's wrong?</title>

      <para><emphasis role="bold">Answer:</emphasis> Your iptables is
      incompatible with your kernel. Either</para>

      <itemizedlist>
        <listitem>
          <para>rebuild iptables using the kernel headers that match your new
          kernel; or</para>
        </listitem>

        <listitem>
          <para>if you don't need policy match support (you are not using the
          IPSEC implementation builtinto the 2.6 kernel) then you can rename
          <filename>/lib/iptables/libipt_policy.so</filename>.</para>
        </listitem>
      </itemizedlist>

      <note>
        <para>Beginning with Shorewall 3.4.0, Shorewall no longer attempts to
        use policy match if you have no IPSEC zones and you have not specified
        the <option>ipsec</option> option on any entry in
        <filename>/etc/shorewall/hosts</filename>. The subject message will
        still appear in your kernel log each time that Shorewall determines
        the capabilities of your kernel/iptables.</para>
      </note>
    </section>

    <section id="faq62">
      <title>(FAQ 62) I have unexplained 30-second pauses during "shorewall
      [re]start". What causes that?</title>

      <para><emphasis role="bold">Answer:</emphasis> This usually happens when
      the firewall uses LDAP Authentication. The solution is to list your LDAP
      server(s) as <emphasis role="bold">critical</emphasis> in <ulink
      url="manpages/shorewall-routestopped.html">/etc/shorewall/routestopped</ulink>.</para>
    </section>

    <section id="faq68">
      <title>(FAQ 68) I have a VM under an OpenVZ system. I can't get rid of
      the following message:</title>

      <para>ERROR: Command "/sbin/iptables -A FORWARD -m state --state
      ESTABLISHED,RELATED -j ACCEPT" failed.</para>

      <para><emphasis role="bold">Answer:</emphasis> At a root shell prompt,
      type the iptables command shown in the error message. If the command
      fails, you OpenVZ Netfilter/iptables configuration is incorrect. Until
      that command can run without error, no stateful iptables firewall will
      be able to run in your VM.</para>
    </section>

    <section id="faq73">
      <title>(FAQ 73) When I stop Shorewall, the firewall is wide open. Isn't
      that a security risk?</title>

      <para>It is important to understand that the scripts in <filename
      class="directory">/etc/init.d</filename> are generally provided by your
      distribution and not by the Shorewall developers. These scripts must
      meet the requirements of the distribution's packaging system which may
      conflict with the requirements of a tight firewall. So when you say
      "…when I stop Shorewall…" it is necessary to distinguish between the
      commands <command>/sbin/shorewall stop</command> and
      <command>/etc/init.d/shorewall stop</command>.</para>

      <para><command>/sbin/shorewall stop</command> places the firewall in a
      <firstterm>safe state</firstterm>, the details of which depend on your
      <filename>/etc/shorewall/routestopped</filename> file (<ulink
      url="manpages/shorewall-routestopped.html">shorewall-routestopped</ulink>(8))
      and on the setting of ADMINISABSENTMINDED in
      <filename>/etc/shorewall/shorewall.conf</filename> (<ulink
      url="manpages/shorewall.conf.html">shorewall.conf</ulink>(8)).</para>

      <para><command>/etc/init.d/shorewall stop</command> may or may not do
      the same thing. In the case of <trademark>Debian</trademark> systems for
      example, that command actually executes <command>/sbin/shorewall
      clear</command> which opens the firewall completely. In other words, in
      the init script's <command>stop</command> reverses the effect of
      <command>start</command>.</para>

      <para>One way to avoid these differences is to install Shorewall from
      the tarballs available from shorewall.net. This places Shorewall outside
      of the control of the packaging system and provides consistent behavior
      between the init scripts and <filename>/sbin/shorewall</filename> (and
      <filename>/sbin/shorewall-lite</filename>). For more information on the
      factors involved when deciding whether to use the Debian package, see
      <ulink url="http://wiki.shorewall.net/wiki/ShorewallOnDebian">this
      article</ulink>.</para>
    </section>

    <section id="faq74">
      <title>(FAQ 74) When I "<command>shorewall start</command>" or
      "<command>shorewall check</command>" on my SuSE 10.0 system, I get FATAL
      ERROR messages and/or the system crashes"</title>

      <para><emphasis role="bold">Answer:</emphasis> These failures result
      from trying to load a particular combination of kernel modules. To work
      around the problem:</para>

      <orderedlist>
        <listitem>
          <para>Copy /usr/share/shorewall/modules to
          /etc/shorewall/modules</para>
        </listitem>

        <listitem>
          <para>Edit /etc/shorewall/modules and remove all entries except for
          those for the helper modules that you need.</para>
        </listitem>
      </orderedlist>
    </section>

    <section id="faq78">
      <title>(FAQ 78) After restart and bootup of my Debian firewall, all
      traffic is blocked for hosts behind the firewall trying to connect out
      onto the net or through the vpn (although i can reach the internal
      firewall interface and obtain dumps etc). Once I issue 'shorewall clear'
      followed by 'shorewall restart' it then works, despite the config not
      changing</title>

      <para><emphasis role="bold">Answer:</emphasis> Set IP_FORWARDING=On in
      <filename><ulink
      url="manpages/shorewall.conf.html">/etc/shorewall/shorewall.conf</ulink></filename>.</para>
    </section>
  </section>

  <section id="MultiISP">
    <title>Multiple ISPs</title>

    <section id="faq57">
      <title>(FAQ 57) I configured two ISPs in Shorewall but when I try to use
      the second one, it doesn't work.</title>

      <para><emphasis role="bold">Answer:</emphasis> The Multi-ISP
      Documentation strongly recommends that you use the <emphasis
      role="bold">balance</emphasis> option on all providers even if you want
      to manually specify which ISP to use. If you don't do that so that your
      main routing table only has one default route, then you must disable
      route filtering. Do not specify the <emphasis
      role="bold">routefilter</emphasis> option on the other interface(s) in
      <filename>/etc/shorewall/interfaces</filename> and disable any
      <emphasis>IP Address Spoofing</emphasis> protection that your
      distribution supplies.</para>
    </section>

    <section id="faq58">
      <title>(FAQ 58) But if I specify 'balance' then won't Shorewall balance
      the traffic between the interfaces? I don't want that!</title>

      <para><emphasis role="bold">Answer:</emphasis> Suppose that you want all
      traffic to go out through ISP1 (mark 1) unless you specify otherwise.
      Then simply add these two rules as the first marking rules in your
      <filename>/etc/shorewall/tcrules</filename> file:</para>

      <programlisting>#MARK         SOURCE           DEST
1:P           0.0.0.0/0
1             $FW
<emphasis>other MARK rules</emphasis></programlisting>

      <para>Now any traffic that isn't marked by one of your other MARK rules
      will have mark = 1 and will be sent via ISP1. That will work whether
      <emphasis role="bold">balance</emphasis> is specified or not!</para>
    </section>
  </section>

  <section>
    <title>Using DNS Names</title>

    <section id="faq79">
      <title>(FAQ 79) Can I use DNS names in Shorewall configuration file
      entries in place of IP addresses?</title>

      <para><emphasis role="bold">Answer</emphasis>: <ulink
      url="configuration_file_basics.htm#dnsnames">Yes</ulink>, but we advise
      strongly against it.</para>
    </section>
  </section>

  <section id="TC">
    <title>Traffic Shaping</title>

    <section id="faq67">
      <title>(FAQ 67) I just configured Shorewall's builtin traffic shaping
      and now Shorewall fails to Start.</title>

      <para>The error I receive is as follows:<programlisting>RTNETLINK answers: No such file or directory
We have an error talking to the kernel
    ERROR: Command "tc filter add dev eth2 parent ffff: protocol ip prio 
                    50 u32 match ip src 0.0.0.0/0 police rate 500kbit burst 10k drop flowid 
                    :1" Failed</programlisting><emphasis
      role="bold">Answer:</emphasis> This message indicates that your kernel
      doesn't have 'traffic policing' support. If your kernel is modularized,
      you may be able to resolve the problem by loading the <emphasis
      role="bold">act_police</emphasis> kernel module. Other kernel modules
      that you will need include:<simplelist>
          <member>cls_u32</member>

          <member>sch_htb</member>

          <member>sch_ingress</member>

          <member>sch_sfq</member>
        </simplelist></para>
    </section>
  </section>

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

    <section id="faq10">
      <title>(FAQ 10) What Distributions does Shorewall work with?</title>

      <para><emphasis role="bold">Answer:</emphasis> Shorewall works with any
      GNU/Linux distribution that includes the <ulink
      url="shorewall_prerequisites.htm">proper prerequisites</ulink>.</para>
    </section>

    <section id="faq11">
      <title>(FAQ 11) What Features does Shorewall have?</title>

      <para><emphasis role="bold">Answer:</emphasis> See the <ulink
      url="shorewall_features.htm">Shorewall Feature List</ulink>.</para>
    </section>

    <section id="faq12">
      <title>(FAQ 12) Is there a GUI?</title>

      <para><emphasis role="bold">Answer:</emphasis> Yes! Shorewall 3.x
      support is available in Webmin 1.300. See <ulink
      url="http://www.webmin.com">http://www.webmin.com</ulink></para>
    </section>

    <section id="faq13">
      <title>(FAQ 13) Why do you call it <quote>Shorewall</quote>?</title>

      <para><emphasis role="bold">Answer:</emphasis> Shorewall is a
      concatenation of <quote> <emphasis>Shore</emphasis>line</quote> (<ulink
      url="http://www.cityofshoreline.com">the city where I live</ulink>) and
      <quote>Fire<emphasis>wall</emphasis> </quote>. The full name of the
      product is actually <quote>Shoreline Firewall</quote> but
      <quote>Shorewall</quote> is much more commonly used.</para>
    </section>

    <section id="faq23">
      <title>(FAQ 23) Why do you use such ugly fonts on your web site?</title>

      <para><emphasis role="bold">Answer:</emphasis> The Shorewall web site is
      almost font neutral (it doesn't explicitly specify fonts except on a few
      pages) so the fonts you see are largely the default fonts configured in
      your browser. If you don't like them then reconfigure your
      browser.</para>
    </section>

    <section id="faq25">
      <title>(FAQ 25) How do I tell which version of Shorewall or Shorewall
      Lite I am running?</title>

      <para><emphasis role="bold">Answer:</emphasis> At the shell prompt,
      type:</para>

      <programlisting><command>/sbin/shorewall[-lite] version</command>      </programlisting>

      <section id="faq25a">
        <title>(FAQ 25a) How do I tell which version of Shorewall-perl and
        Shorewall-shell that I have installed?</title>

        <para><emphasis role="bold">Answer:</emphasis> At the shell prompt,
        type:</para>

        <programlisting><command>/sbin/shorewall version -a</command>    </programlisting>
      </section>
    </section>

    <section id="faq31">
      <title>(FAQ 31) Does Shorewall provide protection against....</title>

      <variablelist>
        <varlistentry>
          <term>IP Spoofing: Sending packets over the WAN interface using an
          internal LAP IP address as the source address?</term>

          <listitem>
            <para><emphasis role="bold">Answer:</emphasis> Yes.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>Tear Drop: Sending packets that contain overlapping
          fragments?</term>

          <listitem>
            <para><emphasis role="bold">Answer:</emphasis> This is the
            responsibility of the IP stack, not the Netfilter-based firewall
            since fragment reassembly occurs before the stateful packet filter
            ever touches each packet.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>Smurf and Fraggle: Sending packets that use the WAN or LAN
          broadcast address as the source address?</term>

          <listitem>
            <para><emphasis role="bold">Answer:</emphasis> Shorewall can be
            configured to do that using the <ulink
            url="blacklisting_support.htm">blacklisting</ulink> facility.
            Shorewall versions 2.0.0 and later filter these packets under the
            <firstterm>nosmurfs</firstterm> interface option in <ulink
            url="manpages/shorewall-interfaces.html">/etc/shorewall/interfaces</ulink>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>Land Attack: Sending packets that use the same address as the
          source and destination address?</term>

          <listitem>
            <para><emphasis role="bold">Answer:</emphasis> Yes, if the <ulink
            url="manpages/shorewall-interfaces.html">routefilter interface
            option</ulink> is selected.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>DOS: - SYN Dos - ICMP Dos - Per-host Dos protection</term>

          <listitem>
            <para><emphasis role="bold">Answer:</emphasis> Shorewall has
            facilities for limiting SYN and ICMP packets. Netfilter as
            included in standard Linux kernels doesn't support per-remote-host
            limiting except by explicit rule that specifies the host IP
            address; that form of limiting is supported by Shorewall.</para>
          </listitem>
        </varlistentry>
      </variablelist>
    </section>

    <section id="faq65">
      <title>(FAQ 65) How do I accomplish failover with Shorewall?</title>

      <para><emphasis role="bold">Answer:</emphasis> <ulink
      url="http://linuxman.wikispaces.com/Clustering+Shorewall">This article
      by Paul Gear</ulink> should help you get started.</para>
    </section>
  </section>

  <section id="RFC1918">
    <title>RFC 1918</title>

    <section id="faq14">
      <title>(FAQ 14) I'm connected via a cable modem and it has an internal
      web server that allows me to configure/monitor it but as expected if I
      enable rfc1918 blocking for my eth0 interface (the Internet one), it
      also blocks the cable modems web server.</title>

      <para>Is there any way it can add a rule before the rfc1918 blocking
      that will let all traffic to and from the 192.168.100.1 address of the
      modem in/out but still block all other rfc1918 addresses?</para>

      <para><emphasis role="bold">Answer:</emphasis> Add the following to
      <filename><ulink
      url="manpages/shorewall-rfc1918.html">/etc/shorewall/rfc1918</ulink></filename>
      (Note: If you are running Shorewall 2.0.0 or later, you may need to
      first copy <filename>/usr/share/shorewall/rfc1918</filename> to
      <filename>/etc/shorewall/rfc1918</filename>):</para>

      <para>Be sure that you add the entry ABOVE the entry for
      192.168.0.0/16.</para>

      <programlisting>#SUBNET        TARGET
192.168.100.1  RETURN</programlisting>

      <note>
        <para>If you add a second IP address to your external firewall
        interface to correspond to the modem address, you must also make an
        entry in <filename>/etc/shorewall/rfc1918</filename> for that address.
        For example, if you configure the address 192.168.100.2 on your
        firewall, then you would add two entries to
        <filename>/etc/shorewall/rfc1918</filename>:</para>

        <programlisting>#SUBNET        TARGET
192.168.100.1  RETURN
192.168.100.2  RETURN</programlisting>
      </note>

      <section id="faq14a">
        <title>(FAQ 14a) Even though it assigns public IP addresses, my ISP's
        DHCP server has an RFC 1918 address. If I enable RFC 1918 filtering on
        my external interface, my DHCP client cannot renew its lease.</title>

        <para><emphasis role="bold">Answer:</emphasis> The solution is the
        same as <xref linkend="faq14" /> above. Simply substitute the IP
        address of your ISPs DHCP server.</para>
      </section>

      <section id="faq14b">
        <title>(FAQ 14b) I connect to the Internet with PPPoE. When I try to
        access the built-in web server in my DSL Modem, I get connection
        Refused.</title>

        <para>I see the following in my log:</para>

        <programlisting>Mar  1 18:20:07 Mail kernel: Shorewall:OUTPUT:REJECT:IN= OUT=eth0 SRC=192.168.1.2 DST=192.168.1.1 LEN=60
TOS=0x00 PREC=0x00 TTL=64 ID=26774 DF PROTO=TCP SPT=32797 DPT=80 WINDOW=5840 RES=0x00 SYN URGP=0 </programlisting>

        <para><emphasis role="bold">Answer:</emphasis> The fact that the
        message is being logged from the OUTPUT chain means that the
        destination IP address is not in any defined zone (see <link
        linkend="faq17">FAQ 17</link>). You need to:</para>

        <orderedlist>
          <listitem>
            <para>Add a zone for the modem in
            <filename>/etc/shorewall/zones</filename>:</para>

            <programlisting>#ZONE    TYPE          OPTIONS
modem    ipv4</programlisting>
          </listitem>

          <listitem>
            <para>Define the zone to be associated with <filename
            class="devicefile">eth0</filename> (or whatever interface connects
            to your modem) in
            <filename>/etc/shorewall/interfaces</filename>:</para>

            <programlisting>#ZONE      INTERFACE   BROADCAST    OPTIONS
modem      eth0        detect</programlisting>
          </listitem>

          <listitem>
            <para>Allow web traffic to the modem in
            <filename>/etc/shorewall/rules</filename>:</para>

            <programlisting>#ACTION    SOURCE      DEST      PROTO     DEST PORT(S)
ACCEPT     fw          modem     tcp       80
ACCEPT     loc         modem     tcp       80</programlisting>
          </listitem>
        </orderedlist>

        <para>Note that many of these ADSL/Cable Modems have no default
        gateway or their default gateway is at a fixed IP address that is
        different from the IP address you have assigned to your external
        interface. In either case, you may have problems browsing the modem
        from your local network even if you have the correct routes
        established on your firewall. This is usually solved by masquerading
        traffic from your local network to the modem.</para>

        <para><filename>/etc/shorewall/masq</filename>:</para>

        <programlisting>#INTERFACE         SOURCE          ADDRESS
eth0               eth1                        # eth1 = interface to local network</programlisting>

        <para>For an example of this when the ADSL/Cable modem is bridged, see
        <ulink url="XenMyWay-Routed.html">my configuration</ulink>. In that
        case, I masquerade using the IP address of my local interface!</para>
      </section>
    </section>
  </section>

  <section id="ALIASES">
    <title>Alias IP Addresses/Virtual Interfaces</title>

    <section id="faq18">
      <title>(FAQ 18) Is there any way to use aliased ip addresses with
      Shorewall, and maintain separate rule sets for different IPs?</title>

      <para><emphasis role="bold">Answer:</emphasis> Yes. See <ulink
      url="Shorewall_and_Aliased_Interfaces.html">Shorewall and Aliased
      Interfaces</ulink>.</para>
    </section>
  </section>

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

    <section id="faq53">
      <title>(FAQ 53) What is Shorewall Lite?</title>

      <para><emphasis role="bold">Answer:</emphasis> Shorewall Lite is a
      companion product to Shorewall and is designed to allow you to maintain
      all Shorewall configuration information on a single system within your
      network. See the <ulink url="CompiledPrograms.html#Lite">Compiled
      Firewall script documentation</ulink> for details.</para>
    </section>

    <section id="faq54">
      <title>(FAQ 54) If I want to use Shorewall Lite, do I also need to
      install Shorewall on the same system?</title>

      <para><emphasis role="bold">Answer:</emphasis> No. In fact, we recommend
      that you do <emphasis role="bold">NOT</emphasis> install Shorewall on
      systems where you wish to use Shorewall Lite. You must have Shorewall
      installed on at least one system within your network in order to use
      Shorewall Lite.</para>
    </section>

    <section id="faq55">
      <title>(FAQ 55) How do I decide which product to use - Shorewall or
      Shorewall Lite?</title>

      <para><emphasis role="bold">Answer:</emphasis> If you plan to have only
      a single firewall system, then Shorewall is the logical choice. I also
      think that Shorewall is the appropriate choice for laptop systems that
      may need to have their firewall configuration changed while on the road.
      In the remaining cases, Shorewall Lite will work very well. At
      shorewall.net, the two laptop systems have the full Shorewall product
      installed as does my personal Linux desktop system. All other Linux
      systems that run a firewall use Shorewall Lite and have their
      configuration directories on my desktop system.</para>
    </section>

    <section id="faq60">
      <title>(FAQ 60) What are the compatibility restrictions between
      Shorewall and Shorewall Lite</title>

      <para><emphasis role="bold">Answer:</emphasis> Beginning with version
      3.2.3, there are no compatibility constraints between Shorewall and
      Shorewall-lite.</para>
    </section>
  </section>

  <section id="Perl">
    <title>Shorewall-Perl</title>

    <section id="faq70">
      <title>(FAQ 70) What is Shorewall-Perl?</title>

      <para><emphasis role="bold">Answer:</emphasis> Shorewall-perl is a
      re-implementation of the Shorewall configuration compiler written in
      Perl.</para>
    </section>

    <section id="faq71">
      <title>(FAQ 71) What are the advantages of using Shorewall-perl?</title>

      <para><emphasis role="bold">Answer:</emphasis></para>

      <itemizedlist>
        <listitem>
          <para>The Shorewall-perl compiler is much faster than the
          Shorewall-shell compiler.</para>
        </listitem>

        <listitem>
          <para>The script generated by the Shorewall-perl compiler uses
          <command>iptables-restore</command> to instantiate the Netfilter
          configuration. So it runs much faster than the script generated by
          the Shorewall-shell compiler and doesn't disable new connections
          during rule set installation.</para>
        </listitem>

        <listitem>
          <para>The Shorewall-perl compiler does more thorough checking of the
          configuration than the Shorewall-shell compiler does.</para>
        </listitem>

        <listitem>
          <para>The error messages produced by the Shorewall-perl compiler are
          better, more consistent and always include the file name and line
          number where the error was detected.</para>
        </listitem>

        <listitem>
          <para>Going forward, the Shorewall-perl compiler will get all
          enhancements; the Shorewall-shell compiler will only get those
          enhancements that are easy to retrofit.</para>
        </listitem>
      </itemizedlist>
    </section>

    <section id="faq72">
      <title>(FAQ 72) Can I switch to using Shorewall-perl without changing my
      Shorewall configuration?</title>

      <para><emphasis role="bold">Answer:</emphasis> Maybe yes, maybe no. See
      the <ulink url="Shorewall-perl.html">Shorewall Perl article</ulink> for
      a list of the incompatibilities between Shorewall-shell and
      Shorewall-perl.</para>
    </section>
  </section>

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

    <section id="faq77">
      <title>(FAQ 77) Shorewall is eating my Asterisk egress traffic!</title>

      <para>Somehow, my firewall config is causing a one-way audio problem in
      Asterisk. If a person calls into the PBX, they cannot hear me speaking,
      but I can hear them. If I plug the Asterisk server directly into the
      router, bypassing the firewall, the problem goes away.</para>

      <para><emphasis role="bold">Answer (requires Shorewall 4.0.6 or
      later):</emphasis> If your kernel version is 2.6.20 or
      earlier:<programlisting>rmmod ip_nat_sip
rmmod ip_conntrack_sip</programlisting>Then change the DONT_LOAD specification
      in your shorewall.conf to:<programlisting>DONT_LOAD=ip_nat_sip,ip_conntrack_sip</programlisting>If
      your kernel version is 2.6.21 or later:<programlisting>rmmod nf_nat_sip
rmmod nf_conntrack_sip</programlisting>Then change the DONT_LOAD specification
      in your shorewall.conf to:<programlisting>DONT_LOAD=nf_nat_sip,nf_conntrack_sip</programlisting>If
      you are running a version of Shorewall earlier than 4.0.6, you can avoid
      loading the sip helper modules by following the suggestions in <link
      linkend="faq59">FAQ 59</link>.</para>
    </section>
  </section>

  <section id="Misc">
    <title>Miscellaneous</title>

    <section id="faq20">
      <title>(FAQ 20) I have just set up a server. Do I have to change
      Shorewall to allow access to my server from the Internet?</title>

      <para><emphasis role="bold">Answer:</emphasis> Yes. Consult the <ulink
      url="shorewall_quickstart_guide.htm">QuickStart guide</ulink> that you
      used during your initial setup for information about how to set up rules
      for your server.</para>
    </section>

    <section id="faq24">
      <title>(FAQ 24) How can I allow connections to, let's say, the ssh port
      only from specific IP Addresses on the Internet?</title>

      <para><emphasis role="bold">Answer:</emphasis> In the SOURCE column of
      the rule, follow <quote>net</quote> by a colon and a list of the
      host/subnet addresses as a comma-separated list.</para>

      <programlisting>net:&lt;ip1&gt;,&lt;ip2&gt;,...</programlisting>

      <example id="Example4">
        <title>Example:</title>

        <programlisting>ACCEPT net:192.0.2.16/28,192.0.2.44 fw tcp 22</programlisting>
      </example>
    </section>

    <section id="faq26">
      <title>(FAQ 26) When I try to use any of the SYN options in nmap on or
      behind the firewall, I get <quote>operation not permitted</quote>. How
      can I use nmap with Shorewall?"</title>

      <para><emphasis role="bold">Answer:</emphasis> Temporarily remove and
      rejNotSyn, dropNotSyn and dropInvalid rules from
      <filename>/etc/shorewall/rules</filename> and restart Shorewall.</para>
    </section>

    <section id="faq27">
      <title>(FAQ 27) I'm compiling a new kernel for my firewall. What should
      I look out for?</title>

      <para><emphasis role="bold">Answer:</emphasis> First take a look at the
      <ulink url="kernel.htm">Shorewall kernel configuration page</ulink>. You
      probably also want to be sure that you have selected the <quote>
      <emphasis role="bold">NAT of local connections (READ HELP)</emphasis>
      </quote> on the Netfilter Configuration menu. Otherwise, DNAT rules with
      your firewall as the source zone won't work with your new kernel.</para>

      <section id="faq27a">
        <title>(FAQ 27a) I just built (or downloaded or otherwise acquired)
        and installed a new kernel and now Shorewall won't start. I know that
        my kernel options are correct.</title>

        <para>The last few lines of <ulink url="troubleshoot.htm">a startup
        trace</ulink> are these:</para>

        <programlisting>+ run_iptables2 -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE
+ '[' 'x-t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE' = 'x-t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.
0/0 -j MASQUERADE' ']'
+ run_iptables -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE
+ iptables -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE
iptables: Invalid argument
+ '[' -z '' ']'
+ stop_firewall
+ set +x</programlisting>

        <para><emphasis role="bold">Answer:</emphasis> Your new kernel
        contains headers that are incompatible with the ones used to compile
        your <command>iptables</command> utility. You need to rebuild
        <command>iptables</command> using your new kernel source.</para>
      </section>
    </section>

    <section id="faq28">
      <title>(FAQ 28) How do I use Shorewall as a Bridging Firewall?</title>

      <para><emphasis role="bold">Answer:</emphasis> Shorewall Bridging
      Firewall support is available — <ulink
      url="bridge-Shorewall-perl.html">check here for details</ulink>.</para>
    </section>

    <section id="faq39">
      <title>(FAQ 39) How do I block connections to a particular domain
      name?</title>

      <para>I tried this rule to block Google's Adsense that you'll find on
      everyone's site. Adsense is a Javascript that people add to their Web
      pages. So I entered the rule:</para>

      <programlisting>#ACTION   SOURCE        DEST                                 PROTO
REJECT    fw            net:pagead2.googlesyndication.com    all</programlisting>

      <para>However, this also sometimes restricts access to "google.com". Why
      is that? Using dig, I found these IPs for domain
      googlesyndication.com:<programlisting>216.239.37.99
216.239.39.99</programlisting>And this for google.com:<programlisting>216.239.37.99
216.239.39.99
216.239.57.99</programlisting>So my guess is that you are not actually
      blocking the domain, but rather the IP being called. So how in the world
      do you block an actual domain name?</para>

      <para><emphasis role="bold">Answer:</emphasis> Packet filters like
      Netfilter base their decisions on the contents of the various protocol
      headers at the front of each packet. Stateful packet filters (of which
      Netfilter is an example) use a combination of header contents and state
      created when the packet filter processed earlier packets. Netfilter (and
      Shorewall's use of Netfilter) also consider the network interface(s)
      where each packet entered and/or where the packet will leave the
      firewall/router.</para>

      <para>When you specify <ulink
      url="configuration_file_basics.htm#dnsnames">a domain name in a
      Shorewall rule</ulink>, the iptables program resolves that name to one
      or more IP addresses and the actual Netfilter rules that are created are
      expressed in terms of those IP addresses. So the rule that you entered
      was equivalent to:</para>

      <para><programlisting>#ACTION   SOURCE        DEST                 PROTO
REJECT    fw            net:216.239.37.99    all
REJECT    fw            net:216.239.39.99    all</programlisting>Given that
      name-based multiple hosting is a common practice (another example:
      lists.shorewall.net and www1.shorewall.net are both hosted on the same
      system with a single IP address), it is not possible to filter
      connections to a particular name by examination of protocol headers
      alone. While some protocols such as <ulink url="FTP.html">FTP</ulink>
      require the firewall to examine and possibly modify packet payload,
      parsing the payload of individual packets doesn't always work because
      the application-level data stream can be split across packets in
      arbitrary ways. This is one of the weaknesses of the 'string match'
      Netfilter extension available in later Linux kernel releases. The only
      sure way to filter on packet content is to proxy the connections in
      question -- in the case of HTTP, this means running something like
      <ulink url="Shorewall_Squid_Usage.html">Squid</ulink>. Proxying allows
      the proxy process to assemble complete application-level messages which
      can then be accurately parsed and decisions can be made based on the
      result.</para>
    </section>

    <section id="faq42">
      <title>(FAQ 42) How can I tell which features my kernel and iptables
      support?</title>

      <para><emphasis role="bold">Answer:</emphasis> Use the
      <command>shorewall[-lite] show capabilities</command> command at a root
      prompt.</para>

      <programlisting>gateway:~# shorewall show capabilities
Loading /usr/share/shorewall/functions...
Processing /etc/shorewall/params ...
Processing /etc/shorewall/shorewall.conf...
Loading Modules...
Shorewall has detected the following iptables/netfilter capabilities:
   NAT: Available
   Packet Mangling: Available
   Multi-port Match: Available
   Extended Multi-port Match: Available
   Connection Tracking Match: Available
   Packet Type Match: Available
   Policy Match: Available
   Physdev Match: Available
   IP range Match: Available
   Recent Match: Available
   Owner Match: Available
   Ipset Match: Available
   ROUTE Target: Available
   Extended MARK Target: Available
   CONNMARK Target: Available
   Connmark Match: Available
   Raw Table: Available
gateway:~#</programlisting>
    </section>

    <section id="faq19">
      <title>(FAQ 19) How do I open the firewall for all traffic to/from the
      LAN?</title>

      <para><emphasis role="bold">Answer:</emphasis> Add these two
      policies:</para>

      <programlisting>#SOURCE            DESTINATION             POLICY            LOG              LIMIT:BURST
#                                                            LEVEL
$FW                loc                     ACCEPT
loc                $FW                     ACCEPT           </programlisting>

      <para>You should also delete any ACCEPT rules from $FW-&gt;loc and
      loc-&gt;$FW since those rules are redundant with the above
      policies.</para>
    </section>
  </section>
</article>