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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>2001-2014</year>

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

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

  <caution>
    <para><emphasis role="bold">This article applies to Shorewall 4.4 and
    later. If you are running a version of Shorewall earlier than Shorewall
    4.4.0 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="faq92">
      <title>(FAQ 92) There are lots of Shorewall packages; which one(s) do I
      install?</title>

      <para><emphasis role="bold">Answer</emphasis>: When first installing
      Shorewall 4.4.0 or later, you must install the <emphasis
      role="bold">shorewall</emphasis> package. If you want to configure an
      IPv6 firewall, you must also install <emphasis
      role="bold">shorewall6</emphasis>.</para>

      <section id="faq92a">
        <title>(FAQ 92a) Someone once told me to install shorewall-perl;
        anything to that?</title>

        <para><emphasis role="bold">Answer</emphasis>: That was good advice in
        Shorewall 4.2 and earlier. In those releases, there were two packages
        that provided the basic firewalling functionality: <emphasis
        role="bold">shorewall-shell</emphasis> and <emphasis
        role="bold">shorewall-perl</emphasis>. Beginning with Shorewall 4.4.0,
        <emphasis role="bold">shorewall-shell</emphasis> is discontinued and
        <emphasis role="bold">shorewall-perl</emphasis> is renamed <emphasis
        role="bold">shorewall</emphasis>.</para>
      </section>
    </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,
        former 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> Beginning with
        Shorewall 4.4, the samples are in the shorewall package and are
        installed in <filename
        class="directory">/usr/share/doc/shorewall/examples/</filename>.</para>
      </section>
    </section>

    <section id="faq14">
      <title>(FAQ 14) I can't find the Shorewall 4.4 shorewall-common,
      shorewall-shell and shorewall-perl packages? Where are they?</title>

      <para><emphasis role="bold">Answer</emphasis>:In Shorewall 4.4, the
      <firstterm>shorewall-shell</firstterm> package was discontinued. The
      <firstterm>shorewall-common</firstterm> and
      <firstterm>shorewall-perl</firstterm> packages were combined to form a
      single <firstterm>shorewall</firstterm> package.</para>
    </section>
  </section>

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

    <section id="faq66">
      <title>(FAQ 66) I'm trying to upgrade to Shorewall 4.x; 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 id="faq34">
      <title>(FAQ 34) I am trying to upgrade to Shorewall 4.4 and I can't find
      the shorewall-common, shorewall-shell and shorewall-perl packages? Where
      are they?</title>

      <para><emphasis role="bold">Answer</emphasis>:In Shorewall 4.4, the
      <firstterm>shorewall-shell</firstterm> package was discontinued. The
      <firstterm>shorewall-common</firstterm> and
      <firstterm>shorewall-perl</firstterm> packages were combined to form a
      single <firstterm>shorewall</firstterm> package. For further
      information, please see the <ulink url="upgrade_issues.htm">upgrade
      issues.</ulink>.</para>
    </section>

    <section id="faq34a">
      <title>(FAQ 34a) I am trying to upgrade to Shorewall 4.4 and I'm getting
      errors when I try to start Shorewall. Where can I find information about
      these issues?</title>

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

    <section id="faq34b">
      <title>(FAQ 34b) I am trying to upgrade to Shorewall 4.4 and I'm seeing
      warning messages when I try to start Shorewall. Where can I find
      information about these issues?</title>

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

    <section id="faq76">
      <title>(FAQ 76) I just upgraded my Debian (Ubuntu, Kubuntu, ...) 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>

      <para><emphasis role="bold">Update</emphasis>: Beginning with Shorewall
      4.4.21, there is a <emphasis role="bold">shorewall update</emphasis>
      command that does a smart merge of your existing shorewall.conf and the
      new one.</para>
    </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 <emphasis>from the net</emphasis> 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>

      <important>
        <para><emphasis role="bold">The above does not work for forwarding
        from the local network. If you want to do that, see <link
        linkend="faq2">FAQ 2</link>.</emphasis></para>
      </important>

      <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
            must be the IP address of the firewall's interface to that system
            unless you use the hack described in <link linkend="faq1f">FAQ
            1f</link>).</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 Wireshark to further diagnose the
                problem.</para>
              </listitem>

              <listitem>
                <para>The traffic is entering your firewall on a different
                interface (interfaces reversed in
                <filename>/etc/shorewall/interfaces</filename>?).</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 address and add the
        following rule, then you will have access on port 4104 from the net
        and on port 22 from your LAN.</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 sequence 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 of making 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><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>and in
        <filename>/etc/shorewall/shorewall.conf</filename>:</para>

        <programlisting>IP_FORWARDING=On</programlisting>

        <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 id="faq1h">
        <title>(FAQ 1h) How do I set shorewall to allow ssh on port 9022 from
        net? SSHD is listening on port 22.</title>

        <para><emphasis role="bold">Answer</emphasis>: Use this rule.</para>

        <programlisting>#ACTION         SOURCE        DEST        PROTO        DEST
#                                                      PORT(S)
REDIRECT        net           22          tcp          9022</programlisting>

        <para>Note that the above rule will also allow connections from the
        net on TCP port 22. If you don't want that, see <link
        linkend="faq1e">FAQ 1e</link>.</para>
      </section>

      <section id="faq1j">
        <title>(FAQ 1j) Why doesn't this DNAT rule work?</title>

        <para>I added this rule but I'm still seeing the log message
        below</para>

        <programlisting>RULE:
DNAT           scnet:172.19.41.2       dmz0:10.199.198.145             udp     2055

LOG:
Sep 21 12:55:37 fw001 kernel: [10357687.114928] Shorewall:scnet2fw:DROP:IN=eth2 OUT=
MAC=00:26:33:dd:aa:05:00:24:f7:19:ce:44:08:00 SRC=172.19.41.2 DST=172.19.1.1 LEN=1492
TOS=0x00 PREC=0x00 TTL=63 ID=23035 PROTO=UDP SPT=6376 DPT=2055 LEN=1472</programlisting>

        <para><emphasis role="bold">Answer</emphasis>: There was already a
        conntrack entry for the failing connection before you added the rule.
        Install the <emphasis role="bold">conntrack</emphasis> utility program
        and use it to delete the entry.</para>

        <programlisting><command>conntrack -D -s 172.19.41.2 -d 172.19.1.1 -p udp -sport 6367 -dport 2055 </command></programlisting>
      </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.<note>
          <para>If you use both 1:1 NAT and SNAT/MASQUERADE, those connections
          that are subject to 1:1 NAT should use ACCEPT rather than DNAT.
          Note, however, that DNAT can be used to override 1:1 NAT so as to
          redirect a connection to a different internal system or port than
          would be the case using 1:1 NAT.</para>
        </note> 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>

      <note>
        <para>The preceding answer should <emphasis>not</emphasis> be
        interpreted to mean that DNAT can only be used in conjunction with
        SNAT. But in common configurations using private local addresses, that
        is the most common usage.</para>
      </note>
    </section>

    <section id="faq8">
      <title>(FAQ 8) I have several external IP addresses and use
      /etc/shorewall/nat to associate them with systems in my DMZ. When I add
      a DNAT rule, say for ports 80 and 443, Shorewall redirects connections
      on those ports for all of my addresses. How can I restrict DNAT to only
      a single address?</title>

      <para><emphasis role="bold">Answer</emphasis>: Specify the external
      address that you want to redirect in the ORIGINAL DEST column.</para>

      <para>Example:</para>

      <programlisting>#ACTION    SOURCE        DEST                   PROTO    DEST PORT   SOURCE    ORIGINAL
#                                                                    PORT      DEST.
DNAT       net           net:192.168.4.22       tcp      80,443      -         <emphasis
          role="bold">206.124.146.178</emphasis></programlisting>
    </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://idallen.com/dnat.txt">Paper about DNAT and
      Linux</ulink>.</para>
    </section>

    <section id="faq48">
      <title>(FAQ 48) How do I Set up a 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. I use a separate DNS server (dnsmasq) here
          at shorewall.net.</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>If you really want to route traffic between two internal systems
      through your firewall, then proceed as described below.<warning>
          <para>All traffic redirected through use of this technique 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        192.168.1.0/24  192.168.1.254   tcp     www</emphasis></programlisting>

          <para>Note: The technique described here is known as
          <firstterm>hairpinning NAT</firstterm> and is described in section 6
          of <ulink url="http://www.faqs.org/rfcs/rfc4787.html">RFC
          4787</ulink>. In that RFC, it is required that the
          <emphasis>external IP address</emphasis> be used as the
          source:</para>

          <programlisting>#INTERFACE              SOURCE          ADDRESS         PROTO   PORT(S)
eth1:192.168.1.5        192.168.1.0/24  <emphasis role="bold">130.151.100.69</emphasis>  tcp     www</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 (and the second one in the previous bullet) 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>.</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.</para>

          <note>
            <para>If your local interface is a bridge, see <link
            linkend="faq2e">FAQ 2e</link> for additional configuration
            steps.</para>
          </note>

          <note>
            <para>For optional interfaces, use the function <emphasis
            role="bold">find_first_interface_address_if_any()</emphasis>
            rather than <emphasis
            role="bold">find_first_interface_address()</emphasis>. The former
            will return 0.0.0.0 if the interface has no configured IP address;
            the latter terminates the calling program.</para>
          </note>

          <note id="Call">
            <para>If you run Shorewall-lite on your firewall, you must use the
            following in the firewall's configuration directory
            <filename>params</filename> file:</para>

            <programlisting><command>ETH0_IP=$(ssh root@firewall "/sbin/shorewall-lite call find_first_interface_address eth0")</command></programlisting>
          </note>
        </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 split DNS. 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 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 of server 192.168.2.2</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/masq</filename>:</para>

          <programlisting>#INTERFACE:         SOURCE           ADDRESS
#ADDRESS
eth2:192.168.1.2    192.168.2.0/24</programlisting>

          <para>In <filename>/etc/shorewall/nat</filename>, be sure that you
          have <quote>Yes</quote> in the ALL INTERFACES column.</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>:</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>For optional interfaces, use the function <emphasis
          role="bold">find_first_interface_address_if_any()</emphasis> rather
          than <emphasis
          role="bold">find_first_interface_address()</emphasis>. The former
          will return 0.0.0.0 if the interface has no configured IP address;
          the latter terminates the calling program.</para>
        </note>

        <note>
          <para>If you use Shorewall-lite, then you need to configure the
          params file in the firewall's configuration directory as described
          <link linkend="Call">above</link>.</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>
        <title>(FAQ 2d) Does Shorewall support hairpinning NAT?</title>

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

        <para>In the case of simple masquerade/SNAT, see <link
        linkend="faq2">FAQ 2</link>.</para>

        <para>For one-to-one (static), NAT, simply place 'Yes' in the ALL
        INTERFACES column of each entry in <ulink
        url="manpages/shorewall-nat.html">/etc/shorewall/nat</ulink>.</para>
      </section>

      <section id="faq2e">
        <title>(FAQ 2e) I have the situation in FAQ 2 but my local interface
        is a bridge and the solution in FAQ 2 doesn't work</title>

        <para><emphasis role="bold">Answer</emphasis>: Assume that the bridge
        is br0 and that eth2 is the bridge port that connects to the LAN
        containing 192.168.1.5</para>

        <para>In addition to the steps in FAQ 2 (replacing eth1 with br0), you
        also need to:</para>

        <orderedlist>
          <listitem>
            <para>Set the <firstterm>hairpin</firstterm> option on
            eth2.</para>

            <programlisting>brctl hairpin br0 eth2 on</programlisting>

            <para>On Debian and derivitives, you can place that command in
            /etc/network/interfaces as a post-up command:</para>

            <programlisting>auto br0
iface br0 inet static
        bridge_ports    eth2
        bridge_fd       0
        bridge_maxwait  0
        address         192.168.1.1
        netmask         255.255.255.0
        <emphasis role="bold">post-up /sbin/brctl hairpin br0 eth2 on</emphasis></programlisting>
          </listitem>

          <listitem>
            <para>Install ebtables if it is not already installed.</para>
          </listitem>

          <listitem>
            <para>Be sure that all traffic going out of eth2 has the correct
            MAC address.</para>

            <programlisting>ebtables -t nat -A POSTROUTING -o eth2 -j snat --to-source <emphasis>br0-MAC-address</emphasis> </programlisting>

            <para>where br0-MAC-address is the MAC address of br0.</para>

            <para>Here's a working example of /etc/shorewall/start that
            executes the above command.</para>

            <programlisting>if [ $(ebtables -t nat -L POSTROUTING | wc -l) -lt 4 ]; then
   <emphasis role="bold">ebtables -t nat -A POSTROUTING -o eth2 -j snat --to-source 0:19:21:d0:61:65</emphasis>
fi</programlisting>
          </listitem>
        </orderedlist>
      </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>

      <note>
        <para>Beginning with Shorewall 4.4.13, you can use the
        <option>blacklist</option> option in <ulink
        url="manpages/shorewall-interfaces.html"><filename>/etc/shorewall/interfaces</filename></ulink>
        to implement blacklisting by destination IP address.</para>
      </note>

      <note>
        <para>Beginning with Shorewall 4.4.26, you can use <ulink
        url="manpages/shorewall-blrules.html">/etc/shorewall/blrules</ulink>
        to implement arbitrary blacklist rules.</para>
      </note>
    </section>

    <section id="faq84">
      <title>(FAQ 84) I put some IPs in the blacklist file in /etc/shorewall
      to block the ips but i'm still getting reports from PSAD from those ips
      saying they're port scanning. Shouldn't being on the blacklist drop all
      packets from those ips?</title>

      <para><emphasis role="bold">Answer</emphasis>: You probably forgot to
      specify the <emphasis role="bold">blacklist</emphasis> option for your
      external interface(s) in <filename><ulink
      url="manpages/shorewall-interfaces.html">/etc/shorewall/interfaces</ulink></filename>.</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.</para>

      <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="faq100">
      <title>(FAQ 100) With Shorewall started, the output of 'iptables -L'
      looks like my firewall is wide open!</title>

      <para><emphasis role="bold">Answer:</emphasis> The problem here is that
      a bare <command>iptables -L</command> command produces totally useless
      output. Use <command>shorewall show</command> instead.</para>

      <note>
        <para>The <command>shorewall show</command> command is a wrapper
        around <command>iptables -L -n -v</command>.</para>
      </note>
    </section>

    <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>

      <para>If you would prefer to 'stealth' port 113, then:</para>

      <itemizedlist>
        <listitem>
          <para>If you are running Shorewall 4.4.20 or earlier, copy
          /<filename>usr/share/shorewall/action.Drop</filename> to
          <filename>/etc/shorewall/</filename> and modify the invocation of
          Auth to <emphasis role="bold">Auth(DROP)</emphasis>.</para>
        </listitem>

        <listitem>
          <para>If you are running Shorewall 4.4.21 or later, in
          shorewall.conf, set DROP_DEFAULT="Drop(-,DROP)". See the <ulink
          url="Actions.html">Action HOWTO</ulink> to learn how that magic
          works.</para>
        </listitem>
      </itemizedlist>

      <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 manpage 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 run 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. You can test this
          by:</para>

          <orderedlist numeration="loweralpha">
            <listitem>
              <para>At a root shell prompt, type 'shorewall clear'.</para>
            </listitem>

            <listitem>
              <para>From a local system, attempt to ping the IP address of the
              Shorewall system's internet (external) interface. If that
              doesn't work, then the default gateway on the system from which
              you pinged is not set correctly.</para>
            </listitem>

            <listitem>
              <para>Be sure to 'shorewall start' after the test.</para>
            </listitem>
          </orderedlist>
        </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
      <option>routeback</option> 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 (or later) 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 the<ulink
      url="bridge-Shorewall-perl.html"> Shorewall-perl bridging
      documentation</ulink> for information about how to configure
      bridge/firewalls.<note>
          <para>Following the instructions in the new bridging documentation
          will not prevent the above message from being issued.</para>
        </note></para>
    </section>

    <section id="faq85">
      <title>(FAQ 85) Shorewall is rejecting connections from my local lan
      because it thinks they are coming from the 'net' zone.</title>

      <para>I'm seeing this in my log:</para>

      <programlisting>Aug 31 16:51:24 fw22 kernel: Shorewall:net2fw:DROP:IN=eth5 OUT= MAC=00:0c:29:74:9c:0c:08:00:20:b2:5f:db:08:00
                                       SRC=10.1.50.14 DST=10.1.50.7 LEN=57 TOS=0x00 PREC=0x00 TTL=255 ID=32302 DF
                                       PROTO=UDP SPT=53289 DPT=53 LEN=37</programlisting>

      <para><emphasis role="bold">Answer</emphasis>: This occurs when the
      external interface and an internal interface are connected to the same
      switch or hub. See <ulink url="FoolsFirewall.html">this article</ulink>
      for details. The solution is to never connect more than one firewall
      interface to the same hub or switch (an obvious exception is that when
      you have a switch that supports VLAN tagging and the interfaces are
      associated with different VLANs).</para>
    </section>
  </section>

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

    <section id="faq91">
      <title>(FAQ 91) I changed the shorewall.conf file in /etc/shorewall/ to
      spit out logs to /var/log/shorewall.log and it's not happening after I
      restart shorewall. LOGFILE=/var/log/shorewall.log &lt;-- that should be
      the correct line, right?</title>

      <para><emphasis role="bold">Answer</emphasis>: No, that is not correct.
      The LOGFILE setting tells Shorewall where to find the log; it does not
      determine where messages are written. See <link linkend="faq6">the next
      FAQ</link>.</para>
    </section>

    <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>It is also possible to <ulink url="shorewall_logging.html">set up
      Shorewall to log all of Netfilter's 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.cert.uni-stuttgart.de.projects/fwlogwatch">fwlogwatch</ulink>.
        It emails me a report each day from my various systems with each
        report summarizing the logged activity on the corresponding system;
        here's a sample:</para>

        <blockquote>
          <programlisting>fwlogwatch summary
Generated Tuesday March 02 08:14:37 PST 2010 by root.
362 (and 455 older than 86400 seconds) of 817 entries in the file "/var/log/ulog/syslogemu.log" are packet logs, 138 have unique characteristics.
First packet log entry: Mar 01 08:16:06, last: Mar 02 08:06:21.
All entries were logged by the same host: "gateway".
All entries have the same target: "-".
Only entries with a count of at least 5 are shown.

net-dmz DROP  eth2 36 packets from 61.158.162.9 to 206.124.146.177
net-fw DROP  eth0 21 packets from 89.163.162.13 to 76.104.233.98
net-fw DROP  eth0 19 packets from 61.184.101.46 to 76.104.233.98
net-fw DROP  eth0 12 packets from 81.157.214.103 to 76.104.233.98
net-fw DROP  eth0 11 packets from 174.37.159.222 to 76.104.233.98
net-fw DROP  eth0 10 packets from 221.195.73.86 to 76.104.233.98
net-dmz DROP  eth2 9 packets from 202.199.158.6 to 206.124.146.177
net-fw DROP  eth2 9 packets from 202.199.158.6 to 206.124.146.176
net-dmz DROP  eth2 9 packets from 202.199.158.6 to 206.124.146.178
net-fw DROP  eth0 6 packets from 221.192.199.35 to 76.104.233.98
net-fw DROP  eth2 5 packets from 61.158.162.9 to 206.124.146.177</programlisting>
        </blockquote>

        <para>Fwlogwatch contains a built-in web server that allows monitoring
        recent activity in summary fashion.</para>
      </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 your logging 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) cat /proc/sys/kernel/prink returns '4 4 1 7' and
        still I get dmesg filled up</title>

        <para><emphasis role="bold">Answer</emphasis>: While we would argue
        that 'dmesg filled up' is not necessarily a problem, the only way to
        eliminate that is to <ulink url="shorewall_logging.html">set up
        Shorewall to log all of Netfilter's messages to a separate
        file</ulink>.</para>
      </section>

      <section id="faq16b">
        <title>(FAQ 16b) 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
        level 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 id="faq16c">
        <title>(FAQ 16c) Shorewall messages are flooding the output of
        'dmesg'; how to I stop that?</title>

        <para><emphasis role="bold">Answer</emphasis>: Switch to using <ulink
        url="???">ulogd</ulink>.</para>
      </section>

      <section id="faq16d">
        <title>(FAQ 16d) I set LOGFILE=/var/log/shorewall but log messages are
        still going to /var/log/messages.</title>

        <para><emphasis role="bold">Answer</emphasis>: See the answer to <link
        linkend="faq16b">FAQ 16b</link> above.</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 id="all2all">
          <term><emphasis role="bold"><replaceable>zone</replaceable>2all,
          <replaceable>zone</replaceable>-all,
          all2<replaceable>zone</replaceable>,
          all-<replaceable>zone</replaceable>, all2all or
          all-all</emphasis></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>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
          role="bold"><replaceable>zone1</replaceable>2<replaceable>zone2</replaceable>
          or <replaceable>zone1-zone2</replaceable></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
          role="bold">@<replaceable>zone1</replaceable>2<replaceable>zone2</replaceable>
          or
          @<replaceable>zone1</replaceable>-<replaceable>zone2</replaceable></emphasis></term>

          <listitem>
            <para>You have a policy for traffic from
            <replaceable>zone1</replaceable> to
            <replaceable>zone2</replaceable> 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
          role="bold"><replaceable>zone1</replaceable>2<replaceable>zone2</replaceable>~,
          <replaceable>zone1</replaceable>-<replaceable>zone2</replaceable>~
          or ~blacklist<replaceable>nn</replaceable></emphasis></term>

          <listitem>
            <para>These are the result of entries in the <ulink
            url="manpages/shorewall-blrules.html">/etc/shorewall/blrules</ulink>
            file.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><emphasis role="bold"><emphasis>interface</emphasis>_mac or
          <emphasis>interface</emphasis>_rec</emphasis></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><emphasis role="bold">blacklist</emphasis></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><emphasis role="bold">INPUT or FORWARD</emphasis></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 or they match the same wildcard entry in
            <ulink
            url="manpages/shorewall-interfaces.html">/etc/shorewall/interfaces</ulink>,
            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>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><emphasis role="bold">OUTPUT</emphasis></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>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><emphasis role="bold">logflags</emphasis></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>

        <varlistentry>
          <term><emphasis role="bold">sfilter</emphasis></term>

          <listitem>
            <para>On systems running Shorewall 4.4.20 or later, either the
            packet matched the <option>filter</option> <ulink
            url="manpages/shorewall-interfaces.html">interface option</ulink>
            or it is being routed out of the same interface on which it
            arrived and the interface does not have the
            <option>routeback</option> or <option>routefilter</option> <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.</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="faq81">
      <title>(FAQ 81) logdrop and logreject don't log.</title>

      <para>I love the ability to type 'shorewall logdrop ww.xx.yy.zz' and
      completely block a particular IP address. However, the log part doesn't
      happen. When I look in the logdrop chain, there is no LOG prefix.</para>

      <para><emphasis role="bold">Answer</emphasis>: You haven't set a value
      for BLACKLIST_LOGLEVEL in <ulink
      url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5).</para>
    </section>

    <section id="faq36">
      <title>(FAQ 36) My log is filling up with these BANDWIDTH
      messages!</title>

      <programlisting>Dec 15 16:47:30 heath-desktop kernel: [17182740.184000] BANDWIDTH_IN:IN=eth1 OUT= MAC=ff:ff:ff:ff:ff:ff:00:01:5c:23:79:02:08:00
                                                        SRC=10.119.248.1 DST=255.255.255.255 LEN=328 TOS=0x00 PREC=0x00 TTL=64
                                                        ID=62081 PROTO=UDP SPT=67 DPT=68 LEN=308
Dec 15 16:47:30 heath-desktop last message repeated 2 times
Dec 15 16:47:30 heath-desktop kernel: [17182740.188000] BANDWIDTH_IN:IN=eth1 OUT= MAC=ff:ff:ff:ff:ff:ff:00:01:5c:23:79:02:08:00
                                                        SRC=10.112.70.1 DST=255.255.255.255 LEN=328 TOS=0x00 PREC=0x00 TTL=64
                                                        ID=62082 PROTO=UDP SPT=67 DPT=68 LEN=308
Dec 15 16:47:30 heath-desktop last message repeated 2 times</programlisting>

      <para><emphasis role="bold">Answer</emphasis>: The Webmin 'bandwidth'
      module adds commands to <filename>/etc/shorewall/start</filename> that
      creates rules to log every packet to/from/through the firewall.
      <emphasis role="bold">DON'T START THE BANDWIDTH SERVICE IN
      WEBMIN!</emphasis></para>

      <para>To correct this situation once it occurs, edit
      <filename>/etc/shorewall/start</filename> and insert 'return 0' prior to
      the BANDWIDTH rules.</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 about 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="faq94">
      <title>(FAQ 94) After I start Shorewall, ps doesn't show any shorewall
      process running. What is the Shorewall daemon called?</title>

      <para><emphasis role="bold">Answer:</emphasis> Shorewall is not a
      daemon. It is a configuration tool that configures your kernel based on
      the contents of <filename>/etc/shorewall/</filename>. Once the
      <command>start</command> command completes, Shorewall has done its job
      and there are no Shorewall processes remaining in the system.</para>
    </section>

    <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
      <command>stop</command> command places the firewall in a safe state;
      connections that are allowed are governed by the setting of
      ADMINISABSENTMINDED in <ulink
      url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5) and the
      contents of <ulink
      url="manpages/shorewall-routestopped.html">shorewall-routestopped</ulink>
      (5). To totally open the firewall, use the <command>clear</command>
      command.</para>
    </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="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="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. An alternative is to set
      LOAD_HELPERS_ONLY=Yes in <ulink
      url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5).</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> See the <ulink
      url="OpenVZ.html">Shorewall OpenVZ article</ulink>.</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>(5))
      and on the setting of ADMINISABSENTMINDED in
      <filename>/etc/shorewall/shorewall.conf</filename> (<ulink
      url="manpages/shorewall.conf.html">shorewall.conf</ulink>(5)).</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, <command>stop</command> reverses the effect of
      <command>start</command>.</para>

      <para>Beginning with Shorewall 4.4, when the Shorewall tarballs are
      installed on a Debian (or derivative) system, the
      <filename>/etc/init.d/shorewall</filename> file is the same as would be
      installed by the .deb. The behavior of <command>/etc/init.d/shorewall
      stop</command> is controlled by the setting of SAFESTOP in
      <filename>/etc/default/shorewall</filename>. When set to 0 (the
      default), the firewall is cleared; when set to 1, the firewall is placed
      in a safe state.</para>
    </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 start' 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 id="faq86">
      <title>(FAQ 86) My distribution (Ubuntu) uses NetworkManager to manage
      my interfaces. I want to specify the upnpclient option for my interfaces
      which requires them to be up and configured when Shorewall starts but
      Shorewall is being started before NetworkManager.</title>

      <para>Answer: I faced a similar problem which I solved as
      follows:</para>

      <itemizedlist>
        <listitem>
          <para>Don't start Shorewall at boot time (Debian and Ubuntu users
          may simply set startup=0 in
          <filename>/etc/default/shorewall</filename>).</para>
        </listitem>

        <listitem>
          <para>In <filename>/etc/network/ip-up.d</filename>, I added a
          <filename>shorewall</filename> script as follows:</para>

          <programlisting>#!/bin/sh

shorewall status &gt; /dev/null 2&gt;&amp;1 || shorewall start # Start Shorewall if it isn't already running</programlisting>

          <para>Be sure to secure the script for execute access.</para>
        </listitem>
      </itemizedlist>

      <variablelist>
        <varlistentry>
          <term>Update:</term>

          <listitem>
            <para>Beginning with Shorewall 4.4.10, there is a new <ulink
            url="Manpages/shorewall-init.html">Shorewall Init Package</ulink>
            that is designed to handle this case.</para>
          </listitem>
        </varlistentry>
      </variablelist>
    </section>

    <section id="faq90">
      <title>(FAQ 90) Shorewall starts fine but after several minutes, it
      stops. Why is it doing that?</title>

      <para><emphasis role="bold">Answer:</emphasis> Shorewall uses the
      presence of a chain named <emphasis>shorewall</emphasis> to indicate
      whether is started or stopped. That chain is created during execution of
      a successful <emphasis role="bold">start</emphasis>, <emphasis
      role="bold">restart</emphasis> or <emphasis
      role="bold">restore</emphasis> command and is removed during <emphasis
      role="bold">stop</emphasis> and <emphasis role="bold">clear</emphasis>.
      If <emphasis role="bold">shorewall status</emphasis> indicates that
      Shorewall is stopped, then something has deleted that chain. Look at the
      output of <emphasis role="bold">shorewall status</emphasis>; if it looks
      like this:</para>

      <blockquote>
        <programlisting>gateway:~# shorewall status
Shorewall-4.4.11 Status at gateway - Wed Jul 21 13:21:41 PDT 2010

Shorewall is <emphasis role="bold">stopped</emphasis>
State:<emphasis role="bold">Started</emphasis> (Tue Jul 20 16:01:49 PDT 2010)

gateway:~#</programlisting>
      </blockquote>

      <para>then it means that something outside of Shorewall has deleted the
      chain. This usually means that you were running another firewall package
      before you installed Shorewall and that other package has replaced
      Shorewall's Netfilter configuration with its own. You must remove (or at
      least disable) the other firewall package and restart Shorewall.</para>

      <blockquote>
        <programlisting>gateway:~# shorewall status
Shorewall-4.4.11 Status at gateway - Wed Jul 21 13:26:29 PDT 2010

Shorewall is <emphasis role="bold">stopped</emphasis>
State:<emphasis role="bold">Stopped</emphasis> (Wed Jul 21 13:26:26 PDT 2010)

gateway:~# </programlisting>
      </blockquote>

      <para>then a <emphasis role="bold">shorewall stop</emphasis> command has
      been executed (if the State shown in the output is <emphasis
      role="bold">Cleared</emphasis>, then a <emphasis role="bold">shorewall
      clear</emphasis> command was executed). Most likely, you have installed
      and configured the <emphasis>shorewall-init</emphasis> package and a
      required interface has gone down.</para>
    </section>

    <section id="faq99">
      <title>(FAQ 99) My /var/lib/shorewall-init.log shows that Shorewall is
      running at boot but after boot 'iptables -L' shows an empty
      configuration</title>

      <para><emphasis role="bold">Answer</emphasis>: This is caused by your
      failure to disable your distributions default iptables configuration
      tool when you installed Shorewall. Look for a service called 'iptables'
      that is being started after Shorewall and disable it.</para>
    </section>

    <section id="faq101">
      <title>(FAQ 101) How can I speed up 'shorewall start' and 'shorewall
      restart' on my slow hardware?</title>

      <para><emphasis role="bold">Answer</emphasis>: There are several steps
      that you can take:</para>

      <orderedlist>
        <listitem>
          <para>If your kernel supports module autoloading (and distribution
          default kernels almost always do), then set LOAD_HELPERS_ONLY=Yes in
          shorewall.conf.</para>
        </listitem>

        <listitem>
          <para>Set AUTOMAKE=Yes in shorewall.conf. This will avoid the
          compilation phase in cases where the configuration has not changed
          since the last time that the configuration was compiled.</para>
        </listitem>

        <listitem>
          <para>Don't set optimization option 8. For example, if you currently
          set OPTIMIZE=31, then change that to OPTIMIZE=23. Optimization
          option 8 combines identical chains which can result in a smaller
          ruleset, but it slows down the compilation of large rulesets.</para>
        </listitem>
      </orderedlist>
    </section>

    <section id="faq103">
      <title>(FAQ 103) Shorewall fails to start at boot but will start
      immediately after</title>

      <para><emphasis role="bold">Answer:</emphasis> This is usually
      associated with SELinux. <ulink
      url="https://lists.fedoraproject.org/pipermail/selinux/2010-June/012680.html">Here</ulink>
      is an example.</para>
    </section>

    <section id="faq104">
      <title>(FAQ 104) I see <emphasis>kernel</emphasis> messages in my log
      when I start or restart Shorewall or Shorewall6</title>

      <para>Example: </para>

      <programlisting>&gt; Oct 1 13:04:39 deb kernel: [ 9570.619744] xt_addrtype: ipv6 does not support BROADCAST matching
</programlisting>

      <para><emphasis role="bold">Answer:</emphasis> These are harmless.
      Shorewall attempts to execute various commands to determine the
      capabiities of your system. If you system doesn't support a command, it
      will generally issue a kernel log message.</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/mangle</filename>
      (<filename>/etc/shorewall/tcrules</filename>) file:</para>

      <programlisting>#ACTION       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_basic</member>

          <member>cls_fw</member>

          <member>cls_u32</member>

          <member>sch_htb</member>

          <member>sch_ingress</member>

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

    <section id="faq97">
      <title>(FAQ 97) I enable Shorewall traffic shaping and now my upload
      rate is way below what I specified</title>

      <para><emphasis role="bold">Answer</emphasis>: This is likely due to TCP
      Segmentation Offload (TSO) and/or Generic Segmentation Offload (GSO)
      being enabled in the network adapter. To verify, install the
      <firstterm>ethtool</firstterm> package and use the -k command:</para>

      <programlisting>root@gateway:~# ethtool -k eth1
Offload parameters for eth1:
rx-checksumming: on
tx-checksumming: on
scatter-gather: on
tcp-segmentation-offload: <emphasis role="bold">on</emphasis>
udp-fragmentation-offload: off
generic-segmentation-offload: <emphasis role="bold">on</emphasis>
generic-receive-offload: off
large-receive-offload: off
ntuple-filters: off
receive-hashing: off
root@gateway:~#</programlisting>

      <para>If that is the case, you can correct the problem by adjusting the
      <replaceable>minburst</replaceable> setting in
      /etc/shorewall/tcinterfaces (simple traffic shaping) or
      /etc/shorewall/tcdevices (complex traffic shaping). We suggest starting
      at 10-12kb and adjust as necessary. Example (simple traffic
      shaping):</para>

      <programlisting>#INTERFACE	TYPE		IN-BANDWIDTH			OUT-BANDWIDTH
eth0		External	50mbit:200kb			5.0mbit:100kb:200ms:100mbit:<emphasis
          role="bold">10kb</emphasis>
</programlisting>

      <para>Alternatively, you can turn off TSO and GSO using this command in
      <filename>/etc/shorewall/init</filename>:</para>

      <programlisting><emphasis role="bold">ethtool -K eth<emphasis>N</emphasis> tso off gso off</emphasis></programlisting>
    </section>

    <section>
      <title>(FAQ 97a) I enable Shorewall traffic shaping and now my download
      rate is way below what I specified</title>

      <para><emphasis role="bold">Answer</emphasis>: This is likely due to
      Generic Receive Offload (GRO) being enabled in the network adapter. To
      verify, install the <firstterm>ethtool</firstterm> package and use the
      -k command:</para>

      <programlisting>root@gateway:/etc/shorewall# ethtool -k eth1
Offload parameters for eth1:
rx-checksumming: on
tx-checksumming: on
scatter-gather: on
tcp-segmentation-offload: on
udp-fragmentation-offload: off
generic-segmentation-offload: on
generic-receive-offload: <emphasis role="bold">on</emphasis>
large-receive-offload: off
ntuple-filters: off
receive-hashing: off
root@gateway:/etc/shorewall# 
</programlisting>

      <para>To work around the issue, use this command:</para>

      <programlisting><emphasis role="bold">ethtool -K eth</emphasis>N <emphasis
          role="bold">gro off</emphasis></programlisting>

      <para>Beginning with Shorewall 4.4.25, another option is available in
      the form of a <firstterm>rate-estimated policing
      filter</firstterm>.</para>

      <para>Example from /etc/shorewall/tcdevices:</para>

      <programlisting>#NUMBER:        IN-BANDWIDTH            OUT-BANDWIDTH   OPTIONS
#INTERFACE
1:COMB_IF       <emphasis role="bold">~20mbit:250ms:4sec</emphasis>      ${UPLOAD}kbit   hfsc,linklayer=ethernet,overhead=0</programlisting>

      <para>To create a rate-estimated filter, precede the bandwidth with a
      tilde ("~"). The optional interval and decay_interval determine how
      often the rate is estimated and how many samples are retained for
      estimating. Please see <ulink
      url="http://ace-host.stuart.id.au/russell/files/tc/doc/estimators.txt">http://ace-host.stuart.id.au/russell/files/tc/doc/estimators.txt</ulink>
      for details.</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 support is
      available in Webmin. See <ulink
      url="http://www.webmin.com">http://www.webmin.com</ulink>. But beware of
      the issue described in <link linkend="faq36">FAQ 36</link>.</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 -a</command>     </programlisting>

      <section id="faq25a">
        <title>(FAQ 25a) It says 4.4.7.5; how do I know if it is
        Shorewall-shell or Shorewall-perl?</title>

        <para><emphasis role="bold">Answer</emphasis>: It is Shorewall-perl.
        Shorewall-shell is discontinued in Shorewall 4.4.</para>
      </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> Shorwall filters
            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> Yes.</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="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 id="faq83">
      <title>(FAQ 83) Is there no way to nest the firewall zone or create
      subzones? I've got a system with Linux-VServers, it's one interface
      (eth0) with multiple IPs</title>

      <para><emphasis role="bold">Answer</emphasis>: Beginning with Shorewall
      4.4.11 Beta 2, you can <ulink url="Vserver.html">create vserver
      zones</ulink> that are nested within the firewall zone.</para>

      <para>Prior to 4.4.11 Beta 2, there is no way to create sub-zones of the
      firewall zone. But you can use shell variables to make vservers easier
      to deal with.</para>

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

      <programlisting>VS1=fw:192.168.2.12
VS2=fw:192.168.2.13
VS3=fw:192.168.2.14</programlisting>

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

      <programlisting>#ACTION      SOURCE         DEST        PROTO         DEST PORT(S)
ACCEPT       $VS1           net         tcp           25
DNAT         net            $VS1        tcp           25
etc...</programlisting>
    </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="Shorewall-Lite.html">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> There are no
      compatibility constraints between Shorewall and Shorewall-lite.</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:</emphasis> There are two things to
      try when VOIP problems are encountered. Both begin with executing two
      <command>rmmod</command> commands.</para>

      <para>If your kernel version is 2.6.20 or earlier:<programlisting>rmmod ip_nat_sip
rmmod ip_conntrack_sip</programlisting>If your kernel version is 2.6.21 or
      later:<programlisting>rmmod nf_nat_sip
rmmod nf_conntrack_sip</programlisting></para>

      <para>The first alternative seems to work for those running recent
      kernels (2.6.26 or later):</para>

      <orderedlist>
        <listitem>
          <para>Copy <filename>/usr/share/shorewall/module</filename>s to
          <filename class="directory">/etc/shorewall</filename>
          (<filename>/usr/share/shorewall/helpers</filename> if you have
          LOAD_HELPERS_ONLY in shorewall.conf).</para>
        </listitem>

        <listitem>
          <para>Edit the copy and change this line:</para>

          <blockquote>
            <para>loadmodule nf_conntrack_sip</para>
          </blockquote>

          <para>to</para>

          <blockquote>
            <para>loadmodule nf_conntrack_sip sip_direct_media=0</para>
          </blockquote>
        </listitem>

        <listitem>
          <para><command>shorewall restart</command></para>
        </listitem>
      </orderedlist>

      <para>The second alternative is to not load the sip helpers:</para>

      <itemizedlist>
        <listitem>
          <para>If you are running kernel 2.6.20 or earlier, then change the
          DONT_LOAD specification in your shorewall.conf to:<programlisting>DONT_LOAD=ip_nat_sip,ip_conntrack_sip</programlisting></para>
        </listitem>

        <listitem>
          <para>If you are running kernel 2.6.21 or later, then change Then
          change the DONT_LOAD specification in your shorewall.conf
          to:<programlisting>DONT_LOAD=nf_nat_sip,nf_conntrack_sip</programlisting></para>
        </listitem>
      </itemizedlist>
    </section>
  </section>

  <section id="faq40">
    <title>IPv6</title>

    <section id="faq80">
      <title>(FAQ 80) Does Shorewall support IPV6?</title>

      <para>Answer: <ulink url="IPv6Support.html">Shorewall IPv6
      support</ulink> is currently available in Shorewall 4.2.4 and
      later.</para>

      <section id="faq80a">
        <title>(FAQ 80a) Why does Shorewall lPv6 Support Require Kernel 2.6.24
        or later?</title>

        <para><emphasis role="bold">Answer:</emphasis> Shorewall implements a
        stateful firewall which requires connection tracking be present in
        ip6tables and in the kernel. Linux kernels before 2.6.20 didn't
        support connection tracking for IPv6. So we could not even start to
        develop Shorewall IPv6 support until 2.6.20 and there were significant
        problems with the facility until at least kernel 2.6.23. When
        distributions began offering IPv6 connection tracking support, it was
        with kernel 2.6.25. So that is what we developed IPv6 support on and
        that's all that we initially tested on. Subsequently, we have tested
        Shorewall6 on Ubuntu Hardy with kernel 2.6.24. If you are running
        2.6.20 or later, you can <emphasis role="bold">try</emphasis> to run
        Shorewall6 by hacking<filename>
        /usr/share/shorewall/prog.footer6</filename> and changing the kernel
        version test to check for your kernel version rather than 2.6.24
        (20624). But after that, you are on your own.</para>

        <programlisting>kernel=$(printf "%2d%02d%02d\n" $(echo $(uname -r) 2&gt; /dev/null | sed 's/-.*//' | tr '.' ' ' ) | head -n1)
if [ $kernel -lt <emphasis role="bold">20624</emphasis> ]; then
    error_message "ERROR: $PRODUCT requires Linux kernel <emphasis role="bold">2.6.24</emphasis> or later"
    status=2
else 
 </programlisting>

        <para>Update: The above logic is found in
        <filename>/usr/share/shorewall/prog.footer</filename> in later
        Shorewall releases.</para>
      </section>
    </section>

    <section>
      <title>(FAQ 40) I have an interface that gets its IPv6 configuration
      from radvd. When I start Shorewall6, I immediately loose my default
      route. Why?</title>

      <para><emphasis role="bold">Answer</emphasis>: You have configured
      forwarding on the interface which disables autoconfiguration of the
      interface. To retain autoconfiguration on the interface when Shorewall6
      starts, specify <emphasis role="bold">forwarding=0</emphasis> in the
      OPTIONS column on the interface's entry in <ulink
      url="manpages6/shorewall6-interfaces.html">shorewall6-interfaces</ulink>
      (5).</para>
    </section>

    <section>
      <title id="faq96">(FAQ 96) I am starting to use ipv6, but on my ipv4 FW,
      when restarting Shorewall . it puts in ip6tables rules. How do i
      dissable that ?</title>

      <para>Answer: This is a two-step process.</para>

      <orderedlist>
        <listitem>
          <para>Set DISABLE_IPV6=No in <ulink
          url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5) and
          restart Shorewall.</para>
        </listitem>

        <listitem>
          <para>Execute these commands at a root shell prompt:</para>

          <itemizedlist>
            <listitem>
              <para>ip6tables -P INPUT ACCEPT</para>
            </listitem>

            <listitem>
              <para>ip6tables -P OUTPUT ACCEPT</para>
            </listitem>

            <listitem>
              <para>ip6tables -P FORWARD ACCEPT</para>
            </listitem>
          </itemizedlist>
        </listitem>
      </orderedlist>

      <para>You will probably want to soon install <ulink
      url="IPv6Support.html">Shorewall6</ulink> so that you have an IPv6
      firewall as well as one for IPv4.</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 any
      <emphasis role="bold">rejNotSyn</emphasis>, <emphasis
      role="bold">dropNotSyn</emphasis>, <emphasis
      role="bold">dropInvalid</emphasis>, <emphasis
      role="bold">NotSyn(...)</emphasis> and <emphasis
      role="bold">Invalid(...)</emphasis> 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>

    <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:~# <command>shorewall show capabilities</command>
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
   Extended Connection Tracking Match Support: Available
   Old Connection Tracking Match Syntax: Not available
   Packet Type Match: Available
   Policy Match: Available
   Physdev Match: Available
   Physdev-is-bridged Support: Available
   Packet length Match: Available
   IP range Match: Available
   Recent Match: Available
   Owner Match: Available
   Ipset Match: Available
   CONNMARK Target: Available
   Extended CONNMARK Target: Available
   Connmark Match: Available
   Extended Connmark Match: Available
   Raw Table: Available
   IPP2P Match: Available
   Old IPP2P Match Syntax: Not available
   CLASSIFY Target: Available
   Extended REJECT: Available
   Repeat match: Available
   MARK Target: Available
   Extended MARK Target: Available
   Mangle FORWARD Chain: Available
   Comments: Available
   Address Type Match: Available
   TCPMSS Match: Available
   Hashlimit Match: Available
   Old Hashlimit Match: Not available
   NFQUEUE Target: Available
   Realm Match: Available
   Helper Match: Available
   Connlimit Match: Available
   Time Match: Available
   Goto Support: Available
   LOGMARK Target: Available
   IPMARK Target: Available
   LOG Target: Available
   Persistent SNAT: Available
gateway:~# </programlisting>

      <para></para>
    </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 id="faq88">
      <title>(FAQ 88) Can I run Snort with Shorewall?</title>

      <para><emphasis role="bold">Answer</emphasis>: Yes. In <emphasis>Network
      Intrusion Detection System (NIDS) mode</emphasis>, Snort is libpcap
      based (like tcpdump) so it doesn't interfere with Shorewall. We have had
      reports that users have also been successful in using Snort in
      <emphasis>inline</emphasis> more with Shorewall, but no HOWTO exists at
      this time.</para>
    </section>

    <section id="faq89">
      <title>(FAQ 89) How do I connect to the web server in my aDSL modem from
      my local LAN?</title>

      <para>Answer: Here's what I did:</para>

      <itemizedlist>
        <listitem>
          <para>My local network is 172.20.1.0/24, so I set the IP address in
          the modem to 172.20.1.2.</para>
        </listitem>

        <listitem>
          <para>The IP address of my firewall's interface to the LAN is
          172.20.1.254. The logical name of the DSL interface is EXT_IF and my
          LAN interface is INT_IF.</para>

          <para>I added the following two configuration entries:</para>

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

          <programlisting>#INTERFACE			SOURCE			ADDRESS

COMMENT DSL Modem

EXT_IF:172.20.1.2	     	0.0.0.0/0		172.20.1.254
</programlisting>

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

          <programlisting>#ADDRESS	INTERFACE	EXTERNAL	HAVEROUTE	PERSISTENT
172.20.1.2	EXT_IF		INT_IF		no		yes
</programlisting>
        </listitem>
      </itemizedlist>

      <para>If you can't change the IP address of your modem and its current
      address isn't in your local network, then you need to change this
      slightly; assuming that the modem IP address is 192.168.1.1:</para>

      <itemizedlist>
        <listitem>
          <para>Do not include an entry in
          <filename>/etc/shorewall/proxyarp</filename>.</para>
        </listitem>

        <listitem>
          <para>Add an IP address in 192.168.1.0/24 to your external interface
          using your configuration's network management tools. For
          Debian-based systems, that means adding this to the interface's
          stanza in <filename>/etc/network/interfaces</filename>:</para>

          <programlisting>    post-up /sbin/ip addr add 192.168.1.254/24 dev <replaceable>external-interface</replaceable></programlisting>
        </listitem>

        <listitem>
          <para>Your entry in <filename>/etc/shorewall/masq</filename> would
          then be:</para>

          <programlisting>#INTERFACE			SOURCE			ADDRESS

COMMENT DSL Modemhttp://ipv6.shorewall.net/SimpleBridge.html

EXT_IF:192.168.1.1	     	0.0.0.0/0		192.168.1.254
</programlisting>
        </listitem>
      </itemizedlist>
    </section>

    <section>
      <title id="faq93">(FAQ 93) I'm not able to use Shorewall to manage a
      bridge. I get the following error: ERROR: BRIDGING=Yes is not supported
      by Shorewall 4.4.13.3.</title>

      <para><emphasis role="bold">Answer:</emphasis> If you want to apply
      firewall rules to the traffic passing between bridge ports, see <ulink
      url="bridge-Shorewall-perl.html">http://www.shorewall.net/bridge-Shorewall-perl.html</ulink>.
      If you simply want to allow all traffic between ports, then see <ulink
      url="SimpleBridge.html">http://www.shorewall.net/SimpleBridge.html</ulink>.</para>
    </section>

    <section id="faq95">
      <title>(FAQ 95) What is this $FW that I see in the configuration files
      and documentation?</title>

      <para><emphasis role="bold">Answer: FW</emphasis> is a <ulink
      url="configuration_file_basics.htm#Variables">shell variable</ulink>
      that expands to the name that you gave to the firewall zone in <ulink
      url="manpages/shorewall-zones.html">shorewall-zones</ulink>(5). The
      default name for the firewall zone is <emphasis
      role="bold">fw</emphasis>:</para>

      <programlisting>#ZONE            TYPE             OPTIONS
<emphasis role="bold">fw</emphasis>               firewall</programlisting>

      <para>So, using the default or sample configurations, writing <emphasis
      role="bold">$FW</emphasis> is the same as writing <emphasis
      role="bold">fw</emphasis>. If you give the firewall zone a different
      name, <emphasis role="bold">gate</emphasis> for example, then writing
      <emphasis role="bold">$FW</emphasis> would be the same as writing
      <emphasis role="bold">gate</emphasis>.</para>

      <programlisting>#ZONE            TYPE             OPTIONS
<emphasis role="bold">gate</emphasis>             firewall</programlisting>

      <section id="faq95a">
        <title>Why was that done?</title>

        <para><emphasis role="bold">Answer:</emphasis> The firewall zone has
        special semantics, so having a way to refer to it in a
        configuration-independent way makes writing the documentation,
        examples, macros, etc. easier.</para>
      </section>
    </section>

    <section id="faq98">
      <title>(FAQ 98) How do I Unsubscribe from the Mailing List</title>

      <para><emphasis role="bold">Answer</emphasis>: There are two
      ways:</para>

      <orderedlist>
        <listitem>
          <para>On the web</para>

          <para>Go to <ulink
          url="https://lists.sourceforge.net/lists/listinfo/shorewall-users">https://lists.sourceforge.net/lists/listinfo/shorewall-users</ulink>.
          At the bottom of the form is a section entitled "<emphasis
          role="bold">Shorewall-users Subscribers</emphasis>". At the bottom
          of that section find:</para>

          <blockquote>
            <para>"To <emphasis role="bold">unsubscribe</emphasis> from
            Shorewall-users, get a password reminder, or change your
            subscription options <emphasis role="bold">enter your subscription
            email address</emphasis>:".</para>
          </blockquote>

          <para>Enter your email address in the box provided and click on the
          "<emphasis role="bold"><ulink url="???">Unsubscribe or edit
          options</ulink></emphasis>" button. That will take you to a second
          form.</para>

          <para>At the top of the second form is a box to <emphasis
          role="bold">enter your password</emphasis> -- enter it there then
          click the <emphasis role="bold">Unsubscribe</emphasis> button in the
          center of the form. You will be unsubscribed.</para>

          <para>If you <emphasis role="bold">don't remember your
          password</emphasis>, click on the <emphasis
          role="bold">Remind</emphasis> button at the bottom of the form and
          your password will be emailed to you.</para>
        </listitem>

        <listitem>
          <para>Via email using this link: <ulink
          url="mailto:shorewall-users-request@lists.sourceforge.net?subject=unsubscribe">mailto:shorewall-users-request@lists.sourceforge.net?subject=unsubscribe</ulink>.
          You will receive a confirmation email shortly; follow the
          instructions in that email.</para>
        </listitem>
      </orderedlist>
    </section>

    <section>
      <title id="faq102">(FAQ 102) What is 'qt'? I see it in some of the older
      documentation.</title>

      <para><emphasis role="bold">Answer</emphasis>: 'qt' stands for 'quiet';
      qt() is a shell function that accepts a command with arguments as
      parameters. It redirects both standard out and standard error to
      /dev/null. It is defined in the Shorewall-core shell library
      lib.common.</para>
    </section>
  </section>
</article>