shorewall_code/docs/FAQ.xml

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

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

      <author>
        <firstname>Tom</firstname>

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

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

    <copyright>
      <year>2001-2006</year>

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

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

  <caution>
    <para><emphasis role="bold">This article applies to Shorewall 3.0 and
    later. If you are running a version of Shorewall earlier than Shorewall
    3.0.0 then please see the documentation for that
    release.</emphasis></para>
  </caution>

  <section>
    <title>Installing Shorewall</title>

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

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

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

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

        <para><quote>For more information about Shorewall usage on Debian
        system please look at /usr/share/doc/shorewall/README.Debian provided
        by [the] shorewall 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 empty. This is
      intentional. The released configuration file skeletons may be found on
      your system in the directory <filename
      class="directory">/usr/share/doc/shorewall/default-config</filename>.
      Simply copy the files you need from that directory to <filename
      class="directory">/etc/shorewall</filename> and modify the
      copies.</para>

      <para>Note that you must copy <filename
      class="directory">/usr/share/doc/shorewall/default-config/shorewall.conf</filename>
      and <filename>/usr/share/doc/shorewall/default-config/modules</filename>
      to <filename class="directory">/etc/shorewall</filename> even if you do
      not modify those files.</para>
    </section>

    <section id="faq44">
      <title>(FAQ 44) I can't install/upgrade the RPM — I keep getting the
      message "error: failed dependencies:iproute is needed..."</title>

      <para><emphasis role="bold">Answer</emphasis>: Read the <ulink
      url="Install.htm">Installation Instructions</ulink>!</para>
    </section>

    <section id="faq50">
      <title>(FAQ 50) When I install/upgrade I get multiple instance of the
      message "warning: user teastep does not exist - using root"</title>

      <para><emphasis role="bold">Answer:</emphasis> You may safely ignore
      this warning message. It was caused by a minor packaging error that has
      since been corrected. It makes no difference to Shorewall's
      operation.</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 first example in the
      <ulink url="Documentation.htm#Rules">rules file documentation</ulink>
      shows how to do port forwarding under Shorewall. The format of a
      port-forwarding rule to a local system is as follows:</para>

      <programlisting>#ACTION    SOURCE      DEST                                   PROTO        DEST PORT
DNAT       net         loc:&lt;l<emphasis>ocal IP address</emphasis>&gt;[:&lt;<emphasis>local port</emphasis>&gt;]  &lt;<emphasis>protocol</emphasis>&gt;   &lt;<emphasis>port #</emphasis>&gt;</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>&lt;external IP&gt;</emphasis> ) on your firewall to an
      internal system:</para>

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

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

      <section id="faq1a">
        <title>(FAQ 1a) Ok -- 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 should be set to the IP address of your
            firewall's internal interface).</para>
          </listitem>

          <listitem>
            <para>Your ISP is blocking that particular port inbound.</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>iptables -t nat -Z</command>
            </quote>. This clears the NetFilter counters in the nat
            table.</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[-lite] show
            nat</command> </quote></para>
          </listitem>

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

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

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

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

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

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

          <listitem>
            <para>If the packet count is non-zero, check your log to see if
            the connection is being dropped or rejected. If it is, then you
            may have a zone definition problem such that the server is in a
            different zone than what is specified in the DEST column. At a
            root promt, type "<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>
        </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>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>: Let's assume the
        following:</para>

        <itemizedlist>
          <listitem>
            <para>External IP address is 206.124.146.176 on <filename
            class="devicefile">eth0</filename>.</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                 
DNAT       loc      dmz:192.168.2.4     tcp      80              -           206.124.146.176</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_interface_address eth0`</command>       </programlisting>

        <para>For users of Shorewall 2.1.0 and later:</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          -         $ETH0_IP</programlisting>
      </section>

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

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

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

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

      <para>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.
      In all other cases, you use ACCEPT unless you need to hijack connections
      as they go through your firewall and handle them on the firewall box
      itself; in that case, you use a REDIRECT rule.</para>
    </section>

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

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

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

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

  <section>
    <title>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 <ulink
          url="shorewall_setup_guide.htm#DNS">Bind Version 9
          <quote>views</quote> </ulink> (or using a separate DNS server for
          local clients) such that www.mydomain.com resolves to 130.141.100.69
          externally and 192.168.1.5 internally. That's what I do here at
          shorewall.net for my local systems that use one-to-one NAT.</para>
        </listitem>
      </itemizedlist>

      <para>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:<warning>
          <para>All traffic redirected through use of this hack will look to
          the server as if it came from the firewall (192.168.1.254) rather
          than from the original client!</para>
        </warning></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              SUBNET          ADDRESS         PROTO   PORT(S)
eth1:192.168.1.5        eth1            192.168.1.254   tcp     www</programlisting>
        </listitem>

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

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

          <para>That rule only works of course if you have a static external
          IP address. If you have a dynamic IP address then include this in
          <filename>/etc/shorewall/params</filename>:</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         -         $ETH0_IP</programlisting>

          <para>Using this technique, you will want to configure your
          DHCP/PPPoE client to automatically restart Shorewall each time that
          you get a new IP address.</para>
        </listitem>
      </itemizedlist>

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

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

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

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

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

        <para>If you don't like those solutions and prefer to stupidly 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>
          <title>Example:</title>

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

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

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

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

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

          <programlisting>#INTERFACE    SUBNETS     ADDRESS
eth2          eth2        192.168.2.254</programlisting>

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

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

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

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

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

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

        <programlisting>#ACTION    SOURCE   DEST                PROTO    DEST PORT(S)    SOURCE      ORIGINAL
#                                                                PORT        DEST                 
DNAT       loc      dmz:192.168.2.4     tcp      80              -           206.124.146.176</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          -         $ETH0_IP</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>
      </section>

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

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

  <section>
    <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/%7Ekadlec/sw/netfilter/newnat-suite/">H.323
      connection tracking/NAT module</ulink> that helps with Netmeeting. Note
      however that one of the Netfilter developers recently posted the
      following:</para>

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

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

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

  <section>
    <title>Open Ports</title>

    <section id="faq0">
      <title>(FAQ 0) 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 communcation 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#id2499092">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#id2499962">Port Forwarding (DNAT)</ulink>,
          and <ulink url="two-interface.htm#id2500428">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#id2499698">Port Forwarding (DNAT)</ulink>
          and <ulink url="three-interface.htm#id2500224">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 zone 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">DROP</emphasis> action (i.e., <emphasis
      role="bold">Auth/DROP</emphasis>). This is necessary to prevent outgoing
      connection problems to services that use the <quote>Auth</quote>
      mechanism for identifying requesting users. That is the only service
      which the default setup rejects.</para>

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

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

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

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

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

        <para><emphasis role="bold">Answer:</emphasis> Rules only govern the
        establishment of new connections. Once a connection is established
        through the firewall it will be usable until disconnected (tcp) or
        until it times out (other protocols). If you stop telnet and try to
        establish a new session your firerwall 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">Here's
        a writeup</ulink> describing a nice integration of Shorewall and
        PortSentry.</para>
      </section>
    </section>

    <section id="faq51">
      <title>(FAQ 51) How do I "Open a Port" with Shorewall</title>

      <para><emphasis role="bold">Answer</emphasis>: It depends…</para>

      <para>If the application serving the port is running on the same system
      as Shorewall then add this rule:</para>

      <programlisting>#ACTION       SOURCE         DEST        PROTO         DEST PORT(S)
ACCEPT        net            $FW         &lt;protocol&gt;    &lt;port number&gt;</programlisting>

      <para>Where &lt;protocol&gt; is either <emphasis>tcp</emphasis> or
      <emphasis>udp</emphasis> and &lt;port number&gt; is the port that you
      wish to "open".</para>

      <para>If the application serving the port is running on one of the
      systems in your local network then please see <link linkend="faq1">FAQ
      1</link>.</para>
    </section>
  </section>

  <section>
    <title>Connection Problems</title>

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

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

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

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

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

        <listitem>
          <para>The entry for the local network in the /etc/shorewall/masq
          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 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>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 <ulink
      url="Documentation.htm#Conf">/etc/shorewall/shorewall.conf</ulink>.</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 /etc/shorewall/interfaces; the bridged Ethernet
      interfaces are not defined to Shorewall. How do I tell Shorewall to
      allow traffic through the bridge?</title>

      <para>Answer: Add the <firstterm>routeback</firstterm> option to
      <filename class="devicefile">br0</filename> in <ulink
      url="Documentation.htm#Interfaces">/etc/shorewall/interfaces</ulink>.</para>

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

  <section>
    <title>Logging</title>

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

      <para><emphasis role="bold">Answer:</emphasis> NetFilter uses the
      kernel's equivalent of syslog (see <quote>man syslog</quote>) to log
      messages. It always uses the LOG_KERN (kern) facility (see <quote>man
      openlog</quote>) and you get to choose the log level (again, see
      <quote>man syslog</quote>) in your <ulink
      url="Documentation.htm#Policy">policies</ulink> and <ulink
      url="Documentation.htm#Rules">rules</ulink>. 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 /etc/syslog.conf, be sure to
      restart syslogd (on a RedHat system, <quote>service syslog
      restart</quote>).</para>

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

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

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

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

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

        <literallayout>
          <ulink url="http://www.shorewall.net/pub/shorewall/parsefw/">http://www.shorewall.net/pub/shorewall/parsefw/</ulink>
          <ulink url="http://www.fireparse.com">http://www.fireparse.com</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>
          <ulink url="http://gege.org/iptables">http://gege.org/iptables</ulink>
          <ulink url="http://home.regit.org/ulogd-php.html">http://home.regit.org/ulogd-php.html</ulink>
        </literallayout>

        <para>I personally use Logwatch. It emails me a report each day from
        my various systems with each report summarizing the logged activity on
        the corresponding system.</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>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>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>
            <title>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>

      <itemizedlist>
        <listitem>
          <para>Find where klogd is being started (it will be from one of the
          files in /etc/init.d -- sysklogd, klogd, ...). Modify that file or
          the appropriate configuration file so that klogd is started with
          <quote>-c <emphasis>&lt;n&gt;</emphasis> </quote> where
          <emphasis>&lt;n&gt;</emphasis> is a log level of 5 or less;
          or</para>
        </listitem>

        <listitem>
          <para>See the <quote>dmesg</quote> man page (<quote>man
          dmesg</quote>). You must add a suitable <quote>dmesg</quote> command
          to your startup scripts or place it in /etc/shorewall/start.</para>
        </listitem>
      </itemizedlist>

      <tip>
        <para>Under RedHat and Mandriva, the max <ulink
        url="shorewall_logging.html">log level</ulink> that is sent to the
        console is specified in /etc/sysconfig/init in the LOGLEVEL variable.
        Set <quote>LOGLEVEL=5</quote> to suppress info (log level 6) messages
        on the console.</para>
      </tip>

      <tip>
        <para>Under Debian, you can set KLOGD=<quote>-c 5</quote> in
        <filename>/etc/init.d/klogd</filename> to suppress info (log level 6)
        messages on the console.</para>
      </tip>

      <tip>
        <para>Under SUSE, add <quote>-c 5</quote> to KLOGD_PARAMS in
        /etc/sysconfig/syslog to suppress info (log level 6) messages on the
        console.</para>
      </tip>
    </section>

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

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

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

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

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

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

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

        <varlistentry id="all2all">
          <term>all2&lt;zone&gt;, &lt;zone&gt;2all or all2all</term>

          <listitem>
            <para>You have a <ulink
            url="Documentation.htm#Policy">policy</ulink> 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="Documentation.htm#Rules">rule</ulink> to that effect.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>&lt;zone1&gt;2&lt;zone2&gt;</term>

          <listitem>
            <para>Either you have a <ulink
            url="Documentation.htm#Policy">policy</ulink> for <emphasis
            role="bold">&lt;zone1&gt;</emphasis> to <emphasis
            role="bold">&lt;zone2&gt;</emphasis> that specifies a log level
            and this packet is being logged under that policy or this packet
            matches a <ulink url="Documentation.htm#Rules">rule</ulink> that
            includes a log level.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>@&lt;source&gt;2&lt;dest&gt;</term>

          <listitem>
            <para>You have a policy for traffic from &lt;<emphasis
            role="bold">source</emphasis>&gt; to &lt;<emphasis
            role="bold">dest</emphasis>&gt; 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>&lt;interface&gt;_mac</term>

          <listitem>
            <para>The packet is being logged under the <emphasis
            role="bold">maclist</emphasis> <ulink
            url="Documentation.htm#Interfaces">interface
            option</ulink>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>logpkt</term>

          <listitem>
            <para>The packet is being logged under the <emphasis
            role="bold">logunclean</emphasis> <ulink
            url="Documentation.htm#Interfaces">interface
            option</ulink>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>badpkt</term>

          <listitem>
            <para>The packet is being logged under the <emphasis
            role="bold">dropunclean</emphasis> <ulink
            url="Documentation.htm#Interfaces">interface option</ulink> as
            specified in the <emphasis role="bold">LOGUNCLEAN</emphasis>
            setting in <ulink url="Documentation.htm#Conf">
            <filename>/etc/shorewall/shorewall.conf</filename>
            </ulink>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>blacklst</term>

          <listitem>
            <para>The packet is being logged because the source IP is
            blacklisted in the <filename> <ulink
            url="Documentation.htm#Blacklist">/etc/shorewall/blacklist</ulink>
            </filename> file.</para>
          </listitem>
        </varlistentry>

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

          <listitem>
            <para>The packet has a source IP address that isn't in any of your
            defined zones (<quote>shorewall[-lite] show zones</quote> and look
            at the printed zone definitions) or the chain is FORWARD and the
            destination IP isn't in any of your defined zones. If the chain is
            FORWARD and the IN and OUT interfaces are the same, then you
            probably need the <emphasis role="bold">routeback</emphasis>
            option on that interface in <filename> <ulink
            url="Documentation.htm#Interfaces">/etc/shorewall/interfaces</ulink>
            </filename> or you need the <emphasis
            role="bold">routeback</emphasis> option in the relevant entry in
            <filename> <ulink
            url="Documentation.htm#Hosts">/etc/shorewall/hosts</ulink>
            </filename>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>OUTPUT</term>

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

        <varlistentry>
          <term>logflags</term>

          <listitem>
            <para>The packet is being logged because it failed the checks
            implemented by the <emphasis role="bold">tcpflags</emphasis>
            <ulink url="Documentation.htm#Interfaces">interface
            option</ulink>.</para>
          </listitem>
        </varlistentry>
      </variablelist>

      <example>
        <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.</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>For additional information about the log message, see <ulink
        url="http://logi.cc/linux/netfilter-log-format.php3">http://logi.cc/linux/netfilter-log-format.php3</ulink>.</para>

        <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 PROTO=ICMP
      TYPE=3 CODE=3 [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> 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 a lot of broken implementations. That is what you
      are seeing with these messages. When Netfilter displays these messages,
      the part before the "[" describes the ICMP packet and the part between
      the "[" and "]" describes the packet for which the ICMP is a
      response.</para>

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

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

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

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

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

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

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

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

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

  <section>
    <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>Answer: See <ulink url="MultiISP.html">this article on Shorewall
      and Routing</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>
    <title>Starting and Stopping</title>

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

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

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

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

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

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

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

      <para>Also, be sure to check the <ulink url="errata.htm">errata</ulink>
      for problems concerning the version of iptables (v1.2.3) shipped with
      RH7.2.</para>

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

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

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

      <para>I just installed Shorewall and when I issue the start 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 eth0 and the local zone is defined as all hosts
      connected through <filename class="devicefile">eth1</filename>. If you
      are running Shorewall 1.4.10 or later, you can consider setting the
      <ulink url="Documentation.htm#Interfaces"> <emphasis
      role="bold">detectnets</emphasis> interface option</ulink> on your local
      interface (<filename class="devicefile">eth1</filename> in the above
      example). That will cause Shorewall to restrict the local zone to only
      those networks routed through that interface.</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>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 to be sure that the
      commands will do what they are intended. Many iptables commands
      published in HOWTOs and other instructional material use the -A command
      which adds the rules to the end of the chain. Most chains that Shorewall
      constructs end with an unconditional DROP, ACCEPT or REJECT rule and any
      rules that you add after that will be ignored. Check <quote>man
      iptables</quote> and look at the -I (--insert) command.</para>
    </section>

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

      <para>Using a light-weight shell such as <command>ash</command> can
      dramatically decrease the time required to <emphasis
      role="bold">start</emphasis> or <emphasis role="bold">restart</emphasis>
      Shorewall. See the SHOREWALL_SHELL variable in <filename> <ulink
      url="Documentation.htm#Conf">shorewall.conf</ulink> </filename>.</para>

      <para>Use a fast terminal emulator -- in particular the KDE konsole
      scrolls much faster than the Gnome terminal. Also use the '-q' option if
      you are restarting remotely or from a slow terminal (or redirect the
      output to a file as in <command>shorewall restart &gt;
      /dev/null</command>).</para>

      <para>Upgrade your hardware. Many people find that even a modest
      increase in CPU and memory speed (e.g. from P3 with SDRAM to P4 with
      DDR) helps dramatically. EM64T-capable CPUs (from either AMD or Intel)
      exhibit quite acceptable restart speeds, even with a fairly complex
      ruleset.</para>

      <para>Shorewall also supports a fast start capability. To use this
      capability:</para>

      <orderedlist>
        <listitem>
          <para>With Shorewall in the <ulink
          url="starting_and_stopping_shorewall.htm">started state</ulink>, run
          <command>shorewall save</command>. This creates the script
          <filename>/var/lib/shorewall/restore</filename>.</para>
        </listitem>

        <listitem>
          <para>Use the <emphasis role="bold">-f </emphasis>option to the
          start command (e.g., <command>shorewall -f start</command>). This
          causes Shorewall to look for the
          <filename>/var/lib/shorewall/restore</filename> script and if that
          script exists, it is run. Running
          <filename>/var/lib/shorewall/restore</filename> takes much less time
          than a full <command>shorewall start</command>.</para>
        </listitem>

        <listitem>
          <para>The <filename>/etc/init.d/shorewall</filename> script that is
          run at boot time uses the <emphasis role="bold">-f</emphasis>
          option.</para>
        </listitem>

        <listitem>
          <para>The <filename>/var/lib/shorewall/restore</filename> script can
          be run any time to restore the firewall. The script may be run
          directly or it may be run indirectly using the <command>shorewall
          restore</command> command.</para>
        </listitem>
      </orderedlist>

      <para>If you change your Shorewall configuration, you must execute a
      <emphasis role="bold">shorewall start</emphasis> (without <emphasis
      role="bold">-f</emphasis>) or <command>shorewall restart</command> prior
      to doing another <command>shorewall save</command>. The
      <command>shorewall save</command> command saves the currently running
      configuration and not the one reflected in your updated configuration
      files.</para>

      <para>Likewise, if you change your Shorewall configuration then once you
      are satisfied that it is working properly, you must do another
      <command>shorewall save</command>. Otherwise at the next reboot, you
      will revert to the old configuration stored in
      <filename>/var/lib/shorewall/restore</filename>.</para>

      <para>Finally, the time that new connections are blocked during
      shorewall restart can be dramatically reduced by upgrading to Shorewall
      3.2 or later. In 3.2 and later releases, <command>shorewall
      [re]start</command> proceeds in two phases:</para>

      <orderedlist>
        <listitem>
          <para>The current configuration is compiled to produce a shell
          program tailored for your configuration.</para>
        </listitem>

        <listitem>
          <para>If compilation is error-free, the compiled program is run to
          [re]start your firewall.</para>
        </listitem>
      </orderedlist>
    </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
      run-levels that you run your firewall system at.</para>
    </section>

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

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

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

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

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

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

  <section>
    <title>About Shorewall</title>

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

      <para>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 it 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
      included in Webmin 1.060 and later versions. See <ulink
      url="http://www.webmin.com">http://www.webmin.com</ulink></para>
    </section>

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

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

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

      <para>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 to I tell which version of Shorewall or Shorewall
      Lite I am running?</title>

      <para>At the shell prompt, type:</para>

      <programlisting><command>/sbin/shorewall[-lite] version</command>      </programlisting>
    </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>Answer: Yes.</para>
          </listitem>
        </varlistentry>

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

          <listitem>
            <para>Answer: 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>Answer: Shorewall can be configured to do that using the
            <ulink url="blacklisting_support.htm">blacklisting</ulink>
            facility. Shorewall versions 2.0.0 and later filter these packets
            under the <firstterm>nosmurfs</firstterm> interface option in
            <ulink
            url="Documentation.htm#Interfaces">/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>Answer: Yes, if the <ulink
            url="Documentation.htm#Interfaces">routefilter interface
            option</ulink> is selected.</para>
          </listitem>
        </varlistentry>

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

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

    <section id="faq36">
      <title>(FAQ 36) Does Shorewall Work with the 2.6 Linux Kernel?</title>

      <para>Shorewall works with the 2.6 Kernels with a couple of
      caveats:</para>

      <itemizedlist>
        <listitem>
          <para>Netfilter/iptables doesn't fully support IPSEC in the 2.6
          Kernels prior to 2.6.16 -- kernel and iptables patches are available
          and the details may be found at the <ulink
          url="IPSEC-2.6.html">Shorewall IPSEC-2.6 page</ulink>.</para>
        </listitem>

        <listitem>
          <para>The 2.6 Kernels do not provide support for the logunclean and
          dropunclean options in
          <filename>/etc/shorewall/interfaces</filename>. Note that support
          for those options was also removed from Shorewall in version
          2.0.0.</para>
        </listitem>
      </itemizedlist>
    </section>
  </section>

  <section>
    <title>RFC 1918</title>

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  <section>
    <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 rulesets for different IPs?</title>

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

  <section>
    <title>Shorewall Lite</title>

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

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

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

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

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

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

  <section>
    <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>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 conections to let's say the ssh port
      only from specific IP Addresses on the internet?</title>

      <para>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>
        <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>Temporarily remove and rejNotSyn, dropNotSyn and dropInvalid rules
      from <filename>/etc/shorewall/rules</filename> and restart
      Shorewall.</para>
    </section>

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

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

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

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

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

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

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

      <para>Shorewall Bridging Firewall support is available — <ulink
      url="bridge.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 examiniation 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 Patch-O-Matic. The only sure way to
      filter on packet content is to proxy the connections in question -- in
      the case of HTTP, this means running something like <ulink
      url="Shorewall_Squid_Usage.html">Squid</ulink>. Proxying allows the
      proxy process to assemble complete application-level messages which can
      then be accurately parsed and decisions can be made based on the
      result.</para>
    </section>

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

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

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

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

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

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

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