shorewall_code/Shorewall-docs2/FAQ.xml

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

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

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

      <author>
        <firstname>Tom</firstname>

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

    <pubdate>2005-03-07</pubdate>

    <copyright>
      <year>2001-2004</year>

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

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

  <section>
    <title>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>
      <title>(FAQ 44) I can't install 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>

  <section>
    <title>Port Forwarding (Port Redirection)</title>

    <section id="faq1">
      <title>(FAQ 1) I want to forward UDP port 7777 to my 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 Mandrake Linux 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 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>
        </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>

    <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>

  <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>If you insist on an IP solution to the accessibility problem
      rather than a DNS solution, then if you are running Shorewall 2.0.0 or
      2.0.1 then please see the <ulink
      url="http://www.shorewall.net/1.4/FAQ.htm#faq2">Shorewall 1.4
      FAQ</ulink>.</para>

      <para>Otherwise, 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 and are running
          Shorewall 1.3.4 through Shorewall 2.0.* then include this in
          <filename>/etc/shorewall/init</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           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 routing 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</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>
        </example>
      </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="http://linux-igd.sourceforge.net">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="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> (Shorewall versions prior
      to 2.0.0 only). The common.def included with version 1.3.x always
      rejects connection requests on TCP port 113 rather than dropping them.
      This is necessary to prevent outgoing connection problems to services
      that use the <quote>Auth</quote> mechanism for identifying requesting
      users. Shorewall also rejects TCP ports 135, 137, 139 and 445 as well as
      UDP ports 137-139. These are ports that are used by Windows (Windows
      <emphasis>can</emphasis> be configured to use the DCE cell locator on
      port 135). Rejecting these connection requests rather than dropping them
      cuts down slightly on the amount of Windows chatter on LAN segments
      connected to the Firewall.</para>

      <para>If you are seeing port 80 being <quote>closed</quote>, that's
      probably your ISP preventing you from running a web server in violation
      of your Service Agreement.</para>

      <tip>
        <para>You can change the default behavior of Shorewall through use of
        an /etc/shorewall/common file. See the <ulink
        url="shorewall_extension_scripts.htm">Extension Script
        Section</ulink>.</para>
      </tip>

      <tip>
        <para>Beginning with Shorewall 1.4.9, Shorewall no longer rejects the
        Windows SMB ports (135-139 and 445) by default and silently drops them
        instead.</para>
      </tip>

      <para><emphasis role="bold">Answer:</emphasis> (Shorewall versions 2.0.0
      and later). 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">RejectAuth</emphasis> action (defined
      in <filename>/usr/share/shorewall/action.RejectAuth</filename>). 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> on a nice integration of Shorewall and
        PortSentry.</para>
      </section>
    </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>
      </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 id="faq40">
      <title>(FAQ 40) Shorewall is Blocking my OpenVPN Tunnel</title>

      <para>I have this entry in <ulink
      url="Documentation.htm#Tunnels">/etc/shorewall/tunnels</ulink>:</para>

      <programlisting># TYPE                  ZONE    GATEWAY         GATEWAY
#                                               ZONE
openvpn:5000            net     69.145.71.133</programlisting>

      <para>Yet I am seeing this log message:</para>

      <programlisting>Oct 12 13:41:03 localhost kernel: Shorewall:net2all:DROP:IN=eth0 OUT=
MAC=00:04:5a:7f:92:9f:00:b0:c2:89:68:e4:08:00 SRC=69.145.71.133
DST=216.187.138.18 LEN=42 TOS=0x00 PREC=0x00 TTL=46 ID=11 DF PROTO=UDP
SPT=33120 DPT=5000 LEN=22</programlisting>

      <para><emphasis role="bold">Answer</emphasis>: Shorewall's <emphasis
      role="bold">openvpn</emphasis> tunnel type assumes that OpenVPN will be
      using the same port (default 5000) for both the source and destination
      port. From the above message, it is clear that the remote client is
      using source port 33120. The solution is to replace your <ulink
      url="Documentation.htm#Tunnels">/etc/shorewall/tunnels</ulink> entry
      with this one:</para>

      <programlisting># TYPE                  ZONE    GATEWAY         GATEWAY
#                                               ZONE
generic:udp:5000        net     69.145.71.133</programlisting>
    </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 ratelimited 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>Beginning with Shorewall version 1.3.12, you can <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 2b) 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>DROP net fw 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 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> If you are running
      Shorewall version 1.4.4 or 1.4.4a then check the <ulink
      url="errata.htm">errata</ulink>. Otherwise:</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 Mandrake, 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>
          </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>newnotsyn</term>

          <listitem>
            <para>The packet is being logged because it is a TCP packet that
            is not part of any current connection yet it is not a syn packet.
            Options affecting the logging of such packets include <emphasis
            role="bold">NEWNOTSYN</emphasis> and <emphasis
            role="bold">LOGNEWNOTSYN</emphasis> in <ulink
            url="Documentation.htm#Conf"><filename>/etc/shorewall/shorewall.conf</filename></ulink>.</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 check</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 check" 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>
            </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 the internet. 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>

  <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>

      <important>
        <para>Anyone with two Internet connections MUST read and understand
        <ulink url="Shorewall_and_Routing.html">this article on Shorewall and
        Routing</ulink>. If you don't, you will be completely lost trying to
        make this work.</para>
      </important>

      <para>Setting this up in Shorewall is easy; setting up the routing is a
      bit harder.</para>

      <para>Assuming that <filename class="devicefile">eth0</filename> and
      <filename class="devicefile">eth1</filename> are the interfaces to the
      two ISPs then:</para>

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

      <programlisting>#ZONE    INTERFACE    BROADCAST       OPTIONS
net      eth0         detect
net      eth1         detect</programlisting>

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

      <programlisting>#SOURCE    DESTINATION    POLICY     LIMIT:BURST
net        net            DROP</programlisting>

      <para>If you have masqueraded hosts, be sure to update
      <filename>/etc/shorewall/masq</filename> to masquerade to both ISPs. For
      example, if you masquerade all hosts connected to <filename
      class="devicefile">eth2</filename> then:</para>

      <programlisting>#INTERFACE       SUBNET            ADDRESS
eth0             eth2
eth1             eth2</programlisting>

      <para><citetitle>There was an article in SysAdmin covering this topic.
      It may be found at <ulink
      url="http://www.samag.com/documents/s=1824/sam0201h/">http://www.samag.com/documents/s=1824/sam0201h/</ulink></citetitle></para>

      <para><citetitle>The following information regarding setting up routing
      for this configuration is reproduced from the <ulink
      url="http://www.lartc.org">LARTC HOWTO</ulink> and has not been verified
      by the author. If you have questions or problems with the instructions
      given below, please post to the <ulink
      url="http://www.lartc.org/#mailinglist">LARTC mailing
      list</ulink>.</citetitle></para>

      <sidebar>
        <para>A common configuration is the following, in which there are two
        providers that connect a local network (or even a single machine) to
        the big Internet.</para>

        <programlisting>                                                                 ________
                                          +------------+        /
                                          |            |       |
                            +-------------+ Provider 1 +-------
        __                  |             |            |     /
    ___/  \_         +------+-------+     +------------+    |
  _/        \__      |     if1      |                      /
 /             \     |              |                      |
| Local network -----+ Linux router |                      |     Internet
 \_           __/    |              |                      |
   \__     __/       |     if2      |                      \
      \___/          +------+-------+     +------------+    |
                            |             |            |     \
                            +-------------+ Provider 2 +-------
                                          |            |       |
                                          +------------+        \________
</programlisting>

        <para>There are usually two questions given this setup.</para>

        <para><emphasis role="bold">Split access</emphasis></para>

        <para>The first is how to route answers to packets coming in over a
        particular provider, say Provider 1, back out again over that same
        provider.</para>

        <para>Let us first set some symbolical names. Let <emphasis
        role="bold">$IF1</emphasis> be the name of the first interface (if1 in
        the picture above) and <emphasis role="bold">$IF2</emphasis> the name
        of the second interface. Then let <emphasis
        role="bold">$IP1</emphasis> be the IP address associated with
        <emphasis role="bold">$IF1</emphasis> and <emphasis
        role="bold">$IP2</emphasis> the IP address associated with <emphasis
        role="bold">$IF2</emphasis>. Next, let <emphasis
        role="bold">$P1</emphasis> be the IP address of the gateway at
        Provider 1, and <emphasis role="bold">$P2</emphasis> the IP address of
        the gateway at provider 2. Finally, let <emphasis
        role="bold">$P1_NET</emphasis> be the IP network <emphasis
        role="bold">$P1</emphasis> is in, and <emphasis
        role="bold">$P2_NET</emphasis> the IP network <emphasis
        role="bold">$P2</emphasis> is in.</para>

        <para>One creates two additional routing tables, say <emphasis
        role="bold">T1</emphasis> and <emphasis role="bold">T2</emphasis>.
        These are added in /etc/iproute2/rt_tables. Then you set up routing in
        these tables as follows:</para>

        <programlisting>ip route add $P1_NET dev $IF1 src $IP1 table T1
ip route add default via $P1 table T1
ip route add $P2_NET dev $IF2 src $IP2 table T2
ip route add default via $P2 table T2</programlisting>

        <para>Nothing spectacular, just build a route to the gateway and build
        a default route via that gateway, as you would do in the case of a
        single upstream provider, but put the routes in a separate table per
        provider. Note that the network route suffices, as it tells you how to
        find any host in that network, which includes the gateway, as
        specified above.</para>

        <para>Next you set up the main routing table. It is a good idea to
        route things to the direct neighbour through the interface connected
        to that neighbour. Note the `src' arguments, they make sure the right
        outgoing IP address is chosen.</para>

        <programlisting>ip route add $P1_NET dev $IF1 src $IP1
ip route add $P2_NET dev $IF2 src $IP2</programlisting>

        <para>Then, your preference for default route:</para>

        <programlisting>ip route add default via $P1</programlisting>

        <para>Next, you set up the routing rules. These actually choose what
        routing table to route with. You want to make sure that you route out
        a given interface if you already have the corresponding source
        address:</para>

        <programlisting>ip rule add from $IP1 table T1
ip rule add from $IP2 table T2</programlisting>

        <para>This set of commands makes sure all answers to traffic coming in
        on a particular interface get answered from that interface.</para>

        <note>
          <para>'If $P0_NET is the local network and $IF0 is its interface,
          the following additional entries are desirable:</para>

          <programlisting format="linespecific">ip route add $P0_NET dev $IF0 table T1
ip route add $P2_NET dev $IF2 table T1
ip route add 127.0.0.0/8 dev lo table T1
ip route add $P0_NET dev $IF0 table T2
ip route add $P1_NET dev $IF1 table T2
ip route add 127.0.0.0/8 dev lo table T2</programlisting>
        </note>

        <para>Now, this is just the very basic setup. It will work for all
        processes running on the router itself, and for the local network, if
        it is masqueraded. If it is not, then you either have IP space from
        both providers or you are going to want to masquerade to one of the
        two providers. In both cases you will want to add rules selecting
        which provider to route out from based on the IP address of the
        machine in the local network.</para>

        <para><emphasis role="bold">Load balancing</emphasis></para>

        <para>The second question is how to balance traffic going out over the
        two providers. This is actually not hard if you already have set up
        split access as above.</para>

        <para>Instead of choosing one of the two providers as your default
        route, you now set up the default route to be a multipath route. In
        the default kernel this will balance routes over the two providers. It
        is done as follows (once more building on the example in the section
        on split-access):</para>

        <programlisting>ip route add default scope global nexthop via $P1 dev $IF1 weight 1 \
     nexthop via $P2 dev $IF2 weight 1</programlisting>

        <para>This will balance the routes over both providers. The <emphasis
        role="bold">weight</emphasis> parameters can be tweaked to favor one
        provider over the other.</para>

        <note>
          <para>balancing will not be perfect, as it is route based, and
          routes are cached. This means that routes to often-used sites will
          always be over the same provider.</para>
        </note>

        <para>Furthermore, if you really want to do this, you probably also
        want to look at Julian Anastasov's patches at <ulink
        url="http://www.ssi.bg/%7Eja/#routes">http://www.ssi.bg/~ja/#routes</ulink>
        , Julian's route patch page. They will make things nicer to work
        with.</para>
      </sidebar>

      <para>The following was contributed by Martin Brown and is an excerpt
      from <citetitle><ulink
      url="http://www.docum.org/stef.coene/qos/faq/cache/44.html">http://www.docum.org/stef.coene/qos/faq/cache/44.html</ulink></citetitle>.</para>

      <sidebar>
        <para>There are two issues requiring different handling when dealing
        with multiple Internet providers on a given network. The below assumes
        that the host which has multiple Internet connections is a
        masquerading (or NATting) host and is at the chokepoint between the
        internal and external networks. For the use of multiple inbound
        connections to the same internal server (public IP A from ISP A and
        public IP B from ISP B both get redirected to the same internal
        server), the ideal solution involves using two private IP addresses on
        the internal server. This leads to an end-to-end uniqueness of public
        IP to private IP and can be easily accomplished by following the
        directions here:</para>

        <para><ulink
        url="http://linux-ip.net/html/adv-multi-internet.html#adv-multi-internet-inbound"><citetitle>http://linux-ip.net/html/adv-multi-internet.html#adv-multi-internet-inbound</citetitle></ulink></para>

        <para>For the use of multiple outbound links to the Internet, there
        are a number of different techniques. The simplest is identified
        here:</para>

        <para><citetitle><ulink
        url="http://linux-ip.net/html/adv-multi-internet.html#adv-multi-internet-outbound">http://linux-ip.net/html/adv-multi-internet.html#adv-multi-internet-outbound</ulink></citetitle></para>

        <para>Better (and more robust) techniques are available after a kernel
        routing patch by Julian Anastasov. See the famous nano-howto.</para>

        <para><citetitle><ulink
        url="http://www.ssi.bg/~ja/">http://www.ssi.bg/~ja/</ulink></citetitle></para>
      </sidebar>
    </section>
  </section>

  <section>
    <title>Starting and Stopping</title>

    <section id="faq7">
      <title>(FAQ 7) When I stop Shorewall using <quote>shorewall
      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 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>Beginning with Shorewall version 2.0.2 Beta 1, Shorewall 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>

      <section id="faq34a">
        <title>(FAQ 34a) I get errors about a host or network not found when I
        run<filename>/var/lib/shorewall/restore</filename>. The
        <command>shorewall restore</command> and <command>shorewall -f
        start</command> commands gives the same result.</title>

        <para>Answer: iptables 1.2.9 is broken with respect to iptables-save
        and the connection tracking match extension. You must patch your
        iptables using the patch available from the <ulink
        url="errata.htm">Shorewall errata page</ulink>.</para>
      </section>
    </section>

    <section id="faq41">
      <title>(FAQ 41) Why do I get modprobe failure messages when I start
      Shorewall?</title>

      <para>When I start shorewall I got the following errors.</para>

      <programlisting>Oct 30 11:13:12 fwr modprobe: modprobe: Can't locate module ipt_conntrack
Oct 30 11:13:17 fwr modprobe: modprobe: Can't locate module ipt_pkttype
Oct 30 11:13:18 fwr modprobe: modprobe: Can't locate module ipt_pkttype
Oct 30 11:13:57 fwr last message repeated 2 times
Oct 30 11:14:06 fwr root: Shorewall Restarted</programlisting>

      <para>The "shorewall status" output seems complying with my rules set.
      Should I worry ? and is there any way to get rid of these errors
      ?</para>

      <para><emphasis role="bold">Answer</emphasis>: You are seeing two
      different things:</para>

      <orderedlist>
        <listitem>
          <para>The normal checking that Shorewall does when it starts.
          Shorewall tries to determine the the capabilities of your 'iptables'
          and kernel and then taylors the ruleset accordingly.</para>
        </listitem>

        <listitem>
          <para>A problem in Shorewall 2.0.3a through 2.0.5 whereby Shorewall
          tried to use the <emphasis>pkttype match</emphasis> feature each
          time that it wanted to generate a rule involving broadcast or
          multicast packets.</para>
        </listitem>
      </orderedlist>

      <para>You can suppress the messages by aliasing the modules mentioned in
      the error messages to off in /etc/modules.conf. Just be sure to review
      these aliases each time that you do a kernel upgrade to be sure that you
      are not disabling a feature in your new kernel that you want to
      use.</para>

      <programlisting>alias ipt_conntrack off
alias ipt_pkttype off</programlisting>

      <para>For users who don't have the pkttype match feature in their
      kernel, I also recommend upgrading to Shorewall 2.0.6 or later and then
      setting PKTTYPE=No in shorewall.conf.</para>
    </section>

    <section>
      <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>

  <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 I am
      running?</title>

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

      <programlisting><command>/sbin/shorewall 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>
      <title>Given that the Debian Stable Release includes Shorewall 1.2.12,
      how can you not support that version?</title>

      <para>The first release of Shorewall was in March of 2001. Shorewall
      1.2.12 was released in May of 2002. It is now the year 2005 and
      Shorewall 2.2 is available. Shorewall 1.2.12 is poorly documented and is
      missing many of the features that Shorewall users find essential today
      and it is silly to continue to run it simply because it is bundled with
      an ancient Debian release.</para>
    </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 -- 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> If you are running a
      version of Shorewall earlier than 1.3.1, create /etc/shorewall/start and
      in it, place the following:</para>

      <programlisting><command>run_iptables -I rfc1918 -s 192.168.100.1 -j ACCEPT</command></programlisting>

      <para>If you are running version 1.3.1 or later, 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    DISPLAY       COMMENTS
modem    ADSLModem     Zone for modem</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>Miscellaneous</title>

    <section id="faq19">
      <title>(FAQ 19) I have added entries to /etc/shorewall/tcrules but they
      don't seem to do anything. Why?</title>

      <para>You probably haven't set TC_ENABLED=Yes in
      /etc/shorewall/shorewall.conf so the contents of the tcrules file are
      simply being ignored.</para>
    </section>

    <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>Edit /etc/shorewall/shorewall.conf and change
      <quote>NEWNOTSYN=No</quote> to <quote>NEWNOTSYN=Yes</quote> then restart
      Shorewall.</para>

      <section id="faq26a">
        <title>(FAQ 26a) When I try to use the <quote>-O</quote> option of
        nmap from the firewall system, I get <quote>operation not
        permitted</quote>. How do I allow this option?</title>

        <para>Add this command to your /etc/shorewall/start file:</para>

        <programlisting><command>run_iptables -D OUTPUT -p ! icmp -m state --state INVALID -j DROP</command></programlisting>
      </section>
    </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>Experimental 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>Answer: At a root prompt, enter the command <command>shorewall
      check</command>. There is a section near the top of the resulting output
      that gives you a synopsis of your kernel/iptables capabilities.</para>

      <programlisting>gateway:/etc/shorewall # shorewall check
Loading /usr/share/shorewall/functions...
Processing /etc/shorewall/params ...
Processing /etc/shorewall/shorewall.conf...
Loading Modules...

Notice:  The 'check' command is unsupported and problem
         reports complaining about errors that it didn't catch
         will not be accepted

Shorewall has detected the following iptables/netfilter capabilities:
   NAT: Available
   Packet Mangling: Available
   Multi-port Match: Available
   Connection Tracking Match: Available
   Packet Type Match: Not available
   Policy Match: Available
   Physdev Match: Available
   IP range Match: Available
Verifying Configuration...
...</programlisting>
    </section>
  </section>
</article>