shorewall_code/docs/troubleshoot.xml

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  <articleinfo>
    <title>Shorewall Troubleshooting Guide</title>

    <author>
      <firstname>Tom</firstname>

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

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

    <copyright>
      <year>2001-2005</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 type="" url="GnuCopyright.htm">GNU Free Documentation
      License</ulink></quote>.</para>
    </legalnotice>
  </articleinfo>

  <section>
    <title><quote>shorewall start</quote> and <quote>shorewall restart</quote>
    Errors</title>

    <para>If you receive an error message when starting or restarting the
    firewall and you can't determine the cause, then do the following:</para>

    <itemizedlist>
      <listitem>
        <para>Make a note of the error message that you see.</para>
      </listitem>

      <listitem>
        <para><command>shorewall debug start 2&gt; /tmp/trace</command></para>
      </listitem>

      <listitem>
        <para>Look at the <filename>/tmp/trace</filename> file and see if that
        helps you determine what the problem is. Be sure you find the place in
        the log where the error message you saw is generated -- If you are
        using Shorewall 1.4.0 or later, you should find the message near the
        end of the log.</para>
      </listitem>

      <listitem>
        <para>If you still can't determine what's wrong then see the <ulink
        url="support.htm">support page</ulink>.</para>
      </listitem>
    </itemizedlist>

    <example>
      <title>Startup Error</title>

      <para>During startup, a user sees the following:</para>

      <programlisting>Adding Common Rules
iptables: No chain/target/match by that name
Terminated</programlisting>

      <para>A search through the trace for <quote>No chain/target/match by
      that name</quote> turned up the following:</para>

      <programlisting>+ echo 'Adding Common Rules'
+ add_common_rules
+ run_iptables -A reject -p tcp -j REJECT --reject-with tcp-reset
++ echo -A reject -p tcp -j REJECT --reject-with tcp-reset
++ sed 's/!/! /g'
+ iptables -A reject -p tcp -j REJECT --reject-with tcp-reset
iptables: No chain/target/match by that name
</programlisting>

      <para>The command that failed was: <quote><command>iptables -A reject -p
      tcp -j REJECT --reject-with tcp-reset</command></quote>. In this case,
      the user had compiled his own kernel and had forgotten to include REJECT
      target support (see <ulink url="kernel.htm">kernel.htm</ulink>)</para>
    </example>
  </section>

  <section>
    <title>Your Network Environment</title>

    <para>Many times when people have problems with Shorewall, the problem is
    actually an ill-conceived network setup. Here are several popular
    snafus:</para>

    <itemizedlist>
      <listitem>
        <para>Port Forwarding where client and server are in the same subnet.
        See <ulink url="FAQ.htm#faq2">FAQ 2</ulink>.</para>
      </listitem>

      <listitem>
        <para>Trying to test net-&gt;loc DNAT rules from inside your firewall.
        You must test these rules from <emphasis
        role="bold">outside</emphasis> your firewall.</para>
      </listitem>

      <listitem>
        <para>Multiple interfaces connected to the same HUB or Switch. Given
        the way that the Linux kernel respond to ARP <quote>who-has</quote>
        requests, this type of setup <emphasis role="bold">does NOT work the
        way that you expect it to</emphasis>. You can test using this kind of
        configuration if you specify the <emphasis
        role="bold">arp_filter</emphasis> option or the <emphasis
        role="bold">arp_ignore</emphasis> option in <filename><ulink
        url="Documentation.htm#Interfaces">/etc/shorewall/interfaces</ulink></filename>
        for all interfaces connected to the common hub/switch. <emphasis
        role="bold">Using such a setup with a production firewall is strongly
        recommended against</emphasis>.</para>
      </listitem>
    </itemizedlist>
  </section>

  <section>
    <title>New Device Doesn't Work?</title>

    <para>If you have just added a new device such as VOIP and it doesn't
    work, be sure that you have assigned it an IP address in your local
    network and that its default gateway has been set to the IP address of
    your internal interface. For many of these devices, the simplest solution
    is to run a DHCP server; running it on your firewall is fine — be sure to
    set the <emphasis role="bold">dhcp</emphasis> option on your internal
    interface in <ulink
    url="Documentation.htm#INterfaces">/etc/shorewall/interfaces</ulink>.</para>
  </section>

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

    <para>One very important thing to remember is that not all connection
    problems are Shorewall configuration problems. If the connection that is
    giving you problems is to or from the firewall system or if it doesn't
    rely on NAT or Proxy ARP then you can often eliminate Shorewall using a
    simple test:</para>

    <itemizedlist>
      <listitem>
        <para><command>/sbin/shorewall clear</command></para>
      </listitem>

      <listitem>
        <para>Try the connection. If it works then the problem is in your
        Shorewall configuration; if the connection still doesn't work then the
        problem is not with Shorewall or the way that it is configured.</para>
      </listitem>

      <listitem>
        <para>Be sure to <command>/sbin/shorewall start</command> after the
        test.</para>
      </listitem>
    </itemizedlist>

    <para>If you still suspect Shorewall and the appropriate policy for the
    connection that you are trying to make is ACCEPT, please DO NOT ADD
    ADDITIONAL ACCEPT RULES TRYING TO MAKE IT WORK. Such additional rules will
    NEVER make it work, they add clutter to your rule set and they represent a
    big security hole in the event that you forget to remove them
    later.</para>

    <para>I also recommend against setting all of your policies to ACCEPT in
    an effort to make something work. That robs you of one of your best
    diagnostic tools - the <quote>Shorewall</quote> messages that Netfilter
    will generate when you try to connect in a way that isn't permitted by
    your rule set.</para>

    <para>Check your log (<quote><command>/sbin/shorewall show
    log</command></quote>). If you don't see Shorewall messages, then your
    problem is probably NOT a Shorewall problem. If you DO see packet
    messages, it may be an indication that you are missing one or more rules
    -- see <ulink url="FAQ.htm#faq17">FAQ 17</ulink>.</para>

    <para>While you are troubleshooting, it is a good idea to clear two
    variables in
    <filename><filename>/etc/shorewall/shorewall.conf</filename></filename>:</para>

    <para><programlisting>LOGRATE=
LOGBURST=""</programlisting>This way, you will see all of the log messages
    being generated (be sure to restart shorewall after clearing these
    variables).</para>

    <example>
      <title>Log Message</title>

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

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

      <itemizedlist>
        <listitem>
          <para>all2all:REJECT - This packet was REJECTed out of the all2all
          chain -- the packet was rejected under the
          <quote>all</quote>-&gt;<quote>all</quote> REJECT policy (see <ulink
          url="FAQ.htm#faq17">FAQ 17</ulink>).</para>
        </listitem>

        <listitem>
          <para>IN=eth2 - the packet entered the firewall via eth2</para>
        </listitem>

        <listitem>
          <para>OUT=eth1 - if accepted, the packet would be sent on
          eth1</para>
        </listitem>

        <listitem>
          <para>SRC=192.168.2.2 - the packet was sent by 192.168.2.2</para>
        </listitem>

        <listitem>
          <para>DST=192.168.1.3 - the packet is destined for
          192.168.1.3</para>
        </listitem>

        <listitem>
          <para>PROTO=UDP - UDP Protocol</para>
        </listitem>

        <listitem>
          <para>DPT=53 - DNS</para>
        </listitem>
      </itemizedlist>

      <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>#ACTION   SOURCE           DEST                  PROTO   DEST
#                                                        PORT(S)
ACCEPT    dmz              loc                   udp     53</programlisting>
    </example>
  </section>

  <section>
    <title>Ping Problems</title>

    <para>Either can't ping when you think you should be able to or are able
    to ping when you think that you shouldn't be allowed? Shorewall's
    <quote>Ping</quote> Management is <ulink url="ping.html">described
    here</ulink>. Here are a couple of tips:</para>

    <itemizedlist>
      <listitem>
        <para>Remember that Shorewall doesn't automatically allow ICMP type 8
        (<quote>ping</quote>) requests to be sent between zones. If you want
        pings to be allowed between zones, you need a rule of the form:</para>

        <programlisting>#ACTION  SOURCE          DEST                  PROTO   DEST
#                                                      PORT(S)
Ping/ACCEPT <emphasis>&lt;source zone&gt;</emphasis>&nbsp;&nbsp; <emphasis>&lt;destination zone&gt;</emphasis></programlisting>

        <para>The ramifications of this can be subtle. For example, if you
        have the following in <filename><ulink
        url="NAT.htm">/etc/shorewall/nat</ulink></filename>:</para>

        <programlisting>#EXTERNAL   INTERFACE  INTERNAL
10.1.1.2&nbsp;&nbsp;&nbsp; eth0&nbsp;&nbsp;&nbsp;    130.252.100.18</programlisting>

        <para>and you ping 130.252.100.18, unless you have allowed icmp type 8
        between the zone containing the system you are pinging from and the
        zone containing 10.1.1.2, the ping requests will be dropped.</para>
      </listitem>

      <listitem>
        <para>Ping requests are subject to logging under your policies. So
        ping floods can cause an equally big flood of log messages. To
        eliminate these, as the last rule in your /etc/shorewall/rules file
        add:</para>

        <programlisting>#ACTION  SOURCE          DEST                  PROTO   DEST
#                                                      PORT(S)
Ping/DROP net             all</programlisting>
      </listitem>
    </itemizedlist>
  </section>

  <section>
    <title>Some Things to Keep in Mind</title>

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

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

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

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

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

  <section>
    <title>Other Gotchas</title>

    <itemizedlist>
      <listitem>
        <para>Seeing rejected/dropped packets logged out of the INPUT or
        FORWARD chains? This means that:</para>

        <orderedlist>
          <listitem>
            <para>your zone definitions are screwed up and the host that is
            sending the packets or the destination host isn't in any zone
            (using an <ulink
            url="Documentation.htm#Hosts"><filename>/etc/shorewall/hosts</filename></ulink>
            file are you?); or</para>
          </listitem>

          <listitem>
            <para>the source and destination hosts are both connected to the
            same interface and you don't have a policy or rule for the source
            zone to or from the destination zone or you haven't set the
            <emphasis role="bold">routeback</emphasis> option for the
            interface in <ulink
            url="Documentation.htm#Interfaces"><filename>/etc/shorewall/interfaces</filename></ulink>.</para>
          </listitem>

          <listitem>
            <para>You have connected two firewall interfaces (from different
            zones) to the same hub or switch.</para>
          </listitem>
        </orderedlist>
      </listitem>

      <listitem>
        <para>If you specify <quote>routefilter</quote> for an interface, that
        interface must be up prior to starting the firewall.</para>
      </listitem>

      <listitem>
        <para>Is your routing correct? For example, internal systems usually
        need to be configured with their default gateway set to the IP address
        of their nearest firewall interface. One often overlooked aspect of
        routing is that in order for two hosts to communicate, the routing
        between them must be set up <emphasis role="bold">in both
        directions</emphasis>. So when setting up routing between <emphasis
        role="bold">A</emphasis> and <emphasis role="bold">B</emphasis>, be
        sure to verify that the route from <emphasis role="bold">B</emphasis>
        back to <emphasis role="bold">A</emphasis> is defined and
        correct.</para>
      </listitem>

      <listitem>
        <para>Do you have your kernel properly configured? <ulink
        url="kernel.htm">Click here to see kernel configuration
        information</ulink>.</para>
      </listitem>
    </itemizedlist>
  </section>

  <section>
    <title>Still Having Problems?</title>

    <para>See the <ulink url="support.htm">Shorewall Support
    Page</ulink>.</para>
  </section>
</article>