shorewall_code/docs/PortKnocking.xml

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  <articleinfo>
    <title>Port Knocking and Other Uses of 'Recent Match'</title>

    <authorgroup>
      <author>
        <firstname>Tom</firstname>

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

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

    <copyright>
      <year>2005</year>

      <year>2006</year>

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

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

  <section>
    <title>What is Port Knocking?</title>

    <para>Port knocking is a technique whereby attempting to connect to port A
    enables access to port B from that same host. For the example on which
    this article is based, see <ulink
    url="http://www.soloport.com/iptables.html">http://www.soloport.com/iptables.html</ulink>
    which should be considered to be part of this documentation.</para>
  </section>

  <section>
    <title>Implementing Port Knocking in Shorewall</title>

    <para>In order to implement this solution, your iptables and kernel must
    support the 'recent match' extension (see <ulink url="FAQ.htm#faq42">FAQ
    42</ulink>). These instructions also assume Shorewall version 2.2.0 or
    later.</para>

    <para>In this example:</para>

    <orderedlist>
      <listitem>
        <para>Attempting to connect to port 1600 enables SSH access. Access is
        enabled for 60 seconds.</para>
      </listitem>

      <listitem>
        <para>Attempting to connect to port 1601 disables SSH access (note
        that in the article linked above, attempting to connect to port 1599
        also disables access. This is an port scan defence as explained in the
        article).</para>
      </listitem>
    </orderedlist>

    <para>To implement that approach:</para>

    <orderedlist>
      <listitem>
        <para>Add an action named SSHKnock (see the <ulink
        url="Actions.html">Action documentation</ulink>). Leave the
        <filename>action.SSHKnock</filename> file empty.</para>
      </listitem>

      <listitem>
        <para>Create /etc/shorewall/SSHKnock with the following
        contents:</para>

        <programlisting>if [ -n "$LEVEL" ]; then
    log_rule_limit $LEVEL $CHAIN SSHKnock ACCEPT "" "$TAG" -A -p tcp --dport 22   -m recent --rcheck --name SSH
    log_rule_limit $LEVEL $CHAIN SSHKnock DROP   "" "$TAG" -A -p tcp --dport ! 22
fi
run_iptables -A $CHAIN -p tcp --dport 22   -m recent --rcheck --seconds 60 --name SSH          -j ACCEPT
run_iptables -A $CHAIN -p tcp --dport 1599 -m recent                       --name SSH --remove -j DROP
run_iptables -A $CHAIN -p tcp --dport 1600 -m recent                       --name SSH --set    -j DROP
run_iptables -A $CHAIN -p tcp --dport 1601 -m recent                       --name SSH --remove -j DROP</programlisting>
      </listitem>

      <listitem>
        <para>Now if you want to protect SSH access to the firewall from the
        Internet, add this rule in
        <filename>/etc/shorewall/rules</filename>:</para>

        <programlisting>#ACTION          SOURCE            DEST           PROTO       DEST PORT(S)
SSHKnock         net               $FW            tcp         22,1599,1600,1601</programlisting>

        <para>If you want to log the DROPs and ACCEPTs done by SSHKnock, you
        can just add a log level as in:</para>

        <programlisting>#ACTION          SOURCE            DEST           PROTO       DEST PORT(S)
SSHKnock:info    net               $FW            tcp         22,1599,1600,1601</programlisting>
      </listitem>

      <listitem>
        <para>If you wish to use SSHKnock with a forwarded connection, you
        must be using Shorewall 2.3.1 or later for fullest protection. Assume
        that you forward port 22 from external IP address 206.124.146.178 to
        internal system 192.168.1.5. In /etc/shorewall/rules:</para>

        <programlisting>#ACTION          SOURCE            DEST            PROTO       DEST PORT(S)  SOURCE      ORIGINAL
#                                                                            PORT(S)     DEST
DNAT-            net               loc:192.168.1.5 tcp         22            -           206.124.146.178
SSHKnock         net               $FW             tcp         1599,1600,1601
SSHKnock         net               loc:192.168.1.5 tcp         22            -           206.124.146.178</programlisting>

        <note>
          <para>You can use SSHKnock with DNAT on earlier releases provided
          that you omit the ORIGINAL DEST entry on the second SSHKnock rule.
          This rule will be quite secure provided that you specify 'norfc1918'
          on your external interface.</para>
        </note>
      </listitem>
    </orderedlist>
  </section>

  <section id="Limit">
    <title>Limiting Per-IP Connection Rate</title>

    <important>
      <para>Debian users. This feature is broken in the Debian version 3.0.7
      of Shorewall (and possibly in other versions). The file
      <filename>/usr/share/shorewall/Limit</filename> was inadvertently
      dropped from the .deb. That file may be obtained from <ulink
      url="???">Shorewall SVN</ulink> and installed manually.</para>
    </important>

    <para>Beginning with Shorewall 3.0.4, Shorewall has a 'Limit' action.
    Limit is invoked with a comma-separated list in place of a logging tag.
    The list has three elements:</para>

    <orderedlist>
      <listitem>
        <para>The name of a 'recent' set; you select the set name which must
        conform to the rules for a valid chain name. Different rules that
        specify the same set name will use the same set of counters.</para>
      </listitem>

      <listitem>
        <para>The number of connections permitted in a specified time
        period.</para>
      </listitem>

      <listitem>
        <para>The time period, expressed in seconds.</para>
      </listitem>
    </orderedlist>

    <para>Connections that exceed the specified rate are dropped.</para>

    <para>For example,to use a recent set name of <emphasis
    role="bold">SSHA</emphasis>, and to limiting SSH to 3 per minute:</para>

    <programlisting>#ACTION                SOURCE            DEST           PROTO       DEST PORT(S)
Limit:none:SSHA,3,60   net               $FW            tcp         22</programlisting>

    <para>If you want dropped connections to be logged at the info level, use
    this rule instead:</para>

    <programlisting>#ACTION                SOURCE            DEST           PROTO       DEST PORT(S)
Limit:info:SSHA,3,60   net               $FW            tcp         22</programlisting>

    <para>To summarize, you pass four pieces of information to the Limit
    action:</para>

    <itemizedlist>
      <listitem>
        <para>The log level. If you don't want to log, specify "none".</para>
      </listitem>

      <listitem>
        <para>The name of the recent set that you want to use ("SSHA" in this
        example).</para>
      </listitem>

      <listitem>
        <para>The maximum number of connections to accept (3 in this
        example).</para>
      </listitem>

      <listitem>
        <para>The number of seconds over which you are willing to accept that
        many connections (60 in this example).</para>
      </listitem>
    </itemizedlist>

    <section>
      <title>How Limit is Implemented</title>

      <para>For those who are curious, the Limit action is implemented in
      Shorewall 3.0 and Shorewall 3.2 as follows:</para>

      <itemizedlist>
        <listitem>
          <para>The file
          <filename>/usr/share/shorewall/action</filename>.Limit is
          empty.</para>
        </listitem>

        <listitem>
          <para>The file <filename>/usr/share/shorewall/Limit</filename> is as
          follows:</para>

          <programlisting>set -- $(separate_list $TAG)

[ $# -eq 3 ] || fatal_error "Rule must include &lt;set name&gt;,&lt;max connections&gt;,&lt;interval&gt; as the log tag"

run_iptables -A $CHAIN -m recent --name $1 --set

if [ -n "$LEVEL" ]; then
    run_iptables -N $CHAIN%
    log_rule_limit $LEVEL $CHAIN% $1 DROP "" "" -A
    run_iptables -A $CHAIN% -j DROP
    run_iptables -A $CHAIN -m recent --name $1 --update --seconds $3 --hitcount $(( $2 + 1 )) -j $CHAIN%
else
    run_iptables -A $CHAIN -m recent --update --name $1 --seconds $3 --hitcount $(( $2 + 1 )) -j DROP
fi

run_iptables -A $CHAIN -j ACCEPT
</programlisting>
        </listitem>
      </itemizedlist>

      <para>In Shorewall 3.3, Limit is made into a built-in action; basically
      that means that the above code now lives inside of Shorewall rather than
      in a separate file.</para>
    </section>
  </section>
</article>