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>
    <title>Limiting Per-IP Connection Rate</title>

    <para>Suppose that you wish to limit the number of connections to port 22
    to 3/minute from individual internet hosts to the firewall.</para>

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

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

        <programlisting>run_iptables -A $CHAIN -m recent --name SSHA --set
if [ -n "$LEVEL" ]; then
    run_iptables -N $CHAIN%
    log_rule_limit $LEVEL $CHAIN% SSHLimit REJECT "" "" -A
    run_iptables -A $CHAIN% -j reject
    run_iptables -A $CHAIN -m recent --name SSHA --update --seconds 60 --hitcount 4 -j $CHAIN%
else
    run_iptables -A $CHAIN -m recent --update --name SSHA --seconds 60 --hitcount 4 -j reject
fi
run_iptables -A $CHAIN -j ACCEPT</programlisting>
      </listitem>

      <listitem>
        <para>Add this rule to /etc/shorewall/rules:</para>

        <programlisting>#ACTION          SOURCE            DEST           PROTO       DEST PORT(S)
SSHLimit         net               $FW            tcp         22</programlisting>

        <para>If you wish to log the rejects at the 'info' level then use this
        rule instead:</para>

        <programlisting>#ACTION          SOURCE            DEST           PROTO       DEST PORT(S)
SSHLimit:info    net               $FW            tcp         22</programlisting>
      </listitem>

      <listitem>
        <para>If you wish to use SSHLimit 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
SSHLimit         net               loc:192.168.1.5 tcp         22            -           206.124.146.178</programlisting>

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

    <para id="Limit">The above can be generalized into a flexible 'Limit'
    target.</para>

    <note>
      <para>'Limit' as described here is included as a standard part of
      Shorewall beginning with version 3.0.4. The following is included to
      show how 'Limit' is implemented; if you are running Shorewall 3.0.4 or
      later, you can omit the following two steps.</para>
    </note>

    <orderedlist>
      <listitem>
        <para>Add an action named Limit. Leave the action.Limit file
        empty.</para>
      </listitem>

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

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

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

if [ -n "$LEVEL" ]; then
    run_iptables -N $CHAIN%
    log_rule_limit $LEVEL $CHAIN% $1 REJECT "" "" -A
    run_iptables -A $CHAIN% -j reject
    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 reject
fi

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

    <para>Now if you want to limit the number of connections to port 22 to
    3/minute from individual internet hosts to the firewall, you can add this
    rule:</para>

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

    <para>If you want rejected 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>As you can see, 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>
</article>