shorewall_code/docs/Actions.xml

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  <articleinfo>
    <title>Actions</title>

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

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

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

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

  <caution>
    <para>This article applies to Shorewall 4.0 and later. If you are running
    a version of Shorewall earlier than Shorewall 4.0.0 then please see the
    documentation for that release.</para>
  </caution>

  <section id="Intro">
    <title>What are Shorewall Actions?</title>

    <para>Shorewall actions allow a symbolic name to be associated with a
    series of one or more iptables rules. The symbolic name may appear in the
    ACTION column of an <filename><ulink
    url="manpages/shorewall-rules.html">/etc/shorewall/rules</ulink></filename>
    file entry, in which case the traffic matching that rules file entry will
    be passed to the series of iptables rules named by the action.</para>

    <para>Actions can be thought of as templates. When an action is invoked in
    an <filename>/etc/shorewall/rules</filename> entry, it may be qualified by
    a logging specification (log level and optionally a log tag). The presence
    of the log level/tag causes a modified series of rules to be generated in
    which each packet/rule match within the action causes a log message to be
    generated.</para>

    <para>There are three types of Shorewall actions:</para>

    <orderedlist>
      <listitem>
        <para>Built-in Actions. These actions are known by the Shorewall code
        itself. They are listed in the comments at the top of the file
        <filename>/usr/share/shorewall/actions.std</filename>.</para>
      </listitem>

      <listitem>
        <para>Standard Actions. These actions are released as part of
        Shorewall. They are listed in the file
        <filename>/usr/share/shorewall/actions.std</filename> and are defined
        in the corresponding action.* files in <filename
        class="directory">/usr/share/shorewall</filename>. Each
        <filename>action.*</filename> file has a comment at the beginning of
        the file that describes what the action does. As an example, here is
        the definition of the <firstterm>AllowSMB</firstterm> standard action
        from Shorewall version 2.2.</para>

        <programlisting>#
# Shorewall 2.2 /usr/share/shorewall/action.AllowSMB
#
#       Allow Microsoft SMB traffic. You need to invoke this action in
#       both directions.
#
######################################################################################
#TARGET  SOURCE         DEST            PROTO   DEST    SOURCE          RATE    USER/
#                                               PORT    PORT(S)         LIMIT   GROUP
ACCEPT   -              -               udp     135,445
ACCEPT   -              -               udp     137:139
ACCEPT   -              -               udp     1024:   137
ACCEPT   -              -               tcp     135,139,445
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE</programlisting>

        <para>If you wish to modify one of the standard actions, do not modify
        the definition in /usr/share/shorewall. Rather, copy the file to
        <filename class="directory">/etc/shorewall</filename> (or somewhere
        else on your CONFIG_PATH) and modify the copy.</para>

        <para>Standard Actions were largely replaced by <ulink
        url="Macros.html">macros</ulink> in Shorewall 3.0 and later major
        versions.</para>
      </listitem>

      <listitem>
        <para>User-defined Actions. These actions are created by end-users.
        They are listed in the file /etc/shorewall/actions and are defined in
        action.* files in /etc/shorewall or in another directory listed in
        your CONFIG_PATH (defined in <ulink
        url="manpages/shorewall.conf.html">/etc/shorewall/shorewall.conf</ulink>).</para>
      </listitem>
    </orderedlist>
  </section>

  <section id="Enabling">
    <title>Enabling the Use of Actions</title>

    <para>In Shorewall version 3.4 and later, to make use of any of the three
    types of actions you must set the USE_ACTIONS option to Yes in
    <filename>/etc/shorewall/shorewall.conf</filename>.</para>
  </section>

  <section id="Default">
    <title>Default Actions (Formerly Common Actions)</title>

    <para>Shorewall allows the association of a <firstterm>default
    action</firstterm> with policies. A separate default action may be
    associated with ACCEPT, DROP and REJECT policies. Default actions provide
    a way to invoke a set of common rules just before the policy is enforced.
    Default actions accomplish two goals:</para>

    <orderedlist>
      <listitem>
        <para>Relieve log congestion. Default actions typically include rules
        to silently drop or reject traffic that would otherwise be logged when
        the policy is enforced.</para>
      </listitem>

      <listitem>
        <para>Ensure correct operation. Default actions can also avoid common
        pitfalls like dropping connection requests on port TCP port 113. If
        these connections are dropped (rather than rejected) then you may
        encounter problems connecting to internet services that utilize the
        AUTH protocol of client authentication<footnote>
            <para>AUTH is actually pretty silly on today's internet but it's
            amazing how many servers still employ it.</para>
          </footnote></para>
      </listitem>
    </orderedlist>

    <para>Shorewall supports default actions for the ACCEPT, REJECT, DROP,
    QUEUE and NFQUEUE policies. These default actions are specified in the
    /etc/shorewall/shorewall.conf file using the ACCEPT_DEFAULT,
    REJECT_DEFAULT, DROP_DEFAULT, QUEUE_DEFAULT and NFQUEUE_DEFAULT options
    respectively. Policies whose default is set to a value of "none" have no
    default action.</para>

    <para></para>

    <para>In addition, the default specified in /etc/shorewall/shorewall.conf
    may be overridden by specifying a different default in the POLICY column
    of <ulink
    url="manpages/shorewall-policy.html">/etc/shorewall/policy</ulink>.</para>

    <warning>
      <para>Entries in the DROP and REJECT default actions <emphasis
      role="bold">ARE NOT THE CAUSE OF CONNECTION PROBLEMS</emphasis>.
      Remember — default actions are only invoked immediately before the
      packet is going to be dropped or rejected anyway!!!</para>
    </warning>
  </section>

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

    <para>Beginning with Shorewall 3.0.4, Shorewall has a 'Limit' <ulink
    url="Actions.html">action</ulink>. 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, use this
    entry in <filename>/etc/shorewall/rules</filename>:</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 id="LimitImp">
      <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>

      <para>For completeness, here's the above
      <filename>/usr/share/shorewall/Limit</filename> for use with
      Shorewall-perl:</para>

      <programlisting>my @tag = split /,/, $tag;

fatal_error 'Limit rules must include &lt;set name&gt;,&lt;max connections&gt;,&lt;interval&gt; as the log tag (' . join( ':', 'Limit', $level eq '' ? 'none' : $level , $tag ) . ')'
    unless @tag == 3;

my $set = $tag[0];

for ( @tag[1,2] ) {
    fatal_error 'Max connections and interval in Limit rules must be numeric (' . join( ':', 'Limit', $level eq '' ? 'none' : $level, $tag ) . ')' unless /^\d+$/
}

my $count = $tag[1] + 1;

add_rule $chainref, "-m recent --name $set --set";

if ( $level ) {
    my $xchainref = new_chain 'filter' , "$chainref-&gt;{name}%";
    log_rule_limit $level, $xchainref, $tag[0], 'DROP', '', '', 'add', '';
    add_rule $xchainref, '-j DROP';
    add_rule $chainref,  "-m recent --name $set --update --seconds $tag[2] --hitcount $count -j $xchainref-&gt;{name}";
} else {
    add_rule $chainref, "-m recent --update --name $set --seconds $tag[2] --hitcount $count -j DROP";
}

add_rule $chainref, '-j ACCEPT';

1; </programlisting>
    </section>
  </section>

  <section id="Defining">
    <title>Defining your own Actions</title>

    <para>Before defining a new action, you should evaluate whether your goal
    can be best accomplished using an action or a
    <firstterm>macro</firstterm>. See <ulink url="Macros.html">this
    article</ulink> for details.</para>

    <para>To define a new action:</para>

    <orderedlist>
      <listitem>
        <para>Add a line to
        <filename><filename>/etc/shorewall/actions</filename></filename> that
        names your new action. Action names must be valid shell variable names
        ((must begin with a letter and be composed of letters, digits and
        underscore characters) as well as valid Netfilter chain names. If you
        intend to log from the action, the name must have a maximum of 11
        characters. It is recommended that the name you select for a new
        action begins with a capital letter; that way, the name won't conflict
        with a Shorewall-defined chain name.</para>

        <para>The name of the action may be optionally followed by a colon
        (<quote>:</quote>) and ACCEPT, DROP or REJECT. When this is done, the
        named action will become the <emphasis>default action </emphasis>for
        policies of type ACCEPT, DROP or REJECT respectively. The default
        action is applied immediately before the policy is enforced (before
        any logging is done under that policy) and is used mainly to suppress
        logging of uninteresting traffic which would otherwise clog your logs.
        The same policy name can appear in multiple actions; the last such
        action for each policy name is the one which Shorewall will
        use.</para>

        <para>Shorewall includes pre-defined actions for DROP and REJECT --
        see above.</para>
      </listitem>

      <listitem>
        <para>Once you have defined your new action name (ActionName), then
        copy /usr/share/shorewall/action.template to
        <filename>/etc/shorewall/action.ActionName</filename> (for example, if
        your new action name is <quote>Foo</quote> then copy
        <filename>/usr/share/shorewall/action.template</filename> to
        <filename>/etc/shorewall/action.Foo</filename>).</para>
      </listitem>

      <listitem>
        <para>Now modify the new file to define the new action.</para>
      </listitem>
    </orderedlist>

    <para>Columns in the action.template file are as follows:</para>

    <itemizedlist>
      <listitem>
        <para>TARGET - Must be ACCEPT, DROP, REJECT, LOG, CONTINUE, QUEUE or
        &lt;<emphasis>action</emphasis>&gt; where
        &lt;<emphasis>action</emphasis>&gt; is a previously-defined action
        (that is, it must precede the action being defined in this file in
        your <filename>/etc/shorewall/actions</filename> file). These actions
        have the same meaning as they do in the
        <filename>/etc/shorewall/rules</filename> file (CONTINUE terminates
        processing of the current action and returns to the point where that
        action was invoked). The TARGET may optionally be followed by a colon
        (<quote>:</quote>) and a syslog log level (e.g, REJECT:info or
        ACCEPT:debugging). This causes the packet to be logged at the
        specified level. You may also specify ULOG (must be in upper case) as
        a log level. This will log to the ULOG target for routing to a
        separate log through use of ulogd (<ulink
        url="http://www.netfilter.org/projects/ulogd/index.html">http://www.netfilter.org/projects/ulogd/index.html</ulink>).</para>

        <para>You may also use a <ulink url="Macros.html">macro</ulink> in
        your action provided that the macro's expansion only results in the
        ACTIONs ACCEPT, DROP, REJECT, LOG, CONTINUE, or QUEUE. See
        <filename>/usr/share/shorewall/Drop</filename> for an example of an
        action that users macros extensively.</para>
      </listitem>

      <listitem>
        <para>SOURCE - Source hosts to which the rule applies. A
        comma-separated list of subnets and/or hosts. Hosts may be specified
        by IP or MAC address; mac addresses must begin with <quote>~</quote>
        and must use <quote>-</quote> as a separator.</para>

        <para>Alternatively, clients may be specified by interface name. For
        example, eth1 specifies a client that communicates with the firewall
        system through eth1. This may be optionally followed by another colon
        (<quote>:</quote>) and an IP/MAC/subnet address as described above
        (e.g., eth1:192.168.1.5).</para>
      </listitem>

      <listitem>
        <para>DEST - Location of Server. Same as above with the exception that
        MAC addresses are not allowed.</para>

        <para>Unlike in the SOURCE column, you may specify a range of up to
        256 IP addresses using the syntax &lt;<emphasis>first
        ip</emphasis>&gt;-&lt;<emphasis>last ip</emphasis>&gt;.</para>
      </listitem>

      <listitem>
        <para>PROTO - Protocol - Must be <quote>tcp</quote>,
        <quote>udp</quote>, <quote>icmp</quote>, a protocol number, or
        <quote>all</quote>.</para>
      </listitem>

      <listitem>
        <para>DEST PORT(S) - Destination Ports. A comma-separated list of Port
        names (from <filename>/etc/services</filename>), port numbers or port
        ranges; if the protocol is <quote>icmp</quote>, this column is
        interpreted as the destination icmp-type(s).</para>

        <para>A port range is expressed as &lt;<emphasis>low
        port</emphasis>&gt;:&lt;<emphasis>high port</emphasis>&gt;.</para>

        <para>This column is ignored if PROTO = "all", but must be entered if
        any of the following fields are supplied. In that case, it is
        suggested that this field contain <quote>-</quote>.</para>

        <para>If your kernel contains multi-port match support, then only a
        single Netfilter rule will be generated if in this list and in the
        CLIENT PORT(S) list below:</para>

        <orderedlist>
          <listitem>
            <para>There are 15 or less ports listed.</para>
          </listitem>

          <listitem>
            <para>No port ranges are included.</para>
          </listitem>
        </orderedlist>

        <para>Otherwise, a separate rule will be generated for each
        port.</para>
      </listitem>

      <listitem>
        <para>SOURCE PORT(S) - Port(s) used by the client. If omitted, any
        source port is acceptable. Specified as a comma-separated list of port
        names, port numbers or port ranges.</para>

        <para>If you don't want to restrict client ports but need to specify
        any of the following fields, then place "-" in this column.</para>

        <para>If your kernel contains multi-port match support, then only a
        single Netfilter rule will be generated if in this list and in the
        DEST PORT(S) list above:</para>

        <orderedlist>
          <listitem>
            <para>There are 15 or less ports listed.</para>
          </listitem>

          <listitem>
            <para>No port ranges are included.</para>
          </listitem>
        </orderedlist>

        <para>Otherwise, a separate rule will be generated for each
        port.</para>
      </listitem>

      <listitem>
        <para>RATE LIMIT - You may rate-limit the rule by placing a value in
        this column:</para>

        <para><programlisting>     &lt;<emphasis>rate</emphasis>&gt;/&lt;<emphasis>interval</emphasis>&gt;[:&lt;<emphasis>burst</emphasis>&gt;]</programlisting>where
        &lt;<emphasis>rate</emphasis>&gt; is the number of connections per
        &lt;<emphasis>interval</emphasis>&gt; (<quote>sec</quote> or
        <quote>min</quote>) and &lt;<emphasis>burst</emphasis>&gt; is the
        largest burst permitted. If no &lt;<emphasis>burst</emphasis>&gt; is
        given, a value of 5 is assumed. There may be no whitespace embedded in
        the specification.</para>

        <para><programlisting>     Example: 10/sec:20</programlisting></para>
      </listitem>

      <listitem>
        <para>USER/GROUP - For output rules (those with the firewall as their
        source), you may control connections based on the effective UID and/or
        GID of the process requesting the connection. This column can contain
        any of the following:</para>

        <simplelist>
          <member>[!]&lt;<emphasis>user number</emphasis>&gt;[:]</member>

          <member>[!]&lt;<emphasis>user name</emphasis>&gt;[:]</member>

          <member>[!]:&lt;<emphasis>group number</emphasis>&gt;</member>

          <member>[!]:&lt;<emphasis>group name</emphasis>&gt;</member>

          <member>[!]&lt;<emphasis>user
          number</emphasis>&gt;:&lt;<emphasis>group
          number</emphasis>&gt;</member>

          <member>[!]&lt;<emphasis>user
          name</emphasis>&gt;:&lt;<emphasis>group
          number</emphasis>&gt;</member>

          <member>[!]&lt;<emphasis>user
          inumber</emphasis>&gt;:&lt;<emphasis>group
          name</emphasis>&gt;</member>

          <member>[!]&lt;<emphasis>user
          name</emphasis>&gt;:&lt;<emphasis>group name</emphasis>&gt;</member>

          <member>[!]+&lt;<emphasis>program name</emphasis>&gt; (Note: support
          for this form was removed from Netfilter in kernel version
          2.6.14).</member>
        </simplelist>
      </listitem>

      <listitem>
        <para>MARK (Added in Shorewall 3.4.4)</para>

        <para><simplelist>
            <member>[!]&lt;<emphasis>value</emphasis>&gt;[/&lt;<emphasis>mask</emphasis>&gt;][:C]</member>
          </simplelist></para>

        <para>Defines a test on the existing packet or connection mark. The
        rule will match only if the test returns true.</para>

        <para>If you don’t want to define a test but need to specify anything
        in the following columns, place a "-" in this field.<simplelist>
            <member>! — Inverts the test (not equal)</member>

            <member>&lt;<emphasis>value</emphasis>&gt; — Value of the packet
            or connection mark.</member>

            <member>&lt;<emphasis>mask</emphasis>&gt; —A mask to be applied to
            the mark before testing.</member>

            <member>:C — Designates a connection mark. If omitted, the packet
            mark’s value is tested. This option is only supported by
            Shorewall-perl</member>
          </simplelist></para>
      </listitem>
    </itemizedlist>

    <para>Omitted column entries should be entered using a dash ("-").</para>

    <para>Example:</para>

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

    <para><programlisting>     LogAndAccept</programlisting><phrase><filename>/etc/shorewall/action.LogAndAccept</filename></phrase><programlisting>     LOG:info
     ACCEPT</programlisting></para>

    <para>To use your action, in <filename>/etc/shorewall/rules</filename> you
    might do something like:</para>

    <programlisting>#ACTION      SOURCE      DEST        PROTO    DEST PORT(S)
LogAndAccept loc         $FW         tcp      22</programlisting>
  </section>

  <section id="Logging">
    <title>Actions and Logging</title>

    <para>Specifying a log level in a rule that specifies a user-defined or
    Shorewall-defined action will cause each rule in the action to be logged
    with the specified level (and tag).</para>

    <para>The extent to which logging of action rules occur is governed by the
    following:</para>

    <orderedlist>
      <listitem>
        <para>When you invoke an action and specify a log level, only those
        rules in the action that have no log level will be changed to log at
        the level specified at the action invocation.</para>

        <para>Example:</para>

        <para>/etc/shorewall/action.foo</para>

        <programlisting>#TARGET      SOURCE     DEST     PROTO    DEST PORT(S)
ACCEPT       -          -        tcp      22
bar:info</programlisting>

        <para>/etc/shorewall/rules:</para>

        <programlisting>#ACTION      SOURCE     DEST     PROTO    DEST PORT(S)
foo:debug    $FW         net</programlisting>

        <para>Logging in the invoke 'foo' action will be as if foo had been
        defined as:</para>

        <programlisting>#TARGET      SOURCE     DEST     PROTO    DEST PORT(S)
ACCEPT:debug -          -        tcp      22
bar:info</programlisting>
      </listitem>

      <listitem>
        <para>If you follow the log level with "!" then logging will be set at
        that level for all rules recursively invoked by the action.</para>

        <para>Example:</para>

        <para>/etc/shorewall/action.foo</para>

        <programlisting>#TARGET      SOURCE     DEST     PROTO    DEST PORT(S)
ACCEPT       -          -        tcp      22
bar:info</programlisting>

        <para>/etc/shorewall/rules:</para>

        <programlisting>#ACTION      SOURCE     DEST     PROTO    DEST PORT(S)
foo:debug!   $FW        net</programlisting>

        <para>Logging in the invoke 'foo' action will be as if foo had been
        defined as:</para>

        <programlisting>#TARGET      SOURCE     DEST     PROTO    DEST PORT(S)
ACCEPT:debug -          -        tcp      22
bar:debug</programlisting>
      </listitem>
    </orderedlist>

    <para>If you define an action 'acton' and you have an
    <filename>/etc/shorewall/acton</filename> script then when that script is
    invoked, the following three variables will be set for use by the
    script:</para>

    <itemizedlist>
      <listitem>
        <para>$CHAIN = the name of the chain where your rules are to be
        placed. When logging is used on an action invocation, Shorewall
        creates a chain with a slightly different name from the action
        itself.</para>
      </listitem>

      <listitem>
        <para>$LEVEL = Log level. If empty, no logging was specified.</para>
      </listitem>

      <listitem>
        <para>$TAG = Log Tag.</para>
      </listitem>
    </itemizedlist>

    <para>Example:</para>

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

    <programlisting>#ACTION          SOURCE           DEST
acton:info:test  $FW              net</programlisting>

    <para>Your /etc/shorewall/acton file will be run with:</para>

    <itemizedlist>
      <listitem>
        <para>$CHAIN="%acton1"</para>
      </listitem>

      <listitem>
        <para>$LEVEL="info"</para>
      </listitem>

      <listitem>
        <para>$TAG="test"</para>
      </listitem>
    </itemizedlist>

    <para>Shorewall-perl sets lexical variables as follows:</para>

    <itemizedlist>
      <listitem>
        <para><emphasis role="bold">$chainref</emphasis> is a reference to the
        chain-table entry for the chain where your rules are to be
        placed.</para>
      </listitem>

      <listitem>
        <para><emphasis role="bold">$level</emphasis> is the log level. If
        false, no logging was specified.</para>
      </listitem>

      <listitem>
        <para><emphasis role="bold">$tag</emphasis> is the log tag.</para>
      </listitem>
    </itemizedlist>

    <para>For an example of how to use these variables in both Shorewall-shell
    and Shorewall-perl, see <ulink url="PortKnocking.html">this
    article</ulink>.</para>
  </section>

  <section id="Extension">
    <title>Creating an Action using an Extension Script</title>

    <para>There may be cases where you wish to create a chain with rules that
    can't be constructed using the tools defined in the action.template. In
    that case, you can use an <ulink
    url="shorewall_extension_scripts.htm">extension script</ulink>.<note>
        <para>If you actually need an action to drop broadcast packets, use
        the <command>dropBcast</command> standard action rather than create
        one like this.</para>
      </note></para>

    <example id="Example">
      <title>An action to drop all broadcast packets</title>

      <para>/etc/shorewall/actions<programlisting>DropBcasts</programlisting></para>

      <para>/etc/shorewall/action.DropBcasts<programlisting># This file is empty</programlisting>When
      using Shorewall-shell:</para>

      <para><blockquote>
          <para>/etc/shorewall/DropBcasts<programlisting>[ -n "$LEVEL" ] &amp;&amp; log_rule_limit $LEVEL $CHAIN DropBcasts DROP   "" "$TAG" -A
run_iptables -A DropBcasts -m pkttype --pkttype broadcast -j DROP</programlisting></para>
        </blockquote>When using Shorewall-Perl:<blockquote>
          <para>/etc/shorewall/DropBcasts<programlisting>use Shorewall::Chains;

log_rule_limit( $level, $chainref, 'DropBcasts', 'DROP', '', $tag, 'add', '' ) if $level ne ''; 
add_rule( $chainref, '-m pkttype --pkttype broadcast -j DROP' );

1;</programlisting></para>
        </blockquote></para>
    </example>

    <para>For a richer example, see <ulink url="PortKnocking.html">this
    article</ulink>.</para>
  </section>
</article>