shorewall_code/docs/Actions.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
<article>
  <!--$Id$-->

  <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>

      <year>2007</year>

      <year>2008</year>

      <year>2009</year>

      <year>2010</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><emphasis role="bold">This article applies to Shorewall 4.3 and
    later. If you are running a version of Shorewall earlier than Shorewall
    4.3.5 then please see the documentation for that
    release.</emphasis></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>For readers familiar with iptables, actions are the way in which you
    can create your own filter-table chains.</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 <filename
        class="directory">/usr/share/shorewall</filename>. 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 have been largely replaced by <ulink
        url="Macros.html">macros</ulink> .</para>
      </listitem>

      <listitem>
        <para>User-defined Actions. These actions are created by end-users.
        They are listed in the file
        <filename>/etc/shorewall/actions</filename> and are defined in
        <filename>action.*</filename> files in <filename
        class="directory">/etc/shorewall</filename> or in another directory
        listed in your CONFIG_PATH (defined in <filename><ulink
        url="manpages/shorewall.conf.html">/etc/shorewall/shorewall.conf</ulink></filename>).</para>
      </listitem>
    </orderedlist>
  </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, REJECT, QUEUE and NFQUEUE 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
    <filename>/etc/shorewall/shorewall.conf</filename> 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 <quote>none</quote> have no default action.</para>

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

    <important>
      <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>
    </important>

    <para>Beginning with Shorewall 4.4.21, the standard Drop and Reject
    options are parameterized. Each has three parameters as follows:</para>

    <informaltable>
      <tgroup cols="4">
        <tbody>
          <row>
            <entry>ACTION</entry>

            <entry>PARAMETER</entry>

            <entry>VALUE</entry>

            <entry>DEFAULT</entry>
          </row>

          <row>
            <entry>Drop</entry>

            <entry>1</entry>

            <entry>Either '-' or 'audit'. 'audit' causes auditing by the
            builtin actions invoked by Drop</entry>

            <entry>-</entry>
          </row>

          <row>
            <entry>Drop</entry>

            <entry>2</entry>

            <entry>Determines what to do with Auth requests</entry>

            <entry>REJECT or A_REJECT depending on the setting of parameter
            1</entry>
          </row>

          <row>
            <entry>Drop</entry>

            <entry>3</entry>

            <entry>Determines what to do with SMB</entry>

            <entry>DROP or A_DROP depending on the setting of parameter
            1</entry>
          </row>

          <row>
            <entry>Reject</entry>

            <entry>1</entry>

            <entry>Either '-' or 'audit'. 'audit' causes auditing by the
            builtin actions invoked by Drop</entry>

            <entry>-</entry>
          </row>

          <row>
            <entry>Reject</entry>

            <entry>2</entry>

            <entry>Determines what to do with Auth requests</entry>

            <entry>REJECT or A_REJECT depending on the setting of parameter
            1</entry>
          </row>

          <row>
            <entry>Reject</entry>

            <entry>3</entry>

            <entry>Determines what to do with SMB</entry>

            <entry>REJECT or A_REJECT depending on the setting of parameter
            1</entry>
          </row>
        </tbody>
      </tgroup>
    </informaltable>

    <para>The parameters may be specified in either shorewall.conf (e.g.,
    DROP_DEFAULT=<emphasis role="bold">Drop(-,DROP)</emphasis> or in the
    POLICY column of <ulink
    url="manpages/shorewall-policy.html">shorewall-policy</ulink>(5) (e.g.,
    DROP:<emphasis role="bold">Drop(audit)</emphasis>:audit).</para>
  </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>/etc/shorewall/actions</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>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 <filename>/usr/share/shorewall/action.template</filename> 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>

    <section>
      <title>Shorewall 4.4.16 and Later.</title>

      <para>Beginning with Shorewall 4.4.16, the columns in action.template
      are the same as those in shorewall-rules (5). The first non-commentary
      line in the template must be</para>

      <programlisting>FORMAT 2</programlisting>

      <para>When using Shorewall 4.4.16 or later, there are no restrictions
      regarding which targets can be used within your action.</para>

      <para>The SOURCE and DEST columns in the action file may not include
      zone names; those are given when the action is invoked.</para>

      <para>Additionally, it is possible to pass parameters to an action, when
      it is invoked in the rules file or in another action.</para>

      <para>Here's a trivial example:</para>

      <para>/etc/shorewall/action.A:</para>

      <programlisting>#TARGET        SOURCE  DEST    PROTO   DEST    SOURCE  ORIGINAL
#                                      PORT(S) PORT(S) DEST
FORMAT 2
$1             -       -       tcp     80      -       1.2.3.4</programlisting>

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

      <programlisting>#TARGET        SOURCE  DEST    PROTO   DEST    SOURCE  ORIGINAL
#                                      PORT(S) PORT(S) DEST

A(REDIRECT)    net     fw</programlisting>

      <para>The above is equivalent to this rule:</para>

      <programlisting>#TARGET        SOURCE  DEST    PROTO   DEST    SOURCE  ORIGINAL
#                                      PORT(S) PORT(S) DEST
REDIRECT       net     -       tcp     80      -       1.2.3.4</programlisting>

      <para>You can 'omit' parameters by using '-'.</para>

      <para>Example: ACTION(REDIRECT,-,info)</para>

      <para>In the above example, $2 would expand to nothing.</para>

      <para>If you want to make '-' a parameter value, use '--' (e.g.,
      ACTION(REDIRECT,--.info)).</para>

      <para>Beginning with Shorewall 4.4.21, you can specify the default
      values of your FORMAT-2 actions:</para>

      <programlisting>DEFAULTS <replaceable>def1</replaceable>,<replaceable>def2</replaceable>,...</programlisting>

      <para>where <replaceable>def1</replaceable> is the default value for the
      first parameter, <replaceable>def2</replaceable> is the default value
      for the second parameter and so on. You can specify an empty default
      using '-' (e.g. DEFAULTS DROP,-,audit).</para>
    </section>

    <section>
      <title>Shorewall 4.4.15 and Earlier.</title>

      <para>Prior to 4.4.16, columns in the
      <filename>action.template</filename> file were as follows:</para>

      <itemizedlist>
        <listitem>
          <para>TARGET - Must be ACCEPT, DROP, REJECT, LOG, CONTINUE, QUEUE or
          an &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/action.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>
        </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 = <quote>all</quote>, 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>
        </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 subsequent fields, then place <quote>-</quote> in this
          column.</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</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 subsequent columns, place a <quote>-</quote> 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
      (<quote>-</quote>).</para>

      <para>Example:</para>

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

      <para><programlisting>     #ACTION             COMMENT (place '# ' below the 'C' in comment followed by
     #                   v        a comment describing the action)
     LogAndAccept        # LOG and ACCEPT a connection</programlisting><emphasis
      role="bold">Note:</emphasis> If your
      <filename>/etc/shorewall/actions</filename> file doesn't have an
      indication where to place the comment, put the <quote>#</quote> in
      column 21.</para>

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

      <para>Placing a comment on the line causes the comment to appear in the
      output of the <command>shorewall show actions</command> command.</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>

  <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 <quote>foo</quote> 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 <quote>!</quote> 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 <quote>foo</quote> 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>
  </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
    <filename>action.template</filename>. 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>If you define an action <quote>acton</quote> and you have an
      <filename>/etc/shorewall/acton</filename> script, the rules compiler
      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>

        <listitem>
          <para><emphasis role="bold">@params</emphasis> is the list of
          parameter values (Shorewall 4.4.16 and later). 'Omitted' parameters
          contain '-'.</para>
        </listitem>
      </itemizedlist>

      <para>Example:</para>

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

      <para>/etc/shorewall/action.DropBcasts<programlisting># This file is empty</programlisting>/etc/shorewall/DropBcasts<programlisting>use Shorewall::Chains;

if ( $level ne '' ) {
    log_rule_limit $level, $chainref, 'dropBcast' , 'DROP', '', $tag, 'add', ' -m addrtype --dst-type BROADCAST ';
    log_rule_limit $level, $chainref, 'dropBcast' , 'DROP', '', $tag, 'add', ' -d 224.0.0.0/4 ';
}

add_rule $chainref, '-m addrtype --dst-type BROADCAST -j DROP';
add_rule $chainref, '-d 224.0.0.0/4 -j DROP';

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

    <para>For a richer example, see the next section.</para>
  </section>

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

    <para>Shorewall supports a <quote>Limit</quote> built-in action. Prior to
    Shorewall 4.4.16, Limit is invoked with a comma-separated list in place of
    a logging tag. Beginning in Shorewall 4.4.16, it may also be invoked with
    a list of three parameters enclosed in parentheses. The list has three
    elements:</para>

    <orderedlist>
      <listitem>
        <para>The name of a <quote>recent</quote> list. You select the list
        name which must conform to the rules for a valid chain name. Different
        rules that specify the same list 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 list name of <emphasis
    role="bold">SSHA</emphasis>, and to limit SSH connections 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>Using Shorewall 4.4.16 or later, you can also invoke the action this
    way:</para>

    <programlisting>#ACTION                SOURCE            DEST           PROTO       DEST PORT(S)
Limit(SSHA,3,60):none  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>Shorewall 4.4.16 and later:<programlisting>#ACTION                SOURCE            DEST           PROTO       DEST PORT(S)
Limit(SSH,3,60):info   net               $FW            tcp         22</programlisting></para>

    <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
        <quote>none</quote>.</para>
      </listitem>

      <listitem>
        <para>The name of the recent list that you want to use
        (<quote>SSHA</quote> 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 in Shorewall 4.4.16 is
      implemented as follows:</para>

      <programlisting>use Shorewall::Chains;

@params = split( /,/, $tag ), $tag='' unless @params;

fatal_error 'Limit rules must include &lt;list name&gt;,&lt;max connections&gt;,&lt;interval&gt; as the log tag or params' unless @params == 3;

my $list = $params[0];

for ( @params[1,2] ) {
    fatal_error 'Max connections and interval in Limit rules must be numeric (' . $_ . ')' unless /^\d+$/
}

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

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

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

add_rule $chainref, '-j ACCEPT';

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