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315 lines
12 KiB
XML
315 lines
12 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<article>
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<!--$Id$-->
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<articleinfo>
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<title>Port Knocking and Other Uses of 'Recent Match'</title>
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<authorgroup>
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<author>
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<firstname>Tom</firstname>
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<surname>Eastep</surname>
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</author>
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</authorgroup>
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<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
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<copyright>
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<year>2005</year>
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<year>2006</year>
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<holder>Thomas M. Eastep</holder>
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</copyright>
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<legalnotice>
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<para>Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License, Version
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1.2 or any later version published by the Free Software Foundation; with
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no Invariant Sections, with no Front-Cover, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled
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<quote><ulink url="GnuCopyright.htm">GNU Free Documentation
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License</ulink></quote>.</para>
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</legalnotice>
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</articleinfo>
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<note>
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<para>The feature described in this article require '<ulink
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url="http://snowman.net/projects/ipt_recent/">Recent Match</ulink>' in
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your iptables and kernel. See the output of <command>shorewall show
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capabilities</command> to see if you have that match.</para>
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</note>
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<section id="What">
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<title>What is Port Knocking?</title>
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<para>Port knocking is a technique whereby attempting to connect to port A
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enables access to port B from that same host. For the example on which
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this article is based, see <ulink
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url="http://www.soloport.com/iptables.html">http://www.soloport.com/iptables.html</ulink>
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which should be considered to be part of this documentation.</para>
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</section>
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<section id="How">
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<title>Implementing Port Knocking in Shorewall</title>
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<para>In order to implement this solution, your iptables and kernel must
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support the 'recent match' extension (see <ulink url="FAQ.htm#faq42">FAQ
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42</ulink>). These instructions also assume Shorewall version 2.2.0 or
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later.</para>
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<para>In this example:</para>
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<orderedlist>
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<listitem>
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<para>Attempting to connect to port 1600 enables SSH access. Access is
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enabled for 60 seconds.</para>
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</listitem>
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<listitem>
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<para>Attempting to connect to port 1601 disables SSH access (note
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that in the article linked above, attempting to connect to port 1599
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also disables access. This is an port scan defence as explained in the
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article).</para>
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</listitem>
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</orderedlist>
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<para>To implement that approach:</para>
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<orderedlist>
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<listitem>
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<para>Add an action named SSHKnock (see the <ulink
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url="Actions.html">Action documentation</ulink>). Leave the
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<filename>action.SSHKnock</filename> file empty.</para>
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</listitem>
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<listitem>
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<para>Create /etc/shorewall/SSHKnock with the following
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contents.</para>
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<para>If using Shorewall-shell:</para>
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<programlisting>if [ -n "$LEVEL" ]; then
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log_rule_limit $LEVEL $CHAIN SSHKnock ACCEPT "" "$TAG" -A -p tcp --dport 22 -m recent --rcheck --name SSH
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log_rule_limit $LEVEL $CHAIN SSHKnock DROP "" "$TAG" -A -p tcp --dport ! 22
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fi
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run_iptables -A $CHAIN -p tcp --dport 22 -m recent --rcheck --seconds 60 --name SSH -j ACCEPT
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run_iptables -A $CHAIN -p tcp --dport 1599 -m recent --name SSH --remove -j DROP
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run_iptables -A $CHAIN -p tcp --dport 1600 -m recent --name SSH --set -j DROP
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run_iptables -A $CHAIN -p tcp --dport 1601 -m recent --name SSH --remove -j DROP</programlisting>
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<para>If using Shorewall-perl:<programlisting>use Shorewall::Chains;
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if ( $level ) {
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log_rule_limit( $level,
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$chainref,
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'SSHKnock',
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'ACCEPT',
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'',
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$tag,
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'add',
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'-p tcp --dport 22 -m recent --rcheck --name SSH );
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log_rule_limit( $level,
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$chainref,
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'SSHKnock'
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'DROP'
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'',
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$tag,
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'add',
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'-p tcp --dport ! 22' );
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}
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add_rule( $chainref, '-p tcp --dport 22 -m recent --rcheck --seconds 60 --name SSH -j ACCEPT' );
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add_rule( $chainref, '-p tcp --dport 1599 -m recent --name SSH --remove -j DROP' );
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add_rule( $chainref, '-p tcp --dport 1600 -m recent --name SSH --set -j DROP' );
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add_rule( $chainref, '-p tcp --dport 1601 -m recent --name SSH --remove -j DROP' );
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1;</programlisting></para>
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</listitem>
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<listitem>
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<para>Now if you want to protect SSH access to the firewall from the
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Internet, add this rule in
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<filename>/etc/shorewall/rules</filename>:</para>
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<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
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SSHKnock net $FW tcp 22,1599,1600,1601</programlisting>
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<para>If you want to log the DROPs and ACCEPTs done by SSHKnock, you
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can just add a log level as in:</para>
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<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
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SSHKnock:info net $FW tcp 22,1599,1600,1601</programlisting>
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</listitem>
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<listitem>
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<para>If you wish to use SSHKnock with a forwarded connection, you
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must be using Shorewall 2.3.1 or later for fullest protection. Assume
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that you forward port 22 from external IP address 206.124.146.178 to
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internal system 192.168.1.5. In /etc/shorewall/rules:</para>
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<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL
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# PORT(S) DEST
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DNAT- net loc:192.168.1.5 tcp 22 - 206.124.146.178
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SSHKnock net $FW tcp 1599,1600,1601
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SSHKnock net loc:192.168.1.5 tcp 22 - 206.124.146.178</programlisting>
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<note>
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<para>You can use SSHKnock with DNAT on earlier releases provided
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that you omit the ORIGINAL DEST entry on the second SSHKnock rule.
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This rule will be quite secure provided that you specify 'norfc1918'
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on your external interface.</para>
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</note>
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</listitem>
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</orderedlist>
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</section>
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<section id="Limit">
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<title>Limiting Per-IP Connection Rate</title>
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<important>
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<para>Debian users. This feature is broken in the Debian version 3.0.7
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of Shorewall (and possibly in other versions). The file
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<filename>/usr/share/shorewall/Limit</filename> was inadvertently
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dropped from the .deb. That file may be obtained from <ulink
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url="???">Shorewall SVN</ulink> and installed manually.</para>
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</important>
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<para>Beginning with Shorewall 3.0.4, Shorewall has a 'Limit' <ulink
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url="Actions.html">action</ulink>. Limit is invoked with a comma-separated
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list in place of a logging tag. The list has three elements:</para>
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<orderedlist>
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<listitem>
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<para>The name of a 'recent' set; you select the set name which must
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conform to the rules for a valid chain name. Different rules that
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specify the same set name will use the same set of counters.</para>
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</listitem>
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<listitem>
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<para>The number of connections permitted in a specified time
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period.</para>
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</listitem>
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<listitem>
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<para>The time period, expressed in seconds.</para>
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</listitem>
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</orderedlist>
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<para>Connections that exceed the specified rate are dropped.</para>
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<para>For example,to use a recent set name of <emphasis
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role="bold">SSHA</emphasis>, and to limiting SSH to 3 per minute, use this
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entry in <filename>/etc/shorewall/rules</filename>:</para>
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<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
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Limit:none:SSHA,3,60 net $FW tcp 22</programlisting>
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<para>If you want dropped connections to be logged at the info level, use
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this rule instead:</para>
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<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
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Limit:info:SSHA,3,60 net $FW tcp 22</programlisting>
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<para>To summarize, you pass four pieces of information to the Limit
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action:</para>
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<itemizedlist>
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<listitem>
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<para>The log level. If you don't want to log, specify "none".</para>
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</listitem>
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<listitem>
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<para>The name of the recent set that you want to use ("SSHA" in this
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example).</para>
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</listitem>
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<listitem>
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<para>The maximum number of connections to accept (3 in this
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example).</para>
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</listitem>
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<listitem>
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<para>The number of seconds over which you are willing to accept that
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many connections (60 in this example).</para>
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</listitem>
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</itemizedlist>
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<section id="LimitImp">
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<title>How Limit is Implemented</title>
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<para>For those who are curious, the Limit action is implemented in
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Shorewall 3.0 and Shorewall 3.2 as follows:</para>
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<itemizedlist>
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<listitem>
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<para>The file
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<filename>/usr/share/shorewall/action</filename>.Limit is
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empty.</para>
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</listitem>
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<listitem>
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<para>The file <filename>/usr/share/shorewall/Limit</filename> is as
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follows:</para>
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<programlisting>set -- $(separate_list $TAG)
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[ $# -eq 3 ] || fatal_error "Rule must include <set name>,<max connections>,<interval> as the log tag"
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run_iptables -A $CHAIN -m recent --name $1 --set
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if [ -n "$LEVEL" ]; then
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run_iptables -N $CHAIN%
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log_rule_limit $LEVEL $CHAIN% $1 DROP "" "" -A
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run_iptables -A $CHAIN% -j DROP
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run_iptables -A $CHAIN -m recent --name $1 --update --seconds $3 --hitcount $(( $2 + 1 )) -j $CHAIN%
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else
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run_iptables -A $CHAIN -m recent --update --name $1 --seconds $3 --hitcount $(( $2 + 1 )) -j DROP
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fi
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run_iptables -A $CHAIN -j ACCEPT</programlisting>
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</listitem>
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</itemizedlist>
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<para>In Shorewall 3.3, Limit is made into a built-in action; basically
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that means that the above code now lives inside of Shorewall rather than
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in a separate file.</para>
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<para>For completeness, here's the above
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<filename>/usr/share/shorewall/Limit</filename> for use with
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Shorewall-perl:</para>
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<programlisting>my @tag = split /,/, $tag;
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fatal_error 'Limit rules must include <set name>,<max connections>,<interval> as the log tag (' . join( ':', 'Limit', $level eq '' ? 'none' : $level , $tag ) . ')'
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unless @tag == 3;
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my $set = $tag[0];
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for ( @tag[1,2] ) {
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fatal_error 'Max connections and interval in Limit rules must be numeric (' . join( ':', 'Limit', $level eq '' ? 'none' : $level, $tag ) . ')' unless /^\d+$/
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}
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my $count = $tag[1] + 1;
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add_rule $chainref, "-m recent --name $set --set";
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if ( $level ) {
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my $xchainref = new_chain 'filter' , "$chainref->{name}%";
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log_rule_limit $level, $xchainref, $tag[0], 'DROP', '', '', 'add', '';
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add_rule $xchainref, '-j DROP';
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add_rule $chainref, "-m recent --name $set --update --seconds $tag[2] --hitcount $count -j $xchainref->{name}";
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} else {
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add_rule $chainref, "-m recent --update --name $set --seconds $tag[2] --hitcount $count -j DROP";
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}
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add_rule $chainref, '-j ACCEPT';
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1; </programlisting>
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</section>
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</section>
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</article> |