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shorewall_code/docs/ConnectionRate.xml
2009-11-03 07:39:27 -08:00

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<?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>Connection Rate Limiting</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
<copyright>
<year>2008</year>
<year>2009</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>Introduction</title>
<para>Shorewall supports several mechanisms for limiting connection rates.
These are described in the following sections.</para>
<para>Rates are expressed in terms of a <firstterm>connections per unit
time</firstterm> and a <filename>burst</filename>. An
<firstterm>interval</firstterm> is calculated by dividing the unit of time
by the number of connections allowed in that unit of time
(<replaceable>connections</replaceable>/{<option>sec</option>|<option>min</option>|<option>hour</option>|<option>day</option>|week|month}[:<replaceable>burst</replaceable>]</para>
<para>Example: 4/min:5</para>
<simplelist>
<member>Connections = 4</member>
<member>Unit of time = 1 minute</member>
<member>Interval = 1 minute/4 = 15 seconds.</member>
<member>Burst = 5</member>
</simplelist>
<para>As each connection arrives,if the burst count is &gt; 0 the
<firstterm>burst</firstterm> count is reduced by one and the connection is
accepted. After each interval (15 seconds) that passes without a
connection arriving, the <firstterm>burst</firstterm> count is incremented
by 1 but is not allowed to exceed its initial setting (5).</para>
<para>By default, the aggregate connection rate is limited. If the
specification is preceeded by "<option>s:</option>" or
"<option>d:</option>", then the rate is limited per SOURCE or per
DESTINATION IP address respectively.</para>
<section>
<title>Policy Rate Limiting</title>
<para>The LIMIT:BURST column in the
<filename>/etc/shorewall/policy</filename> file applies to TCP
connections that are subject to the policy. The limiting is applied
BEFORE the connection request is passed through the rules generated by
entries in <filename>/etc/shorewall/rules</filename>. Those connections
in excess of the limit are logged and dropped.</para>
</section>
<section>
<title>Rules Rate Limiting</title>
<para>The RATE LIMIT column in the
<filename>/etc/shorewall/rules</filename> file allows limiting of
ACCEPT, DNAT and Action rules.</para>
</section>
<section>
<title>Limit Action</title>
<para>The <ulink url="Actions.html#Limit">Limit Action</ulink> is a
legacy mechanism that limits connections per source IP. It does not
support the notion of a burst size.</para>
</section>
</section>
</article>