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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>mod_qos - Additional Information</title>
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<meta name="author" content="Pascal Buchbinder" />
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<body>
<!--
Quality of service module for Apache Web Server.
See http://mod-qos.sourceforge.net/ for further details.
Copyright (C) 2023 Pascal Buchbinder
Licensed to the Apache Software Foundation (ASF) under one or more
contributor license agreements. See the NOTICE file distributed with
this work for additional information regarding copyright ownership.
The ASF licenses this file to You under the Apache License, Version 2.0
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<table>
<tbody>
<tr><td><a href="index.html"><img src="images/mod_qos.gif" alt="mod_qos" title="mod_qos" /></a></td>
<td style="vertical-align: bottom;"><h1>Additional Information</h1></td></tr>
<tr><td> </td>
<td>
<p>
This page gives you additional information on certain aspects of
<a href="index.html">mod_qos</a> to get a better understanding about
how to use the module.
</p>
<hr>
<p>
<ul>
<li><a href="#directives">Directives</a></li>
<li><a href="#rules">Rules</a></li>
<li><a href="#variables">Environment Variables</a></li>
<li><a href="#concurrency">Concurrency Counter</a></li>
<ul>
<li><a href="#QS_LocRequestLimit_Example">Sample Use Case</a></li>
</ul>
<li><a href="#repeat">Repeat Counter</a></li>
<li><a href="#throughput">Throughput Control</a></li>
<ul>
<li><a href="#requestPerSecond">Requests per Second</a></li>
</ul>
<li><a href="#serialization">Serialization</a></li>
<li><a href="#ssi">Error Pages and Server Side Includes (SSI)</a></li>
<li><a href="#UserTracking">User Tracking</a></li>
<li><a href="#RequestStatistics">Request Statistics Using <i>qslog</i></a></li>
</ul>
</p>
<!-- --------------------------------------------------------- -->
<hr>
<a name="directives"></a>
<h2>Directives</h2>
<p>
The module is configured by directives. All directives process the
connection, HTTP request, and response data in a pre-defined sequence.
The following graph shows the order in which the directives work.
</p>
<p>
<a href="images/directive_seq.gif"><img src="images/directive_seq.gif" height="435" width="558" title="directive sequence" alt="directive sequence"/></a>
</p>
<!-- --------------------------------------------------------- -->
<hr>
<a name="rules"></a>
<h2>Rules</h2>
<p>
<a href="index.html">mod_qos</a> allows you to configure different kind of
rules. The main component of a rule is its counter. A rule measures either
the <a href="#concurrency">concurrency</a> (how many times something happens
at the same time), the <a href="#repeat">occurrence </a> (how often does
something happen in a certain amount of time), or the
<a href="#throughput">throughput</a> (sent amount of data or number of
request) and stores this information within that counter.</p>
<p>
Every rule has it's own threshold and maintains its own counter. A rule
is identified by either an URL pattern/matching string or by an
<a href="#variables">environment variable name</a>.
You can configure as many rules as you want. </p>
<img src="images/Rule.png" height="164" width="514" alt="Rule" />
<p>
<i>Note: Some counters are only available once. This applies to the counters of
the rules using the
<code><a href="index.html#QS_Block">QS_Block</a></code>,
<code><a href="index.html#QS_SrvSerialize_var">QS_SrvSerialize</a></code>, and
<code><a href="index.html#QS_Serialize">QS_Serialize</a></code>
<a href="#variables">environment variables</a>.
</i>
</p>
<!-- --------------------------------------------------------- -->
<hr>
<a name="variables"></a>
<h2>Environment Variables</h2>
<p>
The Apache web server provides a mechanism for storing information
in so called <i>environment variables</i>. <a href="index.html">mod_qos</a> uses these
variables to exchange data respectively signalize events between
<a href="#rules">different rules</a> defined by the
<a href="#directives">corresponding directive</a>. These
variables can also be written or read by other Apache modules, such as
<a href="http://modsetenvifplus.sourceforge.net/">mod_setenvifplus <img src="images/link.png"/></a>
or <a href="http://httpd.apache.org/docs/current/mod/mod_setenvif.html">mod_setenvif <img src="images/link.png"/></a>.
</p>
<p>Example:<br>
The <code><a href="http://httpd.apache.org/docs/current/mod/mod_setenvif.html#setenvif">SetEnvIf <img src="images/link.png"/></a></code> directive
is used to set the <i>LimitLogin</i> variable if the request line matches
the <code>^/wp-login.php</code> pattern while the
<a href="index.html#QS_ClientEventLimitCount"><code>QS_ClientEventLimitCount</code></a>
directives increments the <a href="#repeat">repeat counter</a> having the same name
if the variable is present.
<br>
<img src="images/Events.png" height="396" width="795" alt="Event set by SetEnvIf and processed by QS_ClientEventLimitCount" />
</p>
<p>It is also possible to write the values of these variables to your log
file using the format string <code>%{VARNAME}e</code> within the
<code><a href="http://httpd.apache.org/docs/current/mod/mod_log_config.html">TransferLog/CustomLog <img src="images/link.png"/></a></code>
directives. Or you can use them within error pages using
<a href="#ssi">server-side includes (SSI)</a>.
</p>
<p>
<i>Note: Whenever you use a directive (such as
<code><a href="http://modsetenvifplus.sourceforge.net/#SetEnvIfPlus">SetEnvIfPlus <img src="images/link.png"/></a></code>) which can either process request attributes (such
as HTTP headers) or environment variables, you must make sure that
a client can not bypass your rules by sending a request header
with the same name as the environment variable used in your configuration.
Use either the <a href="index.html#QS_RequestHeaderFilter">request header filter</a>
or the <code><a href="index.html#QS_UnsetReqHeader">QS_UnsetReqHeader</a></code>
directive to prevent anyone from sending a request header with the same name as
the variable you have defined.
</i></p>
<!-- --------------------------------------------------------- -->
<hr>
<a name="concurrency"></a>
<h2>Concurrency Counter</h2>
<p>
A "concurrency counter" is used to determine how many times something
happens at the same time (concurrent), e.g. HTTP requests accessing the same
resource/URL at the same time. The rules using this counter type are either
defined by an <a href="#variables">environment variable name</a> or an
URL pattern (regular expression or a string matching the request's URL).
Such a rule automatically increments the counter when the Apache web server
starts to process a matching request and decrements the counter when the
request processing is completed.
</p>
<p>
You have to configure a threshold and the rule's variable name resp. URL
pattern. Requests (or new connections) are denied as soon as the configured
threshold is reached.
</p>
<p>
Directives using this counter type are:
<ul>
<li><a href="index.html#QS_LocRequestLimitMatch"><syntax>QS_LocRequestLimitMatch</syntax></a></li>
<li><a href="index.html#QS_LocRequestLimit"><syntax>QS_LocRequestLimit</syntax></a></li>
<li><a href="index.html#QS_CondLocRequestLimitMatch"><syntax>QS_CondLocRequestLimitMatch</syntax></a></li>
<li><a href="index.html#QS_EventRequestLimit"><syntax>QS_EventRequestLimit</syntax></a></li>
<li><a href="index.html#QS_ClientEventRequestLimit"><syntax>QS_ClientEventRequestLimit</syntax></a></li>
</ul>
Also the
<a href="index.html#QS_SrvMaxConn"><code>QS_SrvMaxConn</code></a>,
<a href="index.html#QS_SrvMaxConnClose"><code>QS_SrvMaxConnClose</code></a>, and
<a href="index.html#QS_SrvMaxConnPerIP"><code>QS_SrvMaxConnPerIP</code></a>
directives use this counter type, although with fewer parameter options.
</p>
<p>
<a name="QS_LocRequestLimit_Example"></a>
<h4>Sample Use Case</h4>
Now let us look at an example to show where these rules can be used.
Let's assume that you have two applications. We call them
"<font color="green">A</font>" and "<font color="blue">B</font>".
Application "<font color="green">A</font>" has been deployed on
path <code>/app/a</code> and "<font color="blue">B</font>"
on <code>/app/b</code>.
<table width="780px">
<tr>
<td> </td>
<td>
As long as the server has enough resources, users can access both
applications the same time without influencing each other.<br>
All requests are processed quickly and the workers (w) become free
again to serve new requests.
<br><br>
</td>
<td>
<small> </small> <img src="images/qsloc1.png" height="216" width="408" alt="trouble-free"/>
</td>
</tr>
<tr>
<td> </td>
<td>
But if the application "<font color="blue">B</font>" becomes slow,
the duration of request processing increases and all workers (w) become busy.
There are no free workers left and application
"<font color="green">A</font>" becomes unavailable because of that.
<br><br>
</td>
<td>
<img src="images/qsloc2.png" height="212" width="422" alt="disruption"/>
</td>
</tr>
<tr>
<td> </td>
<td>
A <a href="index.html#QS_LocRequestLimit"><code>QS_LocRequestLimit</code></a> or
<a href="index.html#QS_LocRequestLimitMatch"><code>QS_LocRequestLimitMatch</code></a>
rule can help in such a situation. mod_qos limits the
number of requests to application "<font color="blue">B</font>"
so that application "<font color="green">A</font>" remains
available even if application "<font color="blue">B</font>"
has performance problems.
</td>
<td>
<img src="images/qsloc3.png" height="244" width="422" alt="disruption"/>
</td>
</tr>
</table>
<br>
Such a scenario can occur due to various infrastructure problems,
e.g., by slow database queries.<br> A similar situation can also
arise through an external influence: if someone penetrates application
"<font color="blue">B</font>" with a <i>HTTP GET / POST flood DoS attack</i>,
then application "<font color="green">A</font>" could also become
unreachable.
A <a href="index.html#QS_LocRequestLimit"><code>QS_LocRequestLimit</code></a>
rule can prevent this.
</p>
<!-- --------------------------------------------------------- -->
<hr>
<a name="repeat"></a>
<h2>Repeat Counter</h2>
<p>
"Repeat counters" limit the number how often (Cr) something is allowed to
happen in a certain amount of time (Td). These rules trigger a timer whenever
the defined event occurs the first time and start to count every subsequent event
until the timer expires. If the event counter reaches the defined limitation,
requests are blocked until the time is up.
</p>
<p>
<img src="images/LimitCount.png" height="432" width="576" alt="reapet counter"/>
</p>
<p>
All repeat counters allow you to define an event which shall
increment the counter if they occur. You also have to configure a
duration Td and the threshold Cr, defining how many events are
allowed within the time Td.</p>
<p>Directive parameter example:<br>
<img src="images/LimitCountExample.png" height="165" width="574" alt="QS_ClientEventLimitCount" />
</p>
<p>
While the counter is automatically cleared (set to 0) when the
time Td is up, you might also configure additional events to
<a href="index.html#_Decrement">decrement</a> or
<a href="index.html#_Clear">clear</a> the counter earlier.
</p>
<p>
The directives using this counter type are:
<ul>
<li><a href="index.html#QS_EventLimitCount"><syntax>QS_EventLimitCount</syntax></a></li>
<li><a href="index.html#QS_CondEventLimitCount"><syntax>QS_CondEventLimitCount</syntax></a></li>
<li><a href="index.html#QS_ClientEventLimitCount"><syntax>QS_ClientEventLimitCount</syntax></a></li>
<li><a href="index.html#QS_CondClientEventLimitCount"><syntax>QS_CondClientEventLimitCount</syntax></a></li>
<li><a href="index.html#QS_ClientEventBlockCount"><syntax>QS_ClientEventBlockCount</syntax></a></li>
</ul>
<i>Note: Some directives support the increase of the counter by more than "1"
taking the variable's value into account.</i>
</p>
<!-- --------------------------------------------------------- -->
<hr>
<a name="throughput"></a>
<h2>Throughput Control</h2>
<p>
Throughput control is implemented by measuring the current usage and
calculating a necessary delay which needs to be applied to the
data processing in order to achieve the desired limitation (closed
loop control system).
</p>
<p>
<img src="images/ClosedLoop.png" height="216" width="478" alt="closed loop"/>
</p>
<p>
<a href="index.html">mod_qos</a> can limit the bandwidth when downloading
data from your web server to the client. This throughput control can
be configured by the following directives:
<ul>
<li><a href="index.html#QS_LocKBytesPerSecLimitMatch"><syntax>QS_LocKBytesPerSecLimitMatch</syntax></a></li>
<li><a href="index.html#QS_LocKBytesPerSecLimit"><syntax>QS_LocKBytesPerSecLimit</syntax></a></li>
<li><a href="index.html#QS_EventKBytesPerSecLimit"><syntax>QS_EventKBytesPerSecLimit</syntax></a></li>
</ul>
<i>Note: Throughput control should always be used with a <a href="#concurrency">concurrency counter</a>
because the added delay increases the number of simultaneous requests.
</i></p>
<a name="requestPerSecond"></a>
<h3>Requests per Second</h3>
<p>
It is also possible to limit the number or requests per second to a
resource. This control function is less accurate than the bandwidth
limitation, since the measurement of the request rate takes longer
(several seconds) and the request delay is more coarse-grained.<br>
The following directive can be used to limit the number of requests
per second:
<ul>
<li><a href="index.html#QS_LocRequestPerSecLimitMatch"><syntax>QS_LocRequestPerSecLimitMatch</syntax></a></li>
<li><a href="index.html#QS_LocRequestPerSecLimit"><syntax>QS_LocRequestPerSecLimit</syntax></a></li>
<li><a href="index.html#QS_EventPerSecLimit"><syntax>QS_EventPerSecLimit</syntax></a></li>
<li><a href="index.html#QS_ClientEventPerSecLimit"><syntax>QS_ClientEventPerSecLimit</syntax></a></li>
</ul>
</p>
<!-- --------------------------------------------------------- -->
<hr>
<a name="serialization"></a>
<h2>Serialization</h2>
<p>
<a href="index.html">mod_qos</a> offers you the option to serialize requests.
Serialization means, that requests are processed one after the other. Incoming
requests are queued if another request is in process and have to wait until
the previous request is finished.
</p>
<p>
<img src="images/Serialization.png" height="336" width="570" alt="serialization"/>
</p>
<p>
Requests, which shall be serialized, are tagged by one of the following
<a href="#variables">environment variables</a>:
<ul>
<li><syntax><a href="index.html#QS_SrvSerialize_var">QS_SrvSerialize</a></syntax></li>
<li><syntax><a href="index.html#QS_Serialize">QS_Serialize</a></syntax></li>
</ul>
</p>
<p>
Serialization might be applied on a per
<a href="index.html#QS_SrvSerialize">server level</a> (serializing all HTTP
requests) or on a <a href="index.html#QS_ClientSerialize">per client level</a>
(serializing multiple requests coming from the same client/IP address).
</p>
<!-- --------------------------------------------------------- -->
<hr>
<a name="ssi"></a>
<a name="SSI"></a>
<h2>Error Pages and Server Side Includes (SSI)</h2>
<p>
Custom error documents to be used by <a href="index.html">mod_qos</a>
are either configured using the
<code><a href="index.html#QS_ErrorPage">QS_ErrorPage</a></code> directive
or the <code><a href="index.html#QS_ErrorPage_Var">QS_ErrorPage</a></code>
variable.<br>
You may also use Apache's <a href="http://httpd.apache.org/docs/current/howto/ssi.html">server-side includes (SSI) <img src="images/link.png"/></a> to generate the content
of the error document dynamically. The <a href="index.html#errorlog">error codes</a> and
other <a href="index.html#variables">variables</a> set by
<a href="index.html">mod_qos</a> can be used.
</p>
<p>
<table border="0" cellspacing="5" cellpadding="10" width="100%">
<tr><td bgcolor="#E2EDE2">
Sample configuration:<br>
<pre>
AddType text/html .shtml
AddOutputFilter INCLUDES .shtml
QS_ErrorPage <a href="#qs_error.shtml">/errorpages/qs_error.shtml</a>
</pre>
</td></tr>
</table>
</p>
<p>
<table border="0" cellspacing="5" cellpadding="10" width="100%">
<tr><td bgcolor="#E2EDE2"><a name="qs_error.shtml"></a>
Sample page:<br>
<pre>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<!--
-- mod_qos sample SSI error page (Apache 2.4)
-->
<meta http-equiv="Cache-Control" content="no-cache, no-store"/>
<meta http-equiv="expires" content="0" />
<title>ERROR - <!--#echo var="REDIRECT_ERROR_NOTES" --></title>
</head>
<body>
<p>
<b><i>sorry - the server was unable to complete your request</i></b>
</p>
<p>
code: mod_qos(<!--#echo var="<a href="index.html#QS_ErrorNotes">QS_ErrorNotes</a>" -->)<br>
<!--#if expr="v('REDIRECT_ERROR_NOTES') =~ /00[0-9]/" -->
reason: initialisation failure
<!--#elif expr="v('REDIRECT_ERROR_NOTES') =~ /0[18][0-9]/" -->
reason: request rule
<!--#elif expr="v('REDIRECT_ERROR_NOTES') =~ /03[0-9]/" -->
reason: connection rule
<!--#elif expr="v('REDIRECT_ERROR_NOTES') =~ /[01]4[0-9]/" -->
reason: request filter
<!--#elif expr="v('REDIRECT_ERROR_NOTES') =~ /05[0-9]/" -->
reason: bandwidth limitation
<!--#elif expr="v('REDIRECT_ERROR_NOTES') =~ /[01]6[0-9]/" -->
reason: client limitation <br>
remaining time: <span id="remaining"><!--#echo var="<a href="index.html#QS_Limit_Remaining">QS_Limit_Remaining</a>" --></span> seconds
<script type="text/javascript">
<!--
setInterval(function () {
var msg = document.getElementById('remaining');
if(msg) {
var value = msg.innerHTML;
var remainTime = value - 1;
if(remainTime < 0) {
window.location = window.location.pathname;
} else {
msg.innerHTML = remainTime;
}
}
}, 1000);
//-->
</script>
<!--#elif expr="v('REDIRECT_ERROR_NOTES') =~ /10[0-9]/" -->
reason: GEO location limitation
<!--#else -->
reason: generic failure
<!--#endif -->
</p>
</body>
</html>
</pre>
</td></tr>
</table>
</p>
<!-- --------------------------------------------------------- -->
<hr>
<a name="UserTracking"></a>
<a name="QS_UserTrackingCookieName"></a>
<h2>User Tracking</h2>
<p>
It might be necessary to identify individual users to define appropriate QoS
rules. For this reason, <a href="index.html">mod_qos</a> can set a cookie
containing a unique identifier. This identifier is then written to the
<code><a href="index.html#mod_qos_user_id">mod_qos_user_id</a></code>
environment variable and you can add it to your log files by the
format string <code>%{mod_qos_user_id}e</code>. This allows you to identify
all requests issued by a user.</p>
<p>
This feature is enabled by the following directive:
<ul>
<li><syntax>QS_UserTrackingCookieName <name> [<path>] [<domain>] ['session'] ['jsredirect']</syntax><br>
The parameter "name" defines the cookie's name and "domain" (optional)
the domain attribute for the Set-Cookie header. The string "session" can
be defined if the cookie should not be stored by the session (but can
be deleted when the user closes this browser).
</li>
</ul>
You can also enforce the browser to support cookies by specifying the
"path" parameter for the <code>QS_UserTrackingCookieName</code> directive.
This parameter defines an error document and mod_qos
answers the request with a redirect (302) to this document when setting
the cookie initially. The browser will follow the redirect and mod_qos
redirects the browser back to the initially requested page if the request
to this error document contains the tracking cookie. If the browser did not
send the cookie, the error document is shown.
</p>
<p> <img src="images/UserTracking.png" height="386" width="410" alt="user tracking cookie enforcement"/></p>
<p>
<i>Note: You can exclude certain clients from this enforcement by
setting the <code>DISABLE_UTC_ENFORCEMENT</code>
<a href="#variables">environment variable</a>
at server level (outside Location), e.g., to allow crawlers not
supporting cookies to access your site.</i>
</p>
<p>
<table border="0" cellspacing="5" cellpadding="10" width="100%">
<tr><td bgcolor="#E2EDE2">
Sample configuration:<br>
<pre>
<a href="index.html#QS_UserTrackingCookieName">QS_UserTrackingCookieName</a> qstrack <a href="#cookiecheck">/errorpages/cookiecheck.html</a> session
<a href="index.html#QS_SessionKey">QS_SessionKey</a> sB.F4_0%D700ahXT2
</pre>
</td></tr>
</table>
</p>
<p>
<table border="0" cellspacing="5" cellpadding="10" width="100%">
<tr><td bgcolor="#E2EDE2"><a name="cookiecheck"></a>
Sample page:<br>
<pre>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Cache-Control" content="no-cache, no-store"/>
<meta http-equiv="expires" content="0" />
<title>Cookie Check Page</title>
</head>
<body>
<h1>Cookie Check Failed</h1>
<p>
Please enable cookies in your browser.<br>
Bitte schalten Sie in Ihrem Browser Cookies ein.<br>
Activez les cookies dans votre navigateur, s'il vous pla&icirc;t.<br>
Por favor active las cookies en su navegador.<br>
Si prega di attivare i cookies nel proprio browser.<br>
</p>
</body>
</html>
</pre>
</td></tr>
</table>
</p>
<!-- --------------------------------------------------------- -->
<hr>
<a name="qslog"></a>
<a name="RequestStatistics"></a>
<h2>Request Statistics Using <i>qslog</i></h2>
<p>
<code><a href="qslog.1.html">qslog</a></code> is a command line tool to
generate usage statistics data from log files. It can generate the data in
real time if defined within your Apache's server configuration.
<table border="0" cellspacing="5" cellpadding="10" width="100%">
<tr><td bgcolor="#E2EDE2">
Sample configuration:<br>
<pre>
CustomLog "|/usr/bin/qslog -o logs/qslog.csv -x -f ISBDk" "%h %>s %b %D %k"
</pre>
</td></tr>
</table>
</p><p>
Or you can use it to process any existing log file using the post processing
option <code>-p</code>. This does not only work for
<a href="http://httpd.apache.org/docs/current/mod/mod_log_config.html">Apache log <img src="images/link.png"/></a> files but for any other log file as well. You just have to know what's in the log
and configure the format string argument <code>-f</code> of the
<code><a href="qslog.1.html">qslog</a></code> command accordingly.
</p>
<p>
Example:<br>
<img src="images/qslogFormat.png" height="381" width="738" alt="log format definition"/>
</p>
<p>
The output shows the data as a function of time: a summary of what happened every minute.
Each line includes all measured values as semicolon spearated name/value pairs (CSV).
</p>
<p>
You can use the spreadsheet program of your choice to process the output:<br><br>
<img src="images/qslog_spreadsheet_example.png" height="413" width="850" alt="spreadsheet"/>
</p>
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</td></tr>
</tbody>
</table>
<br>
<hr>
<SMALL><SMALL>© 2023, Pascal Buchbinder</SMALL></SMALL>
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