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<title>Filters - Apache HTTP Server</title>
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<p class="menu"><a href="/mod/">Modules</a> | <a href="/mod/directives.html">Directives</a> | <a href="/faq/">FAQ</a> | <a href="/glossary.html">Glossary</a> | <a href="/sitemap.html">Sitemap</a></p>
<p class="apache">Apache HTTP Server Version 2.3</p>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="./">Version 2.3</a></div><div id="page-content"><div id="preamble"><h1>Filters</h1>
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<p>This document describes the use of filters in Apache.</p>
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<div id="quickview"><ul id="toc"><li><img alt="" src="/images/down.gif" /> <a href="#intro">Filtering in Apache 2</a></li>
</ul></div>
<div class="section">
<h2><a name="intro" id="intro">Filtering in Apache 2</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="/mod/mod_filter.html">mod_filter</a></code></li><li><code class="module"><a href="/mod/mod_deflate.html">mod_deflate</a></code></li><li><code class="module"><a href="/mod/mod_ext_filter.html">mod_ext_filter</a></code></li><li><code class="module"><a href="/mod/mod_include.html">mod_include</a></code></li><li><code class="module"><a href="/mod/mod_charset_lite.html">mod_charset_lite</a></code></li></ul></td><td><ul><li><code class="directive"><a href="/mod/mod_filter.html#filterchain">FilterChain</a></code></li><li><code class="directive"><a href="/mod/mod_filter.html#filterdeclare">FilterDeclare</a></code></li><li><code class="directive"><a href="/mod/mod_filter.html#filterprotocol">FilterProtocol</a></code></li><li><code class="directive"><a href="/mod/mod_filter.html#filterprovider">FilterProvider</a></code></li><li><code class="directive"><a href="/mod/mod_mime.html#addinputfilter">AddInputFilter</a></code></li><li><code class="directive"><a href="/mod/mod_mime.html#addoutputfilter">AddOutputFilter</a></code></li><li><code class="directive"><a href="/mod/mod_mime.html#removeinputfilter">RemoveInputFilter</a></code></li><li><code class="directive"><a href="/mod/mod_mime.html#removeoutputfilter">RemoveOutputFilter</a></code></li><li><code class="directive"><a href="/mod/mod_ext_filter.html#extfilterdefine">ExtFilterDefine</a></code></li><li><code class="directive"><a href="/mod/mod_ext_filter.html#extfilteroptions">ExtFilterOptions</a></code></li><li><code class="directive"><a href="/mod/core.html#setinputfilter">SetInputFilter</a></code></li><li><code class="directive"><a href="/mod/core.html#setoutputfilter">SetOutputFilter</a></code></li></ul></td></tr></table>
<p>The Filter Chain is available in Apache 2.0 and higher,
and enables applications to process incoming and outgoing data
in a highly flexible and configurable manner, regardless of
where the data comes from. We can pre-process incoming data,
and post-process outgoing data, at will. This is basically
independent of the traditional request processing phases.</p>
<p class="figure">
<img src="images/filter_arch.png" width="569" height="392" alt="Filters can be chained, in a Data Axis orthogonal to request processing" />
</p>
<p>Some examples of filtering in the standard Apache distribution are:</p>
<ul>
<li><code class="module"><a href="/mod/mod_include.html">mod_include</a></code>, implements server-side includes.</li>
<li><code class="module"><a href="/mod/mod_ssl.html">mod_ssl</a></code>, implements SSL encryption (https).</li>
<li><code class="module"><a href="/mod/mod_deflate.html">mod_deflate</a></code>, implements compression/decompression on the fly.</li>
<li><code class="module"><a href="/mod/mod_charset_lite.html">mod_charset_lite</a></code>, transcodes between different character sets.</li>
<li><code class="module"><a href="/mod/mod_ext_filter.html">mod_ext_filter</a></code>, runs an external program as a filter.</li>
</ul>
<p>Apache also uses a number of filters internally to perform
functions like chunking and byte-range handling.</p>
<p>A wider range of applications are implemented by third-party filter
elsewhere. A few of these are:</p>
<ul>
<li>HTML and XML processing and rewriting</li>
<li>XSLT transforms and XIncludes</li>
<li>XML Namespace support</li>
<li>File Upload handling and decoding of HTML Forms</li>
<li>Image processing</li>
<li>Protection of vulnerable applications such as PHP scripts</li>
<li>Text search-and-replace editing</li>
</ul>
<div class="section">
<h2><a name="smart" id="smart">Smart Filtering</a></h2>
<p class="figure">
<img src="images/mod_filter_new.png" width="423" height="331" alt="Smart filtering applies different filter providers according to the state of request processing" />
</p>
<p><code class="module"><a href="/mod/mod_filter.html">mod_filter</a></code>, included in Apache 2.1 and later,
enables the filter chain to be configured dynamically at run time.
So for example you can set up a proxy to rewrite
HTML with an HTML filter and JPEG images with a completely
separate filter, despite the proxy having no prior information
about what the origin server will send. This works by using a
filter harness, that dispatches to different providers according
to the actual contents at runtime. Any filter may be either
inserted directly in the chain and run unconditionally, or
used as a provider and inserted dynamically. For example,</p>
<ul>
<li>an HTML processing filter will only run if the content is
<li>A compression filter will only run if the input is a
compressible type and not already compressed</li>
<li>A charset conversion filter will be inserted if a text
document is not already in the desired charset</li>
</ul>
<div class="section">
<h2><a name="using" id="using">Using Filters</a></h2>
<p>There are two ways to use filtering: Simple and Dynamic.
In general, you should use one or the other; mixing them can
have unexpected consequences (although simple Input filtering
can be mixed freely with either simple or dynamic Output filtering).</p>
<p>The Simple Way is the only way to configure input filters, and is
sufficient for output filters where you need a static filter chain.
Relevant directives are
<code class="directive"><a href="/mod/mod_mime.html#removeinputfilter">RemoveInputFilter</a></code>, and
<code class="directive"><a href="/mod/mod_mime.html#removeoutputfilter">RemoveOutputFilter</a></code>.</p>
<p>The Dynamic Way enables both static and flexible, dynamic configuration
of output filters, as discussed in the <code class="module"><a href="/mod/mod_filter.html">mod_filter</a></code> page.
Relevant directives are
<code class="directive"><a href="/mod/mod_filter.html#filterprovider">FilterProvider</a></code>.</p>
<p>One further directive <code class="directive"><a href="/mod/core.html#addoutputfilterbytype">AddOutputFilterByType</a></code> is still supported,
but may be problematic and is now deprecated. Use dynamic
configuration instead.</p>
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