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<manualpage metafile="filter.xml.meta">

  <title>Filters</title>

  <summary>
    <p>This document describes the use of filters in Apache.</p>
  </summary>

  <section id="intro">
    <title>Filtering in Apache 2</title>
    <related>
      <modulelist>
	<module>mod_filter</module>
        <module>mod_deflate</module>
        <module>mod_ext_filter</module>
        <module>mod_include</module>
	<module>mod_charset_lite</module>
      </modulelist>
      <directivelist>
        <directive module="mod_filter">FilterChain</directive>
        <directive module="mod_filter">FilterDeclare</directive>
        <directive module="mod_filter">FilterProtocol</directive>
        <directive module="mod_filter">FilterProvider</directive>
        <directive module="mod_mime">AddInputFilter</directive>
        <directive module="mod_mime">AddOutputFilter</directive>
        <directive module="mod_mime">RemoveInputFilter</directive>
        <directive module="mod_mime">RemoveOutputFilter</directive>
        <directive module="mod_ext_filter">ExtFilterDefine</directive>
        <directive module="mod_ext_filter">ExtFilterOptions</directive>
        <directive module="core">SetInputFilter</directive>
        <directive module="core">SetOutputFilter</directive>
      </directivelist>
    </related>

<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><module>mod_include</module>, implements server-side includes.</li>
<li><module>mod_ssl</module>, implements SSL encryption (https).</li>
<li><module>mod_deflate</module>, implements compression/decompression on the fly.</li>
<li><module>mod_charset_lite</module>, transcodes between different character sets.</li>
<li><module>mod_ext_filter</module>, 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
modules available from <a
href="http://modules.apache.org/">modules.apache.org</a> and
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>
</section>

<section id="smart">
<title>Smart Filtering</title>
<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><module>mod_filter</module>, 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
text/html or application/xhtml+xml</li>
<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>
</section>

<section id="using">
<title>Using Filters</title>
<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
    <directive module="core">SetInputFilter</directive>,
    <directive module="core">SetOutputFilter</directive>,
    <directive module="mod_mime">AddInputFilter</directive>,
    <directive module="mod_mime">AddOutputFilter</directive>,
    <directive module="mod_mime">RemoveInputFilter</directive>, and
    <directive module="mod_mime">RemoveOutputFilter</directive>.</p>

<p>The Dynamic Way enables both static and flexible, dynamic configuration
of output filters, as discussed in the <module>mod_filter</module> page.
Relevant directives are
    <directive module="mod_filter">FilterChain</directive>,
    <directive module="mod_filter">FilterDeclare</directive>, and
    <directive module="mod_filter">FilterProvider</directive>.</p>

<p>One further directive <directive
module="core">AddOutputFilterByType</directive> is still supported,
but may be problematic and is now deprecated.  Use dynamic
configuration instead.</p>

  </section>
</manualpage>
