docs/manual/filter.xml

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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>
        <module>mod_reflector</module>
        <module>mod_buffer</module>
        <module>mod_data</module>
        <module>mod_ratelimit</module>
        <module>mod_reqtimeout</module>
        <module>mod_request</module>
        <module>mod_sed</module>
        <module>mod_substitute</module>
        <module>mod_xml2enc</module>
        <module>mod_proxy_html</module>
        <module>mod_policy</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_reflector">ReflectorHeader</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="service">

<title>Exposing Filters as an HTTP Service</title>
<p>Filters can be used to process content originating from the client in
addition to processing content originating on the server using the
<module>mod_reflector</module> module.</p>

<p><module>mod_reflector</module> accepts POST requests from clients, and reflects
the content request body received within the POST request back in the response,
passing through the output filter stack on the way back to the client.</p>

<p>This technique can be used as an alternative to a web service running within
an application server stack, where an output filter provides the transformation
required on the request body. For example, the <module>mod_deflate</module>
module might be used to provide a general compression service, or an image
transformation filter might be turned into an image transformation service.</p>

</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="mod_filter">AddOutputFilterByType</directive> is still supported,
but deprecated.  Use dynamic configuration instead.</p>

  </section>
</manualpage>