2.4.x/docs/manual/mod/mod_filter.xml - httpd - Git at Google

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 <modulesynopsis metafile="mod_filter.xml.meta">

 <name>mod_filter</name>
 <description>Context-sensitive smart filter configuration module</description>
 <status>Base</status>
 <sourcefile>mod_filter.c</sourcefile>
 <identifier>filter_module</identifier>
 <compatibility>Version 2.1 and later</compatibility>

 <summary>
     <p>This module enables smart, context-sensitive configuration of
     output content filters.  For example, apache can be configured to
     process different content-types through different filters, even
     when the content-type is not known in advance (e.g. in a proxy).</p>

     <p><module>mod_filter</module> works by introducing indirection into
     the filter chain.  Instead of inserting filters in the chain, we insert
     a filter harness which in turn dispatches conditionally
     to a filter provider.  Any content filter may be used as a provider
     to <module>mod_filter</module>; no change to existing filter modules is
     required (although it may be possible to simplify them).</p>
 </summary>

 <section id="smart"><title>Smart Filtering</title>
     <p>In the traditional filtering model, filters are inserted unconditionally
     using <directive module="mod_mime">AddOutputFilter</directive> and family.
     Each filter then needs to determine whether to run, and there is little
     flexibility available for server admins to allow the chain to be
     configured dynamically.</p>

     <p><module>mod_filter</module> by contrast gives server administrators a
     great deal of flexibility in configuring the filter chain.  In fact,
     filters can be inserted based on complex boolean
     <a href="../expr.html">expressions</a> This generalises the limited
     flexibility offered by <directive>AddOutputFilterByType</directive>.</p>
 </section>

 <section id="terms"><title>Filter Declarations, Providers and Chains</title>
     <p class="figure">
     <img src="../images/mod_filter_old.gif" width="160" height="310"
     alt="[This image displays the traditional filter model]"/><br />
     <dfn>Figure 1:</dfn> The traditional filter model</p>

     <p>In the traditional model, output filters are a simple chain
     from the content generator (handler) to the client.  This works well
     provided the filter chain can be correctly configured, but presents
     problems when the filters need to be configured dynamically based on
     the outcome of the handler.</p>

     <p class="figure">
     <img src="../images/mod_filter_new.gif" width="423" height="331"
     alt="[This image shows the mod_filter model]"/><br />
     <dfn>Figure 2:</dfn> The <module>mod_filter</module> model</p>

     <p><module>mod_filter</module> works by introducing indirection into
     the filter chain.  Instead of inserting filters in the chain, we insert
     a filter harness which in turn dispatches conditionally
     to a filter provider.  Any content filter may be used as a provider
     to <module>mod_filter</module>; no change to existing filter modules
     is required (although it may be possible to simplify them).  There can be
     multiple providers for one filter, but no more than one provider will
     run for any single request.</p>

     <p>A filter chain comprises any number of instances of the filter
     harness, each of which may have any number of providers.  A special
     case is that of a single provider with unconditional dispatch: this
     is equivalent to inserting the provider filter directly into the chain.</p>
 </section>

 <section id="config"><title>Configuring the Chain</title>
     <p>There are three stages to configuring a filter chain with
     <module>mod_filter</module>. For details of the directives, see below.</p>

     <dl>
     <dt>Declare Filters</dt>
     <dd>The <directive module="mod_filter">FilterDeclare</directive> directive
     declares a filter, assigning it a name and filter type.  Required
     only if the filter is not the default type AP_FTYPE_RESOURCE.</dd>

     <dt>Register Providers</dt>
     <dd>The <directive module="mod_filter">FilterProvider</directive>
     directive registers a provider with a filter. The filter may have
     been declared with <directive module="mod_filter"
     >FilterDeclare</directive>; if not, FilterProvider will implicitly
     declare it with the default type AP_FTYPE_RESOURCE. The provider
     must have been
     registered with <code>ap_register_output_filter</code> by some module.
     The final argument to <directive module="mod_filter"
     >FilterProvider</directive> is an expression: the provider will be
     selected to run for a request if and only if the expression evaluates
     to true.  The expression may evaluate HTTP request or response
     headers, environment variables, or the Handler used by this request.
     Unlike earlier versions, mod_filter now supports complex expressions
     involving multiple criteria with AND / OR logic (&amp;&amp; / ||)
     and brackets. The details of the expression syntax are described in
     the <a href="../expr.html">ap_expr documentation</a>.</dd>

     <dt>Configure the Chain</dt>
     <dd>The above directives build components of a smart filter chain,
     but do not configure it to run.  The <directive module="mod_filter"
     >FilterChain</directive> directive builds a filter chain from smart
     filters declared, offering the flexibility to insert filters at the
     beginning or end of the chain, remove a filter, or clear the chain.</dd>
 </dl>
 </section>
 <section id="errordocs"><title>Filtering and Response Status</title>
     <p>mod_filter normally only runs filters on responses with
     HTTP status 200 (OK).  If you want to filter documents with
     other response statuses, you can set the <var>filter-errordocs</var>
     environment variable, and it will work on all responses
     regardless of status.  To refine this further, you can use
     expression conditions with <directive>FilterProvider</directive>.</p>
 </section>
 <section id="upgrade"><title>Upgrading from Apache HTTP Server 2.2 Configuration</title>
     <p>The <directive module="mod_filter">FilterProvider</directive>
     directive has changed from httpd 2.2: the <var>match</var> and
     <var>dispatch</var> arguments are replaced with a single but
     more versatile <var>expression</var>.  In general, you can convert
     a match/dispatch pair to the two sides of an expression, using
     something like:</p>
     <example>"dispatch = 'match'"</example>
     <p>The Request headers, Response headers and Environment variables
     are now interpreted from syntax <var>%{req:foo}</var>,
     <var>%{resp:foo}</var> and <var>%{env:foo}</var> respectively.
     The variables <var>%{HANDLER}</var> and <var>%{CONTENT_TYPE}</var>
     are also supported.</p>
     <p>Note that the match no longer support substring matches.  They can be
     replaced by regular expression matches.</p>
 </section>

 <section id="examples"><title>Examples</title>
     <dl>
     <dt>Server side Includes (SSI)</dt>
     <dd>A simple case of replacing <directive>AddOutputFilterByType</directive>
     <highlight language="config">
 FilterDeclare SSI
 FilterProvider SSI INCLUDES "%{CONTENT_TYPE} =~ m|^text/html|"
 FilterChain SSI
     </highlight>
     </dd>

     <dt>Server side Includes (SSI)</dt>
     <dd>The same as the above but dispatching on handler (classic
     SSI behaviour; .shtml files get processed).
     <highlight language="config">
 FilterProvider SSI INCLUDES "%{HANDLER} = 'server-parsed'"
 FilterChain SSI
     </highlight>
     </dd>

     <dt>Emulating mod_gzip with mod_deflate</dt>
     <dd>Insert INFLATE filter only if "gzip" is NOT in the
     Accept-Encoding header.  This filter runs with ftype CONTENT_SET.
     <highlight language="config">
 FilterDeclare gzip CONTENT_SET
 FilterProvider gzip inflate "%{req:Accept-Encoding} !~ /gzip/"
 FilterChain gzip
     </highlight>
     </dd>

     <dt>Image Downsampling</dt>
     <dd>Suppose we want to downsample all web images, and have filters
     for GIF, JPEG and PNG.
     <highlight language="config">
 FilterProvider unpack jpeg_unpack "%{CONTENT_TYPE} = 'image/jpeg'"
 FilterProvider unpack gif_unpack "%{CONTENT_TYPE} = 'image/gif'"
 FilterProvider unpack png_unpack "%{CONTENT_TYPE} = 'image/png'"

 FilterProvider downsample downsample_filter "%{CONTENT_TYPE} = m|^image/(jpeg|gif|png)|"
 FilterProtocol downsample "change=yes"

 FilterProvider repack jpeg_pack "%{CONTENT_TYPE} = 'image/jpeg'"
 FilterProvider repack gif_pack "%{CONTENT_TYPE} = 'image/gif'"
 FilterProvider repack png_pack "%{CONTENT_TYPE} = 'image/png'"
 &lt;Location "/image-filter"&gt;
     FilterChain unpack downsample repack
 &lt;/Location&gt;
     </highlight>
     </dd>
     </dl>
 </section>

 <section id="protocol"><title>Protocol Handling</title>
     <p>Historically, each filter is responsible for ensuring that whatever
     changes it makes are correctly represented in the HTTP response headers,
     and that it does not run when it would make an illegal change.  This
     imposes a burden on filter authors to re-implement some common
     functionality in every filter:</p>

     <ul>
     <li>Many filters will change the content, invalidating existing content
     tags, checksums, hashes, and lengths.</li>

     <li>Filters that require an entire, unbroken response in input need to
     ensure they don't get byteranges from a backend.</li>

     <li>Filters that transform output in a filter need to ensure they don't
     violate a <code>Cache-Control: no-transform</code> header from the
     backend.</li>

     <li>Filters may make responses uncacheable.</li>
     </ul>

     <p><module>mod_filter</module> aims to offer generic handling of these
     details of filter implementation, reducing the complexity required of
     content filter modules. This is work-in-progress; the
     <directive module="mod_filter">FilterProtocol</directive> implements
     some of this functionality for back-compatibility with Apache 2.0
     modules.  For httpd 2.1 and later, the
     <code>ap_register_output_filter_protocol</code> and
     <code>ap_filter_protocol</code> API enables filter modules to
     declare their own behaviour.</p>

     <p>At the same time, <module>mod_filter</module> should not interfere
     with a filter that wants to handle all aspects of the protocol.  By
     default (i.e. in the absence of any <directive module="mod_filter"
     >FilterProtocol</directive> directives), <module>mod_filter</module>
     will leave the headers untouched.</p>

     <p>At the time of writing, this feature is largely untested,
     as modules in common use are designed to work with 2.0.
     Modules using it should test it carefully.</p>
 </section>

 <directivesynopsis>
 <name>AddOutputFilterByType</name>
 <description>assigns an output filter to a particular media-type</description>
 <syntax>AddOutputFilterByType <var>filter</var>[;<var>filter</var>...]
 <var>media-type</var> [<var>media-type</var>] ...</syntax>
 <contextlist><context>server config</context>
 <context>virtual host</context><context>directory</context>
 <context>.htaccess</context></contextlist>
 <override>FileInfo</override>
 <compatibility>Had severe limitations before
 being moved to <module>mod_filter</module> in version 2.3.7</compatibility>

 <usage>
     <p>This directive activates a particular output <a
     href="../filter.html">filter</a> for a request depending on the
     response <glossary>media-type</glossary>.</p>

     <p>The following example uses the <code>DEFLATE</code> filter, which
     is provided by <module>mod_deflate</module>. It will compress all
     output (either static or dynamic) which is labeled as
     <code>text/html</code> or <code>text/plain</code> before it is sent
     to the client.</p>

     <highlight language="config">
       AddOutputFilterByType DEFLATE text/html text/plain
     </highlight>

     <p>If you want the content to be processed by more than one filter, their
     names have to be separated by semicolons. It's also possible to use one
     <directive>AddOutputFilterByType</directive> directive for each of
     these filters.</p>

     <p>The configuration below causes all script output labeled as
     <code>text/html</code> to be processed at first by the
     <code>INCLUDES</code> filter and then by the <code>DEFLATE</code>
     filter.</p>

     <highlight language="config">
 &lt;Location "/cgi-bin/"&gt;
     Options Includes
     AddOutputFilterByType INCLUDES;DEFLATE text/html
 &lt;/Location&gt;
     </highlight>

 </usage>

 <seealso><directive module="mod_mime">AddOutputFilter</directive></seealso>
 <seealso><directive module="core">SetOutputFilter</directive></seealso>
 <seealso><a href="../filter.html">filters</a></seealso>
 </directivesynopsis>

 <directivesynopsis>
 <name>FilterDeclare</name>
 <description>Declare a smart filter</description>
 <syntax>FilterDeclare <var>filter-name</var> <var>[type]</var></syntax>
 <contextlist><context>server config</context><context>virtual host</context>
 <context>directory</context><context>.htaccess</context></contextlist>
 <override>Options</override>

 <usage>
     <p>This directive declares an output filter together with a
     header or environment variable that will determine runtime
     configuration.  The first argument is a <var>filter-name</var>
     for use in <directive module="mod_filter">FilterProvider</directive>,
     <directive module="mod_filter">FilterChain</directive> and
     <directive module="mod_filter">FilterProtocol</directive> directives.</p>

     <p>The final (optional) argument
     is the type of filter, and takes values of <code>ap_filter_type</code>
     - namely <code>RESOURCE</code> (the default), <code>CONTENT_SET</code>,
     <code>PROTOCOL</code>, <code>TRANSCODE</code>, <code>CONNECTION</code>
     or <code>NETWORK</code>.</p>
 </usage>
 </directivesynopsis>

 <directivesynopsis>
 <name>FilterProvider</name>
 <description>Register a content filter</description>
 <syntax>FilterProvider <var>filter-name</var> <var>provider-name</var>
  <var>expression</var></syntax>
 <contextlist><context>server config</context><context>virtual host</context>
 <context>directory</context><context>.htaccess</context></contextlist>
 <override>Options</override>

 <usage>
     <p>This directive registers a <em>provider</em> for the smart filter.
     The provider will be called if and only if the <var>expression</var>
     declared evaluates to true when the harness is first called.</p>

     <p>
     <var>provider-name</var> must have been registered by loading
     a module that registers the name with
     <code>ap_register_output_filter</code>.
     </p>

     <p><var>expression</var> is an
     <a href="../expr.html">ap_expr</a>.</p>

 </usage>
 <seealso><a href="../expr.html">Expressions in Apache HTTP Server</a>,
 for a complete reference and examples.</seealso>
 <seealso><module>mod_include</module></seealso>
 </directivesynopsis>

 <directivesynopsis>
 <name>FilterChain</name>
 <description>Configure the filter chain</description>
 <syntax>FilterChain [+=-@!]<var>filter-name</var> <var>...</var></syntax>
 <contextlist><context>server config</context><context>virtual host</context>
 <context>directory</context><context>.htaccess</context></contextlist>
 <override>Options</override>

 <usage>
     <p>This configures an actual filter chain, from declared filters.
     <directive>FilterChain</directive> takes any number of arguments,
     each optionally preceded with a single-character control that
     determines what to do:</p>

     <dl>
     <dt><code>+<var>filter-name</var></code></dt>
     <dd>Add <var>filter-name</var> to the end of the filter chain</dd>

     <dt><code>@<var>filter-name</var></code></dt>
     <dd>Insert <var>filter-name</var> at the start of the filter chain</dd>

     <dt><code>-<var>filter-name</var></code></dt>
     <dd>Remove <var>filter-name</var> from the filter chain</dd>

     <dt><code>=<var>filter-name</var></code></dt>
     <dd>Empty the filter chain and insert <var>filter-name</var></dd>

     <dt><code>!</code></dt>
     <dd>Empty the filter chain</dd>

     <dt><code><var>filter-name</var></code></dt>
     <dd>Equivalent to <code>+<var>filter-name</var></code></dd>
     </dl>
 </usage>
 </directivesynopsis>

 <directivesynopsis>
 <name>FilterProtocol</name>
 <description>Deal with correct HTTP protocol handling</description>
 <syntax>FilterProtocol <var>filter-name</var> [<var>provider-name</var>]
     <var>proto-flags</var></syntax>
 <contextlist><context>server config</context><context>virtual host</context>
 <context>directory</context><context>.htaccess</context></contextlist>
 <override>Options</override>

 <usage>
     <p>This directs <module>mod_filter</module> to deal with ensuring the
     filter doesn't run when it shouldn't, and that the HTTP response
     headers are correctly set taking into account the effects of the
     filter.</p>

     <p>There are two forms of this directive.  With three arguments, it
     applies specifically to a <var>filter-name</var> and a
     <var>provider-name</var> for that filter.
     With two arguments it applies to a <var>filter-name</var> whenever the
     filter runs <em>any</em> provider.</p>

     <p>Flags specified with this directive are merged with the flags
     that underlying providers may have registerd with
     <module>mod_filter</module>. For example, a filter may internally specify
     the equivalent of <code>change=yes</code>, but a particular
     configuration of the module can override with <code>change=no</code>.
     </p>

     <p><var>proto-flags</var> is one or more of</p>

     <dl>
     <dt><code>change=yes|no</code></dt>
     <dd>Specifies whether the filter changes the content, including possibly
     the content length.  The "no" argument is supported in 2.4.7 and later.</dd>

     <dt><code>change=1:1</code></dt>
     <dd>The filter changes the content, but will not change the content
     length</dd>

     <dt><code>byteranges=no</code></dt>
     <dd>The filter cannot work on byteranges and requires complete input</dd>

     <dt><code>proxy=no</code></dt>
     <dd>The filter should not run in a proxy context</dd>

     <dt><code>proxy=transform</code></dt>
     <dd>The filter transforms the response in a manner incompatible with
     the HTTP <code>Cache-Control: no-transform</code> header.</dd>

     <dt><code>cache=no</code></dt>
     <dd>The filter renders the output uncacheable (eg by introducing randomised
     content changes)</dd>
     </dl>
 </usage>
 </directivesynopsis>

 <directivesynopsis>
 <name>FilterTrace</name>
 <description>Get debug/diagnostic information from
     <module>mod_filter</module></description>
 <syntax>FilterTrace <var>filter-name</var> <var>level</var></syntax>
 <contextlist><context>server config</context><context>virtual host</context>
 <context>directory</context></contextlist>

 <usage>
     <p>This directive generates debug information from
     <module>mod_filter</module>.
     It is designed to help test and debug providers (filter modules), although
     it may also help with <module>mod_filter</module> itself.</p>

     <p>The debug output depends on the <var>level</var> set:</p>
     <dl>
     <dt><code>0</code> (default)</dt>
     <dd>No debug information is generated.</dd>

     <dt><code>1</code></dt>
     <dd><module>mod_filter</module> will record buckets and brigades
     passing through the filter to the error log, before the provider has
     processed them. This is similar to the information generated by
     <a href="http://apache.webthing.com/mod_diagnostics/">mod_diagnostics</a>.
     </dd>

     <dt><code>2</code> (not yet implemented)</dt>
     <dd>Will dump the full data passing through to a tempfile before the
     provider. <strong>For single-user debug only</strong>; this will not
     support concurrent hits.</dd>
     </dl>
 </usage>
 </directivesynopsis>

 </modulesynopsis>
	<?xml version="1.0"?>
	<!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd">
	<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
	<!-- $LastChangedRevision$ -->

	<!--
	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
	(the "License"); you may not use this file except in compliance with
	the License. You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
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	<modulesynopsis metafile="mod_filter.xml.meta">

	<name>mod_filter</name>
	<description>Context-sensitive smart filter configuration module</description>
	<status>Base</status>
	<sourcefile>mod_filter.c</sourcefile>
	<identifier>filter_module</identifier>
	<compatibility>Version 2.1 and later</compatibility>

	<summary>
	<p>This module enables smart, context-sensitive configuration of
	output content filters. For example, apache can be configured to
	process different content-types through different filters, even
	when the content-type is not known in advance (e.g. in a proxy).</p>

	<p><module>mod_filter</module> works by introducing indirection into
	the filter chain. Instead of inserting filters in the chain, we insert
	a filter harness which in turn dispatches conditionally
	to a filter provider. Any content filter may be used as a provider
	to <module>mod_filter</module>; no change to existing filter modules is
	required (although it may be possible to simplify them).</p>
	</summary>

	<section id="smart"><title>Smart Filtering</title>
	<p>In the traditional filtering model, filters are inserted unconditionally
	using <directive module="mod_mime">AddOutputFilter</directive> and family.
	Each filter then needs to determine whether to run, and there is little
	flexibility available for server admins to allow the chain to be
	configured dynamically.</p>

	<p><module>mod_filter</module> by contrast gives server administrators a
	great deal of flexibility in configuring the filter chain. In fact,
	filters can be inserted based on complex boolean
	<a href="../expr.html">expressions</a> This generalises the limited
	flexibility offered by <directive>AddOutputFilterByType</directive>.</p>
	</section>

	<section id="terms"><title>Filter Declarations, Providers and Chains</title>
	<p class="figure">
	<img src="../images/mod_filter_old.gif" width="160" height="310"
	alt="[This image displays the traditional filter model]"/><br />
	<dfn>Figure 1:</dfn> The traditional filter model</p>

	<p>In the traditional model, output filters are a simple chain
	from the content generator (handler) to the client. This works well
	provided the filter chain can be correctly configured, but presents
	problems when the filters need to be configured dynamically based on
	the outcome of the handler.</p>

	<p class="figure">
	<img src="../images/mod_filter_new.gif" width="423" height="331"
	alt="[This image shows the mod_filter model]"/><br />
	<dfn>Figure 2:</dfn> The <module>mod_filter</module> model</p>

	<p><module>mod_filter</module> works by introducing indirection into
	the filter chain. Instead of inserting filters in the chain, we insert
	a filter harness which in turn dispatches conditionally
	to a filter provider. Any content filter may be used as a provider
	to <module>mod_filter</module>; no change to existing filter modules
	is required (although it may be possible to simplify them). There can be
	multiple providers for one filter, but no more than one provider will
	run for any single request.</p>

	<p>A filter chain comprises any number of instances of the filter
	harness, each of which may have any number of providers. A special
	case is that of a single provider with unconditional dispatch: this
	is equivalent to inserting the provider filter directly into the chain.</p>
	</section>

	<section id="config"><title>Configuring the Chain</title>
	<p>There are three stages to configuring a filter chain with
	<module>mod_filter</module>. For details of the directives, see below.</p>

	<dl>
	<dt>Declare Filters</dt>
	<dd>The <directive module="mod_filter">FilterDeclare</directive> directive
	declares a filter, assigning it a name and filter type. Required
	only if the filter is not the default type AP_FTYPE_RESOURCE.</dd>

	<dt>Register Providers</dt>
	<dd>The <directive module="mod_filter">FilterProvider</directive>
	directive registers a provider with a filter. The filter may have
	been declared with <directive module="mod_filter"
	>FilterDeclare</directive>; if not, FilterProvider will implicitly
	declare it with the default type AP_FTYPE_RESOURCE. The provider
	must have been
	registered with <code>ap_register_output_filter</code> by some module.
	The final argument to <directive module="mod_filter"
	>FilterProvider</directive> is an expression: the provider will be
	selected to run for a request if and only if the expression evaluates
	to true. The expression may evaluate HTTP request or response
	headers, environment variables, or the Handler used by this request.
	Unlike earlier versions, mod_filter now supports complex expressions
	involving multiple criteria with AND / OR logic (&& / \|\|)
	and brackets. The details of the expression syntax are described in
	the <a href="../expr.html">ap_expr documentation</a>.</dd>

	<dt>Configure the Chain</dt>
	<dd>The above directives build components of a smart filter chain,
	but do not configure it to run. The <directive module="mod_filter"
	>FilterChain</directive> directive builds a filter chain from smart
	filters declared, offering the flexibility to insert filters at the
	beginning or end of the chain, remove a filter, or clear the chain.</dd>
	</dl>
	</section>
	<section id="errordocs"><title>Filtering and Response Status</title>
	<p>mod_filter normally only runs filters on responses with
	HTTP status 200 (OK). If you want to filter documents with
	other response statuses, you can set the <var>filter-errordocs</var>
	environment variable, and it will work on all responses
	regardless of status. To refine this further, you can use
	expression conditions with <directive>FilterProvider</directive>.</p>
	</section>
	<section id="upgrade"><title>Upgrading from Apache HTTP Server 2.2 Configuration</title>
	<p>The <directive module="mod_filter">FilterProvider</directive>
	directive has changed from httpd 2.2: the <var>match</var> and
	<var>dispatch</var> arguments are replaced with a single but
	more versatile <var>expression</var>. In general, you can convert
	a match/dispatch pair to the two sides of an expression, using
	something like:</p>
	<example>"dispatch = 'match'"</example>
	<p>The Request headers, Response headers and Environment variables
	are now interpreted from syntax <var>%{req:foo}</var>,
	<var>%{resp:foo}</var> and <var>%{env:foo}</var> respectively.
	The variables <var>%{HANDLER}</var> and <var>%{CONTENT_TYPE}</var>
	are also supported.</p>
	<p>Note that the match no longer support substring matches. They can be
	replaced by regular expression matches.</p>
	</section>

	<section id="examples"><title>Examples</title>
	<dl>
	<dt>Server side Includes (SSI)</dt>
	<dd>A simple case of replacing <directive>AddOutputFilterByType</directive>
	<highlight language="config">
	FilterDeclare SSI
	FilterProvider SSI INCLUDES "%{CONTENT_TYPE} =~ m\|^text/html\|"
	FilterChain SSI
	</highlight>
	</dd>

	<dt>Server side Includes (SSI)</dt>
	<dd>The same as the above but dispatching on handler (classic
	SSI behaviour; .shtml files get processed).
	<highlight language="config">
	FilterProvider SSI INCLUDES "%{HANDLER} = 'server-parsed'"
	FilterChain SSI
	</highlight>
	</dd>

	<dt>Emulating mod_gzip with mod_deflate</dt>
	<dd>Insert INFLATE filter only if "gzip" is NOT in the
	Accept-Encoding header. This filter runs with ftype CONTENT_SET.
	<highlight language="config">
	FilterDeclare gzip CONTENT_SET
	FilterProvider gzip inflate "%{req:Accept-Encoding} !~ /gzip/"
	FilterChain gzip
	</highlight>
	</dd>

	<dt>Image Downsampling</dt>
	<dd>Suppose we want to downsample all web images, and have filters
	for GIF, JPEG and PNG.
	<highlight language="config">
	FilterProvider unpack jpeg_unpack "%{CONTENT_TYPE} = 'image/jpeg'"
	FilterProvider unpack gif_unpack "%{CONTENT_TYPE} = 'image/gif'"
	FilterProvider unpack png_unpack "%{CONTENT_TYPE} = 'image/png'"

	FilterProvider downsample downsample_filter "%{CONTENT_TYPE} = m\|^image/(jpeg\|gif\|png)\|"
	FilterProtocol downsample "change=yes"

	FilterProvider repack jpeg_pack "%{CONTENT_TYPE} = 'image/jpeg'"
	FilterProvider repack gif_pack "%{CONTENT_TYPE} = 'image/gif'"
	FilterProvider repack png_pack "%{CONTENT_TYPE} = 'image/png'"
	<Location "/image-filter">
	FilterChain unpack downsample repack
	</Location>
	</highlight>
	</dd>
	</dl>
	</section>

	<section id="protocol"><title>Protocol Handling</title>
	<p>Historically, each filter is responsible for ensuring that whatever
	changes it makes are correctly represented in the HTTP response headers,
	and that it does not run when it would make an illegal change. This
	imposes a burden on filter authors to re-implement some common
	functionality in every filter:</p>

	<ul>
	<li>Many filters will change the content, invalidating existing content
	tags, checksums, hashes, and lengths.</li>

	<li>Filters that require an entire, unbroken response in input need to
	ensure they don't get byteranges from a backend.</li>

	<li>Filters that transform output in a filter need to ensure they don't
	violate a <code>Cache-Control: no-transform</code> header from the
	backend.</li>

	<li>Filters may make responses uncacheable.</li>
	</ul>

	<p><module>mod_filter</module> aims to offer generic handling of these
	details of filter implementation, reducing the complexity required of
	content filter modules. This is work-in-progress; the
	<directive module="mod_filter">FilterProtocol</directive> implements
	some of this functionality for back-compatibility with Apache 2.0
	modules. For httpd 2.1 and later, the
	<code>ap_register_output_filter_protocol</code> and
	<code>ap_filter_protocol</code> API enables filter modules to
	declare their own behaviour.</p>

	<p>At the same time, <module>mod_filter</module> should not interfere
	with a filter that wants to handle all aspects of the protocol. By
	default (i.e. in the absence of any <directive module="mod_filter"
	>FilterProtocol</directive> directives), <module>mod_filter</module>
	will leave the headers untouched.</p>

	<p>At the time of writing, this feature is largely untested,
	as modules in common use are designed to work with 2.0.
	Modules using it should test it carefully.</p>
	</section>

	<directivesynopsis>
	<name>AddOutputFilterByType</name>
	<description>assigns an output filter to a particular media-type</description>
	<syntax>AddOutputFilterByType <var>filter</var>[;<var>filter</var>...]
	<var>media-type</var> [<var>media-type</var>] ...</syntax>
	<contextlist><context>server config</context>
	<context>virtual host</context><context>directory</context>
	<context>.htaccess</context></contextlist>
	<override>FileInfo</override>
	<compatibility>Had severe limitations before
	being moved to <module>mod_filter</module> in version 2.3.7</compatibility>

	<usage>
	<p>This directive activates a particular output <a
	href="../filter.html">filter</a> for a request depending on the
	response <glossary>media-type</glossary>.</p>

	<p>The following example uses the <code>DEFLATE</code> filter, which
	is provided by <module>mod_deflate</module>. It will compress all
	output (either static or dynamic) which is labeled as
	<code>text/html</code> or <code>text/plain</code> before it is sent
	to the client.</p>

	<highlight language="config">
	AddOutputFilterByType DEFLATE text/html text/plain
	</highlight>

	<p>If you want the content to be processed by more than one filter, their
	names have to be separated by semicolons. It's also possible to use one
	<directive>AddOutputFilterByType</directive> directive for each of
	these filters.</p>

	<p>The configuration below causes all script output labeled as
	<code>text/html</code> to be processed at first by the
	<code>INCLUDES</code> filter and then by the <code>DEFLATE</code>
	filter.</p>

	<highlight language="config">
	<Location "/cgi-bin/">
	Options Includes
	AddOutputFilterByType INCLUDES;DEFLATE text/html
	</Location>
	</highlight>

	</usage>

	<seealso><directive module="mod_mime">AddOutputFilter</directive></seealso>
	<seealso><directive module="core">SetOutputFilter</directive></seealso>
	<seealso><a href="../filter.html">filters</a></seealso>
	</directivesynopsis>

	<directivesynopsis>
	<name>FilterDeclare</name>
	<description>Declare a smart filter</description>
	<syntax>FilterDeclare <var>filter-name</var> <var>[type]</var></syntax>
	<contextlist><context>server config</context><context>virtual host</context>
	<context>directory</context><context>.htaccess</context></contextlist>
	<override>Options</override>

	<usage>
	<p>This directive declares an output filter together with a
	header or environment variable that will determine runtime
	configuration. The first argument is a <var>filter-name</var>
	for use in <directive module="mod_filter">FilterProvider</directive>,
	<directive module="mod_filter">FilterChain</directive> and
	<directive module="mod_filter">FilterProtocol</directive> directives.</p>

	<p>The final (optional) argument
	is the type of filter, and takes values of <code>ap_filter_type</code>
	- namely <code>RESOURCE</code> (the default), <code>CONTENT_SET</code>,
	<code>PROTOCOL</code>, <code>TRANSCODE</code>, <code>CONNECTION</code>
	or <code>NETWORK</code>.</p>
	</usage>
	</directivesynopsis>

	<directivesynopsis>
	<name>FilterProvider</name>
	<description>Register a content filter</description>
	<syntax>FilterProvider <var>filter-name</var> <var>provider-name</var>
	<var>expression</var></syntax>
	<contextlist><context>server config</context><context>virtual host</context>
	<context>directory</context><context>.htaccess</context></contextlist>
	<override>Options</override>

	<usage>
	<p>This directive registers a <em>provider</em> for the smart filter.
	The provider will be called if and only if the <var>expression</var>
	declared evaluates to true when the harness is first called.</p>

	<p>
	<var>provider-name</var> must have been registered by loading
	a module that registers the name with
	<code>ap_register_output_filter</code>.
	</p>

	<p><var>expression</var> is an
	<a href="../expr.html">ap_expr</a>.</p>

	</usage>
	<seealso><a href="../expr.html">Expressions in Apache HTTP Server</a>,
	for a complete reference and examples.</seealso>
	<seealso><module>mod_include</module></seealso>
	</directivesynopsis>

	<directivesynopsis>
	<name>FilterChain</name>
	<description>Configure the filter chain</description>
	<syntax>FilterChain [+=-@!]<var>filter-name</var> <var>...</var></syntax>
	<contextlist><context>server config</context><context>virtual host</context>
	<context>directory</context><context>.htaccess</context></contextlist>
	<override>Options</override>

	<usage>
	<p>This configures an actual filter chain, from declared filters.
	<directive>FilterChain</directive> takes any number of arguments,
	each optionally preceded with a single-character control that
	determines what to do:</p>

	<dl>
	<dt><code>+<var>filter-name</var></code></dt>
	<dd>Add <var>filter-name</var> to the end of the filter chain</dd>

	<dt><code>@<var>filter-name</var></code></dt>
	<dd>Insert <var>filter-name</var> at the start of the filter chain</dd>

	<dt><code>-<var>filter-name</var></code></dt>
	<dd>Remove <var>filter-name</var> from the filter chain</dd>

	<dt><code>=<var>filter-name</var></code></dt>
	<dd>Empty the filter chain and insert <var>filter-name</var></dd>

	<dt><code>!</code></dt>
	<dd>Empty the filter chain</dd>

	<dt><code><var>filter-name</var></code></dt>
	<dd>Equivalent to <code>+<var>filter-name</var></code></dd>
	</dl>
	</usage>
	</directivesynopsis>

	<directivesynopsis>
	<name>FilterProtocol</name>
	<description>Deal with correct HTTP protocol handling</description>
	<syntax>FilterProtocol <var>filter-name</var> [<var>provider-name</var>]
	<var>proto-flags</var></syntax>
	<contextlist><context>server config</context><context>virtual host</context>
	<context>directory</context><context>.htaccess</context></contextlist>
	<override>Options</override>

	<usage>
	<p>This directs <module>mod_filter</module> to deal with ensuring the
	filter doesn't run when it shouldn't, and that the HTTP response
	headers are correctly set taking into account the effects of the
	filter.</p>

	<p>There are two forms of this directive. With three arguments, it
	applies specifically to a <var>filter-name</var> and a
	<var>provider-name</var> for that filter.
	With two arguments it applies to a <var>filter-name</var> whenever the
	filter runs <em>any</em> provider.</p>

	<p>Flags specified with this directive are merged with the flags
	that underlying providers may have registerd with
	<module>mod_filter</module>. For example, a filter may internally specify
	the equivalent of <code>change=yes</code>, but a particular
	configuration of the module can override with <code>change=no</code>.
	</p>

	<p><var>proto-flags</var> is one or more of</p>

	<dl>
	<dt><code>change=yes\|no</code></dt>
	<dd>Specifies whether the filter changes the content, including possibly
	the content length. The "no" argument is supported in 2.4.7 and later.</dd>

	<dt><code>change=1:1</code></dt>
	<dd>The filter changes the content, but will not change the content
	length</dd>

	<dt><code>byteranges=no</code></dt>
	<dd>The filter cannot work on byteranges and requires complete input</dd>

	<dt><code>proxy=no</code></dt>
	<dd>The filter should not run in a proxy context</dd>

	<dt><code>proxy=transform</code></dt>
	<dd>The filter transforms the response in a manner incompatible with
	the HTTP <code>Cache-Control: no-transform</code> header.</dd>

	<dt><code>cache=no</code></dt>
	<dd>The filter renders the output uncacheable (eg by introducing randomised
	content changes)</dd>
	</dl>
	</usage>
	</directivesynopsis>

	<directivesynopsis>
	<name>FilterTrace</name>
	<description>Get debug/diagnostic information from
	<module>mod_filter</module></description>
	<syntax>FilterTrace <var>filter-name</var> <var>level</var></syntax>
	<contextlist><context>server config</context><context>virtual host</context>
	<context>directory</context></contextlist>

	<usage>
	<p>This directive generates debug information from
	<module>mod_filter</module>.
	It is designed to help test and debug providers (filter modules), although
	it may also help with <module>mod_filter</module> itself.</p>

	<p>The debug output depends on the <var>level</var> set:</p>
	<dl>
	<dt><code>0</code> (default)</dt>
	<dd>No debug information is generated.</dd>

	<dt><code>1</code></dt>
	<dd><module>mod_filter</module> will record buckets and brigades
	passing through the filter to the error log, before the provider has
	processed them. This is similar to the information generated by
	<a href="http://apache.webthing.com/mod_diagnostics/">mod_diagnostics</a>.
	</dd>

	<dt><code>2</code> (not yet implemented)</dt>
	<dd>Will dump the full data passing through to a tempfile before the
	provider. <strong>For single-user debug only</strong>; this will not
	support concurrent hits.</dd>
	</dl>
	</usage>
	</directivesynopsis>

	</modulesynopsis>