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<?xml version="1.0" encoding="ISO-8859-1"?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"><html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!-- XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX This file is generated from xml source: DO NOT EDIT XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX --><title>mod_filter - Apache HTTP Server</title><link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /><link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /><link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link href="../images/favicon.ico" rel="shortcut icon" /></head><body><div id="page-header"><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 2.2</p><img alt="" src="../images/feather.gif" /></div><div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div><div id="path"><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 2.2</a> > <a href="./">Modules</a></div><div id="page-content"><div id="preamble"><h1>Apache Module mod_filter</h1><div class="toplang"><p><span>Available Languages: </span><a href="../en/mod/mod_filter.html" title="English"> en </a></p></div><table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Context-sensitive smart filter configuration module</td></tr><tr><th><a href="module-dict.html#Status">Status:</a></th><td>Base</td></tr><tr><th><a href="module-dict.html#ModuleIdentifier">Module營dentifier:</a></th><td>filter_module</td></tr><tr><th><a href="module-dict.html#SourceFile">Source燜ile:</a></th><td>mod_filter.c</td></tr><tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Version 2.1 and later</td></tr></table><h3>Summary</h3> <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><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> 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 <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>; no change to existing filter modules is required (although it may be possible to simplify them).</p></div><div id="quickview"><h3 class="directives">Directives</h3><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#filterchain">FilterChain</a></li><li><img alt="" src="../images/down.gif" /> <a href="#filterdeclare">FilterDeclare</a></li><li><img alt="" src="../images/down.gif" /> <a href="#filterprotocol">FilterProtocol</a></li><li><img alt="" src="../images/down.gif" /> <a href="#filterprovider">FilterProvider</a></li><li><img alt="" src="../images/down.gif" /> <a href="#filtertrace">FilterTrace</a></li></ul><h3>Topics</h3><ul id="topics"><li><img alt="" src="../images/down.gif" /> <a href="#smart">Smart Filtering</a></li><li><img alt="" src="../images/down.gif" /> <a href="#terms">Filter Declarations, Providers and Chains</a></li><li><img alt="" src="../images/down.gif" /> <a href="#config">Configuring the Chain</a></li><li><img alt="" src="../images/down.gif" /> <a href="#examples">Examples</a></li><li><img alt="" src="../images/down.gif" /> <a href="#protocol">Protocol Handling</a></li></ul></div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="smart" id="smart">Smart Filtering</a></h2> <p>In the traditional filtering model, filters are inserted unconditionally using <code class="directive"><a href="../mod/mod_mime.html#addoutputfilter">AddOutputFilter</a></code> 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><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> by contrast gives server administrators a great deal of flexibility in configuring the filter chain. In fact, filters can be inserted based on any Request Header, Response Header or Environment Variable. This generalises the limited flexibility offered by <code class="directive"><a href="../mod/core.html#addoutputfilterbytype">AddOutputFilterByType</a></code>, and fixes it to work correctly with dynamic content, regardless of the content generator. The ability to dispatch based on Environment Variables offers the full flexibility of configuration with <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> to anyone who needs it.</p></div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="terms" id="terms">Filter Declarations, Providers and Chains</a></h2> <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 <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> model</p> <p><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> 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 <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>; 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></div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="config" id="config">Configuring the Chain</a></h2> <p>There are three stages to configuring a filter chain with <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>. For details of the directives, see below.</p> <dl> <dt>Declare Filters</dt> <dd>The <code class="directive"><a href="#filterdeclare">FilterDeclare</a></code> 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 <code class="directive"><a href="#filterprovider">FilterProvider</a></code> directive registers a provider with a filter. The filter may have been declared with <code class="directive"><a href="#filterdeclare">FilterDeclare</a></code>; 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 remaining arguments to <code class="directive"><a href="#filterprovider">FilterProvider</a></code> are a dispatch criterion and a match string. The former may be an HTTP request or response header, an environment variable, or the Handler used by this request. The latter is matched to it for each request, to determine whether this provider will be used to implement the filter for this request.</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 <code class="directive"><a href="#filterchain">FilterChain</a></code> 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></div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="examples" id="examples">Examples</a></h2> <dl> <dt>Server side Includes (SSI)</dt> <dd>A simple case of using <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> in place of <code class="directive"><a href="../mod/core.html#addoutputfilterbytype">AddOutputFilterByType</a></code> <div class="example"><p><code> FilterDeclare SSI<br /> FilterProvider SSI INCLUDES resp=Content-Type $text/html<br /> FilterChain SSI </code></p></div> </dd> <dt>Server side Includes (SSI)</dt> <dd>The same as the above but dispatching on handler (classic SSI behaviour; .shtml files get processed). <div class="example"><p><code> FilterProvider SSI INCLUDES Handler server-parsed<br /> FilterChain SSI </code></p></div> </dd> <dt>Emulating mod_gzip with mod_deflate</dt> <dd>Insert INFLATE filter only if "gzip" is NOT in the Accept-Encoding header. This filter runs with ftype CONTENT_SET. <div class="example"><p><code> FilterDeclare gzip CONTENT_SET<br /> FilterProvider gzip inflate req=Accept-Encoding !$gzip<br /> FilterChain gzip </code></p></div> </dd> <dt>Image Downsampling</dt> <dd>Suppose we want to downsample all web images, and have filters for GIF, JPEG and PNG. <div class="example"><p><code> FilterProvider unpack jpeg_unpack Content-Type $image/jpeg<br /> FilterProvider unpack gif_unpack Content-Type $image/gif<br /> FilterProvider unpack png_unpack Content-Type $image/png<br /> <br /> FilterProvider downsample downsample_filter Content-Type $image<br /> FilterProtocol downsample "change=yes"<br /> <br /> FilterProvider repack jpeg_pack Content-Type $image/jpeg<br /> FilterProvider repack gif_pack Content-Type $image/gif<br /> FilterProvider repack png_pack Content-Type $image/png<br /> <Location /image-filter><br /> <span class="indent"> FilterChain unpack downsample repack<br /> </span> </Location> </code></p></div> </dd> </dl></div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="protocol" id="protocol">Protocol Handling</a></h2> <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><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> 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 <code class="directive"><a href="#filterprotocol">FilterProtocol</a></code> implements some of this functionality for back-compatibility with Apache 2.0⌨️ 快捷键说明
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