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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 15:01:30 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 15:01:30 +0000
commit6beeb1b708550be0d4a53b272283e17e5e35fe17 (patch)
tree1ce8673d4aaa948e5554000101f46536a1e4cc29 /docs/manual/content-negotiation.html.en
parentInitial commit. (diff)
downloadapache2-6beeb1b708550be0d4a53b272283e17e5e35fe17.tar.xz
apache2-6beeb1b708550be0d4a53b272283e17e5e35fe17.zip
Adding upstream version 2.4.57.upstream/2.4.57
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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+<?xml version="1.0" encoding="UTF-8"?>
+<!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>
+<meta content="text/html; charset=UTF-8" http-equiv="Content-Type" />
+<!--
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ This file is generated from xml source: DO NOT EDIT
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ -->
+<title>Content Negotiation - Apache HTTP Server Version 2.4</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 rel="stylesheet" type="text/css" href="./style/css/prettify.css" />
+<script src="./style/scripts/prettify.min.js" type="text/javascript">
+</script>
+
+<link href="./images/favicon.ico" rel="shortcut icon" /></head>
+<body id="manual-page"><div id="page-header">
+<p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p>
+<p class="apache">Apache HTTP Server Version 2.4</p>
+<img alt="" src="./images/feather.png" /></div>
+<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="./images/left.gif" /></a></div>
+<div id="path">
+<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="./">Version 2.4</a></div><div id="page-content"><div id="preamble"><h1>Content Negotiation</h1>
+<div class="toplang">
+<p><span>Available Languages: </span><a href="./en/content-negotiation.html" title="English">&nbsp;en&nbsp;</a> |
+<a href="./fr/content-negotiation.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a> |
+<a href="./ja/content-negotiation.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a> |
+<a href="./ko/content-negotiation.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a> |
+<a href="./tr/content-negotiation.html" hreflang="tr" rel="alternate" title="Türkçe">&nbsp;tr&nbsp;</a></p>
+</div>
+
+
+ <p>Apache HTTPD supports content negotiation as described in
+ the HTTP/1.1 specification. It can choose the best
+ representation of a resource based on the browser-supplied
+ preferences for media type, languages, character set and
+ encoding. It also implements a couple of features to give
+ more intelligent handling of requests from browsers that send
+ incomplete negotiation information.</p>
+
+ <p>Content negotiation is provided by the
+ <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> module, which is compiled in
+ by default.</p>
+</div>
+<div id="quickview"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#about">About Content Negotiation</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#negotiation">Negotiation in httpd</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#methods">The Negotiation Methods</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#better">Fiddling with Quality
+ Values</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#extensions">Extensions to Transparent Content
+Negotiation</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#naming">Note on hyperlinks and naming conventions</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#caching">Note on Caching</a></li>
+</ul><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</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="about" id="about">About Content Negotiation</a></h2>
+
+ <p>A resource may be available in several different
+ representations. For example, it might be available in
+ different languages or different media types, or a combination.
+ One way of selecting the most appropriate choice is to give the
+ user an index page, and let them select. However it is often
+ possible for the server to choose automatically. This works
+ because browsers can send, as part of each request, information
+ about what representations they prefer. For example, a browser
+ could indicate that it would like to see information in French,
+ if possible, else English will do. Browsers indicate their
+ preferences by headers in the request. To request only French
+ representations, the browser would send</p>
+
+<div class="example"><p><code>Accept-Language: fr</code></p></div>
+
+ <p>Note that this preference will only be applied when there is
+ a choice of representations and they vary by language.</p>
+
+ <p>As an example of a more complex request, this browser has
+ been configured to accept French and English, but prefer
+ French, and to accept various media types, preferring HTML over
+ plain text or other text types, and preferring GIF or JPEG over
+ other media types, but also allowing any other media type as a
+ last resort:</p>
+
+<div class="example"><p><code>
+ Accept-Language: fr; q=1.0, en; q=0.5<br />
+ Accept: text/html; q=1.0, text/*; q=0.8, image/gif; q=0.6, image/jpeg; q=0.6, image/*; q=0.5, */*; q=0.1
+</code></p></div>
+
+ <p>httpd supports 'server driven' content negotiation, as
+ defined in the HTTP/1.1 specification. It fully supports the
+ <code>Accept</code>, <code>Accept-Language</code>,
+ <code>Accept-Charset</code> and <code>Accept-Encoding</code>
+ request headers. httpd also supports 'transparent'
+ content negotiation, which is an experimental negotiation
+ protocol defined in RFC 2295 and RFC 2296. It does not offer
+ support for 'feature negotiation' as defined in these RFCs.</p>
+
+ <p>A <strong>resource</strong> is a conceptual entity
+ identified by a URI (RFC 2396). An HTTP server like Apache HTTP Server
+ provides access to <strong>representations</strong> of the
+ resource(s) within its namespace, with each representation in
+ the form of a sequence of bytes with a defined media type,
+ character set, encoding, etc. Each resource may be associated
+ with zero, one, or more than one representation at any given
+ time. If multiple representations are available, the resource
+ is referred to as <strong>negotiable</strong> and each of its
+ representations is termed a <strong>variant</strong>. The ways
+ in which the variants for a negotiable resource vary are called
+ the <strong>dimensions</strong> of negotiation.</p>
+</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="negotiation" id="negotiation">Negotiation in httpd</a></h2>
+
+ <p>In order to negotiate a resource, the server needs to be
+ given information about each of the variants. This is done in
+ one of two ways:</p>
+
+ <ul>
+ <li>Using a type map (<em>i.e.</em>, a <code>*.var</code>
+ file) which names the files containing the variants
+ explicitly, or</li>
+
+ <li>Using a 'MultiViews' search, where the server does an
+ implicit filename pattern match and chooses from among the
+ results.</li>
+ </ul>
+
+ <h3><a name="type-map" id="type-map">Using a type-map file</a></h3>
+
+ <p>A type map is a document which is associated with the handler
+ named <code>type-map</code> (or, for backwards-compatibility with
+ older httpd configurations, the <a class="glossarylink" href="./glossary.html#mime-type" title="see glossary">MIME-type</a>
+ <code>application/x-type-map</code>). Note that to use this
+ feature, you must have a handler set in the configuration that
+ defines a file suffix as <code>type-map</code>; this is best done
+ with</p>
+
+<pre class="prettyprint lang-config">AddHandler type-map .var</pre>
+
+
+ <p>in the server configuration file.</p>
+
+ <p>Type map files should have the same name as the resource
+ which they are describing, followed by the extension
+ <code>.var</code>. In the examples shown below, the resource is
+ named <code>foo</code>, so the type map file is named
+ <code>foo.var</code>.</p>
+
+ <p>This file should have an entry for each available
+ variant; these entries consist of contiguous HTTP-format header
+ lines. Entries for different variants are separated by blank
+ lines. Blank lines are illegal within an entry. It is
+ conventional to begin a map file with an entry for the combined
+ entity as a whole (although this is not required, and if
+ present will be ignored). An example map file is shown below.</p>
+
+ <p>URIs in this file are relative to the location of the type map
+ file. Usually, these files will be located in the same directory as
+ the type map file, but this is not required. You may provide
+ absolute or relative URIs for any file located on the same server as
+ the map file.</p>
+
+<div class="example"><p><code>
+ URI: foo<br />
+<br />
+ URI: foo.en.html<br />
+ Content-type: text/html<br />
+ Content-language: en<br />
+<br />
+ URI: foo.fr.de.html<br />
+ Content-type: text/html;charset=iso-8859-2<br />
+ Content-language: fr, de<br />
+</code></p></div>
+
+ <p>Note also that a typemap file will take precedence over the
+ filename's extension, even when Multiviews is on. If the
+ variants have different source qualities, that may be indicated
+ by the "qs" parameter to the media type, as in this picture
+ (available as JPEG, GIF, or ASCII-art): </p>
+
+<div class="example"><p><code>
+ URI: foo<br />
+<br />
+ URI: foo.jpeg<br />
+ Content-type: image/jpeg; qs=0.8<br />
+<br />
+ URI: foo.gif<br />
+ Content-type: image/gif; qs=0.5<br />
+<br />
+ URI: foo.txt<br />
+ Content-type: text/plain; qs=0.01<br />
+</code></p></div>
+
+ <p>qs values can vary in the range 0.000 to 1.000. Note that
+ any variant with a qs value of 0.000 will never be chosen.
+ Variants with no 'qs' parameter value are given a qs factor of
+ 1.0. The qs parameter indicates the relative 'quality' of this
+ variant compared to the other available variants, independent
+ of the client's capabilities. For example, a JPEG file is
+ usually of higher source quality than an ASCII file if it is
+ attempting to represent a photograph. However, if the resource
+ being represented is an original ASCII art, then an ASCII
+ representation would have a higher source quality than a JPEG
+ representation. A qs value is therefore specific to a given
+ variant depending on the nature of the resource it
+ represents.</p>
+
+ <p>The full list of headers recognized is available in the <a href="mod/mod_negotiation.html#typemaps">mod_negotiation
+ typemap</a> documentation.</p>
+
+
+<h3><a name="multiviews" id="multiviews">Multiviews</a></h3>
+
+ <p><code>MultiViews</code> is a per-directory option, meaning it
+ can be set with an <code class="directive"><a href="./mod/core.html#options">Options</a></code>
+ directive within a <code class="directive"><a href="./mod/core.html#directory">&lt;Directory&gt;</a></code>, <code class="directive"><a href="./mod/core.html#location">&lt;Location&gt;</a></code> or <code class="directive"><a href="./mod/core.html#files">&lt;Files&gt;</a></code> section in
+ <code>httpd.conf</code>, or (if <code class="directive"><a href="./mod/core.html#allowoverride">AllowOverride</a></code> is properly set) in
+ <code>.htaccess</code> files. Note that <code>Options All</code>
+ does not set <code>MultiViews</code>; you have to ask for it by
+ name.</p>
+
+ <p>The effect of <code>MultiViews</code> is as follows: if the
+ server receives a request for <code>/some/dir/foo</code>, if
+ <code>/some/dir</code> has <code>MultiViews</code> enabled, and
+ <code>/some/dir/foo</code> does <em>not</em> exist, then the
+ server reads the directory looking for files named foo.*, and
+ effectively fakes up a type map which names all those files,
+ assigning them the same media types and content-encodings it
+ would have if the client had asked for one of them by name. It
+ then chooses the best match to the client's requirements.</p>
+
+ <p><code>MultiViews</code> may also apply to searches for the file
+ named by the <code class="directive"><a href="./mod/mod_dir.html#directoryindex">DirectoryIndex</a></code> directive, if the
+ server is trying to index a directory. If the configuration files
+ specify</p>
+<pre class="prettyprint lang-config">DirectoryIndex index</pre>
+
+ <p>then the server will arbitrate between <code>index.html</code>
+ and <code>index.html3</code> if both are present. If neither
+ are present, and <code>index.cgi</code> is there, the server
+ will run it.</p>
+
+ <p>If one of the files found when reading the directory does not
+ have an extension recognized by <code>mod_mime</code> to designate
+ its Charset, Content-Type, Language, or Encoding, then the result
+ depends on the setting of the <code class="directive"><a href="./mod/mod_mime.html#multiviewsmatch">MultiViewsMatch</a></code> directive. This
+ directive determines whether handlers, filters, and other
+ extension types can participate in MultiViews negotiation.</p>
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="methods" id="methods">The Negotiation Methods</a></h2>
+
+ <p>After httpd has obtained a list of the variants for a given
+ resource, either from a type-map file or from the filenames in
+ the directory, it invokes one of two methods to decide on the
+ 'best' variant to return, if any. It is not necessary to know
+ any of the details of how negotiation actually takes place in
+ order to use httpd's content negotiation features. However the
+ rest of this document explains the methods used for those
+ interested. </p>
+
+ <p>There are two negotiation methods:</p>
+
+ <ol>
+ <li><strong>Server driven negotiation with the httpd
+ algorithm</strong> is used in the normal case. The httpd
+ algorithm is explained in more detail below. When this
+ algorithm is used, httpd can sometimes 'fiddle' the quality
+ factor of a particular dimension to achieve a better result.
+ The ways httpd can fiddle quality factors is explained in
+ more detail below.</li>
+
+ <li><strong>Transparent content negotiation</strong> is used
+ when the browser specifically requests this through the
+ mechanism defined in RFC 2295. This negotiation method gives
+ the browser full control over deciding on the 'best' variant,
+ the result is therefore dependent on the specific algorithms
+ used by the browser. As part of the transparent negotiation
+ process, the browser can ask httpd to run the 'remote
+ variant selection algorithm' defined in RFC 2296.</li>
+ </ol>
+
+<h3><a name="dimensions" id="dimensions">Dimensions of Negotiation</a></h3>
+
+ <table>
+
+ <tr valign="top">
+ <th>Dimension</th>
+
+ <th>Notes</th>
+ </tr>
+
+ <tr valign="top">
+ <td>Media Type</td>
+
+ <td>Browser indicates preferences with the <code>Accept</code>
+ header field. Each item can have an associated quality factor.
+ Variant description can also have a quality factor (the "qs"
+ parameter).</td>
+ </tr>
+
+ <tr valign="top">
+ <td>Language</td>
+
+ <td>Browser indicates preferences with the
+ <code>Accept-Language</code> header field. Each item can have
+ a quality factor. Variants can be associated with none, one or
+ more than one language.</td>
+ </tr>
+
+ <tr valign="top">
+ <td>Encoding</td>
+
+ <td>Browser indicates preference with the
+ <code>Accept-Encoding</code> header field. Each item can have
+ a quality factor.</td>
+ </tr>
+
+ <tr valign="top">
+ <td>Charset</td>
+
+ <td>Browser indicates preference with the
+ <code>Accept-Charset</code> header field. Each item can have a
+ quality factor. Variants can indicate a charset as a parameter
+ of the media type.</td>
+ </tr>
+ </table>
+
+
+<h3><a name="algorithm" id="algorithm">httpd Negotiation Algorithm</a></h3>
+
+ <p>httpd can use the following algorithm to select the 'best'
+ variant (if any) to return to the browser. This algorithm is
+ not further configurable. It operates as follows:</p>
+
+ <ol>
+ <li>First, for each dimension of the negotiation, check the
+ appropriate <em>Accept*</em> header field and assign a
+ quality to each variant. If the <em>Accept*</em> header for
+ any dimension implies that this variant is not acceptable,
+ eliminate it. If no variants remain, go to step 4.</li>
+
+ <li>
+ Select the 'best' variant by a process of elimination. Each
+ of the following tests is applied in order. Any variants
+ not selected at each test are eliminated. After each test,
+ if only one variant remains, select it as the best match
+ and proceed to step 3. If more than one variant remains,
+ move on to the next test.
+
+ <ol>
+ <li>Multiply the quality factor from the <code>Accept</code>
+ header with the quality-of-source factor for this variants
+ media type, and select the variants with the highest
+ value.</li>
+
+ <li>Select the variants with the highest language quality
+ factor.</li>
+
+ <li>Select the variants with the best language match,
+ using either the order of languages in the
+ <code>Accept-Language</code> header (if present), or else
+ the order of languages in the <code>LanguagePriority</code>
+ directive (if present).</li>
+
+ <li>Select the variants with the highest 'level' media
+ parameter (used to give the version of text/html media
+ types).</li>
+
+ <li>Select variants with the best charset media
+ parameters, as given on the <code>Accept-Charset</code>
+ header line. Charset ISO-8859-1 is acceptable unless
+ explicitly excluded. Variants with a <code>text/*</code>
+ media type but not explicitly associated with a particular
+ charset are assumed to be in ISO-8859-1.</li>
+
+ <li>Select those variants which have associated charset
+ media parameters that are <em>not</em> ISO-8859-1. If
+ there are no such variants, select all variants
+ instead.</li>
+
+ <li>Select the variants with the best encoding. If there
+ are variants with an encoding that is acceptable to the
+ user-agent, select only these variants. Otherwise if
+ there is a mix of encoded and non-encoded variants,
+ select only the unencoded variants. If either all
+ variants are encoded or all variants are not encoded,
+ select all variants.</li>
+
+ <li>Select the variants with the smallest content
+ length.</li>
+
+ <li>Select the first variant of those remaining. This
+ will be either the first listed in the type-map file, or
+ when variants are read from the directory, the one whose
+ file name comes first when sorted using ASCII code
+ order.</li>
+ </ol>
+ </li>
+
+ <li>The algorithm has now selected one 'best' variant, so
+ return it as the response. The HTTP response header
+ <code>Vary</code> is set to indicate the dimensions of
+ negotiation (browsers and caches can use this information when
+ caching the resource). End.</li>
+
+ <li>To get here means no variant was selected (because none
+ are acceptable to the browser). Return a 406 status (meaning
+ "No acceptable representation") with a response body
+ consisting of an HTML document listing the available
+ variants. Also set the HTTP <code>Vary</code> header to
+ indicate the dimensions of variance.</li>
+ </ol>
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="better" id="better">Fiddling with Quality
+ Values</a></h2>
+
+ <p>httpd sometimes changes the quality values from what would
+ be expected by a strict interpretation of the httpd
+ negotiation algorithm above. This is to get a better result
+ from the algorithm for browsers which do not send full or
+ accurate information. Some of the most popular browsers send
+ <code>Accept</code> header information which would otherwise
+ result in the selection of the wrong variant in many cases. If a
+ browser sends full and correct information these fiddles will not
+ be applied.</p>
+
+<h3><a name="wildcards" id="wildcards">Media Types and Wildcards</a></h3>
+
+ <p>The <code>Accept:</code> request header indicates preferences
+ for media types. It can also include 'wildcard' media types, such
+ as "image/*" or "*/*" where the * matches any string. So a request
+ including:</p>
+
+<div class="example"><p><code>Accept: image/*, */*</code></p></div>
+
+ <p>would indicate that any type starting "image/" is acceptable,
+ as is any other type.
+ Some browsers routinely send wildcards in addition to explicit
+ types they can handle. For example:</p>
+
+<div class="example"><p><code>
+ Accept: text/html, text/plain, image/gif, image/jpeg, */*
+</code></p></div>
+ <p>The intention of this is to indicate that the explicitly listed
+ types are preferred, but if a different representation is
+ available, that is ok too. Using explicit quality values,
+ what the browser really wants is something like:</p>
+<div class="example"><p><code>
+ Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01
+</code></p></div>
+ <p>The explicit types have no quality factor, so they default to a
+ preference of 1.0 (the highest). The wildcard */* is given a
+ low preference of 0.01, so other types will only be returned if
+ no variant matches an explicitly listed type.</p>
+
+ <p>If the <code>Accept:</code> header contains <em>no</em> q
+ factors at all, httpd sets the q value of "*/*", if present, to
+ 0.01 to emulate the desired behavior. It also sets the q value of
+ wildcards of the format "type/*" to 0.02 (so these are preferred
+ over matches against "*/*". If any media type on the
+ <code>Accept:</code> header contains a q factor, these special
+ values are <em>not</em> applied, so requests from browsers which
+ send the explicit information to start with work as expected.</p>
+
+
+<h3><a name="exceptions" id="exceptions">Language Negotiation Exceptions</a></h3>
+
+ <p>New in httpd 2.0, some exceptions have been added to the
+ negotiation algorithm to allow graceful fallback when language
+ negotiation fails to find a match.</p>
+
+ <p>When a client requests a page on your server, but the server
+ cannot find a single page that matches the
+ <code>Accept-language</code> sent by
+ the browser, the server will return either a "No Acceptable
+ Variant" or "Multiple Choices" response to the client. To avoid
+ these error messages, it is possible to configure httpd to ignore
+ the <code>Accept-language</code> in these cases and provide a
+ document that does not explicitly match the client's request. The
+ <code class="directive"><a href="./mod/mod_negotiation.html#forcelanguagepriority">ForceLanguagePriority</a></code>
+ directive can be used to override one or both of these error
+ messages and substitute the servers judgement in the form of the
+ <code class="directive"><a href="./mod/mod_negotiation.html#languagepriority">LanguagePriority</a></code>
+ directive.</p>
+
+ <p>The server will also attempt to match language-subsets when no
+ other match can be found. For example, if a client requests
+ documents with the language <code>en-GB</code> for British
+ English, the server is not normally allowed by the HTTP/1.1
+ standard to match that against a document that is marked as simply
+ <code>en</code>. (Note that it is almost surely a configuration
+ error to include <code>en-GB</code> and not <code>en</code> in the
+ <code>Accept-Language</code> header, since it is very unlikely
+ that a reader understands British English, but doesn't understand
+ English in general. Unfortunately, many current clients have
+ default configurations that resemble this.) However, if no other
+ language match is possible and the server is about to return a "No
+ Acceptable Variants" error or fallback to the <code class="directive"><a href="./mod/mod_negotiation.html#languagepriority">LanguagePriority</a></code>, the server
+ will ignore the subset specification and match <code>en-GB</code>
+ against <code>en</code> documents. Implicitly, httpd will add
+ the parent language to the client's acceptable language list with
+ a very low quality value. But note that if the client requests
+ "en-GB; q=0.9, fr; q=0.8", and the server has documents
+ designated "en" and "fr", then the "fr" document will be returned.
+ This is necessary to maintain compliance with the HTTP/1.1
+ specification and to work effectively with properly configured
+ clients.</p>
+
+ <p>In order to support advanced techniques (such as cookies or
+ special URL-paths) to determine the user's preferred language,
+ since httpd 2.0.47 <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> recognizes
+ the <a href="env.html">environment variable</a>
+ <code>prefer-language</code>. If it exists and contains an
+ appropriate language tag, <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> will
+ try to select a matching variant. If there's no such variant,
+ the normal negotiation process applies.</p>
+
+ <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">SetEnvIf Cookie "language=(.+)" prefer-language=$1
+Header append Vary cookie</pre>
+</div>
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="extensions" id="extensions">Extensions to Transparent Content
+Negotiation</a></h2>
+
+<p>httpd extends the transparent content negotiation protocol (RFC
+2295) as follows. A new <code>{encoding ..}</code> element is used in
+variant lists to label variants which are available with a specific
+content-encoding only. The implementation of the RVSA/1.0 algorithm
+(RFC 2296) is extended to recognize encoded variants in the list, and
+to use them as candidate variants whenever their encodings are
+acceptable according to the <code>Accept-Encoding</code> request
+header. The RVSA/1.0 implementation does not round computed quality
+factors to 5 decimal places before choosing the best variant.</p>
+</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="naming" id="naming">Note on hyperlinks and naming conventions</a></h2>
+
+ <p>If you are using language negotiation you can choose between
+ different naming conventions, because files can have more than
+ one extension, and the order of the extensions is normally
+ irrelevant (see the <a href="mod/mod_mime.html#multipleext">mod_mime</a> documentation
+ for details).</p>
+
+ <p>A typical file has a MIME-type extension (<em>e.g.</em>,
+ <code>html</code>), maybe an encoding extension (<em>e.g.</em>,
+ <code>gz</code>), and of course a language extension
+ (<em>e.g.</em>, <code>en</code>) when we have different
+ language variants of this file.</p>
+
+ <p>Examples:</p>
+
+ <ul>
+ <li>foo.en.html</li>
+
+ <li>foo.html.en</li>
+
+ <li>foo.en.html.gz</li>
+ </ul>
+
+ <p>Here some more examples of filenames together with valid and
+ invalid hyperlinks:</p>
+
+ <table class="bordered">
+
+ <tr>
+ <th>Filename</th>
+
+ <th>Valid hyperlink</th>
+
+ <th>Invalid hyperlink</th>
+ </tr>
+
+ <tr>
+ <td><em>foo.html.en</em></td>
+
+ <td>foo<br />
+ foo.html</td>
+
+ <td>-</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.en.html</em></td>
+
+ <td>foo</td>
+
+ <td>foo.html</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.html.en.gz</em></td>
+
+ <td>foo<br />
+ foo.html</td>
+
+ <td>foo.gz<br />
+ foo.html.gz</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.en.html.gz</em></td>
+
+ <td>foo</td>
+
+ <td>foo.html<br />
+ foo.html.gz<br />
+ foo.gz</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.gz.html.en</em></td>
+
+ <td>foo<br />
+ foo.gz<br />
+ foo.gz.html</td>
+
+ <td>foo.html</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.html.gz.en</em></td>
+
+ <td>foo<br />
+ foo.html<br />
+ foo.html.gz</td>
+
+ <td>foo.gz</td>
+ </tr>
+ </table>
+
+ <p>Looking at the table above, you will notice that it is always
+ possible to use the name without any extensions in a hyperlink
+ (<em>e.g.</em>, <code>foo</code>). The advantage is that you
+ can hide the actual type of a document rsp. file and can change
+ it later, <em>e.g.</em>, from <code>html</code> to
+ <code>shtml</code> or <code>cgi</code> without changing any
+ hyperlink references.</p>
+
+ <p>If you want to continue to use a MIME-type in your
+ hyperlinks (<em>e.g.</em> <code>foo.html</code>) the language
+ extension (including an encoding extension if there is one)
+ must be on the right hand side of the MIME-type extension
+ (<em>e.g.</em>, <code>foo.html.en</code>).</p>
+</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="caching" id="caching">Note on Caching</a></h2>
+
+ <p>When a cache stores a representation, it associates it with
+ the request URL. The next time that URL is requested, the cache
+ can use the stored representation. But, if the resource is
+ negotiable at the server, this might result in only the first
+ requested variant being cached and subsequent cache hits might
+ return the wrong response. To prevent this, httpd normally
+ marks all responses that are returned after content negotiation
+ as non-cacheable by HTTP/1.0 clients. httpd also supports the
+ HTTP/1.1 protocol features to allow caching of negotiated
+ responses.</p>
+
+ <p>For requests which come from a HTTP/1.0 compliant client
+ (either a browser or a cache), the directive <code class="directive"><a href="./mod/mod_negotiation.html#cachenegotiateddocs">CacheNegotiatedDocs</a></code> can be
+ used to allow caching of responses which were subject to
+ negotiation. This directive can be given in the server config or
+ virtual host, and takes no arguments. It has no effect on requests
+ from HTTP/1.1 clients.</p>
+
+ <p>For HTTP/1.1 clients, httpd sends a <code>Vary</code> HTTP
+ response header to indicate the negotiation dimensions for the
+ response. Caches can use this information to determine whether a
+ subsequent request can be served from the local copy. To
+ encourage a cache to use the local copy regardless of the
+ negotiation dimensions, set the <code>force-no-vary</code> <a href="env.html#special">environment variable</a>.</p>
+
+</div></div>
+<div class="bottomlang">
+<p><span>Available Languages: </span><a href="./en/content-negotiation.html" title="English">&nbsp;en&nbsp;</a> |
+<a href="./fr/content-negotiation.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a> |
+<a href="./ja/content-negotiation.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a> |
+<a href="./ko/content-negotiation.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a> |
+<a href="./tr/content-negotiation.html" hreflang="tr" rel="alternate" title="Türkçe">&nbsp;tr&nbsp;</a></p>
+</div><div class="top"><a href="#page-header"><img src="./images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&amp;A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Libera.chat, or sent to our <a href="https://httpd.apache.org/lists.html">mailing lists</a>.</div>
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