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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-08 19:09:22 +0000 |
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| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-08 19:09:22 +0000 |
| commit | 2faa747e2303ee774a4b4aace961188e950e185a (patch) | |
| tree | 604e79c7481956ce48f458e3546eaf1090b3ffff /docs/manual/content-negotiation.html.en | |
| parent | Initial commit. (diff) | |
| download | apache2-2faa747e2303ee774a4b4aace961188e950e185a.tar.xz apache2-2faa747e2303ee774a4b4aace961188e950e185a.zip | |
Adding upstream version 2.4.58.upstream/2.4.58
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'docs/manual/content-negotiation.html.en')
| -rw-r--r-- | docs/manual/content-negotiation.html.en | 711 |
1 files changed, 711 insertions, 0 deletions
diff --git a/docs/manual/content-negotiation.html.en b/docs/manual/content-negotiation.html.en new file mode 100644 index 0000000..e64e336 --- /dev/null +++ b/docs/manual/content-negotiation.html.en @@ -0,0 +1,711 @@ +<?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="<-" 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 2.4</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"> en </a> | +<a href="./fr/content-negotiation.html" hreflang="fr" rel="alternate" title="Français"> fr </a> | +<a href="./ja/content-negotiation.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | +<a href="./ko/content-negotiation.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | +<a href="./tr/content-negotiation.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </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"><Directory></a></code>, <code class="directive"><a href="./mod/core.html#location"><Location></a></code> or <code class="directive"><a href="./mod/core.html#files"><Files></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"> en </a> | +<a href="./fr/content-negotiation.html" hreflang="fr" rel="alternate" title="Français"> fr </a> | +<a href="./ja/content-negotiation.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | +<a href="./ko/content-negotiation.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | +<a href="./tr/content-negotiation.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </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&A section. 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