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15 <p class="apache">Apache HTTP Server Version 2.0</p>
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19 <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 2.0</a></div><div id="page-content"><div id="preamble"><h1>Content Negotiation</h1>
21 <p><span>Available Languages: </span><a href="./en/content-negotiation.html" title="English"> en </a> |
22 <a href="./fr/content-negotiation.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
23 <a href="./ja/content-negotiation.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
24 <a href="./ko/content-negotiation.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
25 <a href="./tr/content-negotiation.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
29 <p>Apache supports content negotiation as described in
30 the HTTP/1.1 specification. It can choose the best
31 representation of a resource based on the browser-supplied
32 preferences for media type, languages, character set and
33 encoding. It also implements a couple of features to give
34 more intelligent handling of requests from browsers that send
35 incomplete negotiation information.</p>
37 <p>Content negotiation is provided by the
38 <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> module, which is compiled in
41 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#about">About Content Negotiation</a></li>
42 <li><img alt="" src="./images/down.gif" /> <a href="#negotiation">Negotiation in Apache</a></li>
43 <li><img alt="" src="./images/down.gif" /> <a href="#methods">The Negotiation Methods</a></li>
44 <li><img alt="" src="./images/down.gif" /> <a href="#better">Fiddling with Quality
46 <li><img alt="" src="./images/down.gif" /> <a href="#extensions">Extensions to Transparent Content
48 <li><img alt="" src="./images/down.gif" /> <a href="#naming">Note on hyperlinks and naming conventions</a></li>
49 <li><img alt="" src="./images/down.gif" /> <a href="#caching">Note on Caching</a></li>
51 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
53 <h2><a name="about" id="about">About Content Negotiation</a></h2>
55 <p>A resource may be available in several different
56 representations. For example, it might be available in
57 different languages or different media types, or a combination.
58 One way of selecting the most appropriate choice is to give the
59 user an index page, and let them select. However it is often
60 possible for the server to choose automatically. This works
61 because browsers can send, as part of each request, information
62 about what representations they prefer. For example, a browser
63 could indicate that it would like to see information in French,
64 if possible, else English will do. Browsers indicate their
65 preferences by headers in the request. To request only French
66 representations, the browser would send</p>
68 <div class="example"><p><code>Accept-Language: fr</code></p></div>
70 <p>Note that this preference will only be applied when there is
71 a choice of representations and they vary by language.</p>
73 <p>As an example of a more complex request, this browser has
74 been configured to accept French and English, but prefer
75 French, and to accept various media types, preferring HTML over
76 plain text or other text types, and preferring GIF or JPEG over
77 other media types, but also allowing any other media type as a
80 <div class="example"><p><code>
81 Accept-Language: fr; q=1.0, en; q=0.5<br />
82 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
85 <p>Apache supports 'server driven' content negotiation, as
86 defined in the HTTP/1.1 specification. It fully supports the
87 <code>Accept</code>, <code>Accept-Language</code>,
88 <code>Accept-Charset</code> and<code>Accept-Encoding</code>
89 request headers. Apache also supports 'transparent'
90 content negotiation, which is an experimental negotiation
91 protocol defined in RFC 2295 and RFC 2296. It does not offer
92 support for 'feature negotiation' as defined in these RFCs.</p>
94 <p>A <strong>resource</strong> is a conceptual entity
95 identified by a URI (RFC 2396). An HTTP server like Apache
96 provides access to <strong>representations</strong> of the
97 resource(s) within its namespace, with each representation in
98 the form of a sequence of bytes with a defined media type,
99 character set, encoding, etc. Each resource may be associated
100 with zero, one, or more than one representation at any given
101 time. If multiple representations are available, the resource
102 is referred to as <strong>negotiable</strong> and each of its
103 representations is termed a <strong>variant</strong>. The ways
104 in which the variants for a negotiable resource vary are called
105 the <strong>dimensions</strong> of negotiation.</p>
106 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
107 <div class="section">
108 <h2><a name="negotiation" id="negotiation">Negotiation in Apache</a></h2>
110 <p>In order to negotiate a resource, the server needs to be
111 given information about each of the variants. This is done in
115 <li>Using a type map (<em>i.e.</em>, a <code>*.var</code>
116 file) which names the files containing the variants
119 <li>Using a 'MultiViews' search, where the server does an
120 implicit filename pattern match and chooses from among the
124 <h3><a name="type-map" id="type-map">Using a type-map file</a></h3>
126 <p>A type map is a document which is associated with the
127 handler named <code>type-map</code> (or, for
128 backwards-compatibility with older Apache configurations, the
129 MIME type <code>application/x-type-map</code>). Note that to
130 use this feature, you must have a handler set in the
131 configuration that defines a file suffix as
132 <code>type-map</code>; this is best done with</p>
134 <div class="example"><p><code>AddHandler type-map .var</code></p></div>
136 <p>in the server configuration file.</p>
138 <p>Type map files should have the same name as the resource
139 which they are describing, and have an entry for each available
140 variant; these entries consist of contiguous HTTP-format header
141 lines. Entries for different variants are separated by blank
142 lines. Blank lines are illegal within an entry. It is
143 conventional to begin a map file with an entry for the combined
144 entity as a whole (although this is not required, and if
145 present will be ignored). An example map file is shown below.
146 This file would be named <code>foo.var</code>, as it describes
147 a resource named <code>foo</code>.</p>
149 <div class="example"><p><code>
152 URI: foo.en.html<br />
153 Content-type: text/html<br />
154 Content-language: en<br />
156 URI: foo.fr.de.html<br />
157 Content-type: text/html;charset=iso-8859-2<br />
158 Content-language: fr, de<br />
160 <p>Note also that a typemap file will take precedence over the
161 filename's extension, even when Multiviews is on. If the
162 variants have different source qualities, that may be indicated
163 by the "qs" parameter to the media type, as in this picture
164 (available as JPEG, GIF, or ASCII-art): </p>
166 <div class="example"><p><code>
170 Content-type: image/jpeg; qs=0.8<br />
173 Content-type: image/gif; qs=0.5<br />
176 Content-type: text/plain; qs=0.01<br />
179 <p>qs values can vary in the range 0.000 to 1.000. Note that
180 any variant with a qs value of 0.000 will never be chosen.
181 Variants with no 'qs' parameter value are given a qs factor of
182 1.0. The qs parameter indicates the relative 'quality' of this
183 variant compared to the other available variants, independent
184 of the client's capabilities. For example, a JPEG file is
185 usually of higher source quality than an ASCII file if it is
186 attempting to represent a photograph. However, if the resource
187 being represented is an original ASCII art, then an ASCII
188 representation would have a higher source quality than a JPEG
189 representation. A qs value is therefore specific to a given
190 variant depending on the nature of the resource it
193 <p>The full list of headers recognized is available in the <a href="mod/mod_negotiation.html#typemaps">mod_negotation
194 typemap</a> documentation.</p>
197 <h3><a name="multiviews" id="multiviews">Multiviews</a></h3>
199 <p><code>MultiViews</code> is a per-directory option, meaning it
200 can be set with an <code class="directive"><a href="./mod/core.html#options">Options</a></code>
201 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
202 <code>httpd.conf</code>, or (if <code class="directive"><a href="./mod/core.html#allowoverride">AllowOverride</a></code> is properly set) in
203 <code>.htaccess</code> files. Note that <code>Options All</code>
204 does not set <code>MultiViews</code>; you have to ask for it by
207 <p>The effect of <code>MultiViews</code> is as follows: if the
208 server receives a request for <code>/some/dir/foo</code>, if
209 <code>/some/dir</code> has <code>MultiViews</code> enabled, and
210 <code>/some/dir/foo</code> does <em>not</em> exist, then the
211 server reads the directory looking for files named foo.*, and
212 effectively fakes up a type map which names all those files,
213 assigning them the same media types and content-encodings it
214 would have if the client had asked for one of them by name. It
215 then chooses the best match to the client's requirements.</p>
217 <p><code>MultiViews</code> may also apply to searches for the file
218 named by the <code class="directive"><a href="./mod/mod_dir.html#directoryindex">DirectoryIndex</a></code> directive, if the
219 server is trying to index a directory. If the configuration files
221 <div class="example"><p><code>DirectoryIndex index</code></p></div>
222 <p>then the server will arbitrate between <code>index.html</code>
223 and <code>index.html3</code> if both are present. If neither
224 are present, and <code>index.cgi</code> is there, the server
227 <p>If one of the files found when reading the directory does not
228 have an extension recognized by <code>mod_mime</code> to designate
229 its Charset, Content-Type, Language, or Encoding, then the result
230 depends on the setting of the <code class="directive"><a href="./mod/mod_mime.html#multiviewsmatch">MultiViewsMatch</a></code> directive. This
231 directive determines whether handlers, filters, and other
232 extension types can participate in MultiViews negotiation.</p>
234 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
235 <div class="section">
236 <h2><a name="methods" id="methods">The Negotiation Methods</a></h2>
238 <p>After Apache has obtained a list of the variants for a given
239 resource, either from a type-map file or from the filenames in
240 the directory, it invokes one of two methods to decide on the
241 'best' variant to return, if any. It is not necessary to know
242 any of the details of how negotiation actually takes place in
243 order to use Apache's content negotiation features. However the
244 rest of this document explains the methods used for those
247 <p>There are two negotiation methods:</p>
250 <li><strong>Server driven negotiation with the Apache
251 algorithm</strong> is used in the normal case. The Apache
252 algorithm is explained in more detail below. When this
253 algorithm is used, Apache can sometimes 'fiddle' the quality
254 factor of a particular dimension to achieve a better result.
255 The ways Apache can fiddle quality factors is explained in
256 more detail below.</li>
258 <li><strong>Transparent content negotiation</strong> is used
259 when the browser specifically requests this through the
260 mechanism defined in RFC 2295. This negotiation method gives
261 the browser full control over deciding on the 'best' variant,
262 the result is therefore dependent on the specific algorithms
263 used by the browser. As part of the transparent negotiation
264 process, the browser can ask Apache to run the 'remote
265 variant selection algorithm' defined in RFC 2296.</li>
268 <h3><a name="dimensions" id="dimensions">Dimensions of Negotiation</a></h3>
281 <td>Browser indicates preferences with the <code>Accept</code>
282 header field. Each item can have an associated quality factor.
283 Variant description can also have a quality factor (the "qs"
290 <td>Browser indicates preferences with the
291 <code>Accept-Language</code> header field. Each item can have
292 a quality factor. Variants can be associated with none, one or
293 more than one language.</td>
299 <td>Browser indicates preference with the
300 <code>Accept-Encoding</code> header field. Each item can have
301 a quality factor.</td>
307 <td>Browser indicates preference with the
308 <code>Accept-Charset</code> header field. Each item can have a
309 quality factor. Variants can indicate a charset as a parameter
310 of the media type.</td>
315 <h3><a name="algorithm" id="algorithm">Apache Negotiation Algorithm</a></h3>
317 <p>Apache can use the following algorithm to select the 'best'
318 variant (if any) to return to the browser. This algorithm is
319 not further configurable. It operates as follows:</p>
322 <li>First, for each dimension of the negotiation, check the
323 appropriate <em>Accept*</em> header field and assign a
324 quality to each variant. If the <em>Accept*</em> header for
325 any dimension implies that this variant is not acceptable,
326 eliminate it. If no variants remain, go to step 4.</li>
329 Select the 'best' variant by a process of elimination. Each
330 of the following tests is applied in order. Any variants
331 not selected at each test are eliminated. After each test,
332 if only one variant remains, select it as the best match
333 and proceed to step 3. If more than one variant remains,
334 move on to the next test.
337 <li>Multiply the quality factor from the <code>Accept</code>
338 header with the quality-of-source factor for this variants
339 media type, and select the variants with the highest
342 <li>Select the variants with the highest language quality
345 <li>Select the variants with the best language match,
346 using either the order of languages in the
347 <code>Accept-Language</code> header (if present), or else
348 the order of languages in the <code>LanguagePriority</code>
349 directive (if present).</li>
351 <li>Select the variants with the highest 'level' media
352 parameter (used to give the version of text/html media
355 <li>Select variants with the best charset media
356 parameters, as given on the <code>Accept-Charset</code>
357 header line. Charset ISO-8859-1 is acceptable unless
358 explicitly excluded. Variants with a <code>text/*</code>
359 media type but not explicitly associated with a particular
360 charset are assumed to be in ISO-8859-1.</li>
362 <li>Select those variants which have associated charset
363 media parameters that are <em>not</em> ISO-8859-1. If
364 there are no such variants, select all variants
367 <li>Select the variants with the best encoding. If there
368 are variants with an encoding that is acceptable to the
369 user-agent, select only these variants. Otherwise if
370 there is a mix of encoded and non-encoded variants,
371 select only the unencoded variants. If either all
372 variants are encoded or all variants are not encoded,
373 select all variants.</li>
375 <li>Select the variants with the smallest content
378 <li>Select the first variant of those remaining. This
379 will be either the first listed in the type-map file, or
380 when variants are read from the directory, the one whose
381 file name comes first when sorted using ASCII code
386 <li>The algorithm has now selected one 'best' variant, so
387 return it as the response. The HTTP response header
388 <code>Vary</code> is set to indicate the dimensions of
389 negotiation (browsers and caches can use this information when
390 caching the resource). End.</li>
392 <li>To get here means no variant was selected (because none
393 are acceptable to the browser). Return a 406 status (meaning
394 "No acceptable representation") with a response body
395 consisting of an HTML document listing the available
396 variants. Also set the HTTP <code>Vary</code> header to
397 indicate the dimensions of variance.</li>
400 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
401 <div class="section">
402 <h2><a name="better" id="better">Fiddling with Quality
405 <p>Apache sometimes changes the quality values from what would
406 be expected by a strict interpretation of the Apache
407 negotiation algorithm above. This is to get a better result
408 from the algorithm for browsers which do not send full or
409 accurate information. Some of the most popular browsers send
410 <code>Accept</code> header information which would otherwise
411 result in the selection of the wrong variant in many cases. If a
412 browser sends full and correct information these fiddles will not
415 <h3><a name="wildcards" id="wildcards">Media Types and Wildcards</a></h3>
417 <p>The <code>Accept:</code> request header indicates preferences
418 for media types. It can also include 'wildcard' media types, such
419 as "image/*" or "*/*" where the * matches any string. So a request
422 <div class="example"><p><code>Accept: image/*, */*</code></p></div>
424 <p>would indicate that any type starting "image/" is acceptable,
425 as is any other type.
426 Some browsers routinely send wildcards in addition to explicit
427 types they can handle. For example:</p>
429 <div class="example"><p><code>
430 Accept: text/html, text/plain, image/gif, image/jpeg, */*
432 <p>The intention of this is to indicate that the explicitly listed
433 types are preferred, but if a different representation is
434 available, that is ok too. Using explicit quality values,
435 what the browser really wants is something like:</p>
436 <div class="example"><p><code>
437 Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01
439 <p>The explicit types have no quality factor, so they default to a
440 preference of 1.0 (the highest). The wildcard */* is given a
441 low preference of 0.01, so other types will only be returned if
442 no variant matches an explicitly listed type.</p>
444 <p>If the <code>Accept:</code> header contains <em>no</em> q
445 factors at all, Apache sets the q value of "*/*", if present, to
446 0.01 to emulate the desired behavior. It also sets the q value of
447 wildcards of the format "type/*" to 0.02 (so these are preferred
448 over matches against "*/*". If any media type on the
449 <code>Accept:</code> header contains a q factor, these special
450 values are <em>not</em> applied, so requests from browsers which
451 send the explicit information to start with work as expected.</p>
454 <h3><a name="exceptions" id="exceptions">Language Negotiation Exceptions</a></h3>
456 <p>New in Apache 2.0, some exceptions have been added to the
457 negotiation algorithm to allow graceful fallback when language
458 negotiation fails to find a match.</p>
460 <p>When a client requests a page on your server, but the server
461 cannot find a single page that matches the
462 <code>Accept-language</code> sent by
463 the browser, the server will return either a "No Acceptable
464 Variant" or "Multiple Choices" response to the client. To avoid
465 these error messages, it is possible to configure Apache to ignore
466 the <code>Accept-language</code> in these cases and provide a
467 document that does not explicitly match the client's request. The
468 <code class="directive"><a href="./mod/mod_negotiation.html#forcelanguagepriority">ForceLanguagePriority</a></code>
469 directive can be used to override one or both of these error
470 messages and substitute the servers judgement in the form of the
471 <code class="directive"><a href="./mod/mod_negotiation.html#languagepriority">LanguagePriority</a></code>
474 <p>The server will also attempt to match language-subsets when no
475 other match can be found. For example, if a client requests
476 documents with the language <code>en-GB</code> for British
477 English, the server is not normally allowed by the HTTP/1.1
478 standard to match that against a document that is marked as simply
479 <code>en</code>. (Note that it is almost surely a configuration
480 error to include <code>en-GB</code> and not <code>en</code> in the
481 <code>Accept-Language</code> header, since it is very unlikely
482 that a reader understands British English, but doesn't understand
483 English in general. Unfortunately, many current clients have
484 default configurations that resemble this.) However, if no other
485 language match is possible and the server is about to return a "No
486 Acceptable Variants" error or fallback to the <code class="directive"><a href="./mod/mod_negotiation.html#languagepriority">LanguagePriority</a></code>, the server
487 will ignore the subset specification and match <code>en-GB</code>
488 against <code>en</code> documents. Implicitly, Apache will add
489 the parent language to the client's acceptable language list with
490 a very low quality value. But note that if the client requests
491 "en-GB; q=0.9, fr; q=0.8", and the server has documents
492 designated "en" and "fr", then the "fr" document will be returned.
493 This is necessary to maintain compliance with the HTTP/1.1
494 specification and to work effectively with properly configured
497 <p>In order to support advanced techniques (such as cookies or
498 special URL-paths) to determine the user's preferred language,
499 since Apache 2.0.47 <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> recognizes
500 the <a href="env.html">environment variable</a>
501 <code>prefer-language</code>. If it exists and contains an
502 appropriate language tag, <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> will
503 try to select a matching variant. If there's no such variant,
504 the normal negotiation process applies.</p>
506 <div class="example"><h3>Example</h3><p><code>
507 SetEnvIf Cookie "language=en" prefer-language=en<br />
508 SetEnvIf Cookie "language=fr" prefer-language=fr
511 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
512 <div class="section">
513 <h2><a name="extensions" id="extensions">Extensions to Transparent Content
516 <p>Apache extends the transparent content negotiation protocol (RFC
517 2295) as follows. A new <code>{encoding ..}</code> element is used in
518 variant lists to label variants which are available with a specific
519 content-encoding only. The implementation of the RVSA/1.0 algorithm
520 (RFC 2296) is extended to recognize encoded variants in the list, and
521 to use them as candidate variants whenever their encodings are
522 acceptable according to the <code>Accept-Encoding</code> request
523 header. The RVSA/1.0 implementation does not round computed quality
524 factors to 5 decimal places before choosing the best variant.</p>
525 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
526 <div class="section">
527 <h2><a name="naming" id="naming">Note on hyperlinks and naming conventions</a></h2>
529 <p>If you are using language negotiation you can choose between
530 different naming conventions, because files can have more than
531 one extension, and the order of the extensions is normally
532 irrelevant (see the <a href="mod/mod_mime.html#multipleext">mod_mime</a> documentation
535 <p>A typical file has a MIME-type extension (<em>e.g.</em>,
536 <code>html</code>), maybe an encoding extension (<em>e.g.</em>,
537 <code>gz</code>), and of course a language extension
538 (<em>e.g.</em>, <code>en</code>) when we have different
539 language variants of this file.</p>
548 <li>foo.en.html.gz</li>
551 <p>Here some more examples of filenames together with valid and
552 invalid hyperlinks:</p>
554 <table class="bordered">
559 <th>Valid hyperlink</th>
561 <th>Invalid hyperlink</th>
565 <td><em>foo.html.en</em></td>
574 <td><em>foo.en.html</em></td>
582 <td><em>foo.html.en.gz</em></td>
592 <td><em>foo.en.html.gz</em></td>
602 <td><em>foo.gz.html.en</em></td>
612 <td><em>foo.html.gz.en</em></td>
622 <p>Looking at the table above, you will notice that it is always
623 possible to use the name without any extensions in a hyperlink
624 (<em>e.g.</em>, <code>foo</code>). The advantage is that you
625 can hide the actual type of a document rsp. file and can change
626 it later, <em>e.g.</em>, from <code>html</code> to
627 <code>shtml</code> or <code>cgi</code> without changing any
628 hyperlink references.</p>
630 <p>If you want to continue to use a MIME-type in your
631 hyperlinks (<em>e.g.</em> <code>foo.html</code>) the language
632 extension (including an encoding extension if there is one)
633 must be on the right hand side of the MIME-type extension
634 (<em>e.g.</em>, <code>foo.html.en</code>).</p>
635 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
636 <div class="section">
637 <h2><a name="caching" id="caching">Note on Caching</a></h2>
639 <p>When a cache stores a representation, it associates it with
640 the request URL. The next time that URL is requested, the cache
641 can use the stored representation. But, if the resource is
642 negotiable at the server, this might result in only the first
643 requested variant being cached and subsequent cache hits might
644 return the wrong response. To prevent this, Apache normally
645 marks all responses that are returned after content negotiation
646 as non-cacheable by HTTP/1.0 clients. Apache also supports the
647 HTTP/1.1 protocol features to allow caching of negotiated
650 <p>For requests which come from a HTTP/1.0 compliant client
651 (either a browser or a cache), the directive <code class="directive"><a href="./mod/mod_negotiation.html#cachenegotiateddocs">CacheNegotiatedDocs</a></code> can be
652 used to allow caching of responses which were subject to
653 negotiation. This directive can be given in the server config or
654 virtual host, and takes no arguments. It has no effect on requests
655 from HTTP/1.1 clients.</p>
657 <div class="bottomlang">
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