github.com/krum110487/go-htaccess@v0.0.0-20240316004156-60641c8e7598/tests/data/apache_2_2_34/manual/content-negotiation.html.en (about) 1 <?xml version="1.0" encoding="ISO-8859-1"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 3 <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head> 4 <meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type" /> 5 <!-- 6 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 7 This file is generated from xml source: DO NOT EDIT 8 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 9 --> 10 <title>Content Negotiation - Apache HTTP Server Version 2.2</title> 11 <link href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /> 12 <link href="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /> 13 <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" /> 14 <script src="./style/scripts/prettify.min.js" type="text/javascript"> 15 </script> 16 17 <link href="./images/favicon.ico" rel="shortcut icon" /><link href="http://httpd.apache.org/docs/current/content-negotiation.html" rel="canonical" /></head> 18 <body id="manual-page"><div id="page-header"> 19 <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> 20 <p class="apache">Apache HTTP Server Version 2.2</p> 21 <img alt="" src="./images/feather.gif" /></div> 22 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div> 23 <div id="path"> 24 <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></div><div id="page-content"><div class="retired"><h4>Please note</h4> 25 <p> This document refers to a legacy release (<strong>2.2</strong>) of Apache httpd. The active release (<strong>2.4</strong>) is documented <a href="http://httpd.apache.org/docs/current">here</a>. If you have not already upgraded, please follow <a href="http://httpd.apache.org/docs/current/upgrading.html">this link</a> for more information.</p> 26 <p>You may follow <a href="http://httpd.apache.org/docs/current/content-negotiation.html">this link</a> to go to the current version of this document.</p></div><div id="preamble"><h1>Content Negotiation</h1> 27 <div class="toplang"> 28 <p><span>Available Languages: </span><a href="./en/content-negotiation.html" title="English"> en </a> | 29 <a href="./fr/content-negotiation.html" hreflang="fr" rel="alternate" title="Français"> fr </a> | 30 <a href="./ja/content-negotiation.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | 31 <a href="./ko/content-negotiation.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | 32 <a href="./tr/content-negotiation.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p> 33 </div> 34 35 36 <p>Apache supports content negotiation as described in 37 the HTTP/1.1 specification. It can choose the best 38 representation of a resource based on the browser-supplied 39 preferences for media type, languages, character set and 40 encoding. It also implements a couple of features to give 41 more intelligent handling of requests from browsers that send 42 incomplete negotiation information.</p> 43 44 <p>Content negotiation is provided by the 45 <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> module, which is compiled in 46 by default.</p> 47 </div> 48 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#about">About Content Negotiation</a></li> 49 <li><img alt="" src="./images/down.gif" /> <a href="#negotiation">Negotiation in Apache</a></li> 50 <li><img alt="" src="./images/down.gif" /> <a href="#methods">The Negotiation Methods</a></li> 51 <li><img alt="" src="./images/down.gif" /> <a href="#better">Fiddling with Quality 52 Values</a></li> 53 <li><img alt="" src="./images/down.gif" /> <a href="#extensions">Extensions to Transparent Content 54 Negotiation</a></li> 55 <li><img alt="" src="./images/down.gif" /> <a href="#naming">Note on hyperlinks and naming conventions</a></li> 56 <li><img alt="" src="./images/down.gif" /> <a href="#caching">Note on Caching</a></li> 57 </ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div> 58 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 59 <div class="section"> 60 <h2><a name="about" id="about">About Content Negotiation</a></h2> 61 62 <p>A resource may be available in several different 63 representations. For example, it might be available in 64 different languages or different media types, or a combination. 65 One way of selecting the most appropriate choice is to give the 66 user an index page, and let them select. However it is often 67 possible for the server to choose automatically. This works 68 because browsers can send, as part of each request, information 69 about what representations they prefer. For example, a browser 70 could indicate that it would like to see information in French, 71 if possible, else English will do. Browsers indicate their 72 preferences by headers in the request. To request only French 73 representations, the browser would send</p> 74 75 <div class="example"><p><code>Accept-Language: fr</code></p></div> 76 77 <p>Note that this preference will only be applied when there is 78 a choice of representations and they vary by language.</p> 79 80 <p>As an example of a more complex request, this browser has 81 been configured to accept French and English, but prefer 82 French, and to accept various media types, preferring HTML over 83 plain text or other text types, and preferring GIF or JPEG over 84 other media types, but also allowing any other media type as a 85 last resort:</p> 86 87 <div class="example"><p><code> 88 Accept-Language: fr; q=1.0, en; q=0.5<br /> 89 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 90 </code></p></div> 91 92 <p>Apache supports 'server driven' content negotiation, as 93 defined in the HTTP/1.1 specification. It fully supports the 94 <code>Accept</code>, <code>Accept-Language</code>, 95 <code>Accept-Charset</code> and<code>Accept-Encoding</code> 96 request headers. Apache also supports 'transparent' 97 content negotiation, which is an experimental negotiation 98 protocol defined in RFC 2295 and RFC 2296. It does not offer 99 support for 'feature negotiation' as defined in these RFCs.</p> 100 101 <p>A <strong>resource</strong> is a conceptual entity 102 identified by a URI (RFC 2396). An HTTP server like Apache 103 provides access to <strong>representations</strong> of the 104 resource(s) within its namespace, with each representation in 105 the form of a sequence of bytes with a defined media type, 106 character set, encoding, etc. Each resource may be associated 107 with zero, one, or more than one representation at any given 108 time. If multiple representations are available, the resource 109 is referred to as <strong>negotiable</strong> and each of its 110 representations is termed a <strong>variant</strong>. The ways 111 in which the variants for a negotiable resource vary are called 112 the <strong>dimensions</strong> of negotiation.</p> 113 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 114 <div class="section"> 115 <h2><a name="negotiation" id="negotiation">Negotiation in Apache</a></h2> 116 117 <p>In order to negotiate a resource, the server needs to be 118 given information about each of the variants. This is done in 119 one of two ways:</p> 120 121 <ul> 122 <li>Using a type map (<em>i.e.</em>, a <code>*.var</code> 123 file) which names the files containing the variants 124 explicitly, or</li> 125 126 <li>Using a 'MultiViews' search, where the server does an 127 implicit filename pattern match and chooses from among the 128 results.</li> 129 </ul> 130 131 <h3><a name="type-map" id="type-map">Using a type-map file</a></h3> 132 133 <p>A type map is a document which is associated with the handler 134 named <code>type-map</code> (or, for backwards-compatibility with 135 older Apache configurations, the <a class="glossarylink" href="./glossary.html#mime-type" title="see glossary">MIME-type</a> 136 <code>application/x-type-map</code>). Note that to use this 137 feature, you must have a handler set in the configuration that 138 defines a file suffix as <code>type-map</code>; this is best done 139 with</p> 140 141 <div class="example"><p><code>AddHandler type-map .var</code></p></div> 142 143 <p>in the server configuration file.</p> 144 145 <p>Type map files should have the same name as the resource 146 which they are describing, and have an entry for each available 147 variant; these entries consist of contiguous HTTP-format header 148 lines. Entries for different variants are separated by blank 149 lines. Blank lines are illegal within an entry. It is 150 conventional to begin a map file with an entry for the combined 151 entity as a whole (although this is not required, and if 152 present will be ignored). An example map file is shown below. 153 This file would be named <code>foo.var</code>, as it describes 154 a resource named <code>foo</code>.</p> 155 156 <div class="example"><p><code> 157 URI: foo<br /> 158 <br /> 159 URI: foo.en.html<br /> 160 Content-type: text/html<br /> 161 Content-language: en<br /> 162 <br /> 163 URI: foo.fr.de.html<br /> 164 Content-type: text/html;charset=iso-8859-2<br /> 165 Content-language: fr, de<br /> 166 </code></p></div> 167 <p>Note also that a typemap file will take precedence over the 168 filename's extension, even when Multiviews is on. If the 169 variants have different source qualities, that may be indicated 170 by the "qs" parameter to the media type, as in this picture 171 (available as JPEG, GIF, or ASCII-art): </p> 172 173 <div class="example"><p><code> 174 URI: foo<br /> 175 <br /> 176 URI: foo.jpeg<br /> 177 Content-type: image/jpeg; qs=0.8<br /> 178 <br /> 179 URI: foo.gif<br /> 180 Content-type: image/gif; qs=0.5<br /> 181 <br /> 182 URI: foo.txt<br /> 183 Content-type: text/plain; qs=0.01<br /> 184 </code></p></div> 185 186 <p>qs values can vary in the range 0.000 to 1.000. Note that 187 any variant with a qs value of 0.000 will never be chosen. 188 Variants with no 'qs' parameter value are given a qs factor of 189 1.0. The qs parameter indicates the relative 'quality' of this 190 variant compared to the other available variants, independent 191 of the client's capabilities. For example, a JPEG file is 192 usually of higher source quality than an ASCII file if it is 193 attempting to represent a photograph. However, if the resource 194 being represented is an original ASCII art, then an ASCII 195 representation would have a higher source quality than a JPEG 196 representation. A qs value is therefore specific to a given 197 variant depending on the nature of the resource it 198 represents.</p> 199 200 <p>The full list of headers recognized is available in the <a href="mod/mod_negotiation.html#typemaps">mod_negotiation 201 typemap</a> documentation.</p> 202 203 204 <h3><a name="multiviews" id="multiviews">Multiviews</a></h3> 205 206 <p><code>MultiViews</code> is a per-directory option, meaning it 207 can be set with an <code class="directive"><a href="./mod/core.html#options">Options</a></code> 208 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 209 <code>httpd.conf</code>, or (if <code class="directive"><a href="./mod/core.html#allowoverride">AllowOverride</a></code> is properly set) in 210 <code>.htaccess</code> files. Note that <code>Options All</code> 211 does not set <code>MultiViews</code>; you have to ask for it by 212 name.</p> 213 214 <p>The effect of <code>MultiViews</code> is as follows: if the 215 server receives a request for <code>/some/dir/foo</code>, if 216 <code>/some/dir</code> has <code>MultiViews</code> enabled, and 217 <code>/some/dir/foo</code> does <em>not</em> exist, then the 218 server reads the directory looking for files named foo.*, and 219 effectively fakes up a type map which names all those files, 220 assigning them the same media types and content-encodings it 221 would have if the client had asked for one of them by name. It 222 then chooses the best match to the client's requirements.</p> 223 224 <p><code>MultiViews</code> may also apply to searches for the file 225 named by the <code class="directive"><a href="./mod/mod_dir.html#directoryindex">DirectoryIndex</a></code> directive, if the 226 server is trying to index a directory. If the configuration files 227 specify</p> 228 <div class="example"><p><code>DirectoryIndex index</code></p></div> 229 <p>then the server will arbitrate between <code>index.html</code> 230 and <code>index.html3</code> if both are present. If neither 231 are present, and <code>index.cgi</code> is there, the server 232 will run it.</p> 233 234 <p>If one of the files found when reading the directory does not 235 have an extension recognized by <code>mod_mime</code> to designate 236 its Charset, Content-Type, Language, or Encoding, then the result 237 depends on the setting of the <code class="directive"><a href="./mod/mod_mime.html#multiviewsmatch">MultiViewsMatch</a></code> directive. This 238 directive determines whether handlers, filters, and other 239 extension types can participate in MultiViews negotiation.</p> 240 241 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 242 <div class="section"> 243 <h2><a name="methods" id="methods">The Negotiation Methods</a></h2> 244 245 <p>After Apache has obtained a list of the variants for a given 246 resource, either from a type-map file or from the filenames in 247 the directory, it invokes one of two methods to decide on the 248 'best' variant to return, if any. It is not necessary to know 249 any of the details of how negotiation actually takes place in 250 order to use Apache's content negotiation features. However the 251 rest of this document explains the methods used for those 252 interested. </p> 253 254 <p>There are two negotiation methods:</p> 255 256 <ol> 257 <li><strong>Server driven negotiation with the Apache 258 algorithm</strong> is used in the normal case. The Apache 259 algorithm is explained in more detail below. When this 260 algorithm is used, Apache can sometimes 'fiddle' the quality 261 factor of a particular dimension to achieve a better result. 262 The ways Apache can fiddle quality factors is explained in 263 more detail below.</li> 264 265 <li><strong>Transparent content negotiation</strong> is used 266 when the browser specifically requests this through the 267 mechanism defined in RFC 2295. This negotiation method gives 268 the browser full control over deciding on the 'best' variant, 269 the result is therefore dependent on the specific algorithms 270 used by the browser. As part of the transparent negotiation 271 process, the browser can ask Apache to run the 'remote 272 variant selection algorithm' defined in RFC 2296.</li> 273 </ol> 274 275 <h3><a name="dimensions" id="dimensions">Dimensions of Negotiation</a></h3> 276 277 <table> 278 279 <tr valign="top"> 280 <th>Dimension</th> 281 282 <th>Notes</th> 283 </tr> 284 285 <tr valign="top"> 286 <td>Media Type</td> 287 288 <td>Browser indicates preferences with the <code>Accept</code> 289 header field. Each item can have an associated quality factor. 290 Variant description can also have a quality factor (the "qs" 291 parameter).</td> 292 </tr> 293 294 <tr valign="top"> 295 <td>Language</td> 296 297 <td>Browser indicates preferences with the 298 <code>Accept-Language</code> header field. Each item can have 299 a quality factor. Variants can be associated with none, one or 300 more than one language.</td> 301 </tr> 302 303 <tr valign="top"> 304 <td>Encoding</td> 305 306 <td>Browser indicates preference with the 307 <code>Accept-Encoding</code> header field. Each item can have 308 a quality factor.</td> 309 </tr> 310 311 <tr valign="top"> 312 <td>Charset</td> 313 314 <td>Browser indicates preference with the 315 <code>Accept-Charset</code> header field. Each item can have a 316 quality factor. Variants can indicate a charset as a parameter 317 of the media type.</td> 318 </tr> 319 </table> 320 321 322 <h3><a name="algorithm" id="algorithm">Apache Negotiation Algorithm</a></h3> 323 324 <p>Apache can use the following algorithm to select the 'best' 325 variant (if any) to return to the browser. This algorithm is 326 not further configurable. It operates as follows:</p> 327 328 <ol> 329 <li>First, for each dimension of the negotiation, check the 330 appropriate <em>Accept*</em> header field and assign a 331 quality to each variant. If the <em>Accept*</em> header for 332 any dimension implies that this variant is not acceptable, 333 eliminate it. If no variants remain, go to step 4.</li> 334 335 <li> 336 Select the 'best' variant by a process of elimination. Each 337 of the following tests is applied in order. Any variants 338 not selected at each test are eliminated. After each test, 339 if only one variant remains, select it as the best match 340 and proceed to step 3. If more than one variant remains, 341 move on to the next test. 342 343 <ol> 344 <li>Multiply the quality factor from the <code>Accept</code> 345 header with the quality-of-source factor for this variants 346 media type, and select the variants with the highest 347 value.</li> 348 349 <li>Select the variants with the highest language quality 350 factor.</li> 351 352 <li>Select the variants with the best language match, 353 using either the order of languages in the 354 <code>Accept-Language</code> header (if present), or else 355 the order of languages in the <code>LanguagePriority</code> 356 directive (if present).</li> 357 358 <li>Select the variants with the highest 'level' media 359 parameter (used to give the version of text/html media 360 types).</li> 361 362 <li>Select variants with the best charset media 363 parameters, as given on the <code>Accept-Charset</code> 364 header line. Charset ISO-8859-1 is acceptable unless 365 explicitly excluded. Variants with a <code>text/*</code> 366 media type but not explicitly associated with a particular 367 charset are assumed to be in ISO-8859-1.</li> 368 369 <li>Select those variants which have associated charset 370 media parameters that are <em>not</em> ISO-8859-1. If 371 there are no such variants, select all variants 372 instead.</li> 373 374 <li>Select the variants with the best encoding. If there 375 are variants with an encoding that is acceptable to the 376 user-agent, select only these variants. Otherwise if 377 there is a mix of encoded and non-encoded variants, 378 select only the unencoded variants. If either all 379 variants are encoded or all variants are not encoded, 380 select all variants.</li> 381 382 <li>Select the variants with the smallest content 383 length.</li> 384 385 <li>Select the first variant of those remaining. This 386 will be either the first listed in the type-map file, or 387 when variants are read from the directory, the one whose 388 file name comes first when sorted using ASCII code 389 order.</li> 390 </ol> 391 </li> 392 393 <li>The algorithm has now selected one 'best' variant, so 394 return it as the response. The HTTP response header 395 <code>Vary</code> is set to indicate the dimensions of 396 negotiation (browsers and caches can use this information when 397 caching the resource). End.</li> 398 399 <li>To get here means no variant was selected (because none 400 are acceptable to the browser). Return a 406 status (meaning 401 "No acceptable representation") with a response body 402 consisting of an HTML document listing the available 403 variants. Also set the HTTP <code>Vary</code> header to 404 indicate the dimensions of variance.</li> 405 </ol> 406 407 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 408 <div class="section"> 409 <h2><a name="better" id="better">Fiddling with Quality 410 Values</a></h2> 411 412 <p>Apache sometimes changes the quality values from what would 413 be expected by a strict interpretation of the Apache 414 negotiation algorithm above. This is to get a better result 415 from the algorithm for browsers which do not send full or 416 accurate information. Some of the most popular browsers send 417 <code>Accept</code> header information which would otherwise 418 result in the selection of the wrong variant in many cases. If a 419 browser sends full and correct information these fiddles will not 420 be applied.</p> 421 422 <h3><a name="wildcards" id="wildcards">Media Types and Wildcards</a></h3> 423 424 <p>The <code>Accept:</code> request header indicates preferences 425 for media types. It can also include 'wildcard' media types, such 426 as "image/*" or "*/*" where the * matches any string. So a request 427 including:</p> 428 429 <div class="example"><p><code>Accept: image/*, */*</code></p></div> 430 431 <p>would indicate that any type starting "image/" is acceptable, 432 as is any other type. 433 Some browsers routinely send wildcards in addition to explicit 434 types they can handle. For example:</p> 435 436 <div class="example"><p><code> 437 Accept: text/html, text/plain, image/gif, image/jpeg, */* 438 </code></p></div> 439 <p>The intention of this is to indicate that the explicitly listed 440 types are preferred, but if a different representation is 441 available, that is ok too. Using explicit quality values, 442 what the browser really wants is something like:</p> 443 <div class="example"><p><code> 444 Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01 445 </code></p></div> 446 <p>The explicit types have no quality factor, so they default to a 447 preference of 1.0 (the highest). The wildcard */* is given a 448 low preference of 0.01, so other types will only be returned if 449 no variant matches an explicitly listed type.</p> 450 451 <p>If the <code>Accept:</code> header contains <em>no</em> q 452 factors at all, Apache sets the q value of "*/*", if present, to 453 0.01 to emulate the desired behavior. It also sets the q value of 454 wildcards of the format "type/*" to 0.02 (so these are preferred 455 over matches against "*/*". If any media type on the 456 <code>Accept:</code> header contains a q factor, these special 457 values are <em>not</em> applied, so requests from browsers which 458 send the explicit information to start with work as expected.</p> 459 460 461 <h3><a name="exceptions" id="exceptions">Language Negotiation Exceptions</a></h3> 462 463 <p>New in Apache 2.0, some exceptions have been added to the 464 negotiation algorithm to allow graceful fallback when language 465 negotiation fails to find a match.</p> 466 467 <p>When a client requests a page on your server, but the server 468 cannot find a single page that matches the 469 <code>Accept-language</code> sent by 470 the browser, the server will return either a "No Acceptable 471 Variant" or "Multiple Choices" response to the client. To avoid 472 these error messages, it is possible to configure Apache to ignore 473 the <code>Accept-language</code> in these cases and provide a 474 document that does not explicitly match the client's request. The 475 <code class="directive"><a href="./mod/mod_negotiation.html#forcelanguagepriority">ForceLanguagePriority</a></code> 476 directive can be used to override one or both of these error 477 messages and substitute the servers judgement in the form of the 478 <code class="directive"><a href="./mod/mod_negotiation.html#languagepriority">LanguagePriority</a></code> 479 directive.</p> 480 481 <p>The server will also attempt to match language-subsets when no 482 other match can be found. For example, if a client requests 483 documents with the language <code>en-GB</code> for British 484 English, the server is not normally allowed by the HTTP/1.1 485 standard to match that against a document that is marked as simply 486 <code>en</code>. (Note that it is almost surely a configuration 487 error to include <code>en-GB</code> and not <code>en</code> in the 488 <code>Accept-Language</code> header, since it is very unlikely 489 that a reader understands British English, but doesn't understand 490 English in general. Unfortunately, many current clients have 491 default configurations that resemble this.) However, if no other 492 language match is possible and the server is about to return a "No 493 Acceptable Variants" error or fallback to the <code class="directive"><a href="./mod/mod_negotiation.html#languagepriority">LanguagePriority</a></code>, the server 494 will ignore the subset specification and match <code>en-GB</code> 495 against <code>en</code> documents. Implicitly, Apache will add 496 the parent language to the client's acceptable language list with 497 a very low quality value. But note that if the client requests 498 "en-GB; q=0.9, fr; q=0.8", and the server has documents 499 designated "en" and "fr", then the "fr" document will be returned. 500 This is necessary to maintain compliance with the HTTP/1.1 501 specification and to work effectively with properly configured 502 clients.</p> 503 504 <p>In order to support advanced techniques (such as cookies or 505 special URL-paths) to determine the user's preferred language, 506 since Apache 2.0.47 <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> recognizes 507 the <a href="env.html">environment variable</a> 508 <code>prefer-language</code>. If it exists and contains an 509 appropriate language tag, <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> will 510 try to select a matching variant. If there's no such variant, 511 the normal negotiation process applies.</p> 512 513 <div class="example"><h3>Example</h3><p><code> 514 SetEnvIf Cookie "language=(.+)" prefer-language=$1<br /> 515 Header append Vary cookie 516 </code></p></div> 517 518 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 519 <div class="section"> 520 <h2><a name="extensions" id="extensions">Extensions to Transparent Content 521 Negotiation</a></h2> 522 523 <p>Apache extends the transparent content negotiation protocol (RFC 524 2295) as follows. A new <code>{encoding ..}</code> element is used in 525 variant lists to label variants which are available with a specific 526 content-encoding only. The implementation of the RVSA/1.0 algorithm 527 (RFC 2296) is extended to recognize encoded variants in the list, and 528 to use them as candidate variants whenever their encodings are 529 acceptable according to the <code>Accept-Encoding</code> request 530 header. The RVSA/1.0 implementation does not round computed quality 531 factors to 5 decimal places before choosing the best variant.</p> 532 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 533 <div class="section"> 534 <h2><a name="naming" id="naming">Note on hyperlinks and naming conventions</a></h2> 535 536 <p>If you are using language negotiation you can choose between 537 different naming conventions, because files can have more than 538 one extension, and the order of the extensions is normally 539 irrelevant (see the <a href="mod/mod_mime.html#multipleext">mod_mime</a> documentation 540 for details).</p> 541 542 <p>A typical file has a MIME-type extension (<em>e.g.</em>, 543 <code>html</code>), maybe an encoding extension (<em>e.g.</em>, 544 <code>gz</code>), and of course a language extension 545 (<em>e.g.</em>, <code>en</code>) when we have different 546 language variants of this file.</p> 547 548 <p>Examples:</p> 549 550 <ul> 551 <li>foo.en.html</li> 552 553 <li>foo.html.en</li> 554 555 <li>foo.en.html.gz</li> 556 </ul> 557 558 <p>Here some more examples of filenames together with valid and 559 invalid hyperlinks:</p> 560 561 <table class="bordered"> 562 563 <tr> 564 <th>Filename</th> 565 566 <th>Valid hyperlink</th> 567 568 <th>Invalid hyperlink</th> 569 </tr> 570 571 <tr> 572 <td><em>foo.html.en</em></td> 573 574 <td>foo<br /> 575 foo.html</td> 576 577 <td>-</td> 578 </tr> 579 580 <tr> 581 <td><em>foo.en.html</em></td> 582 583 <td>foo</td> 584 585 <td>foo.html</td> 586 </tr> 587 588 <tr> 589 <td><em>foo.html.en.gz</em></td> 590 591 <td>foo<br /> 592 foo.html</td> 593 594 <td>foo.gz<br /> 595 foo.html.gz</td> 596 </tr> 597 598 <tr> 599 <td><em>foo.en.html.gz</em></td> 600 601 <td>foo</td> 602 603 <td>foo.html<br /> 604 foo.html.gz<br /> 605 foo.gz</td> 606 </tr> 607 608 <tr> 609 <td><em>foo.gz.html.en</em></td> 610 611 <td>foo<br /> 612 foo.gz<br /> 613 foo.gz.html</td> 614 615 <td>foo.html</td> 616 </tr> 617 618 <tr> 619 <td><em>foo.html.gz.en</em></td> 620 621 <td>foo<br /> 622 foo.html<br /> 623 foo.html.gz</td> 624 625 <td>foo.gz</td> 626 </tr> 627 </table> 628 629 <p>Looking at the table above, you will notice that it is always 630 possible to use the name without any extensions in a hyperlink 631 (<em>e.g.</em>, <code>foo</code>). The advantage is that you 632 can hide the actual type of a document rsp. file and can change 633 it later, <em>e.g.</em>, from <code>html</code> to 634 <code>shtml</code> or <code>cgi</code> without changing any 635 hyperlink references.</p> 636 637 <p>If you want to continue to use a MIME-type in your 638 hyperlinks (<em>e.g.</em> <code>foo.html</code>) the language 639 extension (including an encoding extension if there is one) 640 must be on the right hand side of the MIME-type extension 641 (<em>e.g.</em>, <code>foo.html.en</code>).</p> 642 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 643 <div class="section"> 644 <h2><a name="caching" id="caching">Note on Caching</a></h2> 645 646 <p>When a cache stores a representation, it associates it with 647 the request URL. The next time that URL is requested, the cache 648 can use the stored representation. But, if the resource is 649 negotiable at the server, this might result in only the first 650 requested variant being cached and subsequent cache hits might 651 return the wrong response. To prevent this, Apache normally 652 marks all responses that are returned after content negotiation 653 as non-cacheable by HTTP/1.0 clients. Apache also supports the 654 HTTP/1.1 protocol features to allow caching of negotiated 655 responses.</p> 656 657 <p>For requests which come from a HTTP/1.0 compliant client 658 (either a browser or a cache), the directive <code class="directive"><a href="./mod/mod_negotiation.html#cachenegotiateddocs">CacheNegotiatedDocs</a></code> can be 659 used to allow caching of responses which were subject to 660 negotiation. This directive can be given in the server config or 661 virtual host, and takes no arguments. It has no effect on requests 662 from HTTP/1.1 clients.</p> 663 664 <p>For HTTP/1.1 clients, Apache sends a <code>Vary</code> HTTP 665 response header to indicate the negotiation dimensions for the 666 response. Caches can use this information to determine whether a 667 subsequent request can be served from the local copy. To 668 encourage a cache to use the local copy regardless of the 669 negotiation dimensions, set the <code>force-no-vary</code> <a href="env.html#special">environment variable</a>.</p> 670 671 </div></div> 672 <div class="bottomlang"> 673 <p><span>Available Languages: </span><a href="./en/content-negotiation.html" title="English"> en </a> | 674 <a href="./fr/content-negotiation.html" hreflang="fr" rel="alternate" title="Français"> fr </a> | 675 <a href="./ja/content-negotiation.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | 676 <a href="./ko/content-negotiation.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | 677 <a href="./tr/content-negotiation.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p> 678 </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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