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<title>Content Negotiation - Apache HTTP Server</title>
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<p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p>
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<p class="apache">Apache HTTP Server Version 2.2</p>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
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<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 id="preamble"><h1>Content Negotiation</h1>
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<p><span>Available Languages: </span><a href="./en/content-negotiation.html" title="English"> en </a> |
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<a href="./ja/content-negotiation.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
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<a href="./ko/content-negotiation.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
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<p>Apache supports content negotiation as described in
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the HTTP/1.1 specification. It can choose the best
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representation of a resource based on the browser-supplied
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preferences for media type, languages, character set and
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encoding. It also implements a couple of features to give
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more intelligent handling of requests from browsers that send
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incomplete negotiation information.</p>
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<p>Content negotiation is provided by the
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<code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> module, which is compiled in
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<div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#about">About Content Negotiation</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#negotiation">Negotiation in Apache</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#methods">The Negotiation Methods</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#better">Fiddling with Quality
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<li><img alt="" src="./images/down.gif" /> <a href="#extensions">Extensions to Transparent Content
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<li><img alt="" src="./images/down.gif" /> <a href="#naming">Note on hyperlinks and naming conventions</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#caching">Note on Caching</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#more">More Information</a></li>
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<div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<h2><a name="about" id="about">About Content Negotiation</a></h2>
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<p>A resource may be available in several different
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representations. For example, it might be available in
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different languages or different media types, or a combination.
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One way of selecting the most appropriate choice is to give the
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user an index page, and let them select. However it is often
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possible for the server to choose automatically. This works
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because browsers can send, as part of each request, information
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about what representations they prefer. For example, a browser
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could indicate that it would like to see information in French,
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if possible, else English will do. Browsers indicate their
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preferences by headers in the request. To request only French
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representations, the browser would send</p>
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<div class="example"><p><code>Accept-Language: fr</code></p></div>
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<p>Note that this preference will only be applied when there is
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a choice of representations and they vary by language.</p>
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<p>As an example of a more complex request, this browser has
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been configured to accept French and English, but prefer
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French, and to accept various media types, preferring HTML over
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plain text or other text types, and preferring GIF or JPEG over
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other media types, but also allowing any other media type as a
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<div class="example"><p><code>
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Accept-Language: fr; q=1.0, en; q=0.5<br />
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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
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<p>Apache supports 'server driven' content negotiation, as
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defined in the HTTP/1.1 specification. It fully supports the
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<code>Accept</code>, <code>Accept-Language</code>,
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<code>Accept-Charset</code> and<code>Accept-Encoding</code>
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request headers. Apache also supports 'transparent'
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content negotiation, which is an experimental negotiation
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protocol defined in RFC 2295 and RFC 2296. It does not offer
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support for 'feature negotiation' as defined in these RFCs.</p>
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<p>A <strong>resource</strong> is a conceptual entity
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identified by a URI (RFC 2396). An HTTP server like Apache
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provides access to <strong>representations</strong> of the
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resource(s) within its namespace, with each representation in
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the form of a sequence of bytes with a defined media type,
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character set, encoding, etc. Each resource may be associated
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with zero, one, or more than one representation at any given
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time. If multiple representations are available, the resource
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is referred to as <strong>negotiable</strong> and each of its
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representations is termed a <strong>variant</strong>. The ways
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in which the variants for a negotiable resource vary are called
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the <strong>dimensions</strong> of negotiation.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="negotiation" id="negotiation">Negotiation in Apache</a></h2>
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<p>In order to negotiate a resource, the server needs to be
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given information about each of the variants. This is done in
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<li>Using a type map (<em>i.e.</em>, a <code>*.var</code>
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file) which names the files containing the variants
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<li>Using a 'MultiViews' search, where the server does an
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implicit filename pattern match and chooses from among the
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<h3><a name="type-map" id="type-map">Using a type-map file</a></h3>
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<p>A type map is a document which is associated with the handler
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named <code>type-map</code> (or, for backwards-compatibility with
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older Apache configurations, the <a class="glossarylink" href="./glossary.html#mime-type" title="see glossary">MIME-type</a>
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<code>application/x-type-map</code>). Note that to use this
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feature, you must have a handler set in the configuration that
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defines a file suffix as <code>type-map</code>; this is best done
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<div class="example"><p><code>AddHandler type-map .var</code></p></div>
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<p>in the server configuration file.</p>
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<p>Type map files should have the same name as the resource
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which they are describing, and have an entry for each available
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variant; these entries consist of contiguous HTTP-format header
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lines. Entries for different variants are separated by blank
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lines. Blank lines are illegal within an entry. It is
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conventional to begin a map file with an entry for the combined
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entity as a whole (although this is not required, and if
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present will be ignored). An example map file is shown below.
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This file would be named <code>foo.var</code>, as it describes
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a resource named <code>foo</code>.</p>
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<div class="example"><p><code>
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URI: foo.en.html<br />
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Content-type: text/html<br />
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Content-language: en<br />
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URI: foo.fr.de.html<br />
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Content-type: text/html;charset=iso-8859-2<br />
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Content-language: fr, de<br />
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<p>Note also that a typemap file will take precedence over the
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filename's extension, even when Multiviews is on. If the
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variants have different source qualities, that may be indicated
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by the "qs" parameter to the media type, as in this picture
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(available as JPEG, GIF, or ASCII-art): </p>
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<div class="example"><p><code>
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Content-type: image/jpeg; qs=0.8<br />
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Content-type: image/gif; qs=0.5<br />
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Content-type: text/plain; qs=0.01<br />
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<p>qs values can vary in the range 0.000 to 1.000. Note that
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any variant with a qs value of 0.000 will never be chosen.
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Variants with no 'qs' parameter value are given a qs factor of
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1.0. The qs parameter indicates the relative 'quality' of this
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variant compared to the other available variants, independent
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of the client's capabilities. For example, a JPEG file is
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usually of higher source quality than an ASCII file if it is
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attempting to represent a photograph. However, if the resource
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being represented is an original ASCII art, then an ASCII
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representation would have a higher source quality than a JPEG
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representation. A qs value is therefore specific to a given
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variant depending on the nature of the resource it
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<p>The full list of headers recognized is available in the <a href="mod/mod_negotiation.html#typemaps">mod_negotation
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typemap</a> documentation.</p>
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<h3><a name="multiviews" id="multiviews">Multiviews</a></h3>
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<p><code>MultiViews</code> is a per-directory option, meaning it
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can be set with an <code class="directive"><a href="./mod/core.html#options">Options</a></code>
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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
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<code>httpd.conf</code>, or (if <code class="directive"><a href="./mod/core.html#allowoverride">AllowOverride</a></code> is properly set) in
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<code>.htaccess</code> files. Note that <code>Options All</code>
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does not set <code>MultiViews</code>; you have to ask for it by
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<p>The effect of <code>MultiViews</code> is as follows: if the
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server receives a request for <code>/some/dir/foo</code>, if
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<code>/some/dir</code> has <code>MultiViews</code> enabled, and
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<code>/some/dir/foo</code> does <em>not</em> exist, then the
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server reads the directory looking for files named foo.*, and
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effectively fakes up a type map which names all those files,
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assigning them the same media types and content-encodings it
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would have if the client had asked for one of them by name. It
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then chooses the best match to the client's requirements.</p>
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<p><code>MultiViews</code> may also apply to searches for the file
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named by the <code class="directive"><a href="./mod/mod_dir.html#directoryindex">DirectoryIndex</a></code> directive, if the
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server is trying to index a directory. If the configuration files
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<div class="example"><p><code>DirectoryIndex index</code></p></div>
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<p>then the server will arbitrate between <code>index.html</code>
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and <code>index.html3</code> if both are present. If neither
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are present, and <code>index.cgi</code> is there, the server
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<p>If one of the files found when reading the directory does not
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have an extension recognized by <code>mod_mime</code> to designate
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its Charset, Content-Type, Language, or Encoding, then the result
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depends on the setting of the <code class="directive"><a href="./mod/mod_mime.html#multiviewsmatch">MultiViewsMatch</a></code> directive. This
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directive determines whether handlers, filters, and other
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extension types can participate in MultiViews negotiation.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="methods" id="methods">The Negotiation Methods</a></h2>
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<p>After Apache has obtained a list of the variants for a given
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resource, either from a type-map file or from the filenames in
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the directory, it invokes one of two methods to decide on the
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'best' variant to return, if any. It is not necessary to know
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any of the details of how negotiation actually takes place in
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order to use Apache's content negotiation features. However the
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rest of this document explains the methods used for those
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<p>There are two negotiation methods:</p>
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<li><strong>Server driven negotiation with the Apache
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algorithm</strong> is used in the normal case. The Apache
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algorithm is explained in more detail below. When this
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algorithm is used, Apache can sometimes 'fiddle' the quality
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factor of a particular dimension to achieve a better result.
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The ways Apache can fiddle quality factors is explained in
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more detail below.</li>
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<li><strong>Transparent content negotiation</strong> is used
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when the browser specifically requests this through the
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mechanism defined in RFC 2295. This negotiation method gives
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the browser full control over deciding on the 'best' variant,
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the result is therefore dependent on the specific algorithms
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used by the browser. As part of the transparent negotiation
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process, the browser can ask Apache to run the 'remote
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variant selection algorithm' defined in RFC 2296.</li>
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<h3><a name="dimensions" id="dimensions">Dimensions of Negotiation</a></h3>
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<td>Browser indicates preferences with the <code>Accept</code>
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header field. Each item can have an associated quality factor.
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Variant description can also have a quality factor (the "qs"
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<td>Browser indicates preferences with the
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<code>Accept-Language</code> header field. Each item can have
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a quality factor. Variants can be associated with none, one or
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more than one language.</td>
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<td>Browser indicates preference with the
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<code>Accept-Encoding</code> header field. Each item can have
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a quality factor.</td>
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<td>Browser indicates preference with the
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<code>Accept-Charset</code> header field. Each item can have a
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quality factor. Variants can indicate a charset as a parameter
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of the media type.</td>
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<h3><a name="algorithm" id="algorithm">Apache Negotiation Algorithm</a></h3>
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<p>Apache can use the following algorithm to select the 'best'
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variant (if any) to return to the browser. This algorithm is
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not further configurable. It operates as follows:</p>
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<li>First, for each dimension of the negotiation, check the
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appropriate <em>Accept*</em> header field and assign a
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quality to each variant. If the <em>Accept*</em> header for
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any dimension implies that this variant is not acceptable,
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eliminate it. If no variants remain, go to step 4.</li>
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Select the 'best' variant by a process of elimination. Each
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of the following tests is applied in order. Any variants
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not selected at each test are eliminated. After each test,
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if only one variant remains, select it as the best match
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and proceed to step 3. If more than one variant remains,
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move on to the next test.
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<li>Multiply the quality factor from the <code>Accept</code>
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header with the quality-of-source factor for this variants
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media type, and select the variants with the highest
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<li>Select the variants with the highest language quality
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<li>Select the variants with the best language match,
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using either the order of languages in the
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<code>Accept-Language</code> header (if present), or else
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the order of languages in the <code>LanguagePriority</code>
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directive (if present).</li>
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<li>Select the variants with the highest 'level' media
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parameter (used to give the version of text/html media
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<li>Select variants with the best charset media
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parameters, as given on the <code>Accept-Charset</code>
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header line. Charset ISO-8859-1 is acceptable unless
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explicitly excluded. Variants with a <code>text/*</code>
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media type but not explicitly associated with a particular
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charset are assumed to be in ISO-8859-1.</li>
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<li>Select those variants which have associated charset
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media parameters that are <em>not</em> ISO-8859-1. If
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there are no such variants, select all variants
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<li>Select the variants with the best encoding. If there
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are variants with an encoding that is acceptable to the
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user-agent, select only these variants. Otherwise if
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there is a mix of encoded and non-encoded variants,
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select only the unencoded variants. If either all
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variants are encoded or all variants are not encoded,
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select all variants.</li>
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<li>Select the variants with the smallest content
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<li>Select the first variant of those remaining. This
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will be either the first listed in the type-map file, or
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when variants are read from the directory, the one whose
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file name comes first when sorted using ASCII code
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<li>The algorithm has now selected one 'best' variant, so
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return it as the response. The HTTP response header
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<code>Vary</code> is set to indicate the dimensions of
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negotiation (browsers and caches can use this information when
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caching the resource). End.</li>
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<li>To get here means no variant was selected (because none
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are acceptable to the browser). Return a 406 status (meaning
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"No acceptable representation") with a response body
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consisting of an HTML document listing the available
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variants. Also set the HTTP <code>Vary</code> header to
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indicate the dimensions of variance.</li>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="better" id="better">Fiddling with Quality
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<p>Apache sometimes changes the quality values from what would
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be expected by a strict interpretation of the Apache
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negotiation algorithm above. This is to get a better result
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from the algorithm for browsers which do not send full or
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accurate information. Some of the most popular browsers send
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<code>Accept</code> header information which would otherwise
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result in the selection of the wrong variant in many cases. If a
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browser sends full and correct information these fiddles will not
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<h3><a name="wildcards" id="wildcards">Media Types and Wildcards</a></h3>
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<p>The <code>Accept:</code> request header indicates preferences
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for media types. It can also include 'wildcard' media types, such
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as "image/*" or "*/*" where the * matches any string. So a request
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<div class="example"><p><code>Accept: image/*, */*</code></p></div>
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<p>would indicate that any type starting "image/" is acceptable,
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as is any other type.
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Some browsers routinely send wildcards in addition to explicit
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types they can handle. For example:</p>
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<div class="example"><p><code>
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Accept: text/html, text/plain, image/gif, image/jpeg, */*
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<p>The intention of this is to indicate that the explicitly listed
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types are preferred, but if a different representation is
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available, that is ok too. Using explicit quality values,
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what the browser really wants is something like:</p>
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<div class="example"><p><code>
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Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01
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<p>The explicit types have no quality factor, so they default to a
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preference of 1.0 (the highest). The wildcard */* is given a
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low preference of 0.01, so other types will only be returned if
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no variant matches an explicitly listed type.</p>
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<p>If the <code>Accept:</code> header contains <em>no</em> q
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factors at all, Apache sets the q value of "*/*", if present, to
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0.01 to emulate the desired behavior. It also sets the q value of
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wildcards of the format "type/*" to 0.02 (so these are preferred
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over matches against "*/*". If any media type on the
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<code>Accept:</code> header contains a q factor, these special
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values are <em>not</em> applied, so requests from browsers which
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send the explicit information to start with work as expected.</p>
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<h3><a name="exceptions" id="exceptions">Language Negotiation Exceptions</a></h3>
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<p>New in Apache 2.0, some exceptions have been added to the
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negotiation algorithm to allow graceful fallback when language
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negotiation fails to find a match.</p>
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<p>When a client requests a page on your server, but the server
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cannot find a single page that matches the
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<code>Accept-language</code> sent by
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the browser, the server will return either a "No Acceptable
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Variant" or "Multiple Choices" response to the client. To avoid
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these error messages, it is possible to configure Apache to ignore
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the <code>Accept-language</code> in these cases and provide a
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document that does not explicitly match the client's request. The
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<code class="directive"><a href="./mod/mod_negotiation.html#forcelanguagepriority">ForceLanguagePriority</a></code>
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directive can be used to override one or both of these error
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messages and substitute the servers judgement in the form of the
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<code class="directive"><a href="./mod/mod_negotiation.html#languagepriority">LanguagePriority</a></code>
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<p>The server will also attempt to match language-subsets when no
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other match can be found. For example, if a client requests
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documents with the language <code>en-GB</code> for British
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English, the server is not normally allowed by the HTTP/1.1
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standard to match that against a document that is marked as simply
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<code>en</code>. (Note that it is almost surely a configuration
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error to include <code>en-GB</code> and not <code>en</code> in the
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<code>Accept-Language</code> header, since it is very unlikely
481
that a reader understands British English, but doesn't understand
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English in general. Unfortunately, many current clients have
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default configurations that resemble this.) However, if no other
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language match is possible and the server is about to return a "No
485
Acceptable Variants" error or fallback to the <code class="directive"><a href="./mod/mod_negotiation.html#languagepriority">LanguagePriority</a></code>, the server
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will ignore the subset specification and match <code>en-GB</code>
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against <code>en</code> documents. Implicitly, Apache will add
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the parent language to the client's acceptable language list with
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a very low quality value. But note that if the client requests
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"en-GB; q=0.9, fr; q=0.8", and the server has documents
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designated "en" and "fr", then the "fr" document will be returned.
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This is necessary to maintain compliance with the HTTP/1.1
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specification and to work effectively with properly configured
496
<p>In order to support advanced techniques (such as cookies or
497
special URL-paths) to determine the user's preferred language,
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since Apache 2.0.47 <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> recognizes
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the <a href="env.html">environment variable</a>
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<code>prefer-language</code>. If it exists and contains an
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appropriate language tag, <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> will
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try to select a matching variant. If there's no such variant,
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the normal negotiation process applies.</p>
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<div class="example"><h3>Example</h3><p><code>
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SetEnvIf Cookie "language=(.+)" prefer-language=$1
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Header append Vary cookie
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="extensions" id="extensions">Extensions to Transparent Content
515
<p>Apache extends the transparent content negotiation protocol (RFC
516
2295) as follows. A new <code>{encoding ..}</code> element is used in
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variant lists to label variants which are available with a specific
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content-encoding only. The implementation of the RVSA/1.0 algorithm
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(RFC 2296) is extended to recognize encoded variants in the list, and
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to use them as candidate variants whenever their encodings are
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acceptable according to the <code>Accept-Encoding</code> request
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header. The RVSA/1.0 implementation does not round computed quality
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factors to 5 decimal places before choosing the best variant.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="naming" id="naming">Note on hyperlinks and naming conventions</a></h2>
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<p>If you are using language negotiation you can choose between
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different naming conventions, because files can have more than
530
one extension, and the order of the extensions is normally
531
irrelevant (see the <a href="mod/mod_mime.html#multipleext">mod_mime</a> documentation
534
<p>A typical file has a MIME-type extension (<em>e.g.</em>,
535
<code>html</code>), maybe an encoding extension (<em>e.g.</em>,
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<code>gz</code>), and of course a language extension
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(<em>e.g.</em>, <code>en</code>) when we have different
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language variants of this file.</p>
547
<li>foo.en.html.gz</li>
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<p>Here some more examples of filenames together with valid and
551
invalid hyperlinks:</p>
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<table class="bordered">
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<th>Valid hyperlink</th>
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<th>Invalid hyperlink</th>
564
<td><em>foo.html.en</em></td>
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<td><em>foo.en.html</em></td>
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<td><em>foo.html.en.gz</em></td>
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<td><em>foo.en.html.gz</em></td>
601
<td><em>foo.gz.html.en</em></td>
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<td><em>foo.html.gz.en</em></td>
621
<p>Looking at the table above, you will notice that it is always
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possible to use the name without any extensions in a hyperlink
623
(<em>e.g.</em>, <code>foo</code>). The advantage is that you
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can hide the actual type of a document rsp. file and can change
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it later, <em>e.g.</em>, from <code>html</code> to
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<code>shtml</code> or <code>cgi</code> without changing any
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hyperlink references.</p>
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<p>If you want to continue to use a MIME-type in your
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hyperlinks (<em>e.g.</em> <code>foo.html</code>) the language
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extension (including an encoding extension if there is one)
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must be on the right hand side of the MIME-type extension
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(<em>e.g.</em>, <code>foo.html.en</code>).</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="caching" id="caching">Note on Caching</a></h2>
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<p>When a cache stores a representation, it associates it with
639
the request URL. The next time that URL is requested, the cache
640
can use the stored representation. But, if the resource is
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negotiable at the server, this might result in only the first
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requested variant being cached and subsequent cache hits might
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return the wrong response. To prevent this, Apache normally
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marks all responses that are returned after content negotiation
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as non-cacheable by HTTP/1.0 clients. Apache also supports the
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HTTP/1.1 protocol features to allow caching of negotiated
649
<p>For requests which come from a HTTP/1.0 compliant client
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(either a browser or a cache), the directive <code class="directive"><a href="./mod/mod_negotiation.html#cachenegotiateddocs">CacheNegotiatedDocs</a></code> can be
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used to allow caching of responses which were subject to
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negotiation. This directive can be given in the server config or
653
virtual host, and takes no arguments. It has no effect on requests
654
from HTTP/1.1 clients.</p>
656
<p>For HTTP/1.1 clients, Apache sends a <code>Vary</code> HTTP
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response header to indicate the negotiation dimensions for the
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response. Caches can use this information to determine whether a
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subsequent request can be served from the local copy. To
660
encourage a cache to use the local copy regardless of the
661
negotiation dimensions, set the <code>force-no-vary</code> <a href="env.html#special">environment variable</a>.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="more" id="more">More Information</a></h2>
667
<p>For more information about content negotiation, see Alan
668
J. Flavell's <a href="http://ppewww.ph.gla.ac.uk/~flavell/www/lang-neg.html">Language
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Negotiation Notes</a>. But note that this document may not be
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updated to include changes in Apache 2.0.</p>
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<div class="bottomlang">
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<p><span>Available Languages: </span><a href="./en/content-negotiation.html" title="English"> en </a> |
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<a href="./ja/content-negotiation.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
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<a href="./ko/content-negotiation.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
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</div><div id="footer">
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<p class="apache">Copyright 2006 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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<p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p></div>
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