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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>
<p class="apache">Apache HTTP Server Version 2.0</p>
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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 2.0</a></div><div id="page-content"><div id="preamble"><h1>Content Negotiation</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="./en/content-negotiation.html" title="English"> en </a> |
<a href="./fr/content-negotiation.html" hreflang="fr" rel="alternate" title="Fran鏰is"> fr </a> |
<a href="./ja/content-negotiation.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
<a href="./ko/content-negotiation.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
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<p>Apache supports content negotiation as described in
the HTTP/1.1 specification. It can choose the best
representation of a resource based on the browser-supplied
preferences for media type, languages, character set and
encoding. It also implements a couple of features to give
more intelligent handling of requests from browsers that send
incomplete negotiation information.</p>
<p>Content negotiation is provided by the
<code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> module, which is compiled in
by default.</p>
</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#about">About Content Negotiation</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#negotiation">Negotiation in Apache</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#methods">The Negotiation Methods</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#better">Fiddling with Quality
Values</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#extensions">Extensions to Transparent Content
Negotiation</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#naming">Note on hyperlinks and naming conventions</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#caching">Note on Caching</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#more">More Information</a></li>
</ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="about" id="about">About Content Negotiation</a></h2>
<p>A resource may be available in several different
representations. For example, it might be available in
different languages or different media types, or a combination.
One way of selecting the most appropriate choice is to give the
user an index page, and let them select. However it is often
possible for the server to choose automatically. This works
because browsers can send, as part of each request, information
about what representations they prefer. For example, a browser
could indicate that it would like to see information in French,
if possible, else English will do. Browsers indicate their
preferences by headers in the request. To request only French
representations, the browser would send</p>
<div class="example"><p><code>Accept-Language: fr</code></p></div>
<p>Note that this preference will only be applied when there is
a choice of representations and they vary by language.</p>
<p>As an example of a more complex request, this browser has
been configured to accept French and English, but prefer
French, and to accept various media types, preferring HTML over
plain text or other text types, and preferring GIF or JPEG over
other media types, but also allowing any other media type as a
last resort:</p>
<div class="example"><p><code>
Accept-Language: fr; q=1.0, en; q=0.5<br />
Accept: text/html; q=1.0, text/*; q=0.8, image/gif; q=0.6, image/jpeg; q=0.6, image/*; q=0.5, */*; q=0.1
</code></p></div>
<p>Apache supports 'server driven' content negotiation, as
defined in the HTTP/1.1 specification. It fully supports the
<code>Accept</code>, <code>Accept-Language</code>,
<code>Accept-Charset</code> and<code>Accept-Encoding</code>
request headers. Apache also supports 'transparent'
content negotiation, which is an experimental negotiation
protocol defined in RFC 2295 and RFC 2296. It does not offer
support for 'feature negotiation' as defined in these RFCs.</p>
<p>A <strong>resource</strong> is a conceptual entity
identified by a URI (RFC 2396). An HTTP server like Apache
provides access to <strong>representations</strong> of the
resource(s) within its namespace, with each representation in
the form of a sequence of bytes with a defined media type,
character set, encoding, etc. Each resource may be associated
with zero, one, or more than one representation at any given
time. If multiple representations are available, the resource
is referred to as <strong>negotiable</strong> and each of its
representations is termed a <strong>variant</strong>. The ways
in which the variants for a negotiable resource vary are called
the <strong>dimensions</strong> of negotiation.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="negotiation" id="negotiation">Negotiation in Apache</a></h2>
<p>In order to negotiate a resource, the server needs to be
given information about each of the variants. This is done in
one of two ways:</p>
<ul>
<li>Using a type map (<em>i.e.</em>, a <code>*.var</code>
file) which names the files containing the variants
explicitly, or</li>
<li>Using a 'MultiViews' search, where the server does an
implicit filename pattern match and chooses from among the
results.</li>
</ul>
<h3><a name="type-map" id="type-map">Using a type-map file</a></h3>
<p>A type map is a document which is associated with the
handler named <code>type-map</code> (or, for
backwards-compatibility with older Apache configurations, the
MIME type <code>application/x-type-map</code>). Note that to
use this feature, you must have a handler set in the
configuration that defines a file suffix as
<code>type-map</code>; this is best done with</p>
<div class="example"><p><code>AddHandler type-map .var</code></p></div>
<p>in the server configuration file.</p>
<p>Type map files should have the same name as the resource
which they are describing, and have an entry for each available
variant; these entries consist of contiguous HTTP-format header
lines. Entries for different variants are separated by blank
lines. Blank lines are illegal within an entry. It is
conventional to begin a map file with an entry for the combined
entity as a whole (although this is not required, and if
present will be ignored). An example map file is shown below.
This file would be named <code>foo.var</code>, as it describes
a resource named <code>foo</code>.</p>
<div class="example"><p><code>
URI: foo<br />
<br />
URI: foo.en.html<br />
Content-type: text/html<br />
Content-language: en<br />
<br />
URI: foo.fr.de.html<br />
Content-type: text/html;charset=iso-8859-2<br />
Content-language: fr, de<br />
</code></p></div>
<p>Note also that a typemap file will take precedence over the
filename's extension, even when Multiviews is on. If the
variants have different source qualities, that may be indicated
by the "qs" parameter to the media type, as in this picture
(available as JPEG, GIF, or ASCII-art): </p>
<div class="example"><p><code>
URI: foo<br />
<br />
URI: foo.jpeg<br />
Content-type: image/jpeg; qs=0.8<br />
<br />
URI: foo.gif<br />
Content-type: image/gif; qs=0.5<br />
<br />
URI: foo.txt<br />
Content-type: text/plain; qs=0.01<br />
</code></p></div>
<p>qs values can vary in the range 0.000 to 1.000. Note that
any variant with a qs value of 0.000 will never be chosen.
Variants with no 'qs' parameter value are given a qs factor of
1.0. The qs parameter indicates the relative 'quality' of this
variant compared to the other available variants, independent
of the client's capabilities. For example, a JPEG file is
usually of higher source quality than an ASCII file if it is
attempting to represent a photograph. However, if the resource
being represented is an original ASCII art, then an ASCII
representation would have a higher source quality than a JPEG
representation. A qs value is therefore specific to a given
variant depending on the nature of the resource it
represents.</p>
<p>The full list of headers recognized is available in the <a href="mod/mod_negotiation.html#typemaps">mod_negotation
typemap</a> documentation.</p>
<h3><a name="multiviews" id="multiviews">Multiviews</a></h3>
<p><code>MultiViews</code> is a per-directory option, meaning it
can be set with an <code class="directive"><a href="./mod/core.html#options">Options</a></code>
directive within a <code class="directive"><a href="./mod/core.html#directory"><Directory></a></code>, <code class="directive"><a href="./mod/core.html#location"><Location></a></code> or <code class="directive"><a href="./mod/core.html#files"><Files></a></code> section in
<code>httpd.conf</code>, or (if <code class="directive"><a href="./mod/core.html#allowoverride">AllowOverride</a></code> is properly set) in
<code>.htaccess</code> files. Note that <code>Options All</code>
does not set <code>MultiViews</code>; you have to ask for it by
name.</p>
<p>The effect of <code>MultiViews</code> is as follows: if the
server receives a request for <code>/some/dir/foo</code>, if
<code>/some/dir</code> has <code>MultiViews</code> enabled, and
<code>/some/dir/foo</code> does <em>not</em> exist, then the
server reads the directory looking for files named foo.*, and
effectively fakes up a type map which names all those files,
assigning them the same media types and content-encodings it
would have if the client had asked for one of them by name. It
then chooses the best match to the client's requirements.</p>
<p><code>MultiViews</code> may also apply to searches for the file
named by the <code class="directive"><a href="./mod/mod_dir.html#directoryindex">DirectoryIndex</a></code> directive, if the
server is trying to index a directory. If the configuration files
specify</p>
<div class="example"><p><code>DirectoryIndex index</code></p></div>
<p>then the server will arbitrate between <code>index.html</code>
and <code>index.html3</code> if both are present. If neither
are present, and <code>index.cgi</code> is there, the server
will run it.</p>
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