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-<HEAD>
-<TITLE>Apache Content Negotiation</TITLE>
-</HEAD>
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-<H1 ALIGN="CENTER">Content Negotiation</H1>
-
-<P>
-Apache's support for content negotiation has been updated to meet 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 is also implements a
-couple of features to give more intelligent handling of requests from
-browsers which send incomplete negotiation information. <P>
-
-Content negotiation is provided by the
-<A HREF="mod/mod_negotiation.html">mod_negotiation</A> module,
-which is compiled in by default.
-
-<HR>
-
-<H2>About Content Negotiation</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
-
-<PRE>
- Accept-Language: fr
-</PRE>
-
-<P>
-Note that this preference will only be applied when there is a choice
-of representations and they vary by language.
-<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:
-
-<PRE>
- Accept-Language: fr; q=1.0, en; q=0.5
- 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
-</PRE>
-
-Apache 1.2 supports 'server driven' content negotiation, as defined in
-the HTTP/1.1 specification. It fully supports the Accept,
-Accept-Language, Accept-Charset and Accept-Encoding request headers.
-Apache 1.3.4 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>
-
-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.
-
-<H2>Negotiation in Apache</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:
-
-<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> Using a 'MultiViews' search, where the server does an implicit
- filename pattern match and chooses from among the results.
-</UL>
-
-<H3>Using a type-map file</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 a
-
-<PRE>
- AddHandler type-map .var
-</PRE>
-
-in the server configuration file. See the comments in the sample config
-file for more details. <P>
-
-Type map files 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:
-
-<PRE>
- URI: foo
-
- URI: foo.en.html
- Content-type: text/html
- Content-language: en
-
- URI: foo.fr.de.html
- Content-type: text/html;charset=iso-8859-2
- Content-language: fr, de
-</PRE>
-
-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):
-
-<PRE>
- URI: foo
-
- URI: foo.jpeg
- Content-type: image/jpeg; qs=0.8
-
- URI: foo.gif
- Content-type: image/gif; qs=0.5
-
- URI: foo.txt
- Content-type: text/plain; qs=0.01
-</PRE>
-<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>
-The full list of headers recognized is:
-
-<DL>
- <DT> <CODE>URI:</CODE>
- <DD> uri of the file containing the variant (of the given media
- type, encoded with the given content encoding). These are
- interpreted as URLs relative to the map file; they must be on
- the same server (!), and they must refer to files to which the
- client would be granted access if they were to be requested
- directly.
- <DT> <CODE>Content-Type:</CODE>
- <DD> media type --- charset, level and "qs" parameters may be given. These
- are often referred to as MIME types; typical media types are
- <CODE>image/gif</CODE>, <CODE>text/plain</CODE>, or
- <CODE>text/html; level=3</CODE>.
- <DT> <CODE>Content-Language:</CODE>
- <DD> The languages of the variant, specified as an Internet standard
- language tag from RFC 1766 (<EM>e.g.</EM>, <CODE>en</CODE> for English,
- <CODE>kr</CODE> for Korean, <EM>etc.</EM>).
- <DT> <CODE>Content-Encoding:</CODE>
- <DD> If the file is compressed, or otherwise encoded, rather than
- containing the actual raw data, this says how that was done.
- Apache only recognizes encodings that are defined by an
- <A HREF="mod/mod_mime.html#addencoding">AddEncoding</A> directive.
- This normally includes the encodings <CODE>x-compress</CODE>
- for compress'd files, and <CODE>x-gzip</CODE> for gzip'd files.
- The <CODE>x-</CODE> prefix is ignored for encoding comparisons.
- <DT> <CODE>Content-Length:</CODE>
- <DD> The size of the file. Specifying content
- lengths in the type-map allows the server to compare file sizes
- without checking the actual files.
- <DT> <CODE>Description:</CODE>
- <DD> A human-readable textual description of the variant. If Apache cannot
- find any appropriate variant to return, it will return an error
- response which lists all available variants instead. Such a variant
- list will include the human-readable variant descriptions.
-</DL>
-
-<H3>Multiviews</H3>
-
-<P>
-<CODE>MultiViews</CODE> is a per-directory option, meaning it can be set with
-an <CODE>Options</CODE> directive within a <CODE><Directory></CODE>,
-<CODE><Location></CODE> or <CODE><Files></CODE>
-section in <CODE>access.conf</CODE>, or (if <CODE>AllowOverride</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>
-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>
-<CODE>MultiViews</CODE> may also apply to searches for the file named by the
-<CODE>DirectoryIndex</CODE> directive, if the server is trying to
-index a directory. If the configuration files specify
-
-<PRE>
- DirectoryIndex index
-</PRE>
-
-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>
-If one of the files found when reading the directive is a CGI script,
-it's not obvious what should happen. The code gives that case
-special treatment --- if the request was a POST, or a GET with
-QUERY_ARGS or PATH_INFO, the script is given an extremely high quality
-rating, and generally invoked; otherwise it is given an extremely low
-quality rating, which generally causes one of the other views (if any)
-to be retrieved.
-
-<H2>The Negotiation Methods</H2>
-
-After Apache has obtained a list of the variants for a given resource,
-either from a type-map file or from the filenames in the directory, it
-invokes one of two methods to decide on the 'best' variant to
-return, if any. It is not necessary to know any of the details of how
-negotiation actually takes place in order to use Apache's content
-negotiation features. However the rest of this document explains the
-methods used for those interested.
-<P>
-
-There are two negotiation methods:
-
-<OL>
-
-<LI><STRONG>Server driven negotiation with the Apache
-algorithm</STRONG> is used in the normal case. The Apache algorithm is
-explained in more detail below. When this algorithm is used, Apache
-can sometimes 'fiddle' the quality factor of a particular dimension to
-achieve a better result. The ways Apache can fiddle quality factors is
-explained in more detail below.
-
-<LI><STRONG>Transparent content negotiation</STRONG> is used when the
-browser specifically requests this through the mechanism defined in RFC
-2295. This negotiation method gives the browser full control over
-deciding on the 'best' variant, the result is therefore dependent on
-the specific algorithms used by the browser. As part of the
-transparent negotiation process, the browser can ask Apache to run the
-'remote variant selection algorithm' defined in RFC 2296. </UL>
-
-
-<H3>Dimensions of Negotiation</H3>
-
-<TABLE>
-<TR valign="top">
-<TH>Dimension
-<TH>Notes
-<TR valign="top">
-<TD>Media Type
-<TD>Browser indicates preferences with the Accept header field. Each item
-can have an associated quality factor. Variant description can also
-have a quality factor (the "qs" parameter).
-<TR valign="top">
-<TD>Language
-<TD>Browser indicates preferences with the Accept-Language header field.
-Each item can have a quality factor. Variants can be associated with none, one
-or more than one language.
-<TR valign="top">
-<TD>Encoding
-<TD>Browser indicates preference with the Accept-Encoding header field.
-Each item can have a quality factor.
-<TR valign="top">
-<TD>Charset
-<TD>Browser indicates preference with the Accept-Charset header field.
-Each item can have a quality factor.
-Variants can indicate a charset as a parameter of the media type.
-</TABLE>
-
-<H3>Apache Negotiation Algorithm</H3>
-
-<P>
-Apache can use the following algorithm to select the 'best' variant
-(if any) to return to the browser. This algorithm is not
-further configurable. It operates as follows:
-
-<OL>
-<LI>First, for each dimension of the negotiation, check the appropriate
-<EM>Accept*</EM> header field and assign a quality to each
-variant. If the <EM>Accept*</EM> header for any dimension implies that this
-variant is not acceptable, eliminate it. If no variants remain, go
-to step 4.
-
-<LI>Select the 'best' variant by a process of elimination. Each of the
-following tests is applied in order. Any variants not selected at each
-test are eliminated. After each test, if only one variant remains,
-select it as the best match and proceed to step 3. If more than one
-variant remains, move on to the next test.
-
-<OL>
-<LI>Multiply the quality factor from the Accept header with the
- quality-of-source factor for this variant's media type, and select
- the variants with the highest value.
-
-<LI>Select the variants with the highest language quality factor.
-
-<LI>Select the variants with the best language match, using either the
- order of languages in the Accept-Language header (if present), or else
- else the order of languages in the <CODE>LanguagePriority</CODE>
- directive (if present).
-
-<LI>Select the variants with the highest 'level' media parameter
- (used to give the version of text/html media types).
-
-<LI>Select variants with the best charset media parameters,
- as given on the Accept-Charset header line. Charset ISO-8859-1
- is acceptable unless explicitly excluded. Variants with a
- <CODE>text/*</CODE> media type but not explicitly associated
- with a particular charset are assumed to be in ISO-8859-1.
-
-<LI>Select those variants which have associated
- charset media parameters that are <EM>not</EM> ISO-8859-1.
- If there are no such variants, select all variants instead.
-
-<LI>Select the variants with the best encoding. If there are
- variants with an encoding that is acceptable to the user-agent,
- select only these variants. Otherwise if there is a mix of encoded
- and non-encoded variants, select only the unencoded variants.
- If either all variants are encoded or all variants are not encoded,
- select all variants.
-
-<LI>Select the variants with the smallest content length.
-
-<LI>Select the first variant of those remaining. This will be either the
- first listed in the type-map file, or when variants are read from
- the directory, the one whose file name comes first when sorted using
- ASCII code order.
-
-</OL>
-
-<LI>The algorithm has now selected one 'best' variant, so return
- it as the response. The HTTP response header Vary is set to indicate the
- dimensions of negotiation (browsers and caches can use this
- information when caching the resource). End.
-
-<LI>To get here means no variant was selected (because none are acceptable
- to the browser). Return a 406 status (meaning "No acceptable representation")
- with a response body consisting of an HTML document listing the
- available variants. Also set the HTTP Vary header to indicate the
- dimensions of variance.
-
-</OL>
-
-<H2><A NAME="better">Fiddling with Quality Values</A></H2>
-
-<P>
-Apache sometimes changes the quality values from what would be
-expected by a strict interpretation of the Apache negotiation
-algorithm above. This is to get a better result from the algorithm for
-browsers which do not send full or accurate information. Some of the
-most popular browsers send Accept header information which would
-otherwise result in the selection of the wrong variant in many
-cases. If a browser sends full and correct information these fiddles
-will not be applied.
-<P>
-
-<H3>Media Types and Wildcards</H3>
-
-<P>
-The Accept: request header indicates preferences for media types. It
-can also include 'wildcard' media types, such as "image/*" or "*/*"
-where the * matches any string. So a request including:
-<PRE>
- Accept: image/*, */*
-</PRE>
-
-would indicate that any type starting "image/" is acceptable,
-as is any other type (so the first "image/*" is redundant). Some
-browsers routinely send wildcards in addition to explicit types they
-can handle. For example:
-<PRE>
+<?xml version="1.0" encoding="ISO-8859-1"?>
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head>
+<meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type" />
+<!--
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ This file is generated from xml source: DO NOT EDIT
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ -->
+<title>Content Negotiation - Apache HTTP Server Version 2.5</title>
+<link href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
+<link href="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
+<link href="./style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link rel="stylesheet" type="text/css" href="./style/css/prettify.css" />
+<script src="./style/scripts/prettify.min.js" type="text/javascript">
+</script>
+
+<link href="./images/favicon.ico" rel="shortcut icon" /></head>
+<body id="manual-page"><div id="page-header">
+<p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/quickreference.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p>
+<p class="apache">Apache HTTP Server Version 2.5</p>
+<img alt="" src="./images/feather.png" /></div>
+<div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
+<div id="path">
+<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="./">Version 2.5</a></div><div id="page-content"><div id="preamble"><h1>Content Negotiation</h1>
+<div class="toplang">
+<p><span>Available Languages: </span><a href="./en/content-negotiation.html" title="English"> en </a> |
+<a href="./fr/content-negotiation.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
+<a href="./ja/content-negotiation.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
+<a href="./ko/content-negotiation.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
+<a href="./tr/content-negotiation.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
+</div>
+
+
+ <p>Apache HTTPD supports content negotiation as described in
+ the HTTP/1.1 specification. It can choose the best
+ representation of a resource based on the browser-supplied
+ preferences for media type, languages, character set and
+ encoding. It also implements a couple of features to give
+ more intelligent handling of requests from browsers that send
+ incomplete negotiation information.</p>
+
+ <p>Content negotiation is provided by the
+ <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> module, which is compiled in
+ by default.</p>
+</div>
+<div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#about">About Content Negotiation</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#negotiation">Negotiation in httpd</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#methods">The Negotiation Methods</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#better">Fiddling with Quality
+ Values</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#extensions">Extensions to Transparent Content
+Negotiation</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#naming">Note on hyperlinks and naming conventions</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#caching">Note on Caching</a></li>
+</ul><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
+<div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="about" id="about">About Content Negotiation</a> <a title="Permanent link" href="#about" class="permalink">¶</a></h2>
+
+ <p>A resource may be available in several different
+ representations. For example, it might be available in
+ different languages or different media types, or a combination.
+ One way of selecting the most appropriate choice is to give the
+ user an index page, and let them select. However it is often
+ possible for the server to choose automatically. This works
+ because browsers can send, as part of each request, information
+ about what representations they prefer. For example, a browser
+ could indicate that it would like to see information in French,
+ if possible, else English will do. Browsers indicate their
+ preferences by headers in the request. To request only French
+ representations, the browser would send</p>
+
+<div class="example"><p><code>Accept-Language: fr</code></p></div>
+
+ <p>Note that this preference will only be applied when there is
+ a choice of representations and they vary by language.</p>
+
+ <p>As an example of a more complex request, this browser has
+ been configured to accept French and English, but prefer
+ French, and to accept various media types, preferring HTML over
+ plain text or other text types, and preferring GIF or JPEG over
+ other media types, but also allowing any other media type as a
+ last resort:</p>
+
+<div class="example"><p><code>
+ Accept-Language: fr; q=1.0, en; q=0.5<br />
+ Accept: text/html; q=1.0, text/*; q=0.8, image/gif; q=0.6, image/jpeg; q=0.6, image/*; q=0.5, */*; q=0.1
+</code></p></div>
+
+ <p>httpd supports 'server driven' content negotiation, as
+ defined in the HTTP/1.1 specification. It fully supports the
+ <code>Accept</code>, <code>Accept-Language</code>,
+ <code>Accept-Charset</code> and <code>Accept-Encoding</code>
+ request headers. httpd also supports 'transparent'
+ content negotiation, which is an experimental negotiation
+ protocol defined in RFC 2295 and RFC 2296. It does not offer
+ support for 'feature negotiation' as defined in these RFCs.</p>
+
+ <p>A <strong>resource</strong> is a conceptual entity
+ identified by a URI (RFC 2396). An HTTP server like Apache HTTP Server
+ provides access to <strong>representations</strong> of the
+ resource(s) within its namespace, with each representation in
+ the form of a sequence of bytes with a defined media type,
+ character set, encoding, etc. Each resource may be associated
+ with zero, one, or more than one representation at any given
+ time. If multiple representations are available, the resource
+ is referred to as <strong>negotiable</strong> and each of its
+ representations is termed a <strong>variant</strong>. The ways
+ in which the variants for a negotiable resource vary are called
+ the <strong>dimensions</strong> of negotiation.</p>
+</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="negotiation" id="negotiation">Negotiation in httpd</a> <a title="Permanent link" href="#negotiation" class="permalink">¶</a></h2>
+
+ <p>In order to negotiate a resource, the server needs to be
+ given information about each of the variants. This is done in
+ one of two ways:</p>
+
+ <ul>
+ <li>Using a type map (<em>i.e.</em>, a <code>*.var</code>
+ file) which names the files containing the variants
+ explicitly, or</li>
+
+ <li>Using a 'MultiViews' search, where the server does an
+ implicit filename pattern match and chooses from among the
+ results.</li>
+ </ul>
+
+ <h3><a name="type-map" id="type-map">Using a type-map file</a></h3>
+
+ <p>A type map is a document which is associated with the handler
+ named <code>type-map</code> (or, for backwards-compatibility with
+ older httpd configurations, the <a class="glossarylink" href="./glossary.html#mime-type" title="see glossary">MIME-type</a>
+ <code>application/x-type-map</code>). Note that to use this
+ feature, you must have a handler set in the configuration that
+ defines a file suffix as <code>type-map</code>; this is best done
+ with</p>
+
+ <pre class="prettyprint lang-config">AddHandler type-map .var</pre>
+
+
+ <p>in the server configuration file.</p>
+
+ <p>Type map files should have the same name as the resource
+ which they are describing, followed by the extension
+ <code>.var</code>. In the examples shown below, the resource is
+ named <code>foo</code>, so the type map file is named
+ <code>foo.var</code>.</p>
+
+ <p>This file should have an entry for each available
+ variant; these entries consist of contiguous HTTP-format header
+ lines. Entries for different variants are separated by blank
+ lines. Blank lines are illegal within an entry. It is
+ conventional to begin a map file with an entry for the combined
+ entity as a whole (although this is not required, and if
+ present will be ignored). An example map file is shown below.</p>
+
+ <p>URIs in this file are relative to the location of the type map
+ file. Usually, these files will be located in the same directory as
+ the type map file, but this is not required. You may provide
+ absolute or relative URIs for any file located on the same server as
+ the map file.</p>
+
+<div class="example"><p><code>
+ URI: foo<br />
+<br />
+ URI: foo.en.html<br />
+ Content-type: text/html<br />
+ Content-language: en<br />
+<br />
+ URI: foo.fr.de.html<br />
+ Content-type: text/html;charset=iso-8859-2<br />
+ Content-language: fr, de<br />
+</code></p></div>
+
+ <p>Note also that a typemap file will take precedence over the
+ filename's extension, even when Multiviews is on. If the
+ variants have different source qualities, that may be indicated
+ by the "qs" parameter to the media type, as in this picture
+ (available as JPEG, GIF, or ASCII-art): </p>
+
+<div class="example"><p><code>
+ URI: foo<br />
+<br />
+ URI: foo.jpeg<br />
+ Content-type: image/jpeg; qs=0.8<br />
+<br />
+ URI: foo.gif<br />
+ Content-type: image/gif; qs=0.5<br />
+<br />
+ URI: foo.txt<br />
+ Content-type: text/plain; qs=0.01<br />
+</code></p></div>
+
+ <p>qs values can vary in the range 0.000 to 1.000. Note that
+ any variant with a qs value of 0.000 will never be chosen.
+ Variants with no 'qs' parameter value are given a qs factor of
+ 1.0. The qs parameter indicates the relative 'quality' of this
+ variant compared to the other available variants, independent
+ of the client's capabilities. For example, a JPEG file is
+ usually of higher source quality than an ASCII file if it is
+ attempting to represent a photograph. However, if the resource
+ being represented is an original ASCII art, then an ASCII
+ representation would have a higher source quality than a JPEG
+ representation. A qs value is therefore specific to a given
+ variant depending on the nature of the resource it
+ represents.</p>
+
+ <p>The full list of headers recognized is available in the <a href="mod/mod_negotiation.html#typemaps">mod_negotiation
+ typemap</a> documentation.</p>
+
+
+<h3><a name="multiviews" id="multiviews">Multiviews</a></h3>
+
+ <p><code>MultiViews</code> is a per-directory option, meaning it
+ can be set with an <code class="directive"><a href="./mod/core.html#options">Options</a></code>
+ directive within a <code class="directive"><a href="./mod/core.html#directory"><Directory></a></code>, <code class="directive"><a href="./mod/core.html#location"><Location></a></code> or <code class="directive"><a href="./mod/core.html#files"><Files></a></code> section in
+ <code>httpd.conf</code>, or (if <code class="directive"><a href="./mod/core.html#allowoverride">AllowOverride</a></code> is properly set) in
+ <code>.htaccess</code> files. Note that <code>Options All</code>
+ does not set <code>MultiViews</code>; you have to ask for it by
+ name.</p>
+
+ <p>The effect of <code>MultiViews</code> is as follows: if the
+ server receives a request for <code>/some/dir/foo</code>, if
+ <code>/some/dir</code> has <code>MultiViews</code> enabled, and
+ <code>/some/dir/foo</code> does <em>not</em> exist, then the
+ server reads the directory looking for files named foo.*, and
+ effectively fakes up a type map which names all those files,
+ assigning them the same media types and content-encodings it
+ would have if the client had asked for one of them by name. It
+ then chooses the best match to the client's requirements.</p>
+
+ <p><code>MultiViews</code> may also apply to searches for the file
+ named by the <code class="directive"><a href="./mod/mod_dir.html#directoryindex">DirectoryIndex</a></code> directive, if the
+ server is trying to index a directory. If the configuration files
+ specify</p>
+ <pre class="prettyprint lang-config">DirectoryIndex index</pre>
+
+ <p>then the server will arbitrate between <code>index.html</code>
+ and <code>index.html3</code> if both are present. If neither
+ are present, and <code>index.cgi</code> is there, the server
+ will run it.</p>
+
+ <p>If one of the files found when reading the directory does not
+ have an extension recognized by <code>mod_mime</code> to designate
+ its Charset, Content-Type, Language, or Encoding, then the result
+ depends on the setting of the <code class="directive"><a href="./mod/mod_mime.html#multiviewsmatch">MultiViewsMatch</a></code> directive. This
+ directive determines whether handlers, filters, and other
+ extension types can participate in MultiViews negotiation.</p>
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="methods" id="methods">The Negotiation Methods</a> <a title="Permanent link" href="#methods" class="permalink">¶</a></h2>
+
+ <p>After httpd has obtained a list of the variants for a given
+ resource, either from a type-map file or from the filenames in
+ the directory, it invokes one of two methods to decide on the
+ 'best' variant to return, if any. It is not necessary to know
+ any of the details of how negotiation actually takes place in
+ order to use httpd's content negotiation features. However the
+ rest of this document explains the methods used for those
+ interested. </p>
+
+ <p>There are two negotiation methods:</p>
+
+ <ol>
+ <li><strong>Server driven negotiation with the httpd
+ algorithm</strong> is used in the normal case. The httpd
+ algorithm is explained in more detail below. When this
+ algorithm is used, httpd can sometimes 'fiddle' the quality
+ factor of a particular dimension to achieve a better result.
+ The ways httpd can fiddle quality factors is explained in
+ more detail below.</li>
+
+ <li><strong>Transparent content negotiation</strong> is used
+ when the browser specifically requests this through the
+ mechanism defined in RFC 2295. This negotiation method gives
+ the browser full control over deciding on the 'best' variant,
+ the result is therefore dependent on the specific algorithms
+ used by the browser. As part of the transparent negotiation
+ process, the browser can ask httpd to run the 'remote
+ variant selection algorithm' defined in RFC 2296.</li>
+ </ol>
+
+<h3><a name="dimensions" id="dimensions">Dimensions of Negotiation</a></h3>
+
+ <table>
+
+ <tr valign="top">
+ <th>Dimension</th>
+
+ <th>Notes</th>
+ </tr>
+
+ <tr valign="top">
+ <td>Media Type</td>
+
+ <td>Browser indicates preferences with the <code>Accept</code>
+ header field. Each item can have an associated quality factor.
+ Variant description can also have a quality factor (the "qs"
+ parameter).</td>
+ </tr>
+
+ <tr valign="top">
+ <td>Language</td>
+
+ <td>Browser indicates preferences with the
+ <code>Accept-Language</code> header field. Each item can have
+ a quality factor. Variants can be associated with none, one or
+ more than one language.</td>
+ </tr>
+
+ <tr valign="top">
+ <td>Encoding</td>
+
+ <td>Browser indicates preference with the
+ <code>Accept-Encoding</code> header field. Each item can have
+ a quality factor.</td>
+ </tr>
+
+ <tr valign="top">
+ <td>Charset</td>
+
+ <td>Browser indicates preference with the
+ <code>Accept-Charset</code> header field. Each item can have a
+ quality factor. Variants can indicate a charset as a parameter
+ of the media type.</td>
+ </tr>
+ </table>
+
+
+<h3><a name="algorithm" id="algorithm">httpd Negotiation Algorithm</a></h3>
+
+ <p>httpd can use the following algorithm to select the 'best'
+ variant (if any) to return to the browser. This algorithm is
+ not further configurable. It operates as follows:</p>
+
+ <ol>
+ <li>First, for each dimension of the negotiation, check the
+ appropriate <em>Accept*</em> header field and assign a
+ quality to each variant. If the <em>Accept*</em> header for
+ any dimension implies that this variant is not acceptable,
+ eliminate it. If no variants remain, go to step 4.</li>
+
+ <li>
+ Select the 'best' variant by a process of elimination. Each
+ of the following tests is applied in order. Any variants
+ not selected at each test are eliminated. After each test,
+ if only one variant remains, select it as the best match
+ and proceed to step 3. If more than one variant remains,
+ move on to the next test.
+
+ <ol>
+ <li>Multiply the quality factor from the <code>Accept</code>
+ header with the quality-of-source factor for this variants
+ media type, and select the variants with the highest
+ value.</li>
+
+ <li>Select the variants with the highest language quality
+ factor.</li>
+
+ <li>Select the variants with the best language match,
+ using either the order of languages in the
+ <code>Accept-Language</code> header (if present), or else
+ the order of languages in the <code>LanguagePriority</code>
+ directive (if present).</li>
+
+ <li>Select the variants with the highest 'level' media
+ parameter (used to give the version of text/html media
+ types).</li>
+
+ <li>Select variants with the best charset media
+ parameters, as given on the <code>Accept-Charset</code>
+ header line. Charset ISO-8859-1 is acceptable unless
+ explicitly excluded. Variants with a <code>text/*</code>
+ media type but not explicitly associated with a particular
+ charset are assumed to be in ISO-8859-1.</li>
+
+ <li>Select those variants which have associated charset
+ media parameters that are <em>not</em> ISO-8859-1. If
+ there are no such variants, select all variants
+ instead.</li>
+
+ <li>Select the variants with the best encoding. If there
+ are variants with an encoding that is acceptable to the
+ user-agent, select only these variants. Otherwise if
+ there is a mix of encoded and non-encoded variants,
+ select only the unencoded variants. If either all
+ variants are encoded or all variants are not encoded,
+ select all variants.</li>
+
+ <li>Select the variants with the smallest content
+ length.</li>
+
+ <li>Select the first variant of those remaining. This
+ will be either the first listed in the type-map file, or
+ when variants are read from the directory, the one whose
+ file name comes first when sorted using ASCII code
+ order.</li>
+ </ol>
+ </li>
+
+ <li>The algorithm has now selected one 'best' variant, so
+ return it as the response. The HTTP response header
+ <code>Vary</code> is set to indicate the dimensions of
+ negotiation (browsers and caches can use this information when
+ caching the resource). End.</li>
+
+ <li>To get here means no variant was selected (because none
+ are acceptable to the browser). Return a 406 status (meaning
+ "No acceptable representation") with a response body
+ consisting of an HTML document listing the available
+ variants. Also set the HTTP <code>Vary</code> header to
+ indicate the dimensions of variance.</li>
+ </ol>
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="better" id="better">Fiddling with Quality
+ Values</a> <a title="Permanent link" href="#better" class="permalink">¶</a></h2>
+
+ <p>httpd sometimes changes the quality values from what would
+ be expected by a strict interpretation of the httpd
+ negotiation algorithm above. This is to get a better result
+ from the algorithm for browsers which do not send full or
+ accurate information. Some of the most popular browsers send
+ <code>Accept</code> header information which would otherwise
+ result in the selection of the wrong variant in many cases. If a
+ browser sends full and correct information these fiddles will not
+ be applied.</p>
+
+<h3><a name="wildcards" id="wildcards">Media Types and Wildcards</a></h3>
+
+ <p>The <code>Accept:</code> request header indicates preferences
+ for media types. It can also include 'wildcard' media types, such
+ as "image/*" or "*/*" where the * matches any string. So a request
+ including:</p>
+
+<div class="example"><p><code>Accept: image/*, */*</code></p></div>
+
+ <p>would indicate that any type starting "image/" is acceptable,
+ as is any other type.
+ Some browsers routinely send wildcards in addition to explicit
+ types they can handle. For example:</p>
+
+<div class="example"><p><code>
Accept: text/html, text/plain, image/gif, image/jpeg, */*
-</PRE>
-
-The intention of this is to indicate that the explicitly
-listed types are preferred, but if a different representation is
-available, that is ok too. However under the basic algorithm, as given
-above, the */* wildcard has exactly equal preference to all the other
-types, so they are not being preferred. The browser should really have
-sent a request with a lower quality (preference) value for *.*, such
-as:
-<PRE>
+</code></p></div>
+ <p>The intention of this is to indicate that the explicitly listed
+ types are preferred, but if a different representation is
+ available, that is ok too. Using explicit quality values,
+ what the browser really wants is something like:</p>
+<div class="example"><p><code>
Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01
-</PRE>
-
-The explicit types have no quality factor, so they default to a
-preference of 1.0 (the highest). The wildcard */* is given
-a low preference of 0.01, so other types will only be returned if
-no variant matches an explicitly listed type.
-<P>
-
-If the Accept: header contains <EM>no</EM> q factors at all, Apache sets
-the q value of "*/*", if present, to 0.01 to emulate the desired
-behavior. It also sets the q value of wildcards of the format
-"type/*" to 0.02 (so these are preferred over matches against
-"*/*". If any media type on the Accept: header contains a q factor,
-these special values are <EM>not</EM> applied, so requests from browsers
-which send the correct information to start with work as expected.
-
-<H3>Variants with no Language</H3>
-
-<P>
-If some of the variants for a particular resource have a language
-attribute, and some do not, those variants with no language
-are given a very low language quality factor of 0.001.<P>
-
-The reason for setting this language quality factor for
-variant with no language to a very low value is to allow
-for a default variant which can be supplied if none of the
-other variants match the browser's language preferences.
-
-For example, consider the situation with three variants:
-
-<UL>
-<LI>foo.en.html, language en
-<LI>foo.fr.html, language en
-<LI>foo.html, no language
-</UL>
-
-<P>
-The meaning of a variant with no language is that it is
-always acceptable to the browser. If the request Accept-Language
-header includes either en or fr (or both) one of foo.en.html
-or foo.fr.html will be returned. If the browser does not list
-either en or fr as acceptable, foo.html will be returned instead.
-
-<H2>Extensions to Transparent Content Negotiation</H2>
-
-Apache extends the transparent content negotiation protocol (RFC 2295)
-as follows. A new <CODE> {encoding ..}</CODE> element is used in
+</code></p></div>
+ <p>The explicit types have no quality factor, so they default to a
+ preference of 1.0 (the highest). The wildcard */* is given a
+ low preference of 0.01, so other types will only be returned if
+ no variant matches an explicitly listed type.</p>
+
+ <p>If the <code>Accept:</code> header contains <em>no</em> q
+ factors at all, httpd sets the q value of "*/*", if present, to
+ 0.01 to emulate the desired behavior. It also sets the q value of
+ wildcards of the format "type/*" to 0.02 (so these are preferred
+ over matches against "*/*". If any media type on the
+ <code>Accept:</code> header contains a q factor, these special
+ values are <em>not</em> applied, so requests from browsers which
+ send the explicit information to start with work as expected.</p>
+
+
+<h3><a name="exceptions" id="exceptions">Language Negotiation Exceptions</a></h3>
+
+ <p>New in httpd 2.0, some exceptions have been added to the
+ negotiation algorithm to allow graceful fallback when language
+ negotiation fails to find a match.</p>
+
+ <p>When a client requests a page on your server, but the server
+ cannot find a single page that matches the
+ <code>Accept-language</code> sent by
+ the browser, the server will return either a "No Acceptable
+ Variant" or "Multiple Choices" response to the client. To avoid
+ these error messages, it is possible to configure httpd to ignore
+ the <code>Accept-language</code> in these cases and provide a
+ document that does not explicitly match the client's request. The
+ <code class="directive"><a href="./mod/mod_negotiation.html#forcelanguagepriority">ForceLanguagePriority</a></code>
+ directive can be used to override one or both of these error
+ messages and substitute the servers judgement in the form of the
+ <code class="directive"><a href="./mod/mod_negotiation.html#languagepriority">LanguagePriority</a></code>
+ directive.</p>
+
+ <p>The server will also attempt to match language-subsets when no
+ other match can be found. For example, if a client requests
+ documents with the language <code>en-GB</code> for British
+ English, the server is not normally allowed by the HTTP/1.1
+ standard to match that against a document that is marked as simply
+ <code>en</code>. (Note that it is almost surely a configuration
+ error to include <code>en-GB</code> and not <code>en</code> in the
+ <code>Accept-Language</code> header, since it is very unlikely
+ that a reader understands British English, but doesn't understand
+ English in general. Unfortunately, many current clients have
+ default configurations that resemble this.) However, if no other
+ language match is possible and the server is about to return a "No
+ Acceptable Variants" error or fallback to the <code class="directive"><a href="./mod/mod_negotiation.html#languagepriority">LanguagePriority</a></code>, the server
+ will ignore the subset specification and match <code>en-GB</code>
+ against <code>en</code> documents. Implicitly, httpd will add
+ the parent language to the client's acceptable language list with
+ a very low quality value. But note that if the client requests
+ "en-GB; q=0.9, fr; q=0.8", and the server has documents
+ designated "en" and "fr", then the "fr" document will be returned.
+ This is necessary to maintain compliance with the HTTP/1.1
+ specification and to work effectively with properly configured
+ clients.</p>
+
+ <p>In order to support advanced techniques (such as cookies or
+ special URL-paths) to determine the user's preferred language,
+ since httpd 2.0.47 <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> recognizes
+ the <a href="env.html">environment variable</a>
+ <code>prefer-language</code>. If it exists and contains an
+ appropriate language tag, <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> will
+ try to select a matching variant. If there's no such variant,
+ the normal negotiation process applies.</p>
+
+ <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">SetEnvIf Cookie "language=(.+)" prefer-language=$1
+Header append Vary cookie</pre>
+</div>
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="extensions" id="extensions">Extensions to Transparent Content
+Negotiation</a> <a title="Permanent link" href="#extensions" class="permalink">¶</a></h2>
+
+<p>httpd extends the transparent content negotiation protocol (RFC
+2295) as follows. A new <code>{encoding ..}</code> element is used in
variant lists to label variants which are available with a specific
-content-encoding only. The implementation of the
-RVSA/1.0 algorithm (RFC 2296) is extended to recognize encoded
-variants in the list, and to use them as candidate variants whenever
-their encodings are acceptable according to the Accept-Encoding
-request header. The RVSA/1.0 implementation does not round computed
-quality factors to 5 decimal places before choosing the best variant.
-
-<H2>Note on hyperlinks and naming conventions</H2>
-
-<P>
-If you are using language negotiation you can choose between
-different naming conventions, because files can have more than one
-extension, and the order of the extensions is normally irrelevant
-(see <A HREF="mod/mod_mime.html">mod_mime</A> documentation for details).
-<P>
-A typical file has a MIME-type extension (<EM>e.g.</EM>, <SAMP>html</SAMP>),
-maybe an encoding extension (<EM>e.g.</EM>, <SAMP>gz</SAMP>), and of course a
-language extension (<EM>e.g.</EM>, <SAMP>en</SAMP>) when we have different
-language variants of this file.
-
-<P>
-Examples:
-<UL>
-<LI>foo.en.html
-<LI>foo.html.en
-<LI>foo.en.html.gz
-</UL>
-
-<P>
-Here some more examples of filenames together with valid and invalid
-hyperlinks:
-</P>
-
-<TABLE BORDER=1 CELLPADDING=8 CELLSPACING=0>
-<TR>
- <TH>Filename</TH>
- <TH>Valid hyperlink</TH>
- <TH>Invalid hyperlink</TH>
-</TR>
-<TR>
- <TD><EM>foo.html.en</EM></TD>
- <TD>foo<BR>
- foo.html</TD>
- <TD>-</TD>
-</TR>
-<TR>
- <TD><EM>foo.en.html</EM></TD>
- <TD>foo</TD>
- <TD>foo.html</TD>
-</TR>
-<TR>
- <TD><EM>foo.html.en.gz</EM></TD>
- <TD>foo<BR>
- foo.html</TD>
- <TD>foo.gz<BR>
- foo.html.gz</TD>
-</TR>
-<TR>
- <TD><EM>foo.en.html.gz</EM></TD>
- <TD>foo</TD>
- <TD>foo.html<BR>
- foo.html.gz<BR>
- foo.gz</TD>
-</TR>
-<TR>
- <TD><EM>foo.gz.html.en</EM></TD>
- <TD>foo<BR>
- foo.gz<BR>
- foo.gz.html</TD>
- <TD>foo.html</TD>
-</TR>
-<TR>
- <TD><EM>foo.html.gz.en</EM></TD>
- <TD>foo<BR>
- foo.html<BR>
- foo.html.gz</TD>
- <TD>foo.gz</TD>
-</TR>
-</TABLE>
-
-<P>
-Looking at the table above you will notice that it is always possible to
-use the name without any extensions in an hyperlink (<EM>e.g.</EM>, <SAMP>foo</SAMP>).
-The advantage is that you can hide the actual type of a
-document rsp. file and can change it later, <EM>e.g.</EM>, from <SAMP>html</SAMP>
-to <SAMP>shtml</SAMP> or <SAMP>cgi</SAMP> without changing any
-hyperlink references.
-
-<P>
-If you want to continue to use a MIME-type in your hyperlinks (<EM>e.g.</EM>
-<SAMP>foo.html</SAMP>) the language extension (including an encoding extension
-if there is one) must be on the right hand side of the MIME-type extension
-(<EM>e.g.</EM>, <SAMP>foo.html.en</SAMP>).
-
-
-<H2>Note on Caching</H2>
-
-<P>
-When a cache stores a representation, it associates it with the request URL.
-The next time that URL is requested, the cache can use the stored
-representation. But, if the resource is negotiable at the server,
-this might result in only the first requested variant being cached and
-subsequent cache hits might return the wrong response. To prevent this,
-Apache normally marks all responses that are returned after content negotiation
-as non-cacheable by HTTP/1.0 clients. Apache also supports the HTTP/1.1
-protocol features to allow caching of negotiated responses. <P>
-
-For requests which come from a HTTP/1.0 compliant client (either a
-browser or a cache), the directive <TT>CacheNegotiatedDocs</TT> can be
-used to allow caching of responses which were subject to negotiation.
-This directive can be given in the server config or virtual host, and
-takes no arguments. It has no effect on requests from HTTP/1.1 clients.
-
-<!--#include virtual="footer.html" -->
-</BODY>
-</HTML>
+content-encoding only. The implementation of the RVSA/1.0 algorithm
+(RFC 2296) is extended to recognize encoded variants in the list, and
+to use them as candidate variants whenever their encodings are
+acceptable according to the <code>Accept-Encoding</code> request
+header. The RVSA/1.0 implementation does not round computed quality
+factors to 5 decimal places before choosing the best variant.</p>
+</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="naming" id="naming">Note on hyperlinks and naming conventions</a> <a title="Permanent link" href="#naming" class="permalink">¶</a></h2>
+
+ <p>If you are using language negotiation you can choose between
+ different naming conventions, because files can have more than
+ one extension, and the order of the extensions is normally
+ irrelevant (see the <a href="mod/mod_mime.html#multipleext">mod_mime</a> documentation
+ for details).</p>
+
+ <p>A typical file has a MIME-type extension (<em>e.g.</em>,
+ <code>html</code>), maybe an encoding extension (<em>e.g.</em>,
+ <code>gz</code>), and of course a language extension
+ (<em>e.g.</em>, <code>en</code>) when we have different
+ language variants of this file.</p>
+
+ <p>Examples:</p>
+
+ <ul>
+ <li>foo.en.html</li>
+
+ <li>foo.html.en</li>
+
+ <li>foo.en.html.gz</li>
+ </ul>
+
+ <p>Here some more examples of filenames together with valid and
+ invalid hyperlinks:</p>
+
+ <table class="bordered">
+
+ <tr>
+ <th>Filename</th>
+
+ <th>Valid hyperlink</th>
+
+ <th>Invalid hyperlink</th>
+ </tr>
+
+ <tr>
+ <td><em>foo.html.en</em></td>
+
+ <td>foo<br />
+ foo.html</td>
+
+ <td>-</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.en.html</em></td>
+
+ <td>foo</td>
+
+ <td>foo.html</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.html.en.gz</em></td>
+
+ <td>foo<br />
+ foo.html</td>
+
+ <td>foo.gz<br />
+ foo.html.gz</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.en.html.gz</em></td>
+
+ <td>foo</td>
+
+ <td>foo.html<br />
+ foo.html.gz<br />
+ foo.gz</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.gz.html.en</em></td>
+
+ <td>foo<br />
+ foo.gz<br />
+ foo.gz.html</td>
+
+ <td>foo.html</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.html.gz.en</em></td>
+
+ <td>foo<br />
+ foo.html<br />
+ foo.html.gz</td>
+
+ <td>foo.gz</td>
+ </tr>
+ </table>
+
+ <p>Looking at the table above, you will notice that it is always
+ possible to use the name without any extensions in a hyperlink
+ (<em>e.g.</em>, <code>foo</code>). The advantage is that you
+ can hide the actual type of a document rsp. file and can change
+ it later, <em>e.g.</em>, from <code>html</code> to
+ <code>shtml</code> or <code>cgi</code> without changing any
+ hyperlink references.</p>
+
+ <p>If you want to continue to use a MIME-type in your
+ hyperlinks (<em>e.g.</em> <code>foo.html</code>) the language
+ extension (including an encoding extension if there is one)
+ must be on the right hand side of the MIME-type extension
+ (<em>e.g.</em>, <code>foo.html.en</code>).</p>
+</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="caching" id="caching">Note on Caching</a> <a title="Permanent link" href="#caching" class="permalink">¶</a></h2>
+
+ <p>When a cache stores a representation, it associates it with
+ the request URL. The next time that URL is requested, the cache
+ can use the stored representation. But, if the resource is
+ negotiable at the server, this might result in only the first
+ requested variant being cached and subsequent cache hits might
+ return the wrong response. To prevent this, httpd normally
+ marks all responses that are returned after content negotiation
+ as non-cacheable by HTTP/1.0 clients. httpd also supports the
+ HTTP/1.1 protocol features to allow caching of negotiated
+ responses.</p>
+
+ <p>For requests which come from a HTTP/1.0 compliant client
+ (either a browser or a cache), the directive <code class="directive"><a href="./mod/mod_negotiation.html#cachenegotiateddocs">CacheNegotiatedDocs</a></code> can be
+ used to allow caching of responses which were subject to
+ negotiation. This directive can be given in the server config or
+ virtual host, and takes no arguments. It has no effect on requests
+ from HTTP/1.1 clients.</p>
+
+ <p>For HTTP/1.1 clients, httpd sends a <code>Vary</code> HTTP
+ response header to indicate the negotiation dimensions for the
+ response. Caches can use this information to determine whether a
+ subsequent request can be served from the local copy. To
+ encourage a cache to use the local copy regardless of the
+ negotiation dimensions, set the <code>force-no-vary</code> <a href="env.html#special">environment variable</a>.</p>
+
+</div></div>
+<div class="bottomlang">
+<p><span>Available Languages: </span><a href="./en/content-negotiation.html" title="English"> en </a> |
+<a href="./fr/content-negotiation.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
+<a href="./ja/content-negotiation.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
+<a href="./ko/content-negotiation.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
+<a href="./tr/content-negotiation.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
+</div><div class="top"><a href="#page-header"><img src="./images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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projects / apache / blobdiff