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apache-httpd/docs/manual/content-negotiation.xml
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<manualpage metafile="content-negotiation.xml.meta"> | |
<title>Content Negotiation</title> | |
<summary> | |
<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 | |
<module>mod_negotiation</module> module, which is compiled in | |
by default.</p> | |
</summary> | |
<section id="about"><title>About Content Negotiation</title> | |
<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> | |
<example>Accept-Language: fr</example> | |
<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> | |
<example> | |
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 | |
</example> | |
<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> | |
</section> | |
<section id="negotiation"><title>Negotiation in httpd</title> | |
<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> | |
<section id="type-map"><title>Using a type-map file</title> | |
<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 <glossary>MIME-type</glossary> | |
<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> | |
<example>AddHandler type-map .var</example> | |
<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> | |
<example> | |
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 /> | |
</example> | |
<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> | |
<example> | |
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 /> | |
</example> | |
<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> | |
</section> | |
<section id="multiviews"><title>Multiviews</title> | |
<p><code>MultiViews</code> is a per-directory option, meaning it | |
can be set with an <directive module="core">Options</directive> | |
directive within a <directive module="core" | |
type="section">Directory</directive>, <directive module="core" | |
type="section">Location</directive> or <directive module="core" | |
type="section">Files</directive> section in | |
<code>httpd.conf</code>, or (if <directive | |
module="core">AllowOverride</directive> 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 <directive | |
module="mod_dir">DirectoryIndex</directive> directive, if the | |
server is trying to index a directory. If the configuration files | |
specify</p> | |
<example>DirectoryIndex index</example> | |
<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 <directive | |
module="mod_mime">MultiViewsMatch</directive> directive. This | |
directive determines whether handlers, filters, and other | |
extension types can participate in MultiViews negotiation.</p> | |
</section> | |
</section> | |
<section id="methods"><title>The Negotiation Methods</title> | |
<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> | |
<section id="dimensions"><title>Dimensions of Negotiation</title> | |
<table> | |
<columnspec><column width=".15"/><column width=".85"/></columnspec> | |
<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> | |
</section> | |
<section id="algorithm"><title>httpd Negotiation Algorithm</title> | |
<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> | |
</section> | |
</section> | |
<section id="better"><title>Fiddling with Quality | |
Values</title> | |
<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> | |
<section id="wildcards"><title>Media Types and Wildcards</title> | |
<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> | |
<example>Accept: image/*, */*</example> | |
<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> | |
<example> | |
Accept: text/html, text/plain, image/gif, image/jpeg, */* | |
</example> | |
<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> | |
<example> | |
Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01 | |
</example> | |
<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> | |
</section> | |
<section id="exceptions"><title>Language Negotiation Exceptions</title> | |
<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 | |
<directive | |
module="mod_negotiation">ForceLanguagePriority</directive> | |
directive can be used to override one or both of these error | |
messages and substitute the servers judgement in the form of the | |
<directive module="mod_negotiation">LanguagePriority</directive> | |
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 <directive | |
module="mod_negotiation">LanguagePriority</directive>, 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 <module>mod_negotiation</module> recognizes | |
the <a href="env.html">environment variable</a> | |
<code>prefer-language</code>. If it exists and contains an | |
appropriate language tag, <module>mod_negotiation</module> will | |
try to select a matching variant. If there's no such variant, | |
the normal negotiation process applies.</p> | |
<example><title>Example</title> | |
SetEnvIf Cookie "language=(.+)" prefer-language=$1<br /> | |
Header append Vary cookie | |
</example> | |
</section> | |
</section> | |
<section id="extensions"><title>Extensions to Transparent Content | |
Negotiation</title> | |
<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 <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> | |
</section> | |
<section id="naming"><title>Note on hyperlinks and naming conventions</title> | |
<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 border="1" cellpadding="8" cellspacing="0"> | |
<columnspec><column width=".2"/><column width=".2"/> | |
<column width=".2"/></columnspec> | |
<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> | |
</section> | |
<section id="caching"><title>Note on Caching</title> | |
<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 <directive | |
module="mod_negotiation">CacheNegotiatedDocs</directive> 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> | |
</section> | |
</manualpage> |