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<title>Content Negotiation</title>

<summary>

<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

<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>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>

</section>

<section id="negotiation"><title>Negotiation in Apache</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 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>

<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_negotation

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 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>

<p>There are two negotiation methods:</p>

<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>

<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.</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>Apache Negotiation Algorithm</title>

<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:</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>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

<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, 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

<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 Apache 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 Apache 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, Apache 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; qs=0.9, fr; qs=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 Apache 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

</example>

</section>

</section>

<section id="extensions"><title>Extensions to Transparent Content

Negotiation</title>

<p>Apache 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, 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>

<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, Apache 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>

<section id="more"><title>More Information</title>

<p>For more information about content negotiation, see Alan

J. Flavell's <a

href="http://ppewww.ph.gla.ac.uk/~flavell/www/lang-neg.html">Language

Negotiation Notes</a>. But note that this document may not be

updated to include changes in Apache 2.0.</p>

</section>

</manualpage>