Apache HTTP Server Version 2.4
Apache Module mod_negotiation
| Description: | Provides for content negotiation |
|---|---|
| Status: | Base |
| Module Identifier: | negotiation_module |
| Source File: | mod_negotiation.c |
Summary
Content negotiation, or more accurately content selection, is the selection of the document that best matches the clients capabilities, from one of several available documents. There are two implementations of this.
- A type map (a file with the handler
type-map) which explicitly lists the files containing the variants. - A Multiviews search (enabled by the
MultiviewsOptions), where the server does an implicit filename pattern match, and choose from amongst the results.
Bugfix checklist
See also
Type maps
A type map has a format similar to RFC822 mail headers. It contains document descriptions separated by blank lines, with lines beginning with a hash character ('#') treated as comments. A document description consists of several header records; records may be continued on multiple lines if the continuation lines start with spaces. The leading space will be deleted and the lines concatenated. A header record consists of a keyword name, which always ends in a colon, followed by a value. Whitespace is allowed between the header name and value, and between the tokens of value. The headers allowed are:
Content-Encoding:- The encoding of the file. Apache only recognizes
encodings that are defined by an
AddEncodingdirective. This normally includes the encodingsx-compressfor compress'd files, andx-gzipfor gzip'd files. Thex-prefix is ignored for encoding comparisons. Content-Language:- The language(s) of the variant, as an Internet standard
language tag (RFC 1766). An example is
en, meaning English. If the variant contains more than one language, they are separated by a comma. Content-Length:- The length of the file, in bytes. If this header is not present, then the actual length of the file is used.
Content-Type:-
The MIME media type of
the document, with optional parameters. Parameters are
separated from the media type and from one another by a
semi-colon, with a syntax of
name=value. Common parameters include:level- an integer specifying the version of the media type.
For
text/htmlthis defaults to 2, otherwise 0. qs- a floating-point number with a value in the range 0[.000]
to 1[.000], indicating 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 ascii art, then an ascii
file would have a higher source quality than a jpeg file.
All
qsvalues are therefore specific to a given resource.
Example
Content-Type: image/jpeg; qs=0.8 URI:- 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.
Body:- The actual content of the resource may
be included in the type-map file using the Body header. This
header must contain a string that designates a delimiter for
the body content. Then all following lines in the type map
file will be considered part of the resource body until the
delimiter string is found.
Example:
Body:----xyz----
<html>
<body>
<p>Content of the page.</p>
</body>
</html>
----xyz----
Consider, for example, a resource called
document.html which is available in English, French,
and German. The files for each of these are called
document.html.en, document.html.fr, and
document.html.de, respectively. The type map file will
be called document.html.var, and will contain the
following:
URI: document.html
Content-language: en
Content-type: text/html
URI: document.html.en
Content-language: fr
Content-type: text/html
URI: document.html.fr
Content-language: de
Content-type: text/html
URI: document.html.de
All four of these files should be placed in the same directory,
and the .var file should be associated with the
type-map handler with an AddHandler directive:
AddHandler type-map .var
A request for document.html.var in this directory will
result in choosing the variant which most closely matches the language preference
specified in the user's Accept-Language request
header.
If Multiviews is enabled, and MultiviewsMatch is set to "handlers" or "any", a request to
document.html will discover document.html.var and
continue negotiating with the explicit type map.
Other configuration directives, such as Alias can be used to map document.html to
document.html.var.
Multiviews
A Multiviews search is enabled by the Multiviews
Options. If the server receives a
request for /some/dir/foo and
/some/dir/foo does not exist, then the
server reads the directory looking for all 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, and returns that document.
The MultiviewsMatch
directive configures whether Apache will consider files
that do not have content negotiation meta-information assigned
to them when choosing files.
CacheNegotiatedDocs Directive
| Description: | Allows content-negotiated documents to be cached by proxy servers |
|---|---|
| Syntax: | CacheNegotiatedDocs On|Off |
| Default: | CacheNegotiatedDocs Off |
| Context: | server config, virtual host |
| Status: | Base |
| Module: | mod_negotiation |
If set, this directive allows content-negotiated documents to be cached by proxy servers. This could mean that clients behind those proxys could retrieve versions of the documents that are not the best match for their abilities, but it will make caching more efficient.
This directive only applies to requests which come from HTTP/1.0 browsers. HTTP/1.1 provides much better control over the caching of negotiated documents, and this directive has no effect in responses to HTTP/1.1 requests.
ForceLanguagePriority Directive
| Description: | Action to take if a single acceptable document is not found |
|---|---|
| Syntax: | ForceLanguagePriority None|Prefer|Fallback [Prefer|Fallback] |
| Default: | ForceLanguagePriority Prefer |
| Context: | server config, virtual host, directory, .htaccess |
| Override: | FileInfo |
| Status: | Base |
| Module: | mod_negotiation |
The ForceLanguagePriority directive uses
the given LanguagePriority to satisfy
negotiation where the server could otherwise not return a single
matching document.
ForceLanguagePriority Prefer uses
LanguagePriority to serve a one valid result, rather
than returning an HTTP result 300 (MULTIPLE CHOICES) when there
are several equally valid choices. If the directives below were
given, and the user's Accept-Language header assigned
en and de each as quality .500
(equally acceptable) then the first matching variant, en,
will be served.
LanguagePriority en fr de ForceLanguagePriority Prefer
ForceLanguagePriority Fallback uses
LanguagePriority to
serve a valid result, rather than returning an HTTP result 406
(NOT ACCEPTABLE). If the directives below were given, and the user's
Accept-Language only permitted an es
language response, but such a variant isn't found, then the first
variant from the LanguagePriority list below will be served.
LanguagePriority en fr de ForceLanguagePriority Fallback
Both options, Prefer and Fallback, may be
specified, so either the first matching variant from LanguagePriority will be served if
more than one variant is acceptable, or first available document will
be served if none of the variants matched the client's acceptable list
of languages.
See also
LanguagePriority Directive
| Description: | The precedence of language variants for cases where the client does not express a preference |
|---|---|
| Syntax: | LanguagePriority MIME-lang [MIME-lang]
... |
| Context: | server config, virtual host, directory, .htaccess |
| Override: | FileInfo |
| Status: | Base |
| Module: | mod_negotiation |
The LanguagePriority sets the precedence
of language variants for the case where the client does not
express a preference, when handling a Multiviews request. The list
of MIME-lang are in order of decreasing preference.
LanguagePriority en fr de
For a request for foo.html, where
foo.html.fr and foo.html.de both
existed, but the browser did not express a language preference,
then foo.html.fr would be returned.
Note that this directive only has an effect if a 'best'
language cannot be determined by any other means or the ForceLanguagePriority directive
is not None. In general, the client determines the
language preference, not the server.