This module provides for content negotiation.
Source File: mod_negotiation.c
Module Identifier: negotiation_module
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
which explicitly lists the files containing the variants.
- A MultiViews search (enabled by the MultiViews
Option, where the server does an implicit
filename pattern match, and choose from amongst the results.
A type map has the same format as 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:
- The encoding of the file. Apache only recognizes encodings that are
defined by an AddEncoding directive.
This normally includes the encodings
x-compress for compress'd
x-gzip for gzip'd files. The
is ignored for encoding comparisons.
- The language of the variant, as an Internet standard language tag
(RFC 1766). An example is
en, meaning English.
- The length of the file, in bytes. If this header is not present, then
the actual length of the file is used.
- 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
- an integer specifying the version of the media type.
text/html this defaults to 2, otherwise 0.
- a floating-point number with a value in the range 0.0 to 1.0,
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 qs values are therefore specific to a given resource.
Content-Type: image/jpeg; qs=0.8
- The path to the file containing this variant, relative to the map file.
A MultiViews search is enabled by the MultiViews
If the server receives a request for
/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.
Context: server config
Compatibility: CacheNegotiatedDocs is only available
in Apache 1.1 and later.
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
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
Syntax: LanguagePriority MIME-lang
Context: server config, virtual host, directory, .htaccess
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
For a request for
LanguagePriority en fr de
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. Correctly implemented
HTTP/1.1 requests will mean this directive has no effect.