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.
Content negotiation is provided by the mod_negotiation module, which is compiled in by default.
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
Accept-Language: fr
Note that this preference will only be applied when there is a choice of representations and they vary by language.
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:
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
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.
A resource is a conceptual entity identified by a URI (RFC 2396). An HTTP server like Apache provides access to representations 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 negotiable and each of its representations is termed a variant. The ways in which the variants for a negotiable resource vary are called the dimensions of negotiation.
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:
*.var
file) which names the files containing the variants
explicitly, orA type map is a document which is associated with the
handler named type-map (or, for
backwards-compatibility with older Apache configurations, the
mime type application/x-type-map). Note that to
use this feature, you must have a handler set in the
configuration that defines a file suffix as
type-map; this is best done with a
AddHandler type-map .varin the server configuration file. See the comments in the sample config file for more details.
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:
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, deIf 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):
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
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.
The full list of headers recognized is:
URI:Content-Type:image/gif,
text/plain, or
text/html; level=3.Content-Language:en for English, kr for Korean,
etc.).Content-Encoding:x-compress for compress'd files, and
x-gzip for gzip'd files. The x-
prefix is ignored for encoding comparisons.Content-Length:Description:MultiViews is a per-directory option, meaning
it can be set with an Options directive within a
<Directory>, <Location>
or <Files> section in
access.conf, or (if AllowOverride is
properly set) in .htaccess files. Note that
Options All does not set MultiViews;
you have to ask for it by name.
The effect of MultiViews is as follows: if the
server receives a request for /some/dir/foo, if
/some/dir has MultiViews enabled, and
/some/dir/foo does not 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.
MultiViews may also apply to searches for the
file named by the DirectoryIndex directive, if the
server is trying to index a directory. If the configuration
files specify
DirectoryIndex indexthen the server will arbitrate between
index.html
and index.html3 if both are present. If neither
are present, and index.cgi is there, the server
will run it.
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.
There are two negotiation methods:
| Dimension | Notes |
|---|---|
| Media Type | 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). |
| Language | 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. |
| Encoding | Browser indicates preference with the Accept-Encoding header field. Each item can have a quality factor. |
| Charset | 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. |
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:
LanguagePriority directive
(if present).text/* media type
but not explicitly associated with a particular charset
are assumed to be in ISO-8859-1.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.
You should be aware that the error message returned by Apache is necessarily rather terse and might confuse some users (even though it lists the available alternatives). If you want to avoid users seeing this error page, you should organize your documents such that a document in a default language (or with a default encoding etc.) is always returned if a document is not available in any of the languages, encodings etc. the browser asked for.
In particular, if you want a document in a default language to be returned if a document is not available in any of the languages a browser asked for, you should create a document with no language attribute set. See Variants with no Language below for details.
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.
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:
Accept: image/*, */*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:
Accept: text/html, text/plain, image/gif, image/jpeg, */*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:
Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01The 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.
If the Accept: header contains no q factors at all, Apache sets the q value of "*/*", if present, to 0.01 to emulate the desired behaviour. 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 not applied, so requests from browsers which send the correct information to start with work as expected.
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.
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. This allows you to avoid users seeing a "406" error page if their browser is set to only accept languages which you do not offer for the resource that was requested.
For example, consider the situation with Multiviews enabled and three variants:
The meaning of a variant with no language is that it is always
acceptable to the browser. If the request is for foo
and the 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. If the client requests foo.html
instead, then no negotiation will occur since the exact match
will be returned. To avoid this problem, it is sometimes helpful
to name the "no language" variant foo.html.html to assure
that Multiviews and language negotiation will come into play.
{encoding ..} 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.
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 mod_mime documentation for details).
A typical file has a MIME-type extension (e.g., html), maybe an encoding extension (e.g., gz), and of course a language extension (e.g., en) when we have different language variants of this file.
Examples:
Here some more examples of filenames together with valid and invalid hyperlinks:
| Filename | Valid hyperlink | Invalid hyperlink |
|---|---|---|
| foo.html.en | foo foo.html |
- |
| foo.en.html | foo | foo.html |
| foo.html.en.gz | foo foo.html |
foo.gz foo.html.gz |
| foo.en.html.gz | foo | foo.html foo.html.gz foo.gz |
| foo.gz.html.en | foo foo.gz foo.gz.html |
foo.html |
| foo.html.gz.en | foo foo.html foo.html.gz |
foo.gz |
Looking at the table above you will notice that it is always possible to use the name without any extensions in a hyperlink (e.g., foo). The advantage is that you can hide the actual type of a document rsp. file and can change it later, e.g., from html to shtml or cgi without changing any hyperlink references.
If you want to continue to use a MIME-type in your hyperlinks (e.g. foo.html) the language extension (including an encoding extension if there is one) must be on the right hand side of the MIME-type extension (e.g., foo.html.en).
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.
For requests which come from a HTTP/1.0 compliant client (either a browser or a cache), the directive CacheNegotiatedDocs 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.