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
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=0.1Apache 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.
The terms used in content negotiation are: a resource is an item which can be requested of a server, which might be selected as the result of a content negotiation algorithm. If a resource is available in several formats, these are called representations or variants. The ways in which the variants for a particular 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:
*.varfile) which names the files containing the variants explicitly
A type map is a document which is associated with the handler
type-map (or, for backwards-compatibility with
older Apache configurations, the mime type
application/x-type-map). Note that to use this feature,
you've got to have a
SetHandler some place which defines a
file suffix as
type-map; this is best done with a
AddHandler type-map varin
srm.conf. See comments in the sample config files for details.
Type map files have an entry for each available variant; these entries consist of contiguous RFC822-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 between 0.000 and 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 full list of headers recognized is:
krfor Korean, etc.).
x-gzip, as appropriate.
This is a per-directory option, meaning it can be set with an
Options directive within a
access.conf, or (if
is properly set) in
.htaccess files. Note that
Options All does not set
have to ask for it by name. (Fixing this is a one-line change to
The effect of
MultiViews is as follows: if the server
receives a request for
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, and forwards them along.
This applies 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.html3if both are present. If neither are present, and
index.cgiis 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.
In some circumstances, Apache can '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.
|Media Type||Browser indicates preferences on Accept: header. Each item can have an associated quality factor. Variant description can also have a quality factor.|
|Language||Browser indicates preferences on Accept-Language: header. Each item can have a quality factor. Variants can be associated with none, one or more languages.|
|Encoding||Browser indicates preference with Accept-Encoding: header.|
|Charset||Browser indicates preference with Accept-Charset: header. Variants can indicate a charset as a parameter of the media type.|
Apache uses an algorithm to select the 'best' variant (if any) to return to the browser. This algorithm is not configurable. It operates like this:
LanguagePrioritydirective (if present), else the order of languages on the Accept-Language header.
Apache sometimes changes the quality values from what would be expected by a strict interpretation of the 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 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 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. For example, consider the situation with three variants:
The meaning of a variant with no language is that it is always acceptable to the browser. If the request 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 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.
Here some more examples of filenames together with valid and invalid hyperlinks:
|Filename||Valid hyperlink||Invalid hyperlink|
Looking at the table above you will notice that it is always possible to use the name without any extensions in an 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 document, it associates it with the request URL. The next time that URL is requested, the cache can use the stored document, provided it is still within date. But if the resource is subject to content negotiation at the server, this would result in only the first requested variant being cached, and subsequent cache hits could 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.