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The Apache HTTP Server Reference Manual
by Apache Software Foundation
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ISBN 9781906966034
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15.4  Fiddling with Quality Values

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.

15.4.1  Media Types and Wildcards

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. 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. Using explicit quality values, what the browser really wants is something like:

Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01

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.

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 explicit information to start with work as expected.

15.4.2  Language Negotiation Exceptions

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.

When a client requests a page on your server, but the server cannot find a single page that matches the Accept-language 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 Accept-language in these cases and provide a document that does not explicitly match the client’s request. The ForceLanguagePriority directive can be used to override one or both of these error messages and substitute the servers judgement in the form of the LanguagePriority directive.

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 en-GB 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 en. (Note that it is almost surely a configuration error to include en-GB and not en in the Accept-Language 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 LanguagePriority, the server will ignore the subset specification and match en-GB against en 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; q=0.9, fr; q=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.

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 mod_negotiation recognizes the environment variable (p. 1471) prefer-language. If it exists and contains an appropriate language tag, mod_negotiation will try to select a matching variant. If there’s no such variant, the normal negotiation process applies.

Example

SetEnvIf Cookie "language=(.+)" prefer-language=$1
Header append Vary cookie

ISBN 9781906966034The Apache HTTP Server Reference ManualSee the print edition