Apache HTTP Server Version 2.4

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# Content Negotiation

Apache HTTPD supports content negotiation as described in 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 also implements a couple of features to give more intelligent handling of requests from browsers that send incomplete negotiation information.

Content negotiation is provided by the `mod_negotiation` module, which is compiled in by default.

## About Content Negotiation

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

httpd 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. httpd 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 HTTP Server 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.

## Negotiation in httpd

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:

- Using a type map (i.e., a `*.var` file) which names the files containing the variants explicitly, or
- Using a 'MultiViews' search, where the server does an implicit filename pattern match and chooses from among the results.

### Using a type-map file

A type map is a document which is associated with the handler named `type-map` (or, for backwards-compatibility with older httpd 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

    AddHandler type-map .var

in the server configuration file.

Type map files should have the same name as the resource which they are describing, followed by the extension `.var`. In the examples shown below, the resource is named `foo`, so the type map file is named `foo.var`.

This file should 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 shown below.

URIs in this file are relative to the location of the type map file. Usually, these files will be located in the same directory as the type map file, but this is not required. You may provide absolute or relative URIs for any file located on the same server as the map file.

    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, de

Note also that a typemap file will take precedence over the filename's extension, even when Multiviews is on. If a `.var` type map file is found during a Multiviews directory scan, the server will use only the variants listed in that type map file. Any other files in the directory that match the base name but are not listed in the type map will be ignored entirely. For example, if `foo.var` exists and lists only `foo.en.html` and `foo.fr.de.html`, then a file `foo.mn.html` present in the same directory will never be served through negotiation, even if the client requests Mongolian. To make such a variant available, it must be added to the type map file.

This behavior is important for sites that are adding new language variants over time. If you have an existing resource `foo.html` and later add translated variants with language extensions, you should list all variants (including the original) in the type map file. The original file can be assigned its language in the type map without being renamed:

    URI: foo

    URI: foo.html
    Content-type: text/html
    Content-language: en

    URI: foo.fr.html
    Content-type: text/html
    Content-language: fr

If 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 available in the mod_negotiation typemap documentation.

### Multiviews

`MultiViews` is a per-directory option, meaning it can be set with an `Options` directive within a `<Directory>`, `<Location>` or `<Files>` section in `httpd.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 index

then 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 directory does not have an extension recognized by `mod_mime` to designate its Charset, Content-Type, Language, or Encoding, then the result depends on the setting of the `MultiViewsMatch` directive. This directive determines whether handlers, filters, and other extension types can participate in MultiViews negotiation.

## The Negotiation Methods

After httpd has obtained a list of the variants for a given resource, either from a type-map file or from the filenames in the directory, it invokes one of two methods to decide on the 'best' variant to return, if any. It is not necessary to know any of the details of how negotiation actually takes place in order to use httpd's content negotiation features. However the rest of this document explains the methods used for those interested.

There are two negotiation methods:

1. Server driven negotiation with the httpd algorithm is used in the normal case. The httpd algorithm is explained in more detail below. When this algorithm is used, httpd can sometimes 'fiddle' the quality factor of a particular dimension to achieve a better result. The ways httpd can fiddle quality factors is explained in more detail below.
2. Transparent content negotiation is used when the browser specifically requests this through the mechanism defined in RFC 2295. This negotiation method gives the browser full control over deciding on the 'best' variant, the result is therefore dependent on the specific algorithms used by the browser. As part of the transparent negotiation process, the browser can ask httpd to run the 'remote variant selection algorithm' defined in RFC 2296.

### Dimensions of Negotiation

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.

### httpd Negotiation Algorithm

httpd 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:

1. First, for each dimension of the negotiation, check the appropriate Accept* header field and assign a quality to each variant. If the Accept* header for any dimension implies that this variant is not acceptable, eliminate it. If no variants remain, go to step 4.
2. Select the 'best' variant by a process of elimination. Each of the following tests is applied in order. Any variants not selected at each test are eliminated. After each test, if only one variant remains, select it as the best match and proceed to step 3. If more than one variant remains, move on to the next test.
   1. Multiply the quality factor from the `Accept` header with the quality-of-source factor for this variants media type, and select the variants with the highest value.
   2. Select the variants with the highest language quality factor.
   3. Select the variants with the best language match, using either the order of languages in the `Accept-Language` header (if present), or else the order of languages in the `LanguagePriority` directive (if present).
   4. Select the variants with the highest 'level' media parameter (used to give the version of text/html media types).
   5. Select variants with the best charset media parameters, as given on the `Accept-Charset` header line. Charset ISO-8859-1 is acceptable unless explicitly excluded. Variants with a `text/*` media type but not explicitly associated with a particular charset are assumed to be in ISO-8859-1.
   6. Select those variants which have associated charset media parameters that are not ISO-8859-1. If there are no such variants, select all variants instead.
   7. Select the variants with the best encoding. If there are variants with an encoding that is acceptable to the user-agent, select only these variants. Otherwise if there is a mix of encoded and non-encoded variants, select only the unencoded variants. If either all variants are encoded or all variants are not encoded, select all variants.
   8. Select the variants with the smallest content length.
   9. Select the first variant of those remaining. This will be either the first listed in the type-map file, or when variants are read from the directory, the one whose file name comes first when sorted using ASCII code order.
3. The algorithm has now selected one 'best' variant, so return it as the response. The HTTP response header `Vary` is set to indicate the dimensions of negotiation (browsers and caches can use this information when caching the resource). End.
4. 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.

## Fiddling with Quality Values

httpd sometimes changes the quality values from what would be expected by a strict interpretation of the httpd 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.

### 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, httpd 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.

### Language Negotiation Exceptions

New in httpd 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 httpd 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, httpd 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 httpd 2.0.47 `mod_negotiation` recognizes the environment variable `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

## Extensions to Transparent Content Negotiation

httpd extends the transparent content negotiation protocol (RFC 2295) as follows. A new `{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.

## Note on hyperlinks and naming conventions

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 the 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:

- foo.en.html
- foo.html.en
- foo.en.html.gz

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`).

## Note on Caching

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, httpd normally marks all responses that are returned after content negotiation as non-cacheable by HTTP/1.0 clients. httpd 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.

For HTTP/1.1 clients, httpd sends a `Vary` HTTP response header to indicate the negotiation dimensions for the response. Caches can use this information to determine whether a subsequent request can be served from the local copy. To encourage a cache to use the local copy regardless of the negotiation dimensions, set the `force-no-vary` environment variable.