Apache HTTP Server Version 2.4
Apache HTTP Server Version 2.4
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 Negotiation in httpd The Negotiation Methods Fiddling with Quality
Values Extensions to Transparent Content
Negotiation Note on hyperlinks and naming conventions Note on Caching
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.
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 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 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 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.
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:
| 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 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:
Accept
header with the quality-of-source factor for this variants
media type, and select the variants with the highest
value.Accept-Language header (if present), or else
the order of languages in the LanguagePriority
directive (if present).
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.Vary is set to indicate the dimensions of
negotiation (browsers and caches can use this information when
caching the resource). End.
Vary header to
indicate the dimensions of variance.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.
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.
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.
SetEnvIf Cookie "language=(.+)" prefer-language=$1 Header append Vary cookie
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.
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:
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, 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.