</div>
- <p>Apache's supports content negotiation as described in
+ <p>Apache 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
incomplete negotiation information.</p>
<p>Content negotiation is provided by the
- <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> module.
- which is compiled in by default.</p>
+ <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> module, which is compiled in
+ by default.</p>
</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#about">About Content Negotiation</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#negotiation">Negotiation in Apache</a></li>
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
+ 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
<p>Apache 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
+ <code>Accept</code>, <code>Accept-Language</code>,
+ <code>Accept-Charset</code> and<code>Accept-Encoding</code>
request headers. Apache 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. </p>
+ support for 'feature negotiation' as defined in these RFCs.</p>
<p>A <strong>resource</strong> is a conceptual entity
identified by a URI (RFC 2396). An HTTP server like Apache
<p>A type map is a document which is associated with the
handler named <code>type-map</code> (or, for
backwards-compatibility with older Apache configurations, the
- mime type <code>application/x-type-map</code>). Note that to
+ MIME type <code>application/x-type-map</code>). Note that to
use this feature, you must have a handler set in the
configuration that defines a file suffix as
- <code>type-map</code>; this is best done with a</p>
+ <code>type-map</code>; this is best done with</p>
+
<div class="example"><p><code>AddHandler type-map .var</code></p></div>
+
<p>in the server configuration file.</p>
<p>Type map files should have the same name as the resource
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): </p>
+ (available as JPEG, GIF, or ASCII-art): </p>
<div class="example"><p><code>
URI: foo<br />
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
+ 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
+ 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.</p>
<tr valign="top">
<td>Media Type</td>
- <td>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).</td>
+ <td>Browser indicates preferences with the <code>Accept</code>
+ header field. Each item can have an associated quality factor.
+ Variant description can also have a quality factor (the "qs"
+ parameter).</td>
</tr>
<tr valign="top">
<td>Language</td>
- <td>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.</td>
+ <td>Browser indicates preferences with the
+ <code>Accept-Language</code> header field. Each item can have
+ a quality factor. Variants can be associated with none, one or
+ more than one language.</td>
</tr>
<tr valign="top">
<td>Encoding</td>
- <td>Browser indicates preference with the Accept-Encoding
- header field. Each item can have a quality factor.</td>
+ <td>Browser indicates preference with the
+ <code>Accept-Encoding</code> header field. Each item can have
+ a quality factor.</td>
</tr>
<tr valign="top">
<td>Charset</td>
- <td>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.</td>
+ <td>Browser indicates preference with the
+ <code>Accept-Charset</code> header field. Each item can have a
+ quality factor. Variants can indicate a charset as a parameter
+ of the media type.</td>
</tr>
</table>
move on to the next test.
<ol>
- <li>Multiply the quality factor from the Accept header
- with the quality-of-source factor for this variant's
+ <li>Multiply the quality factor from the <code>Accept</code>
+ header with the quality-of-source factor for this variants
media type, and select the variants with the highest
value.</li>
<li>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 <code>LanguagePriority</code> directive
- (if present).</li>
+ <code>Accept-Language</code> header (if present), or else
+ the order of languages in the <code>LanguagePriority</code>
+ directive (if present).</li>
<li>Select the variants with the highest 'level' media
parameter (used to give the version of text/html media
types).</li>
<li>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 <code>text/*</code> media type
- but not explicitly associated with a particular charset
- are assumed to be in ISO-8859-1.</li>
+ parameters, as given on the <code>Accept-Charset</code>
+ header line. Charset ISO-8859-1 is acceptable unless
+ explicitly excluded. Variants with a <code>text/*</code>
+ media type but not explicitly associated with a particular
+ charset are assumed to be in ISO-8859-1.</li>
<li>Select those variants which have associated charset
media parameters that are <em>not</em> ISO-8859-1. If
</li>
<li>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.</li>
+ return it as the response. The HTTP response header
+ <code>Vary</code> is set to indicate the dimensions of
+ negotiation (browsers and caches can use this information when
+ caching the resource). End.</li>
<li>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.</li>
+ variants. Also set the HTTP <code>Vary</code> header to
+ indicate the dimensions of variance.</li>
</ol>
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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.</p>
+ <code>Accept</code> 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.</p>
<h3><a name="wildcards" id="wildcards">Media Types and Wildcards</a></h3>
- <p>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
+ <p>The <code>Accept:</code> 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:</p>
<div class="example"><p><code>Accept: image/*, */*</code></p></div>
low preference of 0.01, so other types will only be returned if
no variant matches an explicitly listed type.</p>
- <p>If the Accept: header contains <em>no</em> 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
- <em>not</em> applied, so requests from browsers which send the
- explicit information to start with work as expected.</p>
+ <p>If the <code>Accept:</code> header contains <em>no</em> 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
+ <code>Accept:</code> header contains a q factor, these special
+ values are <em>not</em> applied, so requests from browsers which
+ send the explicit information to start with work as expected.</p>
<h3><a name="exceptions" id="exceptions">Language Negotiation Exceptions</a></h3>
negotiation fails to find a match.</p>
<p>When a client requests a page on your server, but the server
- cannot find a single page that matches the Accept-language sent by
+ cannot find a single page that matches the
+ <code>Accept-language</code> 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 <code class="directive"><a href="./mod/mod_negotiation.html#forcelanguagepriority">ForceLanguagePriority</a></code>
+ the <code>Accept-language</code> in these cases and provide a
+ document that does not explicitly match the client's request. The
+ <code class="directive"><a href="./mod/mod_negotiation.html#forcelanguagepriority">ForceLanguagePriority</a></code>
directive can be used to override one or both of these error
- messages and substitute the servers judgement in the form of the
+ messages and substitute the servers judgement in the form of the
<code class="directive"><a href="./mod/mod_negotiation.html#languagepriority">LanguagePriority</a></code>
directive.</p>
standard to match that against a document that is marked as simply
<code>en</code>. (Note that it is almost surely a configuration
error to include <code>en-GB</code> and not <code>en</code> 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
+ <code>Accept-Language</code> 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 <code class="directive"><a href="./mod/mod_negotiation.html#languagepriority">LanguagePriority</a></code>, the server
will ignore the subset specification and match <code>en-GB</code>
against <code>en</code> documents. Implicitly, Apache will add
specification and to work effectively with properly configured
clients.</p>
- <p>In order to support advanced techniques (such as Cookies or
+ <p>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 <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> recognizes
the <a href="env.html">environment variable</a>
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.</p>
+acceptable according to the <code>Accept-Encoding</code> request
+header. The RVSA/1.0 implementation does not round computed quality
+factors to 5 decimal places before choosing the best variant.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="naming" id="naming">Note on hyperlinks and naming conventions</a></h2>
<summary>
- <p>Apache's supports content negotiation as described in
+ <p>Apache 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
incomplete negotiation information.</p>
<p>Content negotiation is provided by the
- <module>mod_negotiation</module> module.
- which is compiled in by default.</p>
+ <module>mod_negotiation</module> module, which is compiled in
+ by default.</p>
</summary>
<section id="about"><title>About Content Negotiation</title>
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
+ 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
<p>Apache 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
+ <code>Accept</code>, <code>Accept-Language</code>,
+ <code>Accept-Charset</code> and<code>Accept-Encoding</code>
request headers. Apache 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. </p>
+ support for 'feature negotiation' as defined in these RFCs.</p>
<p>A <strong>resource</strong> is a conceptual entity
identified by a URI (RFC 2396). An HTTP server like Apache
<p>A type map is a document which is associated with the
handler named <code>type-map</code> (or, for
backwards-compatibility with older Apache configurations, the
- mime type <code>application/x-type-map</code>). Note that to
+ MIME type <code>application/x-type-map</code>). Note that to
use this feature, you must have a handler set in the
configuration that defines a file suffix as
- <code>type-map</code>; this is best done with a</p>
+ <code>type-map</code>; this is best done with</p>
+
<example>AddHandler type-map .var</example>
+
<p>in the server configuration file.</p>
<p>Type map files should have the same name as the resource
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): </p>
+ (available as JPEG, GIF, or ASCII-art): </p>
<example>
URI: foo<br />
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
+ 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
+ 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.</p>
<tr valign="top">
<td>Media Type</td>
- <td>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).</td>
+ <td>Browser indicates preferences with the <code>Accept</code>
+ header field. Each item can have an associated quality factor.
+ Variant description can also have a quality factor (the "qs"
+ parameter).</td>
</tr>
<tr valign="top">
<td>Language</td>
- <td>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.</td>
+ <td>Browser indicates preferences with the
+ <code>Accept-Language</code> header field. Each item can have
+ a quality factor. Variants can be associated with none, one or
+ more than one language.</td>
</tr>
<tr valign="top">
<td>Encoding</td>
- <td>Browser indicates preference with the Accept-Encoding
- header field. Each item can have a quality factor.</td>
+ <td>Browser indicates preference with the
+ <code>Accept-Encoding</code> header field. Each item can have
+ a quality factor.</td>
</tr>
<tr valign="top">
<td>Charset</td>
- <td>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.</td>
+ <td>Browser indicates preference with the
+ <code>Accept-Charset</code> header field. Each item can have a
+ quality factor. Variants can indicate a charset as a parameter
+ of the media type.</td>
</tr>
</table>
</section>
move on to the next test.
<ol>
- <li>Multiply the quality factor from the Accept header
- with the quality-of-source factor for this variant's
+ <li>Multiply the quality factor from the <code>Accept</code>
+ header with the quality-of-source factor for this variants
media type, and select the variants with the highest
value.</li>
<li>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 <code>LanguagePriority</code> directive
- (if present).</li>
+ <code>Accept-Language</code> header (if present), or else
+ the order of languages in the <code>LanguagePriority</code>
+ directive (if present).</li>
<li>Select the variants with the highest 'level' media
parameter (used to give the version of text/html media
types).</li>
<li>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 <code>text/*</code> media type
- but not explicitly associated with a particular charset
- are assumed to be in ISO-8859-1.</li>
+ parameters, as given on the <code>Accept-Charset</code>
+ header line. Charset ISO-8859-1 is acceptable unless
+ explicitly excluded. Variants with a <code>text/*</code>
+ media type but not explicitly associated with a particular
+ charset are assumed to be in ISO-8859-1.</li>
<li>Select those variants which have associated charset
media parameters that are <em>not</em> ISO-8859-1. If
</li>
<li>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.</li>
+ return it as the response. The HTTP response header
+ <code>Vary</code> is set to indicate the dimensions of
+ negotiation (browsers and caches can use this information when
+ caching the resource). End.</li>
<li>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.</li>
+ variants. Also set the HTTP <code>Vary</code> header to
+ indicate the dimensions of variance.</li>
</ol>
</section>
</section>
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.</p>
+ <code>Accept</code> 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.</p>
<section id="wildcards"><title>Media Types and Wildcards</title>
- <p>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
+ <p>The <code>Accept:</code> 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:</p>
<example>Accept: image/*, */*</example>
low preference of 0.01, so other types will only be returned if
no variant matches an explicitly listed type.</p>
- <p>If the Accept: header contains <em>no</em> 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
- <em>not</em> applied, so requests from browsers which send the
- explicit information to start with work as expected.</p>
+ <p>If the <code>Accept:</code> header contains <em>no</em> 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
+ <code>Accept:</code> header contains a q factor, these special
+ values are <em>not</em> applied, so requests from browsers which
+ send the explicit information to start with work as expected.</p>
</section>
<section id="exceptions"><title>Language Negotiation Exceptions</title>
negotiation fails to find a match.</p>
<p>When a client requests a page on your server, but the server
- cannot find a single page that matches the Accept-language sent by
+ cannot find a single page that matches the
+ <code>Accept-language</code> 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 <directive
+ the <code>Accept-language</code> in these cases and provide a
+ document that does not explicitly match the client's request. The
+ <directive
module="mod_negotiation">ForceLanguagePriority</directive>
directive can be used to override one or both of these error
- messages and substitute the servers judgement in the form of the
+ messages and substitute the servers judgement in the form of the
<directive module="mod_negotiation">LanguagePriority</directive>
directive.</p>
standard to match that against a document that is marked as simply
<code>en</code>. (Note that it is almost surely a configuration
error to include <code>en-GB</code> and not <code>en</code> 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
+ <code>Accept-Language</code> 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 <directive
module="mod_negotiation">LanguagePriority</directive>, the server
will ignore the subset specification and match <code>en-GB</code>
specification and to work effectively with properly configured
clients.</p>
- <p>In order to support advanced techniques (such as Cookies or
+ <p>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 <module>mod_negotiation</module> recognizes
the <a href="env.html">environment variable</a>
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.</p>
+acceptable according to the <code>Accept-Encoding</code> request
+header. The RVSA/1.0 implementation does not round computed quality
+factors to 5 decimal places before choosing the best variant.</p>
</section>
<section id="naming"><title>Note on hyperlinks and naming conventions</title>
projects / thirdparty / apache / httpd.git / commitdiff
