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This module is contained in the mod_expires.c file, and
is not compiled in by default. It provides for the
generation of Expires headers according to user-specified
criteria.
This module controls the setting of the Expires HTTP
header in server responses. The expiration date can set to be
relative to either the time the source file was last modified, or to
the time of the client access.
The Expires HTTP header is an instruction to the client
about the document's validity and persistence. If cached, the document
may be fetched from the cache rather than from the source until this
time has passed. After that, the cache copy is considered
"expired" and invalid, and a new copy must be obtained from
the source.
Syntax: ExpiresActive boolean
Context: server config, virtual host, directory, .htaccess
Override: Indexes
Status: Extension
Module: mod_expires
This directive enables or disables the generation of the
Expires header for the document realm in question. (That
is, if found in an .htaccess file, for instance, it
applies only to documents generated from that directory.) If set to
Off, no Expires header will be
generated for any document in the realm (unless overridden at a lower
level, such as an .htaccess file overriding a server
config file). If set to On, the header will be
added to served documents according to the criteria defined by the
ExpiresByType
and
ExpiresDefault
directives (q.v.).
Note that this directive does not guarantee that an
Expires header will be generated. If the criteria aren't
met, no header will be sent, and the effect will be as though this
directive wasn't even specified.
Syntax: ExpiresByType mime-type <code>seconds
Context: server config, virtual host, directory, .htaccess
Override: Indexes
Status: Extension
Module: mod_expires
This directive defines the value of the Expires header
generated for documents of the specified type (e.g.,
text/html). The second argument sets the number of
seconds that will be added to a base time to construct the expiration
date.
The base time is either the last modification time of the file, or the
time of the client's access to the document. Which should be used is
specified by the <code> field;
M means that the file's last modification time should
be used as the base time, and A means the client's
access time should be used.
The difference in effect is subtle. If M is used, all current copies of the document in all caches will expire at the same time, which can be good for something like a weekly notice that's always found at the same URL. If A is used, the date of expiration is different for each client; this can be good for image files that don't change very often, particularly for a set of related documents that all refer to the same images (i.e., the images will be accessed repeatedly within a relatively short timespan).
Example:
ExpiresActive On # enable expirations
ExpiresByType image/gif A2592000 # expire GIF images after a month
# in the client's cache
ExpiresByType text/html M604800 # HTML documents are good for a
# week from the time they were
# changed, period
Note that this directive only has effect if ExpiresActive
On has been specified. It overrides, for the specified MIME
type only, any expiration date set by the
ExpiresDefault
directive.
Syntax: ExpiresDefault <code>seconds
Context: server config, virtual host, directory, .htaccess
Override: Indexes
Status: Extension
Module: mod_expires
This directive sets the default algorithm for calculating the expiration time for all documents in the affected realm. It can be overridden on a type-by-type basis by the ExpiresByType directive. See the description of that directive for details about the syntax of the argument.
