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Apache HTTP Server Version 2.0
Apache Module mod_expires
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Summary
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.
Directives
Alternate Interval Syntax
The ExpiresDefault and
ExpiresByType directives
can also be defined in a more readable syntax of the form:
ExpiresDefault "<base> [plus] {<num> <type>}*"
ExpiresByType type/encoding "<base> [plus] {<num> <type>}*"
where <base> is one of:
accessnow(equivalent to 'access')modification
The 'plus' keyword is optional. <num>
should be an integer value [acceptable to atoi()],
and <type> is one of:
yearsmonthsweeksdayshoursminutesseconds
For example, any of the following directives can be used to make documents expire 1 month after being accessed, by default:
ExpiresDefault "access plus 1 month"
ExpiresDefault "access plus 4 weeks"
ExpiresDefault "access plus 30 days"
The expiry time can be fine-tuned by adding several '<num> <type>' clauses:
ExpiresByType text/html "access plus 1 month 15 days 2 hours"
ExpiresByType image/gif "modification plus 5 hours 3 minutes"
Note that if you use a modification date based setting, the Expires header will not be added to content that does not come from a file on disk. This is due to the fact that there is no modification time for such content.
ExpiresActive Directive
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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.
ExpiresByType Directive
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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:
# enable expirations
ExpiresActive On
# expire GIF images after a month in the client's cache
ExpiresByType image/gif A2592000
# HTML documents are good for a week from the time they were changed
ExpiresByType text/html M604800
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.
You can also specify the expiration time calculation using an alternate syntax, described earlier in this document.
ExpiresDefault Directive
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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, and the alternate syntax
description as well.
