Apache HTTP Server Version 2.0
Apache Module mod_expires
| Description: | Generation of Expires and
Cache-Control HTTP headers according to user-specified
criteria |
|---|---|
| Status: | Extension |
| Module Identifier: | expires_module |
| Source File: | mod_expires.c |
Summary
This module controls the setting of the Expires
HTTP header and the max-age directive of the
Cache-Control 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.
These HTTP headers are 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.
To modify Cache-Control directives other than
max-age (see RFC
2616 section 14.9), you can use the Header directive.
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
| Description: | Enables generation of Expires
headers |
|---|---|
| Syntax: | ExpiresActive On|Off |
| Context: | server config, virtual host, directory, .htaccess |
| Override: | Indexes |
| Status: | Extension |
| Module: | mod_expires |
This directive enables or disables the generation of the
Expires and Cache-Control headers 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, the headers will not 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 headers 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 or Cache-Control 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
| Description: | Value of the Expires header configured
by MIME type |
|---|---|
| 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 and the max-age directive of the
Cache-Control 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 Cache-Control:
max-age is calculated by subtracting the request time from
the expiration date and expressing the result in seconds.
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
| Description: | Default algorithm for calculating expiration time |
|---|---|
| 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, and the alternate syntax
description as well.