Apache Module mod_expires

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+ + + +

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.

+ +

Topics

Alternate Interval Syntax

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:

+ +

access
now (equivalent to + 'access')
modification

+ +

The plus keyword is optional. <num> + should be an integer value [acceptable to atoi()], + and <type> is one of:

+ +

years
months
weeks
days
hours
minutes
seconds

+ +

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.

+ +