Description:

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.

When the Expires header is already part of the response generated by the
server, for example when generated by a CGI script or proxied from an origin
server, this module does not change or add an Expires or Cache-Control header.