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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.
  
You can also specify the expiration time calculation using an alternate syntax, described later in this document.
  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.
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:
The 'plus' keyword is optional. <num> should be an integer value [acceptable to atoi()], and <type> is one of:
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.
 
