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351 lines
9.6 KiB
HTML
351 lines
9.6 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<HEAD>
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<TITLE>Apache module mod_expires</TITLE>
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</HEAD>
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<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
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<!--#include virtual="header.html" -->
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<H1 ALIGN="CENTER">Module mod_expires</H1>
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<P>
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This module provides for the generation of <CODE>Expires</CODE> HTTP
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headers according to user-specified criteria.
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</P>
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<P><A
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HREF="module-dict.html#Status"
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REL="Help"
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><STRONG>Status:</STRONG></A> Extension
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<BR>
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<A
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HREF="module-dict.html#SourceFile"
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REL="Help"
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><STRONG>Source File:</STRONG></A> mod_expires.c
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<BR>
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<A
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HREF="module-dict.html#ModuleIdentifier"
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REL="Help"
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><STRONG>Module Identifier:</STRONG></A> expires_module
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<BR>
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<A
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HREF="module-dict.html#Compatibility"
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REL="Help"
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><STRONG>Compatibility:</STRONG></A> Available in Apache 1.2 and later.
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</P>
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<H2>Summary</H2>
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<P>
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This module controls the setting of the <CODE>Expires</CODE> HTTP
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header in server responses. The expiration date can set to be
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relative to either the time the source file was last modified, or to
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the time of the client access.
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</P>
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<P>
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The <CODE>Expires</CODE> HTTP header is an instruction to the client
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about the document's validity and persistence. If cached, the document
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may be fetched from the cache rather than from the source until this
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time has passed. After that, the cache copy is considered
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"expired" and invalid, and a new copy must be obtained from
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the source.
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</P>
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<H2>Directives</H2>
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<P>
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<ul>
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<LI><A
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HREF="#expiresactive"
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>ExpiresActive</A>
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</LI>
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<LI><A
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HREF="#expiresbytype"
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>ExpiresByType</A>
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</LI>
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<LI><A
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HREF="#expiresdefault"
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>ExpiresDefault</A>
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</LI>
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</ul>
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<H2>
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<A NAME="AltSyn">Alternate Interval Syntax</A>
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</H2>
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<P>
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The
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<A
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HREF="#expiresdefault"
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><SAMP>ExpiresDefault</SAMP></A>
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and
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<A
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HREF="#expiresbytype"
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><SAMP>ExpiresByType</SAMP></A>
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directives can also be defined in a more readable syntax of the form:
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</P>
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<DL>
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<DD><CODE>ExpiresDefault "<base> [plus] {<num> <type>}*"
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<BR>
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ExpiresByType type/encoding "<base> [plus]
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{<num> <type>}*"</CODE>
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</DD>
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</DL>
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<P>
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where <base> is one of:
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</P>
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<MENU>
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<LI><SAMP>access</SAMP>
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</LI>
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<LI><SAMP>now</SAMP> (equivalent to '<SAMP>access</SAMP>')
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</LI>
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<LI><SAMP>modification</SAMP>
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</LI>
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</MENU>
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<P>
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The '<SAMP>plus</SAMP>' keyword is optional. <num> should be an
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integer value [acceptable to <SAMP>atoi()</SAMP>], and <type>
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is one of:
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</P>
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<MENU>
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<LI><SAMP>years</SAMP>
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</LI>
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<LI><SAMP>months</SAMP>
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</LI>
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<LI><SAMP>weeks</SAMP>
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</LI>
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<LI><SAMP>days</SAMP>
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</LI>
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<LI><SAMP>hours</SAMP>
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</LI>
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<LI><SAMP>minutes</SAMP>
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</LI>
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<LI><SAMP>seconds</SAMP>
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</LI>
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</MENU>
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<P>
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For example, any of the following directives can be used to make
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documents expire 1 month after being accessed, by default:
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</P>
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<DL>
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<DD><CODE>ExpiresDefault "access plus 1 month"
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<BR>
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ExpiresDefault "access plus 4 weeks"
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<BR>
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ExpiresDefault "access plus 30 days"</CODE>
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</DD>
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</DL>
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<P>
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The expiry time can be fine-tuned by adding several '<num>
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<type>' clauses:
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</P>
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<DL>
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<DD><CODE>ExpiresByType text/html "access plus 1 month 15 days 2 hours"
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<BR>
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ExpiresByType image/gif "modification plus 5 hours 3 minutes"</CODE>
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</DD>
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</DL>
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<P>
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Note that if you use a modification date based setting, the Expires
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header will <STRONG>not</STRONG> be added to content that does
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not come from a file on disk. This is due to the fact that there is
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no modification time for such content.
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<HR>
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<H2><A NAME="expiresactive">
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ExpiresActive directive
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</A></H2>
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<!--%plaintext <?INDEX {\tt ExpiresActive} directive> -->
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<P>
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<A
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HREF="directive-dict.html#Syntax"
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REL="Help"
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><STRONG>Syntax:</STRONG></A> ExpiresActive on|off
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<BR>
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<A
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HREF="directive-dict.html#Context"
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REL="Help"
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><STRONG>Context:</STRONG></A> server config, virtual host, directory,
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.htaccess
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<BR>
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<A
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HREF="directive-dict.html#Override"
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REL="Help"
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><STRONG>Override:</STRONG></A> Indexes
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<BR>
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<A
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HREF="directive-dict.html#Status"
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REL="Help"
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><STRONG>Status:</STRONG></A> Extension
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<BR>
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<A
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HREF="directive-dict.html#Module"
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REL="Help"
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><STRONG>Module:</STRONG></A> mod_expires
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</P>
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<P>
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This directive enables or disables the generation of the
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<CODE>Expires</CODE> header for the document realm in question. (That
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is, if found in an <CODE>.htaccess</CODE> file, for instance, it
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applies only to documents generated from that directory.) If set to
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<EM><CODE>Off</CODE></EM>, no <CODE>Expires</CODE> header will be
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generated for any document in the realm (unless overridden at a lower
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level, such as an <CODE>.htaccess</CODE> file overriding a server
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config file). If set to <EM><CODE>On</CODE></EM>, the header will be
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added to served documents according to the criteria defined by the
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<A
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HREF="#expiresbytype"
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>ExpiresByType</A>
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and
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<A
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HREF="#expiresdefault"
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>ExpiresDefault</A>
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directives (<EM>q.v.</EM>).
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</P>
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<P>
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Note that this directive does not guarantee that an
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<CODE>Expires</CODE> header will be generated. If the criteria aren't
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met, no header will be sent, and the effect will be as though this
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directive wasn't even specified.
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</P>
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<HR>
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<H2><A NAME="expiresbytype">
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ExpiresByType directive
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</A></H2>
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<!--%plaintext <?INDEX {\tt ExpiresByType} directive> -->
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<P>
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<A
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HREF="directive-dict.html#Syntax"
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REL="Help"
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><STRONG>Syntax:</STRONG></A> ExpiresByType <EM>MIME-type
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<code>seconds</EM>
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<BR>
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<A
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HREF="directive-dict.html#Context"
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REL="Help"
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><STRONG>Context:</STRONG></A> server config, virtual host, directory,
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.htaccess
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<BR>
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<A
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HREF="directive-dict.html#Override"
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REL="Help"
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><STRONG>Override:</STRONG></A> Indexes
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<BR>
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<A
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HREF="directive-dict.html#Status"
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REL="Help"
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><STRONG>Status:</STRONG></A> Extension
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<BR>
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<A
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HREF="directive-dict.html#Module"
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REL="Help"
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><STRONG>Module:</STRONG></A> mod_expires
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</P>
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<P>
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This directive defines the value of the <CODE>Expires</CODE> header
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generated for documents of the specified type (<EM>e.g.</EM>,
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<CODE>text/html</CODE>). The second argument sets the number of
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seconds that will be added to a base time to construct the expiration
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date.
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</P>
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<P>
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The base time is either the last modification time of the file, or the
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time of the client's access to the document. Which should be used is
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specified by the <CODE><EM><code></EM></CODE> field;
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<STRONG>M</STRONG> means that the file's last modification time should
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be used as the base time, and <STRONG>A</STRONG> means the client's
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access time should be used.
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</P>
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<P>
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The difference in effect is subtle. If <EM>M</EM> is used, all current
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copies of the document in all caches will expire at the same time,
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which can be good for something like a weekly notice that's always
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found at the same URL. If <EM>A</EM> is used, the date of expiration
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is different for each client; this can be good for image files that
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don't change very often, particularly for a set of related documents
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that all refer to the same images (<EM>i.e.</EM>, the images will be
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accessed repeatedly within a relatively short timespan).
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</P>
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<P>
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<STRONG>Example:</STRONG>
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</P>
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<P>
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<PRE>
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ExpiresActive On # enable expirations
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ExpiresByType image/gif A2592000 # expire GIF images after a month
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# in the client's cache
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ExpiresByType text/html M604800 # HTML documents are good for a
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# week from the time they were
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# changed, period
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</PRE>
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</P>
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<P>
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Note that this directive only has effect if <CODE>ExpiresActive
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On</CODE> has been specified. It overrides, for the specified MIME
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type <EM>only</EM>, any expiration date set by the
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<A
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HREF="#expiresdefault"
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>ExpiresDefault</A>
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directive.
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</P>
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<P>
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You can also specify the expiration time calculation using an
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<A
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HREF="#AltSyn"
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>alternate syntax</A>,
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described later in this document.
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</P>
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<HR>
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<H2><A NAME="expiresdefault">
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ExpiresDefault directive
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</A></H2>
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<!--%plaintext <?INDEX {\tt ExpiresDefault} directive> -->
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<P>
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<A
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HREF="directive-dict.html#Syntax"
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REL="Help"
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><STRONG>Syntax:</STRONG></A> ExpiresDefault <EM><code>seconds</EM>
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<BR>
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<A
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HREF="directive-dict.html#Context"
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REL="Help"
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><STRONG>Context:</STRONG></A> server config, virtual host, directory,
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.htaccess
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<BR>
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<A
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HREF="directive-dict.html#Override"
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REL="Help"
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><STRONG>Override:</STRONG></A> Indexes
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<BR>
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<A
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HREF="directive-dict.html#Status"
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REL="Help"
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><STRONG>Status:</STRONG></A> Extension
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<BR>
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<A
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HREF="directive-dict.html#Module"
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REL="Help"
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><STRONG>Module:</STRONG></A> mod_expires
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</P>
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<P>
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This directive sets the default algorithm for calculating the
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expiration time for all documents in the affected realm. It can be
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overridden on a type-by-type basis by the
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<A
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HREF="#expiresbytype"
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>ExpiresByType</A>
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directive. See the description of that directive for details about
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the syntax of the argument, and the
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<A
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HREF="#AltSyn"
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>alternate syntax</A>
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description as well.
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</P>
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<!--#include virtual="footer.html" -->
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</BODY>
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</HTML>
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