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<modulesynopsis metafile="mod_expires.xml.meta">

<name>mod_expires</name>

<description>Generation of <code>Expires</code> and

<code>Cache-Control</code> HTTP headers according to user-specified

criteria</description>

<status>Extension</status>

<sourcefile>mod_expires.c</sourcefile>

<identifier>expires_module</identifier>

<summary>

<p>This module controls the setting of the <code>Expires</code>

HTTP header and the <code>max-age</code> directive of the

<code>Cache-Control</code> 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.</p>

<p>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.</p>

<p>To modify <code>Cache-Control</code> directives other than

<code>max-age</code> (see <a

href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9">RFC

2616 section 14.9</a>), you can use the <directive

module="mod_headers">Header</directive> directive.</p>

<p> When the <code>Expires</code> 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 <code>Expires</code> or <code>Cache-Control</code> header.</p>

</summary>

<section id="AltSyn"><title>Alternate Interval Syntax</title>

<p>The <directive module="mod_expires">ExpiresDefault</directive> and

<directive module="mod_expires">ExpiresByType</directive> directives

can also be defined in a more readable syntax of the form:</p>

<example>

ExpiresDefault "&lt;base&gt; [plus] {&lt;num&gt;

&lt;type&gt;}*"<br />

ExpiresByType type/encoding "&lt;base&gt; [plus]

{&lt;num&gt; &lt;type&gt;}*"

</example>

<p>where &lt;base&gt; is one of:</p>

<ul>

<li><code>access</code></li>

<li><code>now</code> (equivalent to

'<code>access</code>')</li>

<li><code>modification</code></li>

</ul>

<p>The <code>plus</code> keyword is optional. &lt;num&gt;

should be an integer value [acceptable to <code>atoi()</code>],

and &lt;type&gt; is one of:</p>

<ul>

<li><code>years</code></li>

<li><code>months</code></li>

<li><code>weeks</code></li>

<li><code>days</code></li>

<li><code>hours</code></li>

<li><code>minutes</code></li>

<li><code>seconds</code></li>

</ul>

<p>For example, any of the following directives can be used to

make documents expire 1 month after being accessed, by

default:</p>

<example>

ExpiresDefault "access plus 1 month"<br />

ExpiresDefault "access plus 4 weeks"<br />

ExpiresDefault "access plus 30 days"

</example>

<p>The expiry time can be fine-tuned by adding several

'&lt;num&gt; &lt;type&gt;' clauses:</p>

<example>

ExpiresByType text/html "access plus 1 month 15

days 2 hours"<br />

ExpiresByType image/gif "modification plus 5 hours 3

minutes"

</example>

<p>Note that if you use a modification date based setting, the

Expires header will <strong>not</strong> 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.</p>

</section>

<directivesynopsis>

<name>ExpiresActive</name>

<description>Enables generation of <code>Expires</code>

headers</description>

<syntax>ExpiresActive On|Off</syntax>

<default>ExpiresActive Off</default>

<contextlist><context>server config</context>

<context>virtual host</context><context>directory</context>

<context>.htaccess</context></contextlist>

<override>Indexes</override>

<usage>

<p>This directive enables or disables the generation of the

<code>Expires</code> and <code>Cache-Control</code> headers for

the document realm in question. (That is, if found in an

<code>.htaccess</code> file, for instance, it applies only to

documents generated from that directory.) If set to

<code>Off</code>, the headers will not be generated for any

document in the realm (unless overridden at a lower level, such as

an <code>.htaccess</code> file overriding a server config

file). If set to <code>On</code>, the headers will be added to

served documents according to the criteria defined by the

<directive module="mod_expires">ExpiresByType</directive> and

<directive module="mod_expires">ExpiresDefault</directive>

directives (<em>q.v.</em>).</p>

<p>Note that this directive does not guarantee that an

<code>Expires</code> or <code>Cache-Control</code> 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.</p>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>ExpiresByType</name>

<description>Value of the <code>Expires</code> header configured

by MIME type</description>

<syntax>ExpiresByType <var>MIME-type</var>

<var>&lt;code&gt;seconds</var></syntax>

<contextlist><context>server config</context><context>virtual host</context>

<context>directory</context><context>.htaccess</context></contextlist>

<override>Indexes</override>

<usage>

<p>This directive defines the value of the <code>Expires</code>

header and the <code>max-age</code> directive of the

<code>Cache-Control</code> header generated for documents of the

specified type (<em>e.g.</em>, <code>text/html</code>). The second

argument sets the number of seconds that will be added to a base

time to construct the expiration date. The <code>Cache-Control:

max-age</code> is calculated by subtracting the request time from

the expiration date and expressing the result in seconds.</p>

<p>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><var>&lt;code&gt;</var></code> field; <code>M</code>

means that the file's last modification time should be used as

the base time, and <code>A</code> means the client's access

time should be used.</p>

<p>The difference in effect is subtle. If <code>M</code> 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 <code>A</code> 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 (<em>i.e.</em>, the images will be accessed

repeatedly within a relatively short timespan).</p>

<example><title>Example:</title>

# enable expirations<br />

ExpiresActive On<br />

# expire GIF images after a month in the client's cache<br />

ExpiresByType image/gif A2592000<br />

# HTML documents are good for a week from the<br />

# time they were changed<br />

ExpiresByType text/html M604800

</example>

<p>Note that this directive only has effect if

<code>ExpiresActive On</code> has been specified. It overrides,

for the specified MIME type <em>only</em>, any expiration date

set by the <directive module="mod_expires">ExpiresDefault</directive>

directive.</p>

<p>You can also specify the expiration time calculation using

an <a href="#AltSyn">alternate syntax</a>, described earlier in

this document.</p>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>ExpiresDefault</name>

<description>Default algorithm for calculating expiration time</description>

<syntax>ExpiresDefault <var>&lt;code&gt;seconds</var></syntax>

<contextlist><context>server config</context><context>virtual host</context>

<context>directory</context><context>.htaccess</context></contextlist>

<override>Indexes</override>

<usage>

<p>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 <directive

module="mod_expires">ExpiresByType</directive> directive. See the

description of that directive for details about the syntax of the

argument, and the <a href="#AltSyn">alternate syntax</a>

description as well.</p>

</usage>

</directivesynopsis>

</modulesynopsis>