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

<name>mod_expires</name>

<description>Generation of <code>Expires</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 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>The <code>Expires</code> 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.</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>

<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> header 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>, no <code>Expires</code> header

will 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 header 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> 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 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.</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>