apache/docs/manual/mod/mod_expires.xml

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<!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd">
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<modulesynopsis>

<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 <em><code>Off</code></em>, 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 <em><code>On</code></em>, 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
    <em>MIME-type &lt;code&gt;seconds</em></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><em>&lt;code&gt;</em></code> field; <strong>M</strong>
    means that the file's last modification time should be used as
    the base time, and <strong>A</strong> means the client's access
    time should be used.</p>

    <p>The difference in effect is subtle. If <em>M</em> 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 <em>A</em> 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>

    <p><strong>Example:</strong></p>
<example>
# 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 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 <em>&lt;code&gt;seconds</em></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>