docs/manual/mod/mod_headers.html.en - httpd - Git at Google

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 <title>mod_headers - Apache HTTP Server</title>
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 <p class="apache">Apache HTTP Server Version 2.3</p>
 <img alt="" src="../images/feather.gif" /></div>
 <div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="../images/left.gif" /></a></div>
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 <a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs-project/">Documentation</a> &gt; <a href="../">Version 2.3</a> &gt; <a href="./">Modules</a></div>
 <div id="page-content">
 <div id="preamble"><h1>Apache Module mod_headers</h1>
 <div class="toplang">
 <p><span>Available Languages: </span><a href="../en/mod/mod_headers.html" title="English">&nbsp;en&nbsp;</a> |
 <a href="../ja/mod/mod_headers.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a> |
 <a href="../ko/mod/mod_headers.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a></p>
 </div>
 <table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Customization of HTTP request and response
 headers</td></tr>
 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>headers_module</td></tr>
 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_headers.c</td></tr>
 <tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td><code class="directive"><a href="#requestheader">RequestHeader</a></code>
 is available only in Apache 2.0</td></tr></table>
 <h3>Summary</h3>

     <p>This module provides directives to control and modify HTTP
     request and response headers. Headers can be merged, replaced
     or removed.</p>
 </div>
 <div id="quickview"><h3 class="directives">Directives</h3>
 <ul id="toc">
 <li><img alt="" src="../images/down.gif" /> <a href="#header">Header</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#requestheader">RequestHeader</a></li>
 </ul>
 <h3>Topics</h3>
 <ul id="topics">
 <li><img alt="" src="../images/down.gif" /> <a href="#order">Order of Processing</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#early">Early and Late Processing</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#examples">Examples</a></li>
 </ul></div>
 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="order" id="order">Order of Processing</a></h2>

     <p>The directives provided by <code class="module"><a href="../mod/mod_headers.html">mod_headers</a></code> can
     occur almost anywhere within the server configuration, and can be
     limited in scope by enclosing them in <a href="../sections.html">configuration sections</a>.</p>

     <p>Order of processing is important and is affected both by the
     order in the configuration file and by placement in <a href="../sections.html#mergin">configuration sections</a>. These
     two headers have a different effect if reversed:</p>

     <div class="example"><p><code>
       RequestHeader append MirrorID "mirror 12"<br />
       RequestHeader unset MirrorID
     </code></p></div>

     <p>This way round, the <code>MirrorID</code> header is not set. If
     reversed, the MirrorID header is set to "mirror 12".</p>
 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="early" id="early">Early and Late Processing</a></h2>
     <p><code class="module"><a href="../mod/mod_headers.html">mod_headers</a></code> can be applied either early or late
     in the request.  The normal mode is late, when Request Headers are
     set immediately before running the content generator and Response
     Headers just as the response is sent down the wire.  Always use
     Late mode in an operational server.</p>

     <p>Early mode is designed as a test/debugging aid for developers.
     Directives defined using the <code>early</code> keyword are set
     right at the beginning of processing the request.  This means
     they can be used to simulate different requests and set up test
     cases, but it also means that headers may be changed at any time
     by other modules before generating a Response.</p>

     <p>Because early directives are processed before the request path's
     configuration is traversed, early headers can only be set in a
     main server or virtual host context.  Early directives cannot depend
     on a request path, so they will fail in contexts such as
     <code>&lt;Directory&gt;</code> or <code>&lt;Location&gt;</code>.</p>
 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="examples" id="examples">Examples</a></h2>

     <ol>
       <li>
         Copy all request headers that begin with "TS" to the
         response headers:

         <div class="example"><p><code>
           Header echo ^TS
         </code></p></div>
       </li>

       <li>
         Add a header, <code>MyHeader</code>, to the response including a
         timestamp for when the request was received and how long it
         took to begin serving the request. This header can be used by
         the client to intuit load on the server or in isolating
         bottlenecks between the client and the server.

         <div class="example"><p><code>
           Header add MyHeader "%D %t"
         </code></p></div>

         <p>results in this header being added to the response:</p>

         <div class="example"><p><code>
           MyHeader: D=3775428 t=991424704447256
         </code></p></div>
       </li>

       <li>
         Say hello to Joe

         <div class="example"><p><code>
           Header add MyHeader "Hello Joe. It took %D microseconds \<br />
           for Apache to serve this request."
         </code></p></div>

         <p>results in this header being added to the response:</p>

         <div class="example"><p><code>
           MyHeader: Hello Joe. It took D=3775428 microseconds for Apache
           to serve this request.
         </code></p></div>
       </li>

       <li>
         Conditionally send <code>MyHeader</code> on the response if and
         only if header "MyRequestHeader" is present on the request. This
         is useful for constructing headers in response to some client
         stimulus. Note that this example requires the services of the
         <code class="module"><a href="../mod/mod_setenvif.html">mod_setenvif</a></code> module.

         <div class="example"><p><code>
           SetEnvIf MyRequestHeader value HAVE_MyRequestHeader<br />
           Header add MyHeader "%D %t mytext" env=HAVE_MyRequestHeader<br />
        </code></p></div>

        <p>If the header <code>MyRequestHeader: value</code> is present on
        the HTTP request, the response will contain the following header:</p>

        <div class="example"><p><code>
          MyHeader: D=3775428 t=991424704447256 mytext
        </code></p></div>
       </li>
     </ol>
 </div>
 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="directive-section"><h2><a name="Header" id="Header">Header</a> <a name="header" id="header">Directive</a></h2>
 <table class="directive">
 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Configure HTTP response headers</td></tr>
 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>Header [<var>condition</var>] set|append|add|unset|echo
 <var>header</var> [<var>value</var>] [early|env=[!]<var>variable</var>]</code></td></tr>
 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
 <tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_headers</td></tr>
 </table>
     <p>This directive can replace, merge or remove HTTP response
     headers. The header is modified just after the content handler
     and output filters are run, allowing outgoing headers to be
     modified.</p>

     <p>The optional <var>condition</var> can be either <code>onsuccess</code>
     or <code>always</code>. It determines, which internal header table should be
     operated on. <code>onsuccess</code> stands for <code>2<var>xx</var></code>
     status codes and <code>always</code> for all status codes (including
     <code>2<var>xx</var></code>). Especially if you want to unset headers
     set by certain modules, you should try out, which table is affected.</p>

     <p>The action it performs is determined by the second
     argument. This can be one of the following values:</p>

     <dl>
     <dt><code>set</code></dt>
     <dd>The response header is set, replacing any previous header
     with this name. The <var>value</var> may be a format string.</dd>

     <dt><code>append</code></dt>
     <dd>The response header is appended to any existing header of
     the same name. When a new value is merged onto an existing
     header it is separated from the existing header with a comma.
     This is the HTTP standard way of giving a header multiple values.</dd>

     <dt><code>add</code></dt>
     <dd>The response header is added to the existing set of headers,
     even if this header already exists. This can result in two
     (or more) headers having the same name. This can lead to
     unforeseen consequences, and in general "append" should be
     used instead.</dd>

     <dt><code>unset</code></dt>
     <dd>The response header of this name is removed, if it exists.
     If there are multiple headers of the same name, all will be
     removed. <var>value</var> must be omitted.</dd>

     <dt><code>echo</code></dt>
     <dd>Request headers with this name are echoed back in the
     response headers. <var>header</var> may be a
     <a class="glossarylink" href="../glossary.html#regex" title="see glossary">regular expression</a>.
     <var>value</var> must be omitted.</dd>
     </dl>

     <p>This argument is followed by a <var>header</var> name, which
     can include the final colon, but it is not required. Case is
     ignored for <code>set</code>, <code>append</code>, <code>add</code>
     and <code>unset</code>. The <var>header</var> name for <code>echo</code>
     is case sensitive and may be a <a class="glossarylink" href="../glossary.html#regex" title="see glossary">regular
     expression</a>.</p>

     <p>For <code>add</code>, <code>append</code> and <code>set</code> a
     <var>value</var> is specified as the third argument. If <var>value</var>
     contains spaces, it should be surrounded by doublequotes.
     <var>value</var> may be a character string, a string containing format
     specifiers or a combination of both. The following format specifiers
     are supported in <var>value</var>:</p>

     <table class="bordered"><tr class="header"><th>Format</th><th>Description</th></tr>
 <tr><td><code>%%</code></td>
         <td>The percent sign</td></tr>
 <tr class="odd"><td><code>%t</code></td>
         <td>The time the request was received in Universal Coordinated Time
         since the epoch (Jan. 1, 1970) measured in microseconds. The value
         is preceded by <code>t=</code>.</td></tr>
 <tr><td><code>%D</code></td>
         <td>The time from when the request was received to the time the
         headers are sent on the wire. This is a measure of the duration
         of the request. The value is preceded by <code>D=</code>.</td></tr>
 <tr class="odd"><td><code>%{FOOBAR}e</code></td>
         <td>The contents of the <a href="../env.html">environment
         variable</a> <code>FOOBAR</code>.</td></tr>
 <tr><td><code>%{FOOBAR}s</code></td>
         <td>The contents of the <a href="mod_ssl.html#envvars">SSL environment
         variable</a> <code>FOOBAR</code>, if <code class="module"><a href="../mod/mod_ssl.html">mod_ssl</a></code> is enabled.</td></tr>
 </table>

     <div class="note"><h3>Note</h3>
       <p>The <code>%s</code> format specifier is only available in
       Apache 2.1 and later; it can be used instead of <code>%e</code>
       to avoid the overhead of enabling <code>SSLOptions
       +StdEnvVars</code>.  If <code>SSLOptions +StdEnvVars</code> must
       be enabled anyway for some other reason, <code>%e</code> will be
       more efficient than <code>%s</code>.</p>
     </div>

     <p>The <code class="directive">Header</code> directive may be followed by an
     an additional argument, which may be used to specify conditions under
     which the action will be taken, or may be the keyword <code>early</code>
     to specify <a href="#early">early processing</a>. If the
     <a href="../env.html">environment variable</a> specified in the
     <code>env=<var>...</var></code> argument exists (or if the environment
     variable does not exist and <code>env=!<var>...</var></code> is specified)
     then the action specified by the <code class="directive">Header</code> directive
     will take effect. Otherwise, the directive will have no effect
     on the request.</p>

     <p>Except in <a href="#early">early</a> mode, the
     <code class="directive">Header</code> directives are processed just
     before the response is sent to the network. These means that it is
     possible to set and/or override most headers, except for those headers
     added by the header filter.</p>

 </div>
 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="directive-section"><h2><a name="RequestHeader" id="RequestHeader">RequestHeader</a> <a name="requestheader" id="requestheader">Directive</a></h2>
 <table class="directive">
 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Configure HTTP request headers</td></tr>
 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>RequestHeader set|append|add|unset <var>header</var>
 [<var>value</var>] [early|env=[!]<var>variable</var>]</code></td></tr>
 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
 <tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_headers</td></tr>
 </table>
     <p>This directive can replace, merge or remove HTTP request
     headers. The header is modified just before the content handler
     is run, allowing incoming headers to be modified. The action it
     performs is determined by the first argument. This can be one
     of the following values:</p>

     <dl>
     <dt><code>set</code></dt>
     <dd>The request header is set, replacing any previous header
     with this name</dd>

     <dt><code>append</code></dt>
     <dd>The request header is appended to any existing header of the
     same name. When a new value is merged onto an existing header
     it is separated from the existing header with a comma. This
     is the HTTP standard way of giving a header multiple
     values.</dd>

     <dt><code>add</code></dt>
     <dd>The request header is added to the existing set of headers,
     even if this header already exists. This can result in two
     (or more) headers having the same name. This can lead to
     unforeseen consequences, and in general <code>append</code> should be
     used instead.</dd>

     <dt><code>unset</code></dt>
     <dd>The request header of this name is removed, if it exists. If
     there are multiple headers of the same name, all will be removed.
     <var>value</var> must be omitted.</dd>
     </dl>

     <p>This argument is followed by a header name, which can
     include the final colon, but it is not required. Case is
     ignored. For <code>add</code>, <code>append</code> and
     <code>set</code> a <var>value</var> is given as the third argument. If
     <var>value</var> contains spaces, it should be surrounded by double
     quotes. For unset, no <var>value</var> should be given.
     <var>value</var> may be a character string, a string containing format
     specifiers or a combination of both. The supported format specifiers
     are the same as for the <code class="directive"><a href="#header">Header</a></code>,
     please have a look there for details.</p>

     <p>The <code class="directive">RequestHeader</code> directive may be followed by
     an additional argument, which may be used to specify conditions under
     which the action will be taken, or may be the keyword <code>early</code>
     to specify <a href="#early">early processing</a>. If the
     <a href="../env.html">environment
     variable</a> specified in the <code>env=<var>...</var></code> argument
     exists (or if the environment variable does not exist and
     <code>env=!<var>...</var></code> is specified) then the action specified
     by the <code class="directive">RequestHeader</code> directive will take effect.
     Otherwise, the directive will have no effect on the request.</p>

     <p>Except in <a href="#early">early</a> mode, the
     <code class="directive">RequestHeader</code> directive is processed
     just before the request is run by its handler in the fixup phase.
     This should allow headers generated by the browser, or by Apache
     input filters to be overridden or modified.</p>

 </div>
 </div>
 <div class="bottomlang">
 <p><span>Available Languages: </span><a href="../en/mod/mod_headers.html" title="English">&nbsp;en&nbsp;</a> |
 <a href="../ja/mod/mod_headers.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a> |
 <a href="../ko/mod/mod_headers.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a></p>
 </div><div id="footer">
 <p class="apache">Copyright 1995-2005 The Apache Software Foundation or its licensors, as applicable.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
 <p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div>
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	<title>mod_headers - Apache HTTP Server</title>
	<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
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	<p class="apache">Apache HTTP Server Version 2.3</p>
	<img alt="" src="../images/feather.gif" /></div>
	<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
	<div id="path">
	<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="../">Version 2.3</a> > <a href="./">Modules</a></div>
	<div id="page-content">
	<div id="preamble"><h1>Apache Module mod_headers</h1>
	<div class="toplang">
	<p><span>Available Languages: </span><a href="../en/mod/mod_headers.html" title="English"> en </a> \|
	<a href="../ja/mod/mod_headers.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> \|
	<a href="../ko/mod/mod_headers.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
	</div>
	<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Customization of HTTP request and response
	headers</td></tr>
	<tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
	<tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>headers_module</td></tr>
	<tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_headers.c</td></tr>
	<tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td><code class="directive"><a href="#requestheader">RequestHeader</a></code>
	is available only in Apache 2.0</td></tr></table>
	<h3>Summary</h3>

	<p>This module provides directives to control and modify HTTP
	request and response headers. Headers can be merged, replaced
	or removed.</p>
	</div>
	<div id="quickview"><h3 class="directives">Directives</h3>
	<ul id="toc">
	<li><img alt="" src="../images/down.gif" /> <a href="#header">Header</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#requestheader">RequestHeader</a></li>
	</ul>
	<h3>Topics</h3>
	<ul id="topics">
	<li><img alt="" src="../images/down.gif" /> <a href="#order">Order of Processing</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#early">Early and Late Processing</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#examples">Examples</a></li>
	</ul></div>
	<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="order" id="order">Order of Processing</a></h2>

	<p>The directives provided by <code class="module"><a href="../mod/mod_headers.html">mod_headers</a></code> can
	occur almost anywhere within the server configuration, and can be
	limited in scope by enclosing them in <a href="../sections.html">configuration sections</a>.</p>

	<p>Order of processing is important and is affected both by the
	order in the configuration file and by placement in <a href="../sections.html#mergin">configuration sections</a>. These
	two headers have a different effect if reversed:</p>

	<div class="example"><p><code>
	RequestHeader append MirrorID "mirror 12"<br />
	RequestHeader unset MirrorID
	</code></p></div>

	<p>This way round, the <code>MirrorID</code> header is not set. If
	reversed, the MirrorID header is set to "mirror 12".</p>
	</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="early" id="early">Early and Late Processing</a></h2>
	<p><code class="module"><a href="../mod/mod_headers.html">mod_headers</a></code> can be applied either early or late
	in the request. The normal mode is late, when Request Headers are
	set immediately before running the content generator and Response
	Headers just as the response is sent down the wire. Always use
	Late mode in an operational server.</p>

	<p>Early mode is designed as a test/debugging aid for developers.
	Directives defined using the <code>early</code> keyword are set
	right at the beginning of processing the request. This means
	they can be used to simulate different requests and set up test
	cases, but it also means that headers may be changed at any time
	by other modules before generating a Response.</p>

	<p>Because early directives are processed before the request path's
	configuration is traversed, early headers can only be set in a
	main server or virtual host context. Early directives cannot depend
	on a request path, so they will fail in contexts such as
	<code><Directory></code> or <code><Location></code>.</p>
	</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="examples" id="examples">Examples</a></h2>

	<ol>
	<li>
	Copy all request headers that begin with "TS" to the
	response headers:

	<div class="example"><p><code>
	Header echo ^TS
	</code></p></div>
	</li>

	<li>
	Add a header, <code>MyHeader</code>, to the response including a
	timestamp for when the request was received and how long it
	took to begin serving the request. This header can be used by
	the client to intuit load on the server or in isolating
	bottlenecks between the client and the server.

	<div class="example"><p><code>
	Header add MyHeader "%D %t"
	</code></p></div>

	<p>results in this header being added to the response:</p>

	<div class="example"><p><code>
	MyHeader: D=3775428 t=991424704447256
	</code></p></div>
	</li>

	<li>
	Say hello to Joe

	<div class="example"><p><code>
	Header add MyHeader "Hello Joe. It took %D microseconds \<br />
	for Apache to serve this request."
	</code></p></div>

	<p>results in this header being added to the response:</p>

	<div class="example"><p><code>
	MyHeader: Hello Joe. It took D=3775428 microseconds for Apache
	to serve this request.
	</code></p></div>
	</li>

	<li>
	Conditionally send <code>MyHeader</code> on the response if and
	only if header "MyRequestHeader" is present on the request. This
	is useful for constructing headers in response to some client
	stimulus. Note that this example requires the services of the
	<code class="module"><a href="../mod/mod_setenvif.html">mod_setenvif</a></code> module.

	<div class="example"><p><code>
	SetEnvIf MyRequestHeader value HAVE_MyRequestHeader<br />
	Header add MyHeader "%D %t mytext" env=HAVE_MyRequestHeader<br />
	</code></p></div>

	<p>If the header <code>MyRequestHeader: value</code> is present on
	the HTTP request, the response will contain the following header:</p>

	<div class="example"><p><code>
	MyHeader: D=3775428 t=991424704447256 mytext
	</code></p></div>
	</li>
	</ol>
	</div>
	<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="directive-section"><h2><a name="Header" id="Header">Header</a> <a name="header" id="header">Directive</a></h2>
	<table class="directive">
	<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Configure HTTP response headers</td></tr>
	<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>Header [<var>condition</var>] set\|append\|add\|unset\|echo
	<var>header</var> [<var>value</var>] [early\|env=[!]<var>variable</var>]</code></td></tr>
	<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
	<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
	<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
	<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_headers</td></tr>
	</table>
	<p>This directive can replace, merge or remove HTTP response
	headers. The header is modified just after the content handler
	and output filters are run, allowing outgoing headers to be
	modified.</p>

	<p>The optional <var>condition</var> can be either <code>onsuccess</code>
	or <code>always</code>. It determines, which internal header table should be
	operated on. <code>onsuccess</code> stands for <code>2<var>xx</var></code>
	status codes and <code>always</code> for all status codes (including
	<code>2<var>xx</var></code>). Especially if you want to unset headers
	set by certain modules, you should try out, which table is affected.</p>

	<p>The action it performs is determined by the second
	argument. This can be one of the following values:</p>

	<dl>
	<dt><code>set</code></dt>
	<dd>The response header is set, replacing any previous header
	with this name. The <var>value</var> may be a format string.</dd>

	<dt><code>append</code></dt>
	<dd>The response header is appended to any existing header of
	the same name. When a new value is merged onto an existing
	header it is separated from the existing header with a comma.
	This is the HTTP standard way of giving a header multiple values.</dd>

	<dt><code>add</code></dt>
	<dd>The response header is added to the existing set of headers,
	even if this header already exists. This can result in two
	(or more) headers having the same name. This can lead to
	unforeseen consequences, and in general "append" should be
	used instead.</dd>

	<dt><code>unset</code></dt>
	<dd>The response header of this name is removed, if it exists.
	If there are multiple headers of the same name, all will be
	removed. <var>value</var> must be omitted.</dd>

	<dt><code>echo</code></dt>
	<dd>Request headers with this name are echoed back in the
	response headers. <var>header</var> may be a
	<a class="glossarylink" href="../glossary.html#regex" title="see glossary">regular expression</a>.
	<var>value</var> must be omitted.</dd>
	</dl>

	<p>This argument is followed by a <var>header</var> name, which
	can include the final colon, but it is not required. Case is
	ignored for <code>set</code>, <code>append</code>, <code>add</code>
	and <code>unset</code>. The <var>header</var> name for <code>echo</code>
	is case sensitive and may be a <a class="glossarylink" href="../glossary.html#regex" title="see glossary">regular
	expression</a>.</p>

	<p>For <code>add</code>, <code>append</code> and <code>set</code> a
	<var>value</var> is specified as the third argument. If <var>value</var>
	contains spaces, it should be surrounded by doublequotes.
	<var>value</var> may be a character string, a string containing format
	specifiers or a combination of both. The following format specifiers
	are supported in <var>value</var>:</p>

	<table class="bordered"><tr class="header"><th>Format</th><th>Description</th></tr>
	<tr><td><code>%%</code></td>
	<td>The percent sign</td></tr>
	<tr class="odd"><td><code>%t</code></td>
	<td>The time the request was received in Universal Coordinated Time
	since the epoch (Jan. 1, 1970) measured in microseconds. The value
	is preceded by <code>t=</code>.</td></tr>
	<tr><td><code>%D</code></td>
	<td>The time from when the request was received to the time the
	headers are sent on the wire. This is a measure of the duration
	of the request. The value is preceded by <code>D=</code>.</td></tr>
	<tr class="odd"><td><code>%{FOOBAR}e</code></td>
	<td>The contents of the <a href="../env.html">environment
	variable</a> <code>FOOBAR</code>.</td></tr>
	<tr><td><code>%{FOOBAR}s</code></td>
	<td>The contents of the <a href="mod_ssl.html#envvars">SSL environment
	variable</a> <code>FOOBAR</code>, if <code class="module"><a href="../mod/mod_ssl.html">mod_ssl</a></code> is enabled.</td></tr>
	</table>

	<div class="note"><h3>Note</h3>
	<p>The <code>%s</code> format specifier is only available in
	Apache 2.1 and later; it can be used instead of <code>%e</code>
	to avoid the overhead of enabling <code>SSLOptions
	+StdEnvVars</code>. If <code>SSLOptions +StdEnvVars</code> must
	be enabled anyway for some other reason, <code>%e</code> will be
	more efficient than <code>%s</code>.</p>
	</div>

	<p>The <code class="directive">Header</code> directive may be followed by an
	an additional argument, which may be used to specify conditions under
	which the action will be taken, or may be the keyword <code>early</code>
	to specify <a href="#early">early processing</a>. If the
	<a href="../env.html">environment variable</a> specified in the
	<code>env=<var>...</var></code> argument exists (or if the environment
	variable does not exist and <code>env=!<var>...</var></code> is specified)
	then the action specified by the <code class="directive">Header</code> directive
	will take effect. Otherwise, the directive will have no effect
	on the request.</p>

	<p>Except in <a href="#early">early</a> mode, the
	<code class="directive">Header</code> directives are processed just
	before the response is sent to the network. These means that it is
	possible to set and/or override most headers, except for those headers
	added by the header filter.</p>

	</div>
	<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="directive-section"><h2><a name="RequestHeader" id="RequestHeader">RequestHeader</a> <a name="requestheader" id="requestheader">Directive</a></h2>
	<table class="directive">
	<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Configure HTTP request headers</td></tr>
	<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>RequestHeader set\|append\|add\|unset <var>header</var>
	[<var>value</var>] [early\|env=[!]<var>variable</var>]</code></td></tr>
	<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
	<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
	<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
	<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_headers</td></tr>
	</table>
	<p>This directive can replace, merge or remove HTTP request
	headers. The header is modified just before the content handler
	is run, allowing incoming headers to be modified. The action it
	performs is determined by the first argument. This can be one
	of the following values:</p>

	<dl>
	<dt><code>set</code></dt>
	<dd>The request header is set, replacing any previous header
	with this name</dd>

	<dt><code>append</code></dt>
	<dd>The request header is appended to any existing header of the
	same name. When a new value is merged onto an existing header
	it is separated from the existing header with a comma. This
	is the HTTP standard way of giving a header multiple
	values.</dd>

	<dt><code>add</code></dt>
	<dd>The request header is added to the existing set of headers,
	even if this header already exists. This can result in two
	(or more) headers having the same name. This can lead to
	unforeseen consequences, and in general <code>append</code> should be
	used instead.</dd>

	<dt><code>unset</code></dt>
	<dd>The request header of this name is removed, if it exists. If
	there are multiple headers of the same name, all will be removed.
	<var>value</var> must be omitted.</dd>
	</dl>

	<p>This argument is followed by a header name, which can
	include the final colon, but it is not required. Case is
	ignored. For <code>add</code>, <code>append</code> and
	<code>set</code> a <var>value</var> is given as the third argument. If
	<var>value</var> contains spaces, it should be surrounded by double
	quotes. For unset, no <var>value</var> should be given.
	<var>value</var> may be a character string, a string containing format
	specifiers or a combination of both. The supported format specifiers
	are the same as for the <code class="directive"><a href="#header">Header</a></code>,
	please have a look there for details.</p>

	<p>The <code class="directive">RequestHeader</code> directive may be followed by
	an additional argument, which may be used to specify conditions under
	which the action will be taken, or may be the keyword <code>early</code>
	to specify <a href="#early">early processing</a>. If the
	<a href="../env.html">environment
	variable</a> specified in the <code>env=<var>...</var></code> argument
	exists (or if the environment variable does not exist and
	<code>env=!<var>...</var></code> is specified) then the action specified
	by the <code class="directive">RequestHeader</code> directive will take effect.
	Otherwise, the directive will have no effect on the request.</p>

	<p>Except in <a href="#early">early</a> mode, the
	<code class="directive">RequestHeader</code> directive is processed
	just before the request is run by its handler in the fixup phase.
	This should allow headers generated by the browser, or by Apache
	input filters to be overridden or modified.</p>

	</div>
	</div>
	<div class="bottomlang">
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	<p class="apache">Copyright 1995-2005 The Apache Software Foundation or its licensors, as applicable.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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