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<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> |
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<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></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"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><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><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>Bugfix checklist</h3><ul class="seealso"><li><a href="https://www.apache.org/dist/httpd/CHANGES_2.4">httpd changelog</a></li><li><a href="https://bz.apache.org/bugzilla/buglist.cgi?bug_status=__open__&amp;list_id=144532&amp;product=Apache%20httpd-2&amp;query_format=specific&amp;order=changeddate%20DESC%2Cpriority%2Cbug_severity&amp;component=mod_headers">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&amp;component=mod_headers">Report a bug</a></li></ul><h3>See also</h3>
<ul class="seealso">
<li><a href="#comments_section">Comments</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 directives have a different effect if reversed:</p>

    <pre class="prettyprint lang-config">RequestHeader append MirrorID "mirror 12"
RequestHeader unset MirrorID</pre>


    <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 <em>Request</em> Headers are
    set immediately before running the content generator and <em>Response</em>
    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 class="directive"><a href="../mod/core.html#directory">&lt;Directory&gt;</a></code> or
    <code class="directive"><a href="../mod/core.html#location">&lt;Location&gt;</a></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:

        <pre class="prettyprint lang-config">Header echo ^TS</pre>

      </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.

        <pre class="prettyprint lang-config">Header set MyHeader "%D %t"</pre>


        <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

        <pre class="prettyprint lang-config">Header set MyHeader "Hello Joe. It took %D microseconds for Apache to serve this request."</pre>


        <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 <code>MyRequestHeader</code> 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.

        <pre class="prettyprint lang-config">SetEnvIf MyRequestHeader myvalue HAVE_MyRequestHeader
Header set MyHeader "%D %t mytext" env=HAVE_MyRequestHeader</pre>


        <p>If the header <code>MyRequestHeader: myvalue</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>

      <li>
        Enable DAV to work with Apache running HTTP through SSL hardware
        (<a href="http://svn.haxx.se/users/archive-2006-03/0549.shtml">problem
        description</a>) by replacing <var>https:</var> with
        <var>http:</var> in the <var>Destination</var> header:

        <pre class="prettyprint lang-config">RequestHeader edit Destination ^https: http: early</pre>

      </li>

      <li>
        Set the same header value under multiple nonexclusive conditions,
        but do not duplicate the value in the final header.
        If all of the following conditions applied to a request (i.e.,
        if the <code>CGI</code>, <code>NO_CACHE</code> and
        <code>NO_STORE</code> environment variables all existed for the
        request):

        <pre class="prettyprint lang-config">Header merge Cache-Control no-cache env=CGI
Header merge Cache-Control no-cache env=NO_CACHE
Header merge Cache-Control no-store env=NO_STORE</pre>


        <p>then the response would contain the following header:</p>

        <div class="example"><p><code>
          Cache-Control: no-cache, no-store
        </code></p></div>

        <p>If <code>append</code> was used instead of <code>merge</code>,
        then the response would contain the following header:</p>

        <div class="example"><p><code>
          Cache-Control: no-cache, no-cache, no-store
        </code></p></div>
      </li>
      <li>
        Set a test cookie if and only if the client didn't send us a cookie
        <pre class="prettyprint lang-config">Header set Set-Cookie testcookie "expr=-z %{req:Cookie}"</pre>

      </li>
      <li>
        Append a Caching header for responses with a HTTP status code of 200
        <pre class="prettyprint lang-config">Header append Cache-Control s-maxage=600 "expr=%{REQUEST_STATUS} == 200"</pre>

      </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>] add|append|echo|edit|edit*|merge|set|setifempty|unset|note
<var>header</var> [[expr=]<var>value</var> [<var>replacement</var>]
[early|env=[!]<var>varname</var>|expr=<var>expression</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>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>SetIfEmpty available in 2.4.7 and later, expr=value
available in 2.4.10 and later</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> argument determines which internal
    table of responses headers this directive will operate against:
    <code>onsuccess</code> (default, can be omitted) or <code>always</code>.
    The difference between the two lists is that the headers contained in the
    latter are added to the response even on error, and persisted across
    internal redirects (for example, ErrorDocument handlers).

    Note also that repeating this directive with both conditions makes sense in
    some scenarios because <code>always</code> is not a superset of
    <code>onsuccess</code> with respect to existing headers:</p>

    <ul>
       <li> You're adding a header to a locally generated non-success (non-2xx) response, such
            as a redirect, in which case only the table corresponding to
            <code>always</code> is used in the ultimate response.</li>
       <li> You're modifying or removing a header generated by a CGI script
            or by <code class="module"><a href="../mod/mod_proxy_fcgi.html">mod_proxy_fcgi</a></code>,
            in which case the CGI scripts' headers are in the table corresponding to
            <code>always</code> and not in the default table.</li>
       <li> You're modifying or removing a header generated by some piece of
            the server but that header is not being found by the default
            <code>onsuccess</code> condition.</li>
    </ul>

    <p>This difference between <code>onsuccess</code> and <code>always</code> is
    a feature that resulted as a consequence of how httpd internally stores
    headers for a HTTP response, since it does not offer any "normalized" single
    list of headers. The main problem that can arise if the following concept
    is not kept in mind while writing the configuration is that some HTTP responses
    might end up with the same header duplicated (confusing users or sometimes even
    HTTP clients). For example, suppose that you have a simple PHP proxy setup with
    <code class="module"><a href="../mod/mod_proxy_fcgi.html">mod_proxy_fcgi</a></code> and your backend PHP scripts adds the
    <code>X-Foo: bar</code> header to each HTTP response. As described above,
    <code class="module"><a href="../mod/mod_proxy_fcgi.html">mod_proxy_fcgi</a></code> uses the <code>always</code> table to store
    headers, so a configuration like the following ends up in the wrong result, namely
    having the header duplicated with both values:</p>

    <pre class="prettyprint lang-config"># X-Foo's value is set in the 'onsuccess' headers table
Header set X-Foo: baz</pre>
    

    <p>To circumvent this limitation, there are some known configuration
    patterns that can help, like the following:</p>

        <pre class="prettyprint lang-config"># 'onsuccess' can be omitted since it is the default
Header onsuccess unset X-Foo
Header always set X-Foo "baz"</pre>


    <p>Separately from the <var>condition</var> parameter described above, you
    can limit an action based on HTTP status codes for e.g. proxied or CGI
    requests. See the example that uses %{REQUEST_STATUS} in the section above.</p>

    <p>The action it performs is determined by the first
    argument (second argument if a <var>condition</var> is specified).
    This can be one of the following values:</p>

    <div class="warning"><h3>Warning</h3>
        <p>Please read the difference between <code>always</code>
        and <code>onsuccess</code> headers list described above
        before start reading the actions list, since that important
        concept still applies. Each action, in fact, works as described
        but only on the target headers list.</p>
    </div>

    <dl>
    <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 <code>set</code>,
    <code>append</code> or <code>merge</code> should be used instead.</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>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>

    <dt><code>edit</code></dt>
    <dt><code>edit*</code></dt>
    <dd>If this response header exists, its value is transformed according
    to a <a class="glossarylink" href="../glossary.html#regex" title="see glossary">regular expression</a>
    search-and-replace.  The <var>value</var> argument is a <a class="glossarylink" href="../glossary.html#regex" title="see glossary">regular expression</a>, and the <var>replacement</var>
    is a replacement string, which may contain backreferences or format specifiers.
    The <code>edit</code> form will match and replace exactly once
    in a header value, whereas the <code>edit*</code> form will replace
    <em>every</em> instance of the search pattern if it appears more
    than once.</dd>

    <dt><code>merge</code></dt>
    <dd>The response header is appended to any existing header of
    the same name, unless the value to be appended already appears in the
    header's comma-delimited list of values.  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.
    Values are compared in a case sensitive manner, and after
    all format specifiers have been processed.  Values in double quotes
    are considered different from otherwise identical unquoted values.</dd>

    <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>setifempty</code></dt>
    <dd>The request header is set, but only if there is no previous header
    with this name.
    <div class="note">
    The Content-Type header is a special use case since there might be
    the chance that its value have been determined but the header is not part
    of the response when <code>setifempty</code> is evaluated.
    It is safer to use <code>set</code> for this use case like in the
    following example:
    <pre class="prettyprint lang-config">Header set Content-Type "text/plain" "expr=-z %{CONTENT_TYPE}"</pre>

    </div></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>note</code></dt>
    <dd>The value of the named response <var>header</var> is copied into an
    internal note whose name is given by <var>value</var>.  This is useful
    if a header sent by a CGI or proxied resource is configured to be unset
    but should also be logged.<br />
    Available in 2.4.7 and later.</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>merge</code>,
    <code>add</code>, <code>unset</code> and <code>edit</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>set</code>, <code>append</code>, <code>merge</code> and
    <code>add</code> a <var>value</var> is specified as the next argument.
    If <var>value</var>
    contains spaces, it should be surrounded by double quotes.
    <var>value</var> may be a character string, a string containing
    <code class="module"><a href="../mod/mod_headers.html">mod_headers</a></code> specific format specifiers (and character
    literals), or an <a href="../expr.html">ap_expr</a> expression prefixed
    with <em>expr=</em></p>

    <p> 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>.
        The value is measured in microseconds.</td></tr>
<tr class="odd"><td><code>%l</code></td>
        <td>The current load averages of the actual server itself. It is
        designed to expose the values obtained by <code>getloadavg()</code>
        and this represents the current load average, the 5 minute average, and
        the 15 minute average. The value is preceded by <code>l=</code> with each
        average separated by <code>/</code>.<br />
        Available in 2.4.4 and later.
        </td></tr>
<tr><td><code>%i</code></td>
        <td>The current idle percentage of httpd (0 to 100) based on available
        processes and threads. The value is preceded by <code>i=</code>.<br />
        Available in 2.4.4 and later.
        </td></tr>
<tr class="odd"><td><code>%b</code></td>
        <td>The current busy percentage of httpd (0 to 100) based on available
        processes and threads. The value is preceded by <code>b=</code>.<br />
        Available in 2.4.4 and later.
        </td></tr>
<tr><td><code>%{VARNAME}e</code></td>
        <td>The contents of the <a href="../env.html">environment
        variable</a> <code>VARNAME</code>.</td></tr>
<tr class="odd"><td><code>%{VARNAME}s</code></td>
        <td>The contents of the <a href="mod_ssl.html#envvars">SSL environment
        variable</a> <code>VARNAME</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>

    <div class="note"><h3>Note on expression values</h3>
    <p> When the value parameter uses the <a href="../expr.html">ap_expr</a>
    parser, some expression syntax will differ from examples that evaluate
    <em>boolean</em> expressions such as &lt;If&gt;:</p>
    <ul>
      <li>The starting point of the grammar is 'string' rather than 'expr'.</li>
      <li>Function calls use the %{funcname:arg} syntax rather than
          funcname(arg).</li>
      <li>Multi-argument functions are not currently accessible from this
          starting point</li>
      <li>Quote the entire parameter, such as
          <pre class="prettyprint lang-config">Header set foo-checksum "expr=%{md5:foo}"</pre>

       </li>

    </ul>
    </div>

    <p>For <code>edit</code> there is both a <var>value</var> argument
    which is a <a class="glossarylink" href="../glossary.html#regex" title="see glossary">regular expression</a>,
    and an additional <var>replacement</var> string. As of version 2.4.7
    the replacement string may also contain format specifiers.</p>

    <p>The <code class="directive">Header</code> directive may be followed by
    an additional argument, which may be any of:</p>
    <dl>
    <dt><code>early</code></dt>
    <dd>Specifies <a href="#early">early processing</a>.</dd>
    <dt><code>env=[!]<var>varname</var></code></dt>
    <dd>The directive is applied if and only if the <a href="../env.html">environment variable</a> <code>varname</code> exists.
        A <code>!</code> in front of <code>varname</code> reverses the test,
        so the directive applies only if <code>varname</code> is unset.</dd>
    <dt><code>expr=<var>expression</var></code></dt>
    <dd>The directive is applied if and only if <var>expression</var>
        evaluates to true. Details of expression syntax and evaluation are
        documented in the <a href="../expr.html">ap_expr</a> documentation.
        <pre class="prettyprint lang-config"># This delays the evaluation of the condition clause compared to &lt;If&gt;
Header always set CustomHeader my-value "expr=%{REQUEST_URI} =~ m#^/special_path.php$#"</pre>

        </dd>
    </dl>

    <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. This means that it is
    possible to set and/or override most headers, except for some headers
    added by the HTTP header filter.  Prior to 2.2.12, it was not possible
    to change the Content-Type header with this directive.</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 add|append|edit|edit*|merge|set|setifempty|unset
<var>header</var> [[expr=]<var>value</var> [<var>replacement</var>]
[early|env=[!]<var>varname</var>|expr=<var>expression</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>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>SetIfEmpty available in 2.4.7 and later, expr=value
available in 2.4.10 and later</td></tr>
</table>
    <p>This directive can replace, merge, change 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>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>set</code>,
    <code>append</code> or <code>merge</code> should be used instead.</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>edit</code></dt>
    <dt><code>edit*</code></dt>
    <dd>If this request header exists, its value is transformed according
    to a <a class="glossarylink" href="../glossary.html#regex" title="see glossary">regular expression</a>
    search-and-replace.  The <var>value</var> argument is a <a class="glossarylink" href="../glossary.html#regex" title="see glossary">regular expression</a>, and the <var>replacement</var>
    is a replacement string, which may contain backreferences or format specifiers.
    The <code>edit</code> form will match and replace exactly once
    in a header value, whereas the <code>edit*</code> form will replace
    <em>every</em> instance of the search pattern if it appears more
    than once.</dd>

    <dt><code>merge</code></dt>
    <dd>The request header is appended to any existing header of
    the same name, unless the value to be appended already appears in the
    existing header's comma-delimited list of values.  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.  Values are compared in a case sensitive manner, and after
    all format specifiers have been processed.  Values in double quotes
    are considered different from otherwise identical unquoted values.</dd>

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

    <dt><code>setifempty</code></dt>
    <dd>The request header is set, but only if there is no previous header
    with this name.<br />
    Available in 2.4.7 and later.</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>set</code>, <code>append</code>, <code>merge</code> and
    <code>add</code> a <var>value</var> is given as the third argument. If a
    <var>value</var> contains spaces, it should be surrounded by double
    quotes. For <code>unset</code>, 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.  For <code>edit</code> both
    a <var>value</var> and a <var>replacement</var> are required, and are
    a <a class="glossarylink" href="../glossary.html#regex" title="see glossary">regular expression</a> and a
    replacement string respectively.</p>

    <p>The <code class="directive">RequestHeader</code> directive may be followed by
    an additional argument, which may be any of:</p>
    <dl>
    <dt><code>early</code></dt>
    <dd>Specifies <a href="#early">early processing</a>.</dd>
    <dt><code>env=[!]<var>varname</var></code></dt>
    <dd>The directive is applied if and only if the <a href="../env.html">environment variable</a> <code>varname</code> exists.
        A <code>!</code> in front of <code>varname</code> reverses the test,
        so the directive applies only if <code>varname</code> is unset.</dd>
    <dt><code>expr=<var>expression</var></code></dt>
    <dd>The directive is applied if and only if <var>expression</var>
        evaluates to true. Details of expression syntax and evaluation are
        documented in the <a href="../expr.html">ap_expr</a> documentation.</dd>
    </dl>

    <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>
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