docs/manual/mod/mod_headers.xml

<?xml version="1.0"?>
<!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.xsl"?>
<modulesynopsis>

<name>mod_headers</name>
<description>Customization of HTTP request
    and response headers</description>
<status>Extension</status>
<sourcefile>mod_headers.c</sourcefile>
<identifier>headers_module</identifier>
<compatibility>RequestHeader is available only in Apache 2.0</compatibility>

<summary>
    <p>This module provides directives to control and modify HTTP
    request and response headers. Headers can be merged, replaced
    or removed.</p>
</summary>

<section><title>Order of Processing</title>

    <p>The directives provided by mod_header can occur almost
    anywhere within the server configuration. They are valid in the
    main server config and virtual host sections, inside
    &lt;Directory&gt;, &lt;Location&gt; and &lt;Files&gt; sections,
    and within .htaccess files.</p>

    <p>The directives are processed in the following order:</p>

    <ol>
      <li>main server</li>

      <li>virtual host</li>

      <li>&lt;Directory&gt; sections and .htaccess</li>

      <li>&lt;Location&gt;</li>

      <li>&lt;Files&gt;</li>
    </ol>

    <p>Order is important. These two headers have a different
    effect if reversed:</p>

<example>
RequestHeader append MirrorID "mirror 12"<br />
       RequestHeader unset MirrorID
</example>

    <p>This way round, the MirrorID header is not set. If reversed,
    the MirrorID header is set to "mirror 12".</p>
</section>

<section><title>Example</title>

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

<example>
   Header echo ^TS*
</example></li>

      <li>Add a header, MyHeader, 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.

<example>
   Header add MyHeader "%D %t"
</example>
        results in this header being added to the response:
<example>
   MyHeader: D=3775428 t=991424704447256
</example>
      </li>

      <li>Say hello to Joe

<example>
   Header add MyHeader "Hello Joe. It took %D microseconds for Apache to serve this request."
</example>
        results in this header being added to the response: 
<example>
   MyHeader: Hello Joe. It took D=3775428 microseconds for Apache to serve this request.
</example>
      </li>

      <li>Conditionally send MyHeader 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
      mod_setenvif module.

<example>
   SetEnvIf MyRequestHeader value HAVE_MyRequestHeader<br />
   Header add MyHeader "%D %t mytext" env=HAVE_MyRequestHeader
</example>
        If the header "MyRequestHeader: value" is present on the
        HTTP request, the response will contain the following
        header: 
<example>
   MyHeader: D=3775428 t=991424704447256 mytext
</example>
      </li>
    </ol>
</section>

<directivesynopsis>
<name>RequestHeader</name>
<description>Configure HTTP request headers</description>
<syntax>RequestHeader   set|append|add|unset <em>header</em> 
[<em>value</em>]</syntax>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context></contextlist>
<override>FileInfo</override>

<usage>
    <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>

    <ul>
      <li><strong>set</strong><br />
       The request header is set, replacing any previous header
      with this name</li>

      <li><strong>append</strong><br />
       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.</li>

      <li><strong>add</strong><br />
       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 "append" should be
      used instead.</li>

      <li><strong>unset</strong><br />
       The request header of this name is removed, if it exists. If
      there are multiple headers of the same name, all will be
      removed.</li>
    </ul>

    <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 value is given as the third argument. If
    this value contains spaces, it should be surrounded by double
    quotes. For unset, no value should be given.</p>

    <p>The <directive>RequestHeader</directive> 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>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>Header</name>
<description>Configure HTTP response headers</description>
<syntax>Header set|append|add|unset|echo  <em>header</em> 
[<em>value</em>]</syntax>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context></contextlist>
<override>FileInfo</override>

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

    <ul>
      <li><strong>set</strong><br />
       The response header is set, replacing any previous header
      with this name. The <em>value</em> may be a format
      string.</li>

      <li><strong>append</strong><br />
       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.</li>

      <li><strong>add</strong><br />
       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.</li>

      <li><strong>unset</strong><br />
       The response header of this name is removed, if it exists.
      If there are multiple headers of the same name, all will be
      removed.</li>

      <li><strong>echo</strong><br />
       Request headers with this name are echoed back in the
      response headers. <em>header</em> may be a regular
      expression.</li>
    </ul>

    <p>This argument is followed by a <em>header</em> name, which
    can include the final colon, but it is not required. Case is
    ignored for set, append, add and unset. The <em>header</em>
    name for echo is case sensitive and may be a regular
    expression.</p>

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

<tr><td>%D: </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 "D=".</td></tr>

<tr><td>%{FOOBAR}e:</td> <td>The contents of the <a href="../env.html">environment
variable</a> FOOBAR.</td></tr>
</table>

    <p>When the <directive>Header</directive> directive is used with the
    <code>add</code>, <code>append</code>, or <code>set</code>
    argument, a fourth argument may be used to specify conditions
    under which the action will be taken. If the <a
    href="../env.html">environment variable</a> specified in the
    <code>env=...</code> argument exists (or if the environment
    variable does not exist and <code>env=!...</code> is specified)
    then the action specified by the <directive>Header</directive> directive
    will take effect. Otherwise, the directive will have no effect
    on the request.</p>

    <p>The Header 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>
</usage>
</directivesynopsis>

</modulesynopsis>