path: root/docs/manual/mod/mod_headers.xml

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

<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><directive module="mod_headers">RequestHeader</directive>
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 id="order"><title>Order of Processing</title>

    <p>The directives provided by <module>mod_headers</module> 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>

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

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

<section id="early"><title>Early and Late Processing</title>
    <p><module>mod_headers</module> 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>
</section>

<section id="examples"><title>Examples</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, <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.

        <example>
          Header add MyHeader "%D %t"
        </example>

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

        <example>
          MyHeader: D=3775428 t=991424704447256
        </example>
      </li>

      <li>
        Say hello to Joe

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

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

        <example>
          MyHeader: Hello Joe. It took D=3775428 microseconds for Apache
          to serve this request.
        </example>
      </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
        <module>mod_setenvif</module> module.

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

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

       <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 <var>header</var>
[<var>value</var>] [early|env=[!]<var>variable</var>]</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>

    <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 <directive module="mod_headers">Header</directive>,
    please have a look there for details.</p>

    <p>The <directive>RequestHeader</directive> 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 <directive>RequestHeader</directive> 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
    <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 [<var>condition</var>] set|append|add|unset|echo
<var>header</var> [<var>value</var>] [early|env=[!]<var>variable</var>]</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.</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 regular expression.
    <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 regular expression.</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 border="1" style="zebra">
    <columnspec><column width=".25"/><column width=".75"/></columnspec>
    <tr><th>Format</th><th>Description</th></tr>
    <tr><td><code>%%</code></td>
        <td>The percent sign</td></tr>

    <tr><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><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 <module>mod_ssl</module> is enabled.</td></tr>

    </table>

    <note><title>Note</title>
      <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>
    </note> 

    <p>The <directive>Header</directive> 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 <directive>Header</directive> 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
    <directive>Header</directive> 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>