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<?xml version="1.0" encoding="ISO-8859-1"?>
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<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="../fr/mod/mod_headers.html" hreflang="fr" rel="alternate" title="Français"> fr </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"><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 directives 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 set 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 set 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 <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.
<div class="example"><p><code>
SetEnvIf MyRequestHeader myvalue HAVE_MyRequestHeader<br />
Header set MyHeader "%D %t mytext" env=HAVE_MyRequestHeader
</code></p></div>
<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:
<div class="example"><p><code>
RequestHeader edit Destination ^https: http: early
</code></p></div>
</li>
<li>
Set the same header value under multiple non-exclusive 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):
<div class="example"><p><code>
Header merge Cache-Control no-cache env=CGI<br />
Header merge Cache-Control no-cache env=NO_CACHE<br />
Header merge Cache-Control no-store env=NO_STORE
</code></p></div>
<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
<div class="example"><p><code>
Header set Set-Cookie testcookie !$req{Cookie}
</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>] add|append|echo|edit|merge|set|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 response
headers. The header is modified just after the content handler
and output filters are run, allowing outgoing headers to be
modified.</p>
<p>By default, this directive only affects successful responses (responses
in the <code>2<var>xx</var></code> range). The optional <var>condition</var>
can be either <code>onsuccess</code> (default) or <code>always</code> (all
status codes, including successful responses). A value of <code>always</code>
may be needed to influence headers set by some internal modules even for
successful responses, and is always needed to affect non-<code>2<var>xx</var></code>
responses such as redirects or client errors.</p>
<p>The action it performs is determined by the second
argument. This can be one of the following values:</p>
<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>
<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.</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>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>
</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 third 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 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>.
The value is measured in microseconds.</td></tr>
<tr class="odd"><td><code>%{VARNAME}e</code></td>
<td>The contents of the <a href="../env.html">environment
variable</a> <code>VARNAME</code>.</td></tr>
<tr><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>
<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.</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=[!]varname</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</code></dt>
<dd>An string that matches neither of the above is parsed as an
expression. Details of expression syntax and evaluation are
currently best documented on the <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> page.</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. 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 add|append|edit|edit*|merge|set|unset <var>header</var>
[<var>value</var>] [<var>replacement</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, 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.
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
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>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=[!]varname</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</code></dt>
<dd>An string that matches neither of the above is parsed as an
expression. Details of expression syntax and evaluation are
currently best documented on the <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> page.</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>
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