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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"> en </a> |
<a href="../fr/mod/mod_headers.html" hreflang="fr" rel="alternate" title="Franais"> 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">ModuleIdentifier:</a></th><td>headers_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">SourceFile:</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>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>
<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"><Directory></a></code> or
<code class="directive"><a href="../mod/core.html#location"><Location></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. Despite the
name, the default value of <code>onsuccess</code> does <em>not</em> limit
an <var>action</var> to responses with a 2xx status code. Headers set under
this condition are still used when, for example, a request is <em>successfully</em>
proxied or generated by CGI, even when they have generated a failing status code.</p>
<p>When your action is a function of an existing header, you may need to specify
a condition of <code>always</code>, depending on which internal table the
original header was set in. The table that corresponds to <code>always</code> is
used for locally generated error responses as well as successful responses.
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,
in which case the CGI scripts 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>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>
<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.<br />
Available in 2.4.7 and later.</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 <If>:</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.</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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