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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="../ja/mod/mod_headers.html" hreflang="ja" rel="alternate" title=""> ja </a> |
-<a href="../ko/mod/mod_headers.html" hreflang="ko" rel="alternate" title=""> ko </a></p>
+<a href="../fr/mod/mod_headers.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
+<a href="../ja/mod/mod_headers.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
+<a href="../ko/mod/mod_headers.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
</div>
<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Customization of HTTP request and response
headers</td></tr>
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>
+<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></div>
+</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__&list_id=144532&product=Apache%20httpd-2&query_format=specific&order=changeddate%20DESC%2Cpriority%2Cbug_severity&component=mod_headers">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&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>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>
+ two directives have a different effect if reversed:</p>
+
+ <pre class="prettyprint lang-config">RequestHeader append MirrorID "mirror 12"
+RequestHeader unset MirrorID</pre>
- <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 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
+ 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>
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>
+ <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>
Copy all request headers that begin with "TS" to the
response headers:
- <div class="example"><p><code>
- Header echo ^TS
- </code></p></div>
+ <pre class="prettyprint lang-config">Header echo ^TS</pre>
+
</li>
<li>
the client to intuit load on the server or in isolating
bottlenecks between the client and the server.
- <div class="example"><p><code>
- Header add MyHeader "%D %t"
- </code></p></div>
+ <pre class="prettyprint lang-config">Header set MyHeader "%D %t"</pre>
+
<p>results in this header being added to the response:</p>
<li>
Say hello to Joe
- <div class="example"><p><code>
- Header add MyHeader "Hello Joe. It took %D microseconds \<br />
- for Apache to serve this request."
- </code></p></div>
+ <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>
<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
+ 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>
- SetEnvIf MyRequestHeader value HAVE_MyRequestHeader<br />
- Header add MyHeader "%D %t mytext" env=HAVE_MyRequestHeader<br />
- </code></p></div>
+ Cache-Control: no-cache, no-store
+ </code></p></div>
- <p>If the header <code>MyRequestHeader: value</code> is present on
- the HTTP request, the response will contain the following header:</p>
+ <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>
- <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>
+ 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 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>] set|append|add|unset|echo|edit
-<var>header</var> [<var>value</var>] [early|env=[!]<var>variable</var>]</code></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> 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>
+ <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>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>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
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
+ 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
+ <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.</dd>
+ 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>add</code>
- and <code>unset</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
+ 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>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>
+ <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>
<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 class="odd"><td><code>%{FOOBAR}e</code></td>
+ 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>FOOBAR</code>.</td></tr>
-<tr><td><code>%{FOOBAR}s</code></td>
+ 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>FOOBAR</code>, if <code class="module"><a href="../mod/mod_ssl.html">mod_ssl</a></code> is enabled.</td></tr>
+ 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>
+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>
+
+ <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.</p>
-
- <p>The <code class="directive">Header</code> 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 <code class="directive">Header</code> directive
- will take effect. Otherwise, the directive will have no effect
- on the request.</p>
+ 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 <If>
+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. These means that it is
- possible to set and/or override most headers, except for those headers
- added by the header filter.</p>
+ 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 set|append|add|unset|edit <var>header</var>
-[<var>value</var>] [<var>replacement</var>] [early|env=[!]<var>variable</var>]</code></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
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>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
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>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>
-
- <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>
</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 a
+ 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 unset, no <var>value</var> should be given.
+ 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>,
replacement string respectively.</p>
<p>The <code class="directive">RequestHeader</code> 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 <code class="directive">RequestHeader</code> directive will take effect.
- Otherwise, the directive will have no effect on the request.</p>
+ 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
</div>
<div class="bottomlang">
<p><span>Available Languages: </span><a href="../en/mod/mod_headers.html" title="English"> en </a> |
-<a href="../ja/mod/mod_headers.html" hreflang="ja" rel="alternate" title=""> ja </a> |
-<a href="../ko/mod/mod_headers.html" hreflang="ko" rel="alternate" title=""> ko </a></p>
-</div><div id="footer">
-<p class="apache">Copyright 2006 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
-<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div>
+<a href="../fr/mod/mod_headers.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
+<a href="../ja/mod/mod_headers.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
+<a href="../ko/mod/mod_headers.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
+</div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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+<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/quickreference.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div><script type="text/javascript"><!--//--><![CDATA[//><!--
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