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-<title>mod_headers - Apache HTTP Server</title>
+<title>mod_headers - Apache HTTP Server Version 2.5</title>
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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>
<p class="apache">Apache HTTP Server Version 2.5</p>
-<img alt="" src="../images/feather.gif" /></div>
+<img alt="" src="../images/feather.png" /></div>
<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.5</a> > <a href="./">Modules</a></div>
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><ul class="seealso"><li><a href="#comments_section">Comments</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>
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>
+ <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
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:
- <pre class="prettyprint lang-config">
- Header echo ^TS
- </pre>
+ <pre class="prettyprint lang-config">Header echo ^TS</pre>
</li>
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>
+ <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
- <pre class="prettyprint lang-config">
-Header set MyHeader "Hello Joe. It took %D microseconds for Apache to serve this request."
- </pre>
+ <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>
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>
+ <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
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>
+ <pre class="prettyprint lang-config">RequestHeader edit Destination ^https: http: early</pre>
</li>
<li>
- Set the same header value under multiple non-exclusive conditions,
+ 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
+ <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>
+Header merge Cache-Control no-store env=NO_STORE</pre>
<p>then the response would contain the following header:</p>
</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>
+ <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|unset|note
-<var>header</var> [<var>[expr=]value]</var>] [<var>replacement</var>]
-[early|env=[!]<var>variable</var>]|expr=<var>expression</var>]
+<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
modified.</p>
<p> The optional <var>condition</var> argument determines which internal
- table of responses headers this directive will operate against. Other
- components of the server may have stored their response headers in either
- the table that corresponds to <code>onsuccess</code> or the table that
- corresponds to <code>always</code>. "Always" in this context refers to
- whether headers you add will be sent during both a successful and unsucessful
- response, but if your action is a function of an existing header, you
- will have to read on for further complications.</p>
-
- <p> The default value of <code>onsuccess</code> may need to be changed to
- <code>always</code> under the circumstances similar to those listed below.
+ 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 non-success (non-2xx) response, such
+ <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,
<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>
<dt><code>setifempty</code></dt>
<dd>The request header is set, but only if there is no previous header
- with this name</dd>
+ 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.
<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.</dd>
+ but should also be logged.<br />
+ Available in 2.4.7 and later.</dd>
</dl>
<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
+ <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>
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>.
+ 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>.
+ 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>.
+ 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
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. The replacement string
- may also contain format specifiers.</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>
<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>
+ 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 HTTP header filter, such as Content-Type.</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 add|append|edit|edit*|merge|set|unset
-<var>header</var> [<var>value</var>] [<var>replacement</var>]
-[early|env=[!]<var>variable</var>]|expr=<var>expression</var>]
+<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
<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</dd>
+ 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
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-<p class="apache">Copyright 201
4 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="apache">Copyright 201
7 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>
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