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<H1 ALIGN="CENTER">Module mod_headers</H1>
<p>This module provides for the customization of HTTP request and
response headers.</p>
<P><A
HREF="module-dict.html#Status"
REL="Help"
><STRONG>Status:</STRONG></A> Extension
<BR>
<A
HREF="module-dict.html#SourceFile"
REL="Help"
<BR>
<A
HREF="module-dict.html#ModuleIdentifier"
REL="Help"
><STRONG>Module Identifier:</STRONG></A> headers_module
<BR>
<A
HREF="module-dict.html#Compatibility"
REL="Help"
><STRONG>Compatibility:</STRONG></A> Available in Apache 1.2 and later.
RequestHeader appeared in Apache 2.0.
</P>
<h2>Summary</h2>
This module provides directives to control and modify HTTP
request and response headers. Headers can be merged,
replaced or removed.
<H2>Directives</H2>
<UL>
<LI><A HREF="#requestheader">RequestHeader</A>
<LI><A HREF="#header">Header</A>
</UL>
<H2>Order of Processing</H2>
<p>The directives provided by mod_header can occur almost anywhere within
the server configuration. They are valid in the main server config and virtual
host sections, inside <Directory>, <Location> and <Files>
sections, and within .htaccess files.</p>
<P>The directives are processed in the following order:
<OL>
<LI>main server
<LI>virtual host
<LI><Directory> sections and .htaccess
<LI><Location>
<LI><Files>
</OL>
<p>Order is important. These two headers have a different effect if
reversed:</p>
<blockquote><code>
RequestHeader append MirrorID "mirror 12"<br>
RequestHeader unset MirrorID
</code></blockquote>
<p>This way round, the MirrorID header is not set. If reversed, the
MirrorID header is set to "mirror 12".</P>
<H2>Examples</H2>
<OL>
<LI>Copy all request headers that begin with "TS" to the response headers:</LI>
<PRE>
Header echo ^TS*
</PRE>
<LI>Add a header, MyHeader, 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.</LI>
<PRE>
Header add MyHeader "%D %t"
</PRE>
results in this header being added to the response:
<PRE>
MyHeader: D=3775428 t=991424704447256
</PRE>
<LI>Say hello to Joe</LI>
<PRE>
Header add MyHeader "Hello Joe. It took %D microseconds for Apache to serve this request."
</PRE>
results in this header being added to the response:
<PRE>
MyHeader: Hello Joe. It took D=3775428 microseconds for Apache to serve this request.
</PRE>
<LI>Conditionally send MyHeader on the response if and only if header
"MyRequestHeader" is present on the request. This is useful for
constructing headers in response to some client stimulus. Note that
this example requires the services of the mod_setenvif module.</LI>
<PRE>
SetEnvIf MyRequestHeader value HAVE_MyRequestHeader<BR>
Header add MyHeader "%D %t mytext" env=HAVE_MyRequestHeader
</PRE>
If the header "MyRequestHeader: value" is present on the HTTP request, the response
will contain the following header:
<PRE>
MyHeader: D=3775428 t=991424704447256 mytext
</PRE>
</OL>
<HR>
<H2><A NAME="requestheader">RequestHeader</A> directive</H2>
<A
HREF="directive-dict.html#Syntax"
REL="Help"
><STRONG>Syntax:</STRONG></A> RequestHeader set|append|add
<EM>header</EM> <EM>value</EM><BR>
<A
HREF="directive-dict.html#Syntax"
REL="Help"
><STRONG>Syntax:</STRONG></A> RequestHeader unset <EM>header</EM><BR>
<A
HREF="directive-dict.html#Context"
REL="Help"
.htaccess<BR>
<A
HREF="directive-dict.html#Override"
REL="Help"
><STRONG>Override:</STRONG></A> FileInfo<BR>
<A
HREF="directive-dict.html#Status"
REL="Help"
><STRONG>Status:</STRONG></A> Extension<BR>
<A
HREF="directive-dict.html#Module"
REL="Help"
><STRONG>Module:</STRONG></A> mod_header<P>
<p>This directive can replace, merge or remove HTTP request
headers. The header is modified just before the content handler is
run, allowing incoming headers to be modified. The action it performs
is determined by the first argument. This can be one of the following
values:</p>
<UL>
<LI><STRONG>set</STRONG><BR>
The request header is set, replacing any previous header with this name
<LI><STRONG>append</STRONG><BR>
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.
<LI><STRONG>add</STRONG><BR>
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 "append" should be used instead.
<LI><STRONG>unset</STRONG><BR>
The request header of this name is removed, if it exists. If there are
multiple headers of the same name, all will be removed.
</UL>
<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 value is
given as the third argument. If this value contains spaces, it should
be surrounded by double quotes. For unset, no value should be
given.</p>
<p>The <code>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>
<HR>
<H2><A NAME="header">Header</A> directive</H2>
<A
HREF="directive-dict.html#Syntax"
REL="Help"
><STRONG>Syntax:</STRONG></A> Header set|append|add
<EM>header</EM> <EM>value</EM> <EM>[env=[!]environment-variable]</EM><BR>
<A
HREF="directive-dict.html#Syntax"
REL="Help"
><STRONG>Syntax:</STRONG></A> Header unset <EM>header</EM><BR>
<A
HREF="directive-dict.html#Syntax"
REL="Help"
><STRONG>Syntax:</STRONG></A> Header echo <EM>header</EM><BR>
<A
HREF="directive-dict.html#Context"
REL="Help"
.htaccess<BR>
<A
HREF="directive-dict.html#Override"
REL="Help"
><STRONG>Override:</STRONG></A> FileInfo<BR>
<A
HREF="directive-dict.html#Status"
REL="Help"
><STRONG>Status:</STRONG></A> Extension<BR>
<A
HREF="directive-dict.html#Module"
REL="Help"
><STRONG>Module:</STRONG></A> mod_header<P>
<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. The
action it performs is determined by the first argument. This can be
one of the following values:</p>
<UL>
<LI><STRONG>set</STRONG><BR>
The response header is set, replacing any previous header with this name.
The <EM>value</EM> may be a format string.
<LI><STRONG>append</STRONG><BR>
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.
<LI><STRONG>add</STRONG><BR>
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.
<LI><STRONG>unset</STRONG><BR>
The response header of this name is removed, if it exists. If there are
multiple headers of the same name, all will be removed.
<LI><STRONG>echo</STRONG><BR>
Request headers with this name are echoed back in the response headers.
<EM>header</EM> may be a regular expression.
</UL>
<p>This argument is followed by a <EM>header</EM> name, which can include the
final colon, but it is not required. Case is ignored for set, append, add
and unset. The <EM>header</EM> name for echo is case sensitive and may be a
regular expression.</p>
<P>For <code>add</code>, <code>append</code> and <code>set</code> a
<EM>value</EM> is specified as the third argument. If <EM>value</EM>
contains spaces, it should be surrounded by doublequotes.
<EM>value</EM> may be a character string, a string containing format
specifiers or a combination of both. The following format specifiers
are supported in <EM>value</EM>:
<PRE>
%t: The time the request was received in Universal Coordinated Time
since the epoch (Jan. 1, 1970) measured in microseconds. The
value is preceeded by "t=".
%D: 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 preceeded by "D=".
</PRE>
<p>When the <code>Header</code> directive is used with the
<code>add</code>, <code>append</code>, or <code>set</code> argument, a
fourth argument may be used to specify conditions under which the
action will be taken. If the <a href="/env.html">environment
variable</a> specified in the <code>env=...</code> argument exists (or
if the environment variable does not exist and <code>env=!...</code>
is specified) then the action specified by the <code>Header</code>
directive will take effect. Otherwise, the directive will have no
effect on the request.</p>
<p>The Header directives are processed just before the response is
override most headers, except for those headers added by the header
filter.</p>
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