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<!DOCTYPE modulesynopsis SYSTEM "/style/modulesynopsis.dtd">

<?xml-stylesheet type="text/xsl" href="/style/manual.en.xsl"?>

<modulesynopsis metafile="mod_headers.xml.meta">

<name>mod_headers</name>

<description>Customization of HTTP request and response

headers</description>

<status>Extension</status>

<sourcefile>mod_headers.c</sourcefile>

<identifier>headers_module</identifier>

<compatibility><directive module="mod_headers">RequestHeader</directive>

is available only in Apache 2.0</compatibility>

<summary>

<p>This module provides directives to control and modify HTTP

request and response headers. Headers can be merged, replaced

or removed.</p>

</summary>

<section id="order"><title>Order of Processing</title>

<p>The directives provided by <module>mod_headers</module> 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 headers have a different effect if reversed:</p>

<example>

RequestHeader append MirrorID "mirror 12"<br />

RequestHeader unset MirrorID

</example>

<p>This way round, the <code>MirrorID</code> header is not set. If

reversed, the MirrorID header is set to "mirror 12".</p>

</section>

<section id="early"><title>Early and Late Processing</title>

<p><module>mod_headers</module> 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>&lt;Directory&gt;</code> or <code>&lt;Location&gt;</code>.</p>

</section>

<section id="examples"><title>Examples</title>

<ol>

<li>

Copy all request headers that begin with "TS" to the

response headers:

<example>

Header echo ^TS

</example>

</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.

<example>

Header add MyHeader "%D %t"

</example>

<p>results in this header being added to the response:</p>

<example>

MyHeader: D=3775428 t=991424704447256

</example>

</li>

<li>

Say hello to Joe

<example>

Header add MyHeader "Hello Joe. It took %D microseconds \<br />

for Apache to serve this request."

</example>

<p>results in this header being added to the response:</p>

<example>

MyHeader: Hello Joe. It took D=3775428 microseconds for Apache

to serve this request.

</example>

</li>

<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

stimulus. Note that this example requires the services of the

<module>mod_setenvif</module> module.

<example>

SetEnvIf MyRequestHeader value HAVE_MyRequestHeader<br />

Header add MyHeader "%D %t mytext" env=HAVE_MyRequestHeader<br />

</example>

<p>If the header <code>MyRequestHeader: value</code> is present on

the HTTP request, the response will contain the following header:</p>

<example>

MyHeader: D=3775428 t=991424704447256 mytext

</example>

</li>

</ol>

</section>

<directivesynopsis>

<name>RequestHeader</name>

<description>Configure HTTP request headers</description>

<syntax>RequestHeader set|append|add|unset <var>header</var>

[<var>value</var>] [early|env=[!]<var>variable</var>]</syntax>

<contextlist><context>server config</context><context>virtual host</context>

<context>directory</context><context>.htaccess</context></contextlist>

<override>FileInfo</override>

<usage>

<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>

<dl>

<dt><code>set</code></dt>

<dd>The request header is set, replacing any previous header

with this name</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>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>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>add</code>, <code>append</code> and

<code>set</code> a <var>value</var> is given as the third argument. If

<var>value</var> contains spaces, it should be surrounded by double

quotes. For unset, 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 <directive module="mod_headers">Header</directive>,

please have a look there for details.</p>

<p>The <directive>RequestHeader</directive> 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 <directive>RequestHeader</directive> directive will take effect.

Otherwise, the directive will have no effect on the request.</p>

<p>Except in <a href="#early">early</a> mode, the

<directive>RequestHeader</directive> 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>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>Header</name>

<description>Configure HTTP response headers</description>

<syntax>Header [<var>condition</var>] set|append|add|unset|echo

<var>header</var> [<var>value</var>] [early|env=[!]<var>variable</var>]</syntax>

<contextlist><context>server config</context><context>virtual host</context>

<context>directory</context><context>.htaccess</context></contextlist>

<override>FileInfo</override>

<usage>

<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>

<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>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>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

<glossary ref="regex">regular expression</glossary>.

<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>add</code>

and <code>unset</code>. The <var>header</var> name for <code>echo</code>

is case sensitive and may be a <glossary ref="regex">regular

expression</glossary>.</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>

<table border="1" style="zebra">

<columnspec><column width=".25"/><column width=".75"/></columnspec>

<tr><th>Format</th><th>Description</th></tr>

<tr><td><code>%%</code></td>

<td>The percent sign</td></tr>

<tr><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>.</td></tr>

<tr><td><code>%{FOOBAR}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>

<td>The contents of the <a href="mod_ssl.html#envvars">SSL environment

variable</a> <code>FOOBAR</code>, if <module>mod_ssl</module> is enabled.</td></tr>

</table>

<note><title>Note</title>

<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>

</note>

<p>The <directive>Header</directive> 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 <directive>Header</directive> directive

will take effect. Otherwise, the directive will have no effect

on the request.</p>

<p>Except in <a href="#early">early</a> mode, the

<directive>Header</directive> 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>

</usage>

</directivesynopsis>

</modulesynopsis>