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<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>

<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 directives 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 <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>&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 set 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 set 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 <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

<module>mod_setenvif</module> module.

<example>

SetEnvIf MyRequestHeader myvalue HAVE_MyRequestHeader<br />

Header set MyHeader "%D %t mytext" env=HAVE_MyRequestHeader

</example>

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

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

<example>

MyHeader: D=3775428 t=991424704447256 mytext

</example>

</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:

<example>

RequestHeader edit Destination ^https: http: early

</example>

</li>

<li>

Set the same header value under multiple non-exclusive 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):

<example>

Header merge Cache-Control no-cache env=CGI<br />

Header merge Cache-Control no-cache env=NO_CACHE<br />

Header merge Cache-Control no-store env=NO_STORE

</example>

<p>then the response would contain the following header:</p>

<example>

Cache-Control: no-cache, no-store

</example>

<p>If <code>append</code> was used instead of <code>merge</code>,

then the response would contain the following header:</p>

<example>

Cache-Control: no-cache, no-cache, no-store

</example>

</li>

<li>

Set a test cookie if and only if the client didn't send us a cookie

<example>

Header set Set-Cookie testcookie !$req{Cookie}

</example>

</li>

</ol>

</section>

<directivesynopsis>

<name>RequestHeader</name>

<description>Configure HTTP request headers</description>

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

[<var>value</var>] [<var>replacement</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, 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 <glossary ref="regex">regular expression</glossary>

search-and-replace. The <var>value</var> argument is a <glossary

ref="regex">regular expression</glossary>, and the <var>replacement</var>

is a replacement string, which may contain backreferences.

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

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>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 <directive module="mod_headers">Header</directive>,

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 <glossary ref="regex">regular expression</glossary> and a

replacement string respectively.</p>

<p>The <directive>RequestHeader</directive> 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=[!]varname</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</code></dt>

<dd>An string that matches neither of the above is parsed as an

expression. Details of expression syntax and evaluation are

currently best documented on the <module>mod_filter</module> page.</dd>

</dl>

<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>] add|append|echo|edit|merge|set|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 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. 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

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

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>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

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

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

search-and-replace. The <var>value</var> argument is a <glossary

ref="regex">regular expression</glossary>, and the <var>replacement</var>

is a replacement string, which may contain backreferences.</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>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>

</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 <glossary ref="regex">regular

expression</glossary>.</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 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>.

The value is measured in microseconds.</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><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 <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>For <code>edit</code> there is both a <var>value</var> argument

which is a <glossary ref="regex">regular expression</glossary>,

and an additional <var>replacement</var> string.</p>

<p>The <directive>Header</directive> 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=[!]varname</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</code></dt>

<dd>An string that matches neither of the above is parsed as an

expression. Details of expression syntax and evaluation are

currently best documented on the <module>mod_filter</module> page.</dd>

</dl>

<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 HTTP header filter, such as Content-Type.</p>

</usage>

</directivesynopsis>

</modulesynopsis>