Apache HTTP Server Version 2.5
Apache HTTP Server Version 2.5
| Description: | Customization of HTTP request and response headers |
|---|---|
| Status: | Extension |
| Module Identifier: | headers_module |
| Source File: | mod_headers.c |
This module provides directives to control and modify HTTP request and response headers. Headers can be merged, replaced or removed.
The directives provided by mod_headers can
occur almost anywhere within the server configuration, and can be
limited in scope by enclosing them in configuration sections.
Order of processing is important and is affected both by the order in the configuration file and by placement in configuration sections. These two directives have a different effect if reversed:
RequestHeader append MirrorID "mirror 12" RequestHeader unset MirrorID
This way round, the MirrorID header is not set. If
reversed, the MirrorID header is set to "mirror 12".
mod_headers 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.
Early mode is designed as a test/debugging aid for developers.
Directives defined using the early 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.
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
<Directory> or
<Location>.
Header echo ^TS
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.
Header set MyHeader "%D %t"
results in this header being added to the response:
MyHeader: D=3775428 t=991424704447256
Header set MyHeader "Hello Joe. It took %D microseconds for Apache to serve this request."
results in this header being added to the response:
MyHeader: Hello Joe. It took D=3775428 microseconds for Apache
to serve this request.
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.
SetEnvIf MyRequestHeader myvalue HAVE_MyRequestHeader Header set MyHeader "%D %t mytext" env=HAVE_MyRequestHeader
If the header MyRequestHeader: myvalue is present on
the HTTP request, the response will contain the following header:
MyHeader: D=3775428 t=991424704447256 mytext
RequestHeader edit Destination ^https: http: early
CGI, NO_CACHE and
NO_STORE environment variables all existed for the
request):
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
then the response would contain the following header:
Cache-Control: no-cache, no-store
If append was used instead of merge,
then the response would contain the following header:
Cache-Control: no-cache, no-cache, no-store
Header set Set-Cookie testcookie "expr=-z %{req:Cookie}"
Header append Cache-Control s-maxage=600 "expr=%{REQUEST_STATUS} == 200"
| Description: | Configure HTTP response headers |
|---|---|
| Syntax: | Header [condition] add|append|echo|edit|edit*|merge|set|setifempty|unset|note
header [[expr=]value [replacement]
[early|env=[!]varname|expr=expression]]
|
| Context: | server config, virtual host, directory, .htaccess |
| Override: | FileInfo |
| Status: | Extension |
| Module: | mod_headers |
| Compatibility: | SetIfEmpty available in 2.4.7 and later, expr=value available in 2.4.10 and later |
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 optional condition argument determines which internal
table of responses headers this directive will operate against:
onsuccess (default, can be omitted) or always.
The difference between the two lists is that the headers contained in the
latter are added to the response even on error, and persisted across
internal redirects (for example, ErrorDocument handlers).
Note also that repeating this directive with both conditions makes sense in
some scenarios because always is not a superset of
onsuccess with respect to existing headers:
always is used in the ultimate response.mod_proxy_fcgi,
in which case the CGI scripts' headers are in the table corresponding to
always and not in the default table.onsuccess condition.This difference between onsuccess and always is
a feature that resulted as a consequence of how httpd internally stores
headers for a HTTP response, since it does not offer any "normalized" single
list of headers. The main problem that can arise if the following concept
is not kept in mind while writing the configuration is that some HTTP responses
might end up with the same header duplicated (confusing users or sometimes even
HTTP clients). For example, suppose that you have a simple PHP proxy setup with
mod_proxy_fcgi and your backend PHP scripts adds the
X-Foo: bar header to each HTTP response. As described above,
mod_proxy_fcgi uses the always table to store
headers, so a configuration like the following ends up in the wrong result, namely
having the header duplicated with both values:
# X-Foo's value is set in the 'onsuccess' headers table Header set X-Foo: baz
To circumvent this limitation, there are some known configuration patterns that can help, like the following:
# 'onsuccess' can be omitted since it is the default Header onsuccess unset X-Foo Header always set X-Foo "baz"
Separately from the condition 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.
The action it performs is determined by the first argument (second argument if a condition is specified). This can be one of the following values:
Please read the difference between always
and onsuccess headers list described above
before start reading the actions list, since that important
concept still applies. Each action, in fact, works as described
but only on the target headers list.
addset,
append or merge should be used instead.appendechoeditedit*edit form will match and replace exactly once
in a header value, whereas the edit* form will replace
every instance of the search pattern if it appears more
than once.mergesetsetifemptysetifempty is evaluated.
It is safer to use set for this use case like in the
following example:
Header set Content-Type "text/plain" "expr=-z %{CONTENT_TYPE}"
unsetnoteThis argument is followed by a header name, which
can include the final colon, but it is not required. Case is
ignored for set, append, merge,
add, unset and edit.
The header name for echo
is case sensitive and may be a regular
expression.
For set, append, merge and
add a value is specified as the next argument.
If value
contains spaces, it should be surrounded by double quotes.
value may be a character string, a string containing
mod_headers specific format specifiers (and character
literals), or an ap_expr expression prefixed
with expr=
The following format specifiers are supported in value:
| Format | Description |
|---|---|
%% |
The percent sign |
%t |
The time the request was received in Universal Coordinated Time
since the epoch (Jan. 1, 1970) measured in microseconds. The value
is preceded 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 preceded by D=.
The value is measured in microseconds. |
%l |
The current load averages of the actual server itself. It is
designed to expose the values obtained by getloadavg()
and this represents the current load average, the 5 minute average, and
the 15 minute average. The value is preceded by l= with each
average separated by /.Available in 2.4.4 and later. |
%i |
The current idle percentage of httpd (0 to 100) based on available
processes and threads. The value is preceded by i=.Available in 2.4.4 and later. |
%b |
The current busy percentage of httpd (0 to 100) based on available
processes and threads. The value is preceded by b=.Available in 2.4.4 and later. |
%{VARNAME}e |
The contents of the environment
variable VARNAME. |
%{VARNAME}s |
The contents of the SSL environment
variable VARNAME, if mod_ssl is enabled. |
The %s format specifier is only available in
Apache 2.1 and later; it can be used instead of %e
to avoid the overhead of enabling SSLOptions
+StdEnvVars. If SSLOptions +StdEnvVars must
be enabled anyway for some other reason, %e will be
more efficient than %s.
When the value parameter uses the ap_expr parser, some expression syntax will differ from examples that evaluate boolean expressions such as <If>:
Header set foo-checksum "expr=%{md5:foo}"
For edit there is both a value argument
which is a regular expression,
and an additional replacement string. As of version 2.4.7
the replacement string may also contain format specifiers.
The Header directive may be followed by
an additional argument, which may be any of:
earlyenv=[!]varnamevarname exists.
A ! in front of varname reverses the test,
so the directive applies only if varname is unset.expr=expression# This delays the evaluation of the condition clause compared to <If>
Header always set CustomHeader my-value "expr=%{REQUEST_URI} =~ m#^/special_path.php$#"
Except in early mode, the
Header directives are processed just
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.
| Description: | Configure HTTP request headers |
|---|---|
| Syntax: | RequestHeader add|append|edit|edit*|merge|set|setifempty|unset
header [[expr=]value [replacement]
[early|env=[!]varname|expr=expression]]
|
| Context: | server config, virtual host, directory, .htaccess |
| Override: | FileInfo |
| Status: | Extension |
| Module: | mod_headers |
| Compatibility: | SetIfEmpty available in 2.4.7 and later, expr=value available in 2.4.10 and later |
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:
addset,
append or merge should be used instead.appendeditedit*edit form will match and replace exactly once
in a header value, whereas the edit* form will replace
every instance of the search pattern if it appears more
than once.mergesetsetifemptyunsetThis argument is followed by a header name, which can
include the final colon, but it is not required. Case is
ignored. For set, append, merge and
add a value is given as the third argument. If a
value contains spaces, it should be surrounded by double
quotes. For unset, no value should be given.
value 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 Header,
please have a look there for details. For edit both
a value and a replacement are required, and are
a regular expression and a
replacement string respectively.
The RequestHeader directive may be followed by
an additional argument, which may be any of:
earlyenv=[!]varnamevarname exists.
A ! in front of varname reverses the test,
so the directive applies only if varname is unset.expr=expressionExcept in early mode, the
RequestHeader 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.