Apache Module mod_headers

Description:	Customization of HTTP request and response headers
Status:	Extension
Module Identifier:	headers_module
Source File:	mod_headers.c

Summary

This module provides directives to control and modify HTTP request and response headers. Headers can be merged, replaced or removed.

Topics

Directives

Bugfix checklist

Order of Processing ¶

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>.

Examples ¶

Copy all request headers that begin with "TS" to the response headers:
```
Header echo ^TS
```
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.
```
Header set MyHeader "%D %t"
```
results in this header being added to the response:

MyHeader: D=3775428 t=991424704447256
Say hello to Joe
```
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.
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.
```
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
Enable DAV to work with Apache running HTTP through SSL hardware (problem description) by replacing https: with http: in the Destination header:
```
RequestHeader edit Destination ^https: http: early
```
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 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
Set a test cookie if and only if the client didn't send us a cookie
```
Header set Set-Cookie testcookie "expr=-z %{req:Cookie}"
```

Append a Caching header for responses with a HTTP status code of 200

Header append Cache-Control s-maxage=600 "expr=%{REQUEST_STATUS} == 200"

Header Directive ¶

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. Guidance on when to specify always is provided relative to each action below.

Warning

Carefully read the difference between always and onsuccess for each action listed below as the behavior can be unintuitive and is a frequent source of confusion. Where the guidance suggest repeating the conditions, it is safe to try each experimentally and use the one you find effective to match the pre-existing header.

add

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 set, append or merge should be used instead.

Choosing a condition: Specify a condition of always if you want the header to be included in non-2xx response (such as redirects or errors)

append

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.

Choosing a condition: If the existing header to be appended to was added by this module, you must match the condition parameter that was originally used. Otherwise, you must determine by trial and error whether always should be specified because you can't reliably know which internal table the existing value is present in.

echo

Request headers with this name are echoed back in the response headers. header may be a regular expression. value must be omitted.

Choosing a condition: Specify a condition of always if you want the header to be included in non-2xx response (such as redirects or errors).

edit

edit*

If this response header exists, its value is transformed according to a regular expression search-and-replace. The value argument is a regular expression, and the replacement is a replacement string, which may contain backreferences or format specifiers. The 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.

Choosing a condition: Depending on the origins of the header to be edited, you may have to repeat your edit/edit* directive with both always and onsuccess. Alternatively, determine by experimentation whether a condition of always is necessary.

merge

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.

Choosing a condition: If the target header to be merged was added by this module, you must match the condition parameter that was originally used. Otherwise, you must determine by experimentation whether a condition of always is necessary.

set

The response header is set, replacing any previous header with this name. The value may be a format string.

Choosing a condition: If the target header to be replaced was added by this module, you must match the condition parameter that was originally used. Otherwise, you must determine by experimentation whether a condition always is necessary.

setifempty

The request header is set, but only if there is no previous header with this name.

Choosing a condition: If the target header to conditionally set was added by this module, you must match the condition parameter that was originally used. Otherwise, you must determine by experimentation whether a condition of always is necessary.

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 setifempty 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}"

unset

The response header of this name is removed, if it exists. If there are multiple headers of the same name, all will be removed. value must be omitted.

Choosing a condition: Repeat this directive with both always and onsuccess to be certain the header is unset, or determine by experimentation whether a condition of always is necessary..

note

The value of the named response header is copied into an internal note whose name is given by value. This is useful if a header sent by a CGI or proxied resource is configured to be unset but should also be logged.
Available in 2.4.7 and later.

Choosing a condition: If the target header was added by this module, you must match the condition parameter that was originally used. Otherwise, you must determine by experimentation whether a condition of always is necessary..

This 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.

Note

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.

Note on expression values

When the value parameter uses the ap_expr parser, some expression syntax will differ from examples that evaluate boolean expressions such as <If>:

The starting point of the grammar is 'string' rather than 'expr'.
Function calls use the %{funcname:arg} syntax rather than funcname(arg).
Multi-argument functions are not currently accessible from this starting point

Quote the entire parameter, such as

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:

early

Specifies early processing.

env=[!]varname

The directive is applied if and only if the environment variable varname exists. A ! in front of varname reverses the test, so the directive applies only if varname is unset.

expr=expression

The directive is applied if and only if expression evaluates to true. Details of expression syntax and evaluation are documented in the ap_expr documentation.

# 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.

RequestHeader 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:

add: 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 set, append or merge should be used instead.
append: 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.
edit
edit*: If this request header exists, its value is transformed according to a regular expression search-and-replace. The value argument is a regular expression, and the replacement is a replacement string, which may contain backreferences or format specifiers. The 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.
merge: The request 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.
set: The request header is set, replacing any previous header with this name
setifempty: The request header is set, but only if there is no previous header with this name.
Available in 2.4.7 and later.
unset: The request header of this name is removed, if it exists. If there are multiple headers of the same name, all will be removed. value must be omitted.

This 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:

early: Specifies early processing.
env=[!]varname: The directive is applied if and only if the environment variable varname exists. A ! in front of varname reverses the test, so the directive applies only if varname is unset.
expr=expression: The directive is applied if and only if expression evaluates to true. Details of expression syntax and evaluation are documented in the ap_expr documentation.

Except 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.