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".

Early and Late Processing

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

1. Copy all request headers that begin with "TS" to the response headers:

   Header echo ^TS

2. 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

3. 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.

4. 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

5. 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

6. 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

7. 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}"

8. 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

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. Despite the name, the default value of onsuccess does not limit an action to responses with a 2xx status code. Headers set under this condition are still used when, for example, a request is successfully proxied or generated by CGI, even when they have generated a failing status code.

When your action is a function of an existing header, you may need to specify a condition of always, depending on which internal table the original header was set in. The table that corresponds to always is used for locally generated error responses as well as successful responses. 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:

• You're adding a header to a locally generated non-success (non-2xx) response, such as a redirect, in which case only the table corresponding to always is used in the ultimate response.
• You're modifying or removing a header generated by a CGI script, in which case the CGI scripts are in the table corresponding to always and not in the default table.
• You're modifying or removing a header generated by some piece of the server but that header is not being found by the default onsuccess condition.

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:

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.

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.

echo

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

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.

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.

set

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

setifempty

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

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.

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.

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:

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

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.