Apache Module mod_headers

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:

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 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 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