This is a request header which is put into use at the time of issuing a preflight request in order to let server know or inform about what are all the HTTP headers that will be used when actual request will be made.
This header under consideration is a set of comma-delimited list of all the non-simple headers which are incorporated in the request.
The preflight request is said to be a means of asking authorization for any actual request, even before having made any actual request. The server must look into the headers provided so as to validate whether both, that is, HTTP method along with the headers requested are valid as well as accepted.
Enablement in Apache:
The syntax provided below is added in the configuration file.
Header set Access-Control-Request-Headers “X-Custom-Header”
Enablement in Nginx:
The below mentioned syntax is added in the “nginx.conf” file.
add_header ‘Access-Control-Request-Headers’ ‘ X-Custom-Header ‘;