To find out which request methods a server supports, one can use the curl command-line program to issue an OPTIONS request:

curl -X OPTIONS https://example.org -i

The response then contains an Allow header that holds the allowed methods:

HTTP/1.1 204 No Content
Allow: OPTIONS, GET, HEAD, POST
Cache-Control: max-age=604800
Date: Thu, 13 Oct 2016 11:45:00 GMT
Server: EOS (lax004/2813)

In CORS, a preflight request is sent with the OPTIONS method so that the server can respond if it is acceptable to send the request. In this example, we will request permission for these parameters:

• The Access-Control-Request-Method header sent in the preflight request tells the server that when the actual request is sent, it will have a POST request method.
• The Access-Control-Request-Headers header tells the server that when the actual request is sent, it will have the X-PINGOTHER and Content-Type headers.

OPTIONS /resources/post-here/ HTTP/1.1
Host: bar.example
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Connection: keep-alive
Origin: https://foo.example
Access-Control-Request-Method: POST
Access-Control-Request-Headers: X-PINGOTHER, Content-Type

The server now can respond if it will accept a request under these circumstances. In this example, the server response says that:

Access-Control-Allow-Origin

The https://foo.example origin is permitted to request the bar.example/resources/post-here/ URL via the following:

Access-Control-Allow-Methods

POST, GET, and OPTIONS are permitted methods for the URL. (This header is similar to the Allow response header, but used only for CORS.)

Access-Control-Allow-Headers

Any script inspecting the response is permitted to read the values of the X-PINGOTHER and Content-Type headers.

Access-Control-Max-Age

The above permissions may be cached for 86,400 seconds (1 day).

HTTP/1.1 204 No Content
Date: Mon, 01 Dec 2008 01:15:39 GMT
Server: Apache/2.0.61 (Unix)
Access-Control-Allow-Origin: https://foo.example
Access-Control-Allow-Methods: POST, GET, OPTIONS
Access-Control-Allow-Headers: X-PINGOTHER, Content-Type
Access-Control-Max-Age: 86400
Vary: Accept-Encoding, Origin
Keep-Alive: timeout=2, max=100
Connection: Keep-Alive

• Allow header
• CORS