OPTIONS

OPTIONS *|<request-target>["?"<query>] HTTP/1.1

The request target may be either in 'asterisk form' * indicating the whole server, or a request target as is common with other methods:

*

Indicates that the client wishes to request OPTIONS for the server as a whole, as opposed to a specific named resource of that server.

<request-target>

Identifies the target resource of the request when combined with the information provided in the Host header. This is an absolute path (e.g., /path/to/file.html) in requests to an origin server, and an absolute URL in requests to proxies (e.g., http://www.example.com/path/to/file.html).

<query> Optional

An optional query component preceded by a question-mark ?. Often used to carry identifying information in the form of key=value pairs.

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

This creates the following HTTP request:

OPTIONS / HTTP/2
Host: example.org
User-Agent: curl/8.7.1
Accept: */*

The response 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: content-type,x-pingother

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

X-PINGOTHER and Content-Type are permitted request headers for the URL.

Access-Control-Max-Age

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

HTTP/1.1 200 OK
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

• HTTP request methods
• HTTP response status codes
• HTTP headers
• Allow header
• CORS