Preflight request

A CORS preflight request is a CORS request that checks to see if the CORS protocol is understood.

It is an OPTIONS request using two HTTP request headers: Access-Control-Request-Method and Access-Control-Request-Headers, and the Origin header.

A preflight request is automatically issued by a browser when needed; in normal cases, front-end developers don't need to craft such requests themselves.

For example, a client might be asking a server if it would allow a DELETE request, before actually sending a DELETE request by using a preflight request:

OPTIONS /resource/foo 
Access-Control-Request-Method: DELETE 
Access-Control-Request-Headers: origin, x-requested-with

If the server allows it, then it will respond to the preflight request with a Access-Control-Allow-Methods response header that lists DELETE:

HTTP/1.1 200 OK
Content-Length: 0
Connection: keep-alive
Access-Control-Allow-Methods: POST, GET, OPTIONS, DELETE
Access-Control-Max-Age: 86400

See also

Document Tags and Contributors

 Contributors to this page: teoli, fscholz
 Last updated by: teoli,