How to respond to an HTTP OPTIONS request?

Responding to HTTP OPTIONS Requests

HTTP OPTIONS Request Overview:

An HTTP OPTIONS request is an HTTP method used to obtain the HTTP request methods supported by the server or to query communication options with the web server. It can be used to determine the set of methods supported for a specific URL or the server.

Steps to Respond to OPTIONS Requests:

1. Identify the Requested Resource:

   • The server should first identify the resource requested by the client.
   • If the request targets a specific resource, the server should parse the URI of that resource.
   • If the request is directed at the server itself, the server should consider the common HTTP methods applicable to all resources.

2. Determine Supported Methods:

   • The server should check which HTTP methods it supports, including GET, POST, PUT, DELETE, PATCH, and HEAD.
   • This may depend on the resource type, server configuration, or user permissions.

3. Set Appropriate HTTP Headers:

   • Allow: This header is mandatory and contains a comma-separated list of HTTP methods supported by the server.
   • Access-Control-Allow-Methods: In Cross-Origin Resource Sharing (CORS), this header indicates the methods permitted in cross-origin requests.
   • Access-Control-Allow-Headers: If the client anticipates sending additional headers in the actual request, these headers should be specified here.
   • Access-Control-Max-Age: Specifies the duration for which the result of the OPTIONS request can be cached.
   • Any other headers specific to the server or application, which may pertain to caching policies, security, or other aspects.

4. Return Appropriate Response Codes:

   • Typically, a successful processing of an OPTIONS request should return the 200 OK status code.
   • If the requested resource is not found, it should return 404 Not Found.
   • In case of an internal server error, it should return 500 Internal Server Error.

5. Send the Response:

   • Send the response headers and status code back to the client.
   • OPTIONS requests generally do not require a response body, but it may include one to provide additional descriptive information or server documentation.

Example:

Suppose a client initiates an OPTIONS request for the URL http://example.com/api/data. The following is a simplified example of the response that the server might return.

http
HTTP/1.1 200 OK
Allow: OPTIONS, GET, POST, HEAD
Content-Length: 0
Access-Control-Allow-Methods: OPTIONS, GET, POST
Access-Control-Allow-Headers: X-Custom-Header, Content-Type
Access-Control-Max-Age: 86400

In this example, the server indicates that the client can perform OPTIONS, GET, and POST methods on http://example.com/api/data. Additionally, when handling CORS requests, the server specifies the additional headers allowed in actual requests and the caching duration for the OPTIONS request result.