OWIN preflight requests are not processed

Question

All preflight requests from browsers to my Self-Host OWIN WebAPI are not processed by Middleware. If I make OPTIONS request from Postman they are processed. Why is such a behaviour?

Request URL: http://localhost:9000/api/v1/conversations/create?connectionId=13509f44-eacb-4950-8cc8-71bd37098975

Request Method:OPTIONS

Status Code:401 Unauthorized Remote

Address:[::1]:9000

Accept: /

Accept-Encoding:gzip, deflate, sdch, br

Accept-Language:ru-RU,ru;q=0.8,en-US;q=0.6,en;q=0.4

Access-Control-Request-Headers:content-type

Access-Control-Request-Method:POST

Connection:keep-alive

Host:localhost:9000

Origin: http://localhost:8080

Referer: http://localhost:8080/

User-Agent:Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36

Response Headers for Chrome:

Content-Length:0

Date:Wed, 08 Feb 2017 04:17:26 GMT

Server:Microsoft-HTTPAPI/2.0

WWW-Authenticate:NTLM

Response headers for Postman:

Access-Control-Allow-Credentials →true

Access-Control-Allow-Origin →chrome-extension://fhbjgbiflinjbdggehcddcbncdddomop

Allow →POST

Content-Length →76

Content-Type →application/json; charset=utf-8

Date →Wed, 08 Feb 2017 04:21:02 GMT

Server →Microsoft-HTTPAPI/2.0

I added fake middleware to my appbuilder:

public void BuildWebApi(IAppBuilder appBuilder)
    {
        appBuilder.Use(async (ctx, next) =>
        {
            await next();
        });

and put breakpoint to line "await next()". So breakpoint doesn't stop while browser makes preflight request and stops while postman OPTIONS response.

Answer 1

Solved this by

HttpListener listener = (HttpListener)appBuilder.Properties["System.Net.HttpListener"];
listener.AuthenticationSchemeSelectorDelegate = request =>
{
   if (request.HttpMethod == "OPTIONS")
   {
       return AuthenticationSchemes.Anonymous;
   }
};