# 401 Unauthorized

       The 401 (Unauthorized) status code indicates that the request has not
       been applied because it lacks valid authentication credentials for
       the target resource.  The server generating a 401 response MUST send
       a WWW-Authenticate header field (Section 4.1) containing at least one
       challenge applicable to the target resource.

       If the request included authentication credentials, then the 401
       response indicates that authorization has been refused for those
       credentials.  The user agent MAY repeat the request with a new or
       replaced Authorization header field (Section 4.2).  If the 401
       response contains the same challenge as the prior response, and the
       user agent has already attempted authentication at least once, then
       the user agent SHOULD present the enclosed representation to the
       user, since it usually contains relevant diagnostic information.