#CURLOPT_USERNAME sets the user name to be used in protocol
authentication. You should not use this option together with the (older)
When using Kerberos V5 authentication with a Windows based server, you should include the domain name in order for the server to successfully obtain a Kerberos Ticket. If you don't then the initial part of the authentication handshake may fail.
When using NTLM, the user name can be specified simply as the user name without the domain name should the server be part of a single domain and forest.
To include the domain name use either Down-Level Logon Name or UPN (User Principal Name) formats. For example, EXAMPLE\user and email@example.com respectively.
Some HTTP servers (on Windows) support inclusion of the domain for Basic authentication as well.
To specify the password and login options, along with the user name, use the