MAC or user auth

Hi, I try to configure such a solution: Authorization via MAC Address (with no username required) - if the machine is using a valid IP Address, it is automatically allowed to surf. (I know there is a Calling-Station-id attribute in radcheck) But I need also a support for username/password authentification (via WWW) too. When I try to log in only with MAC, I get a Radius responce "no username", and the machine is denied. How/where in radiusd.conf can I make changes? Thx in advance, Best regards, CoMeC