MAC Auth tied to user

anyone here have experience to implement authorization and accounting from mac to user ? For example: User john is tied to mac aa-bb-cc-dd-ee-ff. every authorization comes from this mac, radius will check session for user john. and do accounting for user john also. So, every packet auth/acct come from NAS, username 'mac' will be replaced to 'user' subsquent process. i think unlang is the solution. anyone here could give us suggest/idea ? Cheers. Johan