kerberos authentication

[Riccardo.Veraldi]

Hello,
I am trying kerberos authentication.
I configured the radiusd.conf

        Auth-Type Kerberos {
                krb5
        }

....

            krb5 {
                keytab = /etc/krb5.keytab
                #service_principal =
        }


than in users

DEFAULT Auth-Type := Kerberos

But I am unable to succesfully authenticate
and I get this error:

rlm_krb5: Attribute "User-Password" is required for authentication.

How could I Solve this problem?
I would like the authentication via 802.1x to point to my kerberos server
instead of a local radius users file authentication (this indeed works 
with EAP-TTLS).
What am I doing wrong ?

should I instead use PAM module and configure PAM
to authenticate using kerberos ?

thanks very much

Rick