pap authentication for freeradius

Dear All I have added an user in /usr/local/etc/users and /etc/shadow but passwod in both place are different. First, I try to authenticate by pap protocol then I can login correctly by supply shell account password but can not login by supply password which reside in users file. Second,I have created another account which exits only in users file but not /etc/shadow then I can login correctly It's seem that rlm_pap module select password which reside in /etc/shadow file before /usr/local/etc/users file. How can I change freeradius configure to select password from users file first? What is different between := and == operator? I have already readed man page but it does not make me clear. Thank You