Problem with remove the realm from User-Name Attribute

Hi, I am trying to remove the realm from User-Name when FreeRADIUS receive the access request. If it have realm in User-Name, the FR will proxy it to other RADIUS server. and i wish the FR will remove the realm from User-Name when it receive any access request from NAS-IP-Address=192.168.1.1 , if using unlang, how to do it thanks, Allen