Named : Trusted ACL & Recursion

vi /etc/named.conf

acl "trusted" {
        192.168.128.0/24;
        localhost;
        localnets;
};

options {  
.......
        allow-query { any; };
        allow-recursion { trusted; };
        allow-query-cache { trusted; };
....
}

To allow only internal user to query recursion , such as yahoo,other domain. It is only allowed access zone's domain for public.