add 2 function:
1. for forwarder names,we return a fixed TTL instead of count by seconds. to minimize the concurrence load cased by DNS requests when the name's TTL timeout
2. for names mapped to multi-ips, our bind can return only one IP in ANSWER SECTION, for nscd can only cache mapped-to-one-IP names.

we can config the 2 function in the follow file  and recompile:
lib/dns/include/dns/types.h

DNS_ANSWER_ONLY_ONE_IP  1              // if return only 1 ip for multi-ip-mapped name . default 1
DNS_DEFAULT_TTL_FOR_NO_AUTH_NAME  500  // fixed