What I'm not sure about ACL on router is, how to survive DNS server under DoS/DDos attack. We suffered from DoS attack last year, and we found the source IPs of that attack locate in our customers IP address blocks. ACL on router could only filter those traffic not meaningful to DNS server, but how about those DDoS attacking packets?

Your router can presumably rate limit the traffic towards the name server to a level the name server can handle. On the name server you can perform further rate limiting on an IP address basis, with for instance FreeBSD ipfw. Steinar Haug, Nethelp consulting, sthaug@nethelp.no