If Freebsd DNS server that served 100 people is crashed. How to move this 100 people to a new FreeBSD DNS server as quickly as possible?
DNS Servers should be very reliable, since they mostly just read a UDP packet, update the local data store and send a UDP packet. Excess load causes UDP packet loss.
DNS servers can be configured in parallel on different hosts, so load is spread and failed servers are skipped. One is master and the rest get their domain data from it over tcp.