Allowing servers with dynamic IP addresses would be a very nice feature. I think there are actually three classes of clients:
- DNS lookup only on start
- DNS lookup on start and when a server becomes unreachable, falseticker, etc.
- DNS lookup before each NTP request
Class 3 has only SNTP clients. In class 1 and 2 are both NTP and SNTP implementations and it may also depend on their configuration (e.g. ntp-4.2.8 configured with the pool directive instead of server).
Servers with dynamic IP could serve class 3 and probably also 2.