NTP users are strongly urged to take immediate action to ensure that their NTP daemon is not susceptible to use in a reflected denial-of-service (DRDoS) attack. Please see the NTP Security Notice for vulnerability and mitigation details, and the Network Time Foundation Blog for more information. (January 2014) See FrequentlyAskedQuestionsDev for discussion of this topic.
NTP Security: MAC (RFC 1305-Appendix C)
How to find out which servers provide such authentication?
-- RalfHauser - 16 Sep 2009