Since OpenBSD 3.6, the base system ships with its own implementation of
OpenBSD 5.7 introduced HTTPS Constraints, which helps to verify the times received
in unauthenticated UDP packets.
servers pool.ntp.org constraints from "https://www.google.com/"
OpenNTPD has become a portable software project and is also available in Linux distributions.
- Authenticated TLS “contraints” in ntpd(8)
- ntpd(8) - OpenBSD manual pages
- ntpd.conf Constraints
- OpenBSD 3.6
- OpenBSD 5.7