There are two levels, or tiers, of Network Time Protocol (NTP) time servers that are available on the Internet. The NTP is defined in Request for Comments (RFC) 1305. The first-level time servers are primarily intended to act as source time servers for second-level time servers.

networking - How to check if port 123 is ready for time TCP port 123 (unknown service): NOT LISTENING. UDP port 123 (ntp service): LISTENING or FILTERED. UDP port 123 (ntp service): LISTENING or FILTERED. I am not sure if the port is available for time sync or not. But our team has confirmed that no firewall should be blocking that. It is not showing "LISTENING" because it is a UDP port? Still blocking ntp ports for outbound 123? | AT&T Jan 17, 2017 Opening UDP port 123 - Cisco Community Re: Opening UDP port 123 NTP should be able to go out across your firewall just like HTTP requests do. Can you crank up NTP logging to make certain that those servers *ever* respond, as some publically listed NTP servers are dormant. How the Windows Time Service Works | Microsoft Docs

Network Time Protocol (NTP) (Building Internet Firewalls

Implementations send and receive timestamps using the User Datagram Protocol (UDP) on port number 123. They can also use broadcasting or multicasting, where clients passively listen to time updates after an initial round-trip calibrating exchange.

22.14. Configure the Firewall to Allow Incoming NTP

Solved: Different between port 123 (ntp) and 37 (timeserve Port 37 has been obsolete for a very long time and is one of the many archaic ports that should be disabled. NTP does not use 37 at all, only port 123. I would disable the following services in inetd.conf immediately, even if the security team has not reported them: daytime time echo discard chargen ntalk uucp help with connecting to NIST NTP server on port 123