Port 123: NTP

What is port 123 used for?

Keeping clocks accurate for logs, certificates and Kerberos.

Is it safe to expose port 123 to the internet?

Public NTP servers can be abused for amplification. Disable monitor commands and restrict queries if you run one.

If port 123 seems closed or unreachable

1. Check that the NTP service is running and listening on port 123, not only on 127.0.0.1.
2. Check the firewall on the host itself (Windows Defender Firewall, iptables, nftables, ufw) for a rule that allows UDP 123.
3. If the host is behind a router, confirm the forwarding rule points to the correct internal address and that your internet provider does not use carrier-grade NAT.
4. Test from outside your own network. A port can be open locally and blocked at the edge, or the other way round.

Using port 123 safely

Port 123 can be exposed when the host exists to provide this service, but add controls: restrict source addresses where you can, require strong authentication and encryption, and monitor the logs for repeated failures.