Port 123: NTP
Port 123 is NTP, which keeps clocks in sync across a network. Accurate time matters for logs, certificates, and Kerberos, so NTP is a quiet but critical service.
| Port number | 123 |
|---|---|
| Service | Network Time Protocol |
| Protocol | UDP |
| Category | Core services |
| Default encryption | No |
How do I check whether port 123 is open?
Port 123 is UDP, which changes the answer: there is no handshake to complete, so nothing outside the host can prove the port is open. Asking the host itself is the reliable route.
| Question | Command | What to know |
|---|---|---|
| Is it open on a remote host? | sudo nmap -sU -p 123 example.com | UDP has no handshake, so there is nothing to complete. nmap infers the state from whether an ICMP port-unreachable comes back, needs root, and is slow and easy to misread. A silent port may be open, filtered, or simply not replying to a probe it does not recognise. |
| What is listening locally? | ss -ulnp | grep ':123 ' | Shows the UDP socket bound to port 123. This is the reliable way to answer the question, because you are asking the host itself rather than guessing from outside. |
Replace example.com with the host you are testing. A blocked port and a port with nothing listening on it look identical from the outside, so if a service should be running, check locally before blaming the firewall.
Frequently asked questions
What is port 123 used for?
Port 123 is NTP, which keeps clocks in sync across a network. Accurate time matters for logs, certificates, and Kerberos, so NTP is a quiet but critical service.
Is port 123 TCP or UDP?
Port 123 (NTP) uses UDP.
Is port 123 secure?
Port 123 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 123 on my firewall?
Only if you specifically need NTP. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.