What port does NTP use?
NTP uses port 123 over UDP.
| Port | Protocol | Name | What it is for |
|---|---|---|---|
| 123 | UDP | NTP | Network Time Protocol — clock synchronisation. |
Exposing it
Another amplification vector. Disable the monlist command and do not run an open NTP server.
Checking it is listening
sudo lsof -i :123 # macOS, Linux
netstat -ano | findstr :123 # Windows
nc -zv host 123 # from elsewhereAn "address already in use" error on a port nothing appears to be holding usually means the socket is in TIME_WAIT rather than that a process has it — the ports reference covers that and the ephemeral range.
Frequently asked questions
What port does NTP use?
NTP uses port 123 over UDP. Network Time Protocol — clock synchronisation.
Is port 123 safe to expose to the internet?
Another amplification vector. Disable the monlist command and do not run an open NTP server.
How do I check whether NTP is listening?
sudo lsof -i :123 on macOS or Linux, netstat -ano | findstr :123 on Windows. From another machine, nc -zv host 123 says whether anything answers.