Port 123: NTP
UDP 123 is the port for Network Time Protocol. Only on deliberate public time servers. Misconfigured NTP (the monlist command) has been used for large amplification attacks.
| Service | Network Time Protocol |
|---|---|
| Protocol | UDP |
| Exposure risk | Medium: expose with care |
| Range | Well-known (0–1023): binding needs root/admin on Unix |
Port 123 is UDP-only. UDP has no handshake, so no online checker can reliably tell open from filtered; test with the service's own client.
What port 123 does
Clocks synchronise with time servers over UDP 123. Accurate time matters for TLS certificate validation, Kerberos, logs and two-factor codes.
Should port 123 be open?
Only on deliberate public time servers. Misconfigured NTP (the monlist command) has been used for large amplification attacks.
Find what's listening on port 123
- Linux:
sudo ss -tulpn | grep :123 - macOS:
sudo lsof -nP -i :123 - Windows:
netstat -ano | findstr :123, then look up the PID in Task Manager.
Open or block it in a firewall
- ufw:
sudo ufw allow 123/udp(ordeny) - firewalld:
sudo firewall-cmd --add-port=123/udp --permanent && sudo firewall-cmd --reload - Windows:
New-NetFirewallRule -DisplayName "Port 123" -Direction Inbound -Protocol UDP -LocalPort 123 -Action Allow
Port assignment per the IANA Service Name and Transport Protocol Port Number Registry. Test several ports at once with the port scanner, or any port with the open port checker.
Questions about port 123
What is port 123 used for?
Is port 123 TCP or UDP?
Is it safe to open port 123?
How do I check if port 123 is open?
ss -ulpn | grep :123 (Linux) or netstat -ano | findstr :123 (Windows).