Port 123 — NTP

Security guidance

Publicly reachable NTP servers are used for amplification attacks. Clients do not need an open inbound port.

Test port 123 yourself

From your own machine, against any host (swap in your domain):

macOS / Linux — netcat

nc -vz example.com 123

Any OS — nmap

nmap -p 123 example.com

Windows — PowerShell

Test-NetConnection example.com -Port 123

open = a service answered on port 123; closed = the host replied but nothing is listening; filtered = a firewall silently dropped the probe. These test from your network — our port checker tests from the outside, the view that matters for security.

Other common ports