Port 123 (UDP) — NTP

Network Time Protocol.

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Misconfigured NTP servers can be used for amplification attacks (disable monlist).

Check if port 123 is open

On the server, see what is listening:

sudo ss -tulpn | grep ':123 '

From another machine, test reachability:

nc -zv your-server.example.com 123

With UFW (Ubuntu/Debian):

sudo ufw deny 123

With firewalld (RHEL/AlmaLinux/Rocky):

sudo firewall-cmd --permanent --remove-port=123/udp && sudo firewall-cmd --reload