good evening, I would like to know how to open the UDP port 123, I need to synchronize the computer clock via the NTP protocol.
Open the port from where, and to where? If your computer is trying to sync to, say, pool.ntp.org, nothing on Neth is going to stop it.
I understood, but using the connection of my phone and creating a hotspot towards the computer, the clock synchronized successfully. instead using the internet connection managed by the firewall, it does not work!
You did not give us enough details, does nethserver is the router of your network for example.
You could do a quick test by stopping the firewall of nethserver
Systemctl stop shorewall
Do a test and restart after the firewall
Systemctl start shorewall
AFAIK Nethserver is already an NTP server.
Just enter the IP of the green (or the host name) of the Nethserver in the “NTP Server” configuration of the device to be synchronized.