Opening the Windows Firewall to Enable NTP Services

If you also want to use NTP for Windows to provide NTP services to the network, the Windows firewall needs to be opened so that NTP queries from clients are not blocked.

NTP network packets are UDP packets, and the NTP service (ntpd.exe) is listening on the well-known NTP port 123, so UDP port 123 has to be opened for incoming NTP requests.

Normally, the firewall settings should be adjusted appropriately by the NTP for Windows installer, but in case this doesn't work, and NTP requests are blocked, here are a few steps to configure the firewall manually.

In the search field of the Windows start center, enter Firewall.

The search results should show an entry labelled Windows Defender Firewall with Extended Settings, which should be used preferably.

Alternatively, there should be an entry Windows Defender Firewall without the Extended settings. This can also be used, but in the next dialog you then have to select Extended Settings.

Generally, the selted option should be run with administrator privileges. On some Windows installations this happens automatically, on others you explicitly have to click on Run as Administrator.

In the dialog that appears after the steps above

• Select Incoming Rules.

• Click on New rule ….

• Select rule type Port.

• Click Next.

• Select “UDP”.\ Under Specific local ports, enter 123.

• Click Next.

• Select Accept connections, IPSec protected as well as not IPSec protected (checked by default).

• Click Next.

• Select all 3 zones to which the rule is to be applied: Domain, Private, Public (checked by by default).

• Click Next.

• Enter a name for the rule, e.g. NTP service, and optionally a description.

• Click Finish.

That's all.

The NTP service should now receive NTP requests from clients on the network and send corresponding replies back to the client, unless restrictions are set in the ntp.conf configuration file that prevent this.

— Martin Burnicki martin.burnicki@meinberg.de, last updated 2025-09-15