NetSecWallFree trialDownload
Port 123 · UDP

Port 123: NTP (time sync)

Network Time Protocol, which keeps the clock right. Windows syncs with time.windows.com by default.

Protocol
UDP
On Windows
svchost.exe (Windows Time service, W32Time)
Risk if open to the internet
Low

Seeing it on your PC

svchost.exe sending UDP 123 every so often is Windows checking the time.

Should you block it?

Allow it outgoing. A wrong clock breaks HTTPS, sign-ins and licence checks. Nothing should need incoming 123.

Check it on your PC

Run this in PowerShell to see what is using port 123 right now, and which program owns it:

Get-NetUDPEndpoint -LocalPort 123 -ErrorAction SilentlyContinue | Select-Object LocalAddress,LocalPort,@{n='Process';e={(Get-Process -Id $_.OwningProcess -ErrorAction SilentlyContinue).ProcessName}}

No output means nothing is using the port at the moment. For every connection at once, paste netstat -ano into the netstat explainer.

Block it in Windows Firewall

In PowerShell run as administrator, this stops other computers from connecting to this port on your PC. It does not stop your own programs connecting out.

New-NetFirewallRule -DisplayName "Block inbound UDP 123" -Direction Inbound -Protocol UDP -LocalPort 123 -Action Block

Undo it with Remove-NetFirewallRule -DisplayName "Block inbound …", using the same name. If you depend on the service on your local network, this rule stops that too.