Port123— NTP
NTP (Network Time Protocol) syncs clocks between clients and servers over UDP 123. A wrong system clock breaks TLS certificate validation, scrambles log ordering and invalidates distributed locks.
Registry assignments, software defaults and local listeners are different things. A port number alone does not identify or secure a service.
Configuration and troubleshooting
To debug time sync: check `timedatectl status` for NTP state, then `chronyc sources -v` (chrony) or `ntpstat` (ntpd) for upstream servers and offset. Firewalls must allow outbound UDP 123; if you run an internal NTP server, allow inbound UDP 123 for clients. Cloud images usually ship chrony pointed at the provider mirror — check before changing it.
Important limitations
NTP is UDP-only: a TCP connectivity test proves nothing about time sync. Public NTP services are abused for amplification attacks (monlist), so restrict queries to your network.
Read-only checks: run on the server host
Choose the commands for your OS. Check TCP and UDP separately. No result is not a lasting availability guarantee; inspect host and container separately.
Linux
sudo ss -lunp 'sport = :123'macOS
sudo lsof -nP -iUDP:123Windows PowerShell
Get-NetUDPEndpoint -LocalPort 123Sources and review
Sources checked on:
This review covers the explanations and sources on this page, not your device or every community software entry below.
Software Using This Port (3 software)
Community software entries: links are references, not individual verification.
Usage Suggestions
- Before using port 123, please confirm it is not occupied by other services
- If you encounter port conflicts, consider using other ports or stopping the occupying service
Frequently asked questions
What is port 123 used for?
Port 123 sits in the Time Services category. This page lists the registered purpose and the software commonly associated with it; to see what is actually listening on a specific machine, check the process with ss/netstat.
Is port 123 TCP or UDP?
The registered protocol on this page is UDP. The same number behaves independently on each protocol: TCP may be listening while UDP is idle, so check them separately.
How do I check if port 123 is in use?
On Linux: sudo ss -tlnp 'sport = :123' for an exact port match. On macOS: sudo lsof -nP -iTCP:123 -sTCP:LISTEN. On Windows PowerShell: Get-NetTCPConnection -LocalPort 123 -State Listen. A listener counts as confirmed only when you can see the bind address and process name; without root/admin you will not see other users' processes.
Is port 123 secure? Should I open it in my firewall?
A port number has no inherent security — it depends on whether the listening service is correctly configured and whether it needs exposure at all. Keep loopback/internal-only services firewalled; where public access is required, pair it with source restrictions, VPN or authentication. The caution section on this page lists port-specific risks.
Quick Info
- Port
- 123
- Protocol
- UDP
- Categories
- Time Services
- Usage Status
- 3 software in use
Related Ports
Need Help?
If you find incorrect information or need to add more, please submit correction information.
Submit Correction