PORT 123 / UDPInfrastructure & CloudExposure Risk: Low

Port 123: NTP — Network Time Protocol Clock Sync

Synchronizes system clocks across servers over packet-switched networks.

Default Service

NTP

Process: ntpd / chronyd

Transport Protocol

UDP

IANA Registered Port 123

Docker Port Flag

-p 123:123/udp

Container to Host Mapping

How to Check & Kill Process on Port 123 (EADDRINUSE Fix)

macOS & Linux (lsof / kill)

# Find process using port 123
sudo lsof -i :123

# Instantly kill process on port 123
lsof -ti:123 | xargs kill -9

Windows (PowerShell & CMD)

# Find PID listening on port 123
netstat -ano | findstr :123

# Force kill by PID
taskkill /PID <PID_NUMBER> /F

Cross-Platform Node.js One-Liner

# Kill port 123 via npx
npx kill-port 123

# Or diagnose via Castov CLI
npx castov fix "EADDRINUSE :::123"

Linux Firewall (UFW) & Connectivity Test

# Test if port 123 is reachable
nc -zv localhost 123

# Allow or deny port 123 in UFW
sudo ufw allow 123/tcp

Frequently Asked Questions about Port 123

What is Port 123 used for?

Port 123 is the standard network port for NTP (Network Time Protocol Clock Sync) operating over UDP. The typical system daemon or process is ntpd / chronyd.

How do I kill a process running on Port 123 (EADDRINUSE)?

On macOS and Linux, run: lsof -ti:123 | xargs kill -9 (or npx kill-port 123). On Windows PowerShell/CMD, run: netstat -ano | findstr :123 and then taskkill /PID <PID> /F.