123
Network time synchronization (NTP)
Network Time Protocol
What is port 123?
NTP (Network Time Protocol) on UDP port 123 synchronizes computer clocks to millisecond accuracy. Every server, router, PC, and mobile device relies on NTP. Accurate time is critical for TLS certificate validation, Kerberos authentication, log correlation, and distributed systems. A publicly accessible NTP server can be abused for NTP amplification DDoS attacks (up to 556x amplification). Common public NTP servers include pool.ntp.org and time.cloudflare.com.
Quick checks
$
nc -zv example.com 123
$
nmap -p 123 example.com