Why is precise clock synchronization critical?

Without accurate time, TLS certificates may fail validation due to perceived expiration, distributed databases experience split-brain anomalies, and multi-system security logs cannot be aligned.

What is NTP reflection attack?

Attackers send small UDP requests to port 123 with a spoofed victim IP. If the server responds with a large data packet, traffic is amplified towards the victim.

Can I test UDP port 123 with a standard TCP port checker?

TCP port checkers cannot test port 123 because NTP operates strictly over UDP. Use 'ntpdate -q ' or 'chronyc sources' to test NTP synchronization.

Home/Open Port Checker/Port 123

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Well-Known Port Reference

Port 123: NTP (Network Time Protocol) (UDP)

Network Time Protocol synchronizing system clocks across network devices to millisecond precision.

Port Number

123

Transport

UDP

Security Risk

Medium

Standard / RFC

RFC 5905

Test Port 123 Reachability Live

Verify whether Port 123 is open, closed, or filtered on your server or public IP address.

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Service Architecture & Function

Port 123 is the standardized UDP port for Network Time Protocol (NTP) daemons (such as chrony and ntpd). Accurate system time is vital for TLS certificate validation, Kerberos authentication, distributed database consensus, and log correlation.

Security Profile & Hardening

NTP servers supporting the legacy 'monlist' command have historically been weaponized in massive UDP reflection DDoS attacks. Always ensure NTP implementations disable 'monlist' and restrict query access to authorized hosts.

Example Firewall Commands:

# UFW (Ubuntu/Debian)

sudo ufw allow 123/udp

# iptables (Direct Kernel)

iptables -A INPUT -p udp --dport 123 -j ACCEPT