Port 123 – NTP Protocol Specification

Technical analysis, security advisories, vulnerability mitigation, and firewall syntax for TCP/UDP port 123 (NTP).

Port Number 123

Transport Protocol UDP

Port Range Type Well-Known (0-1023)

Default Status Restricted / LAN

Service Description & Architectural Context

Network Time Protocol used for microsecond system clock sync.

IANA / RFC Designation:

Port 123 is assigned to NTP under the Internet Assigned Numbers Authority (IANA) service name registry. In enterprise infrastructure and homelab setups, services listening on this port must follow the principle of least privilege.

Security Analysis & Attack Surface Assessment

Disable NTP monlist feature to eliminate DDoS amplification attacks.

Hardening Recommendations:

Do not expose port 123 to public WAN interfaces unless authenticated TLS or VPN wrapping is strictly enforced.
Enforce rate-limiting using Fail2ban, CrowdSec, or cloud edge DDoS protection.
Audit active listening sockets locally: ss -tulpn | grep :123 or netstat -ano | findstr :123.

Firewall Rule Snippets for Port 123

UFW (Ubuntu / Debian Linux)

sudo ufw allow 123/udp

iptables (Standard Linux Kernel)

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

Windows PowerShell (Defender Advanced Firewall)

New-NetFirewallRule -DisplayName "Allow Port 123" -Direction Inbound -LocalPort 123 -Protocol UDP -Action Allow

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