Port 123 (UDP) — Network Time Protocol (NTP)
NTP synchronizes system clocks across servers, routers, Kubernetes nodes, and database clusters within milliseconds of UTC stratum reference clocks.
| Port Number | 123 |
|---|---|
| Transport Protocol | UDP |
| Primary Service | Network Time Protocol (NTP) |
| Standard / Specification | RFC 5905 |
| IANA Port Range | Well-Known System Port (0–1023) |
| Internet Exposure Risk | Legacy monlist commands on unpatched NTP daemons enable severe UDP amplification. |
Firewall Architecture & Hardening Best Practices
Allow outbound UDP/123 from internal hosts to trusted NTP pools or enterprise time appliances, and block inbound public UDP/123 unless operating an hardened public stratum server.
Operational Troubleshooting Note
Clock drift above a few seconds breaks Kerberos tickets, TLS certificate validation, TOTP 2FA codes, and distributed database consensus.
Quick CLI Connectivity & Firewall Reference
# Inspect local UDP/123 socket binding on Linux ss -uapn | grep :123 # Allow restricted CIDR on Linux UFW firewall ufw allow from 10.0.0.0/8 to any port 123 proto udp
Planning firewall allowlists or VPN overlays? Use our IPv4 CIDR Calculator, MTU & TCP MSS Calculator, or WireGuard AllowedIPs Generator.
Browse Common Network & Cloud Ports
Port 22 (SSH) Port 25 (SMTP Mail Relay) Port 53 (Domain Name System) Port 67 (DHCPv4 Server) Port 80 (HTTP Web Traffic) Port 123 (Network Time Protocol) Port 161 (SNMP Polling) Port 179 (Border Gateway Protocol) Port 389 (LDAP Directory) Port 443 (HTTPS & HTTP) Port 445 (SMB Direct File Sharing) Port 465 (SMTPS Implicit TLS Submission) Port 500 (IPsec IKEv2 Key Exchange) Port 514 (Syslog Telemetry) Port 587 (SMTP Mail Submission) Port 636 (LDAPS) Port 853 (DNS over TLS) Port 993 (IMAPS Encrypted Mailbox) Port 1194 (OpenVPN Tunnel) Port 1433 (Microsoft SQL Server) Port 1812 (RADIUS Authentication) Port 2049 (Network File System) Port 2379 (etcd Client API) Port 3306 (MySQL & MariaDB) Port 3389 (Remote Desktop Protocol) Port 4500 (IPsec NAT Traversal) Port 5432 (PostgreSQL Database) Port 6379 (Redis) Port 6443 (Kubernetes API Server) Port 51820 (WireGuard VPN)