NTP
What is Port 123?
Port 123 is used by the Network Time Protocol (NTP), the most widely used protocol for clock synchronization across computer networks. Defined in RFC 5905, NTP can synchronize clocks to within milliseconds over the internet and sub-millisecond on local networks. NTP uses a hierarchical system of time sources called strata, where stratum 0 consists of atomic clocks and GPS receivers, stratum 1 servers connect directly to stratum 0, and so on. Accurate time is critical for security (TLS certificates, Kerberos authentication), distributed systems (database replication, consensus protocols), logging and forensics, financial transactions, and legal compliance. The NTP Pool Project provides free time synchronization via pool.ntp.org, serving billions of queries daily.
Common Uses
- System clock synchronization across networks
- Kerberos and certificate-based authentication timing
- Log correlation and forensic analysis
- Financial transaction timestamping
- Distributed system coordination
Technical Details
Copy-paste commands to check port 123 (NTP) from your terminal
Test connectivity
nmap -p 123 -sU -sV example.com
(echo > /dev/tcp/example.com/123) 2>/dev/null && echo "open" || echo "closed"
Open port in firewall
sudo ufw allow 123/udp
sudo iptables -A INPUT -p udp --dport 123 -j ACCEPT
sudo firewall-cmd --permanent --add-port=123/udp && sudo firewall-cmd --reload
Security Considerations
NTP has been used in massive DDoS amplification attacks because a small query can generate a large response (monlist command). Disable monlist and restrict NTP queries to trusted clients. Use NTPsec or chrony with authentication. NTP spoofing can affect time-sensitive security mechanisms. Keep NTP software updated and use multiple time sources.
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