NTP (Network Time Protocol)

NTP (Network Time Protocol) is a protocol that keeps clocks on computers and devices synchronized with a reference time source, typically accurate to a few milliseconds over the internet. It runs on UDP port 123.

Why it matters

Many systems break quietly when clocks drift. TLS certificates are rejected if the device thinks it is before or after their validity dates, log entries from different servers cannot be correlated, Kerberos authentication fails with skew over about five minutes by default, and time-based one-time passwords stop matching.

NTP organizes servers into strata. Stratum 0 devices are atomic clocks or GPS receivers, stratum 1 servers connect directly to them, and each further level is one hop away. The client compares timestamps in request and reply, estimates network delay and slews the clock gradually rather than jumping it.

Example

$ chronyc tracking
Reference ID    : A29FC801 (time.example.org)
Stratum         : 3
System time     : 0.000012 seconds fast of NTP time

Common mistakes

• Pointing a whole fleet at one server, creating a single point of failure.
• Leaving a public NTP server open to the internet, where it can be abused for DDoS amplification.
• Forgetting that virtual machines paused or restored from snapshots can have wildly wrong clocks.

Related

• UDP
• SSL/TLS
• Latency
• DDoS attack

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