sntp
Query a public NTP server: stratum, offset, round-trip, ref-id, root delay/dispersion. Does not change the system clock.
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NetTests can run this check on a schedule, preserve historical results, compare changes over time, and alert you the moment something breaks.
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What does this tool measure?
It sends an NTP query (UDP port 123) to the specified server and reports: stratum (clock accuracy level), offset (time difference between our server and the NTP server), round-trip delay, reference ID (the stratum-1 source the server syncs to), and root delay/dispersion (total accumulated error from the primary source).
What is NTP stratum?
Stratum describes the distance from the authoritative time source. Stratum 0 — atomic clocks, GPS receivers (not directly queried over NTP). Stratum 1 — servers directly connected to stratum-0 sources. Stratum 2 — servers that sync from stratum-1 (e.g. pool.ntp.org). Each hop adds potential drift. Stratum 16 means unsynchronised.
What is clock offset and what is acceptable?
Offset is the difference between the NTP server's time and the querying system's clock. A well-synced system should have offset under 1 ms. Offsets above 128 ms cause ntpd to refuse synchronisation by default (panic mode). For most applications, offsets under 100 ms are acceptable; Kerberos authentication requires clocks within 5 minutes.
What is the NTP pool?
pool.ntp.org is a large virtual cluster of volunteer time servers. DNS round-robin returns a different IP on each query, distributing load. For production use, add iburst in your ntp.conf and use 0.pool.ntp.org through 3.pool.ntp.org to maintain multiple server associations. Many cloud providers also offer their own NTP endpoints (169.254.169.123 on AWS).