NTP Diagnostic

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I’ve been working quite a bit recently with NTP, so I wanted to record some notes.

I’ve found that when the NTP server is fewer than 4x reached, then it will not choose a source. When it chooses a source, a local source might cause it to flag itself as “unreliable” (leap flag with a bad result). That or a poor stratum (less than client’s fudged is causing a client to ignore a source.

In our case, we saw an SNTP reject on status == 3 (see sntp/main.c::read_packet : “if (failed || data->status == 3”); this “status” is actually the Leap Indicator being 0%11 ( == 3), which was re-instated in RFC-2030 (superseding RFC-958) as an alarm condition (when the last second plus the next second are leap-seconds).

The client is flagging 0x2001 and 0x6001 quite frequently; this is clearly PLL/FLL changeovers, implying that it often sees a “weak” source, perhaps one that jitters too often, and swaps mode.

I’ll add to this over time if I see additional factors. As always — ALWAYS — “ntpq -c opeers” (in whatever form is permitted by your OS) is the best first-step, although “readvar” is also a good first step.

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