Monitoring NTP status

When the ntp package is installed, you can monitor the synchronization status of the machine's clock by running ntptime.

For example, a system with a local clock that is synchronized may report:

ntp_gettime() returns code 0 (OK)
  time de24c0cf.8d5da274  Tue, Feb  6 2018 16:03:27.552, (.552210980),
  maximum error 224455 us, estimated error 383 us, TAI offset 0
ntp_adjtime() returns code 0 (OK)
  modes 0x0 (),
  offset 1279.543 us, frequency 2.500 ppm, interval 1 s,
  maximum error 224455 us, estimated error 383 us,
  status 0x2001 (PLL,NANO),
  time constant 10, precision 0.001 us, tolerance 500 ppm,

Note the following most important parts of output:

maximum error 22455 us: This value is well under the 10-second maximum error required by Kudu
status 0x2001 (PLL,NANO): This indicates that the local clock is synchronized with the true time up to the maximum error above

In contrast, a system without NTP properly configured and running will output something like the following:

ntp_gettime() returns code 5 (ERROR)
  time de24c240.0c006000  Tue, Feb  6 2018 16:09:36.046, (.046881),
  maximum error 16000000 us, estimated error 16000000 us, TAI offset 0
ntp_adjtime() returns code 5 (ERROR)
  modes 0x0 (),
  offset 0.000 us, frequency 2.500 ppm, interval 1 s,
  maximum error 16000000 us, estimated error 16000000 us,
  status 0x40 (UNSYNC),
  time constant 10, precision 1.000 us, tolerance 500 ppm,

The UNSYNC status means that the local clock is not synchronized with the true time. Because of that, the maximum reported error doesn't convey any meaningful estimation of the actual error.

The ntpstat utility reports a summary on the synchronization status of the NTP daemon itself. For example, a system which has ntpd running and synchronized with one of its reference servers may report:

$ ntpstat
synchronised to NTP server (172.18.7.3) at stratum 4
   time correct to within 160 ms
   polling server every 1024 s

Keep in mind that the synchronization status of the NTP daemon itself doesn't reflect the synchronization status of the local clock. The way NTP daemon drives the local clock is subject to many constraints, and it may take the NTP daemon some time to synchronize the local clock after it itself has latched to one of the reference servers.

If more detailed information is needed on the synchronization status of the NTP server (but not the synchronization status of the local clock), the ntpq or ntpdc tools can be used to get detailed information about what NTP server is currently acting as the source of the true time and which are considered as candidates (either viable or not):

$ ntpq -nc lpeers
     remote           refid      st t when poll reach   delay   offset  jitter
------------------------------------------------------------------------------
-108.59.2.24     130.133.1.10     2 u   13   64    1   71.743    0.373   0.016
+192.96.202.120  129.6.15.28      2 u   12   64    1   72.583   -0.426   0.028
-69.10.161.7     204.26.59.157    3 u   11   64    1   15.741    2.641   0.021
-173.255.206.154 45.56.123.24     3 u   10   64    1   43.502    0.199   0.029
-69.195.159.158  128.138.140.44   2 u    9   64    1   53.885   -0.016   0.013
*216.218.254.202 .CDMA.           1 u    6   64    1    1.475   -0.400   0.012
+129.250.35.250  249.224.99.213   2 u    7   64    1    1.342   -0.640   0.018

$ ntpq -nc opeers
     remote           local      st t when poll reach   delay   offset    disp
------------------------------------------------------------------------------
-108.59.2.24     10.17.100.238    2 u   17   64    1   71.743    0.373 187.573
+192.96.202.120  10.17.100.238    2 u   16   64    1   72.583   -0.426 187.594
-69.10.161.7     10.17.100.238    3 u   15   64    1   15.741    2.641 187.569
-173.255.206.154 10.17.100.238    3 u   14   64    1   43.502    0.199 187.580
-69.195.159.158  10.17.100.238    2 u   13   64    1   53.885   -0.016 187.561
*216.218.254.202 10.17.100.238    1 u   10   64    1    1.475   -0.400 187.543
+129.250.35.250  10.17.100.238    2 u   11   64    1    1.342   -0.640 187.588