Occasionaly your NTP client can get out of sync with the source time servers. The following commands can be run to investigate this:

ntpq -p - Checks the configured time source. One of the sources listed should be prefixed with a * (indicating it is the primary source) and the reachability should be 377 . If this is not the case, your NTP daemon is having problems communicating with the source. (For a full list of the meaning of the prefix symbol, see ntpq man page)

ntpq -c as - lists what xntpd thinks of the source time references. Any problems are fairly obvious! If your output does show an error, this can be further diagnosed as follows:

  ntq -c as
  ind assID status  conf reach auth condition  last_event cnt
   1 17064  8034   yes   yes  none    reject   reachable  3
   2 17065  9044   yes   yes  none    reject   reachable  4
   3 17066  9044   yes   yes  none    reject   reachable  4
   4 17067  8034   yes   yes  none    reject   reachable  3

Using the assID, check the flash values

   rv 17064
   assID=17064 status=9044 reach, conf, 4 events, event_reach, 
   srcadr=jssapp01p.justsomestuff.co.uk, srcport=123, dstadr=, 
   dstport=123, leap=00, stratum=2, precision=-24, rootdelay=80.414,
   rootdispersion=77.560, refid=, reach=017, unreach=0,
   hmode=3, pmode=4, hpoll=6, ppoll=6, flash=400 peer_dist, keyid=0, ttl=0,
   offset=474.959, delay=10.215, dispersion=939.297, jitter=10.651,
   reftime=dbbc84d9.090dc0f7  Thu, Oct 27 2016  9:51:21.035,
   org=dbbc8a0a.929474f2  Thu, Oct 27 2016 10:13:30.572,
   rec=dbbc8a0a.1e1db5aa  Thu, Oct 27 2016 10:13:30.117,
   xmt=dbbc8a0a.1b6e0b0a  Thu, Oct 27 2016 10:13:30.107, 
   filtdelay=    10.46   10.41   10.58   10.21    0.00    0.00    0.00    0.00,
   filtoffset=  460.17  465.06  470.11  474.96    0.00    0.00    0.00    0.00,
   filtdisp=      0.00    0.95    1.92    2.88 16000.0 16000.0 16000.0 16000.0

Here flash=400 which indicates distance threshold exceeded, i.e. your NTP source is too far away. flash=800 indicates a peering loop.

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