On 2021-06-24, Jim Pennino <j...@gonzo.specsol.net> wrote: > Jim Pennino <j...@gonzo.specsol.net> wrote: >> I was checking the stability of a new USB GPS refclock on a server which >> is configured to use the GPS, servers from the ntp pool, and another server >> of mine that has a PPS GPS receiver. >> >> I noticed that almost all the pool servers had disappeared. >> >> I then checked other machines that use my "good" server and the ntp >> pool; most all of the pool servers had also disappeared on those >> machines. >> >> This is a mix of PC linux, rasberry pi linux, rasberry pi buster, and >> Windows 10 machines with Meinberg, all with the latest ntp from their >> distros. >> >> Long story short: I realized I had had a network outage and tested the >> theory that was the cause. It was. >> >> It seems that any server in ntp.conf that is specified as a name, as >> the pool servers are, will after a sufficiently long DNS outage just >> disappear and not come back after the outage without restarting ntp. >> >> It would seem to me that ntp should only need to do a DNS lookup on >> startup and from then on continue to use the address found. >> >> But that is not how ntp works. >> >> Anyway, the bottom line is that if the pool is your only source of time >> and if there is a DNS failure for a sufficiently long time, you will >> lilely not have any source of time afterwards. >> >> As for the USB GPS I was testing, it is called a VK-162 G-Mouse >> available from Amazon for $14, uses the Windows 10 native driver so it >> works with Meinberg ntp, and keeps the time within single digit >> milliseconds without any other servers. > > Further testing shows the following: > > I took a machine and ran watch -p -n 10 ntpq -pn to monitor ntp status. > > I then pulled the network connection from the machine. > > After about 7 minutes the pool servers started to disappear. > > If the machine was reconnected to the network within about 15 minutes, > the pool servers would reappear. > > If the machine was off the network for more than about 15 minutes, the > pool servers do NOT reappear until ntp is restarted. >
I suspect it is the number of times that ntpd tries to contact the server and fails rather than the time that is important. You could try putting the server offline and then online again (I use chrony so do not remember if ntpd has that option). > _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
