[ntp:questions] Server Sync Problem

Michael michael.yacc at blueyonder.co.uk
Wed Dec 14 22:37:47 UTC 2005

I have a real headache of a problem with NTP and clients failing to sync 
with the server, so any
help will be greatly appreciated.

I have a Linux solid state GPS based NTP server running NTP 4.2.......
It is rock steady. Server time offset is always < 1ms, usually much less and 
jitter is usually less than .01 ms.

I have many other Linux servers running Gentoo and NTP 4.2.....

3) Problem 1
If I tell these servers to sync directly to the GPS server as clients, they 
do suceed, but then
they lose sync for a few minutes and then find it again etc etc. NTP reports 
in /var/log/messages
that "No servers reachable", but I have sat there with tcpdump running 
watching the UDP
request/responses ticking away with no problems whatsoever. No packets are 
lost at all.
But NTP just doesnt seem to like whats going on.
However, if I make the client "burst" to the server, it never seems to lose 
sync at all. Why I need
to use burst on a LAN when I cant see any packet loss, I dont know but it 
seems to fix the problem.

4) Problem 2
Now, if I sync a server lets call it A, off the GPS server, then try to sync 
the other servers off A, none
of them will sync at all even though NTP can see server A and ntpq reports a 
reach of 377, the offset is
< 1ms and the jitter is < .01ms  and tcpdump sees all the UDP 
request/response packets without any problems.
So communication with the server appears to be fine, but NTP just doesnt 
want to sync up at all.

Anybody got any ideas as to what is happening as I am going slowly mad.



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