[ntp:questions] what happens if ntp server jumps time BIG TIME

Dave Hart davehart at gmail.com
Fri Feb 10 02:32:03 UTC 2012

On Fri, Feb 10, 2012 at 00:16, Chuck Swiger <cswiger at mac.com> wrote:
> On Feb 9, 2012, at 4:05 PM, Chris Albertson wrote:
>> I think what you describe applies to the case where a client NTP
>> starts up and notice the time is very far "off".    But I think (??)
>> the question was that the client is in sync and then the server's time
>> jumps.  I thought -g only applied to start up logic.  I could be
>> wrong.   This situation would never happen on a properly configured
>> network.
> Ah, yes, my response was in the context of client starting up when
> there is a single server, and it is very far off from what the client
> believes is the current time.  From what I recall in earlier testing, if
> ntpd is running and synched, and the remote side does something
> crazy like a multi-year jump, then it is promptly marked and ignored
> as a falseticker.  I'm not sure that I've ever tried doing this with just
> a single server specified, though.

Close but no cigar -- -g allows a single step exceeding the panic
threshold, not necessarily at startup.  So if the client didn't need
to step more than the panic threshold at startup, a single subsequent
panic-exceeding step is allowed.  A second such incident would then
cause the client ntpd to exit.

> I think that -x specifies the max deviation allowed when doing a step
> via settimeofday() rather than slewing the clock with adjtime(), and it
> is limited to a max permissible jump of 600 seconds, except for
> possibly a one-time "big" jump at startup allowed by -g.

-x is equivalent to "tinker step 600", as explained in


This changes the step threshold from 0.128 to 600, and as noted in


any step threshold greater than 0.500 disables the kernel clock
discipline loop (as does "disable kernel").

Let me RTFM that for you,
Dave Hart

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