[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

http://www.eecis.udel.edu/~mills/ntp/html/ntpd.html

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

http://www.eecis.udel.edu/~mills/ntp/html/miscopt.html#tinker

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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