[ntp:questions] Remaining synced on an unsynchronised peer?
E-Mail Sent to this address will be added to the BlackLists
Null at BlackList.Griffin-Technologies.invalid
Wed Dec 2 21:50:24 UTC 2009
michael.buetow at comsoft.de wrote:
> It seems to me that if "prefer" was used, even a
> "freewheeling" ntpd that's lost all upstream reachability
> will still serve the time.
> Ouch, I didn't know "prefer" would do this...
The prefer Peer
"In the prefer scheme the clustering algorithm is modified
so that the prefer peer is never discarded"
"Ordinarily, the combining algorithm computes a weighted average of
the survivor offsets to produce the final synchronization source.
However, if a prefer peer is among the survivors, the combining
algorithm is not used. Instead, the offset of the prefer peer
is used exclusively as the final synchronization source."
"As the selection algorithm scans the associations for selectable
candidates, the modem driver and local driver are segregated
for later, but only if not designated a prefer peer. If so
designated, a driver is included among the candidate population."
"If the prefer peer is among the survivors, it becomes the system
peer and its clock offset and jitter are inherited by the
corresponding system variables. Otherwise, the combining
algorithm computes these variables from the survivor population."
The minsane Option
"The minsane option specifies the minimum number of survivors
required to synchronized the system clock."
"minsane Specify the number of servers used by the selection
algorithm as the minimum to set the system clock.
The default is 1 for legacy purposes; however, for critical
applications the value should be somewhat higher but less
That seems to cover it?
FWIW, I'm currently using tos minclock 6 minsane 4 cohort 1
"The minclock threshold is used by the clustering algorithm
as it casts off outlyer servers until the total remaining
is not more than this value. At the moment, minclock
defaults to three mostly for historic reasons. From
Byzantine agreement principles, it really should be four.
The interesting threshold is minsane, which is the minimum
number of survivors necessary to declare the client
synchronized. It defaults to one in the interest of fast
synchronization, but really should be something higher
like four, assuming that number of servers can always
"To increase your ability to detect and eliminate
falsetickers (especially during startup), you should use
the following configuration command in your /etc/ntp.conf
file: tos minclock 4 minsane 4"
Towards a resilient NTP configuration in NTP4
E-Mail Sent to this address <BlackList at Griffin-Technologies.net>
will be added to the BlackLists.
More information about the questions