| Keeping Sync with a Remote NTP Server 
Posted: 18 May 2005
Background
All computers have their own internal clocks, implemented in hardware and
software. These clocks have acceptable accuracy, but are not anywhere near
perfect - they all exhibit "clock drift," letting their time drift by several
seconds a day. While desktop users may not be inconvenienced by drift, system
administrators cannot tolerate it. When running a server, it is essential to
keep the system's time synchronized with the other servers it interacts
with.
Linux provides a means to keep time in sync via the Network Time
Protocol (NTP). A computer running the NTP client frequently probes one or more
NTP servers and synchronizes its time with that of the server(s).
YaST
provides a means to set-up one or more remote NTP server via the "NTP Client"
module found under "Network Services." A list of public NTP servers can be found
here.
OES servers can also be configured to serve as NTP servers. See the Novell Network Time Protocol Administration Guide for OES (pdf) for
more details.
The ProblemWhen adding a remote time source in YaST, there are always
at least two sources added to /etc/ntp.conf: the local computer and the remote
time source. When the NTP daemon starts, it initially synchronizes the system
time to the time server's time, then periodically performs the same
synchronization in the background. If the time server is not immediately
available, then NTP falls back on the default local time source.
Some
users have observed that their system time is being synchronized to the remote
server's time when xntpd initially starts, but then is only being synchronized
to their local time thereafter. An example /var/log/ntp would look like
this:
2 May 08:11:05 ntpd[2965]: synchronized to LOCAL(0), stratum 10
2 May 08:11:05 ntpd[2965]: kernel time sync disabled 0041
2 May 08:14:17 ntpd[2965]: synchronized to LOCAL(0), stratum 10
2 May 08:26:13 ntpd[2965]: kernel time sync enabled 0001
2 May 10:33:35 ntpd[3489]: synchronized to LOCAL(0), stratum 10
2 May 10:33:35 ntpd[3489]: kernel time sync disabled 0041
Also, executing xntpdc -p would reveal that the time is synchronized
to the LOCAL, not the remote server (see the entry with the '*').
# xntpdc -p
remote local st poll reach delay offset disp
=======================================================================
*LOCAL(0) 127.0.0.1 10 1024 377 0.00000 0.000000 0.11856
=prv-time.provo. 137.65.211.18 2 1024 377 0.00041 -262.1380 0.11470
The SolutionThe solution is to make NTP more persistent when probing
the remote time server. This is done by appending the flags burst and iburst to
the remote server; burst tells NTP to send a burst of eight packets to the
remote server instead of one when the server is reachable, and iburst tells it
to do the same when the server is not reachable. The result is faster and more
reliable synchronizations.
These options can be added in the "NTP Client"
YaST module by selecting "Complex Configuration" then editing the remote server.
The options are added in the "Options" field and are space-delimited. They can
also be added directly to the server in /etc/ntp.conf as follows:
server 137.65.1.123 burst iburst
A healthy /var/log/ntp will then look like this:
29 Apr 15:24:58 ntpd[6892]: synchronized to 137.65.1.123, stratum 2
29 Apr 15:24:58 ntpd[6892]: kernel time sync disabled 0041
29 Apr 15:26:15 ntpd[6892]: kernel time sync enabled 0001
2 May 08:10:41 ntpd[6892]: ntpd exiting on signal 15
2 May 08:13:33 ntpd[6684]: synchronized to 137.65.1.123, stratum 2
2 May 08:13:33 ntpd[6684]: kernel time sync disabled 0041
2 May 08:13:45 ntpd[6684]: kernel time sync enabled 0001
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