[CentOS] CentOs 5.6 and Time Sync

Wed Apr 13 18:50:43 UTC 2011
Cal Webster <cwebster at ec.rr.com>

On Wed, 2011-04-13 at 13:08 -0400, Rob Kampen wrote:
> Mailing List wrote:
> > On 4/13/2011 7:35 AM, Mailing List wrote:
> >> Hi,
> >>
> >>   I have upgraded my Dell C151 to the latest 5.6. I have always used 
> >> ntp to sync this machine and then the rest of the machines in the 
> >> network would sync from it. Since the update I cannot keep the right 
> >> time on the machine. This is with / without ntp. I have attempted 
> >> various scenario's with no luck. I am now trying the old kernel now 
> >> as I type this out. If anyone else has any links or ideas that I 
> >> should check out It would be greatly appreciated.
> >>
> >>      Just a quick note about my setup. I do not use any gui. As 
> >> mentioned I have not had any issues with this machine and it's time 
> >> until I upgrade.
> >>
> >> AMD Athlon(tm) 64 X2 Dual Core Processor 3800+
> >> 3gb of ram.
> >>
> >> TIA.
> >>
> >> Brian.
> >
> >    Just to follow up, I had switched to the old kernel before the 5.6 
> > upgrade, and at this time my clock is working flawlessly.
> >
> > kernel v. 2.6.18-194.32.1.el5 Works as it should...
> > kernel v. 2.6.18-238.5.1.el5 I cannot get my clock accurate.
> >
> there are two other 238 kernels - do they show the same behavior?
> > If there is anything I can do to help solve this IE: information or 
> > test. please let me know.At this point I will just make the old kernel 
> > default boot until there is a kernel update where which I will try again.
> >
> > Brian.

You don't say what version of ntp you are using or whether the system in
question can access the Internet.

Should be: ntp-4.2.2p1-9.el5.centos.2.1.i386



[Main config files]


[Time Sources]

Three time sources you can use for your ntpd:

1. Public or corporate NTP servers

If you have an Internet connection using a public ntp pool is the


2. An accurate external reference clock


Use if you need microsecond or better accuracy and you've got time and
money to setup.

3. Undisciplined local clock on a local computer


This is what I use. You can use this if a few seconds off every few
months is less important than all clients being in sync. If it drifts at
least all clients will drift with it. You can compensate easily for
minor drift too.

As long as all clocks are synced to the same source you should be able
to tolerate being off by even a few minutes from the "real" time. Some
networked services such as Kerberos cannot tolerate differences in time
stamps between client and server. You'll get lots of seemingly arbitrary
faults and errors with AD Windows Domains and Linux-based directory
authentication and such if all participants are not time-synced.

Sounds like you're using your system clock.

Here's how my isolated networks are configured:

[Primary server]

Machine with most stable system clock - uses system clock, compensating
for calculated drift in /var/lib/ntp/drift.

[Secondary servers]

One main RHEL/CentOS server in each of 3 buildings uses primary ntp
server as master and sister servers as peers.


CentOS, Windoze DC, Windows stand-alone, and Unix configured to sync
with secondary ntp servers using the closest first.

[Using undisciplined system clock]

Best way to determine most stable clock is to:

1. turn ntpd off
2. Sync time with accurate server
    I use http://wwp.greenwichmeantime.com/time-zone/usa/eastern-time/
3. Wait days or weeks to check system time against same source
4. Calculate and set drift
5. Start ntpd
6. Check against same time source periodically... every few weeks at
first until it's as close as you can get it.
7. Adjust frequency offset and drift values for local (system) clock
8. Start ntpd and use this as "server" in secondary servers.

Sync secondary servers or just clients off this primary.

There are many ways to configure and implement ntp. You should study the
references and determine which options are best for your specific needs.

If you wish I can provide sample ntp.conf files, and details of my
calibration process. I'm kind of busy right now but I'll throw something
together as time permits and forward if you want.