See MonitoringAndControllingNTPDev for discussion of this topic.

8. Monitoring and Controlling NTP

See TroubleshootingNTP for checking the behavior of a particular machine.

Monitoring NTP

For ongoing monitoring of an NTP network there are a variety of choices, including:

• Have all your machines peer with each other. Then ntpq -p from any machine will show you if any machines are having a problem.
• The tools in the ~/scripts/stats directory of the ntp distribution.
• http://www.nagios.org
  • check_ntpq: Harlan's updated code
• http://people.ee.ethz.ch/~oetiker/webtools/mrtg/
• http://people.ee.ethz.ch/~oetiker/webtools/rrdtool/
• http://cricket.sourceforge.net/
• http://www.orcaware.com/orca/
• http://fortytwo.ch/stats/time/
• http://www.satsignal.eu/ntp/NTPandMRTG.html
• http://www.satsignal.eu/software/net.htm#NTPmonitor - Windows NTP monitoring and plotting
• http://www.netplex-tech.com/software/nocol/
• http://www.netplex-tech.com/snips/
• http://www.meinberg.de/english/sw/time-server-monitor.htm A Windows Application for Monitoring NTP Time Servers
• http://saturn.dennishilberg.com/gathering_data.php
• http://www.wraith.sf.ca.us/ntp/index.html#monitoring

• All of the tools listed at http://people.ee.ethz.ch/~oetiker/webtools/rrdtool/rrdworld/index.html

• whatever else we can find

scripts/stats tools

Foremost among these tools is peer.awk, a script that computes statistics from a peerstats file. It is invoked with:

 % gawk -f peer.awk peerstats

where peerstats is the path and name of the peerstats file to be analyzed. awk or nawk may work as well.

The output looks like this:

% gawk -f peer.awk $NTPSTATS/peerstats.20061120
       ident     cnt     mean     rms      max     delay     dist     disp
==========================================================================
127.127.30.0    5241   -0.000    0.002    0.008    0.000    0.000    0.000
server1         1339    2.055    0.346    4.077   17.993   16.592    3.414
server2         1337    3.542    0.339    3.591   16.791   17.514    3.399
server3         1215    2.196    0.348    2.161   12.954  945.456    4.737
server4         1341    0.897    4.305   36.682    0.538   23.602    3.446
server5         1341    2.580    0.316    3.566   16.445   16.261    3.384
127.127.1.0     1339    0.000    0.000    0.000    0.000    0.000    0.000

The ident column identifies the server by IP address or pseudo IP address. The line beginning with 127.127.30.0 is a such a pseudo IP address. It designates a hardware reference clock using driver thirty - the Motorola Oncore GPS driver. I have replaced actual IP addresses with server1, server2, etc. The cnt column gives the number of samples in the peerstats file for the ident. The mean column contains the arithmetic mean or average of the offsets for that server or reference clock. The rms column contains the root mean square of the offsets. This is a measure of central tendency computed as the square root of the sum of the squares of the offsets divided by the number of samples. The max column contains the offset with the greatest absolute value. The delay column has the mean round trip network delay for a network server. dist is the maximum observed synchronization distance, where synchcronization distance is defined as the dispersion plus one half the round trip delay. disp is defined as the maximum error of the server or peer clock relative to the local clock over the network path between them, in seconds. For all of mean, RMS, max, delay, dist, and disp values closer to 0 (zero) are better.

RRDTOOL notes

RRDTOOL defaults to storing non-negative numbers.

NTP offsets will sometimes go negative. To allow negative numbers to be stored you need to tune your RRD databases:

   rrdtool tune foo.rrd -i ds0:u -i ds1:u

(Do we need to do this for both ds0 and ds1?)

Who is using my NTP server?

You can check which hosts are talking to your time server by using the monlist command of ntpdc, e.g.

   ntpdc -c monlist

Please note that a maximum of 600 entries is supported with current versions of ntpdc. The protocol (or better: the contents of the return packets) used by ntpdc is not standardized, therefore it is recommended to only use ntpdc with a matching ntpd, i.e. both should have the same version number.

To get by this 600 entry limitation, many server operators run client statistics scripts, such as Wayne Schlitt's ntp_clients and ntp_clients_stats scripts, which can be found at http://www.schlitt.net/scripts/ntp/index.html . They work very well, but can use quite a bit of system resources if your client counts are in the high thousands. Examples of these scripts in action can be found at:

• http://www.schlitt.net/ntpstats/ntp_stats.txt
• http://saturn.dennishilberg.com/ntpstats/ntp_clients_stats.php (slightly modified)

Controlling NTP

Start and stop NTP on your server

Depending on your operating system there are a number of possibilities how to start and stop the NTP daemon.

On most Linux machines you can use these commands:

   /etc/init.d/ntpd start      # start NTP daemon
   /etc/init.d/ntpd stop       # stop NTP daemon

Some Linux distributions use xntpd instead of ntpd, even if they are referring to a Version 4 NTP implementation (and not a Version 3, which often has been called xntpd).

On a Windows machine, you can use the commands

   net start ntp
   net stop ntp

if you used the Meinberg Installer for NTP. The NTP Time Server Monitor Application by Meinberg (see above) provides functions to start/stop and restart the service on a graphical interface.