The best way to verify correct operation is using the ntpq and ntpdc utility programs, either on the server itself or from another machine elsewhere in the network. The ntpq program implements the management functions specified in Appendix A of the NTP specification RFC-1305, Appendix A . The ntpdc program implements additional functions not provided in the standard. Both programs can be used to inspect the state variables defined in the specification and, in the case of ntpdc, additional ones of interest. In addition, the ntpdc program can be used to selectively enable and disable some functions of the daemon while the daemon is running.
In extreme cases with elusive bugs, the daemon can operate in two modes, depending on the presence of the -d command-line debug switch. If not present, the daemon detaches from the controlling terminal and proceeds autonomously. If one or more -d switches are present, the daemon does not detach and generates special output useful for debugging. In general, interpretation of this output requires reference to the sources. However, a single -d does produce only mildly cryptic output and can be very useful in finding problems with configuration and network troubles. With a little experience, the volume of output can be reduced by piping the output to grep and specifying the keyword of the trace you want to see.
Some problems are immediately apparent when the daemon first starts running. The most common of these are the lack of a ntp (UDP port 123) in the host /etc/services file. Note that NTP does not use TCP in any form. Other problems are apparent in the system log file. The log file should show the startup banner, some cryptic initialization data, and the computed precision value. The next most common problem is incorrect DNS names. Check that each DNS name used in the configuration file responds to the Unix ping command.
When first started, the daemon normally polls the servers listed in the configuration file at 64-second intervals. In order to allow a sufficient number of samples for the NTP algorithms to reliably discriminate between correctly operating servers and possible intruders, at least four valid messages from at least one server is required before the daemon can not set the local clock. However, if the current local time is greater than 1000 seconds in error from the server time, the daemon will not set the local clock; instead, it will plant a message in the system log and shut down. It is necessary to set the local clock to within 1000 seconds first, either by a time-of-year hardware clock, by first using the ntpdate program or manually be eyeball and wristwatch.
After starting the daemon, run the ntpq program using the -n
switch, which will avoid possible distractions due to name resolution problems.
Use the pe command to display a billboard showing the status of
configured peers and possibly other clients poking the daemon. After operating
for a few minutes, the display should be something like:
remote refid st when poll reach delay offset disp
+184.108.40.206 220.127.116.11 2 131 256 373 9.89 16.28 23.25
*18.104.22.168 .WWVB. 1 137 256 377 280.62 21.74 20.23
-22.214.171.124 126.96.36.199 2 49 128 376 294.14 5.94 17.47
+188.8.131.52 .WWVB. 1 173 256 377 279.95 20.56 16.40
The host addresses shown in the remote column should agree with
the DNS entries in the configuration file, plus any peers not mentioned
in the file at the same or lower than your stratum that happen to be configured
to peer with you. Be prepared for surprises in cases where the peer has
multiple addresses or multiple names. The refid entry shows the
current source of synchronization for each peer, while the st
reveals its stratum and the poll entry the polling interval, in
seconds. The when entry shows the time since the peer was last
heard, normally in seconds, while the reach entry shows the status
of the reachability register (see RFC-1305), which is in octal format.
The remaining entries show the latest delay, offset and dispersion computed
for the peer, in milliseconds.
The tattletale character at the left margin displays the synchronization status of each peer. The currently selected peer is marked *, while additional peers designated acceptable for synchronization, but not currently selected, are marked +. Peers marked * and + are included in a weighted average computation to set the local clock; the data produced by peers marked with other symbols are discarded. See the ntpq documentation for the meaning of these symbols.
Additional details for each peer separately can be determined by the
following procedure. First, use the as command to display an index
of association identifiers, such as
ind assID status conf reach auth condition last_event cnt
1 11670 7414 no yes ok synchr. reachable 1
2 11673 7614 no yes ok sys.peer reachable 1
3 11833 7314 no yes ok outlyer reachable 1
4 11868 7414 no yes ok synchr. reachable 1
Each line in this billboard is associated with the corresponding line the
pe billboard above. Next, use the rv command and the
respective identifier to display a detailed synopsis of the selected peer,
status=7414 reach, auth, sel_sync, 1 event, event_reach
srcadr=184.108.40.206, srcport=123, dstadr=220.127.116.11, dstport=123, keyid=1,
stratum=2, precision=-10, rootdelay=362.00, rootdispersion=21.99,
reftime=af00bb44.849b0000 Fri, Jan 15 1993 4:25:40.517,
delay= 9.89, offset= 16.28, dispersion=23.25, reach=373, valid=8,
hmode=2, pmode=1, hpoll=8, ppoll=10, leap=00, flash=0x0,
org=af00bb48.31a90000 Fri, Jan 15 1993 4:25:44.193,
rec=af00bb48.305e3000 Fri, Jan 15 1993 4:25:44.188,
xmt=af00bb1e.16689000 Fri, Jan 15 1993 4:25:02.087,
filtdelay= 16.40 9.89 140.08 9.63 9.72 9.22 10.79 122.99,
filtoffset= 13.24 16.28 -49.19 16.04 16.83 16.49 16.95 -39.43,
filterror= 16.27 20.17 27.98 31.89 35.80 39.70 43.61 47.52
A detailed explanation of the fields in this billboard are beyond the scope
of this discussion; however, most variables defined in the specification
RFC-1305 can be found. The most useful portion for debugging is the last
three lines, which give the roundtrip delay, clock offset and dispersion
for each of the last eight measurement rounds, all in milliseconds. Note
that the dispersion, which is an estimate of the error, increases as the
age of the sample increases. From these data, it is usually possible to
determine the incidence of severe packet loss, network congestion, and
unstable local clock oscillators. There are no hard and fast rules here,
since every case is unique; however, if one or more of the rounds show
zeros, or if the clock offset changes dramatically in the same direction
for each round, cause for alarm exists.
Finally, the state of the local clock can be determined using the rv
command (without the argument), such as
status=0664 leap_none, sync_ntp, 6 events, event_peer/strat_chg
system="UNIX", leap=00, stratum=2, rootdelay=280.62,
rootdispersion=45.26, peer=11673, refid=18.104.22.168,
reftime=af00bb42.56111000 Fri, Jan 15 1993 4:25:38.336, poll=8,
clock=af00bbcd.8a5de000 Fri, Jan 15 1993 4:27:57.540, phase=21.147,
The most useful data in this billboard show when the clock was last adjusted
reftime, together with its status and most recent exception event.
An explanation of these data is in the specification RFC-1305.
When nothing seems to happen in the pe billboard after some minutes, there may be a network problem. The most common network problem is an access controlled router on the path to the selected peer. No known public NTP time server selectively restricts access at this time, although this may change in future; however, many private networks do. It also may be the case that the server is down or running in unsynchronized mode due to a local problem. Use the ntpq program to spy on its own variables in the same way you can spy on your own.
Once the daemon has set the local clock, it will continuously track the discrepancy between local time and NTP time and adjust the local clock accordingly. There are two components of this adjustment, time and frequency. These adjustments are automatically determined by the clock discipline algorithm, which functions as a hybrid phase/frequency feedback loop. The behavior of this algorithm is carefully controlled to minimize residual errors due to network jitter and frequency variations of the local clock hardware oscillator that normally occur in practice. However, when started for the first time, the algorithm may take some time to converge on the intrinsic frequency error of the host machine.
It has sometimes been the experience that the local clock oscillator frequency error is too large for the NTP discipline algorithm, which can correct frequency errors as large as 30 seconds per day. There are two possibilities that may result in this problem. First, the hardware time- of-year clock chip must be disabled when using NTP, since this can destabilize the discipline process. This is usually done using the tickadj program and the -s command line argument, but other means may be necessary. For instance, in the Sun Solaris kernel, this must be done using a command in the system startup file.
Normally, the daemon will adjust the local clock in small steps in such a way that system and user programs are unaware of its operation. The adjustment process operates continuously as long as the apparent clock error exceeds 128 milliseconds, which for most Internet paths is a quite rare event. If the event is simply an outlyer due to an occasional network delay spike, the correction is simply discarded; however, if the apparent time error persists for an interval of about 20 minutes, the local clock is stepped to the new value (as an option, the daemon can be compiled to slew at an accelerated rate to the new value, rather than be stepped). This behavior is designed to resist errors due to severely congested network paths, as well as errors due to confused radio clocks upon the epoch of a leap second.