NTP Debugging Techniques

Once the NTP software distribution has been compiled and installed and the configuration file constructed, the next step is to verify correct operation and fix any bugs that may result. Usually, the command line that starts the daemon is included in the system startup file, so it is executed only at system boot time; however, the daemon can be stopped and restarted from root at any time. Usually, no command-line arguments are required, unless special actions described in the ntpd page are required. Once started, the daemon will begin sending messages, as specified in the configuration file, and interpreting received messages.

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 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:

ntpq>pe
remote      refid       st t when poll reach   delay  offset   disp
===================================================================
+128.4.2.6  132.249.16.1 2 u  131  256   373    9.89   16.28  23.25
*128.4.1.20 .WWVB.       1 u  137  256   377  280.62   21.74  20.23
-128.8.2.88 128.8.10.1   2 u  49   128   376  294.14    5.94  17.47
+128.4.2.17 .WWVB.       1 u  173  256   377  279.95   20.56  16.40

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

ntpq>as
ind assID status conf reach auth condition last_event cnt
=========================================================
 1  11670   7414   no   yes   ok candidate  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 candidate  reachable   1
 

ntpq>rv 11670
status=7414 reach, auth, sel_sync, 1 event, event_reach
srcadr=128.4.2.6, srcport=123, dstadr=128.4.2.7, dstport=123, keyid=1,
stratum=2, precision=-10, rootdelay=362.00, rootdispersion=21.99,
refid=132.249.16.1,
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

Finally, the state of the local clock can be determined using the rv command (without the argument), such as

ntpq>rv
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=128.4.1.20,
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, freq=13319.46, compliance=2

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 43 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 can 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.

Debugging Checklist

1. Verify the /etc/services file host machine is configured to accept UDP packets on the NTP port 123. NTP is specifically designed to use UDP and does not respond to TCP.
2. Check the system log for ntpd messages about configuration errors, name-lookup failures or initialization problems.
3. Using the ntpdc program and iostats command, verify that the received packets and packets sent counters are incrementing. If the packets send counter does not increment and the configuration file includes designated servers, something may be wrong in the network configuration of the ntpd host. If this counter does increment and packets are actually being sent to the network, but the received packets counter does not increment, something may be wrong in the network or the server may not be responding.
4. If both the packets sent counter and received packets counter do increment, but the rec timestamp in the pe billboard shows far from the current date, received packets are probably being discarded for some reason. There is a handy, undocumented state variable flash visible in the pebillboard. The value is in hex and normally has the value zero (OK). However, if something is wrong, the bits of this variable, reading from the right, correspond to the sanity checks listed in Section 3.4.3 of the NTP specification RFC-1305. A bit other than zero indicates the associated sanity check failed.
5. If the org, rec and xmt timestamps in the pe billboard appear current, but the local clock is not set, as indicated by a stratum number less than 16 in the rv command without arguments, verify that valid clock offset, roundtrip delay and dispersion are displayed for at least one peer. The clock offset should be less than 1000 seconds, the roundtrip delay less than one second and the dispersion less than one second.
6. While the algorithm can tolerate a relatively large frequency error (up to 500 parts per million or 43 seconds per day), various configuration errors (and in some cases kernel bugs) can exceed this tolerance, leading to erratic behavior. This can result in frequent loss of synchronization, together with wildly swinging offsets. Use the ntpdc program (or temporary configuration file) and disable pll command to prevent the ntpd daemon from setting the clock. Using the ntpq or ntpdc programs, watch the apparent offset as it varies over time to determine the intrinsic frequency error. If the error increases by more than 22 milliseconds per 64-second poll interval, the intrinsic frequency must be reduced by some means. The easiest way to do this is with the tickadj program and the -t command line argument.