478 lines
24 KiB
HTML
478 lines
24 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
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<html>
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<head>
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<meta name="generator" content="HTML Tidy, see www.w3.org">
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<title>NTP Debugging Techniques</title>
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</head>
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<body>
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<h3>NTP Debugging Techniques</h3>
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<img align="left" src="pic/pogo.gif" alt="gif"><a href=
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"http://www.eecis.udel.edu/~mills/pictures.htm">from <i>Pogo</i>,
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Walt Kelly</a>
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<p>We make house calls and bring our own bugs.<br clear="left">
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</p>
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<hr>
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<p>Once the NTP software distribution has been compiled and
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installed and the configuration file constructed, the next step is
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to verify correct operation and fix any bugs that may result.
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Usually, the command line that starts the daemon is included in the
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system startup file, so it is executed only at system boot time;
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however, the daemon can be stopped and restarted from root at any
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time. Usually, no command-line arguments are required, unless
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special actions described in the <tt><a href="ntpd.htm">
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ntpd</a></tt> page are required. Once started, the daemon will
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begin sending and receiving messages, as specified in the
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configuration file.</p>
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<h4>Initial Startup</h4>
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<p>The best way to verify correct operation is using the <tt><a
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href="ntpq.htm">ntpq</a></tt> and <tt><a href="ntpdc.htm">
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ntpdc</a></tt> utility programs, either on the server itself or
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from another machine elsewhere in the network. The <tt>ntpq</tt>
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program implements the management functions specified in the NTP
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specification <a href=
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"http://www.eecis.udel.edu/~mills/database/rfc/rfc1305/rfc1305c.ps">
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RFC-1305, Appendix A</a>. The <tt>ntpdc</tt> program implements
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additional functions not provided in the standard. Both programs
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can be used to inspect the state variables defined in the
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specification and, in the case of <tt>ntpdc</tt>, additional ones
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of interest. In addition, the <tt>ntpdc</tt> program can be used to
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selectively reconfigure and enable or disable some functions while
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the daemon is running.</p>
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<p>In extreme cases with elusive bugs, the daemon can operate in
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two modes, depending on the presence of the <tt>-d</tt>
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command-line debug switch. If not present, the daemon detaches from
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the controlling terminal and proceeds autonomously. If one or more
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<tt>-d</tt> switches are present, the daemon does not detach and
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generates special output useful for debugging. In general,
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interpretation of this output requires reference to the sources.
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However, a single <tt>-d</tt> does produce only mildly cryptic
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output and can be very useful in finding problems with
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configuration and network troubles. With a little experience, the
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volume of output can be reduced by piping the output to <tt>
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grep</tt> and specifying the keyword of the trace you want to
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see.</p>
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<p>Some problems are immediately apparent when the daemon first
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starts running. The most common of these are the lack of a UDP port
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for NTP (123) in the Unix <tt>/etc/services</tt> file (or
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equivalent in some systems). Note that NTP does not use TCP in any
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form. Other problems are apparent in the system log file. The log
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file should show the startup banner, some cryptic initialization
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data and the computed precision value. The next most common problem
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is incorrect DNS names. Check that each DNS name used in the
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configuration file exists and that the address responds to the Unix
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<tt>ping</tt> command.</p>
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<p>When first started, the daemon normally polls the servers listed
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in the configuration file at 64-s intervals. In order to allow a
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sufficient number of samples for the NTP algorithms to reliably
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discriminate between correctly operating servers and possible
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intruders, at least four valid messages from the majority of
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servers and peers listed in the configuration file is required
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before the daemon can set the local clock. However, if the
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difference between the client time and server time is greater than
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the panic threshold, which defaults to 1000 s, the daemon will send
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a message to the system log and shut down without setting the
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clock. It is necessary to set the local clock to within the panic
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threshold first, either manually by eyeball and wristwatch and the
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Unix <tt>date</tt> command, or by the <tt>ntpdate</tt> or <tt>ntpd
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-q</tt> commands. The panic threshold can be changed by the <tt>
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tinker panic</tt> command discribed on the <a href="miscopt.htm">
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Miscellaneous Options</a> page. The panic threshold can be disabled
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entirely by the <tt>-g</tt> command line option described on the <a
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href="ntpd.htm">ntpd - Network Time Protocol (NTP) daemon</a>
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page.</p>
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<p>If the difference between local time and server time is less
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than the panic threshold but greater than the step threshold, which
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defaults to 125 ms, the daemon will perform a step adjustment;
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otherwise, it will gradually slew the clock to the nominal time.
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The step threshold can be changed by the <tt>tinker step</tt>
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command discribed on the <a href="miscopt.htm">Miscellaneous
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Options</a> page. The step threshold can be disabled entirely by
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the <tt>-x</tt> command line option described on the <a href=
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"ntpd.htm">ntpd - Network Time Protocol (NTP) daemon</a> page. In
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this case the clock will never be stepped; however, users should
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understand the implications for doing this in a distributed data
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network where all processing must be tightly synchronized. See the
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<a href="leap.htm">NTP Timescale and Leap Seconds</a> page for
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further information. If a step adjustment is made, the clock
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discipline algorithm will start all over again, requiring another
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round of at least four messages as before. This is necessary so
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that all servers and peers operate on the same set of time
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values.</p>
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<p>The clock discipline algorithm is designed to avoid large noise
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spikes that might occur on a congested network or access line. If
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an offset sample exceeds the step threshold, it is ignored and a
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timer started. If a later sample is below the step threshold, the
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counter is reset. However, if the counter is greater than the
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stepout interval, which defaults to 900 s, the next sample will
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step or slew the time as directed. The stepout threshold can be
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changed by the <tt>tinker stepout</tt> command discribed on the <a
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href="miscopt.htm">Miscellaneous Options</a> page.</p>
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<p>If, as discussed later on this page, for some reason the
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hardware clock oscillator frequency error is very large, the time
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errors upon first startup of the daemon may increase over time
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until exceeding the step threshold, which requires another step
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correction. However, due to provisions that reduce vulnerability to
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noise spikes, the second correction will not be done until after
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the stepout threshold. When the frequency error is very large, it
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may take a number of cycles like this until converging on the
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nominal frequency correction. After this, the correction is written
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to the <tt>ntp.drift</tt> file, which is read upon subsequent
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restarts, so the herky-jerky cycles should not recur.</p>
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<h4>Verifying Correct Operation</h4>
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<p>After starting the daemon, run the <tt>ntpq</tt> program using
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the <tt>-n</tt> switch, which will avoid possible distractions due
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to name resolution problems. Use the <tt>pe</tt> command to display
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a billboard showing the status of configured peers and possibly
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other clients poking the daemon. After operating for a few minutes,
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the display should be something like:</p>
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<pre>
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ntpq> pe
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remote refid st t when poll reach delay offset jitter
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=====================================================================
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-isipc6.cairn.ne .GPS1. 1 u 18 64 377 65.592 -5.891 0.044
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+saicpc-isiepc2. pogo.udel.edu 2 u 241 128 370 10.477 -0.117 0.067
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+uclpc.cairn.net pogo.udel.edu 2 u 37 64 177 212.111 -0.551 0.187
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*pogo.udel.edu .GPS1. 1 u 95 128 377 0.607 0.123 0.027
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</pre>
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<p>The host names or addresses shown in the <tt>remote</tt> column
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correspond to the server and peer entries listed in the
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configuration file; however, the DNS names might not agree if the
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names listed are not the canonical DNS names. The <tt>refid</tt>
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column shows the current source of synchronization, while the <tt>
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st</tt> column reveals the stratum, <tt>t</tt> the type (<tt>u</tt>
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= unicast, <tt>m</tt> = multicast, <tt>l</tt> = local, <tt>-</tt> =
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don't know), and <tt>poll</tt> the poll interval in seconds. The
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<tt>when</tt> column shows the time since the peer was last heard
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in seconds, while the <tt>reach</tt> column shows the status of the
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reachability register (see RFC-1305) in octal. The remaining
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entries show the latest delay, offset and jitter in milliseconds.
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Note that in NTP Version 4 what used to be the <tt>dispersion</tt>
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column has been replaced by the <tt>jitter</tt> column.</p>
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<p>The tattletale symbol at the left margin displays the
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synchronization status of each peer. The currently selected peer is
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marked <tt>*</tt>, while additional peers designated acceptable for
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synchronization, but not currently selected, are marked <tt>+</tt>.
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Peers marked <tt>*</tt> and <tt>+</tt> are included in the weighted
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average computation to set the local clock; the data produced by
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peers marked with other symbols are discarded. See the <tt>
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ntpq</tt> page for the meaning of these symbols.</p>
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<p>Additional details for each peer separately can be determined by
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the following procedure. First, use the <tt>as</tt> command to
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display an index of association identifiers, such as</p>
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<pre>
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ntpq> as
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ind assID status conf reach auth condition last_event cnt
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===========================================================
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1 50252 f314 yes yes ok outlyer reachable 1
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2 50253 f414 yes yes ok candidat reachable 1
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3 50254 f414 yes yes ok candidat reachable 1
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4 50255 f614 yes yes ok sys.peer reachable 1
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</pre>
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<p>Each line in this billboard is associated with the corresponding
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line in the <tt>pe</tt> billboard above. The <tt>assID</tt> shows
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the unique identifier for each mobilized association, while the
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<tt>status</tt> column shows the peer status word in hex, as
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defined in the NTP specification. Next, use the <tt>rv</tt> command
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and the respective <tt>assID</tt> identifier to display a detailed
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synopsis for the selected peer, such as</p>
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<pre>
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ntpq> rv 50253
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status=f414 reach, conf, auth, sel_candidat, 1 event, event_reach,
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srcadr=saicpc-isiepc2.cairn.net, srcport=123, dstadr=140.173.1.46,
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dstport=123, keyid=3816249004, stratum=2, precision=-27,
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rootdelay=10.925, rootdispersion=12.848, refid=pogo.udel.edu,
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reftime=bd11b225.133e1437 Sat, Jul 8 2000 13:59:01.075, delay=10.550,
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offset=-1.357, jitter=0.074, dispersion=1.444, reach=377, valid=7,
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hmode=1, pmode=1, hpoll=6, ppoll=7, leap=00, flash=00 ok,
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org=bd11b23c.01385836 Sat, Jul 8 2000 13:59:24.004,
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rec=bd11b23c.02dc8fb8 Sat, Jul 8 2000 13:59:24.011,
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xmt=bd11b21a.ac34c1a8 Sat, Jul 8 2000 13:58:50.672,
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filtdelay= 10.45 10.50 10.63 10.40 10.48 10.43 10.49 11.26,
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filtoffset= -1.18 -1.26 -1.26 -1.35 -1.35 -1.42 -1.54 -1.81,
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filtdisp= 0.51 1.47 2.46 3.45 4.40 5.34 6.33 7.28,
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hostname="miro.time.saic.com", publickey=3171359012, pcookie=0x6629adb2,
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hcookie=0x61f99cdb, initsequence=61, initkey=0x287b649c,
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timestamp=3172053041
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</pre>
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<p>A detailed explanation of the fields in this billboard are
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beyond the scope of this discussion; however, most variables
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defined in the NTP Version 3 specification RFC-1305 are available
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along with others defined for NTP Version 4. This particular
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example was chosen to illustrate probably the most complex
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configuration involving symmetric modes and public-key
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cryptography. As the result of debugging experience, the names and
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values of these variables may change from time to time. An
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explanation of the current set is on the <tt>ntpq</tt> page.</p>
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<p>A useful indicator of miscellaneous problems is the <tt>
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flash</tt> value, which reveals the state of the various sanity
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tests on incoming packets. There are currently eleven bits, one for
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each test, numbered from the right, which is for test 1. If the
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test fails, the corresponding bit is set to one and zero otherwise.
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If any bit is set following each processing step, the packet is
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discarded. The meaning of each test is described on the <tt>
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ntpq</tt> page.</p>
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<p>The three lines identified as <tt>filtdelay</tt>, <tt>
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filtoffset</tt> and <tt>filtdisp</tt> reveal the roundtrip delay,
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clock offset and dispersion for each of the last eight measurement
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rounds, all in milliseconds. Note that the dispersion, which is an
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estimate of the error, increases as the age of the sample
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increases. From these data, it is usually possible to determine the
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incidence of severe packet loss, network congestion, and unstable
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local clock oscillators. There are no hard and fast rules here,
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since every case is unique; however, if one or more of the rounds
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show large values or change radically from one round to another,
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the network is probably congested or lossy.</p>
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<p>Once the daemon has set the local clock, it will continuously
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track the discrepancy between local time and NTP time and adjust
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the local clock accordingly. There are two components of this
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adjustment, time and frequency. These adjustments are automatically
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determined by the clock discipline algorithm, which functions as a
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hybrid phase/frequency feedback loop. The behavior of this
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algorithm is carefully controlled to minimize residual errors due
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to network jitter and frequency variations of the local clock
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hardware oscillator that normally occur in practice. However, when
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started for the first time, the algorithm may take some time to
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converge on the intrinsic frequency error of the host machine.</p>
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<p>The state of the local clock itself can be determined using the
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<tt>rv</tt> command (without the argument), such as</p>
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<pre>
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ntpq> rv
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status=0644 leap_none, sync_ntp, 4 events, event_peer/strat_chg,
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version="ntpd 4.0.99j4-r Fri Jul 7 23:38:17 GMT 2000 (1)",
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processor="i386", system="FreeBSD3.4-RELEASE", leap=00, stratum=2,
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precision=-27, rootdelay=0.552, rootdispersion=12.532, peer=50255,
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refid=pogo.udel.edu,
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reftime=bd11b220.ac89f40a Sat, Jul 8 2000 13:58:56.673, poll=6,
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clock=bd11b225.ee201472 Sat, Jul 8 2000 13:59:01.930, state=4,
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phase=0.179, frequency=44.298, jitter=0.022, stability=0.001,
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hostname="barnstable.udel.edu", publickey=3171372095, params=3171372095,
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refresh=3172016539
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</pre>
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<p>An explanation about most of these variables is in the RFC-1305
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specification. The most useful ones include <tt>clock</tt>, which
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shows when the clock was last adjusted, and <tt>reftime</tt>, which
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shows when the server clock of <tt>refid</tt> was last adjusted.
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The <tt>version</tt>, <tt>processor</tt> and <tt>system</tt> values
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are very helpful when included in bug reports. The mean millisecond
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time offset (<tt>phase</tt>) and deviation (<tt>jitter</tt>)
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monitor the clock quality, while the mean PPM frequency offset
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(<tt>frequency</tt>) and deviation (<tt>stability</tt>) monitor the
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clock stability and serve as a useful diagnostic tool. It has been
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the experience of NTP operators over the years that these data
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represent useful environment and hardware alarms. If the
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motherboard fan freezes up or some hardware bit sticks, the system
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clock is usually the first to notice it.</p>
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<p>Among the new variables added for NTP Version 4 are the <tt>
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hostname</tt>, <tt>publickey</tt>, <tt>params</tt> and <tt>
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refresh</tt>, which are used for the Autokey public-key
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cryptography described on the <a href="authopt.htm">Authentication
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Options</a> page. The values show the filestamps, in NTP seconds,
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that the associated values were created. These are useful in
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diagnosing problems with cryptographic key consistency and ordering
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principles.</p>
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<p>When nothing seems to happen in the <tt>pe</tt> billboard after
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some minutes, there may be a network problem. One common network
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problem is an access controlled router on the path to the selected
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peer or an access controlled server using methods described on the
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<a href="accopt.htm">Access Control Options</a> page. Another
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common problem is that the server is down or running in
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unsynchronized mode due to a local problem. Use the <tt>ntpq</tt>
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program to spy on the server variables in the same way you can spy
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on your own.</p>
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<p>Normally, the daemon will adjust the local clock in small steps
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in such a way that system and user programs are unaware of its
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operation. The adjustment process operates continuously as long as
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the apparent clock error exceeds the step threshold for a period
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longer than the stepout threshold, which for most Internet paths is
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a very rare event. If the event is simply an outlyer due to an
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occasional network delay spike, the correction is simply discarded;
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however, if the apparent time error persists for longer than the
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stepout threshold of about 17 minutes, the local clock is stepped
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or slewed to the new value as directed. This behavior is designed
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to resist errors due to severely congested network paths, as well
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as errors due to confused radio clocks upon the epoch of a leap
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second.</p>
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<h4>Special Problems</h4>
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<p>The frequency tolerance of computer clock oscillators can vary
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widely, which can put a strain on the daemon's ability to
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compensate for the intrinsic frequency error. While the daemon can
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handle frequency errors up to 500 parts-per-million (PPM), or 43
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seconds per day, values much above 100 PPM reduce the headroom and
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increase the time to learn the particular value and record it in
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the <tt>ntp.drift</tt> file. In extreme cases before the particular
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oscillator frequency error has been determined, the residual system
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time offsets can sweep from one extreme to the other of the 128-ms
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tracking window only for the behavior to repeat at 900-s intervals
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until the measurements have converged.</p>
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<p>In order to determine if excessive frequency error is a problem,
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observe the nominal <tt>filtoffset</tt> values for a number of
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rounds and divide by the poll interval. If the result is something
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approaching 500 PPM, there is a good chance that NTP will not work
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properly until the frequency error is reduced by some means. A
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common cause is the hardware time-of-year (TOY) clock chip, which
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must be disabled when NTP disciplines the software clock. For some
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systems this can be done using the <tt><a href="tickadj.htm">
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tickadj</a></tt> utility and the <tt>-s</tt> command line argument.
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For other systems this can be done using a command in the system
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startup file.</p>
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<p>If the TOY chip is not the cause, the problem may be that the
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hardware clock frequency may simply be too slow or two fast. In
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some systems this might require tweaking a trimmer capacitor on the
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motherboard. For other systems the clock frequency can be adjusted
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in increments of 100 PPM using the <tt>tickadj</tt> utility and the
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<tt>-t</tt> command line argument. Note that the <tt>tickadj</tt>
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alters certain kernel variables and, while the utility attempts to
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figure out an acceptable way to do this, there are many cases where
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<tt>tickadj</tt> is incompatible with a running kernel.</p>
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<p>Provisions are included in <tt>ntpd</tt> for access controls
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which deflect unwanted traffic from selected hosts or networks. The
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controls described on the <a href="accopt.htm">Access Control
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Options</a> include detailed packet filter operations based on
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source address and address mask. Normally, filtered packets are
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dropped without notice other than to increment tally counters.
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However, the server can configure to generate what is called a
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kiss-of-death (KOD) packet and send to the client. In case of
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outright access denied, the KOD is the response to the first client
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packet. In this case the client association is permanently disabled
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and the access denied bit (test 4) is set in the flash peer
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variable mentioned above and a message is sent to the system
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log.</p>
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<p>The access control provisions include a limit on the packet rate
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from a host or network. If an incoming packet exceeds the limit, it
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is dropped and a KOD sent to the source. If this occurs after the
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client association has synchronized, the association is not
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disabled, but a message is sent to the system log. See the <a href=
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"accopt.htm">Access Control Options</a> page for further
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informatin.</p>
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<p>In some reported scenarios an access line may show low to
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moderate network delays during some period of the day and moderate
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to high delays during other periods. Often the delay on one
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|
direction of transmission dominates, which can result in large time
|
|
offset errors, sometimes in the range up to a few seconds. It is
|
|
not usually convenient to run <tt>ntpd</tt> throughout the day in
|
|
such scenarios, since this could result in several time steps,
|
|
especially if the condition persists for greater than the stepout
|
|
threshold.</p>
|
|
|
|
<p>The recommended approach in such scenarios is first to calibrate
|
|
the local clock frequency error by running <tt>ntpd</tt> in
|
|
continuous mode during the quiet interval and let it write the
|
|
frequency to the <tt>ntp.drift</tt> file. Then, run <tt>ntpd
|
|
-q</tt> from a cron job each day at some time in the quiet
|
|
interval. In systems with the nanokernel or microkernel performance
|
|
enhancements, including Solaris, Tru64, Linux and FreeBSD, the
|
|
kernel continuously disciplines the frequency so that the residual
|
|
correction produced by <tt>ntpd</tt> is usually less than a few
|
|
milliseconds.</p>
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|
|
|
<h4>Debugging Checklist</h4>
|
|
|
|
If the <tt>ntpq</tt> or <tt>ntpdc</tt> programs do not show that
|
|
messages are being received by the daemon or that received messages
|
|
do not result in correct synchronization, verify the following:
|
|
|
|
<ol>
|
|
<li style="list-style: none"></li>
|
|
|
|
<li>Verify the <tt>/etc/services</tt> 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.</li>
|
|
|
|
<li style="list-style: none"></li>
|
|
|
|
<li>Check the system log for <tt>ntpd</tt> messages about
|
|
configuration errors, name-lookup failures or initialization
|
|
problems.</li>
|
|
|
|
<li style="list-style: none"></li>
|
|
|
|
<li>Verify using <tt>ping</tt> or other utility that packets
|
|
actually do make the round trip between the client and server.
|
|
Verify using <tt>nslookup</tt> or other utility that the DNS server
|
|
names do exist and resolve to valid Internet addresses.</li>
|
|
|
|
<li>Using the <tt>ntpdc</tt> program, verify that the packets
|
|
received and packets sent counters are incrementing. If the sent
|
|
counter does not increment and the configuration file includes
|
|
configured servers, something may be wrong in the host network or
|
|
interface configuration. If this counter does increment, but the
|
|
received counter does not increment, something may be wrong in the
|
|
network or the server NTP daemon may not be running or the server
|
|
itself may be down or not responding.</li>
|
|
|
|
<li style="list-style: none"></li>
|
|
|
|
<li>If both the sent and received counters do increment, but the
|
|
<tt>reach</tt> values in the <tt>pe</tt> billboard with <tt>
|
|
ntpq</tt> continues to show zero, received packets are probably
|
|
being discarded for some reason. If this is the case, the cause
|
|
should be evident from the <tt>flash</tt> variable as discussed
|
|
above and on the <tt>ntpq</tt> page.</li>
|
|
|
|
<li style="list-style: none"></li>
|
|
|
|
<li>If the <tt>reach</tt> values in the <tt>pe</tt> billboard show
|
|
the servers are alive and responding, note the tattletale symbols
|
|
at the left margin, which indicate the status of each server
|
|
resulting from the various grooming and mitigation algorithms. The
|
|
interpretation of these symbols is discussed on the <tt>ntpq</tt>
|
|
page. After a few minutes of operation, one or another of the
|
|
reachable server candidates should show a * tattletale symbol. If
|
|
this doesn't happen, the intersection algorithm, which classifies
|
|
the servers as truechimers or falsetickers, may be unable to find a
|
|
majority of truechimers among the server population.</li>
|
|
|
|
<li style="list-style: none"></li>
|
|
|
|
<li>If all else fails, see the FAQ and/or the discussion and
|
|
briefings at <a href="http://www.eecis.udel.edu/~mills/ntp.htm">
|
|
Network Time Synchronization Project.</a></li>
|
|
</ol>
|
|
|
|
<hr>
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|
<a href="index.htm"><img align="left" src="pic/home.gif" alt=
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|
"gif"></a>
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|
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|
<address><a href="mailto:mills@udel.edu">David L. Mills
|
|
<mills@udel.edu></a></address>
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</body>
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</html>
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