1 SNTP (Simple Network Time Protocol Utility) - Version 1.6
2 ----------------------------------------------------------
4 Please read the file Copyright first. Also note that the file RFC2030.TXT is
5 David Mills's copyright and not the author's - it is just a copy of the RFC
6 that is available from so many Internet archives.
8 RFC 1305 (Network Time Protocol - NTP) is an attempt to provide globally
9 consistent timestamps in an extremely hostile environment; it is fiendishly
10 complicated and an impressive piece of virtuosity. RFC 2030 (Simple Network
11 Time Protocol - SNTP) which supersedes RFC 1769 describes a subset of this that
12 will give excellent accuracy in most environments encountered in practice; it
13 uses only the obvious algorithms that have been used since time immemorial.
15 WARNING: the text version of RFC 1305 is incomplete, and omits the tables that
16 are in the Postscript version. Unfortunately, these contain the only copy of
17 some critical information.
19 draft-mills-sntp-v4-00.txt is the next proposed revision of RFC 2030,
20 and the current goal is to have this code implement that specification.
22 SNTP Servers - Some Little-Known Facts
23 --------------------------------------
25 RFC 2030 states that SNTP clients should be used only at the lowest level,
26 which is good practice. It then states that SNTP servers should be used only
27 at stratum 1 (i.e. top level), which is bizarre! A far saner use of them would
28 be for the very lowest level of server, exporting solely to local clients that
29 do not themselves act as servers to ANY system (e.g. on a Netware server,
30 exporting only to the PCs that it manages).
32 [There is missing language in the previous paragraph. SNTP is designed
33 to be used in 2 cases: as a client at the lowest levels of the timing
34 hierarchy, or as a server of last resort at stratum 1 when connected to
35 a modem or radio clock.]
37 [This is as far as I have updated this file as part of the upgrade.]
39 If the NTP network were being run as a directed acyclic graph (i.e. using SNTP
40 rather than full NTP), with a diameter of D links and a maximum error per link
41 of E, the maximum synchronisation error would be D*E. Reasonable figures for D
42 and E are 5 and 0.1 seconds, so this would be adequate for most uses. Note
43 that the fact that the graph is acyclic is critical, which is one reason why
44 SNTP client/servers must NEVER be embedded WITHIN an NTP network.
46 The other reason is that inserting SNTP client/servers at a low stratum (but
47 not the root) of an NTP network could easily break NTP! See RFC 1305 for why,
48 but don't expect the answer to stand out at you. It would be easy to extend
49 SNTP to a full-function client/server application, thus making it into a true
50 alternative to ntp, but this incompatibility is why it MUST NOT be done.
52 The above does not mean that the SNTP approach is unsatisfactory, but only that
53 it is incompatible with full NTP. The author would favour a complete SNTP
54 network using the SNTP approach, and the statistical error reduction used in
55 SNTP, but it actually addresses a slightly different problem from that
56 addressed by NTP. TANSTAAFL.
58 FINAL WARNING: do NOT use this program to serve NTP requests from outside the
59 systems that you manage. If you do this, and manage to break the time
60 synchronisation on other people's systems, you will be regarded very
61 unfavourably. Actually, this should be possible only if their NTP client is
62 completely broken, because SNTP does its damnedest to declare its packets as
63 the lowest form of NTP timestamp.
67 SNTP and its Assumptions
68 -------------------------
70 SNTP is intended to be a straightforward SNTP daemon/utility that is easy to
71 build on any reasonable Unix platform (and most near-Unix ones), whether or not
72 it has ever been ported to them before. It is intended to answer the following
73 requirements, either by challenge and response or the less reliable broadcast
76 A simple command to run on Unix systems that will check the time
77 and optionally drift compared with a known, local and reliable NTP
78 time server. No privilege is required just to read the time and
81 A client for Unix systems that will synchronise the time from a known,
82 local and reliable NTP time server. This is probably the most common
83 one, and the need that caused the program to be written.
85 A server for Unix systems that are synchronised other than by NTP
86 methods and that need to synchronise other systems by NTP. This is
87 the classroom of PCs with a central server scenario. It is NOT
88 intended to work as a peer with true NTP servers, and won't.
90 A simple method by which two or more Unix systems can keep themselves
91 synchronised using what is becoming a standard protocol. Yes, I know
92 that there are half-a-dozen other such methods.
94 A base for building non-Unix SNTP clients. Some 3/4 of the code
95 (including all of the complicated algorithms and NTP packet handling)
96 should work, unchanged, on any system with an ANSI/ISO C compiler.
98 There are full tracing facilities and a lot of paranoia in the code to check
99 for bad packets (more than in ntp) which may need relaxing in the light of
100 experience. Unfortunately, RFC 1305 does not include a precise description of
101 the data protocol, despite its length, and there are some internal
102 inconsistencies and differences between it and RFC 2030 and ntp's behaviour.
104 WARNING: SNTP has not been tested in conjunction with ntp broadcasts or ntp
105 clients, as the ability to do so was not available to the author. It is very
106 unlikely that it won't work, but you should check. Much of the paranoid code
107 is only partially tested, too, because it is dealing with cases that are very
110 It assumes that the local network is tolerably secure and that any accessible
111 NTP or SNTP servers are trustworthy. It also makes no attempt to check that
112 it has been installed and is being used correctly (e.g. at an appropriate
113 priority) or that the changes it makes have the desired effect. When you first
114 use it, you should both run it in display mode and use the date command as a
117 Furthermore, it does not attempt to solve all of the problems addressed by the
118 NTP protocol and you should NOT use it if any of those problems are likely to
119 cause you serious trouble. If they are, bite the bullet and implement ntp, or
120 buy a fancy time-server.
126 The contents of the distribution are:
129 Copyright - the copyright notice and conditions of use
130 Makefile - the makefile, with comments for several systems
131 header.h - the main header (almost entirely portable)
132 kludges.h - dirty kludges for difficult systems
133 internet.h - a very small header for internet.c and socket.c
134 main.c - most of the source (almost entirely portable)
135 unix.c - just for isatty, sleep and locking
136 internet.c - Internet host and service name lookup
137 socket.c - the Berkeley socket code
138 sntp.1 - the man page
139 RFC2030.TXT - the SNTPv4 specification
141 All you SHOULD need to do is to uncomment the settings in file Makefile for
142 your system or to add new ones. But real life is not always so simple. As
143 POSIX does not yet define sub-second timers, Internet addressing facilities,
144 sockets etc., the code has to rely on the facilities described in the
145 ill-defined and non-standard 'X/Open' documents and the almost totally
146 unspecified 'BSD' extensions.
148 Most hacks should be limited to the compiler options (e.g. setting flags like
149 _XOPEN_SOURCE), but perverse systems may need additions to kludges.h - please
150 report them to the author. See Makefile and kludges.h for documentation on
151 the standard hacks - there only 6, and most are only for obsolete systems.
152 But, generally, using the generic set of C options usually works with no
156 Sick, Bizarre or non-Unix Systems
157 ---------------------------------
159 A very few Unix systems and almost all non-Unix systems may need changes to the
162 If the system doesn't have Berkeley sockets, you will need to replace
163 socket.c and possibly modify internet.h and internet.c. All of the
164 systems for which the author needs this have Berkeley sockets.
166 NTP is supposedly an Internet protocol, but is not Internet specific.
167 For other types of network, you will need to replace internet.c and
168 probably modify internet.h.
170 If the system doesn't have gettimeofday or settimeofday, you will
171 need to modify timing.c. If it doesn't have adjtime (e.g. HP-UX
172 on PA-RISC before 10.0), you can set -DADJTIME_MISSING and the code
173 will compile but the -a option will always give an error.
175 If the system has totally broken signal handling, the program will
176 hang or crash if it can't reach its name server or responses time
177 out. You may be able to improve matters by hacking internet.c and
178 socket.c, but don't bet on it.
180 If the the program won't be able to create files in /etc when
181 updating the clock, you can use another lock file or even set
182 -DLOCKFILE=NULL, which will disable the locking code entirely. On
183 systems that have it, using /var/run would be better than /etc.
185 If the the program hangs when flushing outstanding packets (which
186 you can tell by setting -W), it may help to set -DNONBLOCK_BROKEN.
187 This seems needed only for obsolete systems, like Ultrix.
189 If the system isn't Unix, even vaguely, you will probably need to
190 modify all of the above, and unix.c as well.
192 Note that adjtime is commonly sick, but you don't need to change the
193 code - just use the -r option whan making large corrections (see below
196 Any changes needed to header.h or main.c are bugs. They may be bugs in the
197 code or in the compiler or libraries, but they are bugs. Please prod the
198 people responsible and tell the author, who may be able to bypass them cleanly
199 even if they aren't bugs in his code. The code also makes the following
200 assumptions, which would be quite hard to remove:
202 8-bit bytes. Strictly, neither ANSI/ISO C nor POSIX require these,
203 and there were some very early versions of Unix on systems with other
204 byte sizes. But, without a defined sub-byte facility in C, ....
206 At least 32-bit ints. Well, actually, this wouldn't be too hard to
207 remove. But most Unix programs make this assumption, and I have very
208 little interest in the more rudimentary versions of MS-DOS etc.
210 An ANSI/ISO C compiler. It didn't seem worth writing dual-language
211 code in 1996. Tough luck if you haven't got one.
213 Tolerably efficient floating-point arithmetic, with at least 13 digits
214 (decimal), preferably 15, in the mantissa of doubles. Ditto. If you
215 want to port this to a toaster, please accept my insincerest sympathies
218 A trustworthy local network. It does not check for DNS, Ethernet,
219 packet or other spoofing, and assumes that any accessible NTP or SNTP
220 servers are properly synchronised.
223 Warnings about Installation and Use
224 -----------------------------------
226 Anyone attempting to fiddle with the clock on their system should already know
227 how to write system administration scripts, install daemons and so on. There
230 Don't use the broadcast modes unless you really have to, as the
231 client-server modes are far more reliable. The broadcast modes were
232 implemented more for virtuosity (a.k.a. SNTP conformance) than use.
233 In particular, the error estimates are mere guesses, and may be low
234 or even very low. And even reading broadcasts needs privilege.
236 The program is not intended to be installed setuid or setgid, and
237 doing so is asking for trouble. Its ownerships and access modes are
238 not important. It need not be run by root for merely displaying the
239 time (even in daemon mode).
241 The program does not need to run at a high priority (low in Unix
242 terms!) even when being used to set the clock or as a server, except
243 when the '-r' option is used. However, doing so may improve its
246 Unlike NTP, the SNTP protocol contains no protection against
247 client-server loops. If you set one up, your systems will spin
248 themselves off into a disconnected vortex of unreality!
250 It will get very confused if another process changes the local time
251 while it is running. There is some locking code in unix.c to prevent
252 this program doing this to itself, but it will protect only against
253 some errors. However, the remaining failures should be harmless.
255 Don't run it as a server unless you REALLY know what you are doing.
256 It should be used as a server only on a system that is properly
257 synchronised, by fair means or foul. If it isn't, you will simply
258 perpetrate misinformation. And remember that broadcasts are most
259 unpopular with overloaded administrators of overloaded networks.
261 Watch out for multi-server broadcasts and systems with multiple ports
262 onto the same Ethernet; there is some code to protect against this,
263 but it is still easy to get confused.
265 Don't put the lock file onto an automounted partition or delete it by
266 hand, unless you really want to start two daemons at the same time.
267 Both will probably fail horribly if you do this.
269 The daemon save file is checked fairly carefully, but should be in a
270 reasonably safe directory, unless you want hackers to cause trouble.
271 /tmp is safe enough on most systems, but not all - /etc is better.
274 Installing and Using the Program
275 --------------------------------
277 Start by copying the executable and man page to where you want them. If you
278 want only to display the time and as a replacement for the rdate or date
279 commands, the installation is finished!
281 You can use it as a simple unprivileged command to check the time, quite
282 independently of whether it is running as a time-updating daemon or server, or
283 whether you are running ntp. You can run it in daemon mode without updating
284 the clock, to check for drift, but it may fail if the clock is changed under
285 its feet. Unfortunately, you cannot listen to broadcasts without privilege.
287 If it is used with the -a option to keep the time synchronised, it is best to
288 run it as one of root's cron jobs - for many systems, running it once a day
289 should be adequate, but it will depend on the reliability of the local clock.
290 The author runs it this way with -a and -x - see below.
292 If it is used with the -r option to set the time (instead of the rdate or date
293 commands), it should be used interactively and either on a lightly loaded
294 system or at a high priority. You should then check the result by running it
297 You are advised NOT to run it with the -r option in a cron job, though this is
298 not locked out. If you have to (for example under HP-UX before 10.0), be sure
299 to run it as the highest priority that will not cause other system problems and
300 set the maximum automatic change to as low a value as you can get away with.
302 WARNING: adjtime is more than a bit sick on many systems, and will ignore large
303 corrections, usually without any form of hint that it has done so. It is often
304 (even usually) necessary to reset the clock to approximately the right time
305 using the -r option before using the -a and -x options to keep it correct.
307 It can be started as a time-updating daemon with the -a and -x options (or -r
308 and -x if you must), and will perform some limited drift correction. In this
309 case, start it from any suitable system initialisation script and leave it
310 running. Note that it will stop if it thinks that the time difference or drift
311 has got out of control, and you will need to reset the time and restart it by
314 In daemon mode, it will survive its time server or network disappearing for a
315 while, but will eventually fail, and will fail immediately if the network call
316 returns an unexpected error. If this is a problem, you can start it (say,
317 hourly or nightly) from cron, and it will fail if it is already running
318 (provided that you haven't disabled or deleted the lock file).
320 If it is used as a server, it should be started from any suitable system
321 initialisation script, just like any other daemon. It must be started after
322 the networking, of course. To run it in both server modes, start one copy with
323 the -B option and one with the -S option.
326 Simple Examples of Use
327 ----------------------
329 Many people use it solely to check the time of their system, especially as a
330 cross-check on ntpd. You do not need privilege and it will not cause trouble
331 to the local network, so you can use it on someone else's system! You can
332 specify one server or several. For example:
334 msntp ntp.server.local ntp.server.neighbour
336 You can use it to check how your system is drifting, but it isn't very good at
337 this if the system is drifting very badly (in which case use the previous
338 technique and dc) or if you are running ntp. You do not need privilege and it
339 will not cause trouble to the local network. For example:
341 sntp -x 120 -f /tmp/msntp.state ntp.server.local
343 More generally, it is used to synchronise the clock, in which case you DO need
344 root privilege. It can be used in many ways, but the author favours running it
345 in daemon mode, started from a cron job, which will restart after power cuts
346 with no attention, and send a mail message (if cron is configured to do that)
347 when it fails badly. For example, the author uses a root crontab entry on one
350 15 0 * * * /bin/nice --10 /usr/local/bin/sntp -a -x 480 ntp.server.local
352 If you have a home computer, it can be set up to resynchronise each time you
353 dial up. For example, the author uses a /etc/ppp/ip-up.d/sntp file on his
354 home Linux system of:
358 /bin/nice --10 /usr/local/sbin/sntp -r -P 60 ntp.server.local
360 -a would be better, but adjtime is broken in Linux.
363 Debugging or Hacking the Program
364 --------------------------------
366 Almost everybody who does this is likely to need to modify only the system
367 interfaces. While they are messy, they are pretty simple and have a simple
368 specification. This is documented in comments in the source. This is
371 The main program SHOULD need no attention, though it may need the odd tweak to
372 bypass compiler problems - please report these, if you encounter any. If
373 something looks odd while it is running, start by setting the -v option (lower
374 case), as for investigating network problems, and checking any diagnostics that
375 appear. Note that most of it can be checked in display mode without harming
378 The client will sometimes give up, complaining about inconsistent timestamps or
379 similar. This can be caused by the server being rebooted and similar glitches
380 to the time - unfortunately, there is no reliable way to tell an ignorable
381 fluctuation from a server up the spout. If this happens annoyingly often,
382 the -V option may help tie down the problem. In actual use, it is simplest
383 just to restart the client in a cron job!
385 If it needs more than this, then you will need to debug the source seriously.
386 Start by putting an icepack on your head and pouring yourself a large whisky!
387 While it is commented, it is not well commented, and much of the code interacts
388 in complex and horrible ways. This isn't so much because it lacks 'structure'
389 as because one part needs to make assumptions about the numerical properties of
392 The -W option (upper case) will print out a complete trace of everything it
393 does, and this should be enough to tie down the problem. It does distort the
394 timing a bit, but not usually too badly. However, wading through that amount
395 of gibberish (let alone looking at the source) is not a pleasant task. If you
396 are pretty sure that you have a bug, you may tell the author, and he may ask
397 for a copy of the output - but he will reply rudely if you send thousands of
398 lines of tracing to him by Email!
400 Note that there are a fair number of circumstances where its error recovery
401 could be better, but is left as it is to keep the code simple. Most of these
402 should be pretty rare.
405 Changes in Version 1.2
406 ----------------------
408 The main change was the addition of the daemon mode for drift correction (i.e.
409 the -x option). The daemon code is complex and has a lot of special-casing for
410 strange circumstances, not all of which are testable in practice.
412 A lot of the code was reordered while doing this. The output was slightly
413 different - considerably different with -V.
415 The error estimation for broadcasts was modified, and should bear more relation
416 to reality. It remains a guess, as there is no way to get decent error error
417 estimates under such circumstances.
419 The -B option is now in minutes, and has a different permissible range and
422 The argument consistency checking for broadcasts was tightened up a bit, and a
423 few other internal checks added. These should not affect any reasonable
426 A couple of new functions were added to the portability base, but they don't
427 use any non-standard new facilities. However, the specification of the
428 functions has changed slightly.
431 Changes in Version 1.3
432 ----------------------
434 The main change was the addition of the restarting facility for daemon mode
435 (i.e. the -f option), which is pretty straightforward.
437 There were also a lot of minor changes to the paranoia code in daemon mode, to
438 try to separate out the case of a demented server from network and other
439 'ignorable' problems. These are not entirely successful.
442 Changes in Version 1.4 and 1.5
443 ------------------------------
445 There turned out to be a couple of places where the author misunderstood the
446 specification of NTP, which affect only its use in server mode. The main
447 change is to use stratum 15 instead of stratum 0.
449 And there were some more relaxations of the paranoia code, to allow for more
450 erratic servers, plus a kludge to improve restarting in daemon mode after a
451 period of down time has unsynchronised the clock. There is also an
452 incompatible change to the debugging options to add a new level - the old -V
453 option is now -W, and -V is an intermediate one for debugging daemon mode - but
454 they are both hacker's facilities, and not for normal use.
456 Version 1.5 adds some very minor fixes.
459 Changes in Version 1.6
460 ----------------------
462 The first change is support for multiple server addresses - it uses these in a
463 round-robin fashion. This may be useful when you have access to several
464 servers, all of which are a bit iffy. This means that the restart file format
465 is incompatible with msntp 1.5.
467 It has also been modified to reset itself automatically after detecting an
468 inconsistency in its server's timestamps, because the author got sick of the
469 failures. It writes a comment to syslog (uniquely) in such cases.
471 The ability to query a daemon save file was added.
473 Related to the above, the -E argument has been redefined to mean an error bound
474 on various internal times (which is what it had become, anyway) and a -P option
475 introduced to be what the -E argument was documented to be.
477 The lock and save file handling have been changed to allow defaults to be set
478 at installation time, and to be overridable at run-time. To disable these
479 at either stage, simply set the file names to the null string.
481 And there have been the usual changes for portability, as standards have been
482 modified and/or introduced.
488 There are unlikely to be any, except probably one to fix bugs in version 1.6.
490 I attempted to put support for intermittent connexions (e.g. dial-up) into the
491 daemon mode, but doing so needs so much code reorganisation that it isn't worth
492 it. What needs doing for that is to separate the socket handling from the
493 timekeeping, so that they can be run asynchronously (either in separate
494 processes or threads), and to look up a network name and open a socket only
495 when prodded (and to close it immediately thereafter). So just running it
496 with the -r option is the current best solution.
498 I also attempted to put support for the "Unix 2000" interfaces into the code.
499 Ha, ha. Not merely do very few systems define socklen_t (needed for IPv6
500 support), but "Unix 2000" neither addresses the leap second problem nor even
501 provides an adjtime replacement! Some function like the latter is critical,
502 not so much because of the gradual change, but because of its atomicity;
503 without it, msntp really needs to be made non-interruptible, and that brings in
504 a ghastly number of system-dependencies.
506 Realistically, it needs a complete rewrite before adding any more function.
507 And, worse, the Unix 'standards' need fixing, too.
514 Thanks are due to Douglas M. Wells of Connection Technologies for helping the
515 author with several IP-related conventions, to Sam Nelson of Stirling
516 University for testing it on some very strange systems, and to David Mills for
517 clarifying what the NTP specification really is.
519 Thanks are also due to several other people with locating bugs, finding
520 appropriate options for the Makefile and passing on extension code and
521 suggestions. As I am sure to leave someone out, I shall not name anyone else.
523 Version 1.0 - October 1996.
524 Version 1.1 - November 1996 - mainly portability improvements.
525 Version 1.2 - January 1997 - mainly drift handling, but much reorganisation.
526 Version 1.3 - February 1997 - daemon save file, and some robustness changes.
527 Version 1.4 - May 1997 - relatively minor fixes, more diagnostic levels etc.
528 Version 1.5 - December 1997 - some very minor fixes
529 Version 1.6 - October 2000 - quite a few miscellaneous changes
533 University of Cambridge Computer Laboratory,
534 New Museums Site, Pembroke Street, Cambridge CB2 3QG, England.
535 Email: nmm1@cam.ac.uk
536 Tel.: +44 1223 334761 Fax: +44 1223 334679
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