1
MSNTP (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
The canonical NTP code for Unix is the xntp suite, and is as complicated as
20
would be expected from reading RFC 1305. While its code is moderately clean by
21
Unix and C standards, version 3-5.86 is over 80,000 lines and unavoidably
22
system-dependent. A worse problem is that it is so badly fouled up by its
23
configuration mechanism that ab initio porting could be several weeks' work,
24
even if the new system has all the necessary facilities and is bug free!
27
SNTP Servers - Some Little-Known Facts
28
--------------------------------------
30
RFC 2030 states that SNTP clients should be used only at the lowest level,
31
which is good practice. It then states that SNTP servers should be used only
32
at stratum 1 (i.e. top level), which is bizarre! A far saner use of them would
33
be for the very lowest level of server, exporting solely to local clients that
34
do not themselves act as servers to ANY system (e.g. on a Netware server,
35
exporting only to the PCs that it manages).
37
If the NTP network were being run as a directed acyclic graph (i.e. using SNTP
38
rather than full NTP), with a diameter of D links and a maximum error per link
39
of E, the maximum synchronisation error would be D*E. Reasonable figures for D
40
and E are 5 and 0.1 seconds, so this would be adequate for most uses. Note
41
that the fact that the graph is acyclic is critical, which is one reason why
42
SNTP client/servers must NEVER be embedded WITHIN an NTP network.
44
The other reason is that inserting SNTP client/servers at a low stratum (but
45
not the root) of an NTP network could easily break NTP! See RFC 1305 for why,
46
but don't expect the answer to stand out at you. It would be easy to extend
47
MSNTP to a full-function client/server application, thus making it into a true
48
alternative to xntp, but this incompatibility is why it MUST NOT be done.
50
The above does not mean that the SNTP approach is unsatisfactory, but only that
51
it is incompatible with full NTP. The author would favour a complete SNTP
52
network using the SNTP approach, and the statistical error reduction used in
53
MSNTP, but it actually addresses a slightly different problem from that
54
addressed by NTP. TANSTAAFL.
56
FINAL WARNING: do NOT use this program to serve NTP requests from outside the
57
systems that you manage. If you do this, and manage to break the time
58
synchronisation on other people's systems, you will be regarded very
59
unfavourably. Actually, this should be possible only if their NTP client is
60
completely broken, because MSNTP does its damnedest to declare its packets as
61
the lowest form of NTP timestamp.
65
MSNTP and its Assumptions
66
-------------------------
68
MSNTP is intended to be a straightforward SNTP daemon/utility that is easy to
69
build on any reasonable Unix platform (and most near-Unix ones), whether or not
70
it has ever been ported to them before. It is intended to answer the following
71
requirements, either by challenge and response or the less reliable broadcast
74
A simple command to run on Unix systems that will check the time
75
and optionally drift compared with a known, local and reliable NTP
76
time server. No privilege is required just to read the time and
79
A client for Unix systems that will synchronise the time from a known,
80
local and reliable NTP time server. This is probably the most common
81
one, and the need that caused the program to be written.
83
A server for Unix systems that are synchronised other than by NTP
84
methods and that need to synchronise other systems by NTP. This is
85
the classroom of PCs with a central server scenario. It is NOT
86
intended to work as a peer with true NTP servers, and won't.
88
A simple method by which two or more Unix systems can keep themselves
89
synchronised using what is becoming a standard protocol. Yes, I know
90
that there are half-a-dozen other such methods.
92
A base for building non-Unix SNTP clients. Some 3/4 of the code
93
(including all of the complicated algorithms and NTP packet handling)
94
should work, unchanged, on any system with an ANSI/ISO C compiler.
96
There are full tracing facilities and a lot of paranoia in the code to check
97
for bad packets (more than in xntp) which may need relaxing in the light of
98
experience. Unfortunately, RFC 1305 does not include a precise description of
99
the data protocol, despite its length, and there are some internal
100
inconsistencies and differences between it and RFC 2030 and xntp3-5's
103
WARNING: MSNTP has not been tested in conjunction with xntp broadcasts or xntp
104
clients, as the ability to do so was not available to the author. It is very
105
unlikely that it won't work, but you should check. Much of the paranoid code
106
is only partially tested, too, because it is dealing with cases that are very
109
It assumes that the local network is tolerably secure and that any accessible
110
NTP or SNTP servers are trustworthy. It also makes no attempt to check that
111
it has been installed and is being used correctly (e.g. at an appropriate
112
priority) or that the changes it makes have the desired effect. When you first
113
use it, you should both run it in display mode and use the date command as a
116
Furthermore, it does not attempt to solve all of the problems addressed by the
117
NTP protocol and you should NOT use it if any of those problems are likely to
118
cause you serious trouble. If they are, bite the bullet and implement xntp, or
119
buy a fancy time-server.
125
The contents of the distribution are:
128
Copyright - the copyright notice and conditions of use
129
Makefile - the makefile, with comments for several systems
130
header.h - the main header (almost entirely portable)
131
kludges.h - dirty kludges for difficult systems
132
internet.h - a very small header for internet.c and socket.c
133
main.c - most of the source (almost entirely portable)
134
unix.c - just for isatty, sleep and locking
135
internet.c - Internet host and service name lookup
136
socket.c - the Berkeley socket code
137
msntp.1 - the man page
138
RFC2030.TXT - the SNTPv4 specification
140
All you SHOULD need to do is to uncomment the settings in file Makefile for
141
your system or to add new ones. But real life is not always so simple. As
142
POSIX does not yet define sub-second timers, Internet addressing facilities,
143
sockets etc., the code has to rely on the facilities described in the
144
ill-defined and non-standard 'X/Open' documents and the almost totally
145
unspecified 'BSD' extensions.
147
Most hacks should be limited to the compiler options (e.g. setting flags like
148
_XOPEN_SOURCE), but perverse systems may need additions to kludges.h - please
149
report them to the author. See Makefile and kludges.h for documentation on
150
the standard hacks - there only 6, and most are only for obsolete systems.
151
But, generally, using the generic set of C options usually works with no
155
Sick, Bizarre or non-Unix Systems
156
---------------------------------
158
A very few Unix systems and almost all non-Unix systems may need changes to the
161
If the system doesn't have Berkeley sockets, you will need to replace
162
socket.c and possibly modify internet.h and internet.c. All of the
163
systems for which the author needs this have Berkeley sockets.
165
NTP is supposedly an Internet protocol, but is not Internet specific.
166
For other types of network, you will need to replace internet.c and
167
probably modify internet.h.
169
If the system doesn't have gettimeofday or settimeofday, you will
170
need to modify timing.c. If it doesn't have adjtime (e.g. HP-UX
171
on PA-RISC before 10.0), you can set -DADJTIME_MISSING and the code
172
will compile but the -a option will always give an error.
174
If the system has totally broken signal handling, the program will
175
hang or crash if it can't reach its name server or responses time
176
out. You may be able to improve matters by hacking internet.c and
177
socket.c, but don't bet on it.
179
If the the program won't be able to create files in /etc when
180
updating the clock, you can use another lock file or even set
181
-DLOCKFILE=NULL, which will disable the locking code entirely. On
182
systems that have it, using /var/run would be better than /etc.
184
If the the program hangs when flushing outstanding packets (which
185
you can tell by setting -W), it may help to set -DNONBLOCK_BROKEN.
186
This seems needed only for obsolete systems, like Ultrix.
188
If the system isn't Unix, even vaguely, you will probably need to
189
modify all of the above, and unix.c as well.
191
Note that adjtime is commonly sick, but you don't need to change the
192
code - just use the -r option whan making large corrections (see below
195
Any changes needed to header.h or main.c are bugs. They may be bugs in the
196
code or in the compiler or libraries, but they are bugs. Please prod the
197
people responsible and tell the author, who may be able to bypass them cleanly
198
even if they aren't bugs in his code. The code also makes the following
199
assumptions, which would be quite hard to remove:
201
8-bit bytes. Strictly, neither ANSI/ISO C nor POSIX require these,
202
and there were some very early versions of Unix on systems with other
203
byte sizes. But, without a defined sub-byte facility in C, ....
205
At least 32-bit ints. Well, actually, this wouldn't be too hard to
206
remove. But most Unix programs make this assumption, and I have very
207
little interest in the more rudimentary versions of MS-DOS etc.
209
An ANSI/ISO C compiler. It didn't seem worth writing dual-language
210
code in 1996. Tough luck if you haven't got one.
212
Tolerably efficient floating-point arithmetic, with at least 13 digits
213
(decimal), preferably 15, in the mantissa of doubles. Ditto. If you
214
want to port this to a toaster, please accept my insincerest sympathies
217
A trustworthy local network. It does not check for DNS, Ethernet,
218
packet or other spoofing, and assumes that any accessible NTP or SNTP
219
servers are properly synchronised.
222
Warnings about Installation and Use
223
-----------------------------------
225
Anyone attempting to fiddle with the clock on their system should already know
226
how to write system administration scripts, install daemons and so on. There
229
Don't use the broadcast modes unless you really have to, as the
230
client-server modes are far more reliable. The broadcast modes were
231
implemented more for virtuosity (a.k.a. SNTP conformance) than use.
232
In particular, the error estimates are mere guesses, and may be low
233
or even very low. And even reading broadcasts needs privilege.
235
The program is not intended to be installed setuid or setgid, and
236
doing so is asking for trouble. Its ownerships and access modes are
237
not important. It need not be run by root for merely displaying the
238
time (even in daemon mode).
240
The program does not need to run at a high priority (low in Unix
241
terms!) even when being used to set the clock or as a server, except
242
when the '-r' option is used. However, doing so may improve its
245
Unlike NTP, the SNTP protocol contains no protection against
246
client-server loops. If you set one up, your systems will spin
247
themselves off into a disconnected vortex of unreality!
249
It will get very confused if another process changes the local time
250
while it is running. There is some locking code in unix.c to prevent
251
this program doing this to itself, but it will protect only against
252
some errors. However, the remaining failures should be harmless.
254
Don't run it as a server unless you REALLY know what you are doing.
255
It should be used as a server only on a system that is properly
256
synchronised, by fair means or foul. If it isn't, you will simply
257
perpetrate misinformation. And remember that broadcasts are most
258
unpopular with overloaded administrators of overloaded networks.
260
Watch out for multi-server broadcasts and systems with multiple ports
261
onto the same Ethernet; there is some code to protect against this,
262
but it is still easy to get confused.
264
Don't put the lock file onto an automounted partition or delete it by
265
hand, unless you really want to start two daemons at the same time.
266
Both will probably fail horribly if you do this.
268
The daemon save file is checked fairly carefully, but should be in a
269
reasonably safe directory, unless you want hackers to cause trouble.
270
/tmp is safe enough on most systems, but not all - /etc is better.
273
Installing and Using the Program
274
--------------------------------
276
Start by copying the executable and man page to where you want them. If you
277
want only to display the time and as a replacement for the rdate or date
278
commands, the installation is finished!
280
You can use it as a simple unprivileged command to check the time, quite
281
independently of whether it is running as a time-updating daemon or server, or
282
whether you are running xntp. You can run it in daemon mode without updating
283
the clock, to check for drift, but it may fail if the clock is changed under
284
its feet. Unfortunately, you cannot listen to broadcasts without privilege.
286
If it is used with the -a option to keep the time synchronised, it is best to
287
run it as one of root's cron jobs - for many systems, running it once a day
288
should be adequate, but it will depend on the reliability of the local clock.
289
The author runs it this way with -a and -x - see below.
291
If it is used with the -r option to set the time (instead of the rdate or date
292
commands), it should be used interactively and either on a lightly loaded
293
system or at a high priority. You should then check the result by running it
296
You are advised NOT to run it with the -r option in a cron job, though this is
297
not locked out. If you have to (for example under HP-UX before 10.0), be sure
298
to run it as the highest priority that will not cause other system problems and
299
set the maximum automatic change to as low a value as you can get away with.
301
WARNING: adjtime is more than a bit sick on many systems, and will ignore large
302
corrections, usually without any form of hint that it has done so. It is often
303
(even usually) necessary to reset the clock to approximately the right time
304
using the -r option before using the -a and -x options to keep it correct.
306
It can be started as a time-updating daemon with the -a and -x options (or -r
307
and -x if you must), and will perform some limited drift correction. In this
308
case, start it from any suitable system initialisation script and leave it
309
running. Note that it will stop if it thinks that the time difference or drift
310
has got out of control, and you will need to reset the time and restart it by
313
In daemon mode, it will survive its time server or network disappearing for a
314
while, but will eventually fail, and will fail immediately if the network call
315
returns an unexpected error. If this is a problem, you can start it (say,
316
hourly or nightly) from cron, and it will fail if it is already running
317
(provided that you haven't disabled or deleted the lock file).
319
If it is used as a server, it should be started from any suitable system
320
initialisation script, just like any other daemon. It must be started after
321
the networking, of course. To run it in both server modes, start one copy with
322
the -B option and one with the -S option.
325
Simple Examples of Use
326
----------------------
328
Many people use it solely to check the time of their system, especially as a
329
cross-check on xntpd. You do not need privilege and it will not cause trouble
330
to the local network, so you can use it on someone else's system! You can
331
specify one server or several. For example:
333
msntp ntp.server.local ntp.server.neighbour
335
You can use it to check how your system is drifting, but it isn't very good at
336
this if the system is drifting very badly (in which case use the previous
337
technique and dc) or if you are running xntp. You do not need privilege and it
338
will not cause trouble to the local network. For example:
340
msntp -x 120 -f /tmp/msntp.state ntp.server.local
342
More generally, it is used to synchronise the clock, in which case you DO need
343
root privilege. It can be used in many ways, but the author favours running it
344
in daemon mode, started from a cron job, which will restart after power cuts
345
with no attention, and send a mail message (if cron is configured to do that)
346
when it fails badly. For example, the author uses a root crontab entry on one
349
15 0 * * * /bin/nice --10 /usr/local/bin/msntp -a -x 480 ntp.server.local
351
If you have a home computer, it can be set up to resynchronise each time you
352
dial up. For example, the author uses a /etc/ppp/ip-up.d/msntp file on his
353
home Linux system of:
357
/bin/nice --10 /usr/local/sbin/msntp -r -P 60 ntp.server.local
359
-a would be better, but adjtime is broken in Linux.
362
Debugging or Hacking the Program
363
--------------------------------
365
Almost everybody who does this is likely to need to modify only the system
366
interfaces. While they are messy, they are pretty simple and have a simple
367
specification. This is documented in comments in the source. This is
370
The main program SHOULD need no attention, though it may need the odd tweak to
371
bypass compiler problems - please report these, if you encounter any. If
372
something looks odd while it is running, start by setting the -v option (lower
373
case), as for investigating network problems, and checking any diagnostics that
374
appear. Note that most of it can be checked in display mode without harming
377
The client will sometimes give up, complaining about inconsistent timestamps or
378
similar. This can be caused by the server being rebooted and similar glitches
379
to the time - unfortunately, there is no reliable way to tell an ignorable
380
fluctuation from a server up the spout. If this happens annoyingly often,
381
the -V option may help tie down the problem. In actual use, it is simplest
382
just to restart the client in a cron job!
384
If it needs more than this, then you will need to debug the source seriously.
385
Start by putting an icepack on your head and pouring yourself a large whisky!
386
While it is commented, it is not well commented, and much of the code interacts
387
in complex and horrible ways. This isn't so much because it lacks 'structure'
388
as because one part needs to make assumptions about the numerical properties of
391
The -W option (upper case) will print out a complete trace of everything it
392
does, and this should be enough to tie down the problem. It does distort the
393
timing a bit, but not usually too badly. However, wading through that amount
394
of gibberish (let alone looking at the source) is not a pleasant task. If you
395
are pretty sure that you have a bug, you may tell the author, and he may ask
396
for a copy of the output - but he will reply rudely if you send thousands of
397
lines of tracing to him by Email!
399
Note that there are a fair number of circumstances where its error recovery
400
could be better, but is left as it is to keep the code simple. Most of these
401
should be pretty rare.
404
Changes in Version 1.2
405
----------------------
407
The main change was the addition of the daemon mode for drift correction (i.e.
408
the -x option). The daemon code is complex and has a lot of special-casing for
409
strange circumstances, not all of which are testable in practice.
411
A lot of the code was reordered while doing this. The output was slightly
412
different - considerably different with -V.
414
The error estimation for broadcasts was modified, and should bear more relation
415
to reality. It remains a guess, as there is no way to get decent error error
416
estimates under such circumstances.
418
The -B option is now in minutes, and has a different permissible range and
421
The argument consistency checking for broadcasts was tightened up a bit, and a
422
few other internal checks added. These should not affect any reasonable
425
A couple of new functions were added to the portability base, but they don't
426
use any non-standard new facilities. However, the specification of the
427
functions has changed slightly.
430
Changes in Version 1.3
431
----------------------
433
The main change was the addition of the restarting facility for daemon mode
434
(i.e. the -f option), which is pretty straightforward.
436
There were also a lot of minor changes to the paranoia code in daemon mode, to
437
try to separate out the case of a demented server from network and other
438
'ignorable' problems. These are not entirely successful.
441
Changes in Version 1.4 and 1.5
442
------------------------------
444
There turned out to be a couple of places where the author misunderstood the
445
specification of NTP, which affect only its use in server mode. The main
446
change is to use stratum 15 instead of stratum 0.
448
And there were some more relaxations of the paranoia code, to allow for more
449
erratic servers, plus a kludge to improve restarting in daemon mode after a
450
period of down time has unsynchronised the clock. There is also an
451
incompatible change to the debugging options to add a new level - the old -V
452
option is now -W, and -V is an intermediate one for debugging daemon mode - but
453
they are both hacker's facilities, and not for normal use.
455
Version 1.5 adds some very minor fixes.
458
Changes in Version 1.6
459
----------------------
461
The first change is support for multiple server addresses - it uses these in a
462
round-robin fashion. This may be useful when you have access to several
463
servers, all of which are a bit iffy. This means that the restart file format
464
is incompatible with msntp 1.5.
466
It has also been modified to reset itself automatically after detecting an
467
inconsistency in its server's timestamps, because the author got sick of the
468
failures. It writes a comment to syslog (uniquely) in such cases.
470
The ability to query a daemon save file was added.
472
Related to the above, the -E argument has been redefined to mean an error bound
473
on various internal times (which is what it had become, anyway) and a -P option
474
introduced to be what the -E argument was documented to be.
476
The lock and save file handling have been changed to allow defaults to be set
477
at installation time, and to be overridable at run-time. To disable these
478
at either stage, simply set the file names to the null string.
480
And there have been the usual changes for portability, as standards have been
481
modified and/or introduced.
487
There are unlikely to be any, except probably one to fix bugs in version 1.6.
489
I attempted to put support for intermittent connexions (e.g. dial-up) into the
490
daemon mode, but doing so needs so much code reorganisation that it isn't worth
491
it. What needs doing for that is to separate the socket handling from the
492
timekeeping, so that they can be run asynchronously (either in separate
493
processes or threads), and to look up a network name and open a socket only
494
when prodded (and to close it immediately thereafter). So just running it
495
with the -r option is the current best solution.
497
I also attempted to put support for the "Unix 2000" interfaces into the code.
498
Ha, ha. Not merely do very few systems define socklen_t (needed for IPv6
499
support), but "Unix 2000" neither addresses the leap second problem nor even
500
provides an adjtime replacement! Some function like the latter is critical,
501
not so much because of the gradual change, but because of its atomicity;
502
without it, msntp really needs to be made non-interruptible, and that brings in
503
a ghastly number of system-dependencies.
505
Realistically, it needs a complete rewrite before adding any more function.
506
And, worse, the Unix 'standards' need fixing, too.
513
Thanks are due to Douglas M. Wells of Connection Technologies for helping the
514
author with several IP-related conventions, to Sam Nelson of Stirling
515
University for testing it on some very strange systems, and to David Mills for
516
clarifying what the NTP specification really is.
518
Thanks are also due to several other people with locating bugs, finding
519
appropriate options for the Makefile and passing on extension code and
520
suggestions. As I am sure to leave someone out, I shall not name anyone else.
522
Version 1.0 - October 1996.
523
Version 1.1 - November 1996 - mainly portability improvements.
524
Version 1.2 - January 1997 - mainly drift handling, but much reorganisation.
525
Version 1.3 - February 1997 - daemon save file, and some robustness changes.
526
Version 1.4 - May 1997 - relatively minor fixes, more diagnostic levels etc.
527
Version 1.5 - December 1997 - some very minor fixes
528
Version 1.6 - October 2000 - quite a few miscellaneous changes
532
University of Cambridge Computer Laboratory,
533
New Museums Site, Pembroke Street, Cambridge CB2 3QG, England.
534
Email: nmm1@cam.ac.uk
535
Tel.: +44 1223 334761 Fax: +44 1223 334679
added
removed
sntp/README
