readme.msntp

linux下的获取Internet time的程式

MSNTP (Simple Network Time Protocol Utility) - Version 1.6
----------------------------------------------------------

Please read the file Copyright.msntp first.

RFC 1305 (Network Time Protocol - NTP) is an attempt to provide globally
consistent timestamps in an extremely hostile environment; it is fiendishly
complicated and an impressive piece of virtuosity.  RFC 2030 (Simple Network
Time Protocol - SNTP) which supersedes RFC 1769 describes a subset of this that
will give excellent accuracy in most environments encountered in practice; it
uses only the obvious algorithms that have been used since time immemorial.

WARNING: the text version of RFC 1305 is incomplete, and omits the tables that
are in the Postscript version.  Unfortunately, these contain the only copy of
some critical information.

The canonical NTP code for Unix is the xntp suite, and is as complicated as
would be expected from reading RFC 1305.  While its code is moderately clean by
Unix and C standards, version 3-5.86 is over 80,000 lines and unavoidably
system-dependent.  A worse problem is that it is so badly fouled up by its
configuration mechanism that ab initio porting could be several weeks' work,
even if the new system has all the necessary facilities and is bug free!


SNTP Servers - Some Little-Known Facts
--------------------------------------

RFC 2030 states that SNTP clients should be used only at the lowest level,
which is good practice.  It then states that SNTP servers should be used only
at stratum 1 (i.e. top level), which is bizarre!  A far saner use of them would
be for the very lowest level of server, exporting solely to local clients that
do not themselves act as servers to ANY system (e.g. on a Netware server,
exporting only to the PCs that it manages).

If the NTP network were being run as a directed acyclic graph (i.e. using SNTP
rather than full NTP), with a diameter of D links and a maximum error per link
of E, the maximum synchronisation error would be D*E.  Reasonable figures for D
and E are 5 and 0.1 seconds, so this would be adequate for most uses.  Note
that the fact that the graph is acyclic is critical, which is one reason why
SNTP client/servers must NEVER be embedded WITHIN an NTP network.

The other reason is that inserting SNTP client/servers at a low stratum (but
not the root) of an NTP network could easily break NTP!  See RFC 1305 for why,
but don't expect the answer to stand out at you.  It would be easy to extend
MSNTP to a full-function client/server application, thus making it into a true
alternative to xntp, but this incompatibility is why it MUST NOT be done.

The above does not mean that the SNTP approach is unsatisfactory, but only that
it is incompatible with full NTP.  The author would favour a complete SNTP
network using the SNTP approach, and the statistical error reduction used in
MSNTP, but it actually addresses a slightly different problem from that
addressed by NTP.  TANSTAAFL.

FINAL WARNING: do NOT use this program to serve NTP requests from outside the
systems that you manage.  If you do this, and manage to break the time
synchronisation on other people's systems, you will be regarded very
unfavourably.  Actually, this should be possible only if their NTP client is
completely broken, because MSNTP does its damnedest to declare its packets as
the lowest form of NTP timestamp.



MSNTP and its Assumptions
-------------------------

MSNTP is intended to be a straightforward SNTP daemon/utility that is easy to
build on any reasonable Unix platform (and most near-Unix ones), whether or not
it has ever been ported to them before.  It is intended to answer the following
requirements, either by challenge and response or the less reliable broadcast
method:

    A simple command to run on Unix systems that will check the time
    and optionally drift compared with a known, local and reliable NTP
    time server.  No privilege is required just to read the time and
    estimate the drift.

    A client for Unix systems that will synchronise the time from a known,
    local and reliable NTP time server.  This is probably the most common
    one, and the need that caused the program to be written.

    A server for Unix systems that are synchronised other than by NTP
    methods and that need to synchronise other systems by NTP.  This is
    the classroom of PCs with a central server scenario.  It is NOT
    intended to work as a peer with true NTP servers, and won't.

    A simple method by which two or more Unix systems can keep themselves
    synchronised using what is becoming a standard protocol.  Yes, I know
    that there are half-a-dozen other such methods.

    A base for building non-Unix SNTP clients.  Some 3/4 of the code
    (including all of the complicated algorithms and NTP packet handling)
    should work, unchanged, on any system with an ANSI/ISO C compiler.

There are full tracing facilities and a lot of paranoia in the code to check
for bad packets (more than in xntp) which may need relaxing in the light of
experience.  Unfortunately, RFC 1305 does not include a precise description of
the data protocol, despite its length, and there are some internal
inconsistencies and differences between it and RFC 2030 and xntp3-5's
behaviour.

WARNING: MSNTP has not been tested in conjunction with xntp broadcasts or xntp
clients, as the ability to do so was not available to the author.  It is very
unlikely that it won't work, but you should check.  Much of the paranoid code
is only partially tested, too, because it is dealing with cases that are very
hard to provoke.

It assumes that the local network is tolerably secure and that any accessible
NTP or SNTP servers are trustworthy.  It also makes no attempt to check that
it has been installed and is being used correctly (e.g. at an appropriate
priority) or that the changes it makes have the desired effect.  When you first 
use it, you should both run it in display mode and use the date command as a
cross-check.

Furthermore, it does not attempt to solve all of the problems addressed by the
NTP protocol and you should NOT use it if any of those problems are likely to
cause you serious trouble.  If they are, bite the bullet and implement xntp, or
buy a fancy time-server.


Building SNTP
-------------

All you SHOULD need to do is to uncomment the settings in file Makefile for
your system or to add new ones.  But real life is not always so simple.  As
POSIX does not yet define sub-second timers, Internet addressing facilities,
sockets etc., the code has to rely on the facilities described in the
ill-defined and non-standard 'X/Open' documents and the almost totally
unspecified 'BSD' extensions.

Most hacks should be limited to the compiler options (e.g. setting flags like
_XOPEN_SOURCE), but perverse systems may need additions to kludges.h - please
report them to the author.  See Makefile and kludges.h for documentation on
the standard hacks - there only 6, and most are only for obsolete systems.
But, generally, using the generic set of C options usually works with no
further ado.


Sick, Bizarre or non-Unix Systems
---------------------------------

A very few Unix systems and almost all non-Unix systems may need changes to the
code, such as:

    If the system doesn't have Berkeley sockets, you will need to replace
    socket.c and possibly modify internet.h and internet.c.  All of the
    systems for which the author needs this have Berkeley sockets.

    NTP is supposedly an Internet protocol, but is not Internet specific.
    For other types of network, you will need to replace internet.c and
    probably modify internet.h.

    If the system doesn't have gettimeofday or settimeofday, you will
    need to modify timing.c.  If it doesn't have adjtime (e.g. HP-UX
    on PA-RISC before 10.0), you can set -DADJTIME_MISSING and the code
    will compile but the -a option will always give an error.

    If the system has totally broken signal handling, the program will
    hang or crash if it can't reach its name server or responses time
    out.  You may be able to improve matters by hacking internet.c and
    socket.c, but don't bet on it.

    If the the program won't be able to create files in /etc when
    updating the clock, you can use another lock file or even set
    -DLOCKFILE=NULL, which will disable the locking code entirely.  On
    systems that have it, using /var/run would be better than /etc.

    If the the program hangs when flushing outstanding packets (which
    you can tell by setting -W), it may help to set -DNONBLOCK_BROKEN.
    This seems needed only for obsolete systems, like Ultrix.

    If the system isn't Unix, even vaguely, you will probably need to
    modify all of the above, and unix.c as well.

    Note that adjtime is commonly sick, but you don't need to change the
    code - just use the -r option whan making large corrections (see below
    for more details).

Any changes needed to header.h or main.c are bugs.  They may be bugs in the
code or in the compiler or libraries, but they are bugs.  Please prod the
people responsible and tell the author, who may be able to bypass them cleanly
even if they aren't bugs in his code.  The code also makes the following
assumptions, which would be quite hard to remove:

    8-bit bytes.  Strictly, neither ANSI/ISO C nor POSIX require these,
    and there were some very early versions of Unix on systems with other
    byte sizes.  But, without a defined sub-byte facility in C, ....

    At least 32-bit ints.  Well, actually, this wouldn't be too hard to
    remove.  But most Unix programs make this assumption, and I have very
    little interest in the more rudimentary versions of MS-DOS etc.

    An ANSI/ISO C compiler.  It didn't seem worth writing dual-language
    code in 1996.  Tough luck if you haven't got one.

    Tolerably efficient floating-point arithmetic, with at least 13 digits
    (decimal), preferably 15, in the mantissa of doubles.  Ditto.  If you
    want to port this to a toaster, please accept my insincerest sympathies
    and don't bother me.

    A trustworthy local network.  It does not check for DNS, Ethernet,
    packet or other spoofing, and assumes that any accessible NTP or SNTP
    servers are properly synchronised.


Warnings about Installation and Use
-----------------------------------

Anyone attempting to fiddle with the clock on their system should already know
how to write system administration scripts, install daemons and so on.  There
are a few warnings:

    Don't use the broadcast modes unless you really have to, as the
    client-server modes are far more reliable.  The broadcast modes were
    implemented more for virtuosity (a.k.a. SNTP conformance) than use.
    In particular, the error estimates are mere guesses, and may be low
    or even very low.  And even reading broadcasts needs privilege.

    The program is not intended to be installed setuid or setgid, and
    doing so is asking for trouble.  Its ownerships and access modes are
    not important.  It need not be run by root for merely displaying the
    time (even in daemon mode).

    The program does not need to run at a high priority (low in Unix
    terms!) even when being used to set the clock or as a server, except
    when the '-r' option is  used.  However, doing so may improve its
    accuracy.

    Unlike NTP, the SNTP protocol contains no protection against
    client-server loops.  If you set one up, your systems will spin
    themselves off into a disconnected vortex of unreality!

    It will get very confused if another process changes the local time
    while it is running.  There is some locking code in unix.c to prevent
    this program doing this to itself, but it will protect only against
    some errors.  However, the remaining failures should be harmless.

    Don't run it as a server unless you REALLY know what you are doing.
    It should be used as a server only on a system that is properly
    synchronised, by fair means or foul.  If it isn't, you will simply
    perpetrate misinformation.  And remember that broadcasts are most
    unpopular with overloaded administrators of overloaded networks.

    Watch out for multi-server broadcasts and systems with multiple ports
    onto the same Ethernet; there is some code to protect against this,
    but it is still easy to get confused.

    Don't put the lock file onto an automounted partition or delete it by
    hand, unless you really want to start two daemons at the same time.
    Both will probably fail horribly if you do this.

    The daemon save file is checked fairly carefully, but should be in a
    reasonably safe directory, unless you want hackers to cause trouble.
    /tmp is safe enough on most systems, but not all - /etc is better.


Installing and Using the Program
--------------------------------

Start by copying the executable and man page to where you want them.  If you
want only to display the time and as a replacement for the rdate or date