vsntp README

    See INSTALL for details on how to install and run vsntp.

Obtaining the Newest Information

    vsntp's official website is at...

      vsntp project on GitHub: https://github.com/imacat/vsntp
      vsntp project on SourceForge: https://sf.net/p/vsntp

    You can always download the newest version of vsntp from...

      vsntp download on SourceForge: https://sourceforge.net/projects/vsntp/files
      Tavern IMACAT's FTP directory: https://ftp.imacat.idv.tw/pub/vsntp/

    imacat's PGP public key is at...

      Tavern IMACAT's: https://www.imacat.idv.tw/me/pgpkey.asc

Introduction:

    vsntp is an SNTP client daemon for machines without a sane system
time.  The word "vsntp" stands for "SNTP for Virtual PC".  It was
originally designed for my GNU/Linux server running on Connectix
Virtual PC.  It runs according to RFC 1769 SNTP, connecting the NTP
server on UDP port 123.

    Without Virtual PC Additions, the system time on Virtual PC is
completely insane.  It's RTC (Real Time Clock, or CMOS time, or
hardware clock) is software emulated, which does not seems to be
running.  The GNU/Linux kernel hardly maintains a system time itself.
With smooth run it goes 4 seconds ahead per minute, which is nearly
1.5 hours per day.  That is insane.  You can even tell it with your
eyes.

    David L. Mills' ntp does not work here.  It uses a method that
learns the clock frequency drift first, and adjust the kernel clock
with adjtimex() so that time adjustment goes smoothly, from the point
of view of system and applications.  This assumes an existing fix-
speed system clock.  But this is not the case of Virtual PC.  The
system clock on Virtual PC is software emulated.  It can be faster
or slower now and then, depending on the load of the hosting machine.
There is no fixed clock speed.  The frequency drift does not exist,
then.  It dooms to fail to measure it.

    There is an sntp client that comes with David L. Mills' ntp
package.  It is suggested to be run from crontab.  But crontab runs
by minutes, and Virtual PC goes 4 seconds ahead per minute.  Rolling
back 4 seconds every minute is insane for most applications.  It also
increases system load heavily to run one instance per minute.

    vsntp is a workaround on this.  It runs as a daemon to eliminates
the additional system load on every synchronization. It uses
settimeofday() to synchronize the time.  It synchronizes the time
with an arbitrary interval, so that time can be accurate within a
second.

    There are some defects.  Synchronizing the time too often
introduces heavy network load.  It introduces heavy load on the
target NTP server, too.  You should have a working NTP server nearby
that is owned by you.  Also, since settimeofday() is called so often,
high-accurate time operations like timer, etc., may not run
correctly.

    vsntp uses sleep() as the synchronization scheduler.  Reports show
that on some systems sleep() may not function normally.  If you find
vsntp stops synchronization after running for some time, that the
sleep() is not functioning normally on your system, you may want to
switch to the alarm() scheduler with the "-a" switch.

    If you ever encounter any problem, you may check your syslog.
vsntp logs detailed debugging information to syslog in log level
LOG_DEBUG with facility LOG_DAEMON.  You may turn it on in your
/etc/syslog.conf with the following line:

      daemon.debug  /var/log/debug

and check the /var/log/debug file for the debugging message.  Remember
to remove this afterwards, for the amount of the debugging messages
may be huge and may use up your hard disk in a very short time.  To the
least it may slow down your system for frequent hard disk I/O.

    vsntp was originally written for GNU/Linux.  It uses POSIX
compatible system calls.  It should work on any POSIX compatible
system.  But I have yet only tested it on Cygwin.  Cygwin is known to
work.  I don't have others to test and run on.  Please let me know
(and submit the patch if needed) if you can port it to other systems.
I know it does not work on MSWin32, for the way it handles the PID
file path.

    Please tell me if you have successfully running vsntp on other
virtual machines, like VMWare.

    Generally, please tell me if you are using vsntp.  I would like
to know that I am really doing some good for the world, *^_^*  but not
having fun myself. :p

    This is my first daemon, my first socket program and my first
public-released C program.  Any comment or suggestion is welcome. ^_*'