sntpd is a small SNTP server and client for UNIX systems, implementing RFC 1305 and RFC 4330. Its functionality is only a small subset of ntpd, chrony, OpenNTPd, and xntpd. Since it is much smaller it is also more relevant for embedded systems in need of only a background process to keep the system time in sync (client mode).
sntpd is a fork of ntpclient by Larry Doolittle. As such it implements
a compatibility mode when called with the name ntpclient. The name
has been changed to indicate the expanded feature set, e.g. a background
daemon mode, IPv6, syslog, as well as changes in command line options.
Use the GitHub issue tracker to report bugs. If you want to contribute fixes or new features, see the file CONTRIBUTING.md.
Note: sntpd has limited support for acting as an SNTP server.
The server mode is enabled by default, disable with -p 0.
All arguments are optional, sntpd defaults to use pool.ntp.org.
Usage:
sntpd [options] [SERVER]
-d Dry run, no time correction, useful for debugging
-h Show summary of command line options and exit
-i SEC Check time every interval seconds. Default: 600
-l LEVEL Set log level: none, err, warn, notice (default), info, debug
-n Don't fork. Prevents sntpd from daemonizing by default
Use '-s' with this to use syslog as well, for Finit + systemd
-p PORT SNTP server mode port, default: 123, use 0 to disable
-q USEC Minimum packet delay for transaction, default: 800 usec
-s Use syslog instead of stdout, default unless -n
-t Trust network and server, disable RFC4330 validation
-v Show program version
SERVER Optional NTP server to sync with, default: pool.ntp.org
The sntpd project comes with a compatiblity mode triggered when called
as ntpclient. This mode supports the original command line options.
Mortal users can use the ntpclient tool for monitoring, but not clock
setting (with the -s or -l switches). The -l switch is designed
to be robust in any network environment, but has seen the most extensive
testing in a low latency (less than 2 ms) Ethernet environment. Users
in other environments should study sntpd's behavior, and be prepared to
adjust internal tuning parameters. A long description of how and why to
use sntpd and ntpclient is in the HowTo.md file. sntpd always sends
packets to the server's UDP port 123.
One commonly needed tuning parameter for lock mode is min_delay, the
shortest possible round-trip transaction time. This can be set with the
command line -q switch. The historical default of 800 microseconds
was good for local Ethernet hardware a few years ago. If it is set too
high, you will get a lot of "inconsistent" lines in the log file when
time locking (-l switch). The only true future-proof value is 0, but
that will cause the local time to wander more than it should. Setting
it to 200 is recommended on an end client.
The test.dat file that is part of the source distribution has 200
lines of sample output. Its first few lines, with the output column
headers that are shown when the -d option is chosen, are:
day second elapsed stall skew dispersion freq
36765 00180.386 1398.0 40.3 953773.9 793.5 -1240000
36765 00780.382 1358.0 41.3 954329.0 915.5 -1240000
36765 01380.381 1439.0 56.0 954871.3 915.5 -1240000
- day, second: time of measurement, UTC, relative to NTP epoch (Jan 1, 1900)
- elapsed: total time from query to response (microseconds)
- stall: time the server reports that it sat on the request (microseconds)
- skew: difference between local time and server time (microseconds)
- dispersion: reported by server, see RFC 1305 (microseconds)
- freq: local clock frequency adjustment (Linux only, ppm*65536)
ntclient performs a series of sanity checks on UDP packets received, as
recommended by RFC 4330. If it fails one of these tests, the line
described above is replaced by 36765 01380.381 rejected packet or, if
--enable-debug was selected at configure, one of:
36765 01380.381 rejected packet: LI==3
36765 01380.381 rejected packet: VN<3
36765 01380.381 rejected packet: MODE!=3
36765 01380.381 rejected packet: ORG!=sent
36765 01380.381 rejected packet: XMT==0
36765 01380.381 rejected packet: abs(DELAY)>65536
36765 01380.381 rejected packet: abs(DISP)>65536
36765 01380.381 rejected packet: STRATUM==0
To see the actual values of the rejected packet, start the ntpclient
tool or sntpd with the -d option; this will give a human-readable
printout of every packet received, including the rejected ones. To skip
these checks, use the -t switch.
The file test.dat is suitable for piping into ntpclient -r.
There are more than 200000 samples (lines) archived for study. They are
generally spaced 10 minutes apart, representing over three years of data
logging (from a variety of machines, and not continuous, unfortunately).
If you are interested, contact Larry.
Also included is a version of the adjtimex(1) tool. See its man page
and the HowTo.md file for more information.
Another tool is envelope, which is a perl script that was used for the
lock studies. It's kind of a hack and not worth documenting here.
Some really old Linux systems (e.g., Red Hat EL-3.0 and Ubuntu 4.10)
have a totally broken POSIX clock_settime() implementation. If you
get the following with sntpd -s:
clock_settime: Invalid argument
then configure --enable-obsolete. Linux systems that are even older
will not even compile without that switch set.
- Doesn't understand the LI (Leap second Indicator) field of an NTP packet
- Doesn't interact with
adjtimex(2)status value - Cannot query multiple servers
- Requires Linux
select()semantics, where timeout value is modified
Adherence to RFC 4330 chapter 10, Best practices:
- Enforced, unless someone tinkers with the source code
- No backoff, but no retry either; this isn't TCP
- Not in scope for the upstream source
- Defaults to pool.ntp.org, but is configurable
- Not in scope for the upstream source
- Supported
- Supported, connection to server reopened once a day
- Not supported (scary opportunity to DOS the client)
Please use released, and versioned, tarballs of this project. All releases are available from here:
☞ https://github.com/troglobit/sntpd/releases
sntpd uses the GNU configure & build system:
./configure
makeThe GNU build system use /usr/local as the default install prefix. In
many cases this is useful, but many users expect /usr or /opt. To
install into /usr/sbin/sntpd and /usr/bin/adjtimex:
./configure --prefix=/usr
make
sudo make install-stripThe last command installs, there is also a possiblity to uninstall all files using:
sudo make uninstallFor changing the system clock frequency, only the Linux adjtimex(2)
interface is implemented at this time. Non-Linux systems can only use
the ntpclient tool to measure time differences and set the system clock,
by way of the POSIX 1003.1-2001 standard, the routines clock_gettime()
and clock_settime(). Also, see section Bugs, below.
There are a few compile-time configurations possible. E.g., for older Linux kernels, before the tickless erea (pre 3.0), you want to:
./configure --disable-siocgstampHowever, first try without changing the default. That gives you a full-
featured sntpd and ntpclient tool that use a modern POSIX time API
and works reasonably well with any Linux kernel.
Solaris and other UNIX users may need to adjust the CFLAGS slightly.
For other options, see ./configure --help
If you want to contribute, or try out the latest unreleased features, here is a few things to know about GNU build system:
configure.acand a per-directoryMakefile.amare key filesconfigureandMakefile.inare generated fromautogen.sh, they are not stored in GIT but automatically generated for the release tarballsMakefileis generated byconfigurescript
To build from GIT you first need to clone the repository and run the
autogen.sh script. This requires automake and autoconf to be
installed on your system.
git clone https://github.com/troglobit/sntpd.git
cd sntpd/
./autogen.sh
./configure && makeRemember: GIT sources are a moving target and not recommended for production systems, unless you know what you are doing!
Larry Doolittle created ntpclient and made it freely available under
the terms of the GNU General Public License, version 2. He remains
the official upstream for ntpclient.
This sntpd fork at GitHub is maintained by Joachim Wiberg and adds features like syslog, background daemon, IPv6, and systemd support.