chrony - programs for keeping computer clocks accurate
chrony is a pair of programs for keeping computer clocks accurate.
chronyd is a background (daemon) program and chronyc is a command-line
interface to it. Time reference sources for chronyd can be RFC1305 NTP
servers, human (via keyboard and chronyc), or the computer’s real-time
clock at boot time (Linux only). chronyd can determine the rate at
which the computer gains or loses time and compensate for it while no
external reference is present. Its use of NTP servers can be switched
on and off (through chronyc) to support computers with dial-
up/intermittent access to the Internet, and it can also act as an
RFC1305-compatible NTP server.
chronyc is a command-line interface program which can be used to
monitor chronyd’s performance and to change various operating
parameters whilst it is running.
chronyd’s main function is to obtain measurements of the true (UTC)
time from one of several sources, and correct the system clock
accordingly. It also works out the rate at which the system clock
gains or loses time and uses this information to keep it accurate
between measurements from the reference.
The reference time can be derived from either Network Time Protocol
(NTP) servers, reference clocks, or wristwatch-and-keyboard (via
chronyc). The main source of information about the Network Time
Protocol is http://www.eecis.udel.edu/~ntp.
It is designed so that it can work on computers which only have
intermittent access to reference sources, for example computers which
use a dial-up account to access the Internet. Of course, it will work
on computers with permanent connections too.
In addition, for Linux 2.0.x (for x >= 32) or 2.2 onwards, chronyd can
monitor the system’s real time clock performance, so the system can
maintain accurate time even across reboots.
Typical accuracies available between 2 machines are
On an ethernet LAN : 100-200 microseconds, often much better On a
V32bis dial-up modem connection : 10’s of milliseconds (from one
session to the next)
With a good reference clock the accuracy can reach one microsecond.
chronyd can also operate as an RFC1305-compatible NTP server and peer.
Richard Curnow <email@example.com>
This man-page was written by Jan Schaumann <firstname.lastname@example.org> as
part of "The Missing Man Pages Project". Please see
http://www.netmeister.org/misc/m2p2/index.html for details.
The complete chrony documentation is supplied in texinfo format.