chronyd - chrony background daemon
chrony is a pair of programs for maintaining the accuracy of computer
clocks. chronyd is a background daemon program that can be started at
chronyd is a daemon which runs in background on the system. It obtains
measurements (e.g. via the network) of the system’s offset relative to
other systems, and adjusts the system time accordingly. For isolated
systems, the user can periodically enter the correct time by hand
(using chronyc). In either case, chronyd determines the rate at which
the computer gains or loses time, and compensates for this.
chronyd is usually started at boot-time and requires superuser
If chronyd has been installed to its default location
/usr/local/sbin/chronyd, starting it is simply a matter of entering the
Information messages and warnings will be logged to syslog.
A summary of the options supported by chronyd is included below.
This option will select the SCHED_FIFO real-time scheduler at
the specified priority (which must be between 0 and 100). This
mode is supported only on Linux.
-m This option will lock chronyd into RAM so that it will never be
paged out. This mode is only supported on Linux.
-d When run in this mode, the program will not detach itself from
the terminal, and all messages will be sent to the terminal
instead of to syslog.
This option can be used to specify an alternate location for the
configuration file (default /etc/chrony.conf).
-r This option will reload sample histories for each of the servers
being used. These histories are created by using the dump
command in chronyc, or by setting the dumponexit directive in
the configuration file. This option is useful if you want to
stop and restart chronyd briefly for any reason, e.g. to install
a new version. However, it only makes sense on systems where
the kernel can maintain clock compensation whilst not under
chronyd’s control. The only version where this happens so far
is Linux. On systems where this is not the case, e.g. Solaris
and SunOS the option should not be used.
-s This option will set the system clock from the computer’s real-
time clock. This is analogous to supplying the -s flag to the
/sbin/clock program during the Linux boot sequence.
Support for real-time clocks is limited at present - the
criteria are described in the section on the rtcfile directive
in the documentation supplied with the distribution.
If chronyd cannot support the real time clock on your computer,
this option cannot be used and a warning message will be logged
to the syslog.
If used in conjunction with the -r flag, chronyd will attempt to
preserve the old samples after setting the system clock from the
real time clock. This can be used to allow chronyd to perform
long term averaging of the gain or loss rate across system
reboots, and is useful for dial-up systems that are shut down
when not in use. For this to work well, it relies on chronyd
having been able to determine accurate statistics for the
difference between the real time clock and system clock last
time the computer was on.
When this option is used, chronyd will drop root privileges to
the specified user. So far, it works only on Linux when
compiled with capabilities support.
-v This option displays chronyd’s version number to the terminal
-4 Resolve hostnames only to IPv4 addresses.
-6 Resolve hostnames only to IPv6 addresses.
To report bugs, please visit http://chrony.tuxfamily.org/
chronyd is documented in detail in the documentation supplied with the
distribution (chrony.txt and chrony.texi) and is also available from
chrony(1), chronyc(1), chrony.conf(5), clock(8), xntpd(8), ntpd(8)
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.