Section: System Administration (8)Updated: December 04, 2009Local indexUp
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 boot
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
This option will lock chronyd into RAM so that it will never be paged out.
This mode is only supported on Linux.
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
This option can be used to specify an alternate location for the
configuration file (default /etc/chrony.conf).
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.
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
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.
This option displays chronyd's version number to the terminal and exits