chronyd (8) - Linux Man Pages
chronyd: chrony daemon
chronyd - chrony daemon
chronyd is a daemon for synchronisation of the system clock. It can synchronise the clock with NTP servers, reference clocks (e.g. a GPS receiver), and manual input using wristwatch and keyboard via chronyc. It can also operate as an NTPv4 (RFC 5905) server and peer to provide a time service to other computers in the network.
If no configuration directives are specified on the command line, chronyd will read them from a configuration file. The compiled-in default location of the file is /etc/chrony.conf.
- With this option hostnames will be resolved only to IPv4 addresses and only IPv4 sockets will be created.
- With this option hostnames will be resolved only to IPv6 addresses and only IPv6 sockets will be created.
- This option can be used to specify an alternate location for the configuration file (default /etc/chrony.conf).
- When run in this mode, the program will not detach itself from the terminal.
- 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. When chronyd was compiled with debugging support, this option can be used twice to print also debugging messages.
- When run in this mode, chronyd will set the system clock once and exit. It will not detach from the terminal.
- This option is similar to -q, but it will only print the offset without any corrections of the clock.
- This option will try to reload and then delete files containing sample histories for each of the servers and reference clocks 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 should be used only on systems where the kernel can maintain clock compensation whilst not under chronyd's control (i.e. Linux, FreeBSD, NetBSD and Solaris).
- When this option is used, the initstepslew directive and the makestep directive used with a positive limit will be ignored. This option is useful when restarting chronyd and can be used in conjunction with the -r option.
This option will set the system clock from the computer's real-time clock (RTC)
or to the last modification time of the file specified by the
driftfile directive. Real-time clocks are
supported only on Linux.
If used in conjunction with the -r flag, chronyd will attempt to preserve the old samples after setting the system clock from the RTC. 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 systems with intermittent access to network 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 RTC and system clock last time the computer was on.
If the last modification time of the drift file is later than both the current time and the RTC time, the system time will be set to it to restore the time when chronyd was previously stopped. This is useful on computers that have no RTC or the RTC is broken (e.g. it has no battery).
- This option sets a timeout (in seconds) after which chronyd will exit. If the clock is not synchronised, it will exit with a non-zero status. This is useful with the -q or -Q option to shorten the maximum time waiting for measurements, or with the -r option to limit the time when chronyd is running, but still allow it to adjust the frequency of the system clock.
This option sets the name of the system user to which chronyd will switch
after start in order to drop root privileges. It overrides the
user directive (default chrony).
On Linux, chronyd needs to be compiled with support for the libcap library. On macOS, FreeBSD, NetBSD and Solaris chronyd forks into two processes. The child process retains root privileges, but can only perform a very limited range of privileged system calls on behalf of the parent.
This option configures a system call filter when chronyd is compiled with
support for the Linux secure computing (seccomp) facility. In level 1 the
process is killed when a forbidden system call is made, in level -1 the SYSSIG
signal is thrown instead and in level 0 the filter is disabled (default 0).
It's recommended to enable the filter only when it's known to work on the version of the system where chrony is installed as the filter needs to allow also system calls made from libraries that chronyd is using (e.g. libc) and different versions or implementations of the libraries may make different system calls. If the filter is missing some system call, chronyd could be killed even in normal operation.
- On Linux, this option will select the SCHED_FIFO real-time scheduler at the specified priority (which must be between 0 and 100). On macOS, this option must have either a value of 0 (the default) to disable the thread time constraint policy or 1 for the policy to be enabled. Other systems do not support this option.
- This option will lock chronyd into RAM so that it will never be paged out. This mode is only supported on Linux.
- With this option chronyd will print version number to the terminal and exit.
For instructions on how to report bugs, please visit <URL: https://chrony.tuxfamily.org/ >.
chrony was written by Richard Curnow, Miroslav Lichvar, and others.