Manual: chronyd

CHRONYD(8)					    System Administration					  CHRONYD(8)

NAME
       chronyd - chrony daemon

SYNOPSIS
       chronyd [OPTION]... [DIRECTIVE]...

DESCRIPTION
       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/chrony.conf.

       Informational messages, warnings, and errors will be logged to syslog.

OPTIONS
       -4
	   With this option hostnames will be resolved only to IPv4 addresses and only IPv4 sockets will be created.

       -6
	   With this option hostnames will be resolved only to IPv6 addresses and only IPv6 sockets will be created.

       -f file
	   This option can be used to specify an alternate location for the configuration file. The compiled-in default
	   value is /etc/chrony/chrony.conf.

       -n
	   When run in this mode, the program will not detach itself from the terminal.

       -d
	   When run in this mode, the program will not detach itself from the terminal, and all messages will be written to
	   the terminal instead of syslog. If chronyd was compiled with enabled support for debugging, this option can be
	   used twice to enable debug messages.

       -l file
	   This option enables writing of log messages to a file instead of syslog or the terminal.

       -L level
	   This option specifies the minimum severity level of messages to be written to the log file, syslog, or terminal.
	   The following levels can be specified: -1 (debug, if compiled with enabled support for debugging), 0
	   (informational), 1 (warning), 2 (non-fatal error), and 3 (fatal error). The default value is 0.

       -p
	   When run in this mode, chronyd will print the configuration and exit. It will not detach from the terminal. This
	   option can be used to verify the syntax of the configuration and get the whole configuration, even if it is split
	   into multiple files and read by the include or confdir directive.

       -q
	   When run in this mode, chronyd will set the system clock once and exit. It will not detach from the terminal.

       -Q
	   This option is similar to the -q option, except it only prints the offset without making any corrections of the
	   clock and disables server ports to allow chronyd to be started without root privileges, assuming the
	   configuration does not have any directives which would require them (e.g. refclock, hwtimestamp, rtcfile, etc).

       -r
	   This option will try to reload and then delete files containing sample histories for each of the servers and
	   reference clocks being used. The files are expected to be in the directory specified by the dumpdir 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, illumos, and macOS 10.13 or later).

       -R
	   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.

       -s
	   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).

       -t timeout
	   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.

       -u user
	   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. The compiled-in default value is _chrony.

	   On Linux, chronyd needs to be compiled with support for the libcap library. On macOS, FreeBSD, NetBSD, and
	   illumos 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.

       -U
	   This option disables a check for root privileges to allow chronyd to be started under a non-root user, assuming
	   the process will have all capabilities (e.g. provided by the service manager) and access to all files,
	   directories, and devices, needed to operate correctly in the specified configuration. Note that different
	   capabilities might be needed with different configurations and different Linux kernel versions. Starting chronyd
	   under a non-root user is not recommended when the configuration is not known, or at least limited to specific
	   directives.

       -F level
	   This option configures system call filters loaded by chronyd processes if it was compiled with support for the
	   Linux secure computing (seccomp) facility. Three levels are defined: 0, 1, 2. The filters are disabled at level
	   0. At levels 1 and 2, chronyd will be killed if it makes a system call which is blocked by the filters. The level
	   can be specified as a negative number to trigger the SIGSYS signal instead of SIGKILL, which can be useful for
	   debugging. The default value is 0.

	   At level 1, the filters allow only selected system calls that are normally expected to be made by chronyd. Other
	   system calls are blocked. This level is recommended only if it is known to work on the version of the system
	   where chrony is installed. The filters need to allow also system calls made by libraries that chronyd is using
	   (e.g. libc), but different versions or implementations of the libraries might make different system calls. If the
	   filters are missing a system call, chronyd could be killed even in normal operation.

	   At level 2, the filters block only a small number of specific system calls (e.g. fork and exec). This approach
	   should avoid false positives, but the protection of the system against a compromised chronyd process is much more
	   limited.

	   The filters cannot be enabled with the mailonchange directive.

       -P priority
	   On Linux, FreeBSD, NetBSD, and illumos 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 to
	   disable the thread time constraint policy or 1 for the policy to be enabled. Other systems do not support this
	   option. The default value is 0.

       -m
	   This option will lock chronyd into RAM so that it will never be paged out. This mode is only supported on Linux,
	   FreeBSD, NetBSD, and illumos.

       -x
	   This option disables the control of the system clock. chronyd will not try to make any adjustments of the clock.
	   It will assume the clock is free running and still track its offset and frequency relative to the estimated true
	   time. This option allows chronyd to be started without the capability to adjust or set the system clock (e.g. in
	   some containers) to operate as an NTP server.

       -v, --version
	   With this option chronyd will print version number to the terminal and exit.

       -h, --help
	   With this option chronyd will print a help message to the terminal and exit.

ENVIRONMENT VARIABLES
       LISTEN_FDS
	   On Linux systems, the systemd service manager may pass file descriptors for pre-initialised sockets to chronyd.
	   The service manager allocates and binds the file descriptors, and passes a copy to each spawned instance of the
	   service. This allows for zero-downtime service restarts as the sockets buffer client requests until the service
	   is able to handle them. The service manager sets the LISTEN_FDS environment variable to the number of passed file
	   descriptors.

       NOTIFY_SOCKET
	   The systemd service manager sets this variable for services of the notify type. chronyd sends a message to this
	   socket when it it is fully initialised and ready to accept commands (e.g. from chronyc), with the clock already
	   set if using the -s option or initstepslew directive. It is an alternative to the forking service type, which
	   does not need the PID file. chronyd needs to be started with the -n or -d option to not fork.

FILES
       /etc/chrony/chrony.conf

SEE ALSO
       chronyc(1), chrony.conf(5)

BUGS
       For instructions on how to report bugs, please visit https://chrony-project.org/.

AUTHORS
       chrony was written by Richard Curnow, Miroslav Lichvar, and others.

chrony 4.8						 2025-08-27						  CHRONYD(8)