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.
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 4.6.1 2024-10-08 CHRONYD(8)
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