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/inet/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/inet/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/inet/chrony.confSEE 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 4.5 2023-12-05 CHRONYD(8)
