chrony/doc/chronyd.adoc - chrony - Git at Google

 // This file is part of chrony
 //
 // Copyright (C) Richard P. Curnow  1997-2003
 // Copyright (C) Miroslav Lichvar  2009-2017
 //
 // This program is free software; you can redistribute it and/or modify
 // it under the terms of version 2 of the GNU General Public License as
 // published by the Free Software Foundation.
 //
 // This program is distributed in the hope that it will be useful, but
 // WITHOUT ANY WARRANTY; without even the implied warranty of
 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 // General Public License for more details.
 //
 // You should have received a copy of the GNU General Public License along
 // with this program; if not, write to the Free Software Foundation, Inc.,
 // 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.

 = chronyd(8)
 :doctype: manpage
 :man manual: System Administration
 :man source: chrony @CHRONY_VERSION@

 == 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 _@SYSCONFDIR@/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 _@SYSCONFDIR@/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:
 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 it allows *chronyd* to be
 started without root privileges.

 *-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
 <<chrony.conf.adoc#dumpdir,*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 <<chrony.conf.adoc#initstepslew,*initstepslew*>>
 directive and the <<chrony.conf.adoc#makestep,*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
 <<chrony.conf.adoc#driftfile,*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
 <<chrony.conf.adoc#user,*user*>> directive. The compiled-in default value is
 _@DEFAULT_USER@_.
 +
 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.

 == FILES

 _@SYSCONFDIR@/chrony.conf_

 == SEE ALSO

 <<chronyc.adoc#,*chronyc(1)*>>, <<chrony.conf.adoc#,*chrony.conf(5)*>>

 == BUGS

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

 == AUTHORS

 chrony was written by Richard Curnow, Miroslav Lichvar, and others.
	// This file is part of chrony
	//
	// Copyright (C) Richard P. Curnow 1997-2003
	// Copyright (C) Miroslav Lichvar 2009-2017
	//
	// This program is free software; you can redistribute it and/or modify
	// it under the terms of version 2 of the GNU General Public License as
	// published by the Free Software Foundation.
	//
	// This program is distributed in the hope that it will be useful, but
	// WITHOUT ANY WARRANTY; without even the implied warranty of
	// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
	// General Public License for more details.
	//
	// You should have received a copy of the GNU General Public License along
	// with this program; if not, write to the Free Software Foundation, Inc.,
	// 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.

	= chronyd(8)
	:doctype: manpage
	:man manual: System Administration
	:man source: chrony @CHRONY_VERSION@

	== 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 _@SYSCONFDIR@/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 _@SYSCONFDIR@/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:
	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 it allows chronyd to be
	started without root privileges.

	-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
	<<chrony.conf.adoc#dumpdir,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 <<chrony.conf.adoc#initstepslew,initstepslew>>
	directive and the <<chrony.conf.adoc#makestep,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
	<<chrony.conf.adoc#driftfile,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
	<<chrony.conf.adoc#user,user>> directive. The compiled-in default value is
	_@DEFAULT_USER@_.
	+
	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.

	== FILES

	_@SYSCONFDIR@/chrony.conf_

	== SEE ALSO

	<<chronyc.adoc#,chronyc(1)>>, <<chrony.conf.adoc#,chrony.conf(5)>>

	== BUGS

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

	== AUTHORS

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