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chrony/chrony.texi.in
Miroslav Lichvar 6a9c756cf0 rtc: restore time from driftfile if later than RTC time 
This is useful on computers that have an RTC, but there is no battery to
keep the time when they are turned off and start with the same time on
each boot.
2015-10-06 15:52:36 +02:00

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\input texinfo
@c {{{ Main header stuff
@afourwide
@paragraphindent 0
@setfilename chrony.info
@settitle User guide for the chrony suite
@c @setchapternewpage off
@ifinfo
@dircategory Net Utilities
@direntry
* chrony: (chrony). How to use chronyd and chronyc
* chronyd: (chrony)Starting chronyd. Reference for chronyd
* chronyc: (chrony)Running chronyc. Reference for chronyc
@end direntry
@end ifinfo
@titlepage
@sp 10
@title The chrony suite
@subtitle This manual describes how to use
@subtitle the programs chronyd and chronyc
@author Richard P. Curnow
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1997-1999 Richard P. Curnow
Copyright @copyright{} 2009-2015 Miroslav Lichvar
@end titlepage
@c }}}
@c {{{ Top node
@node Top
@top
@menu
* Introduction:: What the chrony suite does
* Installation:: How to compile and install the software
* Typical scenarios:: How to configure the software for some common cases
* Usage reference:: Reference manual
* GPL:: The GNU General Public License
@end menu
@c }}}
@c {{{ Ch:Introduction
@c {{{ Chapter top
@node Introduction
@chapter Introduction
@menu
* Overview:: What the programs do
* Acknowledgements:: Credit where credit is due
* Availability:: Where to get the software
* Other time synchronisation packages:: Comparision with other software
* Distribution and warranty:: There is no warranty
* Bug reporting:: How to report bugs and make suggestions
* Contributing:: Areas where contributions are particularly welcome
@end menu
@c }}}
@c {{{ S:Overview
@node Overview
@section Overview
chrony is a versatile implementation of the Network Time Protocol (NTP).
It can synchronize the system clock with NTP servers, reference clocks
(e.g. GPS receiver), and manual input using wristwatch and keyboard.
It can also operate as an NTPv4 (RFC 5905) server and peer to provide
a time service to other computers in the network.
It is designed to perform well in a wide range of conditions, including
intermittent network connections, heavily congested networks, changing
temperatures (ordinary computer clocks are sensitive to temperature),
and systems that do not run continuosly, or run on a virtual machine.
Typical accuracy between two machines on a LAN is in tens, or a few
hundreds, of microseconds; over the Internet, accuracy is typically
within a few milliseconds. With a good hardware reference clock
sub-microsecond accuracy is possible.
Two programs are included in chrony, @code{chronyd} is a daemon that can
be started at boot time and @code{chronyc} is a command-line interface
program which can be used to monitor @code{chronyd}'s performance and to
change various operating parameters whilst it is running.
The IP addresses from which @code{chronyc} clients may connect can be tightly
controlled. The default is just the computer that @code{chronyd} itself is
running on.
@c }}}
@c {{{ S:Acknowledgments
@node Acknowledgements
@section Acknowledgements
The @code{chrony} suite makes use of the algorithm known as @emph{RSA
Data Security, Inc. MD5 Message-Digest Algorithm} for authenticating
messages between different machines on the network.
In writing the @code{chronyd} program, extensive use has been made of
RFC 1305 and RFC 5905, written by David Mills. The source code of
the NTP reference implementation has been used to check details of the
protocol.
@c }}}
@c {{{ S:Availability
@node Availability
@section Availability
@menu
* Getting the software:: Where can I get the software from?
* Platforms:: Which platforms will it run on?
@end menu
@node Getting the software
@subsection Getting the software
Links on @uref{http://chrony.tuxfamily.org, the chrony home page}
describe how to obtain the software.
@node Platforms
@subsection Platforms
Although most of the program is portable between
Unix-like systems, there are parts that have to be tailored to each
specific vendor's system. These are the parts that interface with the
operating system's facilities for adjusting the system clock;
different operating systems may provide different function calls to
achieve this, and even where the same function is used it may have
different quirks in its behaviour.
The software is known to work on Linux, FreeBSD, NetBSD, Mac OS X and Solaris.
Closely related systems may work too. Porting the software to other systems
(particularly to those supporting an @code{adjtime} or @code{ntp_adjtime}
system call) should not be difficult, however it requires access to such
systems to test out the driver.
@c }}}
@c {{{ S:Other programs
@node Other time synchronisation packages
@section Relationship to other software packages
@menu
* Comparison with ntpd::
* Comparison with timed::
@end menu
@node Comparison with ntpd
@subsection ntpd
The `reference' implementation of the Network Time Protocol is the
program @code{ntpd}, available via
@uref{http://www.ntp.org/, The NTP home page}.
One of the main differences between @code{ntpd} and @code{chronyd} is in
the algorithms used to control the computer's clock. Things
@code{chronyd} can do better than @code{ntpd}:
@itemize @bullet
@item
@code{chronyd} can perform usefully in an environment where access to
the time reference is intermittent. @code{ntpd} needs regular polling
of the reference to work well.
@item
@code{chronyd} can usually synchronise the clock faster and with better
time accuracy.
@item
@code{chronyd} quickly adapts to sudden changes in the rate of the clock
(e.g. due to changes in the temperature of the crystal oscillator).
@code{ntpd} may need a long time to settle down again.
@item
@code{chronyd} can perform well even when the network is congested for
longer periods of time.
@item
@code{chronyd} in the default configuration never steps the time to not
upset other running programs. @code{ntpd} can be configured to never
step the time too, but it has to use a different means of adjusting the
clock, which has some
disadvantages.
@item
@code{chronyd} can adjust the rate of the clock in a larger range, which
allows it to operate even on machines with broken or unstable clock
(e.g. in some virtual machines).
@end itemize
Things @code{chronyd} can do that @code{ntpd} can't:
@itemize @bullet
@item
@code{chronyd} provides support for isolated networks whether the only
method of time correction is manual entry (e.g. by the administrator
looking at a clock). @code{chronyd} can look at the errors corrected at
different updates to work out the rate at which the computer gains or
loses time, and use this estimate to trim the computer clock
subsequently.
@item
@code{chronyd} provides support to work out the gain or loss rate of the
`real-time clock', i.e. the clock that maintains the time when the
computer is turned off. It can use this data when the system boots to
set the system time from a corrected version of the real-time clock.
These real-time clock facilities are only available on Linux, so far.
@end itemize
Things @code{ntpd} can do that @code{chronyd} can't:
@itemize @bullet
@item
@code{ntpd} supports all operating modes from RFC 5905, including
broadcast, multicast and manycast client / server. It supports the
orphan mode and it also supports authentication based on public-key
cryptography described in RFC 5906.
@item
@code{ntpd} has been ported to more types of computer / operating
system.
@item
@code{ntpd} includes drivers for many reference clocks. @code{chronyd}
relies on other programs (e.g. gpsd) to access the data from the
reference clocks.
@end itemize
@node Comparison with timed
@subsection timed
@code{timed} is a program that is part of the BSD networking suite. It
uses broadcast packets to find all machines running the daemon within a
subnet. The machines elect a master which periodically measures the
system clock offsets of the other computers using ICMP timestamps.
Corrections are sent to each member as a result of this process.
Problems that may arise with @code{timed} are :
@itemize @bullet
@item
Because it uses broadcasts, it is not possible to isolate its
functionality to a particular group of computers; there is a risk of
upsetting other computers on the same network (e.g. where a whole
company is on the same subnet but different departments are independent
from the point of view of administering their computers.)
@item
The update period appears to be 10 minutes. Computers can build up
significant offsets relative to each other in that time. If a
computer can estimate its rate of drift it can keep itself closer to
the other computers between updates by adjusting its clock every few
seconds. @code{timed} does not seem to do this.
@item
@code{timed} does not have any integrated capability for feeding
real-time into its estimates, or for estimating the average rate of time
loss/gain of the machines relative to real-time (unless one of the
computers in the group has access to an external reference and is always
appointed as the `master').
@end itemize
@code{timed} does have the benefit over @code{chronyd} that for isolated
networks of computers, they will track the `majority vote' time. For
such isolated networks, @code{chronyd} requires one computer to be the
`master' with the others slaved to it. If the master has a particular
defective clock, the whole set of computers will tend to slip relative
to real time (but they @emph{will} stay accurate relative to one
another).
@c }}}
@c {{{ S:Rights + warranty
@node Distribution and warranty
@section Distribution rights and (lack of) warranty
Chrony may be distributed in accordance with the GNU General Public License
version 2, reproduced in @xref{GPL}.
@c }}}
@c {{{ S:Bug reporting + suggestions
@node Bug reporting
@section Bug reporting and suggestions
If you think you've found a bug in chrony, or have a suggestion, please let us
know. You can join chrony users mailing list by sending a message with the
subject subscribe to @email{chrony-users-request@@chrony.tuxfamily.org}. Only
subscribers can post to the list.
When you are reporting a bug, please send us all the information you can.
Unfortunately, chrony has proven to be one of those programs where it is very
difficult to reproduce bugs in a different environment. So we may have to
interact with you quite a lot to obtain enough extra logging and tracing to
pin-point the problem in some cases. Please be patient and plan for this!
Of course, if you can debug the problem yourself and send us a source code
patch to fix it, we will be very grateful!
@c }}}
@c {{{ S:Contributions
@node Contributing
@section Contributions
Although chrony is now a fairly mature and established project, there are still
areas that could be improved. If you can program in C and have some expertise
in these areas, you might be able to fill the gaps.
Particular areas that need addressing are :
@enumerate
@item Porting to other Unices
This involves creating equivalents of sys_solaris.c, sys_linux.c etc for the
new system.
@item Porting to Windows NT
A small amount of work on this was done under Cygwin. Only the sorting
out of the include files has really been achieved so far. The two main
areas still to address are
@enumerate
@item The system clock driver.
@item How to make chronyd into an NT service (i.e. what to replace fork(),
setsid() etc with so that chronyd can be automatically started in the system
bootstrap.
@end enumerate
@item More drivers for reference clock support
@end enumerate
@c }}}
@c }}}
@c {{{ Ch:Installation
@node Installation
@chapter Installation
@c {{{ main introduction text
The software is distributed as source code which has to be compiled.
The source code is supplied in the form of a gzipped tar file, which
unpacks to a subdirectory identifying the name and version of the
program.
After unpacking the source code, change directory into it, and type
@example
./configure
@end example
This is a shell script that automatically determines the system type.
There is a single optional parameter, @code{--prefix} which indicates
the directory tree where the software should be installed. For example,
@example
./configure --prefix=/opt/free
@end example
will install the @code{chronyd} daemon into /opt/free/sbin and the
@code{chronyc} control program into /opt/free/bin. The default value for the
prefix is /usr/local.
The configure script assumes you want to use gcc as your compiler.
If you want to use a different compiler, you can configure this way:
@example
CC=cc CFLAGS=-O ./configure --prefix=/opt/free
@end example
for Bourne-family shells, or
@example
setenv CC cc
setenv CFLAGS -O
./configure --prefix=/opt/free
@end example
for C-family shells.
If the software cannot (yet) be built on your system, an error message
will be shown. Otherwise, @file{Makefile} will be generated.
If editline or readline library is available, chronyc will be built with line
editing support. If you don't want this, specify the --disable-readline flag
to configure. Please refer to @pxref{line editing support} for more information.
If a @file{timepps.h} header is available (e.g. from the
@uref{http://linuxpps.org/, LinuxPPS project}), @code{chronyd} will be built with PPS API
reference clock driver. If the header is installed in a location that isn't
normally searched by the compiler, you can add it to the searched locations by
setting @code{CPPFLAGS} variable to @code{-I/path/to/timepps}.
Now type
@example
make
@end example
to build the programs.
If you want to build the manual in plain text, HTML and info versions, type
@example
make docs
@end example
Once the programs have been successfully compiled, they need to be
installed in their target locations. This step normally needs to be
performed by the superuser, and requires the following command to be
entered.
@example
make install
@end example
This will install the binaries and manpages.
To install the plain text, HTML and info versions of the manual, enter the
command
@example
make install-docs
@end example
If you want chrony to appear in the top level info directory listing, you need
to run the @command{install-info} command manually after this step.
@command{install-info} takes 2 arguments. The first is the path to the
@file{chrony.info} file you have just installed. This will be the argument you
gave to --prefix when you configured (@file{/usr/local} by default), with
@file{/share/info/chrony.info} on the end. The second argument is the location of
the file called @file{dir}. This will typically be @file{/usr/share/info/dir}. So
the typical command line would be
@example
install-info /usr/local/share/info/chrony.info /usr/share/info/dir
@end example
Now that the software is successfully installed, the next step is to
set up a configuration file. The default location of the file
is @file{@SYSCONFDIR@/chrony.conf}. Several examples of configuration with
comments are included in the examples directory. Suppose you want to use
public NTP servers from the pool.ntp.org project as your time reference. A
minimal useful configuration file could be
@example
pool pool.ntp.org iburst
makestep 1.0 3
rtcsync
@end example
Then, @code{chronyd} can be run.
@c }}}
@menu
* line editing support:: If libraries are in a non-standard place
* package builders:: Extra options useful to package builders
@end menu
@c {{{ line editing support
@node line editing support
@section Support for line editing libraries
Chronyc can be built with support for line editing, this allows you to use the
cursor keys to replay and edit old commands. Two libraries are supported which
provide such functionality, editline and GNU readline.
Please note that readline since version 6.0 is licensed under GPLv3+ which is
incompatible with chrony's license GPLv2. You should use editline instead if
you don't want to use older readline versions.
The configure script will automatically enable the line editing support if one
of the supported libraries is available. If they are both available, the
editline library will be used.
If you don't want to use it (in which case chronyc will use a minimal command
line interface), invoke configure like this:
@example
./configure --disable-readline other-options...
@end example
If you have editline, readline or ncurses installed in locations that aren't
normally searched by the compiler and linker, you need to use extra options:
@table @samp
@item --with-readline-includes=directory_name
This defines the name of the directory above the one where @file{readline.h}
is. @file{readline.h} is assumed to be in @file{editline} or @file{readline}
subdirectory of the named directory.
@item --with-readline-library=directory_name
This defines the directory containing the @file{libedit.a} or @file{libedit.so}
file, or @file{libreadline.a} or @file{libreadline.so} file.
@item --with-ncurses-library=directory_name
This defines the directory containing the @file{libncurses.a} or
@file{libncurses.so} file.
@end table
@c }}}
@c {{{
@node package builders
@section Extra options for package builders
The configure and make procedures have some extra options that may be useful if
you are building a distribution package for chrony.
The --infodir=DIR option to configure specifies an install directory
for the info files. This overrides the @file{info} subdirectory of the
argument to the --prefix option. For example, you might use
@example
./configure --prefix=/usr --infodir=/usr/share/info
@end example
The --mandir=DIR option to configure specifies an install directory
for the man pages. This overrides the @file{man} subdirectory of the
argument to the --prefix option.
@example
./configure --prefix=/usr --infodir=/usr/share/info --mandir=/usr/share/man
@end example
to set both options together.
The final option is the DESTDIR option to the make command. For example, you
could use the commands
@example
./configure --prefix=/usr --infodir=/usr/share/info --mandir=/usr/share/man
make all docs
make install DESTDIR=./tmp
cd tmp
tar cvf - . | gzip -9 > chrony.tar.gz
@end example
to build a package. When untarred within the root directory, this will install
the files to the intended final locations.
@c }}}
@c }}}
@c {{{ Ch:Typical operating scenarios
@c {{{ Chapter top
@node Typical scenarios
@chapter Typical operating scenarios
@menu
* Computers on the net:: Your computer is on the Internet most of the time
(or on a private network with NTP servers)
* Infrequent connection:: You connect to the Internet sometimes (e.g. via a modem)
* Isolated networks:: You have an isolated network with no reference clocks
* Dial-up home PCs:: Additional considerations if you turn your computer off
when it's not in use
* Configuration options overview:: Overview of some configuration options
@end menu
@c }}}
@c {{{ S:Permanent connection
@node Computers on the net
@section Computers connected to the internet
In this section we discuss how to configure chrony for computers that
are connected to the Internet (or to any network containing true NTP
servers which ultimately derive their time from a reference clock)
permanently or most of the time.
To operate in this mode, you will need to know the names of the NTP
server machines you wish to use. You may be able to find names of
suitable servers by one of the following methods:
@itemize @bullet
@item Your institution may already operate servers on its network.
Contact your system administrator to find out.
@item Your ISP probably has one or more NTP servers available for its
customers.
@item Somewhere under the NTP homepage there is a list of public
stratum 1 and stratum 2 servers. You should find one or more servers
that are near to you --- check that their access policy allows you to
use their facilities.
@item Use public servers from
@uref{http://www.pool.ntp.org/, the pool.ntp.org project}.
@end itemize
Assuming that you have found some servers, you need to set up a
configuration file to run chrony. The (compiled-in) default location
for this file is @file{@SYSCONFDIR@/chrony.conf}. Assuming that your NTP
servers are called @code{foo.example.net}, @code{bar.example.net} and
@code{baz.example.net}, your @file{chrony.conf} file could contain as a minimum
@example
server foo.example.net
server bar.example.net
server baz.example.net
@end example
However, you will probably want to include some of the other directives
described later. The following directives may be particularly useful :
@code{driftfile}, @code{makestep}, @code{rtcsync}. Also, the @code{iburst}
server option is useful to speed up the initial synchronization. The smallest
useful configuration file would look something like
@example
server foo.example.net iburst
server bar.example.net iburst
server baz.example.net iburst
driftfile @CHRONYVARDIR@/drift
makestep 1.0 3
rtcsync
@end example
When using a pool of NTP servers (one name is used for multiple servers which
may change over time), it's better to specify them with the @code{pool}
directive instead of multiple @code{server} directives. The configuration file
could in this case look like
@example
pool pool.ntp.org iburst
driftfile @CHRONYVARDIR@/drift
makestep 1.0 3
rtcsync
@end example
@c }}}
@c {{{ S:Infrequent connection
@node Infrequent connection
@section Infrequent connection to true NTP servers
In this section we discuss how to configure chrony for computers that
have occasional connections to the internet.
@menu
* Configuration for infrequent connections:: How to set up the @code{@SYSCONFDIR@/chrony.conf} file
* Advising chronyd of internet availability:: How to tell chronyd when the link is available
@end menu
@node Configuration for infrequent connections
@subsection Setting up the configuration file for infrequent connections
As in the previous section, you will need access to NTP servers on the
internet. The same remarks apply for how to find them.
In this case, you will need some additional configuration to tell
@code{chronyd} when the connection to the internet goes up and down.
This saves the program from continuously trying to poll the servers when
they are inaccessible.
Again, assuming that your NTP servers are called @code{foo.example.net},
@code{bar.example.net} and @code{baz.example.net}, your @file{chrony.conf} file
would need to contain something like
@example
server foo.example.net
server bar.example.net
server baz.example.net
@end example
However, your computer will keep trying to contact the servers to obtain
timestamps, even whilst offline. If you operate a dial-on-demand
system, things are even worse, because the link to the internet will
keep getting established.
For this reason, it would be better to specify this part of your
configuration file in the following way:
@example
server foo.example.net offline
server bar.example.net offline
server baz.example.net offline
@end example
The @code{offline} keyword indicates that the servers start in an offline
state, and that they should not be contacted until @code{chronyd} receives
notification from @code{chronyc} that the link to the internet is present.
The smallest useful configuration file would look something like
@example
server foo.example.net offline
server bar.example.net offline
server baz.example.net offline
driftfile @CHRONYVARDIR@/drift
makestep 1.0 3
rtcsync
@end example
The next section describes how to tell @code{chronyd} when the internet link
goes up and down.
@node Advising chronyd of internet availability
@subsection How to tell chronyd when the internet link is available.
To tell @code{chronyd} when to start and finish sampling the servers, the
@code{online} and @code{offline} commands of @code{chronyc} need to be used.
To give an example of their use, we assume that @code{pppd} is the
program being used to connect to the internet, and that @code{chronyc} has been
installed at its default location @file{@BINDIR@/chronyc}.
In the file @file{/etc/ppp/ip-up} we add the command sequence
@example
@BINDIR@/chronyc online
@end example
and in the file @file{/etc/ppp/ip-down} we add the sequence
@example
@BINDIR@/chronyc offline
@end example
@code{chronyd's} polling of the servers will now only occur whilst the
machine is actually connected to the Internet.
@c }}}
@c {{{ S:Isolated networks
@node Isolated networks
@section Isolated networks
In this section we discuss how to configure chrony for computers that
never have network conectivity to any computer which ultimately derives
its time from a reference clock.
In this situation, one computer is selected to be the master timeserver.
The other computers are either direct clients of the master, or clients
of clients.
The rate value in the master's drift file needs to be set to the average
rate at which the master gains or loses time. @code{chronyd} includes
support for this, in the form of the @code{manual} directive in the
configuration file and the @code{settime} command in the @code{chronyc}
program.
The @code{smoothtime} directive (@pxref{smoothtime directive}) is useful when
the clocks of the clients need to stay close together when the local time is
adjusted by the @code{settime} command. The smoothing process needs to be
activated by the @code{smoothtime activate} command when the local time is
ready to be served. After that point, any adjustments will be smoothed out.
A typical configuration file for the master (called @code{master}) might be
(assuming the clients are in the 192.168.165.x subnet)
@example
driftfile @CHRONYVARDIR@/drift
local stratum 8
manual
allow 192.168.165
smoothtime 400 0.01
@end example
For the clients the configuration file might be
@example
server master iburst
driftfile @CHRONYVARDIR@/drift
logdir /var/log/chrony
log measurements statistics tracking
@end example
@c }}}
@c {{{ S:Dial-up home PCs
@node Dial-up home PCs
@section The home PC with a dial-up connection
@menu
* Dial-up overview:: General discussion of how the software operates in this mode
* Dial-up configuration:: Typical configuration files
@end menu
@node Dial-up overview
@subsection Assumptions/how the software works
This section considers the home computer which has a dial-up connection.
It assumes that Linux is run exclusively on the computer. Dual-boot
systems may work; it depends what (if anything) the other system does to
the system's real-time clock.
Much of the configuration for this case is discussed earlier
(@pxref{Infrequent connection}). This section addresses specifically
the case of a computer which is turned off between 'sessions'.
In this case, @code{chronyd} relies on the computer's real-time clock
(RTC) to maintain the time between the periods when it is powered up.
The arrangement is shown in the figure below.
@example
@group
trim if required PSTN
+---------------------------+ +----------+
| | | |
v | | |
+---------+ +-------+ +-----+ +---+
| System's| measure error/ |chronyd| |modem| |ISP|
|real-time|------------------->| |-------| | | |
| clock | drift rate +-------+ +-----+ +---+
+---------+ ^ |
| | |
+---------------------------+ --o-----o---
set time at boot up |
+----------+
|NTP server|
+----------+
@end group
@end example
When the computer is connected to the Internet (via the modem),
@code{chronyd} has access to external NTP servers which it makes
measurements from. These measurements are saved, and straight-line fits
are performed on them to provide an estimate of the computer's time
error and rate of gaining/losing time.
When the computer is taken offline from the Internet, the best estimate
of the gain/loss rate is used to free-run the computer until it next
goes online.
Whilst the computer is running, @code{chronyd} makes measurements of the
real-time clock (RTC) (via the @file{/dev/rtc} interface, which must be
compiled into the kernel). An estimate is made of the RTC error at a
particular RTC second, and the rate at which the RTC gains or loses time
relative to true time.
On 2.6 and later kernels, if your motherboard has a HPET, you need to enable the
@samp{HPET_EMULATE_RTC} option in your kernel configuration. Otherwise, chrony
will not be able to interact with the RTC device and will give up using it.
When the computer is powered down, the measurement histories for all the
NTP servers are saved to files (if the @code{dumponexit} directive is
specified in the configuration file), and the RTC tracking information
is also saved to a file (if the @code{rtcfile} directive has been
specified). These pieces of information are also saved if the
@code{dump} and @code{writertc} commands respectively are issued through
@code{chronyc}.
When the computer is rebooted, @code{chronyd} reads the current RTC time
and the RTC information saved at the last shutdown. This information is
used to set the system clock to the best estimate of what its time would
have been now, had it been left running continuously. The measurement
histories for the servers are then reloaded.
The next time the computer goes online, the previous sessions'
measurements can contribute to the line-fitting process, which gives a
much better estimate of the computer's gain/loss rate.
One problem with saving the measurements and RTC data when the machine
is shut down is what happens if there is a power failure; the most
recent data will not be saved. Although @code{chronyd} is robust enough
to cope with this, some performance may be lost. (The main danger
arises if the RTC has been changed during the session, with the
@code{trimrtc} command in @code{chronyc}. Because of this,
@code{trimrtc} will make sure that a meaningful RTC file is saved out
after the change is completed).
The easiest protection against power failure is to put the @code{dump}
and @code{writertc} commands in the same place as the @code{offline}
command is issued to take @code{chronyd} offline; because @code{chronyd}
free-runs between online sessions, no parameters will change
significantly between going offline from the Internet and any power
failure.
A final point regards home computers which are left running for extended
periods and where it is desired to spin down the hard disc when it is
not in use (e.g. when not accessed for 15 minutes). @code{chronyd} has
been planned so it supports such operation; this is the reason why the
RTC tracking parameters are not saved to disc after every update, but
only when the user requests such a write, or during the shutdown
sequence. The only other facility that will generate periodic writes to
the disc is the @code{log rtc} facility in the configuration file; this
option should not be used if you want your disc to spin down.
@node Dial-up configuration
@subsection Typical configuration files.
To illustrate how a dial-up home computer might be configured, example
configuration files are shown in this section.
For the @file{@SYSCONFDIR@/chrony.conf} file, the following can be used as an
example.
@example
server foo.example.net maxdelay 0.4 offline
server bar.example.net maxdelay 0.4 offline
server baz.example.net maxdelay 0.4 offline
logdir /var/log/chrony
log statistics measurements tracking
driftfile @CHRONYVARDIR@/drift
makestep 1.0 3
maxupdateskew 100.0
dumponexit
dumpdir @CHRONYVARDIR@
rtcfile @CHRONYVARDIR@/rtc
@end example
@code{pppd} is used for connecting to the internet. This runs two scripts
@file{/etc/ppp/ip-up} and @file{/etc/ppp/ip-down} when the link goes
online and offline respectively.
The relevant part of the @file{/etc/ppp/ip-up} file is
@example
@BINDIR@/chronyc online
@end example
and the relevant part of the @file{/etc/ppp/ip-down} script is
@example
@BINDIR@/chronyc -m offline dump writertc
@end example
To start @code{chronyd} during the boot sequence, the following
is in @file{/etc/rc.d/rc.local} (this is a Slackware system)
@example
if [ -f @SBINDIR@/chronyd -a -f @SYSCONFDIR@/chrony.conf ]; then
@SBINDIR@/chronyd -r -s
echo "Start chronyd"
fi
@end example
The placement of this command may be important on some systems. In
particular, @code{chronyd} may need to be started before any software
that depends on the system clock not jumping or moving backwards,
depending on the directives in @code{chronyd's} configuration file.
For the system shutdown, @code{chronyd} should receive a SIGTERM several
seconds before the final SIGKILL; the SIGTERM causes the measurement
histories and RTC information to be saved out.
@c }}}
@c {{{ S:Other config options
@node Configuration options overview
@section Other important configuration options
The most common option to include in the configuration file is the
@code{driftfile} option. One of the major tasks of @code{chronyd} is to
work out how fast or how slow the system clock runs relative to real
time - e.g. in terms of seconds gained or lost per day. Measurements
over a long period are usually required to refine this estimate to an
acceptable degree of accuracy. Therefore, it would be bad if
@code{chronyd} had to work the value out each time it is restarted,
because the system clock would not run so accurately whilst the
determination is taking place.
To avoid this problem, @code{chronyd} allows the gain or loss rate to be
stored in a file, which can be read back in when the program is
restarted. This file is called the drift file, and might typically be
stored in @file{@CHRONYVARDIR@/drift}. By specifying an option like the
following
@example
driftfile @CHRONYVARDIR@/drift
@end example
in the configuration file (@file{@SYSCONFDIR@/chrony.conf}), the drift file
facility will be activated.
@c }}}
@c }}}
@c {{{ Ch:Usage reference
@node Usage reference
@chapter Usage reference
@c {{{ Chapter top
@menu
* Starting chronyd:: Command line options for the daemon
* Configuration file:: Format of the configuration file
* Running chronyc:: The run-time configuration program
@end menu
@c }}}
@c {{{ S:Starting chronyd
@node Starting chronyd
@section Starting chronyd
If @code{chronyd} has been installed to its default location
@file{@SBINDIR@/chronyd}, starting it is simply a matter of
entering the command
@example
@SBINDIR@/chronyd
@end example
Information messages and warnings will be logged to syslog.
If no configuration commands are specified on the command line,
@code{chronyd} will read the commands from the configuration file
(default @file{@SYSCONFDIR@/chrony.conf}).
The command line options supported are as follows:
@table @code
@item -n
When run in this mode, the program will not detach itself from the
terminal.
@item -d
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 @code{chronyd} was compiled with debugging support,
this option can be used twice to print also debugging messages.
@item -f <conf-file>
This option can be used to specify an alternate location for the
configuration file (default @file{@SYSCONFDIR@/chrony.conf}).
@item -r
This option will reload sample histories for each of the servers and refclocks being
used. These histories are created by using the @code{dump} command in
@code{chronyc}, or by setting the @code{dumponexit} directive in the
configuration file. This option is useful if you want to stop and
restart @code{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 @code{chronyd's}
control (i.e. Linux, FreeBSD, NetBSD and Solaris).
@item -R
When this option is used, the @code{initstepslew} directive and the
@code{makestep} directive used with a positive limit will be ignored.
This option is useful when restarting @code{chronyd} and can be used
in conjunction with the `-r' option.
@item -s
This option will set the system clock from the computer's real-time clock or
to the last modification time of the file specified by the @code{driftfile}
directive. Real-time clocks are supported only on Linux.
If used in conjunction with the `-r' flag, @code{chronyd} will attempt
to preserve the old samples after setting the system clock from the real
time clock (RTC). This can be used to allow @code{chronyd} to perform long
term averaging of the gain or loss rate across system reboots, and is
useful for dial-up systems that are shut down when not in use. For this
to work well, it relies on @code{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 the current time
and the RTC time, the system time will be set to it to restore the time when
@code{chronyd} was previously stopped. This is useful on computers that have
no RTC or the RTC is broken (e.g. it has no battery).
@item -u <user>
This option sets the name of the system user to which @code{chronyd} will
switch after start in order to drop root privileges. It overrides the
@code{user} directive (default @code{@DEFAULT_USER@}). It may be set to a
non-root user only when @code{chronyd} is compiled with support for Linux
capabilities (libcap) or on NetBSD with the @code{/dev/clockctl} device.
@item -F <level>
This option configures a system call filter when @code{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).
@item -q
When run in this mode, @code{chronyd} will set the system clock once
and exit. It will not detach from the terminal.
@item -Q
This option is similar to `-q', but it will only print the offset and
not correct the clock.
@item -v
This option displays @code{chronyd's} version number to the terminal and
exits.
@item -P <priority>
On Linux, this option will select the SCHED_FIFO real-time scheduler at the
specified priority (which must be between 0 and 100). On Mac OS X, 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.
@item -m
This option will lock chronyd into RAM so that it will never be paged
out. This mode is only supported on Linux.
@item -4
With this option hostnames will be resolved only to IPv4 addresses and only
IPv4 sockets will be created.
@item -6
With this option hostnames will be resolved only to IPv6 addresses and only
IPv6 sockets will be created.
@end table
On systems that support an @file{/etc/rc.local} file for starting
programs at boot time, @code{chronyd} can be started from there.
On systems with a System V style initialisation, a
suitable start/stop script might be as shown below. This might be
placed in the file @file{/etc/rc2.d/S83chrony}.
@example
@group
#!/bin/sh
# This file should have uid root, gid sys and chmod 744
#
killproc() @{ # kill the named process(es)
pid=`/usr/bin/ps -e |
/usr/bin/grep -w $1 |
/usr/bin/sed -e 's/^ *//' -e 's/ .*//'`
[ "$pid" != "" ] && kill $pid
@}
case "$1" in
'start')
if [ -f /opt/free/sbin/chronyd -a -f @SYSCONFDIR@/chrony.conf ]; then
/opt/free/sbin/chronyd
fi
;;
'stop')
killproc chronyd
;;
*)
echo "Usage: /etc/rc2.d/S83chrony @{ start | stop @}"
;;
esac
@end group
@end example
(In both cases, you may want to bear in mind that @code{chronyd} can
step the time when it starts. There may be other programs started at
boot time that could be upset by this, so you may need to consider the
ordering carefully. However, @code{chronyd} will need to start after
daemons providing services that it may require, e.g. the domain name
service.)
@c }}}
@c {{{ S:chronyd configuration file
@node Configuration file
@section The chronyd configuration file
@c {{{ section top
The configuration file is normally called @file{@SYSCONFDIR@/chrony.conf}; in
fact, this is the compiled-in default. However, other locations can be
specified with a command line option.
Each command in the configuration file is placed on a separate line.
The following sections describe each of the commands in turn. The
directives can occur in any order in the file and they are not
case-sensitive.
The configuration commands can also be specified directly on the
@code{chronyd} command line, each argument is parsed as a line and
the configuration file is ignored.
@menu
* comments in config file:: How to write a comment
* acquisitionport directive:: Set NTP client port
* allow directive:: Give access to NTP clients
* bindacqaddress directive:: Limit network interface used by NTP client
* bindaddress directive:: Limit network interface used by NTP server
* bindcmdaddress directive:: Limit network interface used for commands
* broadcast directive:: Make chronyd act as an NTP broadcast server
* clientloglimit directive:: Set client log memory limit
* cmdallow directive:: Give monitoring access to chronyc on other computers
* cmddeny directive:: Deny monitoring access to chronyc on other computers
* cmdport directive:: Set port to use for runtime monitoring
* combinelimit directive:: Limit sources included in combining algorithm
* corrtimeratio directive:: Set correction time ratio
* deny directive:: Deny access to NTP clients
* driftfile directive:: Specify location of file containing drift data
* dumpdir directive:: Specify directory for dumping measurements
* dumponexit directive:: Dump measurements when daemon exits
* fallbackdrift directive:: Specify fallback drift intervals
* hwclockfile directive:: Specify location of hwclock's adjtime file
* include directive:: Include a configuration file
* initstepslew directive:: Trim the system clock on boot-up
* keyfile directive:: Specify location of file containing keys
* leapsecmode directive:: Select leap second handling mode
* leapsectz directive:: Read leap second data from tz database
* local directive:: Allow unsynchronised machine to act as server
* lock_all directive:: Require that chronyd be locked into RAM
* log directive:: Make daemon log certain sets of information
* logbanner directive:: Specify how often is banner written to log files
* logchange directive:: Generate syslog messages if large offsets occur
* logdir directive:: Specify directory for logging
* mailonchange directive:: Send email if a clock correction above a threshold occurs
* makestep directive:: Step system clock if large correction is needed
* manual directive:: Allow manual entry using chronyc's settime cmd
* maxchange directive:: Set maximum allowed offset
* maxclockerror directive:: Set maximum frequency error of local clock
* maxdistance directive:: Set maximum allowed distance of sources
* maxsamples directive:: Set maximum number of samples per source
* maxslewrate directive:: Set maximum slew rate
* maxupdateskew directive:: Stop bad estimates upsetting machine clock
* minsamples directive:: Set minimum number of samples per source
* minsources directive:: Set minimum number of selectable sources to update clock
* noclientlog directive:: Prevent chronyd from gathering data about clients
* peer directive:: Specify an NTP peer
* pidfile directive:: Specify the file where chronyd's pid is written
* pool directive:: Specify an NTP pool
* port directive:: Set NTP server port
* refclock directive:: Specify a reference clock
* reselectdist directive:: Set improvement in distance needed to reselect a source
* rtcautotrim directive:: Specify threshold at which RTC is trimmed automatically
* rtcdevice directive:: Specify name of enhanced RTC device (if not /dev/rtc)
* rtcfile directive:: Specify the file where real-time clock data is stored
* rtconutc directive:: Specify that the real time clock keeps UTC not local time
* rtcsync directive:: Specify that RTC should be automatically synchronised by kernel
* sched_priority directive:: Require real-time scheduling and specify a priority for it
* server directive:: Specify an NTP server
* smoothtime directive:: Smooth served time to keep clients close together
* stratumweight directive:: Specify how important is stratum when selecting source
* tempcomp directive:: Specify temperature sensor and compensation coefficients
* user directive:: Specify user for dropping root privileges
@end menu
@c }}}
@c {{{ comments in config file
@node comments in config file
@subsection Comments in the configuration file
The configuration file may contain comment lines. A comment line is any line
that starts with zero or more spaces followed by any one of the following
characters:
@itemize
@item !
@item ;
@item #
@item %
@end itemize
Any line with this format will be ignored.
@c }}}
@c {{{ acquisitionport directive
@node acquisitionport directive
@subsection acquisitionport
By default, @code{chronyd} uses a separate client socket for each configured
server and their source port is chosen arbitrarily by the operating system.
However, you can use the @code{acquisitionport} directive to explicitly specify
a port and use only one socket (per IPv4/IPv6 address family) for all
configured servers. This may be useful for getting through firewalls. If set
to 0, the source port of the socket will be chosen arbitrarily.
It may be set to the same port as used by the NTP server (@pxref{port
directive}) to use only one socket for all NTP packets.
An example of the @code{acquisitionport} command is
@example
acquisitionport 1123
@end example
This would change the source port used for client requests to udp/1123. You
could then persuade the firewall administrator to let that port through.
@c }}}
@c {{{ allow
@node allow directive
@subsection allow
The @code{allow} command is used to designate a particular subnet from
which NTP clients are allowed to access the computer as an NTP server.
The default is that no clients are allowed access, i.e. @code{chronyd}
operates purely as an NTP client. If the @code{allow} directive is
used, @code{chronyd} will be both a client of its servers, and a server
to other clients.
Examples of use of the command are as follows:
@example
allow foo.example.net
allow 1.2
allow 3.4.5
allow 6.7.8/22
allow 6.7.8.9/22
allow 2001:db8::/32
allow 0/0
allow ::/0
allow
@end example
The first command allows the named node to be an NTP client of this computer.
The second command allows any node with an IPv4 address of the form 1.2.x.y (with
x and y arbitrary) to be an NTP client of this computer. Likewise, the third
command allows any node with an IPv4 address of the form 3.4.5.x to have client
NTP access. The fourth and fifth forms allow access from any node with an IPv4
address of the form 6.7.8.x, 6.7.9.x, 6.7.10.x or 6.7.11.x (with x arbitrary),
i.e. the value 22 is the number of bits defining the specified subnet. (In the
fifth form, the final byte is ignored). The sixth form is used for IPv6
addresses. The seventh and eighth forms allow access by any IPv4 and IPv6 node
respectively. The ninth forms allows access by any node (IPv4 or IPv6).
A second form of the directive, @code{allow all}, has a greater effect,
depending on the ordering of directives in the configuration file. To
illustrate the effect, consider the two examples
@example
allow 1.2.3.4
deny 1.2.3
allow 1.2
@end example
and
@example
allow 1.2.3.4
deny 1.2.3
allow all 1.2
@end example
In the first example, the effect is the same regardles of what order the
three directives are given in. So the 1.2.x.y subnet is allowed access,
except for the 1.2.3.x subnet, which is denied access, however the host
1.2.3.4 is allowed access.
In the second example, the @code{allow all 1.2} directives overrides the
effect of @emph{any} previous directive relating to a subnet within the
specified subnet. Within a configuration file this capability is
probably rather moot; however, it is of greater use for reconfiguration
at run-time via @code{chronyc} (@pxref{allow all command}).
Note, if the @code{initstepslew} directive (@pxref{initstepslew
directive}) is used in the configuration file, each of the computers
listed in that directive must allow client access by this computer for
it to work.
@c }}}
@c {{{ bindacqaddress
@node bindacqaddress directive
@subsection bindacqaddress
The @code{bindacqaddress} directive sets the network interface to which will
@code{chronyd} bind its NTP client sockets. The syntax is similar to the
@code{bindaddress} and @code{bindcmdaddress} directives.
For each of IPv4 and IPv6 protocols, only one @code{bindacqaddress}
directive can be specified.
@c }}}
@c {{{ bindaddress
@node bindaddress directive
@subsection bindaddress
The @code{bindaddress} directive allows you to restrict the network interface
to which @code{chronyd} will listen for NTP requests. This provides an
additional level of access restriction above that available through the
@code{deny} mechanism.
Suppose you have a local ethernet with addresses in the 192.168.1.0
subnet together with an internet connection. The ethernet interface's IP
address is 192.168.1.1. Suppose you want to block all access through the
internet connection. You could add the line
@example
bindaddress 192.168.1.1
@end example
to the configuration file.
For each of IPv4 and IPv6 protocols, only one @code{bindaddress} directive can
be specified. Therefore, it's not useful on computers which should serve NTP
on multiple network interfaces.
@c }}}
@c {{{ bindcmdaddress
@node bindcmdaddress directive
@subsection bindcmdaddress
The @code{bindcmdaddress} directive allows you to specify the network
interface to which @code{chronyd} will listen for monitoring command packets
(issued by @code{chronyc}). This provides an additional level of access
restriction above that available through @code{cmddeny} mechanism.
This directive can also change the path of the Unix domain command socket,
which is used by @code{chronyc} to send configuration commands. The socket
must be in a directory that is accessible only by the root or chrony user. The
directory will be created on start if it doesn't exist. The default path of
the socket is @code{@CHRONYSOCKDIR@/chronyd.sock}.
By default, @code{chronyd} binds to the loopback interface (with addresses
@code{127.0.0.1} and @code{::1}). This blocks all access except from
localhost. To listen for command packets on all interfaces, you can add the
lines
@example
bindcmdaddress 0.0.0.0
bindcmdaddress ::
@end example
to the configuration file.
For each of IPv4 and IPv6 protocols, only one @code{bindcmdaddress}
directive can be specified.
An example that sets the path of the Unix domain command socket is
@example
bindcmdaddress /var/run/chrony/chronyd.sock
@end example
@c }}}
@c {{{ broadcast directive
@node broadcast directive
@subsection broadcast
The @code{broadcast} directive is used to declare a broadcast address to which
chronyd should send packets in NTP broadcast mode (i.e. make chronyd act as a
broadcast server). Broadcast clients on that subnet will be able to
synchronise.
The syntax is as follows
@example
broadcast 30 192.168.1.255
broadcast 60 192.168.2.255 12123
broadcast 60 ff02::101
@end example
In the first example, the destination port defaults to 123/udp (the normal NTP
port). In the second example, the destionation port is specified as 12123.
The first parameter in each case (30 or 60 respectively) is the interval in
seconds between broadcast packets being sent. The second parameter in each
case is the broadcast address to send the packet to. This should correspond to
the broadcast address of one of the network interfaces on the computer where
chronyd is running.
You can have more than 1 @code{broadcast} directive if you have more than 1
network interface onto which you wish to send NTP broadcast packets.
@code{chronyd} itself cannot currently act as a broadcast client; it must always be
configured as a point-to-point client by defining specific NTP servers and
peers. This broadcast server feature is intended for providing a time source
to other NTP software (e.g. various MS Windows clients).
If ntpd is used as the broadcast client, it will try to use a point-to-point
client/server NTP access to measure the round-trip delay. Thus, the broadcast
subnet should also be the subject of an @code{allow} directive (@pxref{allow
directive}).
@c }}}
@c {{{ clientloglimit
@node clientloglimit directive
@subsection clientloglimit
This directive specifies the maximum size of the memory allocated to
log client accesses. When the limit is reached, only information for
clients that have already been logged will be updated. If 0 is
specified, the memory size will be unlimited. The default is 524288
bytes.
An example of the use of this directive is
@example
clientloglimit 1048576
@end example
@c }}}
@c {{{ cmdallow
@node cmdallow directive
@subsection cmdallow
This is similar to the @code{allow} directive (@pxref{allow directive}), except
that it allows monitoring access (rather than NTP client access) to a particular
subnet or host. (By 'monitoring access' is meant that @code{chronyc} can be
run on those hosts and retrieve monitoring data from @code{chronyd} on this
computer.)
The syntax is identical to the @code{allow} directive.
There is also a @code{cmdallow all} directive with similar behaviour to the
@code{allow all} directive (but applying to monitoring access in this case, of
course).
Note that @code{chronyd} has to be configured with the @code{bindcmdaddress}
directive to not listen only on the loopback interface to actually allow remote
access.
@c }}}
@c {{{ cmddeny
@node cmddeny directive
@subsection cmddeny
This is similar to the @code{cmdallow} directive (@pxref{cmdallow directive}),
except that it denies monitoring access to a particular subnet or host,
rather than allowing it.
The syntax is identical.
There is also a @code{cmddeny all} directive with similar behaviour to the
@code{cmdallow all} directive.
@c }}}
@c {{{ cmdport
@node cmdport directive
@subsection cmdport
The @code{cmdport} directive allows the port that is used for run-time
monitoring (via the @code{chronyc} program) to be altered
from its default (323/udp). If set to 0, @code{chronyd} will not open the
port, this is useful to disable the @code{chronyc} access from the internet.
(It does not disable the Unix domain command socket.)
An example shows the syntax
@example
cmdport 257
@end example
This would make @code{chronyd} use 257/udp as its command port.
(@code{chronyc} would need to be run with the @code{-p 257} switch to
inter-operate correctly).
@c }}}
@c {{{ combinelimit
@node combinelimit directive
@subsection combinelimit
When @code{chronyd} has multiple sources available for synchronization, it has
to select one source as the synchronization source. The measured offsets and
frequencies of the system clock relative to the other sources, however, can be
combined with the selected source to improve the accuracy of the system clock.
The @code{combinelimit} directive limits which sources are included in the
combining algorithm. Their synchronization distance has to be shorter than the
distance of the selected source multiplied by the value of the limit. Also,
their measured frequencies have to be close to the frequency of the selected
source.
By default, the limit is 3. Setting the limit to 0 effectively disables the
source combining algorithm and only the selected source will be used to
control the system clock.
The syntax is
@example
combinelimit <limit>
@end example
@c }}}
@c {{{ corrtimeratio
@node corrtimeratio directive
@subsection corrtimeratio
When @code{chronyd} is slewing the system clock to correct an offset, the rate
at which it is slewing adds to the frequency error of the clock. On Linux,
FreeBSD, NetBSD and Solaris this rate can be controlled.
The @code{corrtimeratio} directive sets the ratio between the
duration in which the clock is slewed for an average correction
according to the source history and the interval in which the
corrections are done (usually the NTP polling interval). Corrections
larger than the average take less time and smaller corrections take
more time, the amount of the correction and the correction time are
inversely proportional.
Increasing @code{corrtimeratio} improves the overall frequency error
of the system clock, but increases the overall time error as the
corrections take longer.
By default, the ratio is set to 3, the time accuracy of the clock is
preferred over its frequency accuracy.
The syntax is
@example
corrtimeratio 100
@end example
The maximum allowed slew rate can be set by the @code{maxslewrate}
directive (@pxref{maxslewrate directive}. The current remaining
correction is shown in the @code{tracking} report (@pxref{tracking
command}) as the @code{System time} value.
@c }}}
@c {{{ deny
@node deny directive
@subsection deny
This is similar to the @code{allow} directive (@pxref{allow directive}),
except that it denies NTP client access to a particular subnet or host,
rather than allowing it.
The syntax is identical.
There is also a @code{deny all} directive with similar behaviour to the
@code{allow all} directive.
@c }}}
@c {{{ driftfile
@node driftfile directive
@subsection driftfile
One of the main activities of the @code{chronyd} program is to work out
the rate at which the system clock gains or loses time relative to real
time.
Whenever @code{chronyd} computes a new value of the gain/loss rate, it
is desirable to record it somewhere. This allows @code{chronyd} to
begin compensating the system clock at that rate whenever it is
restarted, even before it has had a chance to obtain an equally good
estimate of the rate during the new run. (This process may take many
minutes, at least).
The driftfile command allows a file to be specified into which
@code{chronyd} can store the rate information. Two parameters are
recorded in the file. The first is the rate at which the system clock
gains or loses time, expressed in parts per million, with gains
positive. Therefore, a value of 100.0 indicates that when the system
clock has advanced by a second, it has gained 100 microseconds on
reality (so the true time has only advanced by 999900 microseconds).
The second is an estimate of the error bound around the first value in
which the true rate actually lies.
An example of the driftfile command is
@example
driftfile @CHRONYVARDIR@/drift
@end example
@c }}}
@c {{{ dumpdir
@node dumpdir directive
@subsection dumpdir
To compute the rate of gain or loss of time, @code{chronyd} has to store
a measurement history for each of the time sources it uses.
Certain systems (Linux, FreeBSD, NetBSD, Solaris) have operating system
support for setting the rate of gain or loss to compensate for known errors.
(On Mac OS X, @code{chronyd} must simulate such a capability by periodically
slewing the system clock forwards or backwards by a suitable amount to
compensate for the error built up since the previous slew).
For such systems, it is possible to save the measurement history across
restarts of @code{chronyd} (assuming no changes are made to the system
clock behaviour whilst it is not running). If this capability is to be
used (via the dumponexit command in the configuration file, or the dump
command in chronyc), the dumpdir command should be used to define the
directory where the measurement histories are saved.
An example of the command is
@example
dumpdir @CHRONYVARDIR@
@end example
A source whose reference id (the IP address for IPv4 sources) is
1.2.3.4 would have its measurement history saved in the file
@file{/var/lib/chrony/1.2.3.4.dat}.
@c }}}
@c {{{ dumponexit
@node dumponexit directive
@subsection dumponexit
If this command is present, it indicates that @code{chronyd} should save
the measurement history for each of its time sources recorded whenever
the program exits. (See the dumpdir command above).
@c }}}
@c {{{ fallbackdrift
@node fallbackdrift directive
@subsection fallbackdrift
Fallback drifts are long-term averages of the system clock drift
calculated over exponentially increasing intervals. They are used
when the clock is no longer synchronised to avoid quickly drifting
away from true time if there was a short-term deviation in the drift
before the synchronisation was lost.
The directive specifies the minimum and maximum interval since last
clock update to switch between fallback drifts. They are defined as a
power of 2 (in seconds). The syntax is as follows
@example
fallbackdrift 16 19
@end example
In this example, the minimum interval is 16 (18 hours) and maximum
interval is 19 (6 days). The system clock frequency will be set to
the first fallback 18 hours after last clock update, to the
second after 36 hours, etc. This might be a good setting to cover
daily and weekly temperature fluctuations.
By default (or if the specified maximum or minimum is 0), no fallbacks
are used and the clock frequency changes only with new measurements from
NTP, reference clocks or manual input.
@c }}}
@c {{{ hwclockfile
@node hwclockfile directive
@subsection hwclockfile
The @code{hwclockfile} directive sets the location of the adjtime file which is
used by the @file{/sbin/hwclock} program. With this directive, @code{chronyd}
will parse the file to find out if the RTC keeps local time or UTC. It
overrides the @code{rtconutc} directive (@pxref{rtconutc directive}).
An example of the command is
@example
hwclockfile /etc/adjtime
@end example
@c }}}
@c {{{ include
@node include directive
@subsection include
The @code{include} directive includes a specified configuration file or
multiple configuration files when a wildcard pattern is specified. This can be
useful when maintaining configuration on multiple hosts to keep the differences
in separate files.
An example of the command is
@example
include @SYSCONFDIR@/chrony.d/*.conf
@end example
@c }}}
@c {{{ initstepslew
@node initstepslew directive
@subsection initstepslew
In normal operation, @code{chronyd} slews the time when it needs to
adjust the system clock. For example, to correct a system clock which
is 1 second slow, @code{chronyd} slightly increases the amount by which the
system clock is advanced on each clock interrupt, until the error is
removed. (Actually, this is done by calling the @code{adjtime()} or
similar system function which does it for us.) Note that at no time
does time run backwards with this method.
On most Unix systems it is not desirable to step the system clock,
because many programs rely on time advancing monotonically forwards.
When the @code{chronyd} daemon is initially started, it is possible that the
system clock is considerably in error. Attempting to correct such an
error by slewing may not be sensible, since it may take several hours
to correct the error by this means.
The purpose of the @code{initstepslew} directive is to allow @code{chronyd} to
make a rapid measurement of the system clock error at boot time, and to
correct the system clock by stepping before normal operation begins.
Since this would normally be performed only at an appropriate point in
the system boot sequence, no other software should be adversely affected
by the step.
If the correction required is less than a specified threshold, a slew is
used instead. This makes it easier to restart @code{chronyd} whilst the
system is in normal operation.
The @code{initstepslew} directive takes a threshold and a list of NTP
servers as arguments. Each of the servers
is rapidly polled several times, and a majority voting mechanism used to
find the most likely range of system clock error that is present. A
step (or slew) is applied to the system clock to correct this error.
@code{chronyd} then enters its normal operating mode.
An example of use of the command is
@example
initstepslew 30 foo.example.net bar.example.net
@end example
where 2 NTP servers are used to make the measurement. The @code{30}
indicates that if the system's error is found to be 30 seconds or less,
a slew will be used to correct it; if the error is above 30 seconds, a
step will be used.
The @code{initstepslew} directive can also be used in an isolated LAN
environment, where the clocks are set manually. The most stable
computer is chosen as the master, and the other computers are slaved to
it. If each of the slaves is configured with the local option (see
below), the master can be set up with an @code{initstepslew} directive
which references some or all of the slaves. Then, if the master machine
has to be rebooted, the slaves can be relied on to 'flywheel' the time
for the master.
The @code{initstepslew} directive is functionally similar to a
combination of the @code{makestep} and @code{server} directives with
the @code{iburst} option. The main difference is that the
@code{initstepslew} servers are used only before normal operation
begins and that the foreground @code{chronyd} process waits for
@code{initstepslew} to finish before exiting. This is useful to
prevent programs started in the boot sequence after @code{chronyd}
from reading the clock before it's stepped.
@c }}}
@c {{{ keyfile
@node keyfile directive
@subsection keyfile
This command is used to specify the location of the file containing
ID/key pairs for authentication of NTP packets.
The format of the command is shown in the example below
@example
keyfile @SYSCONFDIR@/chrony.keys
@end example
The argument is simply the name of the file containing the ID/key
pairs. The format of the file is shown below
@example
10 tulip
11 hyacinth
20 MD5 ASCII:crocus
25 SHA1 HEX:1dc764e0791b11fa67efc7ecbc4b0d73f68a070c
...
@end example
Each line consists of an ID, a name of authentication hash function (optional)
and a password. The ID can be any unsigned integer in the range 1 through
2**32-1. The hash function is MD5 by default, depending on how was
@code{chronyd} compiled, other allowed hash functions may be SHA1, SHA256,
SHA384, SHA512, RMD128, RMD160, RMD256, RMD320, TIGER and WHIRLPOOL. The
password can be encoded as a string of characters not containing a space with
optional @code{ASCII:} prefix or as a hexadecimal number with @code{HEX:}
prefix.
The password is used with the hash function to generate and verify a message
authentication code (MAC) in NTP packets.
For maximum security, it's recommended to use SHA1 or stronger hash function.
The passwords should be random and they should be as long as the output size of
the configured hash function, e.g. 160 bits with SHA1.
These shell commands can be used to generate random MD5 and SHA1 keys on
systems which have the @code{/dev/urandom} device:
@example
echo "1 MD5 HEX:$(tr -d -c '[:xdigit:]' < /dev/urandom | head -c 32)"
echo "1 SHA1 HEX:$(tr -d -c '[:xdigit:]' < /dev/urandom | head -c 40)"
@end example
@c }}}
@c {{{ leapsecmode
@node leapsecmode directive
@subsection leapsecmode
A leap second is an adjustment that is occasionally applied to UTC to keep it
close to the mean solar time. When a leap second is inserted, the last day of
June or December has an extra second 23:59:60.
For computer clocks that is a problem. The Unix time is defined as number of
seconds since 00:00:00 UTC on 1 January 1970 without leap seconds. The system
clock cannot have time 23:59:60, every minute has 60 seconds and every day has
86400 seconds by definition. The inserted leap second is skipped and the clock
is suddenly ahead of UTC by one second. The @code{leapsecmode} directive
selects how that error is corrected. There are four options:
@table @code
@item system
When inserting a leap second, the kernel steps the system clock backwards by
one second when the clock gets to 00:00:00 UTC. When deleting a leap second,
it steps forward by one second when the clock gets to 23:59:59 UTC. This is
the default mode when the system driver supports leap seconds (i.e. on
Linux, FreeBSD, NetBSD and Solaris).
@item step
This is similar to the @code{system} mode, except the clock is stepped by
@code{chronyd} instead of the kernel. It can be useful to avoid bugs in the
kernel code that would be executed in the @code{system} mode. This is the
default mode when the system driver doesn't support leap seconds.
@item slew
The clock is corrected by slewing started at 00:00:00 UTC when a leap second is
inserted or 23:59:59 UTC when a leap second is deleted. This may be preferred
over the @code{system} and @code{step} modes when applications running on the
system are sensitive to jumps in the system time and it's acceptable that the
clock will be off for a longer time. On Linux with the default
@code{maxslewrate} value (@pxref{maxslewrate directive}) the correction takes
12 seconds.
@item ignore
No correction is applied to the clock for the leap second. The clock will be
corrected later in normal operation when new measurements are made and the
estimated offset includes the one second error.
@end table
An example of the command is
@example
leapsecmode slew
@end example
When serving time to NTP clients that can't be configured to correct their
clocks for a leap second by slewing or they would correct them at slightly
different rates when it's necessary to keep them close together, the
@code{slew} mode can be combined with the @code{smoothtime} directive
(@pxref{smoothtime directive}) to enable a server leap smear.
When smearing a leap second, the leap status is suppressed on the server and
the served time is corrected slowly be slewing instead of stepping. The clients
don't need any special configuration as they don't know there is any leap
second and they follow the server time which eventually brings them back to
UTC. Care must be taken to ensure they use for synchronization only NTP
servers which smear the leap second in exactly the same way.
This feature needs to be used carefully, because the server is intentionally
not serving its best estimate of the true time.
A recommended configuration to enable a server leap smear is:
@example
leapsecmode slew
maxslewrate 1000
smoothtime 400 0.001 leaponly
@end example
The first directive is necessary to disable the clock step which would reset
the smoothing process. The second directive limits the slewing rate of the
local clock to 1000 ppm, which improves the stability of the smoothing process
when the local correction starts and ends. The third directive enables the
server time smoothing process. It will start when the clock gets to 00:00:00
UTC and it will take 17 hours 34 minutes to finish. The frequency offset will
be changing by 0.001 ppm per second and will reach maximum of 31.623 ppm. The
@code{leaponly} option makes the duration of the leap smear constant and allows
the clients to safely synchronise with multiple identically configured leap
smearing servers.
@c }}}
@c {{{ leapsectz
@node leapsectz directive
@subsection leapsectz
This directive is used to set the name of the timezone in the system
tz database which @code{chronyd} can use to find out when will the
next leap second occur. It will periodically check if the times
23:59:59 and 23:59:60 are valid on Jun 30 and Dec 31 in the timezone.
A useful timezone is @code{right/UTC}.
This is mainly useful with reference clocks which don't provide the
leap second information. It is not necessary to restart
@code{chronyd} if the tz database is updated with a new leap second at
least 12 hours before the event.
An example of the command is
@example
leapsectz right/UTC
@end example
The following shell command verifies that the timezone contains leap
seconds and can be used with this directive
@example
$ TZ=right/UTC date -d 'Dec 31 2008 23:59:60'
Wed Dec 31 23:59:60 UTC 2008
@end example
@c }}}
@c {{{ local
@node local directive
@subsection local
The local keyword is used to allow @code{chronyd} to appear synchronised
to real time (from the viewpoint of clients polling it), even if it has
no current synchronisation source.
This option is normally used on computers in an isolated network,
where several computers are required to synchronise to one other, this
being the "master" which is kept vaguely in line with real time by
manual input.
An example of the command is
@example
local stratum 10
@end example
The value 10 may be substituted with other values in the range 1
through 15. Stratum 1 indicates a computer that has a true real-time
reference directly connected to it (e.g. GPS, atomic clock etc)
&ndash; such computers are expected to be very close to real time.
Stratum 2 computers are those which have a stratum 1 server; stratum 3
computers have a stratum 2 server and so on.
A large value of 10 indicates that the clock is so many hops away from
a reference clock that its time is fairly unreliable. Put another
way, if the computer ever has access to another computer which is
ultimately synchronised to a reference clock, it will almost certainly
be at a stratum less than 10. Therefore, the choice of a high value
like 10 for the local command prevents the machine's own time from
ever being confused with real time, were it ever to leak out to
clients that have visibility of real servers.
@c }}}
@c {{{ lock_all
@node lock_all directive
@subsection lock_all
The @code{lock_all} directive will lock chronyd into RAM so that it
will never be paged out. This mode is only supported on Linux. This
directive uses the Linux mlockall() system call to prevent @code{chronyd}
from ever being swapped out. This should result in lower and more
consistent latency. It should not have significant impact on
performance as @code{chronyd's} memory usage is modest. The mlockall man
page has more details.
@c }}}
@c {{{ log
@node log directive
@subsection log
@c {{{ section top
The log command indicates that certain information is to be logged.
@table @code
@item measurements
This option logs the raw NTP measurements and related information to a
file called measurements.log.
@item statistics
This option logs information about the regression processing to a file
called statistics.log.
@item tracking
This option logs changes to the estimate of the system's gain or loss
rate, and any slews made, to a file called tracking.log.
@item rtc
This option logs information about the system's real-time clock.
@item refclocks
This option logs the raw and filtered reference clock measurements to
a file called refclocks.log.
@item tempcomp
This option logs the temperature measurements and system rate
compensations to a file called tempcomp.log.
@end table
The files are written to the directory specified by the logdir
command.
An example of the command is
@example
log measurements statistics tracking
@end example
@menu
* measurements log:: The format of the measurements log
* statistics log:: The format of the statistics log
* tracking log:: The format of the tracking log
* RTC log:: The format of the RTC log
* refclocks log:: The format of the refclocks log
* tempcomp log:: The format of the tempcomp log
@end menu
@c }}}
@c {{{ measurements.log
@node measurements log
@subsubsection Measurements log file format
An example line (which actually appears as a single line in the file)
from the measurements log file is shown below.
@example
2014-10-13 05:40:50 158.152.1.76 N 2 111 111 1111 10 10 1.0 \
-4.966e-03 2.296e-01 1.577e-05 1.615e-01 7.446e-03
@end example
The columns are as follows (the quantities in square brackets are the
values from the example line above) :
@enumerate 1
@item
Date [2014-10-13]
@item
Hour:Minute:Second [05:40:50]. Note that the date/time pair is
expressed in UTC, not the local time zone.
@item
IP address of server/peer from which measurement comes [158.152.1.76]
@item
Leap status (@code{N} means normal, @code{+} means that the last minute
of the current month has 61 seconds, @code{-} means that the last minute
of the month has 59 seconds, @code{?} means the remote computer is not
currently synchronised.) [N]
@item
Stratum of remote computer. [2]
@item
RFC 5905 tests 1 through 3 (1=pass, 0=fail) [111]
@item
RFC 5905 tests 5 through 7 (1=pass, 0=fail) [111]
@item
Tests for maximum delay, maximum delay ratio and maximum delay dev ratio,
against defined parameters, and a test for synchronisation loop
(1=pass, 0=fail) [1111]
@item
Local poll [10]
@item
Remote poll [10]
@item
`Score' (an internal score within each polling level used to decide when
to increase or decrease the polling level. This is adjusted based on number
of measurements currently being used for the regression algorithm). [1.0]
@item
The estimated local clock error (`theta' in RFC 5905). Positive
indicates that the local clock is slow of the remote source. [-4.966e-03].
@item
The peer delay (`delta' in RFC 5905). [2.296e-01]
@item
The peer dispersion (`epsilon' in RFC 5905). [1.577e-05]
@item
The root delay (`DELTA' in RFC 5905). [1.615e-01]
@item
The root dispersion (`EPSILON' in RFC 5905). [7.446e-03]
@end enumerate
A banner is periodically written to the log file to indicate the
meanings of the columns.
@c }}}
@c {{{ statistics.log
@node statistics log
@subsubsection Statistics log file format
An example line (which actually appears as a single line in the file)
from the statistics log file is shown below.
@example
1998-07-22 05:40:50 158.152.1.76 6.261e-03 -3.247e-03 \
2.220e-03 1.874e-06 1.080e-06 7.8e-02 16 0 8
@end example
The columns are as follows (the quantities in square brackets are the
values from the example line above) :
@enumerate 1
@item
Date [1998-07-22]
@item
Hour:Minute:Second [05:40:50]. Note that the date/time pair is
expressed in UTC, not the local time zone.
@item
IP address of server/peer from which measurement comes [158.152.1.76]
@item
The estimated standard deviation of the measurements from the source (in
seconds). [6.261e-03]
@item
The estimated offset of the source (in seconds, positive means the local
clock is estimated to be fast, in this case). [-3.247e-03]
@item
The estimated standard deviation of the offset estimate (in
seconds). [2.220e-03]
@item
The estimated rate at which the local clock is gaining or losing time
relative to the source (in seconds per second, positive means the local
clock is gaining). This is relative to the compensation currently being
applied to the local clock, @emph{not} to the local clock without any
compensation. [1.874e-06]
@item
The estimated error in the rate value (in seconds per
second). [1.080e-06].
@item
The ration of |old_rate - new_rate| / old_rate_error. Large values
indicate the statistics are not modelling the source very well. [7.8e-02]
@item
The number of measurements currently being used for the regression
algorithm. [16]
@item
The new starting index (the oldest sample has index 0; this is the
method used to prune old samples when it no longer looks like the
measurements fit a linear model). [0, i.e. no samples discarded this
time]
@item
The number of runs. The number of runs of regression residuals with the
same sign is computed. If this is too small it indicates that the
measurements are no longer represented well by a linear model and that
some older samples need to be discarded. The number of runs for the
data that is being retained is tabulated. Values of approximately half
the number of samples are expected. [8]
@end enumerate
A banner is periodically written to the log file to indicate the
meanings of the columns.
@c }}}
@c {{{ tracking.log
@node tracking log
@subsubsection Tracking log file format
An example line (which actually appears as a single line in the file)
from the tracking log file is shown below.
@example
2012-02-23 05:40:50 158.152.1.76 3 340.529 1.606 1.046e-03 N \
4 6.849e-03 -4.670e-04
@end example
The columns are as follows (the quantities in square brackets are the
values from the example line above) :
@enumerate 1
@item
Date [2012-02-03]
@item
Hour:Minute:Second [05:40:50]. Note that the date/time pair is
expressed in UTC, not the local time zone.
@item
The IP address of the server/peer to which the local system is
synchronised. [158.152.1.76]
@item
The stratum of the local system. [3]
@item
The local system frequency (in ppm, positive means the local system runs
fast of UTC). [340.529]
@item
The error bounds on the frequency (in ppm) [1.606]
@item
The estimated local offset at the epoch (which is rapidly corrected by
slewing the local clock. (In seconds, positive indicates the local
system is fast of UTC). [1.046e-3]
@item
Leap status (@code{N} means normal, @code{+} means that the last minute
of this month has 61 seconds, @code{-} means that the last minute of the month
has 59 seconds, @code{?} means the clock is not currently synchronised.) [N]
@item
The number of combined sources. [4]
@item
The estimated standard deviation of the combined offset (in seconds).
[6.849e-03]
@item
The remaining offset correction from the previous update (in seconds, positive
means the system clock is slow of UTC). [-4.670e-04]
@end enumerate
A banner is periodically written to the log file to indicate the
meanings of the columns.
@c }}}
@c {{{ rtc.log
@node RTC log
@subsubsection Real-time clock log file format
An example line (which actually appears as a single line in the file)
from the measurements log file is shown below.
@example
1998-07-22 05:40:50 -0.037360 1 -0.037434\
-37.948 12 5 120
@end example
The columns are as follows (the quantities in square brackets are the
values from the example line above) :
@enumerate 1
@item
Date [1998-07-22]
@item
Hour:Minute:Second [05:40:50]. Note that the date/time pair is
expressed in UTC, not the local time zone.
@item
The measured offset between the system's real time clock and the system
(@code{gettimeofday()}) time. In seconds, positive indicates that the
RTC is fast of the system time. [-0.037360].
@item
Flag indicating whether the regression has produced valid
coefficients. (1 for yes, 0 for no). [1]
@item
Offset at the current time predicted by the regression process. A large
difference between this value and the measured offset tends to indicate
that the measurement is an outlier with a serious measurement
error. [-0.037434].
@item
The rate at which the RTC is losing or gaining time relative to the
system clock. In ppm, with positive indicating that the RTC is gaining
time. [-37.948]
@item
The number of measurements used in the regression. [12]
@item
The number of runs of regression residuals of the same sign. Low values
indicate that a straight line is no longer a good model of the measured
data and that older measurements should be discarded. [5]
@item
The measurement interval used prior to the measurement being made (in
seconds). [120]
@end enumerate
A banner is periodically written to the log file to indicate the
meanings of the columns.
@c }}}
@c {{{ refclocks.log
@node refclocks log
@subsubsection Refclocks log file format
An example line (which actually appears as a single line in the file)
from the refclocks log file is shown below.
@example
2009-11-30 14:33:27.000000 PPS2 7 N 1 4.900000e-07 -6.741777e-07 1.000e-06
@end example
The columns are as follows (the quantities in square brackets are the
values from the example line above) :
@enumerate 1
@item
Date [2009-11-30]
@item
Hour:Minute:Second.Microsecond [14:33:27.000000]. Note that the
date/time pair is expressed in UTC, not the local time zone.
@item
Reference ID of refclock from which measurement comes. [PPS2]
@item
Sequence number of driver poll within one polling interval for raw
samples, or @code{-} for filtered samples. [7]
@item
Leap status (@code{N} means normal, @code{+} means that the last minute
of the current month has 61 seconds, @code{-} means that the last minute
of the month has 59 seconds). [N]
@item
Flag indicating whether the sample comes from PPS source. (1 for yes,
0 for no, or @code{-} for filtered sample). [1]
@item
Local clock error measured by refclock driver, or @code{-} for
filtered sample. [4.900000e-07]
@item
Local clock error with applied corrections. Positive indicates
that the local clock is slow. [-6.741777e-07]
@item
Assumed dispersion of the sample. [1.000e-06]
@end enumerate
A banner is periodically written to the log file to indicate the
meanings of the columns.
@c }}}
@c {{{ tempcomp.log
@node tempcomp log
@subsubsection Tempcomp log file format
An example line (which actually appears as a single line in the file)
from the tempcomp log file is shown below.
@example
2010-04-19 10:39:48 2.8000e+04 3.6600e-01
@end example
The columns are as follows (the quantities in square brackets are the
values from the example line above) :
@enumerate 1
@item
Date [2010-04-19]
@item
Hour:Minute:Second [10:39:48]. Note that the
date/time pair is expressed in UTC, not the local time zone.
@item
Temperature read from tempcomp file. [2.8000e+04]
@item
Applied compensation in ppm, positive means the system clock is
running faster than it would be without the compensation. [3.6600e-01]
@end enumerate
A banner is periodically written to the log file to indicate the
meanings of the columns.
@c }}}
@c }}}
@c {{{ logbanner
@node logbanner directive
@subsection logbanner
A banner is periodically written to the log files enabled by the
@code{log} directive to indicate the meanings of the columns.
The @code{logbanner} directive specifies after how many entries in the
log file should be the banner written. The default is 32, and 0 can be
used to disable it entirely.
@c }}}
@c {{{ logchange
@node logchange directive
@subsection logchange
This directive forces @code{chronyd} to send a message to syslog if it
makes a system clock adjustment larger than a threshold value. An
example of use is
@example
logchange 0.5
@end example
which would cause a syslog message to be generated a system clock error
of over 0.5 seconds starts to be compensated.
Clock errors detected either via NTP packets or via timestamps entered
via the @code{settime} command of @code{chronyc} are logged.
This directive assumes that syslog messages are appearing where somebody
can see them. This allows that person to see if a large error has
arisen, e.g. because of a fault, or because of faulty timezone handling,
for example when summer time (daylight saving) starts or ends.
@c }}}
@c {{{ logdir
@node logdir directive
@subsection logdir
This directive allows the directory where log files are written to be
specified.
An example of the use of this directive is
@example
logdir /var/log/chrony
@end example
@c }}}
@c {{{ mailonchange
@node mailonchange directive
@subsection mailonchange
This directive defines an email address to which mail should be sent if
chronyd applies a correction exceeding a particular threshold to the
system clock.
An example of use of this directive is
@example
mailonchange root@@localhost 0.5
@end example
This would send a mail message to root if a change of more than 0.5
seconds were applied to the system clock.
This directive can't be used when a system call filter is enabled by the
@code{-F} option as the @code{chronyd} process will not be allowed to fork
and execute the sendmail binary.
@c }}}
@c {{{ makestep
@node makestep directive
@subsection makestep
Normally chronyd will cause the system to gradually correct any time
offset, by slowing down or speeding up the clock as required. In
certain situations, the system clock may be so far adrift that this
slewing process would take a very long time to correct the system clock.
This directive forces @code{chronyd} to step system clock if the
adjustment is larger than a threshold value, but only if there were no
more clock updates since @code{chronyd} was started than a specified
limit (a negative value can be used to disable the limit).
This is particularly useful when using reference clocks, because the
@code{initstepslew} directive (@pxref{initstepslew directive}) works
only with NTP sources.
An example of the use of this directive is
@example
makestep 0.1 10
@end example
This would step system clock if the adjustment is larger than 0.1
seconds, but only in the first ten clock updates.
@c }}}
@c {{{ manual
@node manual directive
@subsection manual
The @code{manual} directive enables support at run-time for the
@code{settime} command in chronyc (@pxref{settime command}). If no
@code{manual} directive is included, any attempt to use the
@code{settime} command in chronyc will be met with an error message.
Note that the @code{settime} command can be enabled at run-time using
the @code{manual} command in chronyc (@pxref{manual command}). (The
idea of the two commands is that the @code{manual} command controls the
manual clock driver's behaviour, whereas the @code{settime} command
allows samples of manually entered time to be provided).
@c }}}
@c {{{ maxchange
@node maxchange directive
@subsection maxchange
This directive sets the maximum allowed offset corrected on a clock
update. The check is performed only after the specified number of
updates to allow a large initial adjustment of the system clock. When
an offset larger than the specified maximum occurs, it will be ignored
for the specified number of times and then @code{chronyd} will give up
and exit (a negative value can be used to never exit). In both cases
a message is sent to syslog.
An example of the use of this directive is
@example
maxchange 1000 1 2
@end example
After the first clock update, @code{chronyd} will check the offset on
every clock update, it will ignore two adjustments larger than 1000
seconds and exit on another one.
@c }}}
@c {{{ maxclockerror
@node maxclockerror directive
@subsection maxclockerror
The @code{maxclockerror} directive sets the maximum assumed frequency
error of the local clock. This is a frequency stability of the clock,
not an absolute frequency error.
By default, the maximum assumed error is set to 1 ppm.
The syntax is
@example
maxclockerror <error-in-ppm>
@end example
Typical values for <error-in-ppm> might be 10 for a low quality clock
to 0.1 for a high quality clock using a temperature compensated
crystal oscillator.
@c }}}
@c {{{ maxdistance
@node maxdistance directive
@subsection maxdistance
The @code{maxdistance} directive sets the maximum allowed root distance of the
sources to not be rejected by the source selection algorithm. The distance
includes the accumulated dispersion, which may be large when the source is no
longer synchronised, and half of the total round-trip delay to the primary
source.
By default, the maximum distance is 3 seconds.
The syntax is
@example
maxdistance <seconds>
@end example
@c }}}
@c {{{ maxsamples
@node maxsamples directive
@subsection maxsamples
The @code{maxsamples} directive sets the default maximum number of samples
@code{chronyd} should keep for each source. This setting can be overriden for
individual sources in the @code{server} and @code{refclock} directives
(@pxref{server directive}, @pxref{refclock directive}). The default value is
0, which disables the configurable limit. The useful range is 4 to 64.
The syntax is
@example
maxsamples <samples>
@end example
@c }}}
@c {{{ maxslewrate
@node maxslewrate directive
@subsection maxslewrate
The @code{maxslewrate} directive sets the maximum rate at which @code{chronyd}
is allowed to slew the time. It limits the slew rate controlled by the
correction time ratio (@pxref{corrtimeratio directive}) and is effective
only on systems where @code{chronyd} is able to control the rate (i.e.
Linux, FreeBSD, NetBSD, Solaris).
For each system there is a maximum frequency offset of the clock that
can be set by the driver. On Linux it's 100000 ppm, on FreeBSD and NetBSD
it's 5000 ppm and on Solaris it is 32500 ppm. Also, due to a kernel
limitation, setting @code{maxslewrate} on FreeBSD and NetBSD to a value between
500 ppm and 5000 ppm will effectively set it to 500 ppm.
By default, the maximum slew rate is set to 83333.333 ppm (one twelfth).
The syntax is
@example
maxslewrate <rate-in-ppm>
@end example
@c }}}
@c {{{ maxupdateskew
@node maxupdateskew directive
@subsection maxupdateskew
One of @code{chronyd's} tasks is to work out how fast or slow the computer's
clock runs relative to its reference sources. In addition, it computes
an estimate of the error bounds around the estimated value.
If the range of error is too large, it probably indicates that the
measurements have not settled down yet, and that the estimated gain or
loss rate is not very reliable.
The @code{maxupdateskew} parameter allows the threshold for determining
whether an estimate may be so unreliable that it should not be used.
By default, the threshold is 1000 ppm.
The syntax is
@example
maxupdateskew <skew-in-ppm>
@end example
Typical values for <skew-in-ppm> might be 100 for a dial-up connection
to servers over a phone line, and 5 or 10 for a computer on a LAN.
It should be noted that this is not the only means of protection against
using unreliable estimates. At all times, @code{chronyd} keeps track of
both the estimated gain or loss rate, and the error bound on the
estimate. When a new estimate is generated following another
measurement from one of the sources, a weighted combination algorithm is
used to update the master estimate. So if @code{chronyd} has an existing
highly-reliable master estimate and a new estimate is generated which
has large error bounds, the existing master estimate will dominate in
the new master estimate.
@c }}}
@c {{{ minsamples
@node minsamples directive
@subsection minsamples
The @code{minsamples} directive sets the default minimum number of samples
@code{chronyd} should keep for each source. This setting can be overriden for
individual sources in the @code{server} and @code{refclock} directives
(@pxref{server directive}, @pxref{refclock directive}). The default value is
0. The useful range is 4 to 64.
The syntax is
@example
minsamples <samples>
@end example
@c }}}
@c {{{ minsources
@node minsources directive
@subsection minsources
The @code{minsources} directive sets the minimum number of sources that need
to be considered as selectable in the source selection algorithm before the
local clock is updated. The default value is 1.
Setting this option to a larger number can be used to improve the reliability.
More sources will have to agree with each other and the clock will not be
updated when only one source (which could be serving wrong time) is reachable.
The syntax is
@example
minsources <sources>
@end example
@c }}}
@c {{{ noclientlog
@node noclientlog directive
@subsection noclientlog
This directive, which takes no arguments, specifies that client accesses
are not to be logged. Normally they are logged, allowing statistics to
be reported using the @code{clients} command in @code{chronyc}.
@c }}}
@c {{{ peer
@node peer directive
@subsection peer
The syntax of this directive is identical to that for the @code{server}
directive (@pxref{server directive}), except that it is used to specify
an NTP peer rather than an NTP server.
Please note that NTP peers that are not configured with a key to enable
authentication are vulnerable to a denial-of-service attack. An attacker
knowing that NTP hosts A and B are peering with each other can send a packet
with random timestamps to host A with source address of B which will set the
NTP state variables on A to the values sent by the attacker. Host A will then
send on its next poll to B a packet with originate timestamp that doesn't match
the transmit timestamp of B and the packet will be dropped. If the attacker
does this periodically for both hosts, they won't be able to synchronize to
each other.
This attack can be prevented by enabling authentication with the key option, or
using the @code{server} directive on both sides to specify the other host as a
server instead of peer, the only drawback is that it will double the network
traffic between the two hosts.
@c }}}
@c {{{ pidfile
@node pidfile directive
@subsection pidfile
chronyd always writes its process ID (pid) to a file, and checks this file on startup to see if another chronyd may already be running on the system. By default, the file used is @code{/var/run/chronyd.pid}. The @code{pidfile} directive allows the name to be changed, e.g.
@example
pidfile /var/tmp/chronyd.pid
@end example
@c }}}
@c {{{ pool
@node pool directive
@subsection pool
The syntax of this directive is similar to that for the @code{server}
directive (@pxref{server directive}), except that it is used to specify a pool
of NTP servers rather than a single NTP server. The pool name is expected to
resolve to multiple addresses which may change over time.
All options valid in the @code{server} directive can be used in this directive
too. There is one option specific to @code{pool} directive: @code{maxsources}
sets the maximum number of sources that can be used from the pool, the default
value is 4.
On start, when the pool name is resolved, @code{chronyd} will add up to 16
sources, one for each resolved address. When the number of sources from which
at least one valid reply was received reaches @code{maxsources}, the other
sources will be removed. When a pool source is unreachable or marked as
falseticker, @code{chronyd} will try to replace the source with a newly
resolved address of the pool.
An example of the pool directive is
@example
pool pool.ntp.org iburst maxsources 3
@end example
@c }}}
@c {{{ port
@node port directive
@subsection port
This option allows you to configure the port on which @code{chronyd}
will listen for NTP requests. The port will be open only when an address is
allowed by the @code{allow} directive or command, an NTP peer is configured, or
the broadcast server mode is enabled.
The compiled in default is udp/123, the standard NTP port. If set to 0,
@code{chronyd} will never open the server port and will operate strictly in a
client-only mode. The source port used in NTP client requests can be set by
the @code{acquisitionport} directive.
An example of the port command is
@example
port 11123
@end example
This would change the NTP port served by @code{chronyd} on the computer to
udp/11123.
@c }}}
@c {{{ refclock
@node refclock directive
@subsection refclock
Reference clocks allows very accurate synchronisation and @code{chronyd}
can function as a stratum 1 server. They are specified by the
@code{refclock} directive. It has two mandatory parameters, a refclock driver
name and a driver specific parameter.
There are currently four drivers included:
@table @code
@item PPS
PPSAPI (pulse per second) driver. The parameter is the path to a PPS
device. Assert events are used by default. Driver option @code{:clear}
can be appended to the path if clear events should be used instead.
As PPS refclock gets only sub-second time information, it needs another
source (NTP or non-PPS refclock) or local directive (@pxref{local
directive}) enabled to work. For example:
@example
refclock PPS /dev/pps0 lock NMEA
refclock SHM 0 offset 0.5 delay 0.2 refid NMEA noselect
@end example
@item SHM
NTP shared memory driver. This driver uses a shared memory segment to
receive data from another daemon which communicates with an actual
reference clock. The parameter is the number of a shared memory segment,
usually 0, 1, 2 or 3. For example:
@example
refclock SHM 1 poll 3 refid GPS1
@end example
A driver option in form @code{:perm=NNN} can be appended to the
segment number to create the segment with permissions other than the
default @code{0600}.
Some examples of applications that can be used as SHM sources are
@uref{http://catb.org/gpsd/, @code{gpsd}}, @code{shmpps} and
@uref{http://www.buzzard.me.uk/jonathan/radioclock.html, @code{radioclk}}.
@item SOCK
Unix domain socket driver. It is similar to the SHM driver, but uses a
different format and uses a socket instead of shared memory. It does not
require polling and it
supports transmitting of PPS data. The parameter is a path to the socket which
will be created by @code{chronyd} and used to receive the messages. The format
of messages sent over the socket is described in the
@code{refclock_sock.c} file.
Recent versions of the @code{gpsd} daemon include support for the SOCK
protocol. The path where the socket should be created is described in the
@code{gpsd(8)} man page. For example:
@example
refclock SOCK /var/run/chrony.ttyS0.sock
@end example
@item PHC
PTP hardware clock (PHC) driver. The parameter is the path to the device of
the PTP clock, which can be synchronised by a PTP daemon (e.g. @code{ptp4l}
from the @uref{http://linuxptp.sourceforge.net/, Linux PTP project}. The PTP
clocks are typically kept in TAI instead of UTC. The @code{offset} option can
be used to compensate for the current UTC/TAI offset. For example:
@example
refclock PHC /dev/ptp0 poll 3 dpoll -2 offset -35
@end example
@end table
The @code{refclock} command also supports a number of subfields (which
may be defined in any order):
@table @code
@item poll
Timestamps produced by refclock drivers are not used immediately, but
they are stored and processed by a median filter in the polling interval
specified by this option. This is defined as a power of 2 and may be
negative to specify a sub-second interval. The
default is 4 (16 seconds). A shorter interval allows @code{chronyd}
to react faster to changes in clock frequency, but it may decrease
the accuracy if the source is too noisy.
@item dpoll
Some drivers don't listen for external events and try to produce
samples in their own polling interval. This is defined as a power of
2 and may be negative to specify a sub-second interval. The default
is 0 (1 second).
@item refid
This option is used to specify a reference id of the refclock, as up
to four ASCII characters. By default, first three characters from
driver name and the number of the refclock are used as refid. Each
refclock must have an unique refid.
@item filter
This option sets the length of the median filter which is used to
reduce noise. With each poll about 40 percent of the stored samples is
discarded and one final sample is calculated as average of the
remaining samples. If the length is 4 or above, at least 4 samples
have to be collected between polls. For lengths below 4, the filter
has to be full. The default is 64.
@item rate
PPS signal frequency (in Hz). This option only controls how the
received pulses are aligned. To actually receive more than one
pulse per second, a negative @code{dpoll} has to be specified (-3 for
5Hz signal). The default is 1.
@item lock
This option can be used to lock a PPS refclock to another refclock
whose reference id is specified by this option. In this mode received
pulses are aligned directly to unfiltered samples from the refclock.
By default, pulses are aligned to local clock, but only when it is
well synchronised.
@item offset
This option can be used to compensate a constant error. The specified
offset (in seconds) is applied to all samples produced by the
refclock. The default is 0.0.
@item delay
This option sets the NTP delay of the source (in seconds). Half of
this value is included in the maximum assumed error which is used in the
source selection algorithm. Increasing the delay is useful to avoid
having no majority in the algorithm or to make it prefer other
sources. The default is 1e-9 (1 nanosecond).
@item precision
Refclock precision (in seconds). The default is 1e-6 (1 microsecond)
for SHM refclock, and 1e-9 (1 nanosecond) for SOCK, PPS and PHC refclocks.
@item maxdispersion
Maximum allowed dispersion for filtered samples (in seconds). Samples
with larger estimated dispersion are ignored. By default, this limit
is disabled.
@item prefer
Prefer this source over sources without prefer option.
@item noselect
Never select this source. This is useful for monitoring or with sources
which are not very accurate, but are locked with a PPS refclock.
@item minsamples
Set the minimum number of samples kept for this source. This overrides the
@code{minsamples} directive (@pxref{minsamples directive}).
@item maxsamples
Set the maximum number of samples kept for this source. This overrides the
@code{maxsamples} directive (@pxref{maxsamples directive}).
@end table
@c }}}
@c {{{ reselectdist
@node reselectdist directive
@subsection reselectdist
When @code{chronyd} selects synchronisation source from available sources, it
will prefer the one with minimum synchronisation distance. However, to
avoid frequent reselecting when there are sources with similar distance, a
fixed distance is added to the distance for sources that are currently not
selected. This can be set with the @code{reselectdist} option. By default, the
distance is 100 microseconds.
The syntax is
@example
reselectdist <dist-in-seconds>
@end example
@c }}}
@c {{{ rtcautotrim
@node rtcautotrim directive
@subsection rtcautotrim
The @code{rtcautotrim} directive is used to keep the real time clock (RTC)
close to the system clock automatically. When the system clock is synchronized
and the estimated error between the two clocks is larger than the specified
threshold, @code{chronyd} will trim the RTC as if the @code{trimrtc}
(@pxref{trimrtc command}) command was issued.
This directive is effective only with the @code{rtcfile} directive.
An example of the use of this directive is
@example
rtcautotrim 30
@end example
This would set the threshold error to 30 seconds.
@c }}}
@c {{{ rtcdevice
@node rtcdevice directive
@subsection rtcdevice
The @code{rtcdevice} directive defines the name of the device file for
accessing the real time clock. By default this is @code{/dev/rtc}, unless the
directive is used to set a different value. This applies to Linux systems with
devfs. An example of use is
@example
rtcdevice /dev/misc/rtc
@end example
@c }}}
@c {{{ rtcfile
@node rtcfile directive
@subsection rtcfile
The @code{rtcfile} directive defines the name of the file in which
@code{chronyd} can save parameters associated with tracking the accuracy
of the system's real-time clock (RTC).
The syntax is illustrated in the following example
@example
rtcfile @CHRONYVARDIR@/rtc
@end example
@code{chronyd} saves information in this file when it exits and when the
@code{writertc} command is issued in @code{chronyc}. The information
saved is the RTC's error at some epoch, that epoch (in seconds since
January 1 1970), and the rate at which the RTC gains or loses time.
So far, the support for real-time clocks is limited - their code is even
more system-specific than the rest of the software. You can only use
the real time clock facilities (the @code{rtcfile} directive and the
@code{-s} command line option to @code{chronyd}) if the following three
conditions apply:
@enumerate 1
@item
You are running Linux version 2.2.x or later.
@item
You have compiled the kernel with extended real-time clock support
(i.e. the @file{/dev/rtc} device is capable of doing useful things).
@item
You don't have other applications that need to make use of
@file{/dev/rtc} at all.
@end enumerate
@c }}}
@c {{{ rtconutc
@node rtconutc directive
@subsection rtconutc
@code{chronyd} assumes by default that the real time clock (RTC) keeps
local time (including any daylight saving changes). This is convenient
on PCs running Linux which are dual-booted with DOS or Windows.
NOTE : IF YOU KEEP THE REAL TIME CLOCK ON LOCAL TIME AND YOUR COMPUTER
IS OFF WHEN DAYLIGHT SAVING (SUMMER TIME) STARTS OR ENDS, THE COMPUTER'S
SYSTEM TIME WILL BE ONE HOUR IN ERROR WHEN YOU NEXT BOOT AND START
CHRONYD.
An alternative is for the RTC to keep Universal Coordinated Time (UTC).
This does not suffer from the 1 hour problem when daylight saving starts
or ends.
If the @code{rtconutc} directive appears, it means the RTC is required
to keep UTC. The directive takes no arguments. It is equivalent to
specifying the @code{-u} switch to the Linux @file{/sbin/hwclock} program.
Note that this setting is overriden when the @code{hwclockfile} directive
(@pxref{hwclockfile directive}) is used.
@c }}}
@c {{{ rtcsync
@node rtcsync directive
@subsection rtcsync
The @code{rtcsync} directive will enable a kernel mode where the
system time is copied to the real time clock (RTC) every 11 minutes.
This directive is supported only on Linux and cannot be used when the
normal RTC tracking is enabled, i.e. when the @code{rtcfile} directive
is used. On other systems this directive does nothing.
@c }}}
@c {{{ sched_priority
@node sched_priority directive
@subsection sched_priority
On Linux, the @code{sched_priority} directive will select the SCHED_FIFO
real-time scheduler at the specified priority (which must be between 0 and
100). On Mac OS X, 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.
On Linux, this directive uses the sched_setscheduler() system call to instruct
the kernel to use the SCHED_FIFO first-in, first-out real-time scheduling
policy for @code{chronyd} with the specified priority.
This means that whenever @code{chronyd} is ready to run it will run,
interrupting whatever else is running unless it is a higher priority
real-time process. This should not impact performance as @code{chronyd's}
resource requirements are modest, but it should result in lower and
more consistent latency since @code{chronyd} will not need to wait for the
scheduler to get around to running it. You should not use this unless
you really need it. The sched_setscheduler man page has more details.
On Mac OS X, this directive uses the thread_policy_set() kernel call to specify
real-time scheduling. As noted for Linux, you should not use this directive
unless you really need it.
@c }}}
@c {{{ server
@node server directive
@subsection server
The @code{server} directive allows NTP servers to be specified. The
client/server relationship is strictly hierarchical : a client may
synchronise its system time to that of the server, but the server's
system time will never be influenced by that of a client.
The @code{server} directive is immediately followed by either the name
of the server, or its IP address. The server command also supports a
number of subfields (which may be defined in any order):
@table @code
@item port
This option allows the UDP port on which the server understands NTP
requests to be specified. For normal servers this option should not be
required (the default is 123, the standard NTP port).
@item minpoll
Although @code{chronyd} will trim the rate at which it samples the
server during normal operation, the user may wish to constrain the
minimum polling interval. This is always defined as a power of 2, so
@code{minpoll 5} would mean that the polling interval cannot drop below 32
seconds. The default is 6 (64 seconds).
@item maxpoll
In a similar way, the user may wish to constrain the maximum polling
interval. Again this is specified as a power of 2, @code{maxpoll 9}
indicates that the polling interval must stay at or below 512 seconds.
The default is 10 (1024 seconds).
@item maxdelay
@code{chronyd} uses the network round-trip delay to the server to
determine how accurate a particular measurement is likely to be. Long
round-trip delays indicate that the request, or the response, or both
were delayed. If only one of the messages was delayed the measurement
error is likely to be substantial.
For small variations in round trip delay, @code{chronyd} uses a
weighting scheme when processing the measurements. However, beyond a
certain level of delay the measurements are likely to be so corrupted as
to be useless. (This is particularly so on dial-up or other slow links,
where a long delay probably indicates a highly asymmetric delay caused
by the response waiting behind a lot of packets related to a download of
some sort).
If the user knows that round trip delays above a certain level should
cause the measurement to be ignored, this level can be defined with the
maxdelay command. For example, @code{maxdelay 0.3} would indicate that
measurements with a round-trip delay of 0.3 seconds or more should be
ignored. The default value is 3 seconds.
@item maxdelayratio
This option is similar to the maxdelay option above. @code{chronyd}
keeps a record of the minimum round-trip delay amongst the previous
measurements that it has buffered. If a measurement has a round trip
delay that is greater than the maxdelayratio times the minimum delay, it
will be rejected.
@item maxdelaydevratio
If a measurement has ratio of the increase in round-trip delay from
the minimum delay amongst the previous measurements to the standard
deviation of the previous measurements that is greater than
maxdelaydevratio, it will be rejected. The default is 10.0.
@item presend
If the timing measurements being made by @code{chronyd} are the only
network data passing between two computers, you may find that some
measurements are badly skewed due to either the client or the server
having to do an ARP lookup on the other party prior to transmitting a
packet. This is more of a problem with long sampling intervals, which
may be similar in duration to the lifetime of entries in the ARP caches
of the machines.
In order to avoid this problem, the @code{presend} option may be used.
It takes a single integer argument, which is the smallest polling
interval for which an extra pair of NTP packets will be exchanged
between the client and the server prior to the actual measurement.
For example, with the following option included in a
@code{server} directive :
@example
presend 9
@end example
when the polling interval is 512 seconds or more, an extra NTP client
packet will be sent to the server a short time (currently 4 seconds)
before making the actual measurement.
@item key
The NTP protocol supports the inclusion of checksums in the packets, to
prevent computers having their system time upset by rogue packets being
sent to them. The checksums are generated as a function of a password,
using the cryptographic hash function set in the key file.
The association between key numbers and passwords is contained in the
keys file, defined by the keyfile command.
If the key option is present, @code{chronyd} will attempt to use
authenticated packets when communicating with this server. The key
number used will be the single argument to the key option (an
unsigned integer in the range 1 through 2**32-1). The server
must have the same password for this key number configured, otherwise no
relationship between the computers will be possible.
@item offline
If the server will not be reachable when @code{chronyd} is started, the
offline option may be specified. @code{chronyd} will not try to poll
the server until it is enabled to do so (by using the online option of
@code{chronyc}).
@item auto_offline
If this option is set, the server will be assumed to have gone offline when 2
requests have been sent to it without receiving a response. This option avoids
the need to run the @code{offline} (@pxref{offline command}) command from
chrony when disconnecting the dial-up link. (It will still be necessary to use
chronyc's @code{online} (@pxref{online command}) command when the link has been
established, to enable measurements to start.)
@item iburst
On start, make four measurements over a short duration (rather than
the usual periodic measurements).
@item minstratum
When the synchronisation source is selected from available sources, sources
with lower stratum are normally preferred. This option can be used to increase
stratum of the source to the specified minimum, so @code{chronyd} will avoid
selecting that source. This is useful with low stratum sources that are known
to be unrealiable or inaccurate and which should be used only when other
sources are unreachable.
@item polltarget
Target number of measurements to use for the regression algorithm which
@code{chronyd} will try to maintain by adjusting polling interval between
@code{minpoll} and @code{maxpoll}. A higher target makes @code{chronyd} prefer
shorter polling intervals. The default is 6 and a useful range is 6 to 60.
@item version
This option sets the NTP version number used in packets sent to the server.
This can be useful when the server runs an old NTP implementation that doesn't
respond to newer versions. The default version number is 4.
@item prefer
Prefer this source over sources without prefer option.
@item noselect
Never select this source. This is particularly useful for monitoring.
@item minsamples
Set the minimum number of samples kept for this source. This overrides the
@code{minsamples} directive (@pxref{minsamples directive}).
@item maxsamples
Set the maximum number of samples kept for this source. This overrides the
@code{maxsamples} directive (@pxref{maxsamples directive}).
@end table
@c }}}
@c {{{ smoothtime
@node smoothtime directive
@subsection smoothtime
The @code{smoothtime} directive can be used to enable smoothing of the time
that @code{chronyd} serves to its clients to make it easier for them to track
it and keep their clocks close together even when large offset or frequency
corrections are applied to the server's clock, for example after being offline
for a longer time.
BE WARNED - the server is intentionally not serving its best estimate of the
true time. If a large offset has been accumulated, it may take a very long
time to smooth it out. This directive should be used only when the clients are
not configured to poll also another NTP server, because they could reject this
server as a falseticker or fail to select a source completely.
The smoothing process is implemented with a quadratic spline function with two
or three pieces. It's independent from any slewing applied to the local system
clock, but the accumulated offset and frequency will be reset when the clock is
corrected by stepping, e.g. by the @code{makestep} directive or command. The
process can be reset without stepping the clock by the @code{smoothtime reset}
command (@pxref{smoothtime command}).
The first two arguments of the directive are the maximum frequency offset of
the smoothed time to the tracked NTP time (in ppm) and the maximum rate at
which the frequency offset is allowed to change (in ppm per second).
@code{leaponly} is an optional third argument which enables a mode where only
leap seconds are smoothed out and normal offset/frequency changes are ignored.
The @code{leaponly} option is useful in a combination with the
@code{leapsecmode slew} option (@pxref{leapsecmode directive}) to allow clients
use multiple time smoothing servers safely.
The smoothing process is activated automatically when 1/10000 of the estimated
skew of the local clock falls below the maximum rate of frequency change. It
can be also activated manually by the @code{smoothtime activate} command,
which is particularly useful when the clock is synchronized only with manual
input and the skew is always larger than the threshold. The @code{smoothing}
command (@pxref{smoothing command}) can be used to monitor the process.
An example suitable for clients using @code{ntpd} and 1024 second polling
interval could be
@example
smoothtime 400 0.001
@end example
An example suitable for clients using @code{chronyd} on Linux could be
@example
smoothtime 50000 0.01
@end example
@c }}}
@c {{{ stratumweight
@node stratumweight directive
@subsection stratumweight
The @code{stratumweight} directive sets how much distance should be added
per stratum to the synchronisation distance when @code{chronyd} selects
the synchronisation source from available sources.
The syntax is
@example
stratumweight <dist-in-seconds>
@end example
By default, the weight is 0.001 seconds. This means that stratum of the
sources in the selection process matters only when the differences between the
distances are in milliseconds.
@c }}}
@c {{{ tempcomp
@node tempcomp directive
@subsection tempcomp
Normally, changes in the rate of drift of the system clock are caused mainly by
changes in the temperature of the crystal oscillator on the mainboard.
If there are temperature measurements available from a sensor close to the
oscillator, the @code{tempcomp} directive can be used to compensate for the
changes in the temperature and improve the stability and accuracy of the clock.
The result depends on many factors, including the resolution of the sensor,
the amount of noise in the measurements, the polling interval of the time
source, the compensation update interval, how well is the compensation
specified, and how close is the sensor to the oscillator. When it's working
well, the frequency reported in the @file{tracking.log} file is more stable and
the maximum reached offset is smaller.
There are two forms of the directive. The first one has six parameters: a
path to the file containing the current temperature from the sensor (in
text format), the compensation update interval (in seconds), and temperature
coefficients T0, k0, k1, k2.
The frequency compensation is calculated (in ppm) as
@code{k0 + (T - T0) * k1 + (T - T0)^2 * k2}
The result has to be between -10 ppm and 10 ppm, otherwise the measurement is
considered invalid and will be ignored. The k0 coefficient can be used to get
the results in that range.
An example of use is
@example
tempcomp /sys/class/hwmon/hwmon0/temp2_input 30 26000 0.0 0.000183 0.0
@end example
The measured temperature will be read from the file in the Linux sysfs
filesystem every 30 seconds. When the temperature is 26000 (26 degrees
Celsius), the frequency correction will be zero. When it is 27000 (27 degrees
Celsius), the clock will be set to run 0.183ppm faster, etc.
The second form has three parameters, the path to the sensor file, the update
interval and a path to a file containing a list of (temperature, compensation)
points, from which the compensation is linearly interpolated or extrapolated.
An example is
@example
tempcomp /sys/class/hwmon/hwmon0/temp2_input 30 /etc/chrony.tempcomp
@end example
where the @file{chrony.tempcomp} file could have
@example
20000 1.0
21000 0.64
22000 0.36
23000 0.16
24000 0.04
25000 0.0
26000 0.04
27000 0.16
28000 0.36
29000 0.64
30000 1.0
@end example
Valid measurements with corresponding compensations are logged to the
@file{tempcomp.log} file if enabled by the @code{log tempcomp} directive.
@c }}}
@c {{{ user
@node user directive
@subsection user
The @code{user} directive sets the name of the system user to which
@code{chronyd} will switch after start in order to drop root privileges.
It may be set to a non-root user only when @code{chronyd} is compiled with
support for Linux capabilities (libcap) or on NetBSD with the
@code{/dev/clockctl} device.
The default value is @code{@DEFAULT_USER@}.
@c }}}
@c }}}
@c {{{ S:Running chronyc
@node Running chronyc
@section Running chronyc
@c {{{ Section top
Chronyc is the program that can be used to reconfigure options within
the @code{chronyd} program whilst it is running. Chronyc can also be
used to generate status reports about the operation of @code{chronyd}.
@menu
* Chronyc basic use:: How to run chronyc
* Chronyc command line options:: Chrony's command line options
* Security with chronyc:: How chronyd restricts access
* Chronyc command reference:: All the commands chronyc supports
@end menu
@c }}}
@c {{{ SS:Chronyc basic use
@node Chronyc basic use
@subsection Basic use
The program chronyc is run by entering
@example
chronyc
@end example
at the command line. The prompt @code{chronyc} is displayed whilst
chronyc is expecting input from the user, when it is being run from a
terminal. If chronyc's input or output are redirected from/to a file,
the prompt is not shown.
When you are finished entering commands, the commands @code{exit} or
@code{quit} will terminate the program. (Entering @key{Control-D} will
also terminate the program.)
@c }}}
@c {{{ SS:Command line options
@node Chronyc command line options
@subsection Command line options
Chronyc supports the following command line options.
@table @code
@item -v
Displays the version number of chronyc on the terminal, and exists.
@item -h <host>
This option allows the user to specify which host (or comma-separated list of
addresses) running the @code{chronyd} program is to be contacted. This allows
for remote monitoring, without having to ssh to the other host first.
The default is to contact @code{chronyd} running on the same host as
that where chronyc is being run.
@item -p <port>
This option allows the user to specify the UDP port number which the
target @code{chronyd} is using for its command & monitoring connections.
This defaults to the compiled-in default; there would rarely be a need
to change this.
@item -n
This option disables resolving IP addresses to hostnames.
@item -d
This option enables printing of debugging messages (if compiled with debugging
support).
@item -4
With this option hostnames will be resolved only to IPv4 addresses.
@item -6
With this option hostnames will be resolved only to IPv6 addresses.
@item -m
With this option multiple commands can be specified on the command line.
Each argument will be interpreted as a whole command.
@item -f <conf-file>
This option is ignored and is provided only for compatibility.
@item -a
This option is ignored and is provided only for compatibility.
@end table
@c }}}
@c {{{ SS:Security with chronyc
@node Security with chronyc
@subsection Security with chronyc
Many of the commands available through chronyc have a fair amount of
power to reconfigure the run-time behaviour of @code{chronyd}. Consequently,
@code{chronyc} is quite dangerous for the integrity of the target
system's clock performance. Having access to @code{chronyd} via @code{chronyc}
is more or less equivalent to being able to modify @code{chronyd's}
configuration file (typically @file{@SYSCONFDIR@/chrony.conf}) and to restart
@code{chronyd}.
@code{chronyc} also provides a number of monitoring (as opposed to
commanding or configuration) commands, which will not affect the behaviour of
@code{chronyd}. However, you may still want to restrict access to these
commands.
There are two ways how @code{chronyc} can access @code{chronyd}. One is the
Internet Protocol (IPv4 or IPv6) and the other is a Unix domain socket, which
is accessible only locally by the root or chrony user (by default
@code{@CHRONYSOCKDIR@/chronyd.sock}).
Only the following monitoring commands are allowed from the internet:
@itemize @bullet
@item @code{activity}
@item @code{manual list}
@item @code{rtcdata}
@item @code{smoothing}
@item @code{sources}
@item @code{sourcestats}
@item @code{tracking}
@item @code{waitsync}.
@end itemize
The set of hosts from which @code{chronyd} will accept these commands can be
restricted. By default, the commands will be accepted only from the localhost
(127.0.0.1 or ::1).
All other commands are allowed only through the Unix domain socket. When sent
over the internet, @code{chronyd} will respond with a @code{Not authorised}
error, even if it's from the localhost.
In @code{chrony} versions before 2.2 the commands had to be authenticated with
a password and they were allowed from the internet, but that is no longer
supported.
By default, @code{chronyc} tries to connect to the Unix domain socket first.
If that fails (e.g. because @code{chronyc} is running under a non-root user),
it will try to connect to 127.0.0.1 and then ::1.
@c }}}
@c {{{ SS:Chronyc command reference
@node Chronyc command reference
@subsection Command reference
@c {{{ Top/menu
This section describes each of the commands available within the chronyc
program. Chronyc offers the user a simple command-line driven
interface.
@menu
* accheck command:: Verifying NTP client access
* activity command:: Check how many NTP servers/peers are online/offline
* add peer command:: Add a new NTP peer
* add server command:: Add a new NTP server
* allow all command:: Allowing NTP client access
* allow command:: Allowing NTP client access
* burst command:: Initiating a rapid set of measurements
* clients command:: Show clients that have accessed the server
* cmdaccheck command:: Verifying monitoring client access
* cmdallow all command:: Allowing monitoring client access
* cmdallow command:: Allowing monitoring client access
* cmddeny all command:: Denying monitoring client access
* cmddeny command:: Denying monitoring client access
* cyclelogs command:: Close and re-open open log files
* delete command:: Remove an NTP server or peer
* deny all command:: Denying NTP client access
* deny command :: Denying NTP client access
* dns command:: Configure how are hostnames and IP addresses resolved
* dump command:: Dump measurement histories to files
* exit command:: Exit from chronyc
* help command:: Generate help summary
* local command:: Let computer be a server when it is unsynchronised
* makestep command:: Correct the system clock by stepping instead of slewing
* manual command:: Enable/disable/configure options for settime
* maxdelay command:: Set max measurement delay for a source
* maxdelaydevratio command:: Set max measurement delay for a source as ratio to deviation
* maxdelayratio command:: Set max measurement delay for a source as ratio
* maxpoll command:: Set maximum polling interval for a source
* maxupdateskew command:: Set safety threshold for clock gain/loss rate
* minpoll command:: Set minimum polling interval for a source
* minstratum command:: Set minimum stratum for a source
* offline command:: Warn that connectivity to a source will be lost
* online command:: Warn that connectivity to a source has been restored
* polltarget command:: Set poll target for a source
* quit command:: Exit from chronyc
* refresh command:: Refresh IP addresses
* reselect command:: Reselect synchronisation source
* reselectdist command:: Set improvement in distance needed to reselect a source
* retries command:: Set maximum number of retries
* rtcdata command:: Display RTC parameters
* settime command:: Provide a manual input of the current time
* smoothing command:: Display current time smoothing state
* smoothtime command:: Reset/activate server time smoothing
* sources command:: Display information about the current set of sources
* sourcestats command:: Display the rate & offset estimation performance of sources
* timeout command:: Set initial response timeout
* tracking command:: Display system clock performance
* trimrtc command:: Correct the RTC time to the current system time
* waitsync command:: Wait until synchronised
* writertc command:: Write the RTC parameters to file
@end menu
@c }}}
@c {{{ accheck
@node accheck command
@subsubsection accheck
This command allows you to check whether client NTP access is allowed
from a particular host.
Examples of use, showing a named host and a numeric IP address, are as
follows:
@example
accheck foo.example.net
accheck 1.2.3.4
accheck 2001:db8::1
@end example
This command can be used to examine the effect of a series of
@code{allow}, @code{allow all}, @code{deny} and @code{deny all} commands
specified either via chronyc, or in @code{chronyd's} configuration file.
@c }}}
@c {{{ activity command
@node activity command
@subsubsection activity
This command reports the number of servers/peers that are online and offline.
If the auto_offline option is used in specifying some of the servers/peers, the
@code{activity} command may be useful for detecting when all of them have
entered the offline state after the PPP link has been disconnected.
The report shows the number of servers/peers in 5 states:
@itemize
@item @code{online} : the server/peer is currently online (i.e. assumed by
chronyd to be reachable)
@item @code{offline} : the server/peer is currently offline (i.e. assumed by
chronyd to be unreachable, and no measurements from it will be attempted.)
@item @code{burst_online} : a burst command has been initiated for the
server/peer and is being performed; after the burst is complete, the
server/peer will be returned to the online state.
@item @code{burst_offline} : a burst command has been initiated for the
server/peer and is being performed; after the burst is complete, the
server/peer will be returned to the offline state.
@item @code{unresolved} : the name of the server/peer wasn't resolved to an
address yet; this server is not visible in the @code{sources} and
@code{sourcestats} reports.
@end itemize
@c }}}
@c {{{ add peer
@node add peer command
@subsubsection add peer
The @code{add peer} command allows a new NTP peer to be added whilst
@code{chronyd} is running.
Following the words @code{add peer}, the syntax of the following
parameters and options is similar to that for the @code{peer}
directive in the configuration file (@pxref{peer directive}).
The following peer options can be set in the command:
@code{port}, @code{minpoll}, @code{maxpoll}, @code{presend},
@code{maxdelayratio}, @code{maxdelay}, @code{key}
An example of using this command is shown below.
@example
add peer foo.example.net minpoll 6 maxpoll 10 key 25
@end example
@c }}}
@c {{{ add server
@node add server command
@subsubsection add server
The @code{add server} command allows a new NTP server to be added whilst
@code{chronyd} is running.
Following the words @code{add server}, the syntax of the following
parameters and options is similar to that for the @code{server}
directive in the configuration file (@pxref{server directive}).
The following server options can be set in the command:
@code{port}, @code{minpoll}, @code{maxpoll}, @code{presend},
@code{maxdelayratio}, @code{maxdelay}, @code{key}
An example of using this command is shown below.
@example
add server foo.example.net minpoll 6 maxpoll 10 key 25
@end example
@c }}}
@c {{{ allow all
@node allow all command
@subsubsection allow all
The effect of the allow command is identical to the @code{allow all}
directive in the configuration file (@pxref{allow directive}).
@c }}}
@c {{{ allow
@node allow command
@subsubsection allow
The effect of the allow command is identical to the @code{allow} directive in
the configuration file (@pxref{allow directive}).
The syntax is illustrated in the following examples:
@example
allow foo.example.net
allow 1.2
allow 3.4.5
allow 6.7.8/22
allow 6.7.8.9/22
allow 2001:db8:789a::/48
allow 0/0
allow ::/0
allow
@end example
The effect of each of these examples is the same as that of the @code{allow}
directive in the configuration file.
@c }}}
@c {{{ burst
@node burst command
@subsubsection burst
The @code{burst} command tells @code{chronyd} to make a set of measurements to
each of its NTP sources over a short duration (rather than the usual
periodic measurements that it makes). After such a burst, @code{chronyd} will
revert to the previous state for each source. This might be either
online, if the source was being periodically measured in the normal way,
or offline, if the source had been indicated as being offline.
(Switching a source between the online and offline states is described
in @ref{online command}, @ref{offline command}).
The syntax of the burst command is as follows
@example
burst <n-good-measurements>/<max-measurements> [<mask>/<masked-address>]
burst <n-good-measurements>/<max-measurements> [<masked-address>/<masked-bits>]
burst <n-good-measurements>/<max-measurements> [<address>]
@end example
The mask and masked-address arguments are optional, in which case
@code{chronyd} will initiate a burst for all of its currently defined sources.
The arguments have the following meaning and format.
@table @code
@item n-good-measurements
This defines the number of good measurements that @code{chronyd} will want to
obtain from each source. A measurement is good if it passes certain
tests, for example, the round trip time to the source must be
acceptable. (This allows @code{chronyd} to reject measurements that are likely
to be bogus.)
@item max-measurements
This defines the maximum number of measurements that @code{chronyd} will
attempt to make, even if the required number of good measurements has
not been obtained.
@item mask
This is an IP address with which the IP address of each of @code{chronyd}'s
sources is to be masked.
@item masked-address
This is an IP address. If the masked IP address of a source matches this value
then the burst command is applied to that source.
@item masked-bits
This can be used with @code{masked-address} for CIDR notation, which is a
shorter alternative to the form with mask.
@item address
This is an IP address or a hostname. The burst command is applied only to that
source.
@end table
If no mask or masked address arguments are provided, every source will
be matched.
An example of the two-argument form of the command is
@example
burst 2/10
@end example
This will cause @code{chronyd} to attempt to get two good measurements from
each source, stopping after two have been obtained, but in no event will
it try more than ten probes to the source.
Examples of the four-argument form of the command are
@example
burst 2/10 255.255.0.0/1.2.0.0
burst 2/10 2001:db8:789a::/48
@end example
In the first case, the two out of ten sampling will only be applied to
sources whose IPv4 addresses are of the form @code{1.2.x.y}, where x and y
are arbitrary. In the second case, the sampling will be applied to sources
whose IPv6 addresses have first 48 bits equal to @code{2001:db8:789a}.
Example of the three-argument form of the command is
@example
burst 2/10 foo.example.net
@end example
@c }}}
@c {{{ clients
@node clients command
@comment node-name, next, previous, up
@subsubsection clients
This command shows a list of all clients that have accessed the server,
through either the NTP or command/monitoring ports. It doesn't include
access to the Unix domain comamnd socket. There are no arguments.
An example of the output is
@example
Hostname Client Peer CmdAuth CmdNorm CmdBad LstN LstC
========================= ====== ====== ====== ====== ====== ==== ====
localhost 0 0 0 1 0 29y 0
aardvark.xxx 4 0 0 0 0 49 29y
badger.xxx 4 0 0 0 0 6 29y
@end example
Each row shows the data for a single host. Only hosts that have passed
the host access checks (set with the @code{allow}, @code{deny},
@code{cmdallow} and @code{cmddeny} commands or configuration file
directives) are logged.
The columns are as follows:
@enumerate 1
@item
The hostname of the client
@item
The number of times the client has accessed the server using an NTP
client mode packet.
@item
The number of times the client has accessed the server using an NTP
symmetric active mode packet.
@item
The number of authenticated command packets that have been processed from the
client. Authentication is no longer supported in command packets, so the
number should be always zero.
@item
The number of unauthenticated command packets that have been processed
from the client.
@item
The number of bad command packets received from the client (not all
forms of bad packet are logged).
@item
Time since the last NTP packet was received
@item
Time since the last command packet was received
@end enumerate
The last two entries will be shown as the time since 1970 if no packet
of that type has ever been received.
@c }}}
@c {{{ cmdaccheck
@node cmdaccheck command
@subsubsection cmdaccheck
This command is similar to the @code{accheck} command, except that it is
used to check whether monitoring access is permitted from a named host.
Examples of use are as follows:
@example
cmdaccheck foo.example.net
cmdaccheck 1.2.3.4
cmdaccheck 2001:db8::1
@end example
@c }}}
@c {{{ cmdallow all
@node cmdallow all command
@subsubsection cmdallow all
This is similar to the @code{allow all} command, except that it is used to
allow particular hosts or subnets to use @code{chronyc} to monitor with
@code{chronyd} on the current host.
@c }}}
@c {{{ cmdallow
@node cmdallow command
@subsubsection cmdallow
This is similar to the @code{allow} command, except that it is used to allow
particular hosts or subnets to use @code{chronyc} to monitor with
@code{chronyd} on the current host.
@c }}}
@c {{{ cmddeny all
@node cmddeny all command
@subsubsection cmddeny all
This is similar to the @code{deny all} command, except that it is used to allow
particular hosts or subnets to use @code{chronyc} to monitor @code{chronyd} on
the current host.
@c }}}
@c {{{ cmddeny
@node cmddeny command
@subsubsection cmddeny
This is similar to the @code{deny} command, except that it is used to allow
particular hosts or subnets to use @code{chronyc} to monitor @code{chronyd} on
the current host.
@c }}}
@c {{{ cyclelogs
@node cyclelogs command
@subsubsection cyclelogs
The @code{cyclelogs} command causes all of @code{chronyd's} open log files to
be closed and re-opened. This allows them to be renamed so that they can be
periodically purged. An example of how to do this is shown below.
@example
% mv /var/log/chrony/measurements.log /var/log/chrony/measurements1.log
% chronyc cyclelogs
% ls -l /var/log/chrony
-rw-r--r-- 1 root root 0 Jun 8 18:17 measurements.log
-rw-r--r-- 1 root root 12345 Jun 8 18:17 measurements1.log
% rm -f measurements1.log
@end example
@c }}}
@c {{{ delete
@node delete command
@subsubsection delete
The @code{delete} command allows an NTP server or peer to be removed
from the current set of sources.
The syntax is illustrated in the examples below.
@example
delete foo.example.net
delete 1.2.3.4
delete 2001:db8::1
@end example
There is one parameter, the name or IP address of the server or peer to
be deleted.
@c }}}
@c {{{ deny all
@node deny all command
@subsubsection deny all
The effect of the allow command is identical to the @code{deny all}
directive in the configuration file (@pxref{deny directive}).
@c }}}
@c {{{ deny
@node deny command
@subsubsection deny
The effect of the allow command is identical to the @code{deny}
directive in the configuration file (@pxref{deny directive}).
The syntax is illustrated in the following examples:
@example
deny foo.example.net
deny 1.2
deny 3.4.5
deny 6.7.8/22
deny 6.7.8.9/22
deny 2001:db8:789a::/48
deny 0/0
deny ::/0
deny
@end example
@c }}}
@c {{{ dns
@node dns command
@subsubsection dns
The @code{dns} command configures how are hostnames and IP addresses resolved in
@code{chronyc}. IP addresses can be resolved to hostnames when printing results
of @code{sources}, @code{sourcestats}, @code{tracking} and @code{clients}
commands. Hostnames are resolved in commands that take an address as argument.
There are five forms of the command:
@table @code
@item dns -n
Disables resolving IP addresses to hostnames. Raw IP addresses will be
displayed.
@item dns +n
Enables resolving IP addresses to hostnames. This is the default unless
@code{chronyc} was started with @code{-n} option.
@item dns -4
Resolves hostnames only to IPv4 addresses.
@item dns -6
Resolves hostnames only to IPv6 addresses.
@item dns -46
Resolves hostnames to both address families. This is the default unless
@code{chronyc} was started with @code{-4} or @code{-6} option.
@end table
@c }}}
@c {{{ dump
@node dump command
@subsubsection dump
The @code{dump} command causes @code{chronyd} to write its current history of
measurements for each of its sources to dump files, either for
inspection or to support the @code{-r} option when @code{chronyd} is restarted.
The @code{dump} command is somewhat equivalent to the @code{dumponexit}
directive in the chrony configuration file. @xref{dumponexit directive}.
To use the @code{dump}, you probably want to configure the name of the
directory into which the dump files will be written. This can only be
done in the configuration file, see @ref{dumpdir directive}.
@c }}}
@c {{{ exit
@node exit command
@subsubsection exit
The exit command exits from chronyc and returns the user to the shell
(same as the quit command).
@c }}}
@c {{{ help
@node help command
@subsubsection help
The help command displays a summary of the commands and their arguments.
@c }}}
@c {{{ local
@node local command
@subsubsection local
The @code{local} command allows @code{chronyd} to be told that it is to appear
as a reference source, even if it is not itself properly synchronised to
an external source. (This can be used on isolated networks, to allow
one computer to be a master time server with the other computers slaving
to it.) The @code{local} command is somewhat equivalent to the
@code{local} directive in the configuration file, see @ref{local directive}.
The syntax is as shown in the following examples.
@example
local stratum 10
local off
@end example
The first example enables the local reference mode on the host, and sets
the stratum at which it should claim to be synchronised.
The second example disables the local reference mode.
@c }}}
@c {{{ makestep
@node makestep command
@subsubsection makestep
Normally chronyd will cause the system to gradually correct any time
offset, by slowing down or speeding up the clock as required. In
certain situations, the system clock may be so far adrift that this
slewing process would take a very long time to correct the system clock.
The @code{makestep} command can be used in this situation. There are two forms
of the command. The first form has no parameters. It tells @code{chronyd} to
cancel any remaining correction that was being slewed and jump the system clock
by the equivalent amount, making it correct immediately.
The second form configures the automatic stepping, similarly to the
@code{makestep} directive (@pxref{makestep directive}). It has two parameters,
stepping threshold (in seconds) and number of future clock updates for which
will be the threshold active. This can be used with the @code{burst} command
to quickly make a new measurement and correct the clock by stepping if needed,
without waiting for @code{chronyd} to complete the measurement and update the
clock.
@example
makestep 0.1 1
burst 1/2
@end example
BE WARNED - certain software will be seriously affected by such jumps to
the system time. (That is the reason why chronyd uses slewing
normally.)
@c }}}
@c {{{ manual
@node manual command
@subsubsection manual
The manual command enables and disables use of the @code{settime}
command (@pxref{settime command}), and is used to modify the behaviour
of the manual clock driver.
Examples of the command are shown below.
@example
manual on
manual off
manual delete 1
manual list
manual reset
@end example
The @code{on} form of the command enables use of the @code{settime}
command.
The @code{off} form of the command disables use of the @code{settime}
command.
The @code{list} form of the command lists all the samples currently
stored in @code{chronyd}. The output is illustrated below.
@example
210 n_samples = 1
# Date Time(UTC) Slewed Original Residual
====================================================
0 27Jan99 22:09:20 0.00 0.97 0.00
@end example
The columns as as follows :
@enumerate 1
@item
The sample index (used for the @code{manual delete} command)
@item
The date and time of the sample
@item
The system clock error when the timestamp was entered, adjusted to allow
for changes made to the system clock since.
@item
The system clock error when the timestamp was entered, as it originally
was (without allowing for changes to the system clock since).
@item
The regression residual at this point, in seconds. This allows
'outliers' to be easily spotted, so that they can be deleted using the
@code{manual delete} command.
@end enumerate
The @code{delete} form of the command deletes a single sample. The
parameter is the index of the sample, as shown in the first column of
the output from @code{manual list}. Following deletion of the data
point, the current error and drift rate are re-estimated from the
remaining data points and the system clock trimmed if necessary. This
option is intended to allow 'outliers' to be discarded, i.e. samples
where the administrator realises he/she has entered a very poor
timestamp.
The @code{reset} form of the command deletes all samples at once. The
system clock is left running as it was before the command was entered.
@c }}}
@c {{{ maxdelay
@node maxdelay command
@subsubsection maxdelay
This allows the @code{maxdelay} option for one of the sources to be
modified, in the same way as specifying the @code{maxdelay} option for
the @code{server} directive in the configuration file (@pxref{server
directive}).
The following examples illustrate the syntax
@example
maxdelay foo.example.net 0.3
maxdelay 1.2.3.4 0.0015
maxdelay 2001:db8::1 0.0015
@end example
The first example sets the maximum network delay allowed for a
measurement to the host @code{foo.example.net} to 0.3 seconds. The second
and third examples set the maximum network delay for a measurement to
the host with IPv4 address @code{1.2.3.4} and the host with IPv6 address
@code{2001:db8::1} to 1.5 milliseconds.
(Any measurement whose network delay exceeds the specified value is
discarded.)
@c }}}
@c {{{ maxdelaydevratio
@node maxdelaydevratio command
@subsubsection maxdelaydevratio
This allows the @code{maxdelaydevratio} option for one of the sources to be
modified, in the same way as specifying the @code{maxdelaydevratio} option
for the @code{server} directive in the configuration file (@pxref{server
directive}).
The following examples illustrate the syntax
@example
maxdelaydevratio foo.example.net 0.1
maxdelaydevratio 1.2.3.4 1.0
maxdelaydevratio 2001:db8::1 100.0
@end example
@c }}}
@c {{{ maxdelayratio
@node maxdelayratio command
@subsubsection maxdelayratio
This allows the @code{maxdelayratio} option for one of the sources to be
modified, in the same way as specifying the @code{maxdelayratio} option
for the @code{server} directive in the configuration file (@pxref{server
directive}).
The following examples illustrate the syntax
@example
maxdelayratio foo.example.net 1.5
maxdelayratio 1.2.3.4 2.0
maxdelayratio 2001:db8::1 2.0
@end example
The first example sets the maximum network delay for a measurement to
the host @code{foo.example.net} to be 1.5 times the minimum delay found
amongst the previous measurements that have been retained. The second
and third examples set the maximum network delay for a measurement to
the host with IPv4 address @code{1.2.3.4} and the host with IPv6
address @code{2001:db8::1} to be double the retained minimum.
As for @code{maxdelay}, any measurement whose network delay is too large
will be discarded.
@c }}}
@c {{{ maxpoll
@node maxpoll command
@subsubsection maxpoll
The @code{maxpoll} command is used to modify the minimum polling
interval for one of the current set of sources. It is equivalent to the
@code{maxpoll} option in the @code{server} directive in the
configuration file (@pxref{server directive}).
The syntax is as follows
@example
maxpoll <host> <new-maxpoll>
@end example
where the host can be specified as either a machine name or
IP address. The new minimum poll is specified as a base-2 logarithm of
the number of seconds between polls (e.g. specify 6 for 64 second
sampling).
An example is
@example
maxpoll foo.example.net 10
@end example
which sets the maximum polling interval for the host @code{foo.example.net}
to 1024 seconds.
Note that the new maximum polling interval only takes effect after the
next measurement has been made.
@c }}}
@c {{{ maxupdateskew
@node maxupdateskew command
@subsubsection maxupdateskew
This command has the same effect as the @code{maxupdateskew} directive
in the configuration file, see @ref{maxupdateskew directive}.
@c }}}
@c {{{ minpoll
@node minpoll command
@subsubsection minpoll
The @code{minpoll} command is used to modify the minimum polling
interval for one of the current set of sources. It is equivalent to the
@code{minpoll} option in the @code{server} directive in the
configuration file (@pxref{server directive}).
The syntax is as follows
@example
minpoll <host> <new-minpoll>
@end example
where the host can be specified as either a machine name or
IP address. The new minimum poll is specified as a base-2 logarithm of
the number of seconds between polls (e.g. specify 6 for 64 second
sampling).
An example is
@example
minpoll foo.example.net 5
@end example
which sets the minimum polling interval for the host @code{foo.example.net}
to 32 seconds.
Note that the new minimum polling interval only takes effect after the
next measurement has been made.
@c }}}
@c {{{ minstratum
@node minstratum command
@subsubsection minstratum
The @code{minstratum} command is used to modify the minimum stratum
for one of the current set of sources. It is equivalent to the
@code{minstratum} option in the @code{server} directive in the
configuration file (@pxref{server directive}).
The syntax is as follows
@example
minstratum <host> <new-min-stratum>
@end example
where the host can be specified as either a machine name or
IP address.
An example is
@example
minpoll foo.example.net 5
@end example
which sets the minimum stratum for the host @code{foo.example.net}
to 5.
Note that the new minimum stratum only takes effect after the
next measurement has been made.
@c }}}
@c {{{ offline
@node offline command
@subsubsection offline
The @code{offline} command is used to warn @code{chronyd} that the network
connection to a particular host or hosts is about to be lost. It should
be used on computers with a dial-up or similar connection to their time
sources, to warn @code{chronyd} that the connection is about to be broken.
An example of how to use @code{offline} in this case is shown in
@ref{Advising chronyd of internet availability}.
Another case where @code{offline} could be used is where a computer
serves time to a local group of computers, and has a permanant
connection to true time servers outside the organisation. However, the
external connection is heavily loaded at certain times of the day and
the measurements obtained are less reliable at those times. In this
case, it is probably most useful to determine the gain/loss rate during
the quiet periods and let the whole network coast through the loaded
periods. The @code{offline} and @code{online} commands can be used to
achieve this. The situation is shown in the figure below.
@example
@group
+----------+
|Ext source|
+----------+
|
|
|/| <-- Link with variable
| reliability
|
+-------------------+
|Local master server|
+-------------------+
|
+---+---+-----+-----+----+----+
| | | | | | |
Local clients
@end group
@end example
If the source to which @code{chronyd} is currently synchronised is indicated
offline in this way, @code{chronyd} will continue to treat it as the
synchronisation source. If the network connection were broken without
the @code{offline} command being used, @code{chronyd} would assume that the
source had failed and would attempt to pick another synchronisation
source.
There are four forms of the @code{offline} command. The first form is a
wildcard, meaning all sources. The second form allows an IP address mask
and a masked address to be specified. The third form uses the CIDR
notation. The fourth form uses an IP address or a hostname. These forms are
illustrated below.
@example
offline
offline 255.255.255.0/1.2.3.0
offline 2001:db8:789a::/48
offline foo.example.net
@end example
The second form means that the @code{offline} command is to be applied
to any source whose IPv4 address is in the @code{1.2.3} subnet. (The host's
address is logically and-ed with the mask, and if the result matches the
masked-address the host is processed). The third form means that the
command is to be applied to all sources whose IPv6 addresses have first
48 bits equal to @code{2001:db8:789a}. The fourth form means that the command
is to be applied only to that one source.
The wildcard form of the address is actually equivalent to
@example
offline 0.0.0.0/0.0.0.0
offline ::/0
@end example
@c }}}
@c {{{ online
@node online command
@subsubsection online
The @code{online} command is opposite in function to the @code{offline}
command. It is used to advise @code{chronyd} that network connectivity to a
particular source or sources has been restored.
The syntax is identical to that of the @code{offline} command, see
@ref{offline command}.
@c }}}
@c {{{ polltarget
@node polltarget command
@subsubsection polltarget
The @code{polltarget} command is used to modify the poll target for
one of the current set of sources. It is equivalent to the
@code{polltarget} option in the @code{server} directive in the
configuration file (@pxref{server directive}).
The syntax is as follows
@example
polltarget <host> <new-poll-target>
@end example
where the host can be specified as either a machine name or
IP address.
An example is
@example
polltarget foo.example.net 12
@end example
which sets the poll target for the host @code{foo.example.net}
to 12.
@c }}}
@c {{{ quit
@node quit command
@subsubsection quit
The quit command exits from chronyc and returns the user to the shell
(same as the exit command).
@c }}}
@c {{{ refresh command
@node refresh command
@subsubsection refresh
The @code{refresh} command can be used to force @code{chronyd} to resolve the
names of configured sources to IP addresses again, e.g. after suspending and
resuming the machine in a different network.
Sources that stop responding will be replaced with newly resolved addresses
automatically after 8 polling intervals, but this command may still be useful
to replace them immediately and not wait until they are marked as unreachable.
@c }}}
@c {{{ reselect command
@node reselect command
@subsubsection reselect
To avoid excessive switching between sources, @code{chronyd} may stay
synchronised to a source even when it is not currently the best one among the
available sources.
The @code{reselect} command can be used to force @code{chronyd} to
reselect the best synchronisation source.
@c }}}
@c {{{ reselectdist command
@node reselectdist command
@subsubsection reselectdist
The @code{reselectdist} command sets the reselect distance. It is equivalent
to the @code{reselectdist} directive in the configuration file
(@pxref{reselectdist directive}).
@c }}}
@c {{{ retries
@node retries command
@subsubsection retries
The @code{retries} command sets the maximum number of retries for
@code{chronyc} requests before giving up. The response timeout is controlled
by @code{timeout} command (@pxref{timeout command}).
The default is 2.
@c }}}
@c {{{ rtcdata
@node rtcdata command
@subsubsection rtcdata
The @code{rtcdata} command displays the current real time clock RTC parameters.
An example output is shown below.
@example
RTC ref time (GMT) : Sat May 30 07:25:56 1998
Number of samples : 10
Number of runs : 5
Sample span period : 549
RTC is fast by : -1.632736 seconds
RTC gains time at : -107.623 ppm
@end example
The fields have the following meaning
@table @code
@item RTC ref time (GMT)
This is the RTC reading the last time its error was measured.
@item Number of samples
This is the number of previous measurements being used to determine the
RTC gain/loss rate.
@item Number of runs
This is the number of runs of residuals of the same sign following the
regression fit for (RTC error) versus (RTC time). A value which is
small indicates that the measurements are not well approximated by a
linear model, and that the algorithm will tend to delete the older
measurements to improve the fit.
@item Sample span period
This is the period that the measurements span (from the oldest to the
newest). Without a unit the value is in seconds; suffixes `m' for
minutes, `h' for hours, `d' for days or `y' for years may be used.
@item RTC is fast by
This is the estimate of how many seconds fast the RTC when it thought
the time was at the reference time (above). If this value is large, you
may (or may not) want to use the @code{trimrtc} command to bring the RTC
into line with the system clock. (Note, a large error will not affect
@code{chronyd's} operation, unless it becomes so big as to start causing
rounding errors.
@item RTC gains time at
This is the amount of time gained (positive) or lost (negative) by the
real time clock for each second that it ticks. It is measured in parts
per million. So if the value shown was +1, suppose the RTC was exactly
right when it crosses a particular second boundary. Then it would be 1
microsecond fast when it crosses its next second boundary.
@end table
@c }}}
@c {{{ settime
@node settime command
@subsubsection settime
The @code{settime} command allows the current time to be entered
manually, if this option has been configured into @code{chronyd}. (It may be
configured either with the @code{manual} directive in the configuration
file (@pxref{manual directive}), or with the @code{manual} command of
chronyc (@pxref{manual command}).
It should be noted that the computer's sense of time will only be as
accurate as the reference you use for providing this input (e.g. your
watch), as well as how well you can time the press of the return key.
Providing your computer's time zone is set up properly, you will be able
to enter a local time (rather than UTC).
The response to a successful @code{settime} command indicates the amount
that the computer's clock was wrong. It should be apparent from this if
you have entered the time wrongly, e.g. with the wrong time zone.
The rate of drift of the system clock is estimated by a regression
process using the entered measurement and all previous measurements
entered during the present run of @code{chronyd}. However, the entered
measurement is used for adjusting the current clock offset (rather than
the estimated intercept from the regression, which is ignored).
Contrast what happens with the @code{manual delete} command, where the
intercept is used to set the current offset (since there is no
measurement that has just been typed in in that case).
The time is parsed by the public domain @file{getdate} algorithm.
Consequently, you can only specify time to the nearest second.
Examples of inputs that are valid are shown below.
@example
settime 16:30
settime 16:30:05
settime Nov 21, 1997 16:30:05
@end example
For a full description of @code{getdate}, get hold of the getdate
documentation (bundled, for example, with the source for GNU tar).
@c }}}
@c {{{ smoothing
@node smoothing command
@subsubsection smoothing
The @code{smoothing} command displays the current state of the NTP server time
smoothing. An example of the output is shown below.
@example
Active : Yes
Offset : +1.000268817 seconds
Frequency : -0.142859 ppm
Wander : -0.010000 ppm per second
Last update : 17.8 seconds ago
Remaining time : 19988.4 seconds
@end example
The fields are explained as follows.
@table @code
@item Active
This shows if the server time smoothing is currently active. Possible values
are @code{Yes} and @code{No}. If the @code{leaponly} option is included in the
@code{smoothtime} directive, @code{(leap second only)} will be shown on the
line.
@item Offset
This is the current offset applied to the time sent to NTP clients. Positive
value means the clients are getting time that's ahead of true time.
@item Frequency
The current frequency offset of the served time. Negative value means the time
observed by clients is running slower than true time.
@item Wander
The current frequency wander of the served time. Negative value means the time
observed by clients is slowing down.
@item Last update
This field shows how long ago was the time smoothing process updated, e.g.
@code{chronyd} accumulated a new measurement.
@item Remaining time
The time it would take for the smoothing process to get to zero offset and
frequency if there were no more updates.
@end table
@c }}}
@c {{{ smoothtime
@node smoothtime command
@subsubsection smoothtime
The @code{smoothtime} command can be used to reset or activate the server time
smoothing process if it is configured with the @code{smoothtime} directive
(@pxref{smoothtime directive}).
The syntax is as follows
@example
smoothtime reset
smoothtime activate
@end example
@c }}}
@c {{{ sources
@node sources command
@subsubsection sources
This command displays information about the current time sources that
@code{chronyd} is accessing.
The optional argument @code{-v} can be specified, meaning @emph{verbose}. In
this case, extra caption lines are shown as a reminder of the meanings of the
columns.
@example
@group
210 Number of sources = 3
MS Name/IP address Stratum Poll Reach LastRx Last sample
===============================================================================
#* GPS0 0 4 377 11 -479ns[ -621ns] +/- 134ns
^? foo.example.net 2 6 377 23 -923us[ -924us] +/- 43ms
^+ bar.example.net 1 6 377 21 -2629us[-2619us] +/- 86ms
@end group
@end example
The columns are as follows:
@table @code
@item M
This indicates the mode of the source. @code{^} means a server,
@code{=} means a peer and @code{#} indicates a locally connected
reference clock.
@item S
This column indicates the state of the sources. @code{*} indicates the
source to which @code{chronyd} is currently synchronised. @code{+}
indicates acceptable sources which are combined with the selected
source. @code{-} indicates acceptable sources which are excluded by
the combining algorithm. @code{?} indicates sources to which
connectivity has been lost or whose packets don't pass all tests.
@code{x} indicates a clock which @code{chronyd}
thinks is is a falseticker (i.e. its time is inconsistent with a
majority of other sources). @code{~} indicates a source whose time
appears to have too much variability. The @code{?} condition is also
shown at start-up, until at least 3 samples have been gathered from it.
@item Name/IP address
This shows the name or the IP address of the source, or refid for
reference clocks.
@item Stratum
This shows the stratum of the source, as reported in its most recently
received sample. Stratum 1 indicates a computer with a locally attached
reference clock. A computer that is synchronised to a stratum 1
computer is at stratum 2. A computer that is synchronised to a stratum
2 computer is at stratum 3, and so on.
@item Poll
This shows the rate at which the source is being polled, as a base-2
logarithm of the interval in seconds. Thus, a value of 6 would indicate
that a measurement is being made every 64 seconds.
@code{chronyd} automatically varies the polling rate in response to prevailing
conditions.
@item Reach
This shows the source's reachability register printed as octal number. The
register has 8 bits and is updated on every received or missed packet from
the source. A value of 377 indicates that a valid reply was received for all
from the last eight transmissions.
@item LastRx
This column shows how long ago the last sample was received from the
source. This is normally in seconds. The letters @code{m}, @code{h},
@code{d} or @code{y} indicate minutes, hours, days or years. A value
of 10 years indicates there were no samples received from this source
yet.
@item Last sample
This column shows the offset between the local clock and the source at
the last measurement. The number in the square brackets shows the
actual measured offset. This may be suffixed by @code{ns} (indicating
nanoseconds), @code{us} (indicating
microseconds), @code{ms} (indicating milliseconds), or @code{s}
(indicating seconds). The number to the left of the square brackets
shows the original measurement, adjusted to allow for any slews applied
to the local clock since. The number following the @code{+/-} indicator
shows the margin of error in the measurement.
Positive offsets indicate that the local clock is fast of the source.
@end table
@c }}}
@c {{{ sourcestats
@node sourcestats command
@subsubsection sourcestats
The @code{sourcestats} command displays information about the drift rate
and offset estimatation process for each of the sources currently being
examined by @code{chronyd}.
The optional argument @code{-v} can be specified, meaning @emph{verbose}. In
this case, extra caption lines are shown as a reminder of the meanings of the
columns.
An example report is
@example
@group
210 Number of sources = 1
Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev
===============================================================================
abc.def.ghi 11 5 46m -0.001 0.045 1us 25us
@end group
@end example
The columns are as follows
@table @code
@item Name/IP Address
This is the name or IP address of the NTP server (or peer) or refid of
the refclock to which the rest of the line relates.
@item NP
This is the number of sample points currently being retained for the
server. The drift rate and current offset are estimated by performing a
linear regression through these points.
@item NR
This is the number of runs of residuals having the same sign following
the last regression. If this number starts to become too small relative
to the number of samples, it indicates that a straight line is no longer
a good fit to the data. If the number of runs is too low,
@code{chronyd} discards older samples and re-runs the regression until
the number of runs becomes acceptable.
@item Span
This is the interval between the oldest and newest samples. If no unit
is shown the value is in seconds. In the example, the interval is 46
minutes.
@item Frequency
This is the estimated residual frequency for the server, in parts per
million. In this case, the computer's clock is estimated to be running
1 part in 10**9 slow relative to the server.
@item Freq Skew
This is the estimated error bounds on @code{Freq} (again in parts per
million).
@item Offset
This is the estimated offset of the source.
@item Std Dev
This is the estimated sample standard deviation.
@end table
@c }}}
@c {{{ timeout
@node timeout command
@subsubsection timeout
The @code{timeout} command sets the initial timeout for @code{chronyc} requests
in milliseconds. If no response is received from @code{chronyd}, the timeout is
doubled and the request is resent. The maximum number of retries is configured
with the @code{retries} command (@pxref{retries command}).
By default, the timeout is 1000 milliseconds.
@c }}}
@c {{{ tracking
@node tracking command
@subsubsection tracking
The @code{tracking} command displays parameters about the system's clock
performance. An example of the output is shown below.
@example
Reference ID : 1.2.3.4 (foo.example.net)
Stratum : 3
Ref time (UTC) : Fri Feb 3 15:00:29 2012
System time : 0.000001501 seconds slow of NTP time
Last offset : -0.000001632 seconds
RMS offset : 0.000002360 seconds
Frequency : 331.898 ppm fast
Residual freq : 0.004 ppm
Skew : 0.154 ppm
Root delay : 0.373169 seconds
Root dispersion : 0.024780 seconds
Update interval : 64.2 seconds
Leap status : Normal
@end example
The fields are explained as follows.
@table @code
@item Reference ID
This is the refid and name (or IP address) if available, of the server to which
the computer is currently synchronised. If this is @code{127.127.1.1}
it means the computer is not synchronised to any external source and
that you have the `local' mode operating (via the @code{local} command
in @code{chronyc} (@pxref{local command}), or the @code{local} directive
in the @file{@SYSCONFDIR@/chrony.conf} file (@pxref{local directive})).
@item Stratum
The stratum indicates how many hops away from a computer with an
attached reference clock we are. Such a computer is a stratum-1
computer, so the computer in the example is two hops away
(i.e. @code{foo.example.net} is a stratum-2 and is synchronised from a stratum-1).
@item Ref time
This is the time (UTC) at which the last measurement from the reference
source was processed.
@item System time
In normal operation, @code{chronyd} @emph{never} steps the system clock,
because any jump in the timescale can have adverse consequences for
certain application programs. Instead, any error in the system clock is
corrected by slightly speeding up or slowing down the system clock until
the error has been removed, and then returning to the system clock's
normal speed. A consequence of this is that there will be a period when
the system clock (as read by other programs using the
@code{gettimeofday()} system call, or by the @code{date} command in the
shell) will be different from @code{chronyd's} estimate of the current
true time (which it reports to NTP clients when it is operating in
server mode). The value reported on this line is the difference due to
this effect.
@item Last offset
This is the estimated local offset on the last clock update.
@item RMS offset
This is a long-term average of the offset value.
@item Frequency
The `frequency' is the rate by which the system's clock would be would
be wrong if @code{chronyd} was not correcting it. It is expressed in
ppm (parts per million). For example, a value of 1ppm would mean that
when the system's clock thinks it has advanced 1 second, it has actually
advanced by 1.000001 seconds relative to true time.
As you can see in the example, the clock in the computer is not a very
good one - it gains about 30 seconds per day!
@item Residual freq
This shows the `residual frequency' for the currently selected reference
source. This reflects any difference between what the measurements from
the reference source indicate the frequency should be and the frequency
currently being used.
The reason this is not always zero is that a smoothing procedure is
applied to the frequency. Each time a measurement from the reference
source is obtained and a new residual frequency computed, the estimated
accuracy of this residual is compared with the estimated accuracy (see
`skew' next) of the existing frequency value. A weighted average is
computed for the new frequency, with weights depending on these
accuracies. If the measurements from the reference source follow a
consistent trend, the residual will be driven to zero over time.
@item Skew
This is the estimated error bound on the the frequency.
@item Root delay
This is the total of the network path delays to the stratum-1 computer
from which the computer is ultimately synchronised.
@item Root dispersion
This is the total dispersion accumulated through all the computers back
to the stratum-1 computer from which the computer is ultimately
synchronised. Dispersion is due to system clock resolution, statistical
measurement variations etc.
An absolute bound on the computer's clock accuracy (assuming the
stratum-1 computer is correct) is given by
@example
clock_error <= root_dispersion + (0.5 * |root_delay|)
@end example
@item Update interval
This is the interval between the last two clock updates.
@item Leap status
This is the leap status, which can be @code{Normal}, @code{Insert second},
@code{Delete second} or @code{Not synchronised}.
@end table
@c }}}
@c {{{ trimrtc
@node trimrtc command
@subsubsection trimrtc
The @code{trimrtc} command is used to correct the system's real time
clock (RTC) to the main system clock. It has no effect if the error
between the two clocks is currently estimated at less than a second (the
resolution of the RTC is only 1 second).
The command takes no arguments. It performs the following steps (if the
RTC is more than 1 second away from the system clock):
@enumerate 1
@item
Remember the currently estimated gain/loss rate of the RTC and flush the
previous measurements.
@item
Step the real time clock to bring it within a second of the system clock.
@item
Make several measurements to accurately determine the new offset between
the RTC and the system clock (i.e. the remaining fraction of a second
error)
@item
Save the RTC parameters to the RTC file (specified with the
@code{rtcfile} directive in the configuration file (@pxref{rtcfile
directive}).
@end enumerate
The last step is done as a precaution against the computer suffering a
power failure before either the daemon exits or the @code{writertc}
command is issued.
@code{chronyd} will still work perfectly well both whilst operating and
across machine reboots even if the @code{trimrtc} command is never used
(and the RTC is allowed to drift away from true time). The
@code{trimrtc} command is provided as a method by which it can be
corrected, in a manner compatible with @code{chronyd} using it to
maintain accurate time across machine reboots.
The @code{trimrtc} command can be executed automatically by @code{chronyd}
with the @code{rtcautotrim} directive (@pxref{rtcautotrim directive}).
@c }}}
@c {{{ waitsync
@node waitsync command
@subsubsection waitsync
The @code{waitsync} command waits for @code{chronyd} to synchronise.
Up to three optional arguments can be specified, the first is the maximum
number of tries in 10 second intervals before giving up and returning a
non-zero error code. When 0 is specified, or there are no arguments, the
number of tries will not be limited.
The second and third arguments are the maximum allowed remaining correction of
the system clock and the maximum allowed skew (in ppm) as reported by the
@code{tracking} command (@pxref{tracking command}) in the @code{System time}
and @code{Skew} fields. If not specified or zero, the value will not be
checked.
An example is
@example
waitsync 60 0.01
@end example
which will wait up to about 10 minutes for @code{chronyd} to synchronise to a
source and the remaining correction to be less than 10 milliseconds.
@c }}}
@c {{{ writertc
@node writertc command
@subsubsection writertc
The @code{writertc} command writes the currently estimated error and
gain/loss rate parameters for the RTC to the RTC file (specified with
the @code{rtcfile} directive (@pxref{rtcfile directive})). This
information is also written automatically when @code{chronyd} is killed
(with SIGHUP, SIGINT, SIGQUIT or SIGTERM) or when the @code{trimrtc}
command is issued.
@c }}}
@c }}}
@c }}}
@c }}}
@c {{{ apx:GNU General Public License
@node GPL
@appendix GNU General Public License
@center GNU GENERAL PUBLIC LICENSE
@center Version 2, June 1991
Copyright (C) 1989, 1991 Free Software Foundation, Inc.,
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
Preamble
The licenses for most software are designed to take away your
freedom to share and change it. By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software--to make sure the software is free for all its users. This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it. (Some other Free Software Foundation software is covered by
the GNU Lesser General Public License instead.) You can apply it to
your programs, too.
When we speak of free software, we are referring to freedom, not
price. Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have. You must make sure that they, too, receive or can get the
source code. And you must show them these terms so they know their
rights.
We protect your rights with two steps: (1) copyright the software, and
(2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.
Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software. If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.
Finally, any free program is threatened constantly by software
patents. We wish to avoid the danger that redistributors of a free
program will individually obtain patent licenses, in effect making the
program proprietary. To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.
The precise terms and conditions for copying, distribution and
modification follow.
GNU GENERAL PUBLIC LICENSE
TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
0. This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License. The "Program", below,
refers to any such program or work, and a "work based on the Program"
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
language. (Hereinafter, translation is included without limitation in
the term "modification".) Each licensee is addressed as "you".
Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope. The act of
running the Program is not restricted, and the output from the Program
is covered only if its contents constitute a work based on the
Program (independent of having been made by running the Program).
Whether that is true depends on what the Program does.
1. You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any warranty;
and give any other recipients of the Program a copy of this License
along with the Program.
You may charge a fee for the physical act of transferring a copy, and
you may at your option offer warranty protection in exchange for a fee.
2. You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:
a) You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.
b) You must cause any work that you distribute or publish, that in
whole or in part contains or is derived from the Program or any
part thereof, to be licensed as a whole at no charge to all third
parties under the terms of this License.
c) If the modified program normally reads commands interactively
when run, you must cause it, when started running for such
interactive use in the most ordinary way, to print or display an
announcement including an appropriate copyright notice and a
notice that there is no warranty (or else, saying that you provide
a warranty) and that users may redistribute the program under
these conditions, and telling the user how to view a copy of this
License. (Exception: if the Program itself is interactive but
does not normally print such an announcement, your work based on
the Program is not required to print an announcement.)
These requirements apply to the modified work as a whole. If
identifiable sections of that work are not derived from the Program,
and can be reasonably considered independent and separate works in
themselves, then this License, and its terms, do not apply to those
sections when you distribute them as separate works. But when you
distribute the same sections as part of a whole which is a work based
on the Program, the distribution of the whole must be on the terms of
this License, whose permissions for other licensees extend to the
entire whole, and thus to each and every part regardless of who wrote it.
Thus, it is not the intent of this section to claim rights or contest
your rights to work written entirely by you; rather, the intent is to
exercise the right to control the distribution of derivative or
collective works based on the Program.
In addition, mere aggregation of another work not based on the Program
with the Program (or with a work based on the Program) on a volume of
a storage or distribution medium does not bring the other work under
the scope of this License.
3. You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the terms of
Sections 1 and 2 above provided that you also do one of the following:
a) Accompany it with the complete corresponding machine-readable
source code, which must be distributed under the terms of Sections
1 and 2 above on a medium customarily used for software interchange; or,
b) Accompany it with a written offer, valid for at least three
years, to give any third party, for a charge no more than your
cost of physically performing source distribution, a complete
machine-readable copy of the corresponding source code, to be
distributed under the terms of Sections 1 and 2 above on a medium
customarily used for software interchange; or,
c) Accompany it with the information you received as to the offer
to distribute corresponding source code. (This alternative is
allowed only for noncommercial distribution and only if you
received the program in object code or executable form with such
an offer, in accord with Subsection b above.)
The source code for a work means the preferred form of the work for
making modifications to it. For an executable work, complete source
code means all the source code for all modules it contains, plus any
associated interface definition files, plus the scripts used to
control compilation and installation of the executable. However, as a
special exception, the source code distributed need not include
anything that is normally distributed (in either source or binary
form) with the major components (compiler, kernel, and so on) of the
operating system on which the executable runs, unless that component
itself accompanies the executable.
If distribution of executable or object code is made by offering
access to copy from a designated place, then offering equivalent
access to copy the source code from the same place counts as
distribution of the source code, even though third parties are not
compelled to copy the source along with the object code.
4. You may not copy, modify, sublicense, or distribute the Program
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense or distribute the Program is
void, and will automatically terminate your rights under this License.
However, parties who have received copies, or rights, from you under
this License will not have their licenses terminated so long as such
parties remain in full compliance.
5. You are not required to accept this License, since you have not
signed it. However, nothing else grants you permission to modify or
distribute the Program or its derivative works. These actions are
prohibited by law if you do not accept this License. Therefore, by
modifying or distributing the Program (or any work based on the
Program), you indicate your acceptance of this License to do so, and
all its terms and conditions for copying, distributing or modifying
the Program or works based on it.
6. Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the
original licensor to copy, distribute or modify the Program subject to
these terms and conditions. You may not impose any further
restrictions on the recipients' exercise of the rights granted herein.
You are not responsible for enforcing compliance by third parties to
this License.
7. If, as a consequence of a court judgment or allegation of patent
infringement or for any other reason (not limited to patent issues),
conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License. If you cannot
distribute so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you
may not distribute the Program at all. For example, if a patent
license would not permit royalty-free redistribution of the Program by
all those who receive copies directly or indirectly through you, then
the only way you could satisfy both it and this License would be to
refrain entirely from distribution of the Program.
If any portion of this section is held invalid or unenforceable under
any particular circumstance, the balance of the section is intended to
apply and the section as a whole is intended to apply in other
circumstances.
It is not the purpose of this section to induce you to infringe any
patents or other property right claims or to contest validity of any
such claims; this section has the sole purpose of protecting the
integrity of the free software distribution system, which is
implemented by public license practices. Many people have made
generous contributions to the wide range of software distributed
through that system in reliance on consistent application of that
system; it is up to the author/donor to decide if he or she is willing
to distribute software through any other system and a licensee cannot
impose that choice.
This section is intended to make thoroughly clear what is believed to
be a consequence of the rest of this License.
8. If the distribution and/or use of the Program is restricted in
certain countries either by patents or by copyrighted interfaces, the
original copyright holder who places the Program under this License
may add an explicit geographical distribution limitation excluding
those countries, so that distribution is permitted only in or among
countries not thus excluded. In such case, this License incorporates
the limitation as if written in the body of this License.
9. The Free Software Foundation may publish revised and/or new versions
of the General Public License from time to time. Such new versions will
be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.
Each version is given a distinguishing version number. If the Program
specifies a version number of this License which applies to it and "any
later version", you have the option of following the terms and conditions
either of that version or of any later version published by the Free
Software Foundation. If the Program does not specify a version number of
this License, you may choose any version ever published by the Free Software
Foundation.
10. If you wish to incorporate parts of the Program into other free
programs whose distribution conditions are different, write to the author
to ask for permission. For software which is copyrighted by the Free
Software Foundation, write to the Free Software Foundation; we sometimes
make exceptions for this. Our decision will be guided by the two goals
of preserving the free status of all derivatives of our free software and
of promoting the sharing and reuse of software generally.
NO WARRANTY
11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS
TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE
PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
REPAIR OR CORRECTION.
12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
END OF TERMS AND CONDITIONS
How to Apply These Terms to Your New Programs
If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest
to attach them to the start of each source file to most effectively
convey the exclusion of warranty; and each file should have at least
the "copyright" line and a pointer to where the full notice is found.
<one line to give the program's name and a brief idea of what it does.>
Copyright (C) <year> <name of author>
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
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.
Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this
when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) year name of author
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.
The hypothetical commands `show w' and `show c' should show the appropriate
parts of the General Public License. Of course, the commands you use may
be called something other than `show w' and `show c'; they could even be
mouse-clicks or menu items--whatever suits your program.
You should also get your employer (if you work as a programmer) or your
school, if any, to sign a "copyright disclaimer" for the program, if
necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program
`Gnomovision' (which makes passes at compilers) written by James Hacker.
<signature of Ty Coon>, 1 April 1989
Ty Coon, President of Vice
This General Public License does not permit incorporating your program into
proprietary programs. If your program is a subroutine library, you may
consider it more useful to permit linking proprietary applications with the
library. If this is what you want to do, use the GNU Lesser General
Public License instead of this License.
@c }}}
@contents
@bye
@c vim:cms=@c\ %s:fdm=marker:fdc=5:syntax=off
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