Manual for version 2.3

1 Introduction

1.1 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, chronyd is a daemon that can be started at boot time and chronyc is a command-line interface program which can be used to monitor chronyd’s performance and to change various operating parameters whilst it is running.

The IP addresses from which chronyc clients may connect can be tightly controlled. The default is just the computer that chronyd itself is running on.

1.2 Acknowledgements

The chrony suite makes use of the algorithm known as RSA Data Security, Inc. MD5 Message-Digest Algorithm for authenticating messages between different machines on the network.

In writing the 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.

1.3 Availability

1.3.1 Getting the software

Links on the chrony home page describe how to obtain the software.

1.3.2 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 adjtime or ntp_adjtime system call) should not be difficult, however it requires access to such systems to test out the driver.

1.4 Relationship to other software packages

1.4.1 ntpd

The ‘reference’ implementation of the Network Time Protocol is the program ntpd, available via The NTP home page.

One of the main differences between ntpd and chronyd is in how they control the computer’s clock. Things chronyd can do better than ntpd:

• chronyd can perform usefully in an environment where access to the time reference is intermittent. ntpd needs regular polling of the reference to work well.
• chronyd can usually synchronise the clock faster and with better time accuracy.
• chronyd quickly adapts to sudden changes in the rate of the clock (e.g. due to changes in the temperature of the crystal oscillator). ntpd may need a long time to settle down again.
• chronyd can perform well even when the network is congested for longer periods of time.
• chronyd in the default configuration never steps the time to not upset other running programs. ntpd can be configured to never step the time too, but in that case it has to use a different means of adjusting the clock (daemon loop instead of kernel discipline), which may have a negative effect on accuracy of the clock.
• 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).
• chronyd is smaller, it uses less memory and it wakes up the CPU only when necessary, which is better for power saving.

Things chronyd can do that ntpd can’t:

• 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). 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.
• 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.

Things ntpd can do that chronyd can’t:

• ntpd supports all operating modes from RFC 5905, including broadcast, multicast, and manycast server/client. However, the broadcast and multicast modes are inherently less accurate and less secure (even with authentication) than the ordinary server/client mode and should generally be avoided.
• ntpd supports the Autokey protocol (RFC 5906) to authenticate servers with public-key cryptography. Note that the protocol has been shown to be insecure and it will be probably replaced with an implementation of the Network Time Security (NTS) specification.
• ntpd supports the orphan mode, which allows synchronisation to a common timescale in isolated networks with multiple servers. With chronyd there can be only one master and all other computers have to be directly or indirectly synchronised to it.
• ntpd has been ported to more operating systems.
• ntpd includes a large number of reference clock drivers. chronyd relies on other programs (e.g. gpsd) to access the timing data via the SHM or SOCK driver.

A comparison of NTP implementations that includes more features and also their performance is on the chrony comparison page.

1.4.2 timed

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 timed are :

• 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.)
• 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. timed does not seem to do this.
• 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’).

timed does have the benefit over chronyd that for isolated networks of computers, they will track the ‘majority vote’ time. For such isolated networks, 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 will stay accurate relative to one another).

1.5 Distribution rights and (lack of) warranty

Chrony may be distributed in accordance with the GNU General Public License version 2, reproduced in See section GNU General Public License.

1.6 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 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!

2 Installation

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

./configure

This is a shell script that automatically determines the system type. There is a single optional parameter, --prefix which indicates the directory tree where the software should be installed. For example,

./configure --prefix=/opt/free

will install the chronyd daemon into /opt/free/sbin and the 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:

CC=cc CFLAGS=-O ./configure --prefix=/opt/free

for Bourne-family shells, or

setenv CC cc
setenv CFLAGS -O
./configure --prefix=/opt/free

for C-family shells.

If the software cannot (yet) be built on your system, an error message will be shown. Otherwise, ‘Makefile’ will be generated.

On Linux, if development files for the libcap library are available, chronyd will be built with support for dropping root privileges. On other systems no extra library is needed. The default user which chronyd should run as can be specified with the --with-user option of the configure script.

If development files for the editline or readline library are 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 see section Support for line editing libraries for more information.

If a ‘timepps.h’ header is available (e.g. from the LinuxPPS project), 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 CPPFLAGS variable to -I/path/to/timepps.

Now type

make

to build the programs.

If you want to build the manual in plain text, HTML and info versions, type

make docs

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.

make install

This will install the binaries and manpages.

To install the plain text, HTML and info versions of the manual, enter the command

make install-docs

If you want chrony to appear in the top level info directory listing, you need to run the install-info command manually after this step. install-info takes 2 arguments. The first is the path to the ‘chrony.info’ file you have just installed. This will be the argument you gave to –prefix when you configured (‘/usr/local’ by default), with ‘/share/info/chrony.info’ on the end. The second argument is the location of the file called ‘dir’. This will typically be ‘/usr/share/info/dir’. So the typical command line would be

install-info /usr/local/share/info/chrony.info /usr/share/info/dir

Now that the software is successfully installed, the next step is to set up a configuration file. The default location of the file is ‘/etc/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

pool pool.ntp.org iburst
makestep 1.0 3
rtcsync

Then, chronyd can be run. For security reasons, it’s recommended to create an unprivileged user for chronyd and specify it with the -u command-line option or the user directive in the configuration file, or set the default user with the --with-user configure option before building.

2.1 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:

./configure --disable-readline other-options...

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:

‘--with-readline-includes=directory_name’

This defines the name of the directory above the one where ‘readline.h’ is. ‘readline.h’ is assumed to be in ‘editline’ or ‘readline’ subdirectory of the named directory.

‘--with-readline-library=directory_name’

This defines the directory containing the ‘libedit.a’ or ‘libedit.so’ file, or ‘libreadline.a’ or ‘libreadline.so’ file.

‘--with-ncurses-library=directory_name’

This defines the directory containing the ‘libncurses.a’ or ‘libncurses.so’ file.

2.2 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 ‘info’ subdirectory of the argument to the –prefix option. For example, you might use

./configure --prefix=/usr --infodir=/usr/share/info

The –mandir=DIR option to configure specifies an install directory for the man pages. This overrides the ‘man’ subdirectory of the argument to the –prefix option.

./configure --prefix=/usr --infodir=/usr/share/info --mandir=/usr/share/man

to set both options together.

The final option is the DESTDIR option to the make command. For example, you could use the commands

./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

to build a package. When untarred within the root directory, this will install the files to the intended final locations.

3 Typical operating scenarios

3.1 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:

• Your institution may already operate servers on its network. Contact your system administrator to find out.
• Your ISP probably has one or more NTP servers available for its customers.
• 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.
• Use public servers from the pool.ntp.org project.

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 ‘/etc/chrony.conf’. Assuming that your NTP servers are called foo.example.net, bar.example.net and baz.example.net, your ‘chrony.conf’ file could contain as a minimum

server foo.example.net
server bar.example.net
server baz.example.net

However, you will probably want to include some of the other directives described later. The following directives may be particularly useful : driftfile, makestep, rtcsync. Also, the iburst server option is useful to speed up the initial synchronization. The smallest useful configuration file would look something like

server foo.example.net iburst
server bar.example.net iburst
server baz.example.net iburst
driftfile /var/lib/chrony/drift
makestep 1.0 3
rtcsync

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 pool directive instead of multiple server directives. The configuration file could in this case look like

pool pool.ntp.org iburst
driftfile /var/lib/chrony/drift
makestep 1.0 3
rtcsync

3.2 Infrequent connection to true NTP servers

In this section we discuss how to configure chrony for computers that have occasional connections to the internet.

3.2.1 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 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 foo.example.net, bar.example.net and baz.example.net, your ‘chrony.conf’ file would need to contain something like

server foo.example.net
server bar.example.net
server baz.example.net

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:

server foo.example.net offline
server bar.example.net offline
server baz.example.net offline

The offline keyword indicates that the servers start in an offline state, and that they should not be contacted until chronyd receives notification from chronyc that the link to the internet is present.

The smallest useful configuration file would look something like

server foo.example.net offline
server bar.example.net offline
server baz.example.net offline
driftfile /var/lib/chrony/drift
makestep 1.0 3
rtcsync

The next section describes how to tell chronyd when the internet link goes up and down.

3.2.2 How to tell chronyd when the internet link is available.

To tell chronyd when to start and finish sampling the servers, the online and offline commands of chronyc need to be used. To give an example of their use, we assume that pppd is the program being used to connect to the internet, and that chronyc has been installed at its default location ‘/usr/local/bin/chronyc’.

In the file ‘/etc/ppp/ip-up’ we add the command sequence

/usr/local/bin/chronyc online

and in the file ‘/etc/ppp/ip-down’ we add the sequence

/usr/local/bin/chronyc offline

chronyd's polling of the servers will now only occur whilst the machine is actually connected to the Internet.

3.3 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. chronyd includes support for this, in the form of the manual directive in the configuration file and the settime command in the chronyc program.

The smoothtime directive (see section smoothtime) is useful when the clocks of the clients need to stay close together when the local time is adjusted by the settime command. The smoothing process needs to be activated by the 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 master) might be (assuming the clients are in the 192.168.165.x subnet)

driftfile /var/lib/chrony/drift
local stratum 8
manual
allow 192.168.165
smoothtime 400 0.01

For the clients the configuration file might be

server master iburst
driftfile /var/lib/chrony/drift
logdir /var/log/chrony
log measurements statistics tracking

3.4 The home PC with a dial-up connection

3.4.1 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 (see section Infrequent connection to true NTP servers). This section addresses specifically the case of a computer which is turned off between ’sessions’.

In this case, 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.

            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|
                                                  +----------+

When the computer is connected to the Internet (via the modem), 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, chronyd makes measurements of the real-time clock (RTC) (via the ‘/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 ‘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 dumponexit directive is specified in the configuration file), and the RTC tracking information is also saved to a file (if the rtcfile directive has been specified). These pieces of information are also saved if the dump and writertc commands respectively are issued through chronyc.

When the computer is rebooted, 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 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 trimrtc command in chronyc. Because of this, 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 dump and writertc commands in the same place as the offline command is issued to take chronyd offline; because 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). 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 log rtc facility in the configuration file; this option should not be used if you want your disc to spin down.

3.4.2 Typical configuration files.

To illustrate how a dial-up home computer might be configured, example configuration files are shown in this section.

For the ‘/etc/chrony.conf’ file, the following can be used as an 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 /var/lib/chrony/drift
makestep 1.0 3
maxupdateskew 100.0
dumponexit
dumpdir /var/lib/chrony
rtcfile /var/lib/chrony/rtc

pppd is used for connecting to the internet. This runs two scripts ‘/etc/ppp/ip-up’ and ‘/etc/ppp/ip-down’ when the link goes online and offline respectively.

The relevant part of the ‘/etc/ppp/ip-up’ file is

/usr/local/bin/chronyc online

and the relevant part of the ‘/etc/ppp/ip-down’ script is

/usr/local/bin/chronyc -m offline dump writertc

To start chronyd during the boot sequence, the following is in ‘/etc/rc.d/rc.local’ (this is a Slackware system)

if [ -f /usr/local/sbin/chronyd -a -f /etc/chrony.conf ]; then
  /usr/local/sbin/chronyd -r -s
  echo "Start chronyd"
fi

The placement of this command may be important on some systems. In particular, 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 chronyd's configuration file.

For the system shutdown, chronyd should receive a SIGTERM several seconds before the final SIGKILL; the SIGTERM causes the measurement histories and RTC information to be saved out.

3.5 Other important configuration options

The most common option to include in the configuration file is the driftfile option. One of the major tasks of 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 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, 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 ‘/var/lib/chrony/drift’. By specifying an option like the following

driftfile /var/lib/chrony/drift

in the configuration file (‘/etc/chrony.conf’), the drift file facility will be activated.

4 Usage reference

4.1 Starting chronyd

If chronyd has been installed to its default location ‘/usr/local/sbin/chronyd’, starting it is simply a matter of entering the command

/usr/local/sbin/chronyd

Information messages and warnings will be logged to syslog.

If no configuration commands are specified on the command line, chronyd will read the commands from the configuration file (default ‘/etc/chrony.conf’).

The command line options supported are as follows:

-n

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

-d

When run in this mode, the program will not detach itself from the terminal, and all messages will be sent to the terminal instead of to syslog. When chronyd was compiled with debugging support, this option can be used twice to print also debugging messages.

-f <conf-file>

This option can be used to specify an alternate location for the configuration file (default ‘/etc/chrony.conf’).

-r

This option will reload sample histories for each of the servers and refclocks being used. These histories are created by using the dump command in chronyc, or by setting the dumponexit directive in the configuration file. This option is useful if you want to stop and restart chronyd briefly for any reason, e.g. to install a new version. However, it should be used only on systems where the kernel can maintain clock compensation whilst not under chronyd's control (i.e. Linux, FreeBSD, NetBSD and Solaris).

-R

When this option is used, the initstepslew directive and the makestep directive used with a positive limit will be ignored. This option is useful when restarting chronyd and can be used in conjunction with the ‘-r’ option.

-s

This option will set the system clock from the computer’s real-time clock or to the last modification time of the file specified by the driftfile directive. Real-time clocks are supported only on Linux.

If used in conjunction with the ‘-r’ flag, chronyd will attempt to preserve the old samples after setting the system clock from the real time clock (RTC). This can be used to allow chronyd to perform long term averaging of the gain or loss rate across system reboots, and is useful for dial-up systems that are shut down when not in use. For this to work well, it relies on chronyd having been able to determine accurate statistics for the difference between the RTC and system clock last time the computer was on.

If the last modification time of the drift file is later than the current time and the RTC time, the system time will be set to it to restore the time when chronyd was previously stopped. This is useful on computers that have no RTC or the RTC is broken (e.g. it has no battery).

-u <user>

This option sets the name of the system user to which chronyd will switch after start in order to drop root privileges. It overrides the user directive (default chrony).

On Linux, chronyd needs to be compiled with support for the libcap library. On Mac OS X, FreeBSD, NetBSD and Solaris chronyd forks into two processes. The child process retains root privileges, but can only perform a very limited range of privileged system calls on behalf of the parent.

-F <level>

This option configures a system call filter when 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).

It’s recommended to enable the filter only when it’s known to work on the version of the system where chrony is installed as the filter needs to allow also system calls made from libraries that chronyd is using (e.g. libc) and different versions or implementations of the libraries may make different system calls. If the filter is missing some system call, chronyd could be killed even in normal operation.

-q

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

-Q

This option is similar to ‘-q’, but it will only print the offset and not correct the clock.

-v

This option displays chronyd's version number to the terminal and exits.

-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.

-m

This option will lock chronyd into RAM so that it will never be paged out. This mode is only supported on Linux.

-4

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

-6

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

On systems that support an ‘/etc/rc.local’ file for starting programs at boot time, 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 ‘/etc/rc2.d/S83chrony’.

#!/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 /etc/chrony.conf ]; then
     /opt/free/sbin/chronyd
   fi
   ;;
'stop')
   killproc chronyd
   ;;
*)
   echo "Usage: /etc/rc2.d/S83chrony { start | stop }"
   ;;
esac

(In both cases, you may want to bear in mind that 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, chronyd will need to start after daemons providing services that it may require, e.g. the domain name service.)

4.2 The chronyd configuration file

The configuration file is normally called ‘/etc/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 chronyd command line, each argument is parsed as a line and the configuration file is ignored.

4.2.1 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:

• !
• ;
• #
• %

Any line with this format will be ignored.

4.2.2 acquisitionport

By default, 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 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 (see section port) to use only one socket for all NTP packets.

An example of the acquisitionport command is

acquisitionport 1123

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.

4.2.3 allow

The 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. chronyd operates purely as an NTP client. If the allow directive is used, chronyd will be both a client of its servers, and a server to other clients.

Examples of use of the command are as follows:

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

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, allow all, has a greater effect, depending on the ordering of directives in the configuration file. To illustrate the effect, consider the two examples

allow 1.2.3.4
deny 1.2.3
allow 1.2

and

allow 1.2.3.4
deny 1.2.3
allow all 1.2

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 allow all 1.2 directives overrides the effect of 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 chronyc (see section allow all).

Note, if the initstepslew directive (see section initstepslew) 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.

4.2.4 bindacqaddress

The bindacqaddress directive sets the network interface to which will chronyd bind its NTP client sockets. The syntax is similar to the bindaddress and bindcmdaddress directives.

For each of IPv4 and IPv6 protocols, only one bindacqaddress directive can be specified.

4.2.5 bindaddress

The bindaddress directive allows you to restrict the network interface to which chronyd will listen for NTP requests. This provides an additional level of access restriction above that available through the 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

bindaddress 192.168.1.1

to the configuration file.

For each of IPv4 and IPv6 protocols, only one bindaddress directive can be specified. Therefore, it’s not useful on computers which should serve NTP on multiple network interfaces.

4.2.6 bindcmdaddress

The bindcmdaddress directive allows you to specify the network interface to which chronyd will listen for monitoring command packets (issued by chronyc). This provides an additional level of access restriction above that available through cmddeny mechanism.

This directive can also change the path of the Unix domain command socket, which is used by 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 /var/run/chrony/chronyd.sock.

By default, chronyd binds to the loopback interface (with addresses 127.0.0.1 and ::1). This blocks all access except from localhost. To listen for command packets on all interfaces, you can add the lines

bindcmdaddress 0.0.0.0
bindcmdaddress ::

to the configuration file.

For each of IPv4 and IPv6 protocols, only one bindcmdaddress directive can be specified.

An example that sets the path of the Unix domain command socket is

bindcmdaddress /var/run/chrony/chronyd.sock

4.2.7 broadcast

The 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

broadcast 30 192.168.1.255
broadcast 60 192.168.2.255 12123
broadcast 60 ff02::101

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 broadcast directive if you have more than 1 network interface onto which you wish to send NTP broadcast packets.

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 allow directive (see section allow).

4.2.8 clientloglimit

This directive specifies the maximum amount of memory that chronyd is allowed to allocate for logging of client accesses. The default limit is 524288 bytes, which allows monitoring of several thousands of addresses at the same time.

In older chrony versions if the limit was set to 0, the memory allocation was unlimited.

An example of the use of this directive is

clientloglimit 1048576

4.2.9 cmdallow

This is similar to the allow directive (see section allow), except that it allows monitoring access (rather than NTP client access) to a particular subnet or host. (By ’monitoring access’ is meant that chronyc can be run on those hosts and retrieve monitoring data from chronyd on this computer.)

The syntax is identical to the allow directive.

There is also a cmdallow all directive with similar behaviour to the allow all directive (but applying to monitoring access in this case, of course).

Note that chronyd has to be configured with the bindcmdaddress directive to not listen only on the loopback interface to actually allow remote access.

4.2.10 cmddeny

This is similar to the cmdallow directive (see section cmdallow), except that it denies monitoring access to a particular subnet or host, rather than allowing it.

The syntax is identical.

There is also a cmddeny all directive with similar behaviour to the cmdallow all directive.

4.2.11 cmdport

The cmdport directive allows the port that is used for run-time monitoring (via the chronyc program) to be altered from its default (323/udp). If set to 0, chronyd will not open the port, this is useful to disable the chronyc access from the internet. (It does not disable the Unix domain command socket.)

An example shows the syntax

cmdport 257

This would make chronyd use 257/udp as its command port. (chronyc would need to be run with the -p 257 switch to inter-operate correctly).

4.2.12 cmdratelimit

This directive enables response rate limiting for command packets. It’s similar to the ratelimit directive (see section ratelimit), except responses to the localhost are never limited and the default interval is 1 (2 seconds), default burst is 16, and default leak rate is 2.

An example of use of the command is

cmdratelimit interval 2

4.2.13 combinelimit

When 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 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

combinelimit <limit>

4.2.14 corrtimeratio

When 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 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 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

corrtimeratio 100

The maximum allowed slew rate can be set by the maxslewrate directive (see section maxslewrate. The current remaining correction is shown in the tracking report (see section tracking) as the System time value.

4.2.15 deny

This is similar to the allow directive (see section allow), 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 deny all directive with similar behaviour to the allow all directive.

4.2.16 driftfile

One of the main activities of the chronyd program is to work out the rate at which the system clock gains or loses time relative to real time.

Whenever chronyd computes a new value of the gain/loss rate, it is desirable to record it somewhere. This allows 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 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

driftfile /var/lib/chrony/drift

4.2.17 dumpdir

To compute the rate of gain or loss of time, 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, 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 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

dumpdir /var/lib/chrony

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 ‘/var/lib/chrony/1.2.3.4.dat’.

4.2.18 dumponexit

If this command is present, it indicates that chronyd should save the measurement history for each of its time sources recorded whenever the program exits. (See the dumpdir command above).

4.2.19 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

fallbackdrift 16 19

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.

4.2.20 hwclockfile

The hwclockfile directive sets the location of the adjtime file which is used by the ‘/sbin/hwclock’ program on Linux. chronyd parses the file to find out if the RTC keeps local time or UTC. It overrides the rtconutc directive (see section rtconutc).

The default value is ‘’.

An example of the command is

hwclockfile /etc/adjtime

4.2.21 include

The 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

include /etc/chrony.d/*.conf

4.2.22 initstepslew

In normal operation, chronyd slews the time when it needs to adjust the system clock. For example, to correct a system clock which is 1 second slow, 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 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 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 initstepslew directive is to allow 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 chronyd whilst the system is in normal operation.

The 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. chronyd then enters its normal operating mode.

An example of use of the command is

initstepslew 30 foo.example.net bar.example.net

where 2 NTP servers are used to make the measurement. The 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 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 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 initstepslew directive is functionally similar to a combination of the makestep and server directives with the iburst option. The main difference is that the initstepslew servers are used only before normal operation begins and that the foreground chronyd process waits for initstepslew to finish before exiting. This is useful to prevent programs started in the boot sequence after chronyd from reading the clock before it’s stepped.

4.2.23 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

keyfile /etc/chrony.keys

The argument is simply the name of the file containing the ID/key pairs. The format of the file is shown below

10 tulip
11 hyacinth
20 MD5 ASCII:crocus
25 SHA1 HEX:1dc764e0791b11fa67efc7ecbc4b0d73f68a070c
 ...

Each line consists of an ID, name of an authentication hash function (optional) and a password. The ID can be any unsigned integer in the range 1 through 2**32-1. The default hash function is MD5. Depending on how chronyd was compiled, other supported functions may be SHA1, SHA256, SHA384, SHA512, RMD128, RMD160, RMD256, RMD320, TIGER and WHIRLPOOL. The password can be specified as a string of characters not containing white space with an optional ASCII: prefix, or as a hexadecimal number with the HEX: prefix. The maximum length of the line is 2047 characters.

The password is used with the hash function to generate and verify a message authentication code (MAC) in NTP packets. It’s recommended to use SHA1 or a stronger hash function with random passwords specified in the hexadecimal format that have at least 128 bits. chronyd will log a warning to syslog on start if a source is specified in the configuration file with a key that has password shorter than 80 bits.

The keygen command of chronyc (see section keygen) can be used to generate random keys for the key file. By default, it generates 160-bit MD5 or SHA1 keys.

4.2.24 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 leapsecmode directive selects how that error is corrected. There are four options:

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).

step

This is similar to the system mode, except the clock is stepped by chronyd instead of the kernel. It can be useful to avoid bugs in the kernel code that would be executed in the system mode. This is the default mode when the system driver doesn’t support leap seconds.

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 system and 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 maxslewrate value (see section maxslewrate) the correction takes 12 seconds.

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.

An example of the command is

leapsecmode slew

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 slew mode can be combined with the smoothtime directive (see section smoothtime) 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:

leapsecmode slew
maxslewrate 1000
smoothtime 400 0.001 leaponly

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 leaponly option makes the duration of the leap smear constant and allows the clients to safely synchronise with multiple identically configured leap smearing servers.

4.2.25 leapsectz

This directive is used to set the name of the timezone in the system tz database which 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 right/UTC. This is mainly useful with reference clocks which don’t provide the leap second information. It is not necessary to restart 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

leapsectz right/UTC

The following shell command verifies that the timezone contains leap seconds and can be used with this directive

$ TZ=right/UTC date -d 'Dec 31 2008 23:59:60'
Wed Dec 31 23:59:60 UTC 2008

4.2.26 local

The local keyword is used to allow 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

local stratum 10

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) – 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.

4.2.27 lock_all

The 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 chronyd from ever being swapped out. This should result in lower and more consistent latency. It should not have significant impact on performance as chronyd's memory usage is modest. The mlockall man page has more details.

4.2.28 log

The log command indicates that certain information is to be logged.

measurements

This option logs the raw NTP measurements and related information to a file called measurements.log.

statistics

This option logs information about the regression processing to a file called statistics.log.

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.

rtc

This option logs information about the system’s real-time clock.

refclocks

This option logs the raw and filtered reference clock measurements to a file called refclocks.log.

tempcomp

This option logs the temperature measurements and system rate compensations to a file called tempcomp.log.

The files are written to the directory specified by the logdir command.

An example of the command is

log measurements statistics tracking

4.2.28.1 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.

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

The columns are as follows (the quantities in square brackets are the values from the example line above) :

1. Date [2014-10-13]
2. Hour:Minute:Second [05:40:50]. Note that the date/time pair is expressed in UTC, not the local time zone.
3. IP address of server/peer from which measurement comes [158.152.1.76]
4. Leap status (N means normal, + means that the last minute of the current month has 61 seconds, - means that the last minute of the month has 59 seconds, ? means the remote computer is not currently synchronised.) [N]
5. Stratum of remote computer. [2]
6. RFC 5905 tests 1 through 3 (1=pass, 0=fail) [111]
7. RFC 5905 tests 5 through 7 (1=pass, 0=fail) [111]
8. 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]
9. Local poll [10]
10. Remote poll [10]
11. ‘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]
12. The estimated local clock error (‘theta’ in RFC 5905). Positive indicates that the local clock is slow of the remote source. [-4.966e-03].
13. The peer delay (‘delta’ in RFC 5905). [2.296e-01]
14. The peer dispersion (‘epsilon’ in RFC 5905). [1.577e-05]
15. The root delay (‘DELTA’ in RFC 5905). [1.615e-01]
16. The root dispersion (‘EPSILON’ in RFC 5905). [7.446e-03]

A banner is periodically written to the log file to indicate the meanings of the columns.

4.2.28.2 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.

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

The columns are as follows (the quantities in square brackets are the values from the example line above) :

1. Date [1998-07-22]
2. Hour:Minute:Second [05:40:50]. Note that the date/time pair is expressed in UTC, not the local time zone.
3. IP address of server/peer from which measurement comes [158.152.1.76]
4. The estimated standard deviation of the measurements from the source (in seconds). [6.261e-03]
5. The estimated offset of the source (in seconds, positive means the local clock is estimated to be fast, in this case). [-3.247e-03]
6. The estimated standard deviation of the offset estimate (in seconds). [2.220e-03]
7. 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, not to the local clock without any compensation. [1.874e-06]
8. The estimated error in the rate value (in seconds per second). [1.080e-06].
9. 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]
10. The number of measurements currently being used for the regression algorithm. [16]
11. 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]
12. 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]

A banner is periodically written to the log file to indicate the meanings of the columns.

4.2.28.3 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.

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

The columns are as follows (the quantities in square brackets are the values from the example line above) :

1. Date [2012-02-03]
2. Hour:Minute:Second [05:40:50]. Note that the date/time pair is expressed in UTC, not the local time zone.
3. The IP address of the server/peer to which the local system is synchronised. [158.152.1.76]
4. The stratum of the local system. [3]
5. The local system frequency (in ppm, positive means the local system runs fast of UTC). [340.529]
6. The error bounds on the frequency (in ppm) [1.606]
7. 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]
8. Leap status (N means normal, + means that the last minute of this month has 61 seconds, - means that the last minute of the month has 59 seconds, ? means the clock is not currently synchronised.) [N]
9. The number of combined sources. [4]
10. The estimated standard deviation of the combined offset (in seconds). [6.849e-03]
11. The remaining offset correction from the previous update (in seconds, positive means the system clock is slow of UTC). [-4.670e-04]

A banner is periodically written to the log file to indicate the meanings of the columns.

4.2.28.4 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.

1998-07-22 05:40:50     -0.037360 1       -0.037434\
          -37.948  12   5  120

The columns are as follows (the quantities in square brackets are the values from the example line above) :

1. Date [1998-07-22]
2. Hour:Minute:Second [05:40:50]. Note that the date/time pair is expressed in UTC, not the local time zone.
3. The measured offset between the system’s real time clock and the system (gettimeofday()) time. In seconds, positive indicates that the RTC is fast of the system time. [-0.037360].
4. Flag indicating whether the regression has produced valid coefficients. (1 for yes, 0 for no). [1]
5. 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].
6. 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]
7. The number of measurements used in the regression. [12]
8. 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]
9. The measurement interval used prior to the measurement being made (in seconds). [120]

A banner is periodically written to the log file to indicate the meanings of the columns.

4.2.28.5 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.

2009-11-30 14:33:27.000000 PPS2    7 N 1  4.900000e-07 -6.741777e-07  1.000e-06

The columns are as follows (the quantities in square brackets are the values from the example line above) :

1. Date [2009-11-30]
2. Hour:Minute:Second.Microsecond [14:33:27.000000]. Note that the date/time pair is expressed in UTC, not the local time zone.
3. Reference ID of refclock from which measurement comes. [PPS2]
4. Sequence number of driver poll within one polling interval for raw samples, or - for filtered samples. [7]
5. Leap status (N means normal, + means that the last minute of the current month has 61 seconds, - means that the last minute of the month has 59 seconds). [N]
6. Flag indicating whether the sample comes from PPS source. (1 for yes, 0 for no, or - for filtered sample). [1]
7. Local clock error measured by refclock driver, or - for filtered sample. [4.900000e-07]
8. Local clock error with applied corrections. Positive indicates that the local clock is slow. [-6.741777e-07]
9. Assumed dispersion of the sample. [1.000e-06]

A banner is periodically written to the log file to indicate the meanings of the columns.

4.2.28.6 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.

2010-04-19 10:39:48  2.8000e+04  3.6600e-01

The columns are as follows (the quantities in square brackets are the values from the example line above) :

1. Date [2010-04-19]
2. Hour:Minute:Second [10:39:48]. Note that the date/time pair is expressed in UTC, not the local time zone.
3. Temperature read from tempcomp file. [2.8000e+04]
4. Applied compensation in ppm, positive means the system clock is running faster than it would be without the compensation. [3.6600e-01]

A banner is periodically written to the log file to indicate the meanings of the columns.

4.2.29 logbanner

A banner is periodically written to the log files enabled by the log directive to indicate the meanings of the columns.

The 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.

4.2.30 logchange

This directive sets the threshold for the adjustment of the system clock that will generate a syslog message.

By default, the threshold is 1 second.

An example of use is

logchange 0.1

which would cause a syslog message to be generated a system clock error of over 0.1 seconds starts to be compensated.

Clock errors detected via NTP packets, reference clocks, or timestamps entered via the settime command of chronyc are logged.

4.2.31 logdir

This directive allows the directory where log files are written to be specified.

An example of the use of this directive is

logdir /var/log/chrony

4.2.32 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

mailonchange root@localhost 0.5

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 -F option as the chronyd process will not be allowed to fork and execute the sendmail binary.

4.2.33 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 chronyd to step system clock if the adjustment is larger than a threshold value, but only if there were no more clock updates since 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 initstepslew directive (see section initstepslew) works only with NTP sources.

An example of the use of this directive is

makestep 0.1 10

This would step system clock if the adjustment is larger than 0.1 seconds, but only in the first ten clock updates.

4.2.34 manual

The manual directive enables support at run-time for the settime command in chronyc (see section settime). If no manual directive is included, any attempt to use the settime command in chronyc will be met with an error message.

Note that the settime command can be enabled at run-time using the manual command in chronyc (see section manual). (The idea of the two commands is that the manual command controls the manual clock driver’s behaviour, whereas the settime command allows samples of manually entered time to be provided).

4.2.35 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 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

maxchange 1000 1 2

After the first clock update, chronyd will check the offset on every clock update, it will ignore two adjustments larger than 1000 seconds and exit on another one.

4.2.36 maxclockerror

The 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

maxclockerror <error-in-ppm>

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.

4.2.37 maxdistance

The 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 root distance is 3 seconds.

Setting maxdistance to a larger value can be useful to allow synchronisation with a server that only has a very infrequent connection to its sources and can accumulate a large dispersion between updates of its clock.

The syntax is

maxdistance <seconds>

4.2.38 maxsamples

The maxsamples directive sets the default maximum number of samples chronyd should keep for each source. This setting can be overriden for individual sources in the server and refclock directives (see section server, see section refclock). The default value is 0, which disables the configurable limit. The useful range is 4 to 64.

The syntax is

maxsamples <samples>

4.2.39 maxslewrate

The maxslewrate directive sets the maximum rate at which chronyd is allowed to slew the time. It limits the slew rate controlled by the correction time ratio (see section corrtimeratio) and is effective only on systems where 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 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

maxslewrate <rate-in-ppm>

4.2.40 maxupdateskew

One of 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 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

maxupdateskew <skew-in-ppm>

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, 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 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.

4.2.41 minsamples

The minsamples directive sets the default minimum number of samples chronyd should keep for each source. This setting can be overriden for individual sources in the server and refclock directives (see section server, see section refclock). The default value is 0. The useful range is 4 to 64.

The syntax is

minsamples <samples>

4.2.42 minsources

The 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

minsources <sources>

4.2.43 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 clients command in chronyc.

4.2.44 peer

The syntax of this directive is identical to that for the server directive (see section server), except that it is used to specify an NTP peer rather than an NTP server.

When a key is specified by the key option to enable authentication, both peers must be configured to use the same key and the same key number.

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 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.

4.2.45 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 /var/run/chronyd.pid. The pidfile directive allows the name to be changed, e.g.

pidfile /var/tmp/chronyd.pid

4.2.46 pool

The syntax of this directive is similar to that for the server directive (see section server), 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 server directive can be used in this directive too. There is one option specific to pool directive: 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, 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 maxsources, the other sources will be removed. When a pool source is unreachable or marked as falseticker, chronyd will try to replace the source with a newly resolved address of the pool.

An example of the pool directive is

pool pool.ntp.org iburst maxsources 3

4.2.47 port

This option allows you to configure the port on which chronyd will listen for NTP requests. The port will be open only when an address is allowed by the 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, 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 acquisitionport directive.

An example of the port command is

port 11123

This would change the NTP port served by chronyd on the computer to udp/11123.

4.2.48 ratelimit

This directive enables response rate limiting for NTP packets. Its purpose is to reduce network traffic with misconfigured or broken NTP clients that are polling the server too frequently. The limits are applied to individual IP addresses. If multiple clients share one IP address (e.g. multiple hosts behind NAT), the sum of their traffic will be limited. If a client that increases its polling rate when it doesn’t receive a reply is detected, its rate limiting will be temporarily suspended to avoid increasing the overall amount of traffic. The maximum number of IP addresses which can be monitored at the same time depends on the memory limit set by the clientloglimit directive.

The ratelimit directive supports a number of subfields (which may be defined in any order):

interval

This option sets the minimum interval between responses. It is defined as a power of 2 in seconds. The default value is 3 (8 seconds). The minimum value is -4 and the maximum value is 12.

burst

This option sets the maximum number of responses that can be send in a burst, temporarily exceeding the limit specified by the interval option. This is useful for clients that make rapid measurements on start (e.g. chronyd with the iburst option). The default value is 8. The minimum value is 1 and the maximum value is 255.

leak

This option sets the rate at which responses are randomly allowed even if the limits specified by the interval and burst options are exceeded. This is necessary to prevent an attacker who is sending requests with a spoofed source address from completely blocking responses to that address. The leak rate is defined as a power of 1/2 and it is 3 by default, i.e. on average at least every eighth request has a response. The minimum value is 1 and the maximum value is 4.

An example use of the command is

ratelimit interval 4 burst 4

This would reduce the response rate for IP addresses that send packets on average more frequently than once per 16 seconds and/or send packets in bursts with more than 4 packets.

4.2.49 refclock

Reference clocks allows very accurate synchronisation and chronyd can function as a stratum 1 server. They are specified by the refclock directive. It has two mandatory parameters, a refclock driver name and a driver specific parameter.

There are currently four drivers included:

PPS

PPSAPI (pulse per second) driver. The parameter is the path to a PPS device. Assert events are used by default. Driver option :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 (see section local) enabled to work. For example:

refclock PPS /dev/pps0 lock NMEA
refclock SHM 0 offset 0.5 delay 0.2 refid NMEA noselect

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:

refclock SHM 1 poll 3 refid GPS1

A driver option in form :perm=NNN can be appended to the segment number to create the segment with permissions other than the default 0600.

Some examples of applications that can be used as SHM sources are gpsd, shmpps and radioclk.

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 chronyd and used to receive the messages. The format of messages sent over the socket is described in the refclock_sock.c file.

Recent versions of the gpsd daemon include support for the SOCK protocol. The path where the socket should be created is described in the gpsd(8) man page. For example:

refclock SOCK /var/run/chrony.ttyS0.sock

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. ptp4l from the Linux PTP project. The PTP clocks are typically kept in TAI instead of UTC. The offset option can be used to compensate for the current UTC/TAI offset. For example:

refclock PHC /dev/ptp0 poll 3 dpoll -2 offset -35

The refclock command also supports a number of subfields (which may be defined in any order):

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 chronyd to react faster to changes in clock frequency, but it may decrease the accuracy if the source is too noisy.

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).

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.

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.

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 dpoll has to be specified (-3 for 5Hz signal). The default is 1.

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.

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.

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).

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.

maxdispersion

Maximum allowed dispersion for filtered samples (in seconds). Samples with larger estimated dispersion are ignored. By default, this limit is disabled.

prefer

Prefer this source over sources without prefer option.

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.

trust

Assume time from this source is always true. It can be rejected as a falseticker in the source selection only if another source with this option doesn’t agree with it.

require

Require that at least one of the sources specified with this option is selectable (i.e. recently reachable and not a falseticker) before updating the clock. Together with the trust option this may be useful to allow a trusted, but not very precise, reference clock to be safely combined with unauthenticated NTP sources in order to improve the accuracy of the clock. They can be selected and used for synchronisation only if they agree with the trusted and required source.

minsamples

Set the minimum number of samples kept for this source. This overrides the minsamples directive (see section minsamples).

maxsamples

Set the maximum number of samples kept for this source. This overrides the maxsamples directive (see section maxsamples).

4.2.50 reselectdist

When 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 reselectdist option. By default, the distance is 100 microseconds.

The syntax is

reselectdist <dist-in-seconds>

4.2.51 rtcautotrim

The 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, chronyd will trim the RTC as if the trimrtc (see section trimrtc) command was issued.

This directive is effective only with the rtcfile directive.

An example of the use of this directive is

rtcautotrim 30

This would set the threshold error to 30 seconds.

4.2.52 rtcdevice

The rtcdevice directive defines the name of the device file for accessing the real time clock. By default this is /dev/rtc, unless the directive is used to set a different value. This applies to Linux systems with devfs. An example of use is

rtcdevice /dev/misc/rtc

4.2.53 rtcfile

The rtcfile directive defines the name of the file in which 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

rtcfile /var/lib/chrony/rtc

chronyd saves information in this file when it exits and when the writertc command is issued in 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 rtcfile directive and the -s command line option to chronyd) if the following three conditions apply:

1. You are running Linux version 2.2.x or later.
2. You have compiled the kernel with extended real-time clock support (i.e. the ‘/dev/rtc’ device is capable of doing useful things).
3. You don’t have other applications that need to make use of ‘/dev/rtc’ at all.

4.2.54 rtconutc

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 rtconutc directive appears, it means the RTC is required to keep UTC. The directive takes no arguments. It is equivalent to specifying the -u switch to the Linux ‘/sbin/hwclock’ program.

Note that this setting is overriden when the hwclockfile directive (see section hwclockfile) is used.

4.2.55 rtcsync

The rtcsync directive enables a mode where the system time is periodically copied to the real time clock (RTC).

On Linux the RTC copy is performed by the kernel every 11 minutes. This directive cannot be used when the normal RTC tracking is enabled, i.e. when the rtcfile directive is used.

On Mac OS X, chronyd will perform the RTC copy every 60 minutes when the system clock is in a synchronised state.

On other systems this directive does nothing.

4.2.56 sched_priority

On Linux, the 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 chronyd with the specified priority. This means that whenever 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 chronyd's resource requirements are modest, but it should result in lower and more consistent latency since 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.

4.2.57 server

The 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 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):

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).

minpoll

Although 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 minpoll 5 would mean that the polling interval cannot drop below 32 seconds. The default is 6 (64 seconds).

maxpoll

In a similar way, the user may wish to constrain the maximum polling interval. Again this is specified as a power of 2, maxpoll 9 indicates that the polling interval must stay at or below 512 seconds. The default is 10 (1024 seconds).

maxdelay

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, 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, 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.

maxdelayratio

This option is similar to the maxdelay option above. 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.

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.

presend

If the timing measurements being made by 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 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 server directive :

presend 9

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.

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, 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.

offline

If the server will not be reachable when chronyd is started, the offline option may be specified. chronyd will not try to poll the server until it is enabled to do so (by using the online option of chronyc).

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 offline (see section offline) command from chrony when disconnecting the dial-up link. (It will still be necessary to use chronyc’s online (see section online) command when the link has been established, to enable measurements to start.)

iburst

On start, make four measurements over a short duration (rather than the usual periodic measurements).

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 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.

polltarget

Target number of measurements to use for the regression algorithm which chronyd will try to maintain by adjusting polling interval between minpoll and maxpoll. A higher target makes chronyd prefer shorter polling intervals. The default is 6 and a useful range is 6 to 60.

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.

prefer

Prefer this source over sources without prefer option.

noselect

Never select this source. This is particularly useful for monitoring.

trust

Assume time from this source is always true. It can be rejected as a falseticker in the source selection only if another source with this option doesn’t agree with it.

require

Require that at least one of the sources specified with this option is selectable (i.e. recently reachable and not a falseticker) before updating the clock. Together with the trust option this may be useful to allow a trusted authenticated source to be safely combined with unauthenticated sources in order to improve the accuracy of the clock. They can be selected and used for synchronisation only if they agree with the trusted and required source.

minsamples

Set the minimum number of samples kept for this source. This overrides the minsamples directive (see section minsamples).

maxsamples

Set the maximum number of samples kept for this source. This overrides the maxsamples directive (see section maxsamples).

4.2.58 smoothtime

The smoothtime directive can be used to enable smoothing of the time that 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 makestep directive or command. The process can be reset without stepping the clock by the smoothtime reset command (see section smoothtime).

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). 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 leaponly option is useful in a combination with the leapsecmode slew option (see section leapsecmode) 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 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 smoothing command (see section smoothing) can be used to monitor the process.

An example suitable for clients using ntpd and 1024 second polling interval could be

smoothtime 400 0.001

An example suitable for clients using chronyd on Linux could be

smoothtime 50000 0.01

4.2.59 stratumweight

The stratumweight directive sets how much distance should be added per stratum to the synchronisation distance when chronyd selects the synchronisation source from available sources.

The syntax is

stratumweight <dist-in-seconds>

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.

4.2.60 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 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 ‘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

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

tempcomp /sys/class/hwmon/hwmon0/temp2_input 30 26000 0.0 0.000183 0.0

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

tempcomp /sys/class/hwmon/hwmon0/temp2_input 30 /etc/chrony.tempcomp

where the ‘chrony.tempcomp’ file could have

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

Valid measurements with corresponding compensations are logged to the ‘tempcomp.log’ file if enabled by the log tempcomp directive.

4.2.61 user

The user directive sets the name of the system user to which chronyd will switch after start in order to drop root privileges.

On Linux, chronyd needs to be compiled with support for the libcap library. On Mac OS X, FreeBSD, NetBSD and Solaris chronyd forks into two processes. The child process retains root privileges, but can only perform a very limited range of privileged system calls on behalf of the parent.

The default value is chrony. The configure script has a --with-user option, which sets the default value.

4.3 Running chronyc

Chronyc is the program that can be used to reconfigure options within the chronyd program whilst it is running. Chronyc can also be used to generate status reports about the operation of chronyd.

4.3.1 Basic use

The program chronyc is run by entering

chronyc

at the command line. The prompt 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 exit or quit will terminate the program. (Entering <Control-D> will also terminate the program.)

4.3.2 Command line options

Chronyc supports the following command line options.

-v

Displays the version number of chronyc on the terminal, and exists.

-h <host>

This option allows the user to specify which host (or comma-separated list of addresses) running the 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 chronyd running on the same host as that where chronyc is being run.

-p <port>

This option allows the user to specify the UDP port number which the target chronyd is using for its command & monitoring connections. This defaults to the compiled-in default; there would rarely be a need to change this.

-n

This option disables resolving IP addresses to hostnames.

-d

This option enables printing of debugging messages (if compiled with debugging support).

-4

With this option hostnames will be resolved only to IPv4 addresses.

-6

With this option hostnames will be resolved only to IPv6 addresses.

-m

With this option multiple commands can be specified on the command line. Each argument will be interpreted as a whole command.

-f <conf-file>

This option is ignored and is provided only for compatibility.

-a

This option is ignored and is provided only for compatibility.

4.3.3 Security with chronyc

Many of the commands available through chronyc have a fair amount of power to reconfigure the run-time behaviour of chronyd. Consequently, chronyc is quite dangerous for the integrity of the target system’s clock performance. Having access to chronyd via chronyc is more or less equivalent to being able to modify chronyd's configuration file (typically ‘/etc/chrony.conf’) and to restart chronyd.

chronyc also provides a number of monitoring (as opposed to commanding or configuration) commands, which will not affect the behaviour of chronyd. However, you may still want to restrict access to these commands.

There are two ways how chronyc can access 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 /var/run/chrony/chronyd.sock).

Only the following monitoring commands are allowed from the internet:

• activity
• manual list
• rtcdata
• smoothing
• sources
• sourcestats
• tracking
• waitsync.

The set of hosts from which 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, chronyd will respond with a Not authorised error, even if it’s from the localhost.

In 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, chronyc tries to connect to the Unix domain socket first. If that fails (e.g. because chronyc is running under a non-root user), it will try to connect to 127.0.0.1 and then ::1.

4.3.4 Command reference

This section describes each of the commands available within the chronyc program. Chronyc offers the user a simple command-line driven interface.

4.3.4.1 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:

accheck foo.example.net
accheck 1.2.3.4
accheck 2001:db8::1

This command can be used to examine the effect of a series of allow, allow all, deny and deny all commands specified either via chronyc, or in chronyd's configuration file.

4.3.4.2 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 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:

• online : the server/peer is currently online (i.e. assumed by chronyd to be reachable)
• offline : the server/peer is currently offline (i.e. assumed by chronyd to be unreachable, and no measurements from it will be attempted.)
• 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.
• 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.
• unresolved : the name of the server/peer wasn’t resolved to an address yet; this server is not visible in the sources and sourcestats reports.

4.3.4.3 add peer

The add peer command allows a new NTP peer to be added whilst chronyd is running.

Following the words add peer, the syntax of the following parameters and options is similar to that for the peer directive in the configuration file (see section peer). The following peer options can be set in the command: port, minpoll, maxpoll, presend, maxdelayratio, maxdelay, key

An example of using this command is shown below.

add peer foo.example.net minpoll 6 maxpoll 10 key 25

4.3.4.4 add server

The add server command allows a new NTP server to be added whilst chronyd is running.

Following the words add server, the syntax of the following parameters and options is similar to that for the server directive in the configuration file (see section server). The following server options can be set in the command: port, minpoll, maxpoll, presend, maxdelayratio, maxdelay, key

An example of using this command is shown below.

add server foo.example.net minpoll 6 maxpoll 10 key 25

4.3.4.5 allow all

The effect of the allow command is identical to the allow all directive in the configuration file (see section allow).

4.3.4.6 allow

The effect of the allow command is identical to the allow directive in the configuration file (see section allow).

The syntax is illustrated in the following examples:

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

The effect of each of these examples is the same as that of the allow directive in the configuration file.

4.3.4.7 burst

The burst command tells 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, 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 online, offline).

The syntax of the burst command is as follows

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>]

The mask and masked-address arguments are optional, in which case chronyd will initiate a burst for all of its currently defined sources.

The arguments have the following meaning and format.

n-good-measurements

This defines the number of good measurements that 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 chronyd to reject measurements that are likely to be bogus.)

max-measurements

This defines the maximum number of measurements that chronyd will attempt to make, even if the required number of good measurements has not been obtained.

mask

This is an IP address with which the IP address of each of chronyd’s sources is to be masked.

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.

masked-bits

This can be used with masked-address for CIDR notation, which is a shorter alternative to the form with mask.

address

This is an IP address or a hostname. The burst command is applied only to that source.

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

burst 2/10

This will cause 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

burst 2/10 255.255.0.0/1.2.0.0
burst 2/10 2001:db8:789a::/48

In the first case, the two out of ten sampling will only be applied to sources whose IPv4 addresses are of the form 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 2001:db8:789a.

Example of the three-argument form of the command is

burst 2/10 foo.example.net

4.3.4.8 clients

This command shows a list of clients that have accessed the server, through either the NTP or command/monitoring ports. It doesn’t include accesses over the Unix domain comamnd socket. There are no arguments.

An example of the output is

Hostname                      NTP   Drop Int IntL Last     Cmd   Drop Int  Last
===============================================================================
localhost                       2      0   2   -   133      15      0  -1     7
foo.example.net                12      0   6   -    23       0      0   -     -

Each row shows the data for a single host. Only hosts that have passed the host access checks (set with the allow, deny, cmdallow and cmddeny commands or configuration file directives) are logged. The intervals are displayed as a power of 2 in seconds.

The columns are as follows:

1. The hostname of the client
2. The number of NTP packets received from the client.
3. The number of NTP packets dropped to limit the response rate.
4. The average interval between NTP packets.
5. The average interval between NTP packets after limiting the response rate.
6. Time since the last NTP packet was received
7. The number of command packets received from the client.
8. The number of command packets dropped to limit the response rate.
9. The average interval between command packets.
10. Time since the last command packet was received.

4.3.4.9 cmdaccheck

This command is similar to the accheck command, except that it is used to check whether monitoring access is permitted from a named host.

Examples of use are as follows:

cmdaccheck foo.example.net
cmdaccheck 1.2.3.4
cmdaccheck 2001:db8::1

4.3.4.10 cmdallow all

This is similar to the allow all command, except that it is used to allow particular hosts or subnets to use chronyc to monitor with chronyd on the current host.

4.3.4.11 cmdallow

This is similar to the allow command, except that it is used to allow particular hosts or subnets to use chronyc to monitor with chronyd on the current host.

4.3.4.12 cmddeny all

This is similar to the deny all command, except that it is used to allow particular hosts or subnets to use chronyc to monitor chronyd on the current host.

4.3.4.13 cmddeny

This is similar to the deny command, except that it is used to allow particular hosts or subnets to use chronyc to monitor chronyd on the current host.

4.3.4.14 cyclelogs

The cyclelogs command causes all of 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.

% 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

4.3.4.15 delete

The 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.

delete foo.example.net
delete 1.2.3.4
delete 2001:db8::1

There is one parameter, the name or IP address of the server or peer to be deleted.

4.3.4.16 deny all

The effect of the allow command is identical to the deny all directive in the configuration file (see section deny).

4.3.4.17 deny

The effect of the allow command is identical to the deny directive in the configuration file (see section deny).

The syntax is illustrated in the following examples:

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

4.3.4.18 dns

The dns command configures how are hostnames and IP addresses resolved in chronyc. IP addresses can be resolved to hostnames when printing results of sources, sourcestats, tracking and clients commands. Hostnames are resolved in commands that take an address as argument.

There are five forms of the command:

dns -n

Disables resolving IP addresses to hostnames. Raw IP addresses will be displayed.

dns +n

Enables resolving IP addresses to hostnames. This is the default unless chronyc was started with -n option.

dns -4

Resolves hostnames only to IPv4 addresses.

dns -6

Resolves hostnames only to IPv6 addresses.

dns -46

Resolves hostnames to both address families. This is the default unless chronyc was started with -4 or -6 option.

4.3.4.19 dump

The dump command causes chronyd to write its current history of measurements for each of its sources to dump files, either for inspection or to support the -r option when chronyd is restarted.

The dump command is somewhat equivalent to the dumponexit directive in the chrony configuration file. See section dumponexit.

To use the 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 dumpdir.

4.3.4.20 exit

The exit command exits from chronyc and returns the user to the shell (same as the quit command).

4.3.4.21 help

The help command displays a summary of the commands and their arguments.

4.3.4.22 keygen

The keygen command generates a key that can be added to the key file (see section keyfile) to allow NTP authentication between server and client, or peers. The key is generated from the /dev/urandom device and it’s printed to standard output.

The command has three optional arguments. The first argument is the key number (by default 1), which will be specified with the key option of the server or peer directives in the configuration file. The second argument is the hash function (by default SHA1 or MD5 if SHA1 is not available) and the third argument is the number of bits the key should have, between 80 and 4096 bits (by default 160 bits).

An example is

keygen 73 SHA1 256

which generates a 256-bit SHA-1 key with number 73. The printed line would then be securely transferred and added to key files on both server and client, or peers.

4.3.4.23 local

The local command allows 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 local command is somewhat equivalent to the local directive in the configuration file, see local.

The syntax is as shown in the following examples.

local stratum 10
local off

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.

4.3.4.24 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 makestep command can be used in this situation. There are two forms of the command. The first form has no parameters. It tells 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 makestep directive (see section makestep). 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 burst command to quickly make a new measurement and correct the clock by stepping if needed, without waiting for chronyd to complete the measurement and update the clock.

makestep 0.1 1
burst 1/2

BE WARNED - certain software will be seriously affected by such jumps to the system time. (That is the reason why chronyd uses slewing normally.)

4.3.4.25 manual

The manual command enables and disables use of the settime command (see section settime), and is used to modify the behaviour of the manual clock driver.

Examples of the command are shown below.

manual on
manual off
manual delete 1
manual list
manual reset

The on form of the command enables use of the settime command.

The off form of the command disables use of the settime command.

The list form of the command lists all the samples currently stored in chronyd. The output is illustrated below.

210 n_samples = 1
#    Date  Time(UTC)    Slewed   Original   Residual
====================================================
 0 27Jan99 22:09:20       0.00       0.97       0.00

The columns as as follows :

1. The sample index (used for the manual delete command)
2. The date and time of the sample
3. The system clock error when the timestamp was entered, adjusted to allow for changes made to the system clock since.
4. The system clock error when the timestamp was entered, as it originally was (without allowing for changes to the system clock since).
5. The regression residual at this point, in seconds. This allows ’outliers’ to be easily spotted, so that they can be deleted using the manual delete command.

The 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 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 reset form of the command deletes all samples at once. The system clock is left running as it was before the command was entered.

4.3.4.26 maxdelay

This allows the maxdelay option for one of the sources to be modified, in the same way as specifying the maxdelay option for the server directive in the configuration file (see section server).

The following examples illustrate the syntax

maxdelay foo.example.net 0.3
maxdelay 1.2.3.4 0.0015
maxdelay 2001:db8::1 0.0015

The first example sets the maximum network delay allowed for a measurement to the host 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 1.2.3.4 and the host with IPv6 address 2001:db8::1 to 1.5 milliseconds.

(Any measurement whose network delay exceeds the specified value is discarded.)

4.3.4.27 maxdelaydevratio

This allows the maxdelaydevratio option for one of the sources to be modified, in the same way as specifying the maxdelaydevratio option for the server directive in the configuration file (see section server).

The following examples illustrate the syntax

maxdelaydevratio foo.example.net 0.1
maxdelaydevratio 1.2.3.4 1.0
maxdelaydevratio 2001:db8::1 100.0

4.3.4.28 maxdelayratio

This allows the maxdelayratio option for one of the sources to be modified, in the same way as specifying the maxdelayratio option for the server directive in the configuration file (see section server).

The following examples illustrate the syntax

maxdelayratio foo.example.net 1.5
maxdelayratio 1.2.3.4 2.0
maxdelayratio 2001:db8::1 2.0

The first example sets the maximum network delay for a measurement to the host 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 1.2.3.4 and the host with IPv6 address 2001:db8::1 to be double the retained minimum.

As for maxdelay, any measurement whose network delay is too large will be discarded.

4.3.4.29 maxpoll

The maxpoll command is used to modify the minimum polling interval for one of the current set of sources. It is equivalent to the maxpoll option in the server directive in the configuration file (see section server).

The syntax is as follows

maxpoll <host> <new-maxpoll>

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

maxpoll foo.example.net 10

which sets the maximum polling interval for the host foo.example.net to 1024 seconds.

Note that the new maximum polling interval only takes effect after the next measurement has been made.

4.3.4.30 maxupdateskew

This command has the same effect as the maxupdateskew directive in the configuration file, see maxupdateskew.

4.3.4.31 minpoll

The minpoll command is used to modify the minimum polling interval for one of the current set of sources. It is equivalent to the minpoll option in the server directive in the configuration file (see section server).

The syntax is as follows

minpoll <host> <new-minpoll>

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

minpoll foo.example.net 5

which sets the minimum polling interval for the host foo.example.net to 32 seconds.

Note that the new minimum polling interval only takes effect after the next measurement has been made.

4.3.4.32 minstratum

The minstratum command is used to modify the minimum stratum for one of the current set of sources. It is equivalent to the minstratum option in the server directive in the configuration file (see section server).

The syntax is as follows

minstratum <host> <new-min-stratum>

where the host can be specified as either a machine name or IP address.

An example is

minpoll foo.example.net 5

which sets the minimum stratum for the host foo.example.net to 5.

Note that the new minimum stratum only takes effect after the next measurement has been made.

4.3.4.33 offline

The offline command is used to warn chronyd that the network connection to a particular host or hosts is about to be lost. It can be used on computers with intermittent connection to their time sources, to warn chronyd that the connection is about to be broken.

An example of how to use offline in this case is shown in How to tell chronyd when the internet link is available..

Another case where 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 offline and online commands can be used to achieve this. The situation is shown in the figure below.

          +----------+
          |Ext source|
          +----------+
              |
              |
              |/| <-- Link with variable
                |     reliability
                |
      +-------------------+
      |Local master server|
      +-------------------+
                |
  +---+---+-----+-----+----+----+
  |   |   |     |     |    |    |
           Local clients

There are four forms of the 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.

offline
offline 255.255.255.0/1.2.3.0
offline 2001:db8:789a::/48
offline foo.example.net

The second form means that the offline command is to be applied to any source whose IPv4 address is in the 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 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

offline 0.0.0.0/0.0.0.0
offline ::/0

4.3.4.34 online

The online command is opposite in function to the offline command. It is used to advise chronyd that network connectivity to a particular source or sources has been restored.

The syntax is identical to that of the offline command, see offline.

4.3.4.35 polltarget

The polltarget command is used to modify the poll target for one of the current set of sources. It is equivalent to the polltarget option in the server directive in the configuration file (see section server).

The syntax is as follows

polltarget <host> <new-poll-target>

where the host can be specified as either a machine name or IP address.

An example is

polltarget foo.example.net 12

which sets the poll target for the host foo.example.net to 12.

4.3.4.36 quit

The quit command exits from chronyc and returns the user to the shell (same as the exit command).

4.3.4.37 refresh

The refresh command can be used to force 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.

4.3.4.38 reselect

To avoid excessive switching between sources, chronyd may stay synchronised to a source even when it is not currently the best one among the available sources.

The reselect command can be used to force chronyd to reselect the best synchronisation source.

4.3.4.39 reselectdist

The reselectdist command sets the reselect distance. It is equivalent to the reselectdist directive in the configuration file (see section reselectdist).

4.3.4.40 retries

The retries command sets the maximum number of retries for chronyc requests before giving up. The response timeout is controlled by timeout command (see section timeout).

The default is 2.

4.3.4.41 rtcdata

The rtcdata command displays the current real time clock RTC parameters.

An example output is shown below.

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

The fields have the following meaning

RTC ref time (GMT)

This is the RTC reading the last time its error was measured.

Number of samples

This is the number of previous measurements being used to determine the RTC gain/loss rate.

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.

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.

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 trimrtc command to bring the RTC into line with the system clock. (Note, a large error will not affect chronyd's operation, unless it becomes so big as to start causing rounding errors.

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.

4.3.4.42 serverstats command

The serverstats command displays how many valid NTP and command requests chronyd as a server received from clients, how many of them were dropped to limit the response rate as configured by the ratelimit and cmdratelimit directives, and how many client log records were dropped due to the memory limit configured by the clientloglimit directive. An example of the output is shown below.

NTP packets received       : 1598
NTP packets dropped        : 8
Command packets received   : 19
Command packets dropped    : 0
Client log records dropped : 0

4.3.4.43 settime

The settime command allows the current time to be entered manually, if this option has been configured into chronyd. (It may be configured either with the manual directive in the configuration file (see section manual), or with the manual command of chronyc (see section manual).

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 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 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 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 ‘getdate’ algorithm. Consequently, you can only specify time to the nearest second.

Examples of inputs that are valid are shown below.

settime 16:30
settime 16:30:05
settime Nov 21, 1997 16:30:05

For a full description of getdate, get hold of the getdate documentation (bundled, for example, with the source for GNU tar).

4.3.4.44 smoothing

The smoothing command displays the current state of the NTP server time smoothing. An example of the output is shown below.

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

The fields are explained as follows.

Active

This shows if the server time smoothing is currently active. Possible values are Yes and No. If the leaponly option is included in the smoothtime directive, (leap second only) will be shown on the line.

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.

Frequency

The current frequency offset of the served time. Negative value means the time observed by clients is running slower than true time.

Wander

The current frequency wander of the served time. Negative value means the time observed by clients is slowing down.

Last update

This field shows how long ago was the time smoothing process updated, e.g. chronyd accumulated a new measurement.

Remaining time

The time it would take for the smoothing process to get to zero offset and frequency if there were no more updates.

4.3.4.45 smoothtime

The smoothtime command can be used to reset or activate the server time smoothing process if it is configured with the smoothtime directive (see section smoothtime).

The syntax is as follows

smoothtime reset
smoothtime activate

4.3.4.46 sources

This command displays information about the current time sources that chronyd is accessing.

The optional argument -v can be specified, meaning verbose. In this case, extra caption lines are shown as a reminder of the meanings of the columns.

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

The columns are as follows:

M

This indicates the mode of the source. ^ means a server, = means a peer and # indicates a locally connected reference clock.

S

This column indicates the state of the sources. * indicates the source to which chronyd is currently synchronised. + indicates acceptable sources which are combined with the selected source. - indicates acceptable sources which are excluded by the combining algorithm. ? indicates sources to which connectivity has been lost or whose packets don’t pass all tests. x indicates a clock which chronyd thinks is is a falseticker (i.e. its time is inconsistent with a majority of other sources). ~ indicates a source whose time appears to have too much variability. The ? condition is also shown at start-up, until at least 3 samples have been gathered from it.

Name/IP address

This shows the name or the IP address of the source, or refid for reference clocks.

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.

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.

chronyd automatically varies the polling rate in response to prevailing conditions.

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.

LastRx

This column shows how long ago the last sample was received from the source. This is normally in seconds. The letters m, h, d or y indicate minutes, hours, days or years. A value of 10 years indicates there were no samples received from this source yet.

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 ns (indicating nanoseconds), us (indicating microseconds), ms (indicating milliseconds), or 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 +/- indicator shows the margin of error in the measurement.

Positive offsets indicate that the local clock is fast of the source.

4.3.4.47 sourcestats

The sourcestats command displays information about the drift rate and offset estimatation process for each of the sources currently being examined by chronyd.

The optional argument -v can be specified, meaning verbose. In this case, extra caption lines are shown as a reminder of the meanings of the columns.

An example report is

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

The columns are as follows

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.

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.

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, chronyd discards older samples and re-runs the regression until the number of runs becomes acceptable.

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.

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.

Freq Skew

This is the estimated error bounds on Freq (again in parts per million).

Offset

This is the estimated offset of the source.

Std Dev

This is the estimated sample standard deviation.

4.3.4.48 timeout

The timeout command sets the initial timeout for chronyc requests in milliseconds. If no response is received from chronyd, the timeout is doubled and the request is resent. The maximum number of retries is configured with the retries command (see section retries).

By default, the timeout is 1000 milliseconds.

4.3.4.49 tracking

The tracking command displays parameters about the system’s clock performance. An example of the output is shown below.

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

The fields are explained as follows.

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 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 local command in chronyc (see section local), or the local directive in the ‘/etc/chrony.conf’ file (see section local)).

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. foo.example.net is a stratum-2 and is synchronised from a stratum-1).

Ref time

This is the time (UTC) at which the last measurement from the reference source was processed.

System time

In normal operation, chronyd 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 gettimeofday() system call, or by the date command in the shell) will be different from 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.

Last offset

This is the estimated local offset on the last clock update.

RMS offset

This is a long-term average of the offset value.

Frequency

The ‘frequency’ is the rate by which the system’s clock would be would be wrong if 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!

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.

Skew

This is the estimated error bound on the the frequency.

Root delay

This is the total of the network path delays to the stratum-1 computer from which the computer is ultimately synchronised.

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

clock_error <= root_dispersion + (0.5 * |root_delay|)

Update interval

This is the interval between the last two clock updates.

Leap status

This is the leap status, which can be Normal, Insert second, Delete second or Not synchronised.

4.3.4.50 trimrtc

The 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):

1. Remember the currently estimated gain/loss rate of the RTC and flush the previous measurements.
2. Step the real time clock to bring it within a second of the system clock.
3. 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)
4. Save the RTC parameters to the RTC file (specified with the rtcfile directive in the configuration file (see section rtcfile).

The last step is done as a precaution against the computer suffering a power failure before either the daemon exits or the writertc command is issued.

chronyd will still work perfectly well both whilst operating and across machine reboots even if the trimrtc command is never used (and the RTC is allowed to drift away from true time). The trimrtc command is provided as a method by which it can be corrected, in a manner compatible with chronyd using it to maintain accurate time across machine reboots.

The trimrtc command can be executed automatically by chronyd with the rtcautotrim directive (see section rtcautotrim).

4.3.4.51 waitsync

The waitsync command waits for chronyd to synchronise.

Up to four optional arguments can be specified, the first is the maximum number of tries 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 tracking command (see section tracking) in the System time and Skew fields. If not specified or zero, the value will not be checked.

The fourth argument is the interval in which the check is repeated. The interval is 10 seconds by default.

An example is

waitsync 60 0.01

which will wait up to about 10 minutes (60 times 10 seconds) for chronyd to synchronise to a source and the remaining correction to be less than 10 milliseconds.

4.3.4.52 writertc

The writertc command writes the currently estimated error and gain/loss rate parameters for the RTC to the RTC file (specified with the rtcfile directive (see section rtcfile)). This information is also written automatically when chronyd is killed (with SIGHUP, SIGINT, SIGQUIT or SIGTERM) or when the trimrtc command is issued.

Appendix A GNU General Public License

GNU GENERAL PUBLIC LICENSE

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.

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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:

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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.

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NO WARRANTY

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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.

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