// This file is part of chrony
//
// Copyright (C) Richard P. Curnow  1997-2003
// Copyright (C) Stephen Wadeley  2016
// Copyright (C) Bryan Christianson  2017
// Copyright (C) Miroslav Lichvar  2009-2024
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of version 2 of the GNU General Public License as
// published by the Free Software Foundation.
//
// This program is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License along
// with this program; if not, write to the Free Software Foundation, Inc.,
// 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.

= chrony.conf(5)
:doctype: manpage
:man manual: Configuration Files
:man source: chrony @CHRONY_VERSION@

== NAME
chrony.conf - chronyd configuration file

== SYNOPSIS
*chrony.conf*

== DESCRIPTION

This file configures the *chronyd* daemon. The compiled-in location is
_@SYSCONFDIR@/chrony.conf_. Other locations can be specified on the
*chronyd* command line with the *-f* option.

Each directive in the configuration file is placed on a separate line. The
following sections describe each of the directives in turn. The directives are
not case-sensitive. Generally, the directives can occur in any order in the file
and if a directive is specified multiple times, only the last one will be
effective. Exceptions are noted in the descriptions.

The configuration directives can also be specified directly on the *chronyd*
command line. In this case each argument is parsed as a new line and the
configuration file is ignored.

While the number of supported directives is large, only a few of them are
typically needed. See the <<examples,*EXAMPLES*>> section for configuration in
typical operating scenarios.

The configuration file might 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.

== DIRECTIVES

=== Time sources

[[server]]*server* _hostname_ [_option_]...::
The *server* directive specifies an NTP server which can be used as a time
source. The client-server relationship is strictly hierarchical: a client might
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 can be specified by its hostname or IP address. If the hostname cannot
be resolved on start, *chronyd* will try it again in increasing intervals, and
also when the <<chronyc.adoc#online,*online*>> command is issued in *chronyc*.
+
The DNS record can change over time. The used address will be replaced with a
newly resolved address when the server becomes unreachable (i.e. no valid
response to last 8 requests), unsynchronised, a falseticker (i.e. does not
agree with a majority of other sources), or the root distance is too large (the
limit can be configured by the <<maxdistance,*maxdistance*>> directive). The
automatic replacement happens at most once per 30 minutes.
+
This directive can be used multiple times to specify multiple servers.
+
The directive supports the following options:
+
*minpoll* _poll_:::
This option specifies the minimum interval between requests sent to the server
as a power of 2 in seconds. For example, *minpoll 5* would mean that the
polling interval should not drop below 32 seconds. The default is 6 (64
seconds), the minimum is -7 (1/128th of a second), and the maximum is 24 (6
months). Note that intervals shorter than 6 (64 seconds) should generally not
be used with public servers on the Internet, because it might be considered
abuse. A sub-second interval will be enabled only when the server is reachable
and the round-trip delay is shorter than 10 milliseconds, i.e. the server
should be in a local network.
*maxpoll* _poll_:::
This option specifies the maximum interval between requests sent to the server
as a power of 2 in seconds. For example, *maxpoll 9* indicates that the polling
interval should stay at or below 9 (512 seconds). The default is 10 (1024
seconds), the minimum is -7 (1/128th of a second), and the maximum is 24 (6
months).
*iburst*:::
With this option, *chronyd* will start with a burst of 4-8 requests in order to
make the first update of the clock sooner. It will also repeat the burst every
time the source is switched from the offline state to online with the
<<chronyc.adoc#online,*online*>> command in *chronyc*.
*burst*:::
With this option, *chronyd* will send a burst of up to 4 requests when it
cannot get a good measurement from the
server. The number of requests in the burst is limited by the current polling
interval to keep the average interval at or above the minimum interval, i.e.
the current interval needs to be at least two times longer than the minimum
interval in order to allow a burst with two requests.
*key* _ID_:::
The NTP protocol supports a message authentication code (MAC) to prevent
computers having their system time upset by rogue packets being sent to them.
The MAC is generated as a function of a key specified in the key file,
which is specified by the <<keyfile,*keyfile*>> directive.
+
The *key* option specifies which key (with an ID in the range 1 through 2^32-1)
should *chronyd* use to authenticate requests sent to the server and verify its
responses. The server must have the same key for this number configured,
otherwise no relationship between the computers will be possible.
+
If the server is running *ntpd* and the output size of the hash function used
by the key is longer than 160 bits (e.g. SHA256), the *version* option needs to
be set to 4 for compatibility.
*nts*:::
This option enables authentication using the Network Time Security (NTS)
mechanism. Unlike with the *key* option, the server and client do not need to
share a key in a key file. NTS has a Key Establishment (NTS-KE) protocol using
the Transport Layer Security (TLS) protocol to get the keys and cookies
required by NTS for authentication of NTP packets.
+
With this option, the hostname specified in the server or pool directive is the
NTS-KE server or pool of NTS-KE servers respectively. The NTP server usually
runs on the same host, but it can be separated from the NTS-KE server (the
hostname or address of the NTP server is provided to the client by the NTS-KE
server).
+
The NTS-KE server can be specified by IP address if it is included in the
server's certificate as a Subject Alternative Name (SAN).
*certset* _ID_:::
This option specifies which set of trusted certificates should be used to verify
the server's certificate when the *nts* option is enabled. Sets of certificates
can be specified with the <<ntstrustedcerts,*ntstrustedcerts*>> directive. The
default set is 0, which by default contains certificates of the system's
default trusted certificate authorities.
*maxdelay* _delay_:::
*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 the 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 wireless networks and 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*
option. For example, *maxdelay 0.3* would indicate that measurements with a
round-trip delay greater than 0.3 seconds should be ignored. The default value
is 3 seconds and the maximum value is 1000 seconds.
*maxdelayratio* _ratio_:::
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
specified ratio times the minimum delay, it will be rejected. By default, this
test is disabled.
*maxdelaydevratio* _ratio_:::
If a measurement has a ratio of the increase in the round-trip delay from the
minimum delay amongst the previous measurements to the standard deviation of
the previous measurements that is greater than the specified ratio, it will be
rejected. The default is 10.0.
*maxdelayquant* _p_:::
This option disables the *maxdelaydevratio* test and specifies the maximum
acceptable delay as a quantile of the round-trip delay instead of a function of
the minimum delay amongst the buffered measurements. If a measurement has a
round-trip delay that is greater than a long-term estimate of the _p_-quantile,
it will be rejected.
+
The specified _p_ value should be between 0.05 and 0.95. For example,
*maxdelayquant 0.2* would indicate that only measurements with the lowest 20
percent of round-trip delays should be accepted. Note that it can take many
measurements for the estimated quantile to reach the expected value. This
option is intended for synchronisation in mostly static local networks with
very short polling intervals and possibly combined with the *filter* option.
By default, this test is disabled in favour of the *maxdelaydevratio* test.
*mindelay* _delay_:::
This option specifies a fixed minimum round-trip delay to be used instead of
the minimum amongst the previous measurements. This can be useful in networks
with static configuration to improve the stability of corrections for
asymmetric jitter, weighting of the measurements, and the *maxdelayratio* and
*maxdelaydevratio* tests. The value should be set accurately in order to have a
positive effect on the synchronisation.
*asymmetry* _ratio_:::
This option specifies the asymmetry of the network jitter on the path to the
source, which is used to correct the measured offset according to the delay.
The asymmetry can be between -0.5 and +0.5. A negative value means the delay of
packets sent to the source is more variable than the delay of packets sent from
the source back. By default, *chronyd* estimates the asymmetry automatically.
*offset* _offset_:::
This option specifies a correction (in seconds) which will be applied to
offsets measured with this source. It's particularly useful to compensate for a
known asymmetry in network delay or timestamping errors. For example, if
packets sent to the source were on average delayed by 100 microseconds more
than packets sent from the source back, the correction would be -0.00005 (-50
microseconds). The default is 0.0.
*minsamples* _samples_:::
Set the minimum number of samples kept for this source. This overrides the
<<minsamples,*minsamples*>> directive.
*maxsamples* _samples_:::
Set the maximum number of samples kept for this source. This overrides the
<<maxsamples,*maxsamples*>> directive.
*filter* _polls_:::
This option enables a median filter to reduce noise in NTP measurements. The
filter will process samples collected in the specified number of polls
into a single sample. It is intended to be used with very short polling
intervals in local networks where it is acceptable to generate a lot of NTP
traffic.
*offline*:::
If the server will not be reachable when *chronyd* is started, the *offline*
option can be specified. *chronyd* will not try to poll the server until it is
enabled to do so (by using the <<chronyc.adoc#online,*online*>> command in
*chronyc*).
*auto_offline*:::
With this option, the server will be assumed to have gone offline when sending
a request fails, e.g. due to a missing route to the network. This option avoids
the need to run the <<chronyc.adoc#offline,*offline*>> command from *chronyc*
when disconnecting the network link. (It will still be necessary to use the
<<chronyc.adoc#online,*online*>> command when the link has been established, to
enable measurements to start.)
*prefer*:::
Prefer this source over other selectable sources without the *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
does not 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 might 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.
*xleave*:::
This option enables the interleaved mode of NTP. It enables the server to
respond with more accurate transmit timestamps (e.g. kernel or hardware
timestamps), which cannot be contained in the transmitted packet itself and
need to refer to a previous packet instead. This can significantly improve the
accuracy and stability of the measurements.
+
The interleaved mode is compatible with servers that support only the basic
mode. Note that even
servers that support the interleaved mode might respond in the basic mode as
the interleaved mode requires the servers to keep some state for each client
and the state might be dropped when there are too many clients (e.g.
<<clientloglimit,*clientloglimit*>> is too small), or it might be overwritten
by other clients that have the same IP address (e.g. computers behind NAT or
someone sending requests with a spoofed source address).
+
The *xleave* option can be combined with the *presend* option in order to
shorten the interval in which the server has to keep the state to be able to
respond in the interleaved mode.
*polltarget* _target_:::
Target number of measurements to use for the regression algorithm which
*chronyd* will try to maintain by adjusting the polling interval between
*minpoll* and *maxpoll*. A higher target makes *chronyd* prefer shorter polling
intervals. The default is 8 and a useful range is from 6 to 60.
*port* _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).
*ntsport* _port_:::
This option specifies the TCP port on which the server is listening for NTS-KE
connections when the *nts* option is enabled. The default is 4460.
*presend* _poll_:::
If the timing measurements being made by *chronyd* are the only network data
passing between two computers, you might 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 might 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 can 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 (2 seconds) before making the actual
measurement.
+
If the *presend* option is used together with the *xleave* option, *chronyd*
will send two extra packets instead of one.
*minstratum* _stratum_:::
When the synchronisation source is selected from available sources, sources
with lower stratum are normally slightly 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 unreliable or inaccurate and which should be used only when other
sources are unreachable.
*version* _version_:::
This option sets the NTP version of packets sent to the server. This can be
useful when the server runs an old NTP implementation that does not respond to
requests using a newer version. The default version depends on other options.
If the *extfield* or *xleave* option is used, the default version is 4. If
those options are not used and the *key* option specifies a key using a hash
function with output size longer than 160 bits (e.g. SHA256), the default
version is 3 for compatibility with older *chronyd* servers. In other cases,
the default version is 4.
*copy*:::
This option specifies that the server and client are closely related, their
configuration does not allow a synchronisation loop to form between them, and
the client is allowed to assume the reference ID and stratum of the server.
This is useful when multiple instances of `chronyd` are running on one computer
(e.g. for security or performance reasons), one primarily operating as a client
to synchronise the system clock and other instances started with the *-x*
option to operate as NTP servers for other computers with their NTP clocks
synchronised to the first instance.
*extfield* _type_:::
This option enables an NTPv4 extension field specified by its type as a
hexadecimal number. It will be included in requests sent to the server and
processed in received responses if the server supports it. Note that some
server implementations do not respond to requests containing an unknown
extension field (*chronyd* as a server responded to such requests since
version 2.0).
+
This option can be used multiple times to enable multiple extension fields.
+
The following extension fields are supported:
+
_F323_::::
An experimental extension field to enable several improvements that were
proposed for the next version of the NTP protocol (NTPv5). The field contains
root delay and dispersion in higher resolution and a monotonic receive
timestamp, which enables a frequency transfer between the server and client to
significantly improve stability of the synchronisation. This field should be
enabled only for servers known to be running *chronyd* version 4.2 or later.
_F324_::::
An experimental extension field to enable the use of the Precision Time
Protocol (PTP) correction field in NTP-over-PTP messages updated by one-step
end-to-end transparent clocks in network switches and routers to significantly
improve accuracy and stability of the synchronisation. NTP-over-PTP can be
enabled by the <<ptpport,*ptpport*>> directive and setting the *port* option to
the PTP port. The corrections are applied only to NTP measurements with HW
timestamps (enabled by the <<hwtimestamp,*hwtimestamp*>> directive). This
field should be enabled only for servers known to be running *chronyd* version
4.5 or later.
*ipv4*:::
*ipv6*:::
These options force *chronyd* to use only IPv4 or IPv6 addresses respectively
for this source. They do not override the *-4* or *-6* option on the *chronyd*
command line.

{blank}:::

[[pool]]*pool* _name_ [_option_]...::
The syntax of this directive is similar to that for the <<server,*server*>>
directive, except that it is used to specify a pool of NTP servers rather than
a single NTP server. The pool name is expected to resolve to multiple addresses
which might change over time.
+
This directive can be used multiple times to specify multiple pools.
+
All options valid in the <<server,*server*>> directive can be used in this
directive too. There is one option specific to the *pool* directive:
+
*maxsources* _sources_:::
This option sets the desired number of sources to be used from the pool.
*chronyd* will repeatedly try to resolve the name until it gets this number of
sources responding to requests. The default value is 4 and the maximum value is
16.
+
An example of the *pool* directive is
+
----
pool pool.ntp.org iburst maxsources 3
----

[[peer]]*peer* _hostname_ [_option_]...::
The syntax of this directive is identical to that for the <<server,*server*>>
directive, except that it specifies a symmetric association with an NTP peer
instead of a client/server association with an NTP server. A single symmetric
association allows the peers to be both servers and clients to each other. This
is mainly useful when the NTP implementation of the peer (e.g. *ntpd*) supports
ephemeral symmetric associations and does not need to be configured with an
address of this host. *chronyd* does not support ephemeral associations.
+
This directive can be used multiple times to specify multiple peers.
+
The following options of the *server* directive do not work in the *peer*
directive: *iburst*, *burst*, *nts*, *presend*, *copy*.
+
When using the *xleave* option, both peers must support and have enabled the
interleaved mode, otherwise the synchronisation will work in one direction
only.
When a key is specified by the *key* option to enable authentication, both
peers must use the same key and the same key number.
+
Note that the symmetric mode is less secure than the client/server mode. A
denial-of-service attack is possible on unauthenticated symmetric associations,
i.e. when the peer was specified without the *key* option. An attacker who does
not see network traffic between two hosts, but knows that they are peering with
each other, can periodically send them unauthenticated packets with spoofed
source addresses in order to disrupt their NTP state and prevent them from
synchronising to each other. When the association is authenticated, an attacker
who does see the network traffic, but cannot prevent the packets from reaching
the other host, can still disrupt the state by replaying old packets. The
attacker has effectively the same power as a man-in-the-middle attacker. A
partial protection against this attack is implemented in *chronyd*, which can
protect the peers if they are using the same polling interval and they never
sent an authenticated packet with a timestamp from future, but it should not be
relied on as it is difficult to ensure the conditions are met. If two hosts
should be able to synchronise to each other in both directions, it is
recommended to use two separate client/server associations (specified by the
<<server,*server*>> directive on both hosts) instead.

[[initstepslew]]*initstepslew* _step-threshold_ [_hostname_]...::
(This directive is deprecated in favour of the <<makestep,*makestep*>>
directive.)
+
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 safer 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 the use of the directive is:
+
----
initstepslew 30 ntp1.example.net ntp2.example.net ntp3.example.net
----
+
where 3 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
primary server and the other computers are its clients. If each of the clients
is configured with the <<local,*local*>> directive, the server can be set up
with an *initstepslew* directive which references some or all of the clients.
Then, if the server machine has to be rebooted, the clients can be relied on to
act analogously to a flywheel and preserve the time for a short period while
the server completes its reboot.
+
The *initstepslew* directive is functionally similar to a combination of the
<<makestep,*makestep*>> and <<server,*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 prevent programs started in
the boot sequence after *chronyd* from reading the clock before it has been
stepped. With the *makestep* directive, the
<<chronyc.adoc#waitsync,*waitsync*>> command of *chronyc* can be used instead.

[[refclock]]*refclock* _driver_ _parameter_[:__option__]... [_option_]...::
The *refclock* directive specifies a hardware reference clock to be used as a
time source. It has two mandatory parameters, a driver name and a
driver-specific parameter. The two parameters are followed by zero or more
refclock options. Some drivers have special options, which can be appended to
the driver-specific parameter using the *:* character.
+
This directive can be used multiple times to specify multiple reference clocks.
+
There are four drivers included in *chronyd*:
+
*PPS*:::
Driver for the kernel PPS (pulse per second) API. The parameter is the path to
the PPS device (typically _/dev/pps?_). As PPS refclocks do not supply full
time, another time source (e.g. NTP server or non-PPS refclock) is needed to
complete samples from the PPS refclock. An alternative is to enable the
<<local,*local*>> directive to allow synchronisation with some unknown but
constant offset. The driver supports the following option:
+
*clear*::::
By default, the PPS refclock uses assert events (rising edge) for
synchronisation. With this option, it will use clear events (falling edge)
instead.
+
{blank}:::
Examples:
+
----
refclock PPS /dev/pps0 lock NMEA refid GPS1
refclock SOCK /var/run/chrony.clk.ttyS0.sock offset 0.5 delay 0.2 refid NMEA noselect
refclock PPS /dev/pps1:clear refid GPS2
----
+
*SOCK*:::
Unix domain socket driver. This driver uses a datagram socket to receive
samples from another application running on the system. The parameter is the
path to the socket, which *chronyd* will create on start. The format of the
messages is described in the _refclock_sock.c_ file in the chrony source code.
+
An application which supports the SOCK protocol is the *gpsd* daemon. It can
provide accurate measurements using the receiver's PPS signal, and since
version 3.25 also (much less accurate) measurements based on the timing of
serial data (e.g. NMEA), which can be useful when the receiver does not provide
a PPS signal, or it cannot be connected to the computer. The paths where *gpsd*
expects the sockets to be created by *chronyd* are described in the *gpsd(8)*
man page. Note that *gpsd* needs to be started after *chronyd* in order to
connect to the socket.
+
Examples:
+
----
refclock SOCK /var/run/chrony.ttyS0.sock refid GPS1 poll 2 filter 4
refclock SOCK /var/run/chrony.clk.ttyUSB0.sock refid GPS2 offset 0.2 delay 0.1
----
+
*SHM*:::
NTP shared memory driver. This driver implements the protocol of the *ntpd*
driver type 28. It is functionally similar to the SOCK driver, but uses a
shared memory segment instead of a socket. The parameter is the unit number,
typically a small number like 0, 1, 2, or 3, from which is derived the key of
the memory segment as 0x4e545030 + unit.
+
The driver supports the following option:
+
*perm*=_mode_::::
This option specifies the permissions of the shared memory segment created by
*chronyd*. They are specified as a numeric mode. The default value is 0600
(read-write access for owner only).
{blank}:::
+
Unlike with the SOCK driver, there is no prescribed order of starting *chronyd*
and the program providing measurements. Both are expected to create the memory
segment if it does not exist. *chronyd* will attach to an existing segment even
if it has a different owner than root or different permissions than the
permissions specified by the *perm* option. The segment needs to be created
before untrusted applications or users can execute code to prevent an attacker
from feeding *chronyd* with false measurements. The owner and permissions of
the segment can be verified with the *ipcs -m* command. For this reason, the
SHM driver is deprecated in favor of SOCK.
+
Examples:
+
----
refclock SHM 0 poll 3 refid GPS1
refclock SHM 1:perm=0644 refid GPS2
----
+
*PHC*:::
PTP hardware clock (PHC) driver. The parameter is the path to the device of
the PTP clock which should be used as a time source. If the clock is kept in
TAI instead of UTC (e.g. it is synchronised by a PTP daemon), the current
UTC-TAI offset needs to be specified by the *offset* option. Alternatively, the
*pps* refclock option can be enabled to treat the PHC as a PPS refclock, using
only the sub-second offset for synchronisation. The driver supports the
following options:
+
*nocrossts*::::
This option disables use of precise cross timestamping.
*extpps*::::
This option enables a PPS mode in which the PTP clock is timestamping pulses
of an external PPS signal connected to the clock. The clock does not need to be
synchronised, but another time source is needed to complete the PPS samples.
Note that some PTP clocks cannot be configured to timestamp only assert or
clear events, and it is necessary to use the *width* option to filter wrong
PPS samples.
*pin*=_index_::::
This option specifies the index of the pin which should be enabled for the
PPS timestamping. If the PHC does not have configurable pins (i.e. the channel
function is fixed), the index needs to be set to -1 to disable the pin
configuration. The default value is 0.
*channel*=_index_::::
This option specifies the index of the channel for the PPS mode. The default
value is 0.
*clear*::::
This option enables timestamping of clear events (falling edge) instead of
assert events (rising edge) in the PPS mode. This may not work with some
clocks.
{blank}:::
+
Examples:
+
----
refclock PHC /dev/ptp0 poll 0 dpoll -2 offset -37
refclock PHC /dev/ptp1:nocrossts poll 3 pps
refclock PHC /dev/ptp2:extpps:pin=1 width 0.2 poll 2
----
+
{blank}::
The *refclock* directive supports the following options:
+
*poll* _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 can 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 the frequency of the system clock, but
it might have a negative effect on its accuracy if the samples have a lot of
jitter.
*dpoll* _dpoll_:::
Some drivers do not listen for external events and try to produce samples in
their own polling interval. This is defined as a power of 2 and can be negative
to specify a sub-second interval. The default is 0 (1 second).
*refid* _refid_:::
This option is used to specify the reference ID of the refclock, as up to four
ASCII characters. The default reference ID is composed from the first three
characters of the driver name and the number of the refclock. Each refclock
must have a unique reference ID.
*lock* _refid_:::
This option can be used to lock a PPS refclock to another refclock, which is
specified by its reference ID. In this mode received PPS samples are paired
directly with raw samples from the specified refclock.
*rate* _rate_:::
This option sets the rate of the pulses in the PPS signal (in Hz). This option
controls how the pulses will be completed with real time. To actually receive
more than one pulse per second, a negative *dpoll* has to be specified (-3 for
a 5Hz signal). The default is 1.
*maxlockage* _pulses_:::
This option specifies in number of pulses how old can be samples from the
refclock specified by the *lock* option to be paired with the pulses.
Increasing this value is useful when the samples are produced at a lower rate
than the pulses. The default is 2.
*width* _width_:::
This option specifies the width of the pulses (in seconds). It is used to
filter PPS samples when the driver provides samples for both rising and falling
edges. Note that it reduces the maximum allowed error of the time source which
completes the PPS samples. If the duty cycle is configurable, 50% should be
preferred in order to maximise the allowed error.
*pps*:::
This options forces *chronyd* to treat any refclock (e.g. SHM or PHC) as a PPS
refclock. This can be useful when the refclock provides time with a variable
offset of a whole number of seconds (e.g. it uses TAI instead of UTC). Another
time source is needed to complete samples from the refclock.
*offset* _offset_:::
This option can be used to compensate for a constant error. The specified
offset (in seconds) is applied to all samples produced by the reference clock.
The default is 0.0.
*delay* _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
source selection or to make it prefer other sources. The default is 1e-9 (1
nanosecond).
*stratum* _stratum_:::
This option sets the NTP stratum of the refclock. This can be useful when the
refclock provides time with a stratum other than 0. The default is 0.
*precision* _precision_:::
This option sets the precision of the reference clock (in seconds). The default
value is the estimated precision of the system clock.
*maxdispersion* _dispersion_:::
Maximum allowed dispersion for filtered samples (in seconds). Samples with
larger estimated dispersion are ignored. By default, this limit is disabled.
*filter* _samples_:::
This option sets the length of the median filter which is used to reduce the
noise in the measurements. With each poll about 40 percent of the stored
samples are discarded and one final sample is calculated as an average of the
remaining samples. If the length is 4 or more, at least 4 samples have to be
collected between polls. For lengths below 4, the filter has to be full. The
default is 64. With drivers that perform their own polling (PPS, PHC, SHM), the
maximum value is adjusted to the number of driver polls per source poll, i.e.
2^(_poll_ - _dpoll_).
*prefer*:::
Prefer this source over other selectable sources without the *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
does not 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 can 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.
*tai*:::
This option indicates that the reference clock keeps time in TAI instead of UTC
and that *chronyd* should correct its offset by the current TAI-UTC offset. The
<<leapsectz,*leapsectz*>> or <<leapseclist,*leapseclist*>> directive must be
used with this option and the database must be kept up to date in order for
this correction to work as expected. This option does not make sense with PPS
refclocks.
*local*:::
This option specifies that the reference clock is an unsynchronised clock which
is more stable than the system clock (e.g. TCXO, OCXO, or atomic clock) and
it should be used as a local standard to stabilise the system clock. The
refclock will bypass the source selection. There should be at most one refclock
specified with this option and it should have the shortest polling interval
among all configured sources.
*minsamples* _samples_:::
Set the minimum number of samples kept for this source. This overrides the
<<minsamples,*minsamples*>> directive.
*maxsamples* _samples_:::
Set the maximum number of samples kept for this source. This overrides the
<<maxsamples,*maxsamples*>> directive.

[[manual]]*manual*::
The *manual* directive enables support at run-time for the
<<chronyc.adoc#settime,*settime*>> command in *chronyc*. 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 <<chronyc.adoc#manual,*manual*>> command in *chronyc*. (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.)

[[acquisitionport]]*acquisitionport* _port_::
By default, *chronyd* as an NTP client opens a new socket for each request with
the source port chosen randomly by the operating system. The *acquisitionport*
directive can be used to specify the source port and use only one socket (per
IPv4 or IPv6 address family) for all configured servers. This can be useful for
getting through some firewalls. It should not be used if not necessary as there
is a small impact on security of the client. If set to 0, the source port of
the permanent socket will be chosen randomly by the operating system.
+
It can be set to the same port as is used by the NTP server (which can be
configured with the <<port,*port*>> directive) to use only one socket for all
NTP packets.
+
An example of the *acquisitionport* directive is:
+
----
acquisitionport 1123
----
+
This would change the source port used for client requests to UDP port 1123.
You could then persuade the firewall administrator to open that port.

[[bindacqaddress]]*bindacqaddress* _address_::
The *bindacqaddress* directive specifies a local IP address to which
*chronyd* will bind its NTP and NTS-KE client sockets. The syntax is similar to
the <<bindaddress,*bindaddress*>> and <<bindcmdaddress,*bindcmdaddress*>>
directives.
+
For each of the IPv4 and IPv6 protocols, only one *bindacqaddress* directive
can be specified.

[[bindacqdevice]]*bindacqdevice* _interface_::
The *bindacqdevice* directive binds the client sockets to a network device
specified by the interface name. This can be useful when the local address is
dynamic, or to enable an NTP source specified with a link-local IPv6 address.
This directive can specify only one interface and it is supported on Linux
only.
+
An example of the directive is:
+
----
bindacqdevice eth0
----

[[dscp]]*dscp* _point_::
The *dscp* directive sets the Differentiated Services Code Point (DSCP) in
transmitted NTP packets to the specified value. It can improve stability of NTP
measurements in local networks where switches or routers are configured to
prioritise forwarding of packets with specific DSCP values. The default value
is 0 and the maximum value is 63.
+
An example of the directive (setting the Expedited Forwarding class) is:
+
----
dscp 46
----

[[dumpdir]]*dumpdir* _directory_::
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.
+
All supported systems, with the exception of macOS 10.12 and earlier, have
operating system support for setting the rate of gain or loss to compensate for
known errors.
(On macOS 10.12 and earlier, *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). The *dumpdir* directive defines the
directory where the measurement histories are saved when *chronyd* exits,
or the <<chronyc.adoc#dump,*dump*>> command in *chronyc* is issued.
+
If the directory does not exist, it will be created automatically.
+
The *-r* option of *chronyd* enables loading of the dump files on start. All
dump files found in the directory will be removed after start, even if the *-r*
option is not present.
+
An example of the directive is:
+
----
dumpdir @CHRONYRUNDIR@
----
+
A source whose IP address is _1.2.3.4_ would have its measurement history saved
in the file _@CHRONYRUNDIR@/1.2.3.4.dat_. History of reference clocks is saved
to files named by their reference ID in form of _refid:XXXXXXXX.dat_.

[[maxsamples]]*maxsamples* _samples_::
The *maxsamples* directive sets the default maximum number of samples that
*chronyd* should keep for each source. This setting can be overridden for
individual sources in the <<server,*server*>> and <<refclock,*refclock*>>
directives. The default value is 0, which disables the configurable limit. The
useful range is 4 to 64.
+
As a special case, setting *maxsamples* to 1 disables frequency tracking in
order to make the sources immediately selectable with only one sample. This can
be useful when *chronyd* is started with the *-q* or *-Q* option.

[[minsamples]]*minsamples* _samples_::
The *minsamples* directive sets the default minimum number of samples that
*chronyd* should keep for each source. This setting can be overridden for
individual sources in the <<server,*server*>> and <<refclock,*refclock*>>
directives. The default value is 6. The useful range is 4 to 64.
+
Forcing *chronyd* to keep more samples than it would normally keep reduces
noise in the estimated frequency and offset, but slows down the response to
changes in the frequency and offset of the clock. The offsets in the
<<chronyc.adoc#tracking,*tracking*>> and
<<chronyc.adoc#sourcestats,*sourcestats*>> reports (and the _tracking.log_ and
_statistics.log_ files) may be smaller than the actual offsets.

[[ntsaeads1]]*ntsaeads* _ID_...::
This directive specifies a list of IDs of Authenticated Encryption with
Associated Data (AEAD) algorithms enabled for NTS authentication of NTP
messages. The algorithms are specified in decreasing order of priority.
Algorithms that are not supported by the installed version of the crypto
library (Nettle, GnuTLS) are ignored.
+
The following IDs are supported:
+
* 15: AES-SIV-CMAC-256
* 30: AES-128-GCM-SIV
{blank}::
+
The default list of IDs is _30 15_. AES-128-GCM-SIV is prefered over
AES-SIV-CMAC-256 for shorter keys, which makes NTS cookies shorter and improves
reliability of NTS in networks that block or limit rate of longer NTP messages.
+
The ID of the used algorithm is reported for each server by the
<<chronyc.adoc#authdata,*authdata*>> command.
+
An example of the directive is:
+
----
ntsaeads 15
----
+
This list is used also by the <<ntsaeads2,NTS server>>.

[[ntsdumpdir1]]*ntsdumpdir* _directory_::
This directive specifies a directory for the client to save NTS cookies it
received from the server in order to avoid making an NTS-KE request when
*chronyd* is started again. The cookies are saved separately for each NTP
source in files named by the IP address of the NTS-KE server (e.g.
_1.2.3.4.nts_). By default, the client does not save the cookies.
+
If the directory does not exist, it will be created automatically.
+
An example of the directive is:
+
----
ntsdumpdir @CHRONYVARDIR@
----
+
This directory is used also by the <<ntsdumpdir2,NTS server>> to save keys.

[[ntsrefresh]]*ntsrefresh* _interval_::
This directive specifies the maximum interval between NTS-KE handshakes (in
seconds) in order to refresh the keys authenticating NTP packets. The default
value is 2419200 (4 weeks) and the maximum value is 2^31-1 (68 years).
+
The interval must be longer than polling intervals of all configured NTP
sources using NTS, otherwise the source with a longer polling interval will
refresh the keys on each poll and no NTP packets will be exchanged.

[[ntstrustedcerts]]*ntstrustedcerts* [_set-ID_] _file_|_directory_::
This directive specifies a file or directory containing trusted certificates
(in the PEM format) which are needed to verify certificates of NTS-KE servers,
e.g. certificates of trusted certificate authorities (CA) or self-signed
certificates of the servers.
+
The optional _set-ID_ argument is a number in the range 0 through 2^32-1, which
selects the set of certificates where certificates from the specified file
or directory are added. The default ID is 0, which is a set containing the
system's default trusted CAs (unless the *nosystemcert* directive is present).
All other sets are empty by default. A set of certificates can be selected for
verification of an NTS server by the *certset* option in the *server* or *pool*
directive.
+
This directive can be used multiple times to specify one or more sets of
trusted certificates, each containing certificates from one or more files
and/or directories.
+
It is not necessary to restart *chronyd* in order to reload the certificates if
they change (e.g. after a renewal).
+
An example is:
+
----
ntstrustedcerts /etc/pki/nts/ca1.example.net.crt
ntstrustedcerts 1 /etc/pki/nts/ca2.example.net.crt
ntstrustedcerts 1 /etc/pki/nts/ca3.example.net.crt
ntstrustedcerts 2 /etc/pki/nts/ntp2.example.net.crt
----

[[nosystemcert]]*nosystemcert*::
This directive disables the system's default trusted CAs. Only certificates
specified by the *ntstrustedcerts* directive will be trusted.

[[nocerttimecheck]]*nocerttimecheck* _limit_::
This directive disables the checks of the activation and expiration times of
certificates for the specified number of clock updates. It allows the NTS
authentication mechanism to be used on computers which start with wrong time
(e.g. due to not having an RTC or backup battery). Disabling the time checks
has important security implications and should be used only as a last resort,
preferably with a minimal number of trusted certificates. The default value is
0, which means the time checks are always enabled.
+
An example of the directive is:
+
----
nocerttimecheck 1
----
+
This would disable the time checks until the clock is updated for the first
time, assuming the first update corrects the clock and later checks can work
with correct time.

[[refresh]]*refresh* _interval_::
This directive specifies the interval (in seconds) between refreshing IP
addresses of NTP sources specified by hostname. If the hostname no longer
resolves to the currently used address, it will be replaced with one of the new
addresses to avoid using a server which is no longer intended for service, even
if it is still responding correctly and would not be replaced as unreachable.
Only one source is refreshed at a time. The default value is 1209600 (2 weeks)
and the maximum value is 2^31-1 (68 years). A value of 0 disables the periodic
refreshment.
+
The <<chronyc.adoc#refresh,*refresh*>> command can be used to refresh all
sources immediately.

=== Source selection

[[authselectmode]]*authselectmode* _mode_::
NTP sources can be specified with the *key* or *nts* option to enable
authentication to limit the impact of man-in-the-middle attacks. The
attackers can drop or delay NTP packets (up to the *maxdelay* and
<<maxdistance,*maxdistance*>> limits), but they cannot modify the timestamps
contained in the packets. The attack can cause only a limited slew or step, and
also cause the clock to run faster or slower than real time (up to double of
the <<maxdrift,*maxdrift*>> limit).
+
When authentication is enabled for an NTP source, it is important to disable
unauthenticated NTP sources which could be exploited in the attack, e.g. if
they are not reachable only over a trusted network. Alternatively, the source
selection can be configured with the *require* and *trust* options to
synchronise to the unauthenticated sources only if they agree with the
authenticated sources and might have a positive impact on the accuracy of the
clock. Note that in this case the impact of the attack is higher. The attackers
cannot cause an arbitrarily large step or slew, but they have more control over
the frequency of the clock and can cause *chronyd* to report false information,
e.g. a significantly smaller root delay and dispersion.
+
This directive determines the default selection options for authenticated and
unauthenticated sources in order to simplify the configuration with the
configuration file and *chronyc* commands. It sets a policy for authentication.
+
Sources specified with the *noselect* option are ignored (not counted as either
authenticated or unauthenticated), and they always have only the selection
options specified in the configuration.
+
There are four modes:
+
*require*:::
Authentication is strictly required for NTP sources in this mode. If any
unauthenticated NTP sources are specified, they will automatically get the
*noselect* option to prevent them from being selected for synchronisation.
*prefer*:::
In this mode, authentication is optional and preferred. If it is enabled for at
least one NTP source, all unauthenticated NTP sources will get the *noselect*
option.
*mix*:::
In this mode, authentication is optional and synchronisation to a mix of
authenticated and unauthenticated NTP sources is allowed. If both authenticated
and unauthenticated NTP sources are specified, all authenticated NTP sources
and reference clocks will get the *require* and *trust* options to prevent
synchronisation to unauthenticated NTP sources if they do not agree with a
majority of the authenticated sources and reference clocks. This is the default
mode.
*ignore*:::
In this mode, authentication is ignored in the source selection. All sources
will have only the selection options that were specified in the configuration
file, or *chronyc* command. This was the behaviour of *chronyd* in versions
before 4.0.
{blank}::
+
As an example, the following configuration using the default *mix* mode:
+
----
server ntp1.example.net nts
server ntp2.example.net nts
server ntp3.example.net
refclock SOCK /var/run/chrony.ttyS0.sock
----
+
is equivalent to the following configuration using the *ignore* mode:
+
----
authselectmode ignore
server ntp1.example.net nts require trust
server ntp2.example.net nts require trust
server ntp3.example.net
refclock /var/run/chrony.ttyS0.sock require trust
----

[[combinelimit]]*combinelimit* _limit_::
When *chronyd* has multiple sources available for synchronisation, it has to
select one source as the synchronisation 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 synchronisation 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.
If the selected source was specified with the *prefer* option, it can be
combined only with other sources specified with this option.
+
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.

[[maxdistance]]*maxdistance* _distance_::
The *maxdistance* directive sets the maximum root distance of a source to be
acceptable for synchronisation of the clock. Sources that have a distance
larger than the specified distance will be rejected. The distance estimates the
maximum error of the source. It includes the root dispersion and half of the
root delay (round-trip time) accumulated on the path 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.

[[maxjitter]]*maxjitter* _jitter_::
The *maxjitter* directive sets the maximum allowed jitter of the sources to not
be rejected by the source selection algorithm. This prevents synchronisation
with sources that have a small root distance, but their time is too variable.
+
By default, the maximum jitter is 1 second.

[[minsources]]*minsources* _sources_::
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 incorrect time) is
reachable.

[[reselectdist]]*reselectdist* _distance_::
When *chronyd* selects a synchronisation source from available sources, it
will prefer the one with the shortest 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* directive. By default, the
distance is 100 microseconds.

[[stratumweight]]*stratumweight* _distance_::
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.
+
By default, the weight is 0.001 seconds. This means that the stratum of the sources
in the selection process matters only when the differences between the
distances are in milliseconds.

=== System clock

[[clockprecision]]*clockprecision* _precision_::
The *clockprecision* directive specifies the precision of the system clock (in
seconds). It is used by *chronyd* to estimate the minimum noise in NTP
measurements and randomise low-order bits of timestamps in NTP responses. By
default, the precision is measured on start as the minimum time to read the
clock.
+
The measured value works well in most cases. However, it generally
overestimates the precision and it can be sensitive to the CPU speed, which can
change over time to save power. In some cases with a high-precision clocksource
(e.g. the Time Stamp Counter of the CPU) and hardware timestamping, setting the
precision on the server to a smaller value can improve stability of clients'
NTP measurements. The server's precision is reported on clients by the
<<chronyc.adoc#ntpdata,*ntpdata*>> command.
+
An example setting the precision to 8 nanoseconds is:
+
----
clockprecision 8e-9
----

[[corrtimeratio]]*corrtimeratio* _ratio_::
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 all supported
systems, with the exception of macOS 12 and earlier, 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 maximum allowed slew rate can be set by the <<maxslewrate,*maxslewrate*>>
directive. The current remaining correction is shown in the
<<chronyc.adoc#tracking,*tracking*>> report as the *System time* value.

[[driftfile]]*driftfile* _file_::
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 or 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
can take many minutes, at least.)
+
The *driftfile* directive 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 in 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 directive is:
+
----
driftfile @CHRONYVARDIR@/drift
----

[[fallbackdrift]]*fallbackdrift* _min-interval_ _max-interval_::
Fallback drifts are long-term averages of the system clock drift calculated
over exponentially increasing intervals. They are used to avoid quickly
drifting away from true time when the clock was not updated for a longer period
of time and there was a short-term deviation in the drift before the updates
stopped.
+
The directive specifies the minimum and maximum interval since the 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 the 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, and so on. This
might be a good setting to cover frequency changes due to daily and weekly
temperature fluctuations. When the frequency is set to a fallback, the state of
the clock will change to '`Not synchronised`'.
+
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 sources,
reference clocks, or manual input.

[[leapsecmode]]*leapsecmode* _mode_::
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. all supported
systems with the exception of macOS 12 and earlier).
*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 does not 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 might be
preferred over the *system* and *step* modes when applications running on the
system are sensitive to jumps in the system time and it is acceptable that the
clock will be off for a longer time. On Linux with the default
<<maxslewrate,*maxslewrate*>> value 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. This option is particularly
useful when multiple *chronyd* instances are running on the system, one
controlling the system clock and others started with the *-x* option, which
should rely on the first instance to correct the system clock and ignore it for
the correction of their own NTP clock running on top of the system clock.
{blank}::
+
When serving time to NTP clients that cannot be configured to correct their
clocks for a leap second by slewing, or to clients that would correct at
slightly different rates when it is necessary to keep them close together, the
*slew* mode can be combined with the <<smoothtime,*smoothtime*>> directive to
enable a server leap smear.
+
When smearing a leap second, the leap status is suppressed on the server and
the served time is corrected slowly by slewing instead of stepping. The clients
do not need any special configuration as they do not 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 only NTP servers which smear the
leap second in exactly the same way for synchronisation.
+
This feature must 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 0.001024 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 62500 seconds (about 17.36 hours) to finish. The frequency
offset will be changing by 0.001024 ppm per second and will reach a maximum of
32 ppm in 31250 seconds. 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.
+
The duration of the leap smear can be calculated from the specified wander as
+
----
duration = sqrt(4 / wander)
----

[[leapsectz]]*leapsectz* _timezone_::
This directive specifies a timezone in the system timezone database which
*chronyd* can use to determine when will the next leap second occur and what is
the current offset between TAI and UTC. It will periodically check if 23:59:59
and 23:59:60 are valid times in the timezone. This normally works with the
_right/UTC_ timezone.
+
When a leap second is announced, the timezone needs to be updated at least 12
hours before the leap second. It is not necessary to restart *chronyd*.
+
This directive is useful with reference clocks and other time sources which do
not announce leap seconds, or announce them too late for an NTP server to
forward them to its own clients. Clients of leap smearing servers must not
use this directive.
+
It is also useful when the system clock is required to have correct TAI-UTC
offset. Note that the offset is set only when leap seconds are handled by the
kernel, i.e. <<leapsecmode,*leapsecmode*>> is set to *system*.
+
The specified timezone is not used as an exclusive source of information about
leap seconds. If a majority of time sources announce on the last day of June or
December that a leap second should be inserted or deleted, it will be accepted
even if it is not included in the timezone.
+
An example of the directive 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
----

[[leapseclist]]*leapseclist* _file_::
This directive specifies the path to a file containing a list of leap seconds
and TAI-UTC offsets in NIST/IERS format. It is recommended to use
the file _leap-seconds.list_ usually included with the system timezone
database. The behaviour of this directive is otherwise equivalent to
<<leapsectz,*leapsectz*>>.
+
An example of this directive is:
+
----
leapseclist /usr/share/zoneinfo/leap-seconds.list
----

[[makestep]]*makestep* _threshold_ _limit_::
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,
e.g. when *chronyd* is initially started, the system clock might 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 the 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 the specified limit. A negative value disables
the limit.
+
On most systems it is desirable to step the system clock only on boot, before
starting programs that rely on time advancing monotonically forwards.
+
An example of the use of this directive is:
+
----
makestep 0.1 3
----
+
This would step the system clock if the adjustment is larger than 0.1 seconds, but
only in the first three clock updates.

[[maxchange]]*maxchange* _offset_ _start_ _ignore_::
This directive sets the maximum offset to be accepted on a clock update. The
offset is measured relative to the current estimate of the true time, which is
different from the system time if a previous slew did not finish.
+
The check is enabled after the specified number of clock updates to allow a
large initial offset to be corrected on start. Offsets larger than the
specified maximum will be ignored for the specified number of times. Another
large offset will cause *chronyd* to give up and exit. A negative value
can be used to disable the limit to ignore all large offsets. A syslog message
will be generated when an offset is ignored or it causes the exit.
+
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.

[[maxclockerror]]*maxclockerror* _error-in-ppm_::
The *maxclockerror* directive sets the maximum assumed frequency error that the
system clock can gain on its own between clock updates. It describes the
stability of the clock.
+
By default, the maximum error is 1 ppm.
+
Typical values for _error-in-ppm_ might be 10 for a low quality clock and 0.1
for a high quality clock using a temperature compensated crystal oscillator.

[[maxdrift]]*maxdrift* _drift-in-ppm_::
This directive specifies the maximum assumed drift (frequency error) of the
system clock. It limits the frequency adjustment that *chronyd* is allowed to
use to correct the measured drift. It is an additional limit to the maximum
adjustment that can be set by the system driver (100000 ppm on Linux, 500 ppm
on FreeBSD, NetBSD, and macOS 10.13+, 32500 ppm on illumos).
+
By default, the maximum assumed drift is 500000 ppm, i.e. the adjustment is
limited by the system driver rather than this directive.

[[maxupdateskew]]*maxupdateskew* _skew-in-ppm_::
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* directive sets the threshold for determining whether an
estimate might be so unreliable that it should not be used. By default, the
threshold is 1000 ppm.
+
Typical values for _skew-in-ppm_ might be 100 for NTP sources polled over a
wireless network, and 10 or smaller for sources on a local wired network.
+
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 existing estimate. If it has
significantly smaller error bounds than the new estimate, the existing estimate
will dominate in the new combined value.

[[maxslewrate]]*maxslewrate* _rate-in-ppm_::
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 (which can be set by the <<corrtimeratio,*corrtimeratio*>> directive) and
is effective only on systems where *chronyd* is able to control the rate (i.e.
all supported systems with the exception of macOS 12 or earlier).
+
For each system there is a maximum frequency offset of the clock that can be set
by the driver. On Linux it is 100000 ppm, on FreeBSD, NetBSD and macOS 10.13+ it
is 5000 ppm, and on illumos it is 32500 ppm. Also, due to a kernel limitation,
setting *maxslewrate* on FreeBSD, NetBSD, macOS 10.13+ 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).

[[tempcomp]]
*tempcomp* _file_ _interval_ _T0_ _k0_ _k1_ _k2_::
*tempcomp* _file_ _interval_ _points-file_::
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 motherboard.
+
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 the compensation is specified, and
how close the sensor is to the oscillator. When it is 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
+
----
comp = 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 adjusted to
keep the compensation in that range.
+
An example of the 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 faster by 0.183 ppm, 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 _/etc/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,*log tempcomp*>> directive.

=== NTP server

[[allow]]*allow* [*all*] [_subnet_]::
The *allow* directive is used to designate a particular subnet from which NTP
clients are allowed to access the computer as an NTP server. It also controls
access of NTS-KE clients when NTS is enabled on the 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.
+
This directive can be used multiple times.
+
Examples of the use of the directive are as follows:
+
----
allow 1.2.3.4
allow 3.4.5.0/24
allow 3.4.5
allow 2001:db8::/32
allow 0/0
allow ::/0
allow
----
+
The first directive allows access from an IPv4 address. The second directive
allows access from all computers in an IPv4 subnet specified in the CIDR
notation. The third directive specifies the same subnet using a simpler
notation where the prefix length is determined by the number of dots. The
fourth directive specifies an IPv6 subnet. The fifth and sixth directives allow
access from all IPv4 and IPv6 addresses respectively. The seventh directive
allows access from all addresses (both 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 1.2.3.0/24
allow 1.2.0.0/16
----
+
and
+
----
allow 1.2.3.4
deny 1.2.3.0/24
allow all 1.2.0.0/16
----
+
In the first example, the effect is the same regardless of what order the three
directives are given in. So the _1.2.0.0/16_ subnet is allowed access, except
for the _1.2.3.0/24_ subnet, which is denied access, however the host _1.2.3.4_
is allowed access.
+
In the second example, the *allow all 1.2.0.0/16* directive 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*
with the <<chronyc.adoc#allow,*allow all*>> command.
+
The rules are internally represented as a tree of tables with one level per
four bits of the IPv4 or IPv6 address. The order of the *allow* and *deny*
directives matters if they modify the same records of one table, i.e. if one
subnet is included in the other subnet and their prefix lengths are at the same
level. For example, _1.2.3.0/28_ and _1.2.3.0/29_ are in different tables, but
_1.2.3.0/25_ and _1.2.3.0/28_ are in the same table. The configuration can be
verified for individual addresses with the <<chronyc.adoc#accheck,*accheck*>>
command in *chronyc*.
+
A hostname can be specified in the directives instead of the IP address, but
the name must be resolvable when *chronyd* is started, i.e. the network is
already up and DNS is working. If the hostname resolves to multiple addresses,
only the first address (in the order returned by the system resolver) will be
allowed or denied.
+
Note, if the <<initstepslew,*initstepslew*>> directive is used in the
configuration file, each of the computers listed in that directive must allow
client access by this computer for it to work.

[[deny]]*deny* [*all*] [_subnet_]::
This is similar to the <<allow,*allow*>> directive, except that it denies NTP
and NTS-KE client access to a particular subnet or host, rather than allowing
it.
+
The syntax is identical and the directive can be used multiple times too.
+
There is also a *deny all* directive with similar behaviour to the *allow all*
directive.

[[bindaddress]]*bindaddress* _address_::
The *bindaddress* directive binds the sockets on which *chronyd* listens for
NTP and NTS-KE requests to a local address of the computer. On systems other
than Linux, the address of the computer needs to be already configured when
*chronyd* is started.
+
An example of the use of the directive is:
+
----
bindaddress 192.168.1.1
----
+
Currently, for each of the IPv4 and IPv6 protocols, only one *bindaddress*
directive can be specified. Therefore, it is not useful on computers which
should serve NTP on multiple network interfaces.

[[binddevice]]*binddevice* _interface_::
The *binddevice* directive binds the NTP and NTS-KE server sockets to a network
device specified by the interface name. This directive can specify only one
interface and it is supported on Linux only.
+
An example of the directive is:
+
----
binddevice eth0
----

[[broadcast]]*broadcast* _interval_ _address_ [_port_]::
The *broadcast* directive is used to declare a broadcast address to which
chronyd should send packets in the NTP broadcast mode (i.e. make *chronyd* act
as a broadcast server). Broadcast clients on that subnet will be able to
synchronise.
+
This directive can be used multiple times to specify multiple addresses.
+
The syntax is as follows:
+
----
broadcast 32 192.168.1.255
broadcast 64 192.168.2.255 12123
broadcast 64 ff02::101
----
+
In the first example, the destination port defaults to UDP port 123 (the normal NTP
port). In the second example, the destination port is specified as 12123. The
first parameter in each case (32 or 64 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 want to send NTP broadcast packets.
+
*chronyd* itself cannot 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
implementations.
+
If *ntpd* is used as the broadcast client, it will try to measure the
round-trip delay between the server and client with normal client mode packets.
Thus, the broadcast subnet should also be the subject of an <<allow,*allow*>>
directive.

[[clientloglimit]]*clientloglimit* _limit_::
This directive specifies the maximum amount of memory that *chronyd* is allowed
to allocate for logging of client accesses and the state that *chronyd* as an
NTP server needs to support the interleaved mode for its clients. The default
limit is 524288 bytes, which enables monitoring of up to 4096 IP addresses at
the same time and holding NTP timestamps for up to 4096 clients using the
interleaved mode (depending on uniformity of their polling interval). The
number of addresses and timestamps is always a power of 2. The maximum
effective value is 2147483648 (2 GB), which corresponds to 16777216 addresses
and timestamps.
+
An example of the use of this directive is:
+
----
clientloglimit 1048576
----

[[noclientlog]]*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 <<chronyc.adoc#clients,*clients*>> command in *chronyc*. This option
also effectively disables server support for the NTP interleaved mode.

[[local]]*local* [_option_]...::
The *local* directive enables a local reference mode, which allows *chronyd*
operating as an NTP server to appear synchronised to real time (from the
viewpoint of clients polling it), even when it was never synchronised or
the last update of the clock happened a long time ago.
+
This directive is normally used in an isolated network, where computers are
required to be synchronised to one another, but not necessarily to real time.
The server can be kept vaguely in line with real time by manual input.
+
The *local* directive has the following options:
+
*stratum* _stratum_:::
This option sets the stratum of the server which will be reported to clients
when the local reference is active. The specified value is in the range 1
through 15, and the default value is 10. It should be larger than the maximum
expected stratum in the network when external NTP servers are accessible.
+
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 value
of 10 indicates that the clock is so many hops away from a reference clock that
its time is fairly unreliable.
*distance* _distance_:::
This option sets the threshold for the root distance which will activate the local
reference. If *chronyd* was synchronised to some source, the local reference
will not be activated until its root distance reaches the specified value (the
rate at which the distance is increasing depends on how well the clock was
tracking the source). The default value is 1 second.
+
The current root distance can be calculated from root delay and root dispersion
(reported by the <<chronyc.adoc#tracking,*tracking*>> command in *chronyc*) as:
+
----
distance = delay / 2 + dispersion
----
*activate* _distance_:::
This option sets an activating root distance for the local reference. The
local reference will not be used until the root distance drops below the
configured value for the first time. This can be used to prevent the local
reference from being activated on a server which has never been synchronised
with an upstream server. The default value of 0.0 causes no activating
distance to be used, such that the local reference is always eligible for
activation.
*orphan*:::
This option enables a special '`orphan`' mode, where sources with stratum equal
to the local _stratum_ are assumed to not serve real time. They are ignored
unless no other source is selectable and their reference IDs are smaller than
the local reference ID.
+
This allows multiple servers in the network to use the same *local*
configuration and to be synchronised to one another, without confusing clients
that poll more than one server. Each server needs to be configured to poll all
other servers with the *local* directive. This ensures only the server with the
smallest reference ID has the local reference active and others are
synchronised to it. If that server stops responding, the server with the second
smallest reference ID will take over when its local reference mode activates
(root distance reaches the threshold configured by the *distance* option).
+
The *orphan* mode is compatible with the *ntpd*'s orphan mode (enabled by the
*tos orphan* command).
{blank}::
+
An example of the directive is:
+
----
local stratum 10 orphan distance 0.1 activate 0.5
----

[[ntpsigndsocket]]*ntpsigndsocket* _directory_::
This directive specifies the location of the Samba *ntp_signd* socket when it
is running as a Domain Controller (DC). If *chronyd* is compiled with this
feature, responses to MS-SNTP clients will be signed by the *smbd* daemon.
+
Note that MS-SNTP requests are not authenticated and any client that is allowed
to access the server by the <<allow,*allow*>> directive, or the
<<chronyc.adoc#allow,*allow*>> command in *chronyc*, can get an MS-SNTP
response signed with a trust account's password and try to crack the password
in a brute-force attack. Access to the server should be carefully controlled.
+
An example of the directive is:
+
----
ntpsigndsocket /var/lib/samba/ntp_signd
----

[[ntsport]]*ntsport* _port_::
This directive specifies the TCP port on which *chronyd* will provide the NTS
Key Establishment (NTS-KE) service. The default port is 4460.
+
The port will be open only when a certificate and key is specified by the
*ntsservercert* and *ntsserverkey* directives.

[[ntsservercert]]*ntsservercert* _file_::
This directive specifies a file containing a certificate in the PEM format
for *chronyd* to operate as an NTS server. The file should also include
any intermediate certificates that the clients will need to validate the
server's certificate. The file needs to be readable by the user under which
*chronyd* is running after dropping root privileges.
+
This directive can be used multiple times to specify multiple certificates for
different names of the server.
+
The files are loaded only once. *chronyd* needs to be restarted in order to
load a renewed certificate. The <<ntsdumpdir,*ntsdumpdir*>> and
<<dumpdir,*dumpdir*>> directives with the *-r* option of *chronyd* are
recommended for a near-seamless server operation.

[[ntsserverkey]]*ntsserverkey* _file_::
This directive specifies a file containing a private key in the PEM format
for *chronyd* to operate as an NTS server. The file needs to be readable by
the user under which *chronyd* is running after dropping root privileges. For
security reasons, it should not be readable by other users.
+
This directive can be used multiple times to specify multiple keys. The number
of keys must be the same as the number of certificates and the corresponding
files must be specified in the same order.

[[ntsprocesses]]*ntsprocesses* _processes_::
This directive specifies how many helper processes will *chronyd* operating
as an NTS server start for handling client NTS-KE requests in order to improve
performance with multi-core CPUs and multithreading. If set to 0, no helper
process will be started and all NTS-KE requests will be handled by the main
*chronyd* process. The default value is 1.

[[maxntsconnections]]*maxntsconnections* _connections_::
This directive specifies the maximum number of concurrent NTS-KE connections
per process that the NTS server will accept. The default value is 100. The
maximum practical value is half of the system *FD_SETSIZE* constant (usually
1024).

[[ntsaeads2]]*ntsaeads* _ID_...::
This directive specifies a list of IDs of Authenticated Encryption with
Associated Data (AEAD) algorithms enabled for NTS authentication of NTP
messages. *chronyd* as a server uses the first enabled algorithm from the list
provided by the client. Algorithms that are not supported by the installed
version of the crypto library (Nettle, GnuTLS) are ignored.
+
The following IDs are supported:
+
* 15: AES-SIV-CMAC-256
* 30: AES-128-GCM-SIV
{blank}::
+
The default list of IDs is _30 15_. AES-128-GCM-SIV is prefered over
AES-SIV-CMAC-256 for shorter keys, which makes NTS cookies shorter and improves
reliability of NTS in networks that block or limit rate of longer NTP messages.
+
An example of the directive is:
+
----
ntsaeads 15
----
+
This list is used also by the <<ntsaeads1,NTS client>>.
+
Note the the NTS specification (RFC 8915) requires servers to support
AES-SIV-CMAC-256, i.e. 15 should be always included in the specified list.
+
The AES-128-GCM-SIV keys used by *chronyd* do not comply to RFC 8915 for
compatibility with older *chrony* clients unless the use of compliant keys is
negotiated with an
https://chrony-project.org/doc/spec/nts-compliant-128gcm.html[NTS-KE record].
Support for this record was added in version 4.6.1. As a client, *chronyd* can
interoperate with a server that uses compliant keys, but does not support the
negotiation, if it responds to incorrectly authenticated requests with an NTS
NAK.

[[ntsdumpdir2]]*ntsdumpdir* _directory_::
This directive specifies a directory where *chronyd* operating as an NTS server
can save the keys which encrypt NTS cookies provided to clients. The keys are
saved to a single file named _ntskeys_. When *chronyd* is restarted, reloading
the keys allows the clients to continue using old cookies and avoids a storm of
NTS-KE requests. By default, the server does not save the keys.
+
An example of the directive is:
+
----
ntsdumpdir @CHRONYVARDIR@
----
+
This directory is used also by the <<ntsdumpdir1,NTS client>> to save NTS cookies.

[[ntsntpserver]]*ntsntpserver* _hostname_::
This directive specifies the hostname (as a fully qualified domain name) or
address of the NTP server(s) which is
provided in the NTS-KE response to the clients. It allows the NTS-KE server to
be separated from the NTP server. However, the servers need to share the keys,
i.e. external key management needs to be enabled by setting
<<ntsrotate,*ntsrotate*>> to 0. By default, no hostname or address is provided
to the clients, which means they should use the same server for NTS-KE and NTP.

[[ntsrotate]]*ntsrotate* _interval_::
This directive specifies the rotation interval (in seconds) of the server key
which encrypts the NTS cookies. New keys are generated automatically from the
_/dev/urandom_ device. The server keeps two previous keys to give the clients
time to get new cookies encrypted by the latest key. The interval is measured
as the server's operating time, i.e. the actual interval can be longer if
*chronyd* is not running continuously. The default interval is 604800 seconds
(1 week). The maximum value is 2^31-1 (68 years).
+
The automatic rotation of the keys can be disabled by setting *ntsrotate* to 0.
In this case the keys are assumed to be managed externally. *chronyd* will not
save the keys to the _ntskeys_ file and will reload the keys from the file when
the <<chronyc.adoc#rekey,*rekey*>> command is issued in *chronyc*. The file can
be periodically copied from another server running *chronyd* (which does
not have *ntsrotate* set to 0) in order to have one or more servers dedicated
to NTS-KE. The file includes the subsequent key to which the NTS-KE server will
switch on the next rotation, i.e. the process copying and reloading the file
does not need to be timed precisely (it can be delayed by up to one rotation
interval). The NTS-KE servers need to be configured with the
<<ntsntpserver,*ntsntpserver*>> directive to point the clients to the right NTP
server.
+
An example of the directive is:
+
----
ntsrotate 2592000
----

[[port]]*port* _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,*allow*>> directive or the <<chronyc.adoc#allow,*allow*>> command in
*chronyc*, an NTP peer is configured, or the broadcast server mode is enabled.
+
The default value is 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,*acquisitionport*>> directive.

[[ratelimit]]*ratelimit* [_option_]...::
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 does not 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,*clientloglimit*>>
directive.
+
The *ratelimit* directive supports a number of options (which can be defined
in any order):
+
*interval* _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 -19 (524288 packets per second) and the maximum value is 12 (one packet per
4096 seconds). Note that with values below -4 the rate limiting is coarse
(responses are allowed in bursts, even if the interval between them is shorter
than the specified interval).
*burst* _responses_:::
This option sets the maximum number of responses that can be sent 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* _rate_:::
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 2 by default, i.e. on average at
least every fourth request has a response. The minimum value is 1 and the
maximum value is 4.
*kod* _rate_:::
This option sets the rate at which Kiss-o'-Death (KoD) RATE responses are
randomly sent when the limits specified by the *interval* and *burst* options
are exceeded. It is an additional stream of responses to the *leak* option. A
KoD RATE response is a request for the client to reduce its polling rate. Few
implementations actually support it. The rate is defined as a power of 1/2. The
default value is 0, which means disabled. The minimum value is 0 and the
maximum value is 4.
{blank}::
+
An example use of the directive is:
+
----
ratelimit interval 1 burst 16
----
+
This would reduce the response rate for IP addresses sending packets on average
more than once per 2 seconds, or sending packets in bursts of more than 16
packets, by up to 75% (with default *leak* of 2).

[[ntsratelimit]]*ntsratelimit* [_option_]...::
This directive enables rate limiting of NTS-KE requests. It is similar to the
<<ratelimit,*ratelimit*>> directive, except the default interval is 6
(1 connection per 64 seconds) and the *kod* option is not supported.
+
An example of the use of the directive is:
+
----
ntsratelimit interval 3 burst 1
----

[[smoothtime]]*smoothtime* _max-freq_ _max-wander_ [*leaponly*]::
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 can take a very long time
to smooth it out. This directive should be used only when the clients are not
configured to also poll 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 is 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,*makestep*>> directive or the
<<chronyc.adoc#makestep,*makestep*>> command in *chronyc*. The process can be
reset without stepping the clock by the <<chronyc.adoc#smoothtime,*smoothtime
reset*>> command.
+
The first two arguments of the directive are the maximum frequency offset of
the smoothed time to the tracked NTP time (in ppm) and the maximum rate at
which the frequency offset is allowed to change (in ppm per second). *leaponly*
is an optional third argument which enables a mode where only leap seconds are
smoothed out and normal offset and frequency changes are ignored. The *leaponly*
option is useful in a combination with the <<leapsecmode,*leapsecmode slew*>>
directive to allow the clients to 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 <<chronyc.adoc#smoothtime,*smoothtime
activate*>> command, which is particularly useful when the clock is
synchronised only with manual input and the skew is always larger than the
threshold. The <<chronyc.adoc#smoothing,*smoothing*>> command 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
----

=== Command and monitoring access

[[bindcmdaddress]]*bindcmdaddress* _address_::
The *bindcmdaddress* directive specifies a local IP address to which *chronyd*
will bind the UDP socket listening for monitoring command packets (issued
by *chronyc*). On systems other than Linux, the address of the interface needs
to be already configured when *chronyd* is started.
+
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 does not exist. The compiled-in default
path of the socket is _@CHRONYRUNDIR@/chronyd.sock_. The socket can be
disabled by setting the path to _/_.
+
By default, *chronyd* binds the UDP sockets to the addresses _127.0.0.1_ and
_::1_ (i.e. the loopback interface). 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 the IPv4, IPv6, and Unix domain 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
----

[[bindcmddevice]]*bindcmddevice* _interface_::
The *bindcmddevice* directive binds the UDP command sockets to a network device
specified by the interface name. This directive can specify only one interface
and it is supported on Linux only.
+
An example of the directive is:
+
----
bindcmddevice eth0
----

[[cmdallow]]*cmdallow* [*all*] [_subnet_]::
This is similar to the <<allow,*allow*>> directive, except that it allows
monitoring access (rather than NTP client access) to a particular subnet or
host. (By '`monitoring access`' is meant that *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,*bindcmdaddress*>> directive to not listen only on the
loopback interface to actually allow remote access.

[[cmddeny]]*cmddeny* [*all*] [_subnet_]::
This is similar to the <<cmdallow,*cmdallow*>> directive, except that it denies
monitoring access to a particular subnet or host, rather than allowing it.
+
The syntax is identical.
+
There is also a *cmddeny all* directive with similar behaviour to the *cmdallow
all* directive.

[[cmdport]]*cmdport* _port_::
The *cmdport* directive allows the port that is used for run-time monitoring
(via the *chronyc* program) to be altered from its default (323). If set to 0,
*chronyd* will not open the port, which disables remote *chronyc* access (with
a non-default *bindcmdaddress*) and local access for unprivileged users. It
does not disable the Unix domain command socket.
+
An example shows the syntax:
+
----
cmdport 257
----
+
This would make *chronyd* use UDP 257 as its command port. (*chronyc* would
need to be run with the *-p 257* option to inter-operate correctly.)

[[cmdratelimit]]*cmdratelimit* [_option_]...::
This directive enables response rate limiting for command packets. It is
similar to the <<ratelimit,*ratelimit*>> directive, except responses to
localhost are never limited, the default interval is -4 (16 packets per
second), and the *kod* option is not supported.
+
An example of the use of the directive is:
+
----
cmdratelimit interval 2
----

=== Real-time clock (RTC)

[[hwclockfile]]*hwclockfile* _file_::
The *hwclockfile* directive sets the location of the adjtime file which is
used by the *hwclock* program on Linux. *chronyd* parses the file to find out
if the RTC keeps local time or UTC. It overrides the <<rtconutc,*rtconutc*>>
directive.
+
The compiled-in default value is '_@DEFAULT_HWCLOCK_FILE@_'.
+
An example of the directive is:
+
----
hwclockfile /etc/adjtime
----

[[rtcautotrim]]*rtcautotrim* _threshold_::
The *rtcautotrim* directive is used to keep the RTC close to the system clock
automatically. When the system clock is synchronised and the estimated error
between the two clocks is larger than the specified threshold, *chronyd* will
trim the RTC as if the <<chronyc.adoc#trimrtc,*trimrtc*>> command in *chronyc*
was issued. The trimming operation is accurate to only about 1 second, which is
the minimum effective threshold.
+
This directive is effective only with the <<rtcfile,*rtcfile*>> directive.
+
An example of the use of this directive is:
+
----
rtcautotrim 30
----
+
This would set the threshold error to 30 seconds.

[[rtcdevice]]*rtcdevice* _device_::
The *rtcdevice* directive sets the path to the device file for accessing the
RTC. The default path is _@DEFAULT_RTC_DEVICE@_.

[[rtcfile]]*rtcfile* _file_::
The *rtcfile* directive defines the name of the file in which *chronyd* can
save parameters associated with tracking the accuracy of the RTC.
+
An example of the directive is:
+
----
rtcfile @CHRONYVARDIR@/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 RTC
facilities (the <<rtcfile,*rtcfile*>> directive and the *-s* command-line
option to *chronyd*) if the following three conditions apply:
+
. You are running Linux.
. The kernel is compiled with extended real-time clock support (i.e. the
  _/dev/rtc_ device is capable of doing useful things).
. You do not have other applications that need to make use of _/dev/rtc_ at all.

[[rtconutc]]*rtconutc*::
*chronyd* assumes by default that the RTC keeps local time (including any
daylight saving changes). This is convenient on PCs running Linux which are
dual-booted with Windows.
+
If you keep the RTC 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 *hwclock* program.
+
Note that this setting is overridden by the <<hwclockfile,*hwclockfile*>> file
and is not relevant for the <<rtcsync,*rtcsync*>> directive.

[[rtcsync]]*rtcsync*::
The *rtcsync* directive enables a mode where the system time is periodically
copied to the RTC and *chronyd* does not try to track its drift. This directive
cannot be used with the <<rtcfile,*rtcfile*>> directive.
+
On Linux, the RTC copy is performed by the kernel every 11 minutes.
+
On macOS, <<chronyd,*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.

=== Logging

[[log]]*log* [_option_]...::
The *log* directive indicates that certain information is to be logged.
The log files are written to the directory specified by the <<logdir,*logdir*>>
directive. A banner is periodically written to the files to indicate the
meanings of the columns.
+
*rawmeasurements*:::
This option logs the raw NTP measurements and related information to a file
called _measurements.log_. An entry is made for each packet received from the
source. This can be useful when debugging a problem. An example line (which
actually appears as a single line in the file) from the log file is shown
below.
+
----
2016-11-09 05:40:50 203.0.113.15    N  2 111 111 1111  10 10 1.0 \
   -4.966e-03  2.296e-01  1.577e-05  1.615e-01  7.446e-03 CB00717B 4B D K
----
+
The columns are as follows (the quantities in square brackets are the values
from the example line above):
+
. Date [2015-10-13]
. Hour:Minute:Second. Note that the date-time pair is expressed in UTC, not the
  local time zone. [05:40:50]
. IP address of server or peer from which measurement came [203.0.113.15]
. 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]
. Stratum of remote computer. [2]
. RFC 5905 tests 1 through 3 (1=pass, 0=fail) [111]
. RFC 5905 tests 5 through 7 (1=pass, 0=fail) [111]
. Results of the *maxdelay*, *maxdelayratio*, and *maxdelaydevratio* (or
  *maxdelayquant*) tests, and a test for synchronisation loop (1=pass,
  0=fail). The first test from these four also checks the server precision,
  response time, validity of the measured offset, and whether an interleaved
  response is acceptable for synchronisation. [1111]
. Local poll [10]
. Remote poll [10]
. '`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]
. The estimated local clock error (_theta_ in RFC 5905). Positive indicates
  that the local clock is slow of the remote source. [-4.966e-03]
. The peer delay (_delta_ in RFC 5905). [2.296e-01]
. The peer dispersion (_epsilon_ in RFC 5905). [1.577e-05]
. The root delay (_DELTA_ in RFC 5905). [1.615e-01]
. The root dispersion (_EPSILON_ in RFC 5905). [7.446e-03]
. Reference ID of the server's source as a hexadecimal number. [CB00717B]
. NTP mode of the received packet (_1_=active peer, _2_=passive peer,
  _4_=server, _B_=basic, _I_=interleaved). [4B]
. Source of the local transmit timestamp
  (_D_=daemon, _K_=kernel, _H_=hardware). [D]
. Source of the local receive timestamp
  (_D_=daemon, _K_=kernel, _H_=hardware). [K]
+
*measurements*:::
This option is identical to the *rawmeasurements* option, except it logs only
valid measurements from synchronised sources, i.e. measurements which passed
the RFC 5905 tests 1 through 7. This can be useful for producing graphs of the
source's performance.
+
*statistics*:::
This option logs information about the regression processing to a file called
_statistics.log_. An example line (which actually appears as a single line in
the file) from the log file is shown below.
+
----
2016-08-10 05:40:50 203.0.113.15     6.261e-03 -3.247e-03 \
     2.220e-03  1.874e-06  1.080e-06 7.8e-02  16   0   8  0.00
----
+
The columns are as follows (the quantities in square brackets are the values
from the example line above):
+
. Date [2015-07-22]
. Hour:Minute:Second. Note that the date-time pair is expressed in
  UTC, not the local time zone. [05:40:50]
. IP address of server or peer from which measurement comes [203.0.113.15]
. The estimated standard deviation of the measurements from the source (in
  seconds). [6.261e-03]
. The estimated offset of the source (in seconds, positive means the local
  clock is estimated to be fast, in this case). [-3.247e-03]
. The estimated standard deviation of the offset estimate (in seconds).
  [2.220e-03]
. 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]
. The estimated error in the rate value (in seconds per second). [1.080e-06].
. The ratio of |old_rate - new_rate| / old_rate_error. Large values
  indicate the statistics are not modelling the source very well. [7.8e-02]
. The number of measurements currently being used for the regression
  algorithm. [16]
. 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]
. 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]
. The estimated or configured asymmetry of network jitter on the path to the
  source which was used to correct the measured offsets. The asymmetry can be
  between -0.5 and +0.5. A negative value means the delay of packets sent to
  the source is more variable than the delay of packets sent from the source
  back. [0.00, i.e. no correction for asymmetry]
+
*selection*:::
This option logs information about selection of sources for synchronisation to
a file called _selection.log_. Note that the rate of entries written to this
file grows quadratically with the number of specified sources (each measurement
triggers the selection for all sources). An example line (which actually
appears as a single line in the file) from the log file is shown below.
+
----
2022-05-01 02:01:20 203.0.113.15    * -----  377  1.00  \
     4.228e+01 -1.575e-04  1.239e-04
----
+
The columns are as follows (the quantities in square brackets are the values
from the example line above):
+
. Date [2022-05-01]
. Hour:Minute:Second. Note that the date-time pair is expressed in
  UTC, not the local time zone. [02:01:20]
. IP address or reference ID of the source. [203.0.113.15]
. State of the source indicated with one of the following symbols. [*]
{blank}::::
Not considered selectable for synchronisation:
* _N_ - has the *noselect* option.
* _s_ - is not synchronised.
* _M_ - does not have enough measurements.
* _d_ - has a root distance larger than the maximum distance (configured by the
        <<maxdistance,*maxdistance*>> directive).
* _~_ - has a jitter larger than the maximum jitter (configured by the
        <<maxjitter,*maxjitter*>> directive).
* _w_ - waits for other sources to get out of the _M_ state.
* _S_ - has older measurements than other sources.
* _O_ - has a stratum equal or larger than the orphan stratum (configured by
        the <<local,*local*>> directive).
* _T_ - does not fully agree with sources that have the *trust* option.
* _x_ - does not agree with other sources (falseticker).
{blank}::::
Considered selectable for synchronisation, but not currently used:
* _W_ - waits for other sources to be selectable (required by the
        <<minsources,*minsources*>> directive, or the *require* option of
        another source).
* _P_ - another selectable source is preferred due to the *prefer* option.
* _U_ - waits for a new measurement (after selecting a different best source).
* _D_ - has, or recently had, a root distance which is too large to be combined
        with other sources (configured by the <<combinelimit,*combinelimit*>>
        directive).
{blank}::::
Used for synchronisation of the local clock:
* _+_ - combined with the best source.
* _*_ - selected as the best source to update the reference data (e.g. root
        delay, root dispersion).
. Current effective selection options of the source. which can be different
  from the configured options due to the authentication selection mode
  (configured by the <<authselectmode,*authselectmode*>> directive). [-----]
* _N_ indicates the *noselect* option.
* _P_ indicates the *prefer* option.
* _T_ indicates the *trust* option.
* _R_ indicates the *require* option.
. Reachability register printed as an 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. [377]
. Current score against the source in the _*_ state. The scoring system avoids
  frequent reselection when multiple sources have a similar root distance. A
  value larger than 1 indicates this source was better than the _*_ source in
  recent selections. If the score reaches 10, the best source will be reselected
  and the scores will be reset to 1. [1.00]
. Interval since the last measurement of the source in seconds. [4.228e+01]
. Lower endpoint of the interval which was expected to contain the true offset
  of the local clock determined by the root distance of the source. [-1.575e-04]
. Upper endpoint of the interval which was expected to contain the true offset
  of the local clock determined by the root distance of the source. [1.239e-04]
+
*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_. An example line (which
actually appears as a single line in the file) from the log file is shown
below.
+
----
2017-08-22 13:22:36 203.0.113.15     2     -3.541      0.075 -8.621e-06 N \
            2  2.940e-03 -2.084e-04  1.534e-02  3.472e-04  8.304e-03
----
+
The columns are as follows (the quantities in square brackets are the
values from the example line above) :
+
. Date [2017-08-22]
. Hour:Minute:Second. Note that the date-time pair is expressed in UTC, not the
  local time zone. [13:22:36]
. The IP address of the server or peer to which the local system is synchronised.
  [203.0.113.15]
. The stratum of the local system. [2]
. The local system frequency (in ppm, positive means the local system runs fast
  of UTC). [-3.541]
. The error bounds on the frequency (in ppm). [0.075]
. The estimated local offset at the epoch, which is normally corrected by
  slewing the local clock (in seconds, positive indicates the clock is fast of
  UTC). [-8.621e-06]
. 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]
. The number of combined sources. [2]
. The estimated standard deviation of the combined offset (in seconds).
  [2.940e-03]
. The remaining offset correction from the previous update (in seconds,
  positive means the system clock is slow of UTC). [-2.084e-04]
. The total of the network path delays to the reference clock to which
  the local clock is ultimately synchronised (in seconds). [1.534e-02]
. The total dispersion accumulated through all the servers back to the
  reference clock to which the local clock is ultimately synchronised
  (in seconds). [3.472e-04]
. The maximum estimated error of the system clock in the interval since the
  previous update (in seconds). It includes the offset, remaining offset
  correction, root delay, and dispersion from the previous update with the
  dispersion which accumulated in the interval. [8.304e-03]
+
*rtc*:::
This option logs information about the system's real-time clock. An example
line (which actually appears as a single line in the file) from the _rtc.log_
file is shown below.
+
----
2015-07-22 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):
+
. Date [2015-07-22]
. Hour:Minute:Second. Note that the date-time pair is expressed in UTC, not the
  local time zone. [05:40:50]
. The measured offset between the RTC and the system clock in seconds.
  Positive indicates that the RTC is fast of the system time [-0.037360].
. Flag indicating whether the regression has produced valid coefficients.
  (1 for yes, 0 for no). [1]
. 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]
. 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]
. The number of measurements used in the regression. [12]
. 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]
. The measurement interval used prior to the measurement being made (in
  seconds). [120]
+
*refclocks*:::
This option logs the raw and filtered reference clock measurements to a file
called _refclocks.log_. An example line (which actually appears as a single
line in the file) from the 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):
+
. Date [2009-11-30]
. Hour:Minute:Second.Microsecond. Note that the date-time pair is expressed in
  UTC, not the local time zone. [14:33:27.000000]
. Reference ID of the reference clock from which the measurement came. [PPS2]
. Sequence number of driver poll within one polling interval for raw samples,
  or _-_ for filtered samples. [7]
. 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]
. Flag indicating whether the sample comes from PPS source. (1 for yes,
  0 for no, or _-_ for filtered sample). [1]
. Local clock error measured by reference clock driver, or _-_ for filtered sample.
  [4.900000e-07]
. Local clock error with applied corrections. Positive indicates that the local
  clock is slow. [-6.741777e-07]
. Assumed dispersion of the sample. [1.000e-06]
+
*tempcomp*:::
This option logs the temperature measurements and system rate compensations to
a file called _tempcomp.log_. An example line (which actually appears as a
single line in the file) from the log file is shown below.
+
----
2015-04-19 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):
+
. Date [2015-04-19]
. Hour:Minute:Second. Note that the date-time pair is expressed in UTC, not the
  local time zone. [10:39:48]
. Temperature read from the sensor. [2.8000e+04]
. Applied compensation in ppm, positive means the system clock is running
  faster than it would be without the compensation. [3.6600e-01]
+
{blank}::
An example of the directive is:
+
----
log measurements statistics tracking
----

[[logbanner]]*logbanner* _entries_::
A banner is periodically written to the log files enabled by the <<log,*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.

[[logchange]]*logchange* _threshold_::
This directive sets the threshold for the adjustment of the system clock that
will generate a syslog message. Clock errors detected via NTP packets,
reference clocks, or timestamps entered via the
<<chronyc.adoc#settime,*settime*>> command of *chronyc* are logged.
+
By default, the threshold is 1 second.
+
An example of the use is:
+
----
logchange 0.1
----
+
which would cause a syslog message to be generated if a system clock error of over
0.1 seconds starts to be compensated.

[[logdir]]*logdir* _directory_::
This directive specifies the directory for writing log files enabled by the
*log* directive. If the directory does not exist, it will be created
automatically.
+
An example of the use of this directive is:
+
----
logdir /var/log/chrony
----

[[mailonchange]]*mailonchange* _email_ _threshold_::
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 the 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 cannot 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.

=== Miscellaneous

[[confdir]]*confdir* _directory_...::
The *confdir* directive includes configuration files with the _.conf_ suffix
from a directory. The files are included in the lexicographical order of the
file names.
+
Multiple directories (up to 10) can be specified with a single *confdir*
directive. In this case, if multiple directories contain a file with the same
name, only the first file in the order of the specified directories will be
included. This enables a fragmented configuration where existing fragments can
be replaced by adding files to a different directory.
+
This directive can be used multiple times.
+
An example of the directive is:
+
----
confdir @SYSCONFDIR@/chrony.d
----

[[sourcedir]]*sourcedir* _directory_...::
The *sourcedir* directive is identical to the *confdir* directive, except the
configuration files have the _.sources_ suffix, they can only specify NTP
sources (i.e. the *server*, *pool*, and *peer* directives), they are expected
to have all lines terminated by the newline character, and they can be
reloaded by the <<chronyc.adoc#reload,*reload sources*>> command in
*chronyc*. It is particularly useful with dynamic sources like NTP servers
received from a DHCP server, which can be written to a file specific to the
network interface by a networking script.
+
This directive can be used multiple times.
+
An example of the directive is:
+
----
sourcedir /var/run/chrony-dhcp
----

[[include]]*include* _pattern_::
The *include* directive includes a configuration file, or multiple configuration
files if a wildcard pattern is specified. Unlike with the *confdir* directive,
the full name of the files needs to be specified and at least one file is
required to exist.
+
This directive can be used multiple times.
+
An example of the directive is:
+
----
include @SYSCONFDIR@/chrony.d/*.conf
----

[[hwtimestamp]]*hwtimestamp* _interface_ [_option_]...::
This directive enables hardware timestamping of NTP packets sent to and
received from the specified network interface. The network interface controller
(NIC) uses its own clock to accurately timestamp the actual transmissions and
receptions, avoiding processing and queueing delays in the kernel, network
driver, and hardware. This can significantly improve the accuracy of the
timestamps and the measured offset, which is used for synchronisation of the
system clock. In order to get the best results, both sides receiving and
sending NTP packets (i.e. server and client, or two peers) need to use HW
timestamping. If the server or peer supports the interleaved mode, it needs to
be enabled by the *xleave* option in the <<server,*server*>> or the
<<peer,*peer*>> directive.
+
This directive is supported on Linux 3.19 and newer. The NIC must support HW
timestamping, which can be verified with the *ethtool -T* command. The list of
capabilities should include _hardware-raw-clock_, _hardware-transmit_, and
_hardware-receive_. The receive filter _all_, or _ntp_, is necessary for
timestamping of received NTP packets. Timestamping of packets received on
bridged and bonded interfaces is supported on Linux 4.13 and newer. If HW
timestamping does not work for received packets, *chronyd* will use kernel
receive timestamps instead. Transmit-only HW timestamping can still be useful
to improve stability of the synchronisation.
+
*chronyd* does not synchronise the NIC clock. It assumes the clock is running
free. Multiple instances of *chronyd* can use the same interface with enabled
HW timestamping. Applications which need HW timestamping with a synchronised
clock (e.g. a PTP daemon) should use a virtual clock running on top of the
physical clock created by writing to _/sys/class/ptp/ptpX/n_vclocks_. This
feature is available on Linux 5.14 and newer.
+
If the kernel supports software timestamping, it will be enabled for all
interfaces automatically.
+
The source of timestamps (i.e. hardware, kernel, or daemon) is indicated on the
client side in the _measurements.log_ file (if enabled by the <<log,*log*>>
directive) and the <<chronyc.adoc#ntpdata,*ntpdata*>> report. On the server
side, the number of served timestamps from each source is provided in the
<<chronyc.adoc#serverstats,*serverstats*>> report.
+
This directive can be used multiple times to enable HW timestamping on multiple
interfaces. If the specified interface is _*_, *chronyd* will try to enable HW
timestamping on all available interfaces.
+
The *hwtimestamp* directive has the following options:
+
*minpoll* _poll_:::
This option specifies the minimum interval between readings of the NIC clock.
It's defined as a power of 2. It should correspond to the minimum polling
interval of all NTP sources and the minimum expected polling interval of NTP
clients. The default value is 0 (1 second), the minimum value is -6 (1/64th
of a second), and the maximum value is 20 (about 12 days).
*maxpoll* _poll_:::
This option specifies the maximum interval between readings of the NIC clock,
as a power of 2. The default value is *minpoll* + 1, i.e. 1 (2 seconds) with
the default *minpoll* of 0. The minimum and maximum values are the same as with
the *minpoll* option.
*minsamples* _samples_:::
This option specifies the minimum number of readings kept for tracking of the
NIC clock. The default value is 2.
*maxsamples* _samples_:::
This option specifies the maximum number of readings kept for tracking of the
NIC clock. The default value is 16.
*precision* _precision_:::
This option specifies the assumed precision of reading of the NIC clock. The
default value is 100e-9 (100 nanoseconds).
*txcomp* _compensation_:::
This option specifies the difference in seconds between the actual transmission
time at the physical layer and the reported transmit timestamp. This value will
be added to transmit timestamps obtained from the NIC. The default value is 0.
*rxcomp* _compensation_:::
This option specifies the difference in seconds between the reported receive
timestamp and the actual reception time at the physical layer. This value will
be subtracted from receive timestamps obtained from the NIC. The default value
is 0.
*nocrossts*:::
Some hardware can precisely cross timestamp the NIC clock with the system
clock. This option disables the use of the cross timestamping.
*rxfilter* _filter_:::
This option selects the receive timestamping filter. The _filter_ can be one of
the following:
_all_::::
Enables timestamping of all received packets.
_ntp_::::
Enables timestamping of received NTP packets.
_ptp_::::
Enables timestamping of received PTP packets.
_none_::::
Disables timestamping of received packets.
{blank}:::
The most specific filter for timestamping of NTP packets supported by the NIC
is selected by default. Some NICs can timestamp PTP packets only. By default,
they will be configured with the _none_ filter and expected to provide hardware
timestamps for transmitted packets only. Timestamping of PTP packets is useful
with NTP-over-PTP enabled by the <<chrony.conf.adoc#ptpport,*ptpport*>>
directive, or when another application is receiving PTP packets on the
interface. Forcing timestamping of all packets with the _all_ filter could be
useful if the NIC supported both the _all_ and _ntp_ filters, and it should
timestamp both NTP and PTP packets, or NTP packets on a different UDP port.
{blank}::
+
Examples of the directive are:
+
----
hwtimestamp eth0
hwtimestamp eth1 txcomp 300e-9 rxcomp 645e-9
hwtimestamp *
----

[[hwtstimeout]]*hwtstimeout* _timeout_::
If hardware timestamping is used with a close NTP server, or the NIC or its
driver is slow in providing the transmit timestamp of NTP requests, a response
from the server can be received before the transmit timestamp of the request.
To avoid calculating the offset with a less accurate transmit timestamp,
*chronyd* can save the response for later processing and wait for the hardware
transmit timestamp. There is no guarantee that the timestamp will be provided
(NICs typically have a limited rate of transmit timestamping). This directive
configures how long should *chronyd* wait for the timestamp after receiving a
valid response from the server. If a second valid response is received from the
server while waiting for the timestamp, they will be both processed
immediately.
+
The default value is 0.001 seconds, which should be sufficient with most
hardware. If you frequently see kernel transmit timestamps in the
_measurements.log_ file or <<chronyc.adoc#ntpdata,*ntpdata*>> report, and it is
not a server handling a high rate of requests in the interleaved mode on the
same interface (which would compete with timestamping of the server's own
requests), increasing the timeout to 0.01 or possibly even longer might help.
Note that the maximum timeout is limited by the NTP polling interval.

[[keyfile]]*keyfile* _file_::
This directive is used to specify the location of the file containing symmetric
keys which are shared between NTP servers and clients, or peers, in order to
authenticate NTP packets with a message authentication code (MAC) using a
cryptographic hash function or cipher.
+
The format of the directive is shown in the example below:
+
----
keyfile @SYSCONFDIR@/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:933F62BE1D604E68A81B557F18CFA200483F5B70
30 AES128 HEX:7EA62AE64D190114D46D5A082F948EC1
31 AES256 HEX:37DDCBC67BB902BCB8E995977FAB4D2B5642F5B32EBCEEE421921D97E5CBFE39
 ...
----
+
Each line consists of an ID, optional type, and key.
+
The ID can be any positive integer in the range 1 through 2^32-1.
+
The type is a name of a cryptographic hash function or cipher which is used to
generate and verify the MAC. The default type is *MD5*, which is always
supported.
If *chronyd* was built with enabled support for hashing using a crypto library
(Nettle, GnuTLS, NSS, or LibTomCrypt), the following functions are available: *MD5*,
*SHA1*, *SHA256*, *SHA384*, *SHA512*. Depending on which library and version is
*chronyd* using, some of the following hash functions and ciphers may
also be available:
*SHA3-224*, *SHA3-256*, *SHA3-384*, *SHA3-512*, *TIGER*, *WHIRLPOOL*, *AES128*,
*AES256*.
+
The key can be specified as a string of ASCII 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.
If the type is a cipher, the length of the key must match the cipher (i.e. 128
bits for AES128 and 256 bits for AES256).
+
It is recommended to use randomly generated keys, specified in the hexadecimal
format, which are at least 128 bits long (i.e. they have at least 32 characters
after the *HEX:* prefix). *chronyd* will log a warning to syslog on start if a
source is specified in the configuration file with a key shorter than 80 bits.
+
The recommended key types are AES ciphers and SHA3 hash functions. MD5 should
be avoided unless no other type is supported on the server and client, or
peers. A major weakness of MD5 for the NTP MAC is a length extension attack,
where a man-in-the-middle attacker can add arbitrary extension fields to the
NTP message and update the MAC to pass the verification of the extended
message. The *extfield* option (enabling processing of the specified extension
field) should not be used for NTP sources authenticated with an MD5 key.
+
The <<chronyc.adoc#keygen,*keygen*>> command of *chronyc* can be used to
generate random keys for the key file. By default, it generates 160-bit MD5 or
SHA1 keys.
+
For security reasons, the file should be readable only by root and the user
under which *chronyd* is normally running (to allow *chronyd* to re-read the
file when the <<chronyc.adoc#rekey,*rekey*>> command is issued by *chronyc*).

[[lock_all]]*lock_all*::
The *lock_all* directive will lock the *chronyd* process into RAM so that it
will never be paged out. This can result in lower and more consistent latency.
The directive is supported on Linux, FreeBSD, NetBSD, and illumos.

[[pidfile]]*pidfile* _file_::
Unless *chronyd* is started with the *-Q* option, it writes its process ID
(PID) to a file, and checks this file on startup to see if another *chronyd*
might already be running on the system. By default, the file used is
_@DEFAULT_PID_FILE@_. The *pidfile* directive allows the name to be changed,
e.g.:
+
----
pidfile /run/chronyd.pid
----
+
Setting this directive to _/_ disables writing and checking of the PID file.

[[ptpport]]*ptpport* _port_::
The *ptpport* directive enables *chronyd* to send and receive NTP messages
contained in PTP event messages (NTP-over-PTP) to enable hardware timestamping
on NICs which cannot timestamp NTP packets, but can timestamp unicast PTP
packets, and also use corrections provided by PTP one-step end-to-end
transparent clocks in network switches and routers. The port recognized by the
NICs and PTP transparent clocks is 319 (PTP event port). The default value is 0
(disabled).
+
The NTP-over-PTP support is experimental. The protocol and configuration can
change in future. It should be used only in local networks.
+
The PTP port will be open even if *chronyd* is not configured to operate as a
server or client. The directive does not change the default protocol of
specified NTP sources. Each NTP source that should use NTP-over-PTP needs to
be specified with the *port* option set to the PTP port. To actually enable
hardware timestamping on NICs which can timestamp PTP packets only, the
*rxfilter* option of the *hwtimestamp* directive needs to be set to _ptp_. The
extension field _F324_ needs to be enabled to use the corrections provided by
the PTP transparent clocks.
+
An example of client configuration is:
+
----
server ntp1.example.net minpoll 0 maxpoll 0 xleave port 319 extfield F324
hwtimestamp * rxfilter ptp
ptpport 319
----

[[ptpdomain]]*ptpdomain* _domain_::
The *ptpdomain* directive sets the PTP domain number of transmitted and
accepted NTP-over-PTP messages. Messages from other domains are ignored.
The default is 123, the minimum is 0, and the maximum is 255.

[[sched_priority]]*sched_priority* _priority_::
On Linux, FreeBSD, NetBSD, and illumos, the *sched_priority* directive will
select the SCHED_FIFO real-time scheduler at the specified priority (which must
be between 0 and 100). On macOS, this option must have either a value of 0 (the
default) to disable the thread time constraint policy or 1 for the policy to be
enabled.
+
On systems other than macOS, this directive uses the *pthread_setschedparam()*
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* 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 *pthread_setschedparam(3)* man page has
more details.
+
On macOS, this directive uses the *thread_policy_set()* kernel call to
specify real-time scheduling. As noted above, you should not use this directive
unless you really need it.

[[user]]*user* _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 macOS, FreeBSD, NetBSD and illumos *chronyd* forks into two processes.
The child process retains root privileges, but can only perform a very limited
range of privileged system calls on behalf of the parent.
+
The compiled-in default value is _@DEFAULT_USER@_.

[[examples]]
== EXAMPLES

=== NTP client with permanent connection to NTP servers

This section shows how to configure *chronyd* 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 servers
you want to use. You might be able to find names of suitable servers by one of
the following methods:

* Your institution might 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 https://www.pool.ntp.org/[pool.ntp.org] project.

Assuming that your NTP servers are called _ntp1.example.net_, _ntp2.example.net_
and _ntp3.example.net_, your _chrony.conf_ file could contain as a minimum:

----
server ntp1.example.net
server ntp2.example.net
server ntp3.example.net
----

However, you will probably want to include some of the other directives. The
<<driftfile,*driftfile*>>, <<makestep,*makestep*>> and <<rtcsync,*rtcsync*>>
might be particularly useful. Also, the *iburst* option of the
<<server,*server*>> directive is useful to speed up the initial
synchronisation. The smallest useful configuration file would look something
like:

----
server ntp1.example.net iburst
server ntp2.example.net iburst
server ntp3.example.net iburst
driftfile @CHRONYVARDIR@/drift
makestep 1.0 3
rtcsync
----

When using a pool of NTP servers (one name is used for multiple servers which
might change over time), it is better to specify them with the <<pool,*pool*>>
directive instead of multiple *server* directives. The configuration file could
in this case look like:

----
pool pool.ntp.org iburst
driftfile @CHRONYVARDIR@/drift
makestep 1.0 3
rtcsync
----

If the servers (or pool) support the Network Time Security (NTS)
authentication mechanism and *chronyd* is compiled with NTS support, the *nts*
option will enable a secure synchronisation to the servers. The configuration
file could look like:

----
server ntp1.example.net iburst nts
server ntp2.example.net iburst nts
server ntp3.example.net iburst nts
driftfile @CHRONYVARDIR@/drift
makestep 1.0 3
rtcsync
----

=== NTP client with infrequent connection to NTP servers

This section shows how to configure *chronyd* for computers that have
occasional connections to NTP servers. In this case, you will need some
additional configuration to tell *chronyd* when the connection 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 _ntp1.example.net_,
_ntp2.example.net_ and _ntp3.example.net_, your _chrony.conf_ file would now
contain:

----
server ntp1.example.net offline
server ntp2.example.net offline
server ntp3.example.net offline
driftfile @CHRONYVARDIR@/drift
makestep 1.0 3
rtcsync
----

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. To tell *chronyd* when to
start and finish sampling the servers, the <<chronyc.adoc#online,*online*>> and
<<chronyc.adoc#offline,*offline*>> commands of *chronyc* need to be used.

To give an example of their use, assuming that *pppd* is the program being
used to connect to the Internet and that *chronyc* has been installed at
_@BINDIR@/chronyc_, the script _/etc/ppp/ip-up_ would include:

----
@BINDIR@/chronyc online
----

and the script _/etc/ppp/ip-down_ would include:

----
@BINDIR@/chronyc offline
----

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

=== Isolated networks

This section shows how to configure *chronyd* for computers that never have
network connectivity to any computer which ultimately derives its time from a
reference clock.

In this situation, one computer is selected to be the primary timeserver. The
other computers are either direct clients of the server, or clients of clients.

The <<local,*local*>> directive enables a local reference mode, which allows
*chronyd* to appear synchronised even when it is not.

The rate value in the server's drift file needs to be set to the average rate
at which the server gains or loses time. *chronyd* includes support for this,
in the form of the <<manual,*manual*>> directive and the
<<chronyc.adoc#settime,*settime*>> command in the *chronyc* program.

If the server is rebooted, *chronyd* can re-read the drift rate from the drift
file. However, the server has no accurate estimate of the current time. To get
around this, the system can be configured so that the server can initially set
itself to a '`majority-vote`' of selected clients' times; this allows the
clients to '`flywheel`' the server while it is rebooting.

The <<smoothtime,*smoothtime*>> directive is useful when the clocks of the
clients need to stay close together when the local time is adjusted by the
<<chronyc.adoc#settime,*settime*>> command. The smoothing process needs to be
activated by the <<chronyc.adoc#smoothtime,*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 server (called _ntp.local_) might be
(assuming the clients and the server are in the _192.168.165.x_ subnet):

----
initstepslew 1 client1 client3 client6
driftfile @CHRONYVARDIR@/drift
local stratum 8
manual
allow 192.168.165.0/24
smoothtime 400 0.01
rtcsync
----

For the clients that have to resynchronise the server when it restarts,
the configuration file might be:

----
server ntp.local iburst
driftfile @CHRONYVARDIR@/drift
allow 192.168.165.0/24
makestep 1.0 3
rtcsync
----

The rest of the clients would be the same, except that the *allow* directive is
not required.

If there is no suitable computer to be designated as the primary server, or
there is a requirement to keep the clients synchronised even when it fails, the
*orphan* option of the *local* directive enables a special mode where the
server is selected from multiple computers automatically. They all need to use
the same *local* configuration and poll one another. The server with the
smallest reference ID (which is based on its IP address) will take the role of
the primary server and others will be synchronised to it. When it fails, the
server with the second smallest reference ID will take over and so on.

A configuration file for the first server might be (assuming there are three
servers called _ntp1.local_, _ntp2.local_, and _ntp3.local_):

----
initstepslew 1 ntp2.local ntp3.local
server ntp2.local
server ntp3.local
driftfile @CHRONYVARDIR@/drift
local stratum 8 orphan
manual
allow 192.168.165.0/24
rtcsync
----

The other servers would be the same, except the hostnames in the *initstepslew*
and *server* directives would be modified to specify the other servers. Their
clients might be configured to poll all three servers.

=== RTC tracking

This section considers a computer which has occasional connections to the
Internet and is turned off between '`sessions`'. In this case, *chronyd* relies
on the computer's RTC to maintain the time between the periods when it is
powered up. It assumes that Linux is run exclusively on the computer. Dual-boot
systems might work; it depends what (if anything) the other system does to the
RTC. On 2.6 and later kernels, if your motherboard has a HPET, you will need to
enable the *HPET_EMULATE_RTC* option in your kernel configuration. Otherwise,
*chronyd* will not be able to interact with the RTC device and will give up
using it.

When the computer is connected to the Internet, *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 or losing time.

When the computer is taken offline from the Internet, the best estimate of the
gain or loss rate is used to free-run the computer until it next goes online.

Whilst the computer is running, *chronyd* makes measurements of the 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.

When the computer is powered down, the measurement histories for all the NTP
servers are saved to files, and the RTC tracking information is also
saved to a file (if the <<rtcfile,*rtcfile*>> directive has been specified).
These pieces of information are also saved if the <<chronyc.adoc#dump,*dump*>>
and <<chronyc.adoc#writertc,*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 or 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 might 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 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 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.

To illustrate how a computer might be configured for this case, example
configuration files are shown.

For the _chrony.conf_ file, the following can be used as an example.

----
server ntp1.example.net maxdelay 0.4 offline
server ntp2.example.net maxdelay 0.4 offline
server ntp3.example.net maxdelay 0.4 offline
logdir /var/log/chrony
log statistics measurements tracking
driftfile @CHRONYVARDIR@/drift
makestep 1.0 3
maxupdateskew 100.0
dumpdir @CHRONYVARDIR@
rtcfile @CHRONYVARDIR@/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:

----
@BINDIR@/chronyc online
----

and the relevant part of the _/etc/ppp/ip-down_ script is:

----
@BINDIR@/chronyc -m offline dump writertc
----

*chronyd* is started during the boot sequence with the *-r* and *-s* options.
It might 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.

=== Public NTP server

*chronyd* can be configured to operate as a public NTP server, e.g. to join the
https://www.pool.ntp.org/en/join.html[pool.ntp.org] project. The configuration
is similar to the NTP client with permanent connection, except it needs to
allow client access from all addresses. It is recommended to find at least four
good servers (e.g. from the pool, or on the NTP homepage). If the server has a
hardware reference clock (e.g. a GPS receiver), it can be specified by the
<<refclock,*refclock*>> directive.

The amount of memory used for logging client accesses can be increased in order
to enable clients to use the interleaved mode even when the server has a large
number of clients, and better support rate limiting if it is enabled by the
<<ratelimit,*ratelimit*>> directive. The system timezone database, if it is
kept up to date and includes the _right/UTC_ timezone, can be used as a
reliable source to determine when a leap second will be applied to UTC. The
*-r* option with the <<dumpdir,*dumpdir*>> directive shortens the time in which
*chronyd* will not be able to serve time to its clients when it needs to be
restarted (e.g. after upgrading to a newer version, or a change in the
configuration).

The configuration file could look like:

----
server ntp1.example.net iburst
server ntp2.example.net iburst
server ntp3.example.net iburst
server ntp4.example.net iburst
makestep 1.0 3
rtcsync
allow
clientloglimit 100000000
leapsectz right/UTC
driftfile @CHRONYVARDIR@/drift
dumpdir @CHRONYRUNDIR@
----

== SEE ALSO

<<chronyc.adoc#,*chronyc(1)*>>, <<chronyd.adoc#,*chronyd(8)*>>

== BUGS

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

== AUTHORS

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