CHRONY.CONF(5) Configuration Files CHRONY.CONF(5)
NAME
chrony.conf - chronyd configuration file
SYNOPSIS
chrony.conf
DESCRIPTION
This file configures the chronyd daemon. The compiled-in location is
/etc/chrony.conf, but 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 can occur in any order in the file and they are not
case-sensitive.
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 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 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 directive is immediately followed by either the name of
the server, or its IP address. The server 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
-6 (1/64th 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 -6 (1/64th of a second), and the maximum is 24 (6
months).
iburst
With this option, the interval between the first four requests
sent to the server will be 2 seconds or less instead of the
interval specified by the minpoll option, which allows chronyd
to make the first update of the clock shortly after start.
burst
With this option, chronyd will shorten the interval between up
to four requests to 2 seconds or less 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 password specified in the key file, which is specified by
the 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.
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
dial-up or other slow links, where a long delay probably
indicates a highly asymmetric delay caused by the response
waiting behind a lot of packets related to a download of some
sort).
If the user knows that round trip delays above a certain level
should cause the measurement to be ignored, this level can be
defined with the maxdelay option. For example, maxdelay 0.3
would indicate that measurements with a round-trip delay of 0.3
seconds or more should be ignored. The default value is 3
seconds 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 maxdelayratio
times the minimum delay, it will be rejected.
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.
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 directive.
maxsamples samples
Set the maximum number of samples kept for this source. This
overrides the maxsamples directive.
filter samples
This option enables a median filter to reduce noise in NTP
measurements. The filter will reduce the specified number of
samples to 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
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
offline command from chronyc when disconnecting the network
link. (It will still be necessary to use the online command
when the link has been established, to enable measurements to
start.)
prefer
Prefer this source over 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 an interleaved mode which allows the server
or the peer to send transmit timestamps captured after the
actual transmission (e.g. when the server or the peer is
running chronyd with software (kernel) or hardware
timestamping). This can significantly improve the accuracy of
the measurements.
The interleaved mode is compatible with servers that support
only the basic mode, but peers must both support and have
enabled the interleaved mode, otherwise the synchronisation
will work only in one direction. 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 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).
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.
The presend option cannot be used in the peer directive. If it
is used 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 whether a key is
specified by the key option and which authentication hash
function the key is using. If the output size of the hash
function is longer than 160 bits, the default version is 3 for
compatibility with older chronyd servers. Otherwise, the
default version is 4.
pool name [option]...
The syntax of this directive is similar to that for the 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.
All options valid in the server directive can be used in this
directive too. There is one option specific to the pool directive:
maxsources sets the maximum number of sources that can be used from
the pool, the default value is 4.
On start, when the pool name is resolved, chronyd will add up to 16
sources, one for each resolved address. When the number of sources
from which at least one valid reply was received reaches the number
specified by the maxsources option, the other sources will be
removed. When a pool source is unreachable, marked as a
falseticker, or has a distance larger than the limit set by the
maxdistance directive, chronyd will try to replace the source with
a newly resolved address from the pool.
An example of the pool directive is
pool pool.ntp.org iburst maxsources 3
peer hostname [option]...
The syntax of this directive is identical to that for the 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.
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 directive on both hosts) instead.
initstepslew step-threshold [hostname]...
In normal operation, chronyd slews the time when it needs to adjust
the system clock. For example, to correct a system clock which is 1
second slow, chronyd slightly increases the amount by which the
system clock is advanced on each clock interrupt, until the error
is removed. Note that at no time does time run backwards with this
method.
On most Unix systems it is not desirable to step the system clock,
because many programs rely on time advancing monotonically
forwards.
When the chronyd daemon is initially started, it is possible that
the system clock is considerably in error. Attempting to correct
such an error by slewing might not be sensible, since it might take
several hours to correct the error by this means.
The purpose of the initstepslew directive is to allow chronyd to
make a rapid measurement of the system clock error at boot time,
and to correct the system clock by stepping before normal operation
begins. Since this would normally be performed only at an
appropriate point in the system boot sequence, no other software
should be adversely affected by the step.
If the correction required is less than a specified threshold, a
slew is used instead. This makes it 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 foo.example.net bar.example.net
where 2 NTP servers are used to make the measurement. The 30
indicates that if the system's error is found to be 30 seconds or
less, a slew will be used to correct it; if the error is above 30
seconds, a step will be used.
The initstepslew directive can also be used in an isolated LAN
environment, where the clocks are set manually. The most stable
computer is chosen as the master, and the other computers are
slaved to it. If each of the slaves is configured with the local
directive, the master can be set up with an initstepslew directive
which references some or all of the slaves. Then, if the master
machine has to be rebooted, the slaves can be relied on to act
analogously to a flywheel and preserve the time for a short period
while the master completes its reboot.
The initstepslew directive is functionally similar to a combination
of the makestep and server directives with the iburst option. The
main difference is that the initstepslew servers are used only
before normal operation begins and that the foreground chronyd
process waits for initstepslew to finish before exiting. This is
useful to prevent programs started in the boot sequence after
chronyd from reading the clock before it has been stepped.
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 (separated by the : and , characters).
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
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.
Examples:
refclock PPS /dev/pps0 lock NMEA refid GPS
refclock SHM 0 offset 0.5 delay 0.2 refid NMEA noselect
refclock PPS /dev/pps1:clear refid GPS2
SHM
NTP shared memory driver. This driver uses a shared memory
segment to receive samples from another process (e.g. gpsd).
The parameter is the number of the shared memory segment,
typically a small number like 0, 1, 2, or 3. 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).
Examples:
refclock SHM 0 poll 3 refid GPS1
refclock SHM 1:perm=0644 refid GPS2
SOCK
Unix domain socket driver. It is similar to the SHM driver, but
samples are received from a Unix domain socket instead of
shared memory and the messages have a different format. The
parameter is the path to the socket, which chronyd creates on
start. An advantage over the SHM driver is that SOCK does not
require polling and it can receive PPS samples with incomplete
time. 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. The path where gpsd expects the socket to be created is
described in the gpsd(8) man page. For example:
refclock SOCK /var/run/chrony.ttyS0.sock
PHC
PTP hardware clock (PHC) driver. The parameter is the path to
the device of the PTP clock which 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 to which is
connected the PPS signal. 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.
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
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.
prefer
Prefer this source over 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 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.
minsamples samples
Set the minimum number of samples kept for this source. This
overrides the minsamples directive.
maxsamples samples
Set the maximum number of samples kept for this source. This
overrides the maxsamples directive.
manual
The manual directive enables support at run-time for the 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
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 port
By default, chronyd uses a separate client socket for each
configured server and their source port is chosen arbitrarily by
the operating system. However, you can use the acquisitionport
directive to explicitly specify a port and use only one socket (per
IPv4 or IPv6 address family) for all configured servers. This can
be useful for getting through some firewalls. If set to 0, the
source port of the socket will be chosen arbitrarily.
It can be set to the same port as is used by the NTP server (which
can be configured with the 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 address
The bindacqaddress directive sets the network interface to which
chronyd will bind its NTP client sockets. The syntax is similar to
the bindaddress and bindcmdaddress directives.
For each of the IPv4 and IPv6 protocols, only one bindacqaddress
directive can be specified.
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 dump command in chronyc is issued.
An example of the directive is:
dumpdir /var/run/chrony
A source whose IP address is 1.2.3.4 would have its measurement
history saved in the file /var/run/chrony/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 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 and refclock
directives. The default value is 0, which disables the configurable
limit. The useful range is 4 to 64.
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 and 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 tracking and sourcestats reports (and the
tracking.log and statistics.log files) may be smaller than the
actual offsets.
Source selection
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.
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 distance
The maxdistance directive sets the maximum allowed root distance of
the sources to not be rejected by the source selection algorithm.
The distance includes the accumulated dispersion, which might be
large when the source is no longer synchronised, and half of the
total round-trip delay to the primary source.
By default, the maximum root distance is 3 seconds.
Setting maxdistance to a larger value can be useful to allow
synchronisation with a server that only has a very infrequent
connection to its sources and can accumulate a large dispersion
between updates of its clock.
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 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 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 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
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
directive. The current remaining correction is shown in the
tracking report as the System time value.
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 /var/lib/chrony/drift
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 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
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.
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 directive to enable a server leap smear.
When smearing a leap second, the leap status is suppressed on the
server and the served time is corrected slowly be slewing instead
of stepping. The clients 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 0.001 leaponly
The first directive is necessary to disable the clock step which
would reset the smoothing process. The second directive limits the
slewing rate of the local clock to 1000 ppm, which improves the
stability of the smoothing process when the local correction starts
and ends. The third directive enables the server time smoothing
process. It will start when the clock gets to 00:00:00 UTC and it
will take 17 hours 34 minutes to finish. The frequency offset will
be changing by 0.001 ppm per second and will reach a maximum of
31.623 ppm. The leaponly option makes the duration of the leap
smear constant and allows the clients to safely synchronise with
multiple identically configured leap smearing servers.
leapsectz timezone
This directive specifies a timezone in the system tz 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 typically 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 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
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, 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 a specified
limit (a negative value can be used to disable the limit).
This is particularly useful when using reference clocks, because
the initstepslew directive works only with NTP sources.
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 offset start ignore
This directive sets the maximum allowed offset corrected on a clock
update. The check is performed only after the specified number of
updates to allow a large initial adjustment of the system clock.
When an offset larger than the specified maximum occurs, it will be
ignored for the specified number of times and then chronyd will
give up and exit (a negative value can be used to never exit). In
both cases a message is sent to syslog.
An example of the use of this directive is:
maxchange 1000 1 2
After the first clock update, chronyd will check the offset on
every clock update, it will ignore two adjustments larger than 1000
seconds and exit on another one.
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 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 Solaris).
By default, the maximum assumed drift is 500000 ppm, i.e. the
adjustment is limited by the system driver rather than this
directive.
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 a dial-up
connection to servers over a phone line, and 5 or 10 for a computer
on a LAN.
It should be noted that this is not the only means of protection
against using unreliable estimates. At all times, chronyd keeps
track of both the estimated gain or loss rate, and the error bound
on the estimate. When a new estimate is generated following another
measurement from one of the sources, a weighted combination
algorithm is used to update the master estimate. So if chronyd has
an existing highly-reliable master estimate and a new estimate is
generated which has large error bounds, the existing master
estimate will dominate in the new master estimate.
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
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 Solaris 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.
In early beta releases of macOS 13 this capability is disabled
because of a system kernel bug. When the kernel bug is fixed,
chronyd will detect this and re-enable the capability (see above
limitations) with no recompilation required.
By default, the maximum slew rate is set to 83333.333 ppm (one
twelfth).
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
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 tempcomp directive.
NTP server
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.
The default is that no clients are allowed access, i.e. chronyd
operates purely as an NTP client. If the allow directive is used,
chronyd will be both a client of its servers, and a server to other
clients.
Examples of the use of the directive are as follows:
allow 1.2.3.4
allow 1.2
allow 3.4.5
allow 6.7.8/22
allow 6.7.8.9/22
allow 2001:db8::/32
allow 0/0
allow ::/0
allow
The first directive allows a node with IPv4 address 1.2.3.4 to be
an NTP client of this computer. The second directive allows any
node with an IPv4 address of the form 1.2.x.y (with x and y
arbitrary) to be an NTP client of this computer. Likewise, the
third directive allows any node with an IPv4 address of the form
3.4.5.x to have client NTP access. The fourth and fifth forms allow
access from any node with an IPv4 address of the form 6.7.8.x,
6.7.9.x, 6.7.10.x or 6.7.11.x (with x arbitrary), i.e. the value 22
is the number of bits defining the specified subnet. In the fifth
form, the final byte is ignored. The sixth form is used for IPv6
addresses. The seventh and eighth forms allow access by any IPv4
and IPv6 node respectively. The ninth forms allows access by any
node (IPv4 or IPv6).
A second form of the directive, allow all, has a greater effect,
depending on the ordering of directives in the configuration file.
To illustrate the effect, consider the two examples:
allow 1.2.3.4
deny 1.2.3
allow 1.2
and
allow 1.2.3.4
deny 1.2.3
allow all 1.2
In the first example, the effect is the same regardless of what
order the three directives are given in. So the 1.2.x.y subnet is
allowed access, except for the 1.2.3.x subnet, which is denied
access, however the host 1.2.3.4 is allowed access.
In the second example, the allow all 1.2 directives overrides the
effect of any previous directive relating to a subnet within the
specified subnet. Within a configuration file this capability is
probably rather moot; however, it is of greater use for
reconfiguration at run-time via chronyc with the allow all command.
The directive allows a hostname to be specified instead of an IP
address, but the name must be resolvable when chronyd is started
(i.e. chronyd needs to be started when the network is already up
and DNS is working).
Note, if the 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 [all] [subnet]
This is similar to the allow directive, except that it denies NTP
client access to a particular subnet or host, rather than allowing
it.
The syntax is identical.
There is also a deny all directive with similar behaviour to the
allow all directive.
bindaddress address
The bindaddress directive binds the socket on which chronyd listens
for NTP 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.
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.
The syntax is as follows:
broadcast 30 192.168.1.255
broadcast 60 192.168.2.255 12123
broadcast 60 ff02::101
In the first example, the destination port defaults to UDP port 123
(the normal NTP port). In the second example, the destination port
is specified as 12123. The first parameter in each case (30 or 60
respectively) is the interval in seconds between broadcast packets
being sent. The second parameter in each case is the broadcast
address to send the packet to. This should correspond to the
broadcast address of one of the network interfaces on the computer
where chronyd is running.
You can have more than 1 broadcast directive if you have more than
1 network interface onto which you 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 directive.
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 is
sufficient for monitoring about four thousand clients at the same
time.
In older chrony versions if the limit was set to 0, the memory
allocation was unlimited.
An example of the use of this directive is:
clientloglimit 1048576
noclientlog
This directive, which takes no arguments, specifies that client
accesses are not to be logged. Normally they are logged, allowing
statistics to be reported using the clients command in chronyc.
This option also effectively disables server support for the NTP
interleaved mode.
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 tracking command in chronyc)
as:
distance = delay / 2 + dispersion
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. When that server fails, another will
take over.
The orphan mode is compatible with the ntpd's orphan mode
(enabled by the tos orphan command).
An example of the directive is:
local stratum 10 orphan
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 directive, or the
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
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 directive or the 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 directive.
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 directive.
The ratelimit directive supports a number of options (which can be
defined in any order):
interval
This option sets the minimum interval between responses. It is
defined as a power of 2 in seconds. The default value is 3 (8
seconds). The minimum value is -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
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
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.
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).
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 directive or the makestep command in
chronyc. The process can be reset without stepping the clock by the
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 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
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 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 address
The bindcmdaddress directive allows you to specify an IP address of
an interface on which chronyd will listen 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
/var/run/chrony/chronyd.sock. The socket can be disabled by setting
the path to /.
By default, chronyd binds to the loopback interface (with addresses
127.0.0.1 and ::1). This blocks all access except from localhost.
To listen for command packets on all interfaces, you can add the
lines:
bindcmdaddress 0.0.0.0
bindcmdaddress ::
to the configuration file.
For each of 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
cmdallow [all] [subnet]
This is similar to the 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
directive to not listen only on the loopback interface to actually
allow remote access.
cmddeny [all] [subnet]
This is similar to the 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 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, this is useful
to disable chronyc access from the Internet. (It does not disable
the Unix domain command socket.)
An example shows the syntax:
cmdport 257
This would make chronyd use UDP 257 as its command port. (chronyc
would need to be run with the -p 257 switch to inter-operate
correctly.)
cmdratelimit [option]...
This directive enables response rate limiting for command packets.
It is similar to the ratelimit directive, except responses to
localhost are never limited and the default interval is -4 (16
packets per second).
An example of the use of the directive is:
cmdratelimit interval 2
Real-time clock (RTC)
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 directive.
The compiled-in default value is '/etc/adjtime'.
An example of the directive is:
hwclockfile /etc/adjtime
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 trimrtc
command in chronyc was issued.
This directive is effective only with the rtcfile directive.
An example of the use of this directive is:
rtcautotrim 30
This would set the threshold error to 30 seconds.
rtcdevice device
The rtcdevice directive sets the path to the device file for
accessing the RTC. The default path is /dev/rtc.
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 /var/lib/chrony/rtc
chronyd saves information in this file when it exits and when the
writertc command is issued in chronyc. The information saved is the
RTC's error at some epoch, that epoch (in seconds since January 1
1970), and the rate at which the RTC gains or loses time.
So far, the support for real-time clocks is limited; their code is
even more system-specific than the rest of the software. You can
only use the RTC facilities (the rtcfile directive and the -s
command-line option to chronyd) if the following three conditions
apply:
1. You are running Linux.
2. The kernel is compiled with extended real-time clock support
(i.e. the /dev/rtc device is capable of doing useful things).
3. You do not have other applications that need to make use of
/dev/rtc at all.
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 when the hwclockfile directive
is specified.
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
directive.
On Linux, the RTC copy is performed by the kernel every 11 minutes.
On macOS, 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 [option]...
The log directive indicates that certain information is to be
logged. The log files are written to the directory specified by the
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):
1. Date [2015-10-13]
2. Hour:Minute:Second. Note that the date-time pair is
expressed in UTC, not the local time zone. [05:40:50]
3. IP address of server or peer from which measurement came
[203.0.113.15]
4. Leap status (N means normal, + means that the last minute
of the current month has 61 seconds, - means that the last
minute of the month has 59 seconds, ? means the remote
computer is not currently synchronised.) [N]
5. Stratum of remote computer. [2]
6. RFC 5905 tests 1 through 3 (1=pass, 0=fail) [111]
7. RFC 5905 tests 5 through 7 (1=pass, 0=fail) [111]
8. Tests for maximum delay, maximum delay ratio and maximum
delay dev ratio, against defined parameters, and a test for
synchronisation loop (1=pass, 0=fail) [1111]
9. Local poll [10]
10. Remote poll [10]
11. `Score' (an internal score within each polling level used
to decide when to increase or decrease the polling level.
This is adjusted based on number of measurements currently
being used for the regression algorithm). [1.0]
12. The estimated local clock error (theta in RFC 5905).
Positive indicates that the local clock is slow of the
remote source. [-4.966e-03]
13. The peer delay (delta in RFC 5905). [2.296e-01]
14. The peer dispersion (epsilon in RFC 5905). [1.577e-05]
15. The root delay (DELTA in RFC 5905). [1.615e-01]
16. The root dispersion (EPSILON in RFC 5905). [7.446e-03]
17. Reference ID of the server's source as a hexadecimal
number. [CB00717B]
18. NTP mode of the received packet (1=active peer, 2=passive
peer, 4=server, B=basic, I=interleaved). [4B]
19. Source of the local transmit timestamp (D=daemon,
K=kernel, H=hardware). [D]
20. 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):
1. Date [2015-07-22]
2. Hour:Minute:Second. Note that the date-time pair is
expressed in UTC, not the local time zone. [05:40:50]
3. IP address of server or peer from which measurement comes
[203.0.113.15]
4. The estimated standard deviation of the measurements from
the source (in seconds). [6.261e-03]
5. The estimated offset of the source (in seconds, positive
means the local clock is estimated to be fast, in this
case). [-3.247e-03]
6. The estimated standard deviation of the offset estimate (in
seconds). [2.220e-03]
7. The estimated rate at which the local clock is gaining or
losing time relative to the source (in seconds per second,
positive means the local clock is gaining). This is
relative to the compensation currently being applied to the
local clock, not to the local clock without any
compensation. [1.874e-06]
8. The estimated error in the rate value (in seconds per
second). [1.080e-06].
9. The ratio of |old_rate - new_rate| / old_rate_error. Large
values indicate the statistics are not modelling the source
very well. [7.8e-02]
10. The number of measurements currently being used for the
regression algorithm. [16]
11. The new starting index (the oldest sample has index 0;
this is the method used to prune old samples when it no
longer looks like the measurements fit a linear model). [0,
i.e. no samples discarded this time]
12. The number of runs. The number of runs of regression
residuals with the same sign is computed. If this is too
small it indicates that the measurements are no longer
represented well by a linear model and that some older
samples need to be discarded. The number of runs for the
data that is being retained is tabulated. Values of
approximately half the number of samples are expected. [8]
13. 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]
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) :
1. Date [2017-08-22]
2. Hour:Minute:Second. Note that the date-time pair is
expressed in UTC, not the local time zone. [13:22:36]
3. The IP address of the server or peer to which the local
system is synchronised. [203.0.113.15]
4. The stratum of the local system. [2]
5. The local system frequency (in ppm, positive means the
local system runs fast of UTC). [-3.541]
6. The error bounds on the frequency (in ppm). [0.075]
7. 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]
8. Leap status (N means normal, + means that the last minute
of this month has 61 seconds, - means that the last minute
of the month has 59 seconds, ? means the clock is not
currently synchronised.) [N]
9. The number of combined sources. [2]
10. The estimated standard deviation of the combined offset
(in seconds). [2.940e-03]
11. The remaining offset correction from the previous update
(in seconds, positive means the system clock is slow of
UTC). [-2.084e-04]
12. The total of the network path delays to the reference
clock to which the local clock is ultimately synchronised
(in seconds). [1.534e-02]
13. 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]
14. 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):
1. Date [2015-07-22]
2. Hour:Minute:Second. Note that the date-time pair is
expressed in UTC, not the local time zone. [05:40:50]
3. 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].
4. Flag indicating whether the regression has produced valid
coefficients. (1 for yes, 0 for no). [1]
5. Offset at the current time predicted by the regression
process. A large difference between this value and the
measured offset tends to indicate that the measurement is
an outlier with a serious measurement error. [-0.037434]
6. The rate at which the RTC is losing or gaining time
relative to the system clock. In ppm, with positive
indicating that the RTC is gaining time. [-37.948]
7. The number of measurements used in the regression. [12]
8. The number of runs of regression residuals of the same
sign. Low values indicate that a straight line is no longer
a good model of the measured data and that older
measurements should be discarded. [5]
9. The measurement interval used prior to the measurement
being made (in seconds). [120]
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):
1. Date [2009-11-30]
2. Hour:Minute:Second.Microsecond. Note that the date-time
pair is expressed in UTC, not the local time zone.
[14:33:27.000000]
3. Reference ID of the reference clock from which the
measurement came. [PPS2]
4. Sequence number of driver poll within one polling interval
for raw samples, or - for filtered samples. [7]
5. Leap status (N means normal, + means that the last minute
of the current month has 61 seconds, - means that the last
minute of the month has 59 seconds). [N]
6. Flag indicating whether the sample comes from PPS source.
(1 for yes, 0 for no, or - for filtered sample). [1]
7. Local clock error measured by reference clock driver, or -
for filtered sample. [4.900000e-07]
8. Local clock error with applied corrections. Positive
indicates that the local clock is slow. [-6.741777e-07]
9. Assumed dispersion of the sample. [1.000e-06]
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):
1. Date [2015-04-19]
2. Hour:Minute:Second. Note that the date-time pair is
expressed in UTC, not the local time zone. [10:39:48]
3. Temperature read from the sensor. [2.8000e+04]
4. Applied compensation in ppm, positive means the system
clock is running faster than it would be without the
compensation. [3.6600e-01]
An example of the directive is:
log measurements statistics tracking
logbanner entries
A banner is periodically written to the log files enabled by the
log directive to indicate the meanings of the columns.
The logbanner directive specifies after how many entries in the log
file should be the banner written. The default is 32, and 0 can be
used to disable it entirely.
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
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 directory
This directive allows the directory where log files are written to
be specified.
An example of the use of this directive is:
logdir /var/log/chrony
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
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 or the peer directive.
This directive is supported on Linux. The NIC must support HW
timestamping, which can be verified with the ethtool -T command.
The list of capabilities should include
SOF_TIMESTAMPING_RAW_HARDWARE, SOF_TIMESTAMPING_TX_HARDWARE, and
SOF_TIMESTAMPING_RX_HARDWARE. Receive filter HWTSTAMP_FILTER_ALL,
or HWTSTAMP_FILTER_NTP_ALL, is necessary for timestamping of
received packets. When chronyd is running, no other process (e.g. a
PTP daemon) should be working with the NIC clock.
If the kernel supports software timestamping, it will be enabled
for all interfaces. The source of timestamps (i.e. hardware,
kernel, or daemon) is indicated in the measurements.log file if
enabled by the log measurements directive, and the ntpdata report
in chronyc.
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 two. 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) and the minimum value is -6
(1/64th of a second).
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.
none
Disables timestamping of received packets.
The most specific filter for timestamping NTP packets which is
supported by the NIC is selected by default. Some NICs can
timestamp only PTP packets, which limits the selection to the
none filter. Forcing timestamping of all packets with the all
filter when the NIC supports both all and ntp filters can be
useful when packets are received from or on a non-standard UDP
port (e.g. specified by the port directive).
Examples of the directive are:
hwtimestamp eth0
hwtimestamp eth1 txcomp 300e-9 rxcomp 645e-9
hwtimestamp *
include pattern
The include directive includes a configuration file or multiple
configuration files if a wildcard pattern is specified. This can be
useful when maintaining configuration on multiple hosts to keep the
differences in separate files.
An example of the directive is:
include /etc/chrony.d/*.conf
keyfile file
This directive is used to specify the location of the file
containing ID-key pairs for authentication of NTP packets.
The format of the directive is shown in the example below:
keyfile /etc/chrony.keys
The argument is simply the name of the file containing the ID-key
pairs. The format of the file is shown below:
10 tulip
11 hyacinth
20 MD5 ASCII:crocus
25 SHA1 HEX:1dc764e0791b11fa67efc7ecbc4b0d73f68a070c
...
Each line consists of an ID, name of an authentication hash
function (optional), and a password. The ID can be any unsigned
integer in the range 1 through 2^32-1. The default hash function is
MD5, which is always supported.
If chronyd was built with enabled support for hashing using a
crypto library (nettle, nss, or libtomcrypt), the following
functions are available: MD5, SHA1, SHA256, SHA384, SHA512.
Depending on which library and version is chronyd using, some or
all of the following functions may also be available: SHA3-224,
SHA3-256, SHA3-384, SHA3-512, RMD128, RMD160, RMD256, RMD320,
TIGER, WHIRLPOOL.
The password can be specified as a string of characters not
containing white space with an optional ASCII: prefix, or as a
hexadecimal number with the HEX: prefix. The maximum length of the
line is 2047 characters.
The password is used with the hash function to generate and verify
a message authentication code (MAC) in NTP packets. It is
recommended to use SHA1, or stronger, hash function with random
passwords specified in the hexadecimal format that have at least
128 bits. chronyd will log a warning to syslog on start if a source
is specified in the configuration file with a key that has password
shorter than 80 bits.
The keygen command of chronyc 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 rekey command is issued by chronyc).
lock_all
The lock_all directive will lock chronyd into RAM so that it will
never be paged out. This mode is only supported on Linux. This
directive uses the Linux mlockall() system call to prevent chronyd
from ever being swapped out. This should result in lower and more
consistent latency. It should not have significant impact on
performance as chronyd's memory usage is modest. The mlockall(2)
man page has more details.
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 /var/run/chrony/chronyd.pid. The pidfile directive
allows the name to be changed, e.g.:
pidfile /run/chronyd.pid
sched_priority priority
On Linux, the sched_priority directive will select the SCHED_FIFO
real-time scheduler at the specified priority (which must be
between 0 and 100). On 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. Other systems do not support this
option.
On Linux, this directive uses the sched_setscheduler() system call
to instruct the kernel to use the SCHED_FIFO first-in, first-out
real-time scheduling policy for chronyd with the specified
priority. This means that whenever chronyd is ready to run it will
run, interrupting whatever else is running unless it is a higher
priority real-time process. This should not impact performance as
chronyd resource requirements are modest, but it should result in
lower and more consistent latency since chronyd will not need to
wait for the scheduler to get around to running it. You should not
use this unless you really need it. The sched_setscheduler(2) man
page has more details.
On macOS, this directive uses the thread_policy_set() kernel call
to specify real-time scheduling. As noted for Linux, you should not
use this directive unless you really need it.
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 Solaris chronyd forks into
two processes. The child process retains root privileges, but can
only perform a very limited range of privileged system calls on
behalf of the parent.
The compiled-in default value is chrony.
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:
o Your institution might already operate servers on its network.
Contact your system administrator to find out.
o Your ISP probably has one or more NTP servers available for its
customers.
o 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.
o Use public servers from the pool.ntp.org <http://www.pool.ntp.org/>;
project.
Assuming that your NTP servers are called foo.example.net,
bar.example.net and baz.example.net, your chrony.conf file could
contain as a minimum:
server foo.example.net
server bar.example.net
server baz.example.net
However, you will probably want to include some of the other
directives. The driftfile, makestep and rtcsync might be particularly
useful. Also, the iburst option of the server directive is useful to
speed up the initial synchronisation. The smallest useful configuration
file would look something like:
server foo.example.net iburst
server bar.example.net iburst
server baz.example.net iburst
driftfile /var/lib/chrony/drift
makestep 1.0 3
rtcsync
When using a pool of NTP servers (one name is used for multiple servers
which might change over time), it is better to specify them with the
pool directive instead of multiple server directives. The configuration
file could in this case look like:
pool pool.ntp.org iburst
driftfile /var/lib/chrony/drift
makestep 1.0 3
rtcsync
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 foo.example.net,
bar.example.net and baz.example.net, your chrony.conf file would now
contain:
server foo.example.net offline
server bar.example.net offline
server baz.example.net offline
driftfile /var/lib/chrony/drift
makestep 1.0 3
rtcsync
The 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 online
and 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 /usr/bin/chronyc, the script /etc/ppp/ip-up would include:
/usr/bin/chronyc online
and the script /etc/ppp/ip-down would include:
/usr/bin/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 conectivity to any computer which ultimately derives its
time from a reference clock.
In this situation, one computer is selected to be the master
timeserver. The other computers are either direct clients of the
master, or clients of clients.
The local directive enables a local reference mode, which allows
chronyd to appear synchronised even when it is not.
The rate value in the master's drift file needs to be set to the
average rate at which the master gains or loses time. chronyd includes
support for this, in the form of the manual directive and the settime
command in the chronyc program.
If the master is rebooted, chronyd can re-read the drift rate from the
drift file. However, the master has no accurate estimate of the current
time. To get around this, the system can be configured so that the
master can initially set itself to a `majority-vote' of selected
clients' times; this allows the clients to `flywheel' the master while
it is rebooting.
The smoothtime directive is useful when the clocks of the clients need
to stay close together when the local time is adjusted by the settime
command. The smoothing process needs to be activated by the smoothtime
activate command when the local time is ready to be served. After that
point, any adjustments will be smoothed out.
A typical configuration file for the master (called master) might be
(assuming the clients and the master are in the 192.168.165.x subnet):
initstepslew 1 client1 client3 client6
driftfile /var/lib/chrony/drift
local stratum 8
manual
allow 192.168.165.0/24
smoothtime 400 0.01
rtcsync
For the clients that have to resynchronise the master when it restarts,
the configuration file might be:
server master iburst
driftfile /var/lib/chrony/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 master, 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 master 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 master 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 master1, master2, and master3):
initstepslew 1 master2 master3
server master2
server master3
driftfile /var/lib/chrony/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 directive has been specified).
These pieces of information are also saved if the dump and writertc
commands respectively are issued through chronyc.
When the computer is rebooted, chronyd reads the current RTC time and
the RTC information saved at the last shutdown. This information is
used to set the system clock to the best estimate of what its time
would have been now, had it been left running continuously. The
measurement histories for the servers are then reloaded.
The next time the computer goes online, the previous sessions'
measurements can contribute to the line-fitting process, which gives a
much better estimate of the computer's gain 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 foo.example.net maxdelay 0.4 offline
server bar.example.net maxdelay 0.4 offline
server baz.example.net maxdelay 0.4 offline
logdir /var/log/chrony
log statistics measurements tracking
driftfile /var/lib/chrony/drift
makestep 1.0 3
maxupdateskew 100.0
dumpdir /var/lib/chrony
rtcfile /var/lib/chrony/rtc
pppd is used for connecting to the Internet. This runs two scripts
/etc/ppp/ip-up and /etc/ppp/ip-down when the link goes online and
offline respectively.
The relevant part of the /etc/ppp/ip-up file is:
/usr/bin/chronyc online
and the relevant part of the /etc/ppp/ip-down script is:
/usr/bin/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 pool.ntp.org <http://www.pool.ntp.org/en/join.html>; 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
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 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 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 foo.example.net iburst
server bar.example.net iburst
server baz.example.net iburst
server qux.example.net iburst
makestep 1.0 3
rtcsync
allow
clientloglimit 100000000
leapsectz right/UTC
driftfile /var/lib/chrony/drift
dumpdir /var/run/chrony
SEE ALSO
chronyc(1), chronyd(8)
BUGS
For instructions on how to report bugs, please visit <https://
chrony.tuxfamily.org/>.
AUTHORS
chrony was written by Richard Curnow, Miroslav Lichvar, and others.
chrony 3.4 2018-09-19 CHRONY.CONF(5)
Linux-Distributionen
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