| // This file is part of chrony |
| // |
| // Copyright (C) Richard P. Curnow 1997-2003 |
| // Copyright (C) Stephen Wadeley 2016 |
| // Copyright (C) Bryan Christianson 2017 |
| // Copyright (C) Miroslav Lichvar 2009-2017 |
| // |
| // This program is free software; you can redistribute it and/or modify |
| // it under the terms of version 2 of the GNU General Public License as |
| // published by the Free Software Foundation. |
| // |
| // This program is distributed in the hope that it will be useful, but |
| // WITHOUT ANY WARRANTY; without even the implied warranty of |
| // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| // General Public License for more details. |
| // |
| // You should have received a copy of the GNU General Public License along |
| // with this program; if not, write to the Free Software Foundation, Inc., |
| // 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. |
| |
| = chrony.conf(5) |
| :doctype: manpage |
| :man manual: Configuration Files |
| :man source: chrony @CHRONY_VERSION@ |
| |
| == NAME |
| chrony.conf - chronyd configuration file |
| |
| == SYNOPSIS |
| *chrony.conf* |
| |
| == DESCRIPTION |
| |
| This file configures the *chronyd* daemon. The compiled-in location is |
| _@SYSCONFDIR@/chrony.conf_, 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,*EXAMPLES*>> section for configuration in |
| typical operating scenarios. |
| |
| The configuration file might contain comment lines. A comment line is any line |
| that starts with zero or more spaces followed by any one of the following |
| characters: *!*, *;*, *#*, *%*. Any line with this format will be ignored. |
| |
| == DIRECTIVES |
| |
| === Time sources |
| |
| [[server]]*server* _hostname_ [_option_]...:: |
| The *server* directive specifies an NTP server which can be used as a time |
| source. The client-server relationship is strictly hierarchical: a client might |
| synchronise its system time to that of the server, but the server's system time |
| will never be influenced by that of a client. |
| + |
| The *server* 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 -4 (1/16th 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. |
| *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 0 (1 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 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 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,*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,*minsamples*>> directive. |
| *maxsamples* _samples_::: |
| Set the maximum number of samples kept for this source. This overrides the |
| <<maxsamples,*maxsamples*>> directive. |
| *offline*::: |
| If the server will not be reachable when *chronyd* is started, the *offline* |
| option can be specified. *chronyd* will not try to poll the server until it is |
| enabled to do so (by using the <<chronyc.adoc#online,*online*>> command in |
| *chronyc*). |
| *auto_offline*::: |
| With this option, the server will be assumed to have gone offline when two |
| requests have been sent to it without receiving a response. This option avoids |
| the need to run the <<chronyc.adoc#offline,*offline*>> command from *chronyc* |
| when disconnecting the network link, if it is safe to assume that the requests |
| and responses will not be dropped in the network, e.g. in a trusted local |
| network. (It will still be necessary to use the <<chronyc.adoc#online,*online*>> |
| command when the link has been established, to enable measurements to start.) |
| *prefer*::: |
| Prefer this source over 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,*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]]*pool* _name_ [_option_]...:: |
| The syntax of this directive is similar to that for the <<server,*server*>> |
| directive, except that it is used to specify a pool of NTP servers rather than |
| a single NTP server. The pool name is expected to resolve to multiple addresses |
| which might change over time. |
| + |
| All options valid in the <<server,*server*>> directive can be used in this |
| directive too. There is one option specific to the *pool* directive: |
| *maxsources* 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,*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]]*peer* _hostname_ [_option_]...:: |
| The syntax of this directive is identical to that for the <<server,*server*>> |
| directive, except that it specifies a symmetric association with an NTP peer |
| instead of a client/server association with an NTP server. A single symmetric |
| association allows the peers to be both servers and clients to each other. This |
| is mainly useful when the NTP implementation of the peer (e.g. *ntpd*) supports |
| ephemeral symmetric associations and does not need to be configured with an |
| address of this host. *chronyd* does not support ephemeral associations. |
| + |
| When a key is specified by the *key* option to enable authentication, both |
| peers must use the same key and the same key number. |
| + |
| Note that the symmetric mode is less secure than the client/server mode. A |
| denial-of-service attack is possible on unauthenticated symmetric associations, |
| i.e. when the peer was specified without the *key* option. An attacker who does |
| not see network traffic between two hosts, but knows that they are peering with |
| each other, can periodically send them unauthenticated packets with spoofed |
| source addresses in order to disrupt their NTP state and prevent them from |
| synchronising to each other. When the association is authenticated, an attacker |
| who does see the network traffic, but cannot prevent the packets from reaching |
| the other host, can still disrupt the state by replaying old packets. The |
| attacker has effectively the same power as a man-in-the-middle attacker. A |
| partial protection against this attack is implemented in *chronyd*, which can |
| protect the peers if they are using the same polling interval and they never |
| sent an authenticated packet with a timestamp from future, but it should not be |
| relied on as it is difficult to ensure the conditions are met. If two hosts |
| should be able to synchronise to each other in both directions, it is |
| recommended to use two separate client/server associations (specified by the |
| <<server,*server*>> directive on both hosts) instead. |
| |
| [[initstepslew]]*initstepslew* _step-threshold_ [_hostname_]...:: |
| 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,*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,*makestep*>> and <<server,*server*>> directives with the *iburst* |
| option. The main difference is that the *initstepslew* servers are used only |
| before normal operation begins and that the foreground *chronyd* process waits |
| for *initstepslew* to finish before exiting. This is useful to prevent programs |
| started in the boot sequence after *chronyd* from reading the clock before it |
| has been stepped. |
| |
| [[refclock]]*refclock* _driver_ _parameter_[:__option__,...] [_option_]...:: |
| The *refclock* directive specifies a hardware reference clock to be used as a |
| time source. It has two mandatory parameters, a driver name and a |
| driver-specific parameter. The two parameters are followed by zero or more |
| refclock options. Some drivers have special options, which can be appended to |
| the driver-specific parameter (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,*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,*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,*minsamples*>> directive. |
| *maxsamples* _samples_::: |
| Set the maximum number of samples kept for this source. This overrides the |
| <<maxsamples,*maxsamples*>> directive. |
| |
| [[manual]]*manual*:: |
| The *manual* directive enables support at run-time for the |
| <<chronyc.adoc#settime,*settime*>> command in *chronyc*. If no *manual* |
| directive is included, any attempt to use the *settime* command in *chronyc* |
| will be met with an error message. |
| + |
| Note that the *settime* command can be enabled at run-time using |
| the <<chronyc.adoc#manual,*manual*>> command in *chronyc*. (The idea of the two |
| commands is that the *manual* command controls the manual clock driver's |
| behaviour, whereas the *settime* command allows samples of manually entered |
| time to be provided.) |
| |
| [[acquisitionport]]*acquisitionport* _port_:: |
| By default, *chronyd* 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,*port*>> directive) to use only one socket for all |
| NTP packets. |
| + |
| An example of the *acquisitionport* directive is: |
| + |
| ---- |
| acquisitionport 1123 |
| ---- |
| + |
| This would change the source port used for client requests to UDP port 1123. |
| You could then persuade the firewall administrator to open that port. |
| |
| [[bindacqaddress]]*bindacqaddress* _address_:: |
| The *bindacqaddress* directive sets the network interface to which |
| *chronyd* will bind its NTP client sockets. The syntax is similar to the |
| <<bindaddress,*bindaddress*>> and <<bindcmdaddress,*bindcmdaddress*>> |
| directives. |
| + |
| For each of the IPv4 and IPv6 protocols, only one *bindacqaddress* directive |
| can be specified. |
| |
| [[dumpdir]]*dumpdir* _directory_:: |
| To compute the rate of gain or loss of time, *chronyd* has to store a |
| measurement history for each of the time sources it uses. |
| + |
| All supported systems, with the exception of macOS 10.12 and earlier, have |
| operating system support for setting the rate of gain or loss to compensate for |
| known errors. |
| (On macOS 10.12 and earlier, *chronyd* must simulate such a capability by |
| periodically slewing the system clock forwards or backwards by a suitable amount |
| to compensate for the error built up since the previous slew.) |
| + |
| For such systems, it is possible to save the measurement history across |
| restarts of *chronyd* (assuming no changes are made to the system clock |
| behaviour whilst it is not running). The *dumpdir* directive defines the |
| directory where the measurement histories are saved when *chronyd* exits, |
| or the <<chronyc.adoc#dump,*dump*>> command in *chronyc* is issued. |
| + |
| An example of the directive is: |
| + |
| ---- |
| dumpdir @CHRONYRUNDIR@ |
| ---- |
| + |
| A source whose IP address is _1.2.3.4_ would have its measurement history saved |
| in the file _@CHRONYRUNDIR@/1.2.3.4.dat_. History of reference clocks is saved |
| to files named by their reference ID in form of _refid:XXXXXXXX.dat_. |
| |
| [[maxsamples]]*maxsamples* _samples_:: |
| The *maxsamples* directive sets the default maximum number of samples that |
| *chronyd* should keep for each source. This setting can be overridden for |
| individual sources in the <<server,*server*>> and <<refclock,*refclock*>> |
| directives. The default value is 0, which disables the configurable limit. The |
| useful range is 4 to 64. |
| |
| [[minsamples]]*minsamples* _samples_:: |
| The *minsamples* directive sets the default minimum number of samples that |
| *chronyd* should keep for each source. This setting can be overridden for |
| individual sources in the <<server,*server*>> and <<refclock,*refclock*>> |
| directives. The default value is 6. The useful range is 4 to 64. |
| |
| === Source selection |
| |
| [[combinelimit]]*combinelimit* _limit_:: |
| When *chronyd* has multiple sources available for synchronisation, it has to |
| select one source as the synchronisation source. The measured offsets and |
| frequencies of the system clock relative to the other sources, however, can be |
| combined with the selected source to improve the accuracy of the system clock. |
| + |
| The *combinelimit* directive limits which sources are included in the combining |
| algorithm. Their synchronisation distance has to be shorter than the distance |
| of the selected source multiplied by the value of the limit. Also, their |
| measured frequencies have to be close to the frequency of the selected source. |
| + |
| By default, the limit is 3. Setting the limit to 0 effectively disables the |
| source combining algorithm and only the selected source will be used to control |
| the system clock. |
| |
| [[maxdistance]]*maxdistance* _distance_:: |
| The *maxdistance* directive sets the maximum 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]]*maxjitter* _jitter_:: |
| The *maxjitter* directive sets the maximum allowed jitter of the sources to not |
| be rejected by the source selection algorithm. This prevents synchronisation |
| with sources that have a small root distance, but their time is too variable. |
| + |
| By default, the maximum jitter is 1 second. |
| |
| [[minsources]]*minsources* _sources_:: |
| The *minsources* directive sets the minimum number of sources that need to be |
| considered as selectable in the source selection algorithm before the local |
| clock is updated. The default value is 1. |
| + |
| Setting this option to a larger number can be used to improve the reliability. |
| More sources will have to agree with each other and the clock will not be |
| updated when only one source (which could be serving incorrect time) is |
| reachable. |
| |
| [[reselectdist]]*reselectdist* _distance_:: |
| When *chronyd* selects a synchronisation source from available sources, it |
| will prefer the one with the shortest synchronisation distance. However, to |
| avoid frequent reselecting when there are sources with similar distance, a |
| fixed distance is added to the distance for sources that are currently not |
| selected. This can be set with the *reselectdist* directive. By default, the |
| distance is 100 microseconds. |
| |
| [[stratumweight]]*stratumweight* _distance_:: |
| The *stratumweight* directive sets how much distance should be added per |
| stratum to the synchronisation distance when *chronyd* selects the |
| synchronisation source from available sources. |
| + |
| By default, the weight is 0.001 seconds. This means that the stratum of the sources |
| in the selection process matters only when the differences between the |
| distances are in milliseconds. |
| |
| === System clock |
| |
| [[corrtimeratio]]*corrtimeratio* _ratio_:: |
| When *chronyd* is slewing the system clock to correct an offset, the rate at |
| which it is slewing adds to the frequency error of the clock. On all supported |
| systems, with the exception of macOS 12 and earlier, this rate can be |
| controlled. |
| + |
| The *corrtimeratio* directive sets the ratio between the duration in which the |
| clock is slewed for an average correction according to the source history and |
| the interval in which the corrections are done (usually the NTP polling |
| interval). Corrections larger than the average take less time and smaller |
| corrections take more time, the amount of the correction and the correction |
| time are inversely proportional. |
| + |
| Increasing *corrtimeratio* improves the overall frequency error of the system |
| clock, but increases the overall time error as the corrections take longer. |
| + |
| By default, the ratio is set to 3, the time accuracy of the clock is preferred |
| over its frequency accuracy. |
| + |
| The maximum allowed slew rate can be set by the <<maxslewrate,*maxslewrate*>> |
| directive. The current remaining correction is shown in the |
| <<chronyc.adoc#tracking,*tracking*>> report as the *System time* value. |
| |
| [[driftfile]]*driftfile* _file_:: |
| One of the main activities of the *chronyd* program is to work out the rate at |
| which the system clock gains or loses time relative to real time. |
| + |
| Whenever *chronyd* computes a new value of the gain or loss rate, it is desirable |
| to record it somewhere. This allows *chronyd* to begin compensating the system |
| clock at that rate whenever it is restarted, even before it has had a chance to |
| obtain an equally good estimate of the rate during the new run. (This process |
| can take many minutes, at least.) |
| + |
| The *driftfile* directive allows a file to be specified into which *chronyd* |
| can store the rate information. Two parameters are recorded in the file. The |
| first is the rate at which the system clock gains or loses time, expressed in |
| parts per million, with gains positive. Therefore, a value of 100.0 indicates |
| that when the system clock has advanced by a second, it has gained 100 |
| microseconds in reality (so the true time has only advanced by 999900 |
| microseconds). The second is an estimate of the error bound around the first |
| value in which the true rate actually lies. |
| + |
| An example of the driftfile directive is: |
| + |
| ---- |
| driftfile @CHRONYVARDIR@/drift |
| ---- |
| |
| [[fallbackdrift]]*fallbackdrift* _min-interval_ _max-interval_:: |
| Fallback drifts are long-term averages of the system clock drift calculated |
| over exponentially increasing intervals. They are used to avoid quickly |
| drifting away from true time when the clock was not updated for a longer period |
| of time and there was a short-term deviation in the drift before the updates |
| stopped. |
| + |
| The directive specifies the minimum and maximum interval since the last clock |
| update to switch between fallback drifts. They are defined as a power of 2 (in |
| seconds). The syntax is as follows: |
| + |
| ---- |
| fallbackdrift 16 19 |
| ---- |
| + |
| In this example, the minimum interval is 16 (18 hours) and the maximum interval is |
| 19 (6 days). The system clock frequency will be set to the first fallback 18 |
| hours after last clock update, to the second after 36 hours, and so on. This |
| might be a good setting to cover frequency changes due to daily and weekly |
| temperature fluctuations. When the frequency is set to a fallback, the state of |
| the clock will change to '`Not synchronised`'. |
| + |
| By default (or if the specified maximum or minimum is 0), no fallbacks are used |
| and the clock frequency changes only with new measurements from NTP sources, |
| reference clocks, or manual input. |
| |
| [[leapsecmode]]*leapsecmode* _mode_:: |
| A leap second is an adjustment that is occasionally applied to UTC to keep it |
| close to the mean solar time. When a leap second is inserted, the last day of |
| June or December has an extra second 23:59:60. |
| + |
| For computer clocks that is a problem. The Unix time is defined as number of |
| seconds since 00:00:00 UTC on 1 January 1970 without leap seconds. The system |
| clock cannot have time 23:59:60, every minute has 60 seconds and every day has |
| 86400 seconds by definition. The inserted leap second is skipped and the clock |
| is suddenly ahead of UTC by one second. The *leapsecmode* directive selects how |
| that error is corrected. There are four options: |
| + |
| *system*::: |
| When inserting a leap second, the kernel steps the system clock backwards by |
| one second when the clock gets to 00:00:00 UTC. When deleting a leap second, it |
| steps forward by one second when the clock gets to 23:59:59 UTC. This is the |
| default mode when the system driver supports leap seconds (i.e. all supported |
| systems with the exception of macOS 12 and earlier). |
| *step*::: |
| This is similar to the *system* mode, except the clock is stepped by |
| *chronyd* instead of the kernel. It can be useful to avoid bugs in the kernel |
| code that would be executed in the *system* mode. This is the default mode |
| when the system driver does not support leap seconds. |
| *slew*::: |
| The clock is corrected by slewing started at 00:00:00 UTC when a leap second |
| is inserted or 23:59:59 UTC when a leap second is deleted. This might be |
| preferred over the *system* and *step* modes when applications running on the |
| system are sensitive to jumps in the system time and it is acceptable that the |
| clock will be off for a longer time. On Linux with the default |
| <<maxslewrate,*maxslewrate*>> value the correction takes 12 seconds. |
| *ignore*::: |
| No correction is applied to the clock for the leap second. The clock will be |
| corrected later in normal operation when new measurements are made and the |
| estimated offset includes the one second error. |
| :: |
| + |
| When serving time to NTP clients that cannot be configured to correct their |
| clocks for a leap second by slewing, or to clients that would correct at |
| slightly different rates when it is necessary to keep them close together, the |
| *slew* mode can be combined with the <<smoothtime,*smoothtime*>> directive to |
| enable a server leap smear. |
| + |
| When smearing a leap second, the leap status is suppressed on the server and |
| the served time is corrected slowly 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]]*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,*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]]*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,*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]]*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]]*maxclockerror* _error-in-ppm_:: |
| The *maxclockerror* directive sets the maximum assumed frequency error that the |
| system clock can gain on its own between clock updates. It describes the |
| stability of the clock. |
| + |
| By default, the maximum error is 1 ppm. |
| + |
| Typical values for _error-in-ppm_ might be 10 for a low quality clock and 0.1 |
| for a high quality clock using a temperature compensated crystal oscillator. |
| |
| [[maxdrift]]*maxdrift* _drift-in-ppm_:: |
| This directive specifies the maximum assumed drift (frequency error) of the |
| system clock. It limits the frequency adjustment that *chronyd* is allowed to |
| use to correct the measured drift. It is an additional limit to the maximum |
| adjustment that can be set by the system driver (100000 ppm on Linux, 500 ppm |
| on FreeBSD, NetBSD, and macOS 10.13+, 32500 ppm on 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]]*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]]*maxslewrate* _rate-in-ppm_:: |
| The *maxslewrate* directive sets the maximum rate at which *chronyd* is allowed |
| to slew the time. It limits the slew rate controlled by the correction time |
| ratio (which can be set by the <<corrtimeratio,*corrtimeratio*>> directive) and |
| is effective only on systems where *chronyd* is able to control the rate (i.e. |
| all supported systems with the exception of macOS 12 or earlier). |
| + |
| For each system there is a maximum frequency offset of the clock that can be set |
| by the driver. On Linux it is 100000 ppm, on FreeBSD, NetBSD and macOS 10.13+ it |
| is 5000 ppm, and on 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]] |
| *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,*log tempcomp*>> directive. |
| |
| === NTP server |
| |
| [[allow]]*allow* [*all*] [_subnet_]:: |
| The *allow* directive is used to designate a particular subnet from which NTP |
| clients are allowed to access the computer as an NTP server. |
| + |
| 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 |
| <<chronyc.adoc#allow,*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,*initstepslew*>> directive is used in the |
| configuration file, each of the computers listed in that directive must allow |
| client access by this computer for it to work. |
| |
| [[deny]]*deny* [*all*] [_subnet_]:: |
| This is similar to the <<allow,*allow*>> directive, except that it denies NTP |
| 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]]*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]]*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,*allow*>> |
| directive. |
| |
| [[clientloglimit]]*clientloglimit* _limit_:: |
| This directive specifies the maximum amount of memory that *chronyd* is allowed |
| to allocate for logging of client accesses and the state that *chronyd* as an |
| NTP server needs to support the interleaved mode for its clients. The default |
| limit is 524288 bytes, which 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]]*noclientlog*:: |
| This directive, which takes no arguments, specifies that client accesses are |
| not to be logged. Normally they are logged, allowing statistics to be reported |
| using the <<chronyc.adoc#clients,*clients*>> command in *chronyc*. This option |
| also effectively disables server support for the NTP interleaved mode. |
| |
| [[local]]*local* [_option_]...:: |
| The *local* directive enables a local reference mode, which allows *chronyd* |
| operating as an NTP server to appear synchronised to real time (from the |
| viewpoint of clients polling it), even when it was never synchronised or |
| the last update of the clock happened a long time ago. |
| + |
| This directive is normally used in an isolated network, where computers are |
| required to be synchronised to one another, but not necessarily to real time. |
| The server can be kept vaguely in line with real time by manual input. |
| + |
| The *local* directive has the following options: |
| + |
| *stratum* _stratum_::: |
| This option sets the stratum of the server which will be reported to clients |
| when the local reference is active. The specified value is in the range 1 |
| through 15, and the default value is 10. It should be larger than the maximum |
| expected stratum in the network when external NTP servers are accessible. |
| + |
| Stratum 1 indicates a computer that has a true real-time reference directly |
| connected to it (e.g. GPS, atomic clock, etc.), such computers are expected to |
| be very close to real time. Stratum 2 computers are those which have a stratum |
| 1 server; stratum 3 computers have a stratum 2 server and so on. A value |
| of 10 indicates that the clock is so many hops away from a reference clock that |
| its time is fairly unreliable. |
| *distance* _distance_::: |
| This option sets the threshold for the root distance which will activate the local |
| reference. If *chronyd* was synchronised to some source, the local reference |
| will not be activated until its root distance reaches the specified value (the |
| rate at which the distance is increasing depends on how well the clock was |
| tracking the source). The default value is 1 second. |
| + |
| The current root distance can be calculated from root delay and root dispersion |
| (reported by the <<chronyc.adoc#tracking,*tracking*>> command in *chronyc*) as: |
| + |
| ---- |
| distance = delay / 2 + dispersion |
| ---- |
| *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]]*ntpsigndsocket* _directory_:: |
| This directive specifies the location of the Samba *ntp_signd* socket when it |
| is running as a Domain Controller (DC). If *chronyd* is compiled with this |
| feature, responses to MS-SNTP clients will be signed by the *smbd* daemon. |
| + |
| Note that MS-SNTP requests are not authenticated and any client that is allowed |
| to access the server by the <<allow,*allow*>> directive, or the |
| <<chronyc.adoc#allow,*allow*>> command in *chronyc*, can get an MS-SNTP |
| response signed with a trust account's password and try to crack the password |
| in a brute-force attack. Access to the server should be carefully controlled. |
| + |
| An example of the directive is: |
| + |
| ---- |
| ntpsigndsocket /var/lib/samba/ntp_signd |
| ---- |
| |
| [[port]]*port* _port_:: |
| This option allows you to configure the port on which *chronyd* will listen for |
| NTP requests. The port will be open only when an address is allowed by the |
| <<allow,*allow*>> directive or the <<chronyc.adoc#allow,*allow*>> command in |
| *chronyc*, an NTP peer is configured, or the broadcast server mode is enabled. |
| + |
| The default value is 123, the standard NTP port. If set to 0, *chronyd* will |
| never open the server port and will operate strictly in a client-only mode. The |
| source port used in NTP client requests can be set by the |
| <<acquisitionport,*acquisitionport*>> directive. |
| |
| [[ratelimit]]*ratelimit* [_option_]...:: |
| This directive enables response rate limiting for NTP packets. Its purpose is |
| to reduce network traffic with misconfigured or broken NTP clients that are |
| polling the server too frequently. The limits are applied to individual IP |
| addresses. If multiple clients share one IP address (e.g. multiple hosts behind |
| NAT), the sum of their traffic will be limited. If a client that increases its |
| polling rate when it does not receive a reply is detected, its rate limiting |
| will be temporarily suspended to avoid increasing the overall amount of |
| traffic. The maximum number of IP addresses which can be monitored at the same |
| time depends on the memory limit set by the <<clientloglimit,*clientloglimit*>> |
| directive. |
| + |
| The *ratelimit* directive supports a number of options (which can be defined |
| in any order): |
| + |
| *interval*::: |
| 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]]*smoothtime* _max-freq_ _max-wander_ [*leaponly*]:: |
| The *smoothtime* directive can be used to enable smoothing of the time that |
| *chronyd* serves to its clients to make it easier for them to track it and keep |
| their clocks close together even when large offset or frequency corrections are |
| applied to the server's clock, for example after being offline for a longer |
| time. |
| + |
| BE WARNED: The server is intentionally not serving its best estimate of the |
| true time. If a large offset has been accumulated, it can take a very long time |
| to smooth it out. This directive should be used only when the clients are not |
| configured to also poll another NTP server, because they could reject this |
| server as a falseticker or fail to select a source completely. |
| + |
| The smoothing process is implemented with a quadratic spline function with two |
| or three pieces. It is independent from any slewing applied to the local system |
| clock, but the accumulated offset and frequency will be reset when the clock is |
| corrected by stepping, e.g. by the <<makestep,*makestep*>> directive or the |
| <<chronyc.adoc#makestep,*makestep*>> command in *chronyc*. The process can be |
| reset without stepping the clock by the <<chronyc.adoc#smoothtime,*smoothtime |
| reset*>> command. |
| + |
| The first two arguments of the directive are the maximum frequency offset of |
| the smoothed time to the tracked NTP time (in ppm) and the maximum rate at |
| which the frequency offset is allowed to change (in ppm per second). *leaponly* |
| is an optional third argument which enables a mode where only leap seconds are |
| smoothed out and normal offset and frequency changes are ignored. The *leaponly* |
| option is useful in a combination with the <<leapsecmode,*leapsecmode slew*>> |
| directive to allow the clients to use multiple time smoothing servers safely. |
| + |
| The smoothing process is activated automatically when 1/10000 of the estimated |
| skew of the local clock falls below the maximum rate of frequency change. It |
| can be also activated manually by the <<chronyc.adoc#smoothtime,*smoothtime |
| activate*>> command, which is particularly useful when the clock is |
| synchronised only with manual input and the skew is always larger than the |
| threshold. The <<chronyc.adoc#smoothing,*smoothing*>> command can be used to |
| monitor the process. |
| + |
| An example suitable for clients using *ntpd* and 1024 second polling interval |
| could be: |
| + |
| ---- |
| smoothtime 400 0.001 |
| ---- |
| + |
| An example suitable for clients using *chronyd* on Linux could be: |
| + |
| ---- |
| smoothtime 50000 0.01 |
| ---- |
| |
| === Command and monitoring access |
| |
| [[bindcmdaddress]]*bindcmdaddress* _address_:: |
| The *bindcmdaddress* directive 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 _@CHRONYRUNDIR@/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]]*cmdallow* [*all*] [_subnet_]:: |
| This is similar to the <<allow,*allow*>> directive, except that it allows |
| monitoring access (rather than NTP client access) to a particular subnet or |
| host. (By '`monitoring access`' is meant that *chronyc* can be run on those |
| hosts and retrieve monitoring data from *chronyd* on this computer.) |
| + |
| The syntax is identical to the *allow* directive. |
| + |
| There is also a *cmdallow all* directive with similar behaviour to the *allow |
| all* directive (but applying to monitoring access in this case, of course). |
| + |
| Note that *chronyd* has to be configured with the |
| <<bindcmdaddress,*bindcmdaddress*>> directive to not listen only on the |
| loopback interface to actually allow remote access. |
| |
| [[cmddeny]]*cmddeny* [*all*] [_subnet_]:: |
| This is similar to the <<cmdallow,*cmdallow*>> directive, except that it denies |
| monitoring access to a particular subnet or host, rather than allowing it. |
| + |
| The syntax is identical. |
| + |
| There is also a *cmddeny all* directive with similar behaviour to the *cmdallow |
| all* directive. |
| |
| [[cmdport]]*cmdport* _port_:: |
| The *cmdport* directive allows the port that is used for run-time monitoring |
| (via the *chronyc* program) to be altered from its default (323). If set to 0, |
| *chronyd* will not open the port, 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]]*cmdratelimit* [_option_]...:: |
| This directive enables response rate limiting for command packets. It is |
| similar to the <<ratelimit,*ratelimit*>> directive, except responses to |
| localhost are never limited 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]]*hwclockfile* _file_:: |
| The *hwclockfile* directive sets the location of the adjtime file which is |
| used by the *hwclock* program on Linux. *chronyd* parses the file to find out |
| if the RTC keeps local time or UTC. It overrides the <<rtconutc,*rtconutc*>> |
| directive. |
| + |
| The compiled-in default value is '_@DEFAULT_HWCLOCK_FILE@_'. |
| + |
| An example of the directive is: |
| + |
| ---- |
| hwclockfile /etc/adjtime |
| ---- |
| |
| [[rtcautotrim]]*rtcautotrim* _threshold_:: |
| The *rtcautotrim* directive is used to keep the RTC close to the system clock |
| automatically. When the system clock is synchronised and the estimated error |
| between the two clocks is larger than the specified threshold, *chronyd* will |
| trim the RTC as if the <<chronyc.adoc#trimrtc,*trimrtc*>> command in *chronyc* |
| was issued. |
| + |
| This directive is effective only with the <<rtcfile,*rtcfile*>> directive. |
| + |
| An example of the use of this directive is: |
| + |
| ---- |
| rtcautotrim 30 |
| ---- |
| + |
| This would set the threshold error to 30 seconds. |
| |
| [[rtcdevice]]*rtcdevice* _device_:: |
| The *rtcdevice* directive sets the path to the device file for accessing the |
| RTC. The default path is _@DEFAULT_RTC_DEVICE@_. |
| |
| [[rtcfile]]*rtcfile* _file_:: |
| The *rtcfile* directive defines the name of the file in which *chronyd* can |
| save parameters associated with tracking the accuracy of the RTC. |
| + |
| An example of the directive is: |
| + |
| ---- |
| rtcfile @CHRONYVARDIR@/rtc |
| ---- |
| + |
| *chronyd* saves information in this file when it exits and when the *writertc* |
| command is issued in *chronyc*. The information saved is the RTC's error at |
| some epoch, that epoch (in seconds since January 1 1970), and the rate at which |
| the RTC gains or loses time. |
| + |
| So far, the support for real-time clocks is limited; their code is even more |
| system-specific than the rest of the software. You can only use the RTC |
| facilities (the <<rtcfile,*rtcfile*>> directive and the *-s* command-line |
| option to *chronyd*) if the following three conditions apply: |
| + |
| . You are running Linux. |
| . The kernel is compiled with extended real-time clock support (i.e. the |
| _/dev/rtc_ device is capable of doing useful things). |
| . You do not have other applications that need to make use of _/dev/rtc_ at all. |
| |
| [[rtconutc]]*rtconutc*:: |
| *chronyd* assumes by default that the RTC keeps local time (including any |
| daylight saving changes). This is convenient on PCs running Linux which are |
| dual-booted with Windows. |
| + |
| If you keep the RTC on local time and your computer is off when daylight saving |
| (summer time) starts or ends, the computer's system time will be one hour in |
| error when you next boot and start chronyd. |
| + |
| An alternative is for the RTC to keep Universal Coordinated Time (UTC). This |
| does not suffer from the 1 hour problem when daylight saving starts or ends. |
| + |
| If the *rtconutc* directive appears, it means the RTC is required to keep UTC. |
| The directive takes no arguments. It is equivalent to specifying the *-u* |
| switch to the Linux *hwclock* program. |
| + |
| Note that this setting is overridden when the <<hwclockfile,*hwclockfile*>> |
| directive is specified. |
| |
| [[rtcsync]]*rtcsync*:: |
| The *rtcsync* directive enables a mode where the system time is periodically |
| copied to the RTC and *chronyd* does not try to track its drift. This directive |
| cannot be used with the <<rtcfile,*rtcfile*>> directive. |
| + |
| On Linux, the RTC copy is performed by the kernel every 11 minutes. |
| + |
| On macOS, <<chronyd,*chronyd*>> will perform the RTC copy every 60 minutes |
| when the system clock is in a synchronised state. |
| + |
| On other systems this directive does nothing. |
| |
| === Logging |
| |
| [[log]]*log* [_option_]...:: |
| The *log* directive indicates that certain information is to be logged. |
| The log files are written to the directory specified by the <<logdir,*logdir*>> |
| directive. A banner is periodically written to the files to indicate the |
| meanings of the columns. |
| + |
| *rawmeasurements*::: |
| This option logs the raw NTP measurements and related information to a file |
| called _measurements.log_. An entry is made for each packet received from the |
| source. This can be useful when debugging a problem. An example line (which |
| actually appears as a single line in the file) from the log file is shown |
| below. |
| + |
| ---- |
| 2016-11-09 05:40:50 203.0.113.15 N 2 111 111 1111 10 10 1.0 \ |
| -4.966e-03 2.296e-01 1.577e-05 1.615e-01 7.446e-03 CB00717B 4B D K |
| ---- |
| + |
| The columns are as follows (the quantities in square brackets are the values |
| from the example line above): |
| + |
| . Date [2015-10-13] |
| . Hour:Minute:Second. Note that the date-time pair is expressed in UTC, not the |
| local time zone. [05:40:50] |
| . IP address of server or peer from which measurement came [203.0.113.15] |
| . Leap status (_N_ means normal, _+_ means that the last minute of the current |
| month has 61 seconds, _-_ means that the last minute of the month has 59 |
| seconds, _?_ means the remote computer is not currently synchronised.) [N] |
| . Stratum of remote computer. [2] |
| . RFC 5905 tests 1 through 3 (1=pass, 0=fail) [111] |
| . RFC 5905 tests 5 through 7 (1=pass, 0=fail) [111] |
| . 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] |
| . Local poll [10] |
| . Remote poll [10] |
| . '`Score`' (an internal score within each polling level used to decide when to |
| increase or decrease the polling level. This is adjusted based on number of |
| measurements currently being used for the regression algorithm). [1.0] |
| . The estimated local clock error (_theta_ in RFC 5905). Positive indicates |
| that the local clock is slow of the remote source. [-4.966e-03] |
| . The peer delay (_delta_ in RFC 5905). [2.296e-01] |
| . The peer dispersion (_epsilon_ in RFC 5905). [1.577e-05] |
| . The root delay (_DELTA_ in RFC 5905). [1.615e-01] |
| . The root dispersion (_EPSILON_ in RFC 5905). [7.446e-03] |
| . Reference ID of the server's source as a hexadecimal number. [CB00717B] |
| . NTP mode of the received packet (_1_=active peer, _2_=passive peer, |
| _4_=server, _B_=basic, _I_=interleaved). [4B] |
| . Source of the local transmit timestamp |
| (_D_=daemon, _K_=kernel, _H_=hardware). [D] |
| . Source of the local receive timestamp |
| (_D_=daemon, _K_=kernel, _H_=hardware). [K] |
| + |
| *measurements*::: |
| This option is identical to the *rawmeasurements* option, except it logs only |
| valid measurements from synchronised sources, i.e. measurements which passed |
| the RFC 5905 tests 1 through 7. This can be useful for producing graphs of the |
| source's performance. |
| + |
| *statistics*::: |
| This option logs information about the regression processing to a file called |
| _statistics.log_. An example line (which actually appears as a single line in |
| the file) from the log file is shown below. |
| + |
| ---- |
| 2016-08-10 05:40:50 203.0.113.15 6.261e-03 -3.247e-03 \ |
| 2.220e-03 1.874e-06 1.080e-06 7.8e-02 16 0 8 0.00 |
| ---- |
| + |
| The columns are as follows (the quantities in square brackets are the values |
| from the example line above): |
| + |
| . Date [2015-07-22] |
| . Hour:Minute:Second. Note that the date-time pair is expressed in |
| UTC, not the local time zone. [05:40:50] |
| . IP address of server or peer from which measurement comes [203.0.113.15] |
| . The estimated standard deviation of the measurements from the source (in |
| seconds). [6.261e-03] |
| . The estimated offset of the source (in seconds, positive means the local |
| clock is estimated to be fast, in this case). [-3.247e-03] |
| . The estimated standard deviation of the offset estimate (in seconds). |
| [2.220e-03] |
| . The estimated rate at which the local clock is gaining or losing time |
| relative to the source (in seconds per second, positive means the local clock |
| is gaining). This is relative to the compensation currently being applied to |
| the local clock, _not_ to the local clock without any compensation. |
| [1.874e-06] |
| . The estimated error in the rate value (in seconds per second). [1.080e-06]. |
| . The ratio of |old_rate - new_rate| / old_rate_error. Large values |
| indicate the statistics are not modelling the source very well. [7.8e-02] |
| . The number of measurements currently being used for the regression |
| algorithm. [16] |
| . The new starting index (the oldest sample has index 0; this is the method |
| used to prune old samples when it no longer looks like the measurements fit a |
| linear model). [0, i.e. no samples discarded this time] |
| . The number of runs. The number of runs of regression residuals with the same |
| sign is computed. If this is too small it indicates that the measurements are |
| no longer represented well by a linear model and that some older samples need |
| to be discarded. The number of runs for the data that is being retained is |
| tabulated. Values of approximately half the number of samples are expected. |
| [8] |
| . The estimated or configured asymmetry of network jitter on the path to the |
| source which was used to correct the measured offsets. The asymmetry can be |
| between -0.5 and +0.5. A negative value means the delay of packets sent to |
| the source is more variable than the delay of packets sent from the source |
| back. [0.00, i.e. no correction for asymmetry] |
| + |
| *tracking*::: |
| This option logs changes to the estimate of the system's gain or loss rate, and |
| any slews made, to a file called _tracking.log_. An example line (which |
| actually appears as a single line in the file) from the log file is shown |
| below. |
| + |
| ---- |
| 2017-08-22 13:22:36 203.0.113.15 2 -3.541 0.075 -8.621e-06 N \ |
| 2 2.940e-03 -2.084e-04 1.534e-02 3.472e-04 8.304e-03 |
| ---- |
| + |
| The columns are as follows (the quantities in square brackets are the |
| values from the example line above) : |
| + |
| . Date [2017-08-22] |
| . Hour:Minute:Second. Note that the date-time pair is expressed in UTC, not the |
| local time zone. [13:22:36] |
| . The IP address of the server or peer to which the local system is synchronised. |
| [203.0.113.15] |
| . The stratum of the local system. [2] |
| . The local system frequency (in ppm, positive means the local system runs fast |
| of UTC). [-3.541] |
| . The error bounds on the frequency (in ppm). [0.075] |
| . The estimated local offset at the epoch, which is normally corrected by |
| slewing the local clock (in seconds, positive indicates the clock is fast of |
| UTC). [-8.621e-06] |
| . Leap status (_N_ means normal, _+_ means that the last minute of this month |
| has 61 seconds, _-_ means that the last minute of the month has 59 seconds, |
| _?_ means the clock is not currently synchronised.) [N] |
| . The number of combined sources. [2] |
| . The estimated standard deviation of the combined offset (in seconds). |
| [2.940e-03] |
| . The remaining offset correction from the previous update (in seconds, |
| positive means the system clock is slow of UTC). [-2.084e-04] |
| . The total of the network path delays to the reference clock to which |
| the local clock is ultimately synchronised (in seconds). [1.534e-02] |
| . The total dispersion accumulated through all the servers back to the |
| reference clock to which the local clock is ultimately synchronised |
| (in seconds). [3.472e-04] |
| . The maximum estimated error of the system clock in the interval since the |
| previous update (in seconds). It includes the offset, remaining offset |
| correction, root delay, and dispersion from the previous update with the |
| dispersion which accumulated in the interval. [8.304e-03] |
| + |
| *rtc*::: |
| This option logs information about the system's real-time clock. An example |
| line (which actually appears as a single line in the file) from the _rtc.log_ |
| file is shown below. |
| + |
| ---- |
| 2015-07-22 05:40:50 -0.037360 1 -0.037434\ |
| -37.948 12 5 120 |
| ---- |
| + |
| The columns are as follows (the quantities in square brackets are the |
| values from the example line above): |
| + |
| . Date [2015-07-22] |
| . Hour:Minute:Second. Note that the date-time pair is expressed in UTC, not the |
| local time zone. [05:40:50] |
| . The measured offset between the RTC and the system clock in seconds. |
| Positive indicates that the RTC is fast of the system time [-0.037360]. |
| . Flag indicating whether the regression has produced valid coefficients. |
| (1 for yes, 0 for no). [1] |
| . Offset at the current time predicted by the regression process. A large |
| difference between this value and the measured offset tends to indicate that |
| the measurement is an outlier with a serious measurement error. [-0.037434] |
| . The rate at which the RTC is losing or gaining time relative to the system |
| clock. In ppm, with positive indicating that the RTC is gaining time. |
| [-37.948] |
| . The number of measurements used in the regression. [12] |
| . The number of runs of regression residuals of the same sign. Low values |
| indicate that a straight line is no longer a good model of the measured data |
| and that older measurements should be discarded. [5] |
| . The measurement interval used prior to the measurement being made (in |
| seconds). [120] |
| + |
| *refclocks*::: |
| This option logs the raw and filtered reference clock measurements to a file |
| called _refclocks.log_. An example line (which actually appears as a single |
| line in the file) from the log file is shown below. |
| + |
| ---- |
| 2009-11-30 14:33:27.000000 PPS2 7 N 1 4.900000e-07 -6.741777e-07 1.000e-06 |
| ---- |
| + |
| The columns are as follows (the quantities in square brackets are the values |
| from the example line above): |
| + |
| . Date [2009-11-30] |
| . Hour:Minute:Second.Microsecond. Note that the date-time pair is expressed in |
| UTC, not the local time zone. [14:33:27.000000] |
| . Reference ID of the reference clock from which the measurement came. [PPS2] |
| . Sequence number of driver poll within one polling interval for raw samples, |
| or _-_ for filtered samples. [7] |
| . Leap status (_N_ means normal, _+_ means that the last minute of the current |
| month has 61 seconds, _-_ means that the last minute of the month has 59 |
| seconds). [N] |
| . Flag indicating whether the sample comes from PPS source. (1 for yes, |
| 0 for no, or _-_ for filtered sample). [1] |
| . Local clock error measured by reference clock driver, or _-_ for filtered sample. |
| [4.900000e-07] |
| . Local clock error with applied corrections. Positive indicates that the local |
| clock is slow. [-6.741777e-07] |
| . Assumed dispersion of the sample. [1.000e-06] |
| + |
| *tempcomp*::: |
| This option logs the temperature measurements and system rate compensations to |
| a file called _tempcomp.log_. An example line (which actually appears as a |
| single line in the file) from the log file is shown below. |
| + |
| ---- |
| 2015-04-19 10:39:48 2.8000e+04 3.6600e-01 |
| ---- |
| + |
| The columns are as follows (the quantities in square brackets are the values |
| from the example line above): |
| + |
| . Date [2015-04-19] |
| . Hour:Minute:Second. Note that the date-time pair is expressed in UTC, not the |
| local time zone. [10:39:48] |
| . Temperature read from the sensor. [2.8000e+04] |
| . Applied compensation in ppm, positive means the system clock is running |
| faster than it would be without the compensation. [3.6600e-01] |
| + |
| :: |
| An example of the directive is: |
| + |
| ---- |
| log measurements statistics tracking |
| ---- |
| |
| [[logbanner]]*logbanner* _entries_:: |
| A banner is periodically written to the log files enabled by the <<log,*log*>> |
| directive to indicate the meanings of the columns. |
| + |
| The *logbanner* directive specifies after how many entries in the log file |
| should be the banner written. The default is 32, and 0 can be used to disable |
| it entirely. |
| |
| [[logchange]]*logchange* _threshold_:: |
| This directive sets the threshold for the adjustment of the system clock that |
| will generate a syslog message. Clock errors detected via NTP packets, |
| reference clocks, or timestamps entered via the |
| <<chronyc.adoc#settime,*settime*>> command of *chronyc* are logged. |
| + |
| By default, the threshold is 1 second. |
| + |
| An example of the use is: |
| + |
| ---- |
| logchange 0.1 |
| ---- |
| + |
| which would cause a syslog message to be generated if a system clock error of over |
| 0.1 seconds starts to be compensated. |
| |
| [[logdir]]*logdir* _directory_:: |
| This directive 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]]*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]]*hwtimestamp* _interface_ [_option_]...:: |
| This directive enables hardware timestamping of NTP packets sent to and |
| received from the specified network interface. The network interface controller |
| (NIC) uses its own clock to accurately timestamp the actual transmissions and |
| receptions, avoiding processing and queueing delays in the kernel, network |
| driver, and hardware. This can significantly improve the accuracy of the |
| timestamps and the measured offset, which is used for synchronisation of the |
| system clock. In order to get the best results, both sides receiving and |
| sending NTP packets (i.e. server and client, or two peers) need to use HW |
| timestamping. If the server or peer supports the interleaved mode, it needs to |
| be enabled by the *xleave* option in the <<server,*server*>> or the |
| <<peer,*peer*>> directive. |
| + |
| This directive is supported on Linux 3.19 and newer. The NIC must support HW |
| timestamping, which can be verified with the *ethtool -T* command. The list of |
| capabilities should include _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. Timestamping of packets received from bridged |
| and bonded interfaces is supported on Linux 4.13 and newer. 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,*log |
| measurements*>> directive, and the <<chronyc.adoc#ntpdata,*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). |
| *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]]*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 @SYSCONFDIR@/chrony.d/*.conf |
| ---- |
| |
| [[keyfile]]*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 @SYSCONFDIR@/chrony.keys |
| ---- |
| + |
| The argument is simply the name of the file containing the ID-key pairs. The |
| format of the file is shown below: |
| + |
| ---- |
| 10 tulip |
| 11 hyacinth |
| 20 MD5 ASCII:crocus |
| 25 SHA1 HEX: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 <<chronyc.adoc#keygen,*keygen*>> command of *chronyc* can be used to |
| generate random keys for the key file. By default, it generates 160-bit MD5 or |
| SHA1 keys. |
| |
| [[lock_all]]*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]]*pidfile* _file_:: |
| *chronyd* always writes its process ID (PID) to a file, and checks this file on |
| startup to see if another *chronyd* might already be running on the system. By |
| default, the file used is _@DEFAULT_PID_FILE@_. The *pidfile* directive |
| allows the name to be changed, e.g.: |
| + |
| ---- |
| pidfile /run/chronyd.pid |
| ---- |
| |
| [[sched_priority]]*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* _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 _@DEFAULT_USER@_. |
| |
| [[examples]] |
| == EXAMPLES |
| |
| === NTP client with permanent connection to NTP servers |
| |
| This section shows how to configure *chronyd* for computers that are connected |
| to the Internet (or to any network containing true NTP servers which ultimately |
| derive their time from a reference clock) permanently or most of the time. |
| |
| To operate in this mode, you will need to know the names of the NTP servers |
| you want to use. You might be able to find names of suitable servers by one of |
| the following methods: |
| |
| * Your institution might already operate servers on its network. |
| Contact your system administrator to find out. |
| * Your ISP probably has one or more NTP servers available for its |
| customers. |
| * Somewhere under the NTP homepage there is a list of public |
| stratum 1 and stratum 2 servers. You should find one or more servers that are |
| near to you. Check that their access policy allows you to use their |
| facilities. |
| * Use public servers from the http://www.pool.ntp.org/[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,*driftfile*>>, <<makestep,*makestep*>> and <<rtcsync,*rtcsync*>> |
| might be particularly useful. Also, the *iburst* option of the |
| <<server,*server*>> directive is useful to speed up the initial |
| synchronisation. The smallest useful configuration file would look something |
| like: |
| |
| ---- |
| server foo.example.net iburst |
| server bar.example.net iburst |
| server baz.example.net iburst |
| driftfile @CHRONYVARDIR@/drift |
| makestep 1.0 3 |
| rtcsync |
| ---- |
| |
| When using a pool of NTP servers (one name is used for multiple servers which |
| might change over time), it is better to specify them with the <<pool,*pool*>> |
| directive instead of multiple *server* directives. The configuration file could |
| in this case look like: |
| |
| ---- |
| pool pool.ntp.org iburst |
| driftfile @CHRONYVARDIR@/drift |
| makestep 1.0 3 |
| rtcsync |
| ---- |
| |
| === 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 @CHRONYVARDIR@/drift |
| makestep 1.0 3 |
| rtcsync |
| ---- |
| |
| The *offline* keyword indicates that the servers start in an offline state, and |
| that they should not be contacted until *chronyd* receives notification from |
| *chronyc* that the link to the Internet is present. To tell *chronyd* when to |
| start and finish sampling the servers, the <<chronyc.adoc#online,*online*>> and |
| <<chronyc.adoc#offline,*offline*>> commands of *chronyc* need to be used. |
| |
| To give an example of their use, assuming that *pppd* is the program being |
| used to connect to the Internet and that *chronyc* has been installed at |
| _@BINDIR@/chronyc_, the script _/etc/ppp/ip-up_ would include: |
| |
| ---- |
| @BINDIR@/chronyc online |
| ---- |
| |
| and the script _/etc/ppp/ip-down_ would include: |
| |
| ---- |
| @BINDIR@/chronyc offline |
| ---- |
| |
| *chronyd*'s polling of the servers would now only occur whilst the machine is |
| actually connected to the Internet. |
| |
| === Isolated networks |
| |
| This section shows how to configure *chronyd* for computers that never have |
| network 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,*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,*manual*>> directive and the |
| <<chronyc.adoc#settime,*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,*smoothtime*>> directive is useful when the clocks of the |
| clients need to stay close together when the local time is adjusted by the |
| <<chronyc.adoc#settime,*settime*>> command. The smoothing process needs to be |
| activated by the <<chronyc.adoc#smoothtime,*smoothtime activate*>> command when |
| the local time is ready to be served. After that point, any adjustments will be |
| smoothed out. |
| |
| A typical configuration file for the 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 @CHRONYVARDIR@/drift |
| local stratum 8 |
| manual |
| allow 192.168.165.0/24 |
| smoothtime 400 0.01 |
| rtcsync |
| ---- |
| |
| For the clients that have to resynchronise the master when it restarts, |
| the configuration file might be: |
| |
| ---- |
| server master iburst |
| driftfile @CHRONYVARDIR@/drift |
| allow 192.168.165.0/24 |
| makestep 1.0 3 |
| rtcsync |
| ---- |
| |
| The rest of the clients would be the same, except that the *allow* directive is |
| not required. |
| |
| If there is no suitable computer to be designated as the 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 @CHRONYVARDIR@/drift |
| local stratum 8 orphan |
| manual |
| allow 192.168.165.0/24 |
| rtcsync |
| ---- |
| |
| The other servers would be the same, except the hostnames in the *initstepslew* |
| and *server* directives would be modified to specify the other servers. Their |
| clients might be configured to poll all three servers. |
| |
| === RTC tracking |
| |
| This section considers a computer which has occasional connections to the |
| Internet and is turned off between '`sessions`'. In this case, *chronyd* relies |
| on the computer's RTC to maintain the time between the periods when it is |
| powered up. It assumes that Linux is run exclusively on the computer. Dual-boot |
| systems might work; it depends what (if anything) the other system does to the |
| RTC. On 2.6 and later kernels, if your motherboard has a HPET, you will need to |
| enable the *HPET_EMULATE_RTC* option in your kernel configuration. Otherwise, |
| *chronyd* will not be able to interact with the RTC device and will give up |
| using it. |
| |
| When the computer is connected to the Internet, *chronyd* has access to |
| external NTP servers which it makes measurements from. These measurements are |
| saved, and straight-line fits are performed on them to provide an estimate of |
| the computer's time error and rate of gaining or losing time. |
| |
| When the computer is taken offline from the Internet, the best estimate of the |
| gain or loss rate is used to free-run the computer until it next goes online. |
| |
| Whilst the computer is running, *chronyd* makes measurements of the RTC (via |
| the _/dev/rtc_ interface, which must be compiled into the kernel). An estimate |
| is made of the RTC error at a particular RTC second, and the rate at which the |
| RTC gains or loses time relative to true time. |
| |
| When the computer is powered down, the measurement histories for all the NTP |
| servers are saved to files, and the RTC tracking information is also |
| saved to a file (if the <<rtcfile,*rtcfile*>> directive has been specified). |
| These pieces of information are also saved if the <<chronyc.adoc#dump,*dump*>> |
| and <<chronyc.adoc#writertc,*writertc*>> commands respectively are issued |
| through *chronyc*. |
| |
| When the computer is rebooted, *chronyd* reads the current RTC time and the RTC |
| information saved at the last shutdown. This information is used to set the |
| system clock to the best estimate of what its time would have been now, had it |
| been left running continuously. The measurement histories for the servers are |
| then reloaded. |
| |
| The next time the computer goes online, the previous sessions' measurements can |
| contribute to the line-fitting process, which gives a much better estimate of |
| the computer's gain or loss rate. |
| |
| One problem with saving the measurements and RTC data when the machine is shut |
| down is what happens if there is a power failure; the most recent data will not |
| be saved. Although *chronyd* is robust enough to cope with this, some |
| performance might be lost. (The main danger arises if the RTC has been changed |
| during the session, with the *trimrtc* command in *chronyc*. Because of this, |
| *trimrtc* will make sure that a meaningful RTC file is saved after the |
| change is completed). |
| |
| The easiest protection against power failure is to put the *dump* and |
| *writertc* commands in the same place as the *offline* command is issued to |
| take *chronyd* offline; because *chronyd* free-runs between online sessions, no |
| parameters will change significantly between going offline from the Internet |
| and any power failure. |
| |
| A final point regards computers which are left running for extended periods and |
| where it is desired to spin down the hard disc when it is not in use (e.g. when |
| not accessed for 15 minutes). *chronyd* has been planned so it supports such |
| operation; this is the reason why the RTC tracking parameters are not saved to |
| disc after every update, but only when the user requests such a write, or |
| during the shutdown sequence. The only other facility that will generate |
| periodic writes to the disc is the *log rtc* facility in the configuration |
| file; this option should not be used if you want your disc to spin down. |
| |
| To illustrate how a computer might be configured for this case, example |
| configuration files are shown. |
| |
| For the _chrony.conf_ file, the following can be used as an example. |
| |
| ---- |
| server foo.example.net maxdelay 0.4 offline |
| server bar.example.net maxdelay 0.4 offline |
| server baz.example.net maxdelay 0.4 offline |
| logdir /var/log/chrony |
| log statistics measurements tracking |
| driftfile @CHRONYVARDIR@/drift |
| makestep 1.0 3 |
| maxupdateskew 100.0 |
| dumpdir @CHRONYVARDIR@ |
| rtcfile @CHRONYVARDIR@/rtc |
| ---- |
| |
| *pppd* is used for connecting to the Internet. This runs two scripts |
| _/etc/ppp/ip-up_ and _/etc/ppp/ip-down_ when the link goes online and offline |
| respectively. |
| |
| The relevant part of the _/etc/ppp/ip-up_ file is: |
| |
| ---- |
| @BINDIR@/chronyc online |
| ---- |
| |
| and the relevant part of the _/etc/ppp/ip-down_ script is: |
| |
| ---- |
| @BINDIR@/chronyc -m offline dump writertc |
| ---- |
| |
| *chronyd* is started during the boot sequence with the *-r* and *-s* options. |
| It might need to be started before any software that depends on the system clock |
| not jumping or moving backwards, depending on the directives in *chronyd*'s |
| configuration file. |
| |
| For the system shutdown, *chronyd* should receive a SIGTERM several seconds |
| before the final SIGKILL; the SIGTERM causes the measurement histories and RTC |
| information to be saved. |
| |
| === Public NTP server |
| |
| *chronyd* can be configured to operate as a public NTP server, e.g. to join the |
| http://www.pool.ntp.org/en/join.html[pool.ntp.org] project. The configuration |
| is similar to the NTP client with permanent connection, except it needs to |
| allow client access from all addresses. It is recommended to find at least four |
| good servers (e.g. from the pool, or on the NTP homepage). If the server has a |
| hardware reference clock (e.g. a GPS receiver), it can be specified by the |
| <<refclock,*refclock*>> directive. |
| |
| The amount of memory used for logging client accesses can be increased in order |
| to enable clients to use the interleaved mode even when the server has a large |
| number of clients, and better support rate limiting if it is enabled by the |
| <<ratelimit,*ratelimit*>> directive. The system timezone database, if it is |
| kept up to date and includes the _right/UTC_ timezone, can be used as a |
| reliable source to determine when a leap second will be applied to UTC. The |
| *-r* option with the <<dumpdir,*dumpdir*>> directive shortens the time in which |
| *chronyd* will not be able to serve time to its clients when it needs to be |
| restarted (e.g. after upgrading to a newer version, or a change in the |
| configuration). |
| |
| The configuration file could look like: |
| |
| ---- |
| server 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 @CHRONYVARDIR@/drift |
| dumpdir @CHRONYRUNDIR@ |
| ---- |
| |
| == SEE ALSO |
| |
| <<chronyc.adoc#,*chronyc(1)*>>, <<chronyd.adoc#,*chronyd(8)*>> |
| |
| == BUGS |
| |
| For instructions on how to report bugs, please visit |
| https://chrony.tuxfamily.org/. |
| |
| == AUTHORS |
| |
| chrony was written by Richard Curnow, Miroslav Lichvar, and others. |