18.3. Using chrony
18.3.1. Installing chrony
~]#The default location for the chrony daemon is
yum install chrony
/usr/sbin/chronyd. The command line utility will be installed to
18.3.2. Checking the Status of chronyd
chronyd, issue the following command:
systemctl status chronydchronyd.service - NTP client/server Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled) Active: active (running) since Wed 2013-06-12 22:23:16 CEST; 11h ago
18.3.3. Starting chronyd
chronyd, issue the following command as
systemctl start chronyd
chronydstarts automatically at system start, issue the following command as
systemctl enable chronyd
18.3.4. Stopping chronyd
chronyd, issue the following command as
systemctl stop chronyd
chronydfrom starting automatically at system start, issue the following command as
systemctl disable chronyd
18.3.5. Checking if chrony is Synchronized
184.108.40.206. Checking chrony Tracking
~]$The fields are as follows:
chronyc trackingReference ID : CB00710F (foo.example.net) Stratum : 3 Ref time (UTC) : Fri Jan 27 09:49:17 2017 System time : 0.000006523 seconds slow of NTP time Last offset : -0.000006747 seconds RMS offset : 0.000035822 seconds Frequency : 3.225 ppm slow Residual freq : 0.000 ppm Skew : 0.129 ppm Root delay : 0.013639022 seconds Root dispersion : 0.001100737 seconds Update interval : 64.2 seconds Leap status : Normal
- Reference ID
- This is the reference ID and name (or
IPaddress) if available, of the server to which the computer is currently synchronized. Reference ID is a hexadecimal number to avoid confusion with IPv4 addresses.
- The stratum indicates how many hops away from a computer with an attached reference clock we are. Such a computer is a stratum-1 computer, so the computer in the example is two hops away (that is to say, a.b.c is a stratum-2 and is synchronized from a stratum-1).
- Ref time
- This is the time (UTC) at which the last measurement from the reference source was processed.
- System time
- In normal operation,
chronydnever steps the system clock, because any jump in the timescale can have adverse consequences for certain application programs. Instead, any error in the system clock is corrected by slightly speeding up or slowing down the system clock until the error has been removed, and then returning to the system clock’s normal speed. A consequence of this is that there will be a period when the system clock (as read by other programs using the
gettimeofday()system call, or by the date command in the shell) will be different from
chronyd's estimate of the current true time (which it reports to
NTPclients when it is operating in server mode). The value reported on this line is the difference due to this effect.
- Last offset
- This is the estimated local offset on the last clock update.
- RMS offset
- This is a long-term average of the offset value.
- The “frequency” is the rate by which the system’s clock would be wrong if
chronydwas not correcting it. It is expressed in ppm (parts per million). For example, a value of 1 ppm would mean that when the system’s clock thinks it has advanced 1 second, it has actually advanced by 1.000001 seconds relative to true time.
- Residual freq
- This shows the “residual frequency” for the currently selected reference source. This reflects any difference between what the measurements from the reference source indicate the frequency should be and the frequency currently being used. The reason this is not always zero is that a smoothing procedure is applied to the frequency. Each time a measurement from the reference source is obtained and a new residual frequency computed, the estimated accuracy of this residual is compared with the estimated accuracy (see
skewnext) of the existing frequency value. A weighted average is computed for the new frequency, with weights depending on these accuracies. If the measurements from the reference source follow a consistent trend, the residual will be driven to zero over time.
- This is the estimated error bound on the frequency.
- Root delay
- This is the total of the network path delays to the stratum-1 computer from which the computer is ultimately synchronized. Root delay values are printed in nanosecond resolution. In certain extreme situations, this value can be negative. (This can arise in a symmetric peer arrangement where the computers’ frequencies are not tracking each other and the network delay is very short relative to the turn-around time at each computer.)
- Root dispersion
- This is the total dispersion accumulated through all the computers back to the stratum-1 computer from which the computer is ultimately synchronized. Dispersion is due to system clock resolution, statistical measurement variations etc. Root dispersion values are printed in nanosecond resolution.
- Leap status
- This is the leap status, which can be Normal, Insert second, Delete second or Not synchronized.
220.127.116.11. Checking chrony Sources
chronydis accessing. The optional argument -v can be specified, meaning verbose. In this case, extra caption lines are shown as a reminder of the meanings of the columns.
~]$The columns are as follows:
chronyc sources210 Number of sources = 3 MS Name/IP address Stratum Poll Reach LastRx Last sample =============================================================================== #* GPS0 0 4 377 11 -479ns[ -621ns] +/- 134ns ^? a.b.c 2 6 377 23 -923us[ -924us] +/- 43ms ^+ d.e.f 1 6 377 21 -2629us[-2619us] +/- 86ms
- This indicates the mode of the source.
^means a server,
=means a peer and
#indicates a locally connected reference clock.
- This column indicates the state of the sources. “*” indicates the source to which
chronydis currently synchronized. “+” indicates acceptable sources which are combined with the selected source. “-” indicates acceptable sources which are excluded by the combining algorithm. “?” indicates sources to which connectivity has been lost or whose packets do not pass all tests. “x” indicates a clock which
chronydthinks is a falseticker (its time is inconsistent with a majority of other sources). “~” indicates a source whose time appears to have too much variability. The “?” condition is also shown at start-up, until at least 3 samples have been gathered from it.
- Name/IP address
- This shows the name or the
IPaddress of the source, or reference ID for reference clock.
- This shows the stratum of the source, as reported in its most recently received sample. Stratum 1 indicates a computer with a locally attached reference clock. A computer that is synchronized to a stratum 1 computer is at stratum 2. A computer that is synchronized to a stratum 2 computer is at stratum 3, and so on.
- This shows the rate at which the source is being polled, as a base-2 logarithm of the interval in seconds. Thus, a value of 6 would indicate that a measurement is being made every 64 seconds.
chronydautomatically varies the polling rate in response to prevailing conditions.
- This shows the source’s reach register printed as an octal number. The register has 8 bits and is updated on every received or missed packet from the source. A value of 377 indicates that a valid reply was received for all of the last eight transmissions.
- This column shows how long ago the last sample was received from the source. This is normally in seconds. The letters
yindicate minutes, hours, days or years. A value of 10 years indicates there were no samples received from this source yet.
- Last sample
- This column shows the offset between the local clock and the source at the last measurement. The number in the square brackets shows the actual measured offset. This may be suffixed by
ms(indicating milliseconds), or
s(indicating seconds). The number to the left of the square brackets shows the original measurement, adjusted to allow for any slews applied to the local clock since. The number following the
+/-indicator shows the margin of error in the measurement. Positive offsets indicate that the local clock is ahead of the source.
18.104.22.168. Checking chrony Source Statistics
sourcestatscommand displays information about the drift rate and offset estimation process for each of the sources currently being examined by
chronyd. The optional argument
-vcan be specified, meaning verbose. In this case, extra caption lines are shown as a reminder of the meanings of the columns.
~]$The columns are as follows:
chronyc sourcestats210 Number of sources = 1 Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev =============================================================================== abc.def.ghi 11 5 46m -0.001 0.045 1us 25us
- Name/IP address
- This is the name or
IPaddress of the
NTPserver (or peer) or reference ID of the reference clock to which the rest of the line relates.
- This is the number of sample points currently being retained for the server. The drift rate and current offset are estimated by performing a linear regression through these points.
- This is the number of runs of residuals having the same sign following the last regression. If this number starts to become too small relative to the number of samples, it indicates that a straight line is no longer a good fit to the data. If the number of runs is too low,
chronyddiscards older samples and re-runs the regression until the number of runs becomes acceptable.
- This is the interval between the oldest and newest samples. If no unit is shown the value is in seconds. In the example, the interval is 46 minutes.
- This is the estimated residual frequency for the server, in parts per million. In this case, the computer’s clock is estimated to be running 1 part in 109 slow relative to the server.
- Freq Skew
- This is the estimated error bounds on Freq (again in parts per million).
- This is the estimated offset of the source.
- Std Dev
- This is the estimated sample standard deviation.
18.3.6. Manually Adjusting the System Clock
rtcfiledirective is used, the real-time clock should not be manually adjusted. Random adjustments would interfere with chrony's need to measure the rate at which the real-time clock drifts.