Adding upstream version 3.4.upstream/3.4upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+Frequently Asked Questions
+
+Table of Contents
+
+  o 1. chrony compared to other programs
+      ? 1.1. How does chrony compare to ntpd?
+  o 2. Configuration issues
+      ? 2.1. What is the minimum recommended configuration for an NTP client?
+      ? 2.2. How do I make an NTP server from an NTP client?
+      ? 2.3. I have several computers on a LAN. Should be all clients of an
+        external server?
+      ? 2.4. Must I specify servers by IP address if DNS is not available on
+        chronyd start?
+      ? 2.5. How can I make chronyd more secure?
+      ? 2.6. How can I improve the accuracy of the system clock with NTP
+        sources?
+      ? 2.7. Does chronyd have an ntpdate mode?
+      ? 2.8. Can chronyd be configured to control the clock like ntpd?
+      ? 2.9. What happened to the commandkey and generatecommandkey directives?
+  o 3. Computer is not synchronising
+      ? 3.1. Behind a firewall?
+      ? 3.2. Are NTP servers specified with the offline option?
+      ? 3.3. Is chronyd allowed to step the system clock?
+      ? 3.4. Using a Windows NTP server?
+      ? 3.5. Using a PPS reference clock?
+  o 4. Issues with chronyc
+      ? 4.1. I keep getting the error 506 Cannot talk to daemon
+      ? 4.2. I keep getting the error 501 Not authorised
+      ? 4.3. Why does chronyc tracking always print an IPv4 address as
+        reference ID?
+      ? 4.4. Is the chronyc / chronyd protocol documented anywhere?
+  o 5. Real-time clock issues
+      ? 5.1. What is the real-time clock (RTC)?
+      ? 5.2. I want to use chronyd's RTC support. Must I disable hwclock?
+      ? 5.3. I just keep getting the 513 RTC driver not running message
+      ? 5.4. I get Could not open /dev/rtc, Device or resource busy in my
+        syslog file
+      ? 5.5. What if my computer does not have an RTC or backup battery?
+  o 6. NTP-specific issues
+      ? 6.1. Can chronyd be driven from broadcast/multicast NTP servers?
+      ? 6.2. Can chronyd transmit broadcast NTP packets?
+      ? 6.3. Can chronyd keep the system clock a fixed offset away from real
+        time?
+      ? 6.4. What happens if the network connection is dropped without using
+        chronyc's offline command first?
+  o 7. Operating systems
+      ? 7.1. Does chrony support Windows?
+      ? 7.2. Are there any plans to support Windows?
+
+1. chrony compared to other programs
+
+1.1. How does chrony compare to ntpd?
+
+chronyd was designed to work well in a wide range of conditions and it can
+usually synchronise the system clock faster and with better time accuracy. It
+doesn't implement some of the less useful NTP modes like broadcast client or
+multicast server/client.
+
+If your computer is connected to the Internet only for few minutes at a time,
+the network connection is often congested, you turn your computer off or
+suspend it frequently, the clock is not very stable (e.g. there are rapid
+changes in the temperature or it's a virtual machine), or you want to use NTP
+on an isolated network with no hardware reference clocks in sight, chrony will
+probably work much better for you.
+
+For a more detailed comparison of features and performance, see the comparison
+page on the chrony website.
+
+2. Configuration issues
+
+2.1. What is the minimum recommended configuration for an NTP client?
+
+First, the client needs to know which NTP servers it should ask for the current
+time. They are specified by the server or pool directive. The pool directive
+can be used for names that resolve to multiple addresses. For good reliability
+the client should have at least three servers. The iburst option speeds up the
+initial synchronisation.
+
+To stabilise the initial synchronisation on the next start, the estimated drift
+of the system clock is saved to a file specified by the driftfile directive.
+
+If the system clock can be far from the true time after boot for any reason,
+chronyd should be allowed to correct it quickly by stepping instead of slewing,
+which would take a very long time. The makestep directive does that.
+
+In order to keep the real-time clock (RTC) close to the true time, so the
+system time is reasonably close to the true time when it's initialised on the
+next boot from the RTC, the rtcsync directive enables a mode in which the
+system time is periodically copied to the RTC. It is supported on Linux and
+macOS.
+
+If you want to use public NTP servers from the pool.ntp.org project, the
+minimal chrony.conf file could be:
+
+pool pool.ntp.org iburst
+driftfile /var/lib/chrony/drift
+makestep 1 3
+rtcsync
+
+2.2. How do I make an NTP server from an NTP client?
+
+You need to add an allow directive to the chrony.conf file in order to open the
+NTP port and allow chronyd to reply to client requests. allow with no specified
+subnet allows access from all IPv4 and IPv6 addresses.
+
+2.3. I have several computers on a LAN. Should be all clients of an external
+server?
+
+The best configuration is usually to make one computer the server, with the
+others as clients of it. Add a local directive to the server's chrony.conf
+file. This configuration will be better because
+
+  o the load on the external connection is less
+
+  o the load on the external NTP server(s) is less
+
+  o if your external connection goes down, the computers on the LAN will
+    maintain a common time with each other.
+
+2.4. Must I specify servers by IP address if DNS is not available on chronyd
+start?
+
+No. Starting from version 1.25, chronyd will keep trying to resolve the names
+specified by the server, pool, and peer directives in an increasing interval
+until it succeeds. The online command can be issued from chronyc to force
+chronyd to try to resolve the names immediately.
+
+2.5. How can I make chronyd more secure?
+
+If you don't need to serve time to NTP clients or peers, you can add port 0 to
+the chrony.conf file to completely disable the NTP server functionality and
+prevent NTP requests from reaching chronyd. Starting from version 2.0, the NTP
+server port is open only when client access is allowed by the allow directive
+or command, an NTP peer is configured, or the broadcast directive is used.
+
+If you don't need to use chronyc remotely, you can add the following directives
+to the configuration file to bind the command sockets to the loopback
+interface. This is done by default since version 2.0.
+
+bindcmdaddress 127.0.0.1
+bindcmdaddress ::1
+
+If you don't need to use chronyc at all or you need to run chronyc only under
+the root or chrony user (which can access chronyd through a Unix domain socket
+since version 2.2), you can disable the internet command sockets completely by
+adding cmdport 0 to the configuration file.
+
+You can specify an unprivileged user with the -u option, or the user directive
+in the chrony.conf file, to which chronyd will switch after start in order to
+drop root privileges. The configure script has a --with-user option, which sets
+the default user. On Linux, chronyd needs to be compiled with support for the
+libcap library. On other systems, 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.
+
+Also, if chronyd is compiled with support for the Linux secure computing
+(seccomp) facility, you can enable a system call filter with the -F option. It
+will significantly reduce the kernel attack surface and possibly prevent kernel
+exploits from the chronyd process if it's compromised. It's recommended to
+enable the filter only when it's known to work on the version of the system
+where chrony is installed as the filter needs to allow also system calls made
+from libraries that chronyd is using (e.g. libc) and different versions or
+implementations of the libraries may make different system calls. If the filter
+is missing some system call, chronyd could be killed even in normal operation.
+
+2.6. How can I improve the accuracy of the system clock with NTP sources?
+
+Select NTP servers that are well synchronised, stable and close to your
+network. It's better to use more than one server, three or four is usually
+recommended as the minimum, so chronyd can detect servers that serve false time
+and combine measurements from multiple sources.
+
+If you have a network card with hardware timestamping supported on Linux, it
+can be enabled by the hwtimestamp directive in the chrony.conf file. It should
+make local receive and transmit timestamps of NTP packets much more accurate.
+
+There are also useful options which can be set in the server directive, they
+are minpoll, maxpoll, polltarget, maxdelay, maxdelayratio, maxdelaydevratio,
+and xleave.
+
+The first three options set the minimum and maximum allowed polling interval,
+and how should be the actual interval adjusted in the specified range. Their
+default values are 6 (64 seconds) for minpoll, 10 (1024 seconds) for maxpoll
+and 8 (samples) for polltarget. The default values should be used for general
+servers on the Internet. With your own NTP servers, or if you have permission
+to poll some servers more frequently, setting these options for shorter polling
+intervals may significantly improve the accuracy of the system clock.
+
+The optimal polling interval depends mainly on two factors, stability of the
+network latency and stability of the system clock (which mainly depends on the
+temperature sensitivity of the crystal oscillator and the maximum rate of the
+temperature change).
+
+Generally, if the sourcestats command usually reports a small number of samples
+retained for a source (e.g. fewer than 16), a shorter polling interval should
+be considered. If the number of samples is usually at the maximum of 64, a
+longer polling interval may work better.
+
+An example of the directive for an NTP server on the Internet that you are
+allowed to poll frequently could be
+
+server foo.example.net minpoll 4 maxpoll 6 polltarget 16
+
+An example using shorter polling intervals with a server located in the same
+LAN could be
+
+server ntp.local minpoll 2 maxpoll 4 polltarget 30
+
+The maxdelay options are useful to ignore measurements with an unusally large
+delay (e.g. due to congestion in the network) and improve the stability of the
+synchronisation. The maxdelaydevratio option could be added to the example with
+local NTP server
+
+server ntp.local minpoll 2 maxpoll 4 polltarget 30 maxdelaydevratio 2
+
+If your server supports the interleaved mode (e.g. it is running chronyd), the
+xleave option should be added to the server directive in order to allow the
+server to send the client more accurate transmit timestamps (kernel or
+preferably hardware). For example:
+
+server ntp.local minpoll 2 maxpoll 4 xleave
+
+When combined with local hardware timestamping, good network switches, and even
+shorter polling intervals, a sub-microsecond accuracy and stability of a few
+tens of nanoseconds may be possible. For example:
+
+server ntp.local minpoll 0 maxpoll 0 xleave
+hwtimestamp eth0
+
+If it is acceptable for NTP clients in the network to send requests at an
+excessive rate, a sub-second polling interval may be specified. A median filter
+can be enabled in order to update the clock at a reduced rate with more stable
+measurements. For example:
+
+server ntp.local minpoll -6 maxpoll -6 filter 15 xleave
+hwtimestamp eth0 minpoll -6
+
+2.7. Does chronyd have an ntpdate mode?
+
+Yes. With the -q option chronyd will set the system clock once and exit. With
+the -Q option it will print the measured offset without setting the clock. If
+you don't want to use a configuration file, NTP servers can be specified on the
+command line. For example:
+
+# chronyd -q 'pool pool.ntp.org iburst'
+
+2.8. Can chronyd be configured to control the clock like ntpd?
+
+It is not possible to perfectly emulate ntpd, but there are some options that
+can configure chronyd to behave more like ntpd.
+
+In the following example the minsamples directive slows down the response to
+changes in the frequency and offset of the clock. The maxslewrate and
+corrtimeratio directives reduce the maximum frequency error due to an offset
+correction and the maxdrift directive reduces the maximum assumed frequency
+error of the clock. The makestep directive enables a step threshold and the
+maxchange directive enables a panic threshold. The maxclockerror directive
+increases the minimum dispersion rate.
+
+minsamples 32
+maxslewrate 500
+corrtimeratio 100
+maxdrift 500
+makestep 0.128 -1
+maxchange 1000 1 1
+maxclockerror 15
+
+2.9. What happened to the commandkey and generatecommandkey directives?
+
+They were removed in version 2.2. Authentication is no longer supported in the
+command protocol. Commands that required authentication are now allowed only
+through a Unix domain socket, which is accessible only by the root and chrony
+users. If you need to configure chronyd remotely or locally without the root
+password, please consider using ssh and/or sudo to run chronyc under the root
+or chrony user on the host where chronyd is running.
+
+3. Computer is not synchronising
+
+This is the most common problem. There are a number of reasons, see the
+following questions.
+
+3.1. Behind a firewall?
+
+Check the Reach value printed by the chronyc's sources command. If it's zero,
+it means chronyd did not get any valid responses from the NTP server you are
+trying to use. If there is a firewall between you and the server, the packets
+may be blocked. Try using a tool like wireshark or tcpdump to see if you're
+getting any responses from the server.
+
+When chronyd is receiving responses from the servers, the output of the sources
+command issued few minutes after chronyd start might look like this:
+
+210 Number of sources = 3
+MS Name/IP address         Stratum Poll Reach LastRx Last sample
+===============================================================================
+^* foo.example.net               2   6   377    34   +484us[ -157us] +/-   30ms
+^- bar.example.net               2   6   377    34    +33ms[  +32ms] +/-   47ms
+^+ baz.example.net               3   6   377    35  -1397us[-2033us] +/-   60ms
+
+3.2. Are NTP servers specified with the offline option?
+
+Check that you're using chronyc's online and offline commands appropriately.
+The activity command prints the number of sources that are currently online and
+offline. For example:
+
+200 OK
+3 sources online
+0 sources offline
+0 sources doing burst (return to online)
+0 sources doing burst (return to offline)
+0 sources with unknown address
+
+3.3. Is chronyd allowed to step the system clock?
+
+By default, chronyd adjusts the clock gradually by slowing it down or speeding
+it up. If the clock is too far from the true time, it will take a long time to
+correct the error. The System time value printed by the chronyc's tracking
+command is the remaining correction that needs to be applied to the system
+clock.
+
+The makestep directive can be used to allow chronyd to step the clock. For
+example, if chrony.conf had
+
+makestep 1 3
+
+the clock would be stepped in the first three updates if its offset was larger
+than one second. Normally, it's recommended to allow the step only in the first
+few updates, but in some cases (e.g. a computer without an RTC or virtual
+machine which can be suspended and resumed with an incorrect time) it may be
+necessary to allow the step on any clock update. The example above would change
+to
+
+makestep 1 -1
+
+3.4. Using a Windows NTP server?
+
+A common issue with Windows NTP servers is that they report a very large root
+dispersion (e.g. three seconds or more), which causes chronyd to ignore the
+server for being too inaccurate. The sources command may show a valid
+measurement, but the server is not selected for synchronisation. You can check
+the root dispersion of the server with the chronyc's ntpdata command.
+
+The maxdistance value needs to be increased in chrony.conf to enable
+synchronisation to such a server. For example:
+
+maxdistance 16.0
+
+3.5. Using a PPS reference clock?
+
+A pulse-per-second (PPS) reference clock requires a non-PPS time source to
+determine which second of UTC corresponds to each pulse. If it is another
+reference clock specified with the lock option in the refclock directive, the
+offset between the two reference clocks must be smaller than 0.2 seconds in
+order for the PPS reference clock to work. With NMEA reference clocks it is
+common to have a larger offset. It needs to be corrected with the offset
+option.
+
+One approach to find out a good value of the offset option is to configure the
+reference clocks with the noselect option and compare them to an NTP server.
+For example, if the sourcestats command showed
+
+Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
+==============================================================================
+PPS0                        0   0     0     +0.000   2000.000     +0ns  4000ms
+NMEA                       58  30   231    -96.494     38.406   +504ms  6080us
+foo.example.net             7   3   200     -2.991     16.141   -107us   492us
+
+the offset of the NMEA source would need to be increased by about 0.504
+seconds. It does not have to be very accurate. As long as the offset of the
+NMEA reference clock stays below 0.2 seconds, the PPS reference clock should be
+able to determine the seconds corresponding to the pulses and allow the samples
+to be used for synchronisation.
+
+4. Issues with chronyc
+
+4.1. I keep getting the error 506 Cannot talk to daemon
+
+When accessing chronyd remotely, make sure that the chrony.conf file (on the
+computer where chronyd is running) has a cmdallow entry for the computer you
+are running chronyc on and an appropriate bindcmdaddress directive. This isn't
+necessary for localhost.
+
+Perhaps chronyd is not running. Try using the ps command (e.g. on Linux, ps
+-auxw) to see if it's running. Or try netstat -a and see if the ports 123/udp
+and 323/udp are listening. If chronyd is not running, you may have a problem
+with the way you are trying to start it (e.g. at boot time).
+
+Perhaps you have a firewall set up in a way that blocks packets on port 323/
+udp. You need to amend the firewall configuration in this case.
+
+4.2. I keep getting the error 501 Not authorised
+
+Since version 2.2, the password command doesn't do anything and chronyc needs
+to run locally under the root or chrony user, which are allowed to access the
+chronyd's Unix domain command socket.
+
+With older versions, you need to authenticate with the password command first
+or use the -a option to authenticate automatically on start. The configuration
+file needs to specify a file which contains keys (keyfile directive) and which
+key in the key file should be used for chronyc authentication (commandkey
+directive).
+
+4.3. Why does chronyc tracking always print an IPv4 address as reference ID?
+
+The reference ID is a 32-bit value and in versions before 3.0 it was printed in
+quad-dotted notation, even if the reference source did not actually have an
+IPv4 address. For IPv4 addresses, the reference ID is equal to the address, but
+for IPv6 addresses it is the first 32 bits of the MD5 sum of the address. For
+reference clocks, the reference ID is the value specified with the refid option
+in the refclock directive.
+
+Since version 3.0, the reference ID is printed as a hexadecimal number to avoid
+confusion with IPv4 addresses.
+
+If you need to get the IP address of the current reference source, use the -n
+option to disable resolving of IP addresses and read the second field (printed
+in parentheses) on the Reference ID line.
+
+4.4. Is the chronyc / chronyd protocol documented anywhere?
+
+Only by the source code. See cmdmon.c (chronyd side) and client.c (chronyc
+side).
+
+5. Real-time clock issues
+
+5.1. What is the real-time clock (RTC)?
+
+This is the clock which keeps the time even when your computer is turned off.
+It is used to initialise the system clock on boot. It normally doesn't drift
+more than few seconds per day.
+
+There are two approaches how chronyd can work with it. One is to use the
+rtcsync directive, which tells chronyd to enable a kernel mode which sets the
+RTC from the system clock every 11 minutes. chronyd itself won't touch the RTC.
+If the computer is not turned off for a long time, the RTC should still be
+close to the true time when the system clock will be initialised from it on the
+next boot.
+
+The other option is to use the rtcfile directive, which tells chronyd to
+monitor the rate at which the RTC gains or loses time. When chronyd is started
+with the -s option on the next boot, it will set the system time from the RTC
+and also compensate for the drift it has measured previously. The rtcautotrim
+directive can be used to keep the RTC close to the true time, but it's not
+strictly necessary if its only purpose is to set the system clock when chronyd
+is started on boot. See the documentation for details.
+
+5.2. I want to use chronyd's RTC support. Must I disable hwclock?
+
+The hwclock program is often set-up by default in the boot and shutdown scripts
+with many Linux installations. With the kernel RTC synchronisation (rtcsync
+directive), the RTC will be set also every 11 minutes as long as the system
+clock is synchronised. If you want to use chronyd's RTC monitoring (rtcfile
+directive), it's important to disable hwclock in the shutdown procedure. If you
+don't, it will over-write the RTC with a new value, unknown to chronyd. At the
+next reboot, chronyd started with the -s option will compensate this (wrong)
+time with its estimate of how far the RTC has drifted whilst the power was off,
+giving a meaningless initial system time.
+
+There is no need to remove hwclock from the boot process, as long as chronyd is
+started after it has run.
+
+5.3. I just keep getting the 513 RTC driver not running message
+
+For the real-time clock support to work, you need the following three things
+
+  o an RTC in your computer
+
+  o a Linux kernel with enabled RTC support
+
+  o an rtcfile directive in your chrony.conf file
+
+5.4. I get Could not open /dev/rtc, Device or resource busy in my syslog file
+
+Some other program running on the system may be using the device.
+
+5.5. What if my computer does not have an RTC or backup battery?
+
+In this case you can still use the -s option to set the system clock to the
+last modification time of the drift file, which should correspond to the system
+time when chronyd was previously stopped. The initial system time will be
+increasing across reboots and applications started after chronyd will not
+observe backward steps.
+
+6. NTP-specific issues
+
+6.1. Can chronyd be driven from broadcast/multicast NTP servers?
+
+No, the broadcast/multicast client mode is not supported and there is currently
+no plan to implement it. While the mode may be useful to simplify configuration
+of clients in large networks, it is inherently less accurate and less secure
+(even with authentication) than the ordinary client/server mode.
+
+When configuring a large number of clients in a network, it is recommended to
+use the pool directive with a DNS name which resolves to addresses of multiple
+NTP servers. The clients will automatically replace the servers when they
+become unreachable, or otherwise unsuitable for synchronisation, with new
+servers from the pool.
+
+Even with very modest hardware, an NTP server can serve time to hundreds of
+thousands of clients using the ordinary client/server mode.
+
+6.2. Can chronyd transmit broadcast NTP packets?
+
+Yes, the broadcast directive can be used to enable the broadcast server mode to
+serve time to clients in the network which support the broadcast client mode
+(it's not supported in chronyd, see the previous question).
+
+6.3. Can chronyd keep the system clock a fixed offset away from real time?
+
+Yes. Starting from version 3.0, an offset can be specified by the offset option
+for all time sources in the chrony.conf file.
+
+6.4. What happens if the network connection is dropped without using chronyc's
+offline command first?
+
+chronyd will keep trying to access the sources that it thinks are online, and
+it will take longer before new measurements are actually made and the clock is
+corrected when the network is connected again. If the sources were set to
+offline, chronyd would make new measurements immediately after issuing the
+online command.
+
+Unless the network connection lasts only few minutes (less than the maximum
+polling interval), the delay is usually not a problem, and it may be acceptable
+to keep all sources online all the time.
+
+7. Operating systems
+
+7.1. Does chrony support Windows?
+
+No. The chronyc program (the command-line client used for configuring chronyd
+while it is running) has been successfully built and run under Cygwin in the
+past. chronyd is not portable, because part of it is very system-dependent. It
+needs adapting to work with Windows' equivalent of the adjtimex() call, and it
+needs to be made to work as a service.
+
+7.2. Are there any plans to support Windows?
+
+We have no plans to do this. Anyone is welcome to pick this work up and
+contribute it back to the project.
+
+Last updated 2018-09-19 16:38:15 CEST