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doc: update FAQ

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Miroslav Lichvar 2015-08-04 17:23:27 +02:00
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doc/faq.adoc
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@ -4,25 +4,29 @@
Frequently Asked Questions
==========================
== Chrony compared to other programs
== +chrony+ compared to other programs
=== How does +chrony+ compare to +ntpd+?
+chrony+ can usually synchronise the system clock faster and with better time
accuracy, but it doesn't implement all NTP features, e.g. broadcast/multicast
mode, or authentication based on public-key cryptography. For a more detailed
comparison, see the http://chrony.tuxfamily.org/comparison.html[comparison
page] on the chrony website and section
+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.
For a more detailed comparison of features and performance, see the
http://chrony.tuxfamily.org/comparison.html[comparison page] on the +chrony+
website and the
http://chrony.tuxfamily.org/manual.html#Comparison-with-ntpd[Comparison with
ntpd] in the manual.
ntpd] section in the manual.
If your computer connects to the 'net only for few minutes at a time, you turn
your Linux computer off or suspend it frequently, the clock is not very stable
(e.g. it is a virtual machine), or you want to use NTP on an isolated network
with no hardware clocks in sight, +chrony+ will probably work much better for
you.
If your computer is connected to the internet only for few minutes at a time,
the network connection is often congested, you turn your Linux 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.
The original reason +chrony+ was written was that ntpd (called xntpd at the
The original reason +chrony+ was written was that +ntpd+ (called +xntpd+ at the
time) could not to do anything sensible on a PC which was connected to the 'net
only for about 5 minutes once or twice a day, mainly to upload/download email
and news. The requirements were
@ -39,7 +43,7 @@ and news. The requirements were
information to set the system clock correctly at boot up.
Also, when working with isolated networks with no true time references at all
ntpd was found to give no help with managing the local clock's gain/loss rate
+ntpd+ was found to give no help with managing the local clock's gain/loss rate
on the NTP master node (which was set from watch). Some automated support was
added to +chrony+ to deal with this.
@ -63,7 +67,7 @@ the hostnames specified in the +server+ and +peer+ directives in
increasing intervals until it succeeds. The +online+ command can be
issued from +chronyc+ to try to resolve them immediately.
=== How can I make chronyd more secure?
=== 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
@ -146,11 +150,11 @@ following questions.
=== Behind a firewall?
If there is a firewall between you and the NTP server you're trying to use, the
packets may be blocked. Try using a tool like wireshark or tcpdump to see if
packets may be blocked. Try using a tool like +wireshark+ or +tcpdump+ to see if
you're getting responses from the server. If you have an external modem, see
if the receive light blinks straight after the transmit light (when the link is
quiet apart from the NTP traffic.) Try adding +log measurements+ to the
'chrony.conf' file and look in the 'measurements.log' file after +chrony+ has
'chrony.conf' file and look in the 'measurements.log' file after +chronyd+ has
been running for a short period. See if any measurements appear.
=== Are NTP servers specified with the +offline+ option?
@ -159,6 +163,32 @@ Check that you're using +chronyc+\'s +online+ and +offline+ commands
appropriately. Again, check in 'measurements.log' to see if you're getting any
data back from the server.
=== 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 correct 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 RTC or virtual
machine which can be suspended and resumed with incorrect time) it may be
necessary to allow the step at any clock update. The example above would
change to
----
makestep 1 -1
----
== Issues with +chronyc+
=== I keep getting the error +506 Cannot talk to daemon+
@ -186,21 +216,37 @@ Only by the source code :-) See 'cmdmon.c' (+chronyd+ side) and 'client.c'
=== What is the real-time clock (RTC)?
This is the clock which keeps the time even when your computer is turned off.
It works with 1 second resolution. +chronyd+ can monitor the rate at which the
real-time clock gains or loses time, and compensate for it when you set the
system time from it at the next reboot. See the documentation for details.
It is used to initialize the system clock on boot. It normally doesn't drift
more than few seconds per day.
=== I want to use +chronyd+'s real-time clock support. Must I disable hwclock?
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 initialized from
it on the next boot.
The hwclock program is often set-up by default in the boot and shutdown scripts
with many Linux installations. If you want to use +chronyd+'s real-time clock
support, the important thing is 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+ 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.
The other option is to use the +rtcfile+ directive, which will tell +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.
There is no need to remove hwclock from the boot process, as long as +chronyd+
=== 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.
=== I just keep getting the +513 RTC driver not running+ message
@ -212,33 +258,24 @@ things
* enhanced RTC support compiled into the kernel
* an +rtcfile+ directive in your 'chrony.conf' file
== Microsoft Windows
=== 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.
=== 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.
== NTP-specific issues
=== Can +chrony+ be driven from broadcast NTP servers?
=== Can +chronyd+ be driven from broadcast NTP servers?
No, this NTP mode is not implemented yet.
No, the broadcast client mode is not supported and there is currently no plan
to implement it. The broadcast and multicast modes are inherently less
accurate and less secure (even with authentication) than the ordinary
server/client mode and they are not as useful as they used to be. Even with
very modest hardware a single NTP server can serve time to hundreds of
thousands of clients using the ordinary mode.
=== Can chronyd transmit broadcast NTP packets (e.g. to synchronise other computers on a private LAN)?
=== Can +chronyd+ transmit broadcast NTP packets?
Yes. Starting from version 1.17, +chrony+ has this capability.
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).
=== Can +chrony+ keep the system clock a fixed offset away from real time?
=== Can +chronyd+ keep the system clock a fixed offset away from real time?
This is not possible as the program currently stands.
@ -271,3 +308,19 @@ aspect of the Solaris kernel has changed which prevents the same
technique working. We no longer have root access to any Solaris
machines to work on this, and we are reliant on somebody developing the
patch and testing it.
== Microsoft Windows
=== 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.
=== 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.