<title>Frequently asked questions</title>
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This is a set of questions and answers to common problems and issues.
As I receive more emails about the software, I will add new questions
to this page.
The author can be reached by email
include the word "chrony" in your subject line if possible (so that my
mail reader can keep my mail sorted by topic)!
S: Administrative issues
Q: Where can I get chrony source code?
Via the home page, see below.
Q: Are there any packaged versions of chrony?
I am aware of packages for Debian, Mandrake and Redhat. I am not personally
involved with how these are built or distributed.
Q: Where is the home page?
It is currently at <a href="http://chrony.sunsite.dk/">http://chrony.sunsite.dk/</a>.
Q: Is there a mailing list?
Yes, it's currently at firstname.lastname@example.org. There is a low-volume
list called chrony-announce which is just for announcements of new releases or
similar matters of high importance. You can join the lists by sending a
message to <a href="mailto:email@example.com">firstname.lastname@example.org</a> or
<a href="mailto:email@example.com">firstname.lastname@example.org</a> respectively.
For those who want to contribute to the development of chrony, there is a
developers' mailing list. You can subscribe by sending mail to
Q: What licence is applied to chrony?
Starting from version 1.15, chrony is licensed under the GNU General Public
License. Versions prior to 1.15 were licensed under a custom BSD-like
If you want to use parts of chrony in non-free software, you will need to use
older versions of the source code. Alternatively, contact me - I may be
prepared to licence parts of the source code to suit your purposes. I am quite
sympathetic to projects licensed under other free/open-source (but non-GPL)
licences, as well as to commercial projects which are of a single-customer
"turnkey" nature (as opposed to mass-market "shrink-wrap" or "floating-licence"
S: Chrony compared to other programs
Q: How does chrony compare to xntpd?
If your computer is permenently connected, or connected for long periods (that
is, for the several hours it takes xntpd to settle down), or you need to
support hardware reference clocks to your computer, then xntpd will work fine.
Apart from not supporting hardware clocks, chrony will work fine too.
If your computer connects to the 'net for 5 minutes once a day (or something
like that), or you turn your (Linux v2.0) computer off when you're not using
it, or you want to use NTP on an isolated network with no hardware clocks in
sight, chrony will work much better for you.
The reason I wrote chrony was that I could not get xntpd to do
anything sensible on my PC at home, which is connected to the 'net for
about 5 minutes once or twice a day, mainly to upload/download email
and news. Nowadays it is also turned off for 22-23 hours a day, when
not in use. I wanted a program which would :
- slew the time to correct it when I go online and NTP servers become visible
- determine the rate at which the computer gains or loses time and use this
information to keep it reasonably correct between connects to the 'net. This
has to be done using a method that does not care about the intermittent
availability of the references or the fact the computer is turned off between
groups of measurements..
- maintain the time across reboots, by working out the error and drift rate of
the computer's real-time clock and using this information to set the system
clock correctly at boot up. (In the last few months, it became impossible for
me to leave my computer powered permanently.)
Also, when working with isolated networks with no true time references
at all, I found xntpd gave me no help with managing the local clock's
gain/loss rate on the NTP master node (which I set from my watch). I
added some automated support in chrony to deal with this.
S: Compilation issues
Q:How do I apply source patches?
Sometimes I release source patches rather than a full version when I need to
provide a fix for small problems. Supposing you have chrony-1.X.tar.gz and a
source patch chrony-1.X-1.X.1.gz. The steps required are:
tar xzvf ../chrony-1.X.tar.gz
gunzip < ../../chrony-1.X-1.X.1.gz | patch -p1
Q:Can I compile chrony with an ANSI-C compiler that is not GCC v2.x?
I have had reports that chrony can be compiled with GCC v1.42, by using the
following trick when running make
make CC='gcc -D__FUNCTION__=\"function_not_available\"'
(this gets around the lack of a __FUNCTION__ macro in GCC v1.)
The same trick may be enough to allow other compilers to be used.
Q: I get errors like 'client.c:44: readline/readline.h: file not found'
Read the section about 'readline' in the INSTALL file or in chrony.txt. You
may need to disable readline support (e.g. if you haven't got readline
installed at all, or just don't want it), or specify the location of the
readline files (e.g. if you've installed them in a non-standard place).
Q: I have RedHat 7.3 and can't compile rtc_linux.c (error in spinlock.h)
The following solution has been found for this. Enter the following 3 commands
mv linux linux.rh
ln -s /usr/src/linux/include/linux ./linux
The problem seems to be that RedHat provide their own kernel header files in
/usr/include/linux. Besides differing from those used by your current kernel,
if you compiled it yourself, they also seem to have been changed in a way that
causes a problem compiling chrony. Chrony compiles fine with standard kernel
There have also been reports that just replacing the file
/usr/src/linux/spinlock.h by the equivalent file from a vanilla kernel source
tree is sufficient to fix the problem.
Note : from version 1.21 onwards, this problem no longer exists. The kernel
header files are no longer included.
S: Selection of NTP servers
Q: I have several computers on a LAN. Should I make one the master, or make them all clients of an external server?
I think the best configuration is to make one computer the master, with the
others as clients of it. Add a 'local' directive to the master's chrony.conf
file. This configuration will be better because
* the load on the external connection is less
* the load on the external NTP server(s) is less
* if your external connection goes down, the computers on the LAN will maintain
a common time with each other.
S: Addressing issues
Q: I get the following error message : "Could not get IP adress for localhost"
Add a line like the following to your /etc/hosts file
Q: I have problems if I put the names of my NTP servers in the chrony.conf file.
If you have no connection to the Internet at boot time, chrony won't be able to
turn the names into IP addresses when it starts. There seem to be 2 solutions:
1. Put the numeric IP addresses in the chrony.conf file
2. Put the server->IP address mappings in your /etc/hosts file and ensure that
/etc/host.conf reads 'order hosts,bind'.
The problem is that chronyd (currently) isn't designed in a way that allows
hostname->IP address lookups during normal operation. I hope to work on this
problem very soon.
S: My computer is not synchronising.
This is the most common problem. There are a number of reasons, see the
Q: 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 etherfind 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
been running for a short period. See if any measurements appear.
Most people run chronyd on the firewall itself, to avoid all issues of UDP
packet forwarding and/or masquerading.
Q: Do you have a non-permanant (i.e. intermittent) Internet connection?
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.
Q: In measurements.log, do the '7' and '8' flag columns always show zero?
Do you have a 'local stratum X' directive in the chrony.conf file? If X is
lower than the stratum of the server you're trying to use, this situation will
arise. You should always make X quite high (e.g. 10) in this directive.
S: Issues with chronyd
Q: chronyd crashes after a syslog message "adjtimex failed for set frequency"
The usual cause is that the kernel is running with a different value of 'HZ'
(the timer interrupt rate) than the value that was found in the kernel header
files when chrony was compiled. The chrony.conf file can include options to
modify the HZ value (see the discussion of linux_hz and linux_freq_scale in the
documentation), however the problem is to find the value of HZ being used.
At the end of the chrony v1.18 section of the <a href="./download.php">download page</a>
you'll find instructions on how to do this.
This will be fixed in version 1.19, by getting chronyd to auto-detect the
kernel's value rather than relying on the compiled-in default.
S: Issues with chronyc
Q: I keep getting the error '510 No command access from this host --- Reply not authenticated'.
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. This
shouldn't be necessary for localhost, but some people still seem to need an
entry like 'cmdallow 127.0.0.1'. (It would be good to understand why problem
only affects some people).
Q: I cannot log in from chronyc to carry out privileged tasks.
This is the second most common problem.
Perhaps your /etc/chrony.keys file is badly formatted. Make sure that the
final line has a line feed at the end, otherwise the key on that line will work
as though the last character is missing. (Note, this bug was fixed in version
Q: When I enter a command and hit <Return>, chronyc hangs
This probably means that chronyc cannot communicate with chronyd.
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.
Q: Is the chronyc<->chronyd protocol documented anywhere?
Only by the source code :-) See cmdmon.c (chronyd side) and client.c (chronyc
S: Real-time clock issues.
Q: 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.
Q: I want to use chronyd's real-time clock support. Must I disable hwclock?
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 <b>shutdown</b>
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.
There is no need to remove hwclock from the boot process, as long as chronyd is
started after it has run.
Q: 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:
* a kernel that is supported (e.g. 2.2 onwards)
* enhanced RTC support compiled into the kernel
* an 'rtcfile' directive in your chrony.conf file.
S: Problems with isolated networks.
Q: When I use the 'settime' command, chronyd crashes.
If you enter times that are too far away from the real time, chronyd will
think the system clock runs fast or slow by an excessive amount. The required
compensation factor will be outside the bounds for the adjtimex() system call.
chronyd will crash when it tries to apply such an excessive adjustment.
S: Microsoft Windows
Q: 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 an NT service.
Q: Are there any plans to support Windows?
I have no personal plans to do this. I have neither the time nor the
Windows programming expertise. Some time ago I did start work on a port which
I was developing under Cygwin. Anyone is welcome to pick this work up and
contribute it back to the project.
Q: What alternative NTP clients are there for Windows?
Some of the names I've seen mentioned are
- NetTime (nettime.sourceforge.net)
S: NTP-specific issues
Q: Can chrony be driven from broadcast NTP servers?
No. I remember looking at how they worked when I was first writing chrony.
Since the 'target market' then was dial-up systems, broadcast packets were not
relevant so I didn't bother working out how to deal with the complexities of
doing the delay estimation.
I no longer have root access to a LAN environment to develop and test broadcast
server support. Neither have I the time to work on this. I would be very
happy to accept a patch from anyone who can develop, test and debug the
Q: Can chronyd transmit broadcast NTP packets (e.g. to synchronise other computers on a private LAN)?
Yes. Starting from version 1.17, chrony has this capability.
Q: Can chrony keep the system clock a fixed offset away from real time?
I have not experimented much, but I don't believe this would be possible as
the program currently stands.
Q: What happens if the network connection is dropped without using chronyc's 'offline' command first?
In this case chronyd will keep trying to access the server(s) that it thinks
are online. Eventually it will decide that they are unreachable and no longer
consider itself synchronised to them. If you have other computers on your LAN
accessing the computer that is affected this way, they too will become
'unsynchronised', unless you have the 'local' directive set up on the master
The 'auto_offline' option to the 'server' entry in the chrony.conf file may be
useful to avoid this situation.
Q: Can I get the source via CVS from anywhere?
Yes. See <a href="http://chrony.sunsite.dk/cvs.php">http://chrony.sunsite.dk/cvs.php</a> for information. Currently there is
only anonymous read-only access. I keep the master copy on my own PC, which is
more convenient for me because I don't have to connect to the Internet to do
CVS operations on the files. So for now, there is no read-write access for
other developers. Please email me your patches + documentation instead.
S: Linux-specific issues
Q: Why does the source code include kernel header files?
The program needs to see the definitions of structures used to interact with
the real time clock (via /dev/rtc) and with the adjtimex() system call. Sadly
this has led to a number of compilation problems with newer kernels which have
been increasingly hard to fix in a way that makes the code compilable on all
Linux kernel versions (from 2.0 up anyway, I doubt 1.x still works.) Hopefully
the situation will not deteriorate further with future kernel versions.
Q: I get "Could not open /dev/rtc, Device or resource busy" in my syslog file.
Check that you haven't accidentally got two copies of chronyd running (perhaps
defined in different start-up scripts.)
S: Solaris-specific issues
Q: On Solaris 2.8, I get an error message about not being able to open kvm to change dosynctodr.
(The dosynctodr variable controls whether Solaris couples the equivalent of its
BIOS clock into its system clock at regular intervals). The Solaris port of
chrony was developed in the Solaris 2.5 era. Some aspect of the Solaris kernel
has changed which prevents the same technique working. I no longer have root
access to any Solaris machines to work on this, and am reliant on somebody
developing the patch and testing it. A good starting point would be to see if
xntpd has been modified to work for Solaris 2.8.
<a href="mailto:email@example.com?subject=chrony">the author</a>'s
<a href="http://www.rc0.org.uk/">main page</a>