chrony_3_3/doc/faq.adoc

// This file is part of chrony
//
// Copyright (C) Richard P. Curnow  1997-2003
// Copyright (C) Miroslav Lichvar  2014-2016
//
// 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.

= Frequently Asked Questions
:toc:
:numbered:

== `chrony` compared to other programs

=== 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
https://chrony.tuxfamily.org/comparison.html[comparison page] on the `chrony`
website.

== Configuration issues

=== 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
http://www.pool.ntp.org/[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
----

=== 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.

=== 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

* 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.

=== 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.

=== 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.

=== 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).

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 very short polling intervals for 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 larger 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, the `xleave` option should be
added to the `server` directive in order to allow the server to send the
client more accurate hardware or kernel transmit timestamps. When combined with
local hardware timestamping, sub-microsecond accuracy may be possible. An
example could be

----
server ntp.local minpoll 2 maxpoll 2 xleave
hwtimestamp eth0
----

=== 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'
----

=== 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.

== Computer is not synchronising

This is the most common problem. There are a number of reasons, see the
following questions.

=== 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
----

=== 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
----

=== 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
----

== Issues with `chronyc`

=== 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.

=== 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).

=== 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.

=== Is the `chronyc` / `chronyd` protocol documented anywhere?

Only by the source code. See _cmdmon.c_ (`chronyd` side) and _client.c_
(`chronyc` side).

== Real-time clock issues

=== 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.

=== 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

For the real-time clock support to work, you need the following three
things

* an RTC in your computer
* a Linux kernel with enabled RTC support
* an `rtcfile` directive in your _chrony.conf_ file

=== 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.

=== 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.

== NTP-specific issues

=== 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.

=== 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).

=== 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.

=== 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.

== Operating systems

=== 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.