404 lines
18 KiB
Text
404 lines
18 KiB
Text
// This file is part of chrony
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//
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// Copyright (C) Richard P. Curnow 1997-2003
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// Copyright (C) Miroslav Lichvar 2014-2016
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//
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// This program is free software; you can redistribute it and/or modify
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// it under the terms of version 2 of the GNU General Public License as
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// published by the Free Software Foundation.
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//
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// This program is distributed in the hope that it will be useful, but
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// WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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// General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License along
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// with this program; if not, write to the Free Software Foundation, Inc.,
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// 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
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= Frequently Asked Questions
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:toc:
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:numbered:
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== `chrony` compared to other programs
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=== How does `chrony` compare to `ntpd`?
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`chronyd` was designed to work well in a wide range of conditions and it can
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usually synchronise the system clock faster and with better time accuracy. It
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doesn't implement some of the less useful NTP modes like broadcast client or
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multicast server/client.
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If your computer is connected to the Internet only for few minutes at a time,
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the network connection is often congested, you turn your computer off or
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suspend it frequently, the clock is not very stable (e.g. there are rapid
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changes in the temperature or it's a virtual machine), or you want to use NTP
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on an isolated network with no hardware reference clocks in sight, `chrony`
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will probably work much better for you.
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For a more detailed comparison of features and performance, see the
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https://chrony.tuxfamily.org/comparison.html[comparison page] on the `chrony`
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website.
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== Configuration issues
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=== What is the minimum recommended configuration for an NTP client?
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First, the client needs to know which NTP servers it should ask for the current
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time. They are specified by the `server` or `pool` directive. The `pool`
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directive can be used for names that resolve to multiple addresses. For good
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reliability the client should have at least three servers. The `iburst` option
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speeds up the initial synchronisation.
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To stabilize the initial synchronisation on the next start, the estimated drift
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of the system clock is saved to a file specified by the `driftfile` directive.
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If the system clock can be far from the true time after boot for any reason,
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`chronyd` should be allowed to correct it quickly by stepping instead of
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slewing, which would take a very long time. The `makestep` directive does
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that.
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In order to keep the real-time clock (RTC) close to the true time, so the
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system time is reasonably close to the true time when it's initialized on the
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next boot from the RTC, the `rtcsync` directive enables a mode in which the
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system time is periodically copied to the RTC. It is supported on Linux and Mac
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OS X.
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If you want to use public NTP servers from the
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http://www.pool.ntp.org/[pool.ntp.org] project, the minimal _chrony.conf_ file
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could be:
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----
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pool pool.ntp.org iburst
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driftfile /var/lib/chrony/drift
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makestep 1 3
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rtcsync
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----
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=== How do I make an NTP server from an NTP client?
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You need to add an `allow` directive to the _chrony.conf_ file in order to open
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the NTP port and allow `chronyd` to reply to client requests. `allow` with no
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specified subnet allows all IPv4 and IPv6 addresses.
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=== I have several computers on a LAN. Should be all clients of an external server?
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The best configuration is usually to make one computer the server, with
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the others as clients of it. Add a `local` directive to the server's
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_chrony.conf_ file. This configuration will be better because
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* the load on the external connection is less
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* the load on the external NTP server(s) is less
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* if your external connection goes down, the computers on the LAN
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will maintain a common time with each other.
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=== Must I specify servers by IP address if DNS is not available on chronyd start?
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No. Starting from version 1.25, `chronyd` will keep trying to resolve
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the names specified by the `server`, `pool`, and `peer` directives in an
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increasing interval until it succeeds. The `online` command can be issued from
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`chronyc` to try to resolve them immediately.
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=== How can I make `chronyd` more secure?
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If you don't need to serve time to NTP clients or peers, you can add `port 0`
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to the _chrony.conf_ file to completely disable the NTP server functionality
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and prevent NTP requests from reaching `chronyd`. Starting from version 2.0,
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the NTP server port is open only when client access is allowed by the `allow`
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directive or command, an NTP peer is configured, or the `broadcast` directive
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is used.
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If you don't need to use `chronyc` remotely, you can add the following
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directives to the configuration file to bind the command sockets to the
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loopback interface. This is done by default since version 2.0.
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----
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bindcmdaddress 127.0.0.1
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bindcmdaddress ::1
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----
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If you don't need to use `chronyc` at all or you need to run `chronyc` only
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under the root or _chrony_ user (which can access `chronyd` through a Unix
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domain socket since version 2.2), you can disable the internet command sockets
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completely by adding `cmdport 0` to the configuration file.
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You can specify an unprivileged user with the `-u` option, or the `user`
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directive in the _chrony.conf_ file, to which `chronyd` will switch after start
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in order to drop root privileges. The configure script has a `--with-user`
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option, which sets the default user. On Linux, `chronyd` needs to be compiled
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with support for the `libcap` library. On other systems, `chronyd` forks into
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two processes. The child process retains root privileges, but can only perform
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a very limited range of privileged system calls on behalf of the parent.
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Also, if `chronyd` is compiled with support for the Linux secure computing
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(seccomp) facility, you can enable a system call filter with the `-F` option.
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It will significantly reduce the kernel attack surface and possibly prevent
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kernel exploits from the `chronyd` process if it's compromised. It's
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recommended to enable the filter only when it's known to work on the version of
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the system where `chrony` is installed as the filter needs to allow also system
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calls made from libraries that `chronyd` is using (e.g. libc) and different
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versions or implementations of the libraries may make different system calls.
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If the filter is missing some system call, `chronyd` could be killed even in
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normal operation.
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=== How can I improve the accuracy of the system clock with NTP sources?
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Select NTP servers that are well synchronised, stable and close to your
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network. It's better to use more than one server, three or four is usually
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recommended as the minimum, so `chronyd` can detect servers that serve false
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time and combine measurements from multiple sources.
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There are also useful options which can be set in the `server` directive, they
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are `minpoll`, `maxpoll`, `polltarget`, `maxdelay`, `maxdelayratio` and
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`maxdelaydevratio`.
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The first three options set the minimum and maximum allowed polling interval,
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and how should be the actual interval adjusted in the specified range. Their
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default values are 6 (64 seconds) for `minpoll`, 10 (1024 seconds) for
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`maxpoll` and 6 (samples) for `polltarget`. The default values should be used
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for general servers on the Internet. With your own NTP servers or if have
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permission to poll some servers more frequently, setting these options for
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shorter polling intervals may significantly improve the accuracy of the system
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clock.
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The optimal polling interval depends mainly on two factors, stability of the
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network latency and stability of the system clock (which mainly depends on the
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temperature sensitivity of the crystal oscillator and the maximum rate of the
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temperature change).
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An example of the directive for an NTP server on the Internet that you are
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allowed to poll frequently could be
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----
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server foo.example.net minpoll 4 maxpoll 6 polltarget 16
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----
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An example using very short polling intervals for a server located in the same
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LAN could be
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----
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server ntp.local minpoll 2 maxpoll 4 polltarget 30
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----
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The maxdelay options are useful to ignore measurements with larger delay (e.g.
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due to congestion in the network) and improve the stability of the
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synchronisation. The `maxdelaydevratio` option could be added to the example
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with local NTP server
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----
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server ntp.local minpoll 2 maxpoll 4 polltarget 30 maxdelaydevratio 2
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----
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=== What happened to the `commandkey` and `generatecommandkey` directives?
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They were removed in version 2.2. Authentication is no longer supported in the
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command protocol. Commands that required authentication are now allowed only
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through a Unix domain socket, which is accessible only by the root and _chrony_
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users. If you need to configure `chronyd` remotely or locally without the root
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password, please consider using ssh and/or sudo to run `chronyc` under the root
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or _chrony_ user on the host where `chronyd` is running.
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== Computer is not synchronising
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This is the most common problem. There are a number of reasons, see the
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following questions.
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=== Behind a firewall?
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Check the `Reach` value printed by the ``chronyc``'s `sources` command. If it's
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zero, it means `chronyd` did not get any valid responses from the NTP server
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you are trying to use. If there is a firewall between you and the server, the
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packets may be blocked. Try using a tool like `wireshark` or `tcpdump` to see
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if you're getting any responses from the server.
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When `chronyd` is receiving responses from the servers, the output of the
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`sources` command issued few minutes after `chronyd` start might look like
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this:
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----
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210 Number of sources = 3
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MS Name/IP address Stratum Poll Reach LastRx Last sample
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===============================================================================
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^* foo.example.net 2 6 377 34 +484us[ -157us] +/- 30ms
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^- bar.example.net 2 6 377 34 +33ms[ +32ms] +/- 47ms
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^+ baz.example.net 3 6 377 35 -1397us[-2033us] +/- 60ms
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----
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=== Are NTP servers specified with the `offline` option?
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Check that you're using ``chronyc``'s `online` and `offline` commands
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appropriately. Again, check in _measurements.log_ to see if you're getting any
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data back from the server.
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=== Is `chronyd` allowed to step the system clock?
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By default, `chronyd` adjusts the clock gradually by slowing it down or
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speeding it up. If the clock is too far from the true time, it will take
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a long time to correct the error. The `System time` value printed by the
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``chronyc``'s `tracking` command is the remaining correction that needs to be
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applied to the system clock.
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The `makestep` directive can be used to allow `chronyd` to step the clock. For
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example, if _chrony.conf_ had
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----
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makestep 1 3
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----
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the clock would be stepped in the first three updates if its offset was larger
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than one second. Normally, it's recommended to allow the step only in the first
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few updates, but in some cases (e.g. a computer without an RTC or virtual
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machine which can be suspended and resumed with an incorrect time) it may be
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necessary to allow the step on any clock update. The example above would change
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to
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----
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makestep 1 -1
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----
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== Issues with `chronyc`
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=== I keep getting the error `506 Cannot talk to daemon`
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When accessing `chronyd` remotely, make sure that the _chrony.conf_ file (on
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the computer where `chronyd` is running) has a `cmdallow` entry for the
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computer you are running `chronyc` on and an appropriate `bindcmdaddress`
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directive. This isn't necessary for localhost.
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Perhaps `chronyd` is not running. Try using the `ps` command (e.g. on Linux,
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`ps -auxw`) to see if it's running. Or try `netstat -a` and see if the ports
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123/udp and 323/udp are listening. If `chronyd` is not running, you may have a
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problem with the way you are trying to start it (e.g. at boot time).
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Perhaps you have a firewall set up in a way that blocks packets on port
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323/udp. You need to amend the firewall configuration in this case.
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=== I keep getting the error `501 Not authorised`
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Since version 2.2, the `password` command doesn't do anything and `chronyc`
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needs to run locally under the root or _chrony_ user, which are allowed to
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access the ``chronyd``'s Unix domain command socket.
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With older versions, you need to authenticate with the `password` command first
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or use the `-a` option to authenticate automatically on start. The
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configuration file needs to specify a file which contains keys (`keyfile`
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directive) and which key in the key file should be used for `chronyc`
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authentication (`commandkey` directive).
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=== Why does `chronyc tracking` always print an IPv4 address as reference ID?
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The reference ID is a 32-bit value and is always printed in quad-dotted
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notation, even if the reference source doesn't have an IPv4 address. For IPv4
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addresses, the reference ID is equal to the address, but for IPv6 addresses it
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is the first 32 bits of the MD5 sum of the address. For reference clocks, the
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reference ID is the value specified with the `refid` option in the `refclock`
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directive.
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If you need to get the IP address of the current reference source, use the `-n`
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option to disable resolving of IP addresses and read the second field (printed
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in parentheses) on the `Reference ID` line.
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=== Is the `chronyc` / `chronyd` protocol documented anywhere?
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Only by the source code. See _cmdmon.c_ (`chronyd` side) and _client.c_
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(`chronyc` side).
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== Real-time clock issues
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=== What is the real-time clock (RTC)?
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This is the clock which keeps the time even when your computer is turned off.
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It is used to initialize the system clock on boot. It normally doesn't drift
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more than few seconds per day.
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There are two approaches how `chronyd` can work with it. One is to use the
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`rtcsync` directive, which tells `chronyd` to enable a kernel mode which sets
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the RTC from the system clock every 11 minutes. `chronyd` itself won't touch
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the RTC. If the computer is not turned off for a long time, the RTC should
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still be close to the true time when the system clock will be initialized from
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it on the next boot.
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The other option is to use the `rtcfile` directive, which tells `chronyd` to
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monitor the rate at which the RTC gains or loses time. When `chronyd` is
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started with the `-s` option on the next boot, it will set the system time from
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the RTC and also compensate for the drift it has measured previously. The
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`rtcautotrim` directive can be used to keep the RTC close to the true time, but
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it's not strictly necessary if its only purpose is to set the system clock when
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`chronyd` is started on boot. See the documentation for details.
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=== I want to use ``chronyd``'s RTC support. Must I disable `hwclock`?
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The `hwclock` program is often set-up by default in the boot and shutdown
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scripts with many Linux installations. With the kernel RTC synchronisation
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(`rtcsync` directive), the RTC will be set also every 11 minutes as long as the
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system clock is synchronised. If you want to use ``chronyd``'s RTC monitoring
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(`rtcfile` directive), it's important to disable `hwclock` in the shutdown
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procedure. If you don't, it will over-write the RTC with a new value, unknown
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to `chronyd`. At the next reboot, `chronyd` started with the `-s` option will
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compensate this (wrong) time with its estimate of how far the RTC has drifted
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whilst the power was off, giving a meaningless initial system time.
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There is no need to remove `hwclock` from the boot process, as long as `chronyd`
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is started after it has run.
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=== I just keep getting the `513 RTC driver not running` message
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For the real-time clock support to work, you need the following three
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things
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* an RTC in your computer
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* a Linux kernel with enabled RTC support
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* an `rtcfile` directive in your _chrony.conf_ file
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=== I get `Could not open /dev/rtc, Device or resource busy` in my syslog file
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Some other program running on the system may be using the device.
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== NTP-specific issues
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=== Can `chronyd` be driven from broadcast NTP servers?
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No, the broadcast client mode is not supported and there is currently no plan
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to implement it. The broadcast and multicast modes are inherently less
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accurate and less secure (even with authentication) than the ordinary
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server/client mode and they are not as useful as they used to be. Even with
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very modest hardware a single NTP server can serve time to hundreds of
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thousands of clients using the ordinary mode.
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=== Can `chronyd` transmit broadcast NTP packets?
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Yes, the `broadcast` directive can be used to enable the broadcast server mode
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to serve time to clients in the network which support the broadcast client mode
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(it's not supported in `chronyd`, see the previous question).
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=== Can `chronyd` keep the system clock a fixed offset away from real time?
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This is not possible as the program currently stands.
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=== What happens if the network connection is dropped without using ``chronyc``'s `offline` command first?
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`chronyd` will keep trying to access the server(s) that it thinks are online.
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When the network is connected again, it will take some time (on average half of
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the maximum polling interval) before new measurements are made and the clock is
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corrected. If the servers were set to offline and the `online` command was
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issued when the network was connected, `chronyd` would make new measurements
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immediately.
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The `auto_offline` option to the `server` entry in the _chrony.conf_ file may
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be useful to switch the servers to the offline state automatically.
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== Operating systems
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=== Does `chrony` support Windows?
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No. The `chronyc` program (the command-line client used for configuring
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`chronyd` while it is running) has been successfully built and run under
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Cygwin in the past. `chronyd` is not portable, because part of it is
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very system-dependent. It needs adapting to work with Windows'
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equivalent of the adjtimex() call, and it needs to be made to work as a
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service.
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=== Are there any plans to support Windows?
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We have no plans to do this. Anyone is welcome to pick this work up and
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contribute it back to the project.
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