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Things `chronyd` can do that `ntpd` cannot do:
`chronyd` provides support for isolated networks where the only method of time correction is manual entry. For example, by the administrator looking at a clock. `chronyd` can examine the errors corrected at different updates to estimate the rate at which the computer gains or loses time, and use this estimate to trim the computer clock subsequently.
`chronyd` provides support to work out the rate of gain or loss of the real-time clock, the hardware clock, that maintains the time when the computer is turned off. It can use this data when the system boots to set the system time using an adjusted value of the time taken from the real-time clock. This is, at time of writing, only available in Linux.
Things `ntpd` can do that `chronyd` cannot do:
`ntpd` fully supports `NTP` version 4 ([citetitle]_RFC 5905_), including broadcast, multicast, manycast clients and servers, and the orphan mode. It also supports extra authentication schemes based on public-key cryptography ([citetitle]_RFC 5906_). `chronyd` uses `NTP` version 3 ([citetitle]_RFC 1305_), which is compatible with version 4.
`ntpd` includes drivers for many reference clocks whereas `chronyd` relies on other programs, for example [application]*gpsd*, to access the data from the reference clocks.
Choosing Between NTP Daemons
[application]*Chrony* should be considered for all systems which are frequently suspended or otherwise intermittently disconnected and reconnected to a network. Mobile and virtual systems for example.
The `NTP` daemon (`ntpd`) should be considered for systems which are normally kept permanently on. Systems which are required to use broadcast or multicast `IP`, or to perform authentication of packets with the `Autokey` protocol, should consider using `ntpd`. [application]*Chrony* only supports symmetric key authentication using a message authentication code (MAC) with MD5, SHA1 or stronger hash functions, whereas `ntpd` also supports the `Autokey` authentication protocol which can make use of the PKI system. `Autokey` is described in [citetitle]_RFC 5906_.
Understanding chrony and Its Configuration
Understanding chronyd
The [application]*chrony* daemon, `chronyd`, running in user space, makes adjustments to the system clock which is running in the kernel. It does this by consulting external time sources, using the `NTP` protocol, when ever network access allows it to do so. When external references are not available, `chronyd` will use the last calculated drift stored in the drift file. It can also be commanded manually to make corrections, by [application]*chronyc*.
Understanding chronyc
The [application]*chrony* daemon, `chronyd`, can be controlled by the command line utility [application]*chronyc*. This utility provides a command prompt which allows entering of a number of commands to make changes to `chronyd`. The default configuration is for `chronyd` to only accept commands from a local instance of [application]*chronyc*, but [application]*chronyc* can be used to alter the configuration so that `chronyd` will allow external control. [application]*chronyc* can be run remotely after first configuring `chronyd` to accept remote connections. The `IP` addresses allowed to connect to `chronyd` should be tightly controlled.
Understanding the chrony Configuration Commands
The default configuration file for `chronyd` is `/etc/chrony.conf`. The [option]`-f` option can be used to specify an alternate configuration file path. See the `chronyd` man page for further options. For a complete list of the directives that can be used see [citetitle]_link:++http://chrony.tuxfamily.org/manual.html#Configuration-file++[http://chrony.tuxfamily.org/manual.html#Configuration-file]_. Below is a selection of configuration options:
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allow
Optionally specify a host, subnet, or network from which to allow `NTP` connections to a machine acting as `NTP` server. The default is not to allow connections. Examples:
[subs="quotes"]
allow server1.example.com
Use this form to specify a particular host, by its host name, to be allowed access.
allow 192.0.2.0/24
Use this form to specify a particular network to be allowed access.
allow 2001:db8::/32
Use this form to specify an `IPv6` address to be allowed access.
cmdallow
This is similar to the [command]#allow# directive (see section [command]#allow#), except that it allows control access (rather than `NTP` client access) to a particular subnet or host. (By "control access" is meant that [application]*chronyc* can be run on those hosts and successfully connect to `chronyd` on this computer.) The syntax is identical. There is also a [command]#cmddeny all# directive with similar behavior to the [command]#cmdallow all# directive.
dumpdir
Path to the directory to save the measurement history across restarts of `chronyd` (assuming no changes are made to the system clock behavior whilst it is not running). If this capability is to be used (via the [command]#dumponexit# command in the configuration file, or the [command]#dump# command in [application]*chronyc*), the [command]#dumpdir# command should be used to define the directory where the measurement histories are saved.

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The default configuration file for `chronyd` is `/etc/chrony.conf`. The [option]`-f` option can be used to specify an alternate configuration file path. See the `chronyd` man page for further options. For a complete list of the directives that can be used see [citetitle]_link:++http://chrony.tuxfamily.org/manual.html#Configuration-file++[http://chrony.tuxfamily.org/manual.html#Configuration-file]_. Below is a selection of configuration options:
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./pages/servers/Configuring_NTP_Using_the_chrony_Suite.adoc:72
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pot/f26/pages/servers/Configuring_NTP_Using_the_chrony_Suite.pot, string 30