Difference between revisions of "Systemd-timesyncd"

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Revision as of 00:28, 30 December 2014

From the systemd mailing list:

systemd-timesyncd is a daemon that has been added for synchronizing the system clock across the network. It implements an SNTP client. In contrast to NTP implementations such as chrony or the NTP reference server this only implements a client side, and does not bother with the full NTP complexity, focusing only on querying time from one remote server and synchronizing the local clock to it. Unless you intend to serve NTP to networked clients or want to connect to local hardware clocks this simple NTP client should be more than appropriate for most installations. The daemon runs with minimal privileges, and has been hooked up with networkd to only operate when network connectivity is available. The daemon saves the current clock to disk every time a new NTP sync has been acquired, and uses this to possibly correct the system clock early at bootup, in order to accommodate for systems that lack an RTC such as the Raspberry Pi and embedded devices, and make sure that time monotonically progresses on these systems, even if it is not always correct. To make use of this daemon a new system user and group "systemd-timesync" needs to be created on installation of systemd.

Installation

The systemd-timesyncd service is available with systemd >= 213. To start and enable it:

# timedatectl set-ntp true 
Tip: Before systemd 216 systemd-timesyncd required systemd-networkd to be started (without further configuration) for notification of network up-/down events. This is reportedly not universally the case anymore, for example it now receives the events from dhcpcd and NetworkManager as well, but may still be required depending on network configuration or used network management tool.

Configuration

When starting, systemd-timesyncd will read the configuration file from /etc/systemd/timesyncd.conf. As of systemd 217 it looks like this:

/etc/systemd/timesyncd.conf
[Time]
#NTP=
#FallbackNTP=0.arch.pool.ntp.org 1.arch.pool.ntp.org 2.arch.pool.ntp.org 3.arch.pool.ntp.org

To add time servers or change the provided ones, uncomment the relevant line and list their host name or IP separated by a space. For example, you can use any servers provided by the NTP pool project or use the default Arch ones (also provided by the NTP pool project):

/etc/systemd/timesyncd.conf
[Time]
NTP=0.arch.pool.ntp.org 1.arch.pool.ntp.org 2.arch.pool.ntp.org 3.arch.pool.ntp.org
FallbackNTP=0.pool.ntp.org 1.pool.ntp.org 0.fr.pool.ntp.org

Further to the daemon configuration, NTP servers may also be provided via a systemd-networkd configuration with a NTP= option or, dynamically, via a DHCP server (since systemd 216).

The NTP server to be used will be determined using the following rules:

  • Any per-interface NTP servers obtained from systemd-networkd.service(8) configuration or via DHCP take precedence.
  • The NTP servers defined in /etc/systemd/timesyncd.conf will be appended to the per-interface list at runtime and the daemon will contact the servers in turn until one is found that responds.
  • If no NTP server information is acquired after completing those steps, the NTP server host names or IP addresses defined in FallbackNTP= will be used.

See also