Difference between revisions of "Network Time Protocol daemon"
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Revision as of 15:29, 22 May 2013
zh-CN:Network Time Protocol daemon This article describes how to set up and run NTPd (Network Time Protocol daemon), the most common method to synchronize the software clock of a GNU/Linux system with internet time servers using the Network Time Protocol; if set up correctly, NTPd can make your computer act as a time server itself.
Contents
Installation
Install ntp, available in the Official Repositories.
Configuration
Configuring connection to NTP servers
The first thing you define in your /etc/ntp.conf is the servers your machine will synchronize to.
NTP servers are classified in a hierarchical system with many levels called strata: the devices which are considered independent time sources are classified as stratum 0 sources; the servers directly connected to stratum 0 devices are classified as stratum 1 sources; servers connected to stratum 1 sources are then classified as stratum 2 sources and so on.
It has to be understood that a server's stratum cannot be taken as an indication of its accuracy or reliability. Typically, stratum 2 servers are used for general synchronization purposes: if you do not already know the servers you are going to connect to, you should use the pool.ntp.org servers (alternate link) and choose the server pool that is closest to your location.
The following lines are just an example:
/etc/ntp.conf
server 0.pool.ntp.org iburst server 1.pool.ntp.org iburst server 2.pool.ntp.org iburst server 3.pool.ntp.org iburst
The iburst option is recommended, and sends a burst of packets only if it cannot obtain a connection with the first attempt. The burst option always does this, even on the first attempt, and should never be used without explicit permission and may result in blacklisting.
Configuring your own NTP server
If setting up an NTP server, you need to add local clock as a server, so that, in case it loses internet access, it will continue serving time to the network; add local clock as a stratum 10 server (using the fudge command) so that it will never be used unless internet access is lost:
server 127.127.1.0 fudge 127.127.1.0 stratum 10
Next, define the rules that will allow clients to connect to your service (localhost is considered a client too) using the restrict command; you should already have a line like this in your file:
restrict default nomodify nopeer noquery
This restricts everyone from modifying anything and prevents everyone from querying the status of your time server: nomodify prevents reconfiguring your ntpd (with ntpq or ntpdc), and noquery prevents dumping status data from your ntpd (also with ntpq or ntpdc).
You can also add other options:
restrict default kod nomodify notrap nopeer noquery
Full docs for the "restrict" option are in man ntp_acc. See https://support.ntp.org/bin/view/Support/AccessRestrictions for detailed instructions.
Following this line, you need to tell ntpd what to allow through into your server; the following line is enough if you are not configuring an NTP server:
restrict 127.0.0.1
If you want to force DNS resolution to the IPv6 namespace, write -6 before the IP address or host name (-4 forces IPv4 instead), for example:
restrict -6 default kod nomodify notrap nopeer noquery restrict -6 ::1 # ::1 is the IPv6 equivalent for 127.0.0.1
Lastly, specify the drift file (which keeps track of your clock's time deviation) and optionally the log file location:
driftfile /var/lib/ntp/ntp.drift logfile /var/log/ntp.log
A very basic configuration file will look like this (all comments have been stripped out for clarity):
/etc/ntp.conf
server 0.pool.ntp.org iburst server 1.pool.ntp.org iburst server 2.pool.ntp.org iburst server 3.pool.ntp.org iburst restrict default kod nomodify notrap nopeer noquery restrict -6 default kod nomodify notrap nopeer noquery restrict 127.0.0.1 restrict -6 ::1 driftfile /var/lib/ntp/ntp.drift logfile /var/log/ntp.log
Other resources about NTP configuration
In conclusion, never forget man pages: man ntp.conf is likely to answer any doubts you could still have (see also the related man pages: man {ntpd|ntp_auth|ntp_mon|ntp_acc|ntp_clock|ntp_misc}).
Using without daemon
To synchronize your system clock just once, without starting the NTP daemon, run:
# ntpd -qg
The -g option allows shifting the clock further than the panic threshold (15 min by default) without a warning. Note that such offset is abnormal and might indicate either wrong timezone setting, clock chip failure, or simply a very long period of neglect. If in these cases you would rather not set the clock and print an error to syslog, remove -g.
After updating the system clock, store the time to the hardware clock so that it is preserved when rebooting:
# hwclock -w
Synchronize once per boot
Write a oneshot systemd unit:
/etc/systemd/system/ntp-once.service
[Unit] Description=Network Time Service (once) After=network.target nss-lookup.target [Service] Type=oneshot ExecStart=/usr/bin/ntpd -g -u ntp:ntp ; /sbin/hwclock -w [Install] WantedBy=multi-user.targetand enable it:
# systemctl enable ntp-once
Note that a systemd unit of the type oneshot executes once only. Hence the ntpd -q option should not be used in this case.
Running as a daemon
To start ntpd:
# systemctl start ntpd
To enable ntpd at startup:
# systemctl enable ntpd
Or alternatively with the command:
# timedatectl set-ntp 1
Check whether the daemon is synchronizing correctly
Before you can use the ntpq command you'll need to use pacman and install the libedit package. Then use ntpq to see the list of configured peers:
$ ntpq -np
The delay, offset and jitter columns should be non-zero. The servers ntpd is synchronizing with are prefixed by an asterisk. It can take several minutes before ntpd selects a server to synchronize with; try checking after 17 minutes (1024 seconds).
NetworkManager
ntpd can be brought up/down along with a network connection through the use of NetworkManager's dispatcher scripts. You will need to install networkmanager-dispatcher-ntpd from the official repositories.
Running in a chroot
Edit /etc/conf.d/ntpd.conf and change
NTPD_ARGS="-g -u ntp:ntp"
to
NTPD_ARGS="-g -i /var/lib/ntp -u ntp:ntp"
Then, edit /etc/ntp.conf to change the driftfile path such that it is relative to the chroot directory, rather than to the real system root. Change:
driftfile /var/lib/ntp/ntp.drift
to
driftfile /ntp.drift
Create a suitable chroot environment so that getaddrinfo() will work by creating pertinent directories and files (as root):
# mkdir /var/lib/ntp/etc /var/lib/ntp/lib /var/lib/ntp/proc # touch /var/lib/ntp/etc/resolv.conf /var/lib/ntp/etc/services
and by bind-mounting the aformentioned files:
/etc/fstab
... #ntpd chroot mounts /etc/resolv.conf /var/lib/ntp/etc/resolv.conf none bind 0 0 /etc/services /var/lib/ntp/etc/services none bind 0 0 /lib /var/lib/ntp/lib none bind 0 0 /proc /var/lib/ntp/proc none bind 0 0
# mount -a
Finally, restart the daemon again:
# systemctl restart ntpd
It is relatively difficult to be sure that your driftfile configuration is actually working without waiting a while, as ntpd does not read or write it very often. If you get it wrong, it will log an error; if you get it right, it will update the timestamp. If you do not see any errors about it after a full day of running, and the timestamp is updated, you should be confident of success.
Alternatives
An alternative to NTPd is Chrony, a dial-up friendly and specifically designed for systems that are not online all the time.
See also
- Time (for more information on computer timekeeping)