Difference between revisions of "Network Time Protocol daemon"
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=== Netctl === | === Netctl === |
Revision as of 05:00, 16 February 2014
zh-CN:Network Time Protocol daemon Network Time Protocol is the most common method to synchronize the software clock of a GNU/Linux system with internet time servers. It is designed to mitigate the effects of variable network latency and can usually maintain time to within tens of milliseconds over the public Internet. The accuracy on local area networks is even better, up to one millisecond.
The NTP Project provides a reference implementation of the protocol called simply NTP. An alternative to NTP is Chrony, a dial-up friendly and specifically designed for systems that are not online all the time, and OpenNTPD, part of the OpenBSD project and currently not maintained for Linux.
This article further describes how to set up and run the NTP daemon, both as a client and as a server.
Installation
Install ntp, available in the official repositories.
Configuration
The main daemon is ntpd, which is configured in /etc/ntp.conf
.
The ntp package provides a default configuration file that should make ntpd work out of the box in client mode, without requiring custom configuration.
Configuring connection to NTP servers
If you want to configure /etc/ntp.conf
manually, first thing you define 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 (alternative 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 ntpd (with ntpq or ntpdc), and noquery
prevents dumping status data from ntpd (also with ntpq or ntpdc).
You can also add other options:
restrict default kod nomodify notrap nopeer noquery
noserve
to stop serving time. It will also block time synchronization since it blocks all packets except ntpq and ntpdc queries.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:
/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 manual pages: man ntp.conf
is likely to answer any doubts you could still have (see also the related manual 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 ntpd, run:
# ntpd -q
Using the -q
flag causes ntpd to set the time once and quit, rather than running as a daemon.
ntpdate
.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 ; /usr/bin/hwclock -w [Install] WantedBy=multi-user.target
and enable it.
ntpd -q
option should not be used in this case.Netctl
To synchronize your system clock along with a network connection through the use with Netctl. You can append the following line to your netctl profile.
ExecUpPost='/usr/bin/ntpd -gq || true'
Wicd
Executing ntpd once after successful connection of Wicd can be achieved by creating a script in the appropriate postconnect
directory:
/etc/wicd/scripts/postconnect/ntpd
#!/bin/bash /usr/bin/ntpd -gq
Running as a daemon
Start ntpd.service
with systemctl. To have it started at boot also enable it, or alternatively use the command:
# timedatectl set-ntp 1
Check whether the daemon is synchronizing correctly
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
The ntpd daemon 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 ntpd
daemon again.
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.
See also
- Time (for more information on computer timekeeping)
- http://www.ntp.org/
- http://support.ntp.org/
- http://www.pool.ntp.org/
- http://www.eecis.udel.edu/~mills/ntp/html/index.html
- http://www.akadia.com/services/ntp_synchronize.html