Difference between revisions of "Network Time Protocol daemon"

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Revision as of 22:44, 1 September 2011

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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.

Installation

Template:Package Official is available from the [extra] repository.

Configuration

The first thing you define in your Template:Filename 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:

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 if it cannot obtain a connection with the first attempt. The burst option always sends a burst of packets, even on the first attempt. The burst option should never be used without explicit permission and may result in blacklisting.

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
Note: This still allows other people to query your time server. You need to add noserve to stop serving time.

Full docs for the "restrict" option are in Template:Codeline. See http://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):

Template:File

Note: Defining the log file is not mandatory, but it is always a good idea to have feedback for ntpd operations.

In conclusion, never forget man pages: Template:Codeline is likely to answer any doubts you could still have (see also the related man pages: Template:Codeline).

Template:Gentoo

Running as a daemon

Starting ntpd

ntpd sets 11 minute mode, which syncs the system clock to hardware every 11 minutes. The hwclock daemon measures hardware clock drift and syncs it, which conflicts with ntpd.

Stop the hwclock daemon (if it is running):

Template:Cli

Start the ntpd daemon: Template:Cli

Add ntpd to your DAEMONS array so it starts automatically on boot and make sure hwclock is disabled: Template:File

NetworkManager

Note: ntpd should still be running when the network is down if the hwclock daemon is disabled, so you should not use this.

ntpd can be brought up/down along with a network connection through the use of NetworkManager's dispatcher scripts. You can install the needed script from [community]:

Template:Cli

Running as non-root user

When compiled with --enable-linux-caps, ntp can be run as a non-root user for increased security (the vanilla Arch Linux package has this enabled).

Note: Before attempting this, make sure ntp has already created Template:Filename.

Create ntp group and ntp user:

Template:Cli

Change ownership of the ntp directory to the ntp user/group:

Template:Cli

Edit Template:Filename and change

NTPD_ARGS="-g"

to

NTPD_ARGS="-g -u ntp:ntp"

Finally, restart the daemon:

Template:Cli

Running in a chroot

Note: Before attempting this, complete the previous section on running as non-root, since chroots are relatively useless at securing processes running as root.

Edit Template:Filename and change

NTPD_ARGS="-g -u ntp:ntp"

to

NTPD_ARGS="-g -i /var/lib/ntp -u ntp:ntp"

Then, edit Template:Filename 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

Finally, restart the daemon again:

Template:Cli

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.

Syncing the clock without running the daemon

If what you want is just synchronize your system clock at boot time without running ntpd as a daemon, you can add this line to your Template:Filename:

ntpd -qg &

This behavior mimics that of the ntpdate program, which is now deprecated.

Note: In order for this method to work you have to make sure that, when Template:Filename is executed, the network connection has already been initialized (for example you should not background essential network-related daemons in Template:Filename)
Warning:
  • Using this method is highly discouraged on servers and in general on machines that need to run continuously for more than 2 or 3 days, as the system clock will be updated only once at boot time.
  • Running "Template:Codeline" as a cron event is to be completely avoided, unless you are perfectly aware of how your running applications would react to instantaneous system time changes.

Check that the DAEMONS array in Template:Filename includes hwclock, to ensure the hardware clock is periodically updated:

Template:File

Warning: If something other already takes care of updating the hardware clock, for example another operating system in dual boot, you should avoid starting hwclock.

Alternatives

An available alternative to NTPd is OpenNTPD, part of the OpenBSD project and currently not maintained for Linux.

See also

  • Time (for more information on computer timekeeping)

External links