# 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..

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. If you want to use the defaults for a start, you can skip to #Usage.

The following describes main configuration items to customize. Also refer to the manual pages: `man ntp.conf` and the related `man {ntpd|ntp_auth|ntp_mon|ntp_acc|ntp_clock|ntp_misc}`.

### 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) (you can set up to stratum 15) so that it will never be used unless internet access is lost:

    server 127.127.1.1
    fudge  127.127.1.1 stratum 12

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

Note: This still allows other people to query your time server. You need to add `noserve` to stop serving time. It will also block time synchronization since it blocks all packets except ntpq and ntpdc queries.

If you want to change any of these, see the full docs for the "restrict" option in `man ntp_acc`, the detailed ntp instructions and #As a daemon.

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

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

### Running in a chroot

Note: ntpd should be started as non-root (default in the Arch Linux package) before attempting to jail it in a chroot, since chroots are relatively useless at securing processes running as root.

Create a new directory `/etc/systemd/system/ntpd.service.d/` if it doesn't exist and a file named `customexec.conf` inside with the following content:

    [Service]
    ExecStart=
    ExecStart=/usr/bin/ntpd -g -i /var/lib/ntp -u ntp:ntp -p /run/ntpd.pid

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. Once it restarted you can verify that the daemon process is chrooted by checking where `/proc/{PID}/root` symlinks to:

    # ps -C ntpd | awk '{print $1}' | sed 1d | while read -r PID; do ls -l /proc/$PID/root; done

should now link to `/var/lib/ntp` instead of `/`.

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.

## Usage

This section will deal with the usage of ntpd and the other commands provided by the ntp package.

### As a daemon

Warning: As of February 2014, ntpd, when used as a daemon, was vulnerable to DDoS attacks in some configurations. Arch's default configuration for ntpd was not vulnerable to CVE-2013-5211 because `noquery` disables responses for the `monlist` command. If a user removed `noquery` from the configuration, and does not add `disable monlist`, then that user is vulnerable with ntpd prior to version 4.2.7. See the arch-dev mailing list post and the aforementioned CVE report for more information. The fixed package has been available since May 6, 2014. It was decided to leave this warning in place for users not updating as frequently for at least a month.

The basic command to start the NTP daemon is:

    # ntpd

However, it will run as root in the background. Hence, it should always be started specifying the user option:

    # ntpd -u ntp:ntp

See also #systemd services.

#### Check that the daemon is working correctly

Use ntpq to see the list of configured peers:

    $ ntpq -p

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).

### Without daemon

System time can be synchronized without using the daemon as well. However, this would not be suitable for machines that run without rebooting for more than a few days. To synchronize the system clock just once without starting ntpd in the background, run:

    # ntpd -q

Using the `-q` flag causes the ntpd daemon to set the time once and quit, i.e. not fork to the background. If the operation is unsuccessful, your system clock will not be synchronized.

Note: This has the same effect as the now deprecated ntpdate.

The system time also will not be synchronized if the ntp-server's time differs from the system clock by more than a given threshold (so-called panic-gate), in order to protect running system processes. However, the option `-g` can be used to disable the threshold and allow it to be exceeded, e.g. when the time is set for the first time or if the hardware clock is faulty.

## Autostarting

### systemd services

ntpd can be autostarted by systemd at boot.

#### Start the daemon at boot

The ntp package provides `ntpd.service` for systemd. Enable it with systemctl to start the daemon at boot.

Alternatively use the command:

    # timedatectl set-ntp 1

#### Synchronize once per boot

Warning: Using this method is discouraged on servers, and in general on machines that run without rebooting for more than a few days.

The ntp package provides the `ntpdate.service`. Enable it with systemd to synchronize time once per boot. This oneshot service is the useful option, if the system does not require the daemon to remain active after one successful time synchronization.

If the synchronized time should be written to the hardware clock as well, configure the provided unit as described in systemd#Editing provided unit files:

    /etc/systemd/system/ntpdate.service.d/hwclock.conf

    [Service]
    ExecStart=/usr/bin/hwclock -w

before starting it.

### At network connection

ntpd can also be started by your network manager, so that the daemon only runs when the computer is online.

#### Netctl

Append the following lines to your netctl profile:

    ExecUpPost='/usr/bin/ntpd || true'
    ExecDownPre='killall ntpd || true'

Note: You are advised to customize the options for the ntpd command as explained in #Usage.

#### NetworkManager

The ntpd daemon can be brought up/down along with a network connection through the use of NetworkManager's dispatcher scripts. The networkmanager-dispatcher-ntpd from the official repositories installs one, pre-configured to start and stop the ntpd service with a connection.

#### Wicd

For Wicd, create a start script in the `postconnect` directory and a stop script in the `predisconnect` directory. Remember to make them executable:

    /etc/wicd/scripts/postconnect/ntpd

    #!/bin/bash

    /usr/bin/ntpd &
    -or-
    systemctl start ntpd &

    /etc/wicd/scripts/predisconnect/ntpd

    #!/bin/bash

    killall ntpd &
    -or-
    systemctl stop ntpd &

Note: You are advised to customize the options for the ntpd command as explained in #Usage.

See also Wicd#Scripts.

## See also

- Time (for more information on computer timekeeping)
- http://www.ntp.org/
- https://support.ntp.org/
- http://www.pool.ntp.org/
- https://www.eecis.udel.edu/~mills/ntp/html/index.html
- http://www.akadia.com/services/ntp_synchronize.html

Category:

- Networking