2.1.1.4 Ensure ntp is configured - restrict -4

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Information

ntp is a daemon which implements the Network Time Protocol (NTP). It is designed to synchronize system clocks across a variety of systems and use a source that is highly accurate. More information on NTP can be found at http://www.ntp.org. ntp can be configured to be a client and/or a server.

Notes:

If chrony or systemd-timesyncd are used, ntp should be removed and this section skipped

This recommendation only applies if ntp is in use on the system

Only one time synchronization method should be in use on the system

Rationale:

If ntp is in use on the system proper configuration is vital to ensuring time synchronization is working properly.

Solution

Remove and/or disable additional time synchronization methods:

Run the following command to remove chrony:

apt purge chrony

Run the following command to stop and mask systemd-timesyncd:

# systemctl --now mask systemd-timesyncd

Configure ntp:

Add or edit restrict lines in /etc/ntp.conf to match the following:

restrict -4 default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery

Add or edit server or pool lines to /etc/ntp.conf as appropriate:

server <remote-server>

Configure ntp to run as the ntp user by adding or editing the /etc/init.d/ntp file:

RUNASUSER=ntp

See Also

https://workbench.cisecurity.org/files/3228