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22.16.12. Configuring Symmetric Authentication Using a Key

To configure symmetric authentication using a key, add the following option to the end of a server or peer command: 
key number
where number is in the range 1 to 65534 inclusive. This option enables the use of a message authentication code (MAC) in packets. This option is for use with the peer, server, broadcast, and manycastclient commands.
The option can be used in the /etc/ntp.conf file as follows: 
server 192.168.1.1 key 10
broadcast 192.168.1.255 key 20
manycastclient 239.255.254.254 key 30
See also Section 22.6, “Authentication Options for NTP”.