Using the motd file

The /etc/motd file will present a message banner to anyone who logs in to a system through Secure Shell. On your CentOS machine, an empty motd file is already there. On your Ubuntu machine, the motd file isn't there, but it's a simple matter to create one. Either way, open the file in your text editor and create your message. Save the file and test it by remotely logging in through Secure Shell. You should see something like:

maggie@192.168.0.100's password:
Last login: Sat Oct 7 20:51:09 2017
Warning: Authorized Users Only!

All others will be prosecuted.
[maggie@localhost ~]$

motd stands for Message of the Day.