I was just wondering on CentOs how can I change the 'greeting' that the server output when a user logs in via ssh?
Perhaps you are looking to update the content that is part of the Message of the Day (MOTD). I believe this is /etc/motd on Centos.
I assume you mean ssh.
Modify /etc/issue as root to put whatever text you wish.
Check out also
inside /etc/ssh/sshd_config: with this you can also display messages before login completes.
/etc/motd is the file you are looking for.