How can I display a banner when someone attempts to ssh to my server?

This can be set with the Banner directive in sshd_config.

First create a banner to display upon connection:

root@root [~]# vi /etc/sshd_banner.txt

Set the Banner directive to point to this file and restart sshd:

root@root [~]# cd /etc/ssh/

root@root [/etc/ssh]# cp sshd_config sshd_config.save

root@root [/etc/ssh]# vi sshd_config

root@root [/etc/ssh]# diff sshd_config sshd_config.save

93c93

< Banner /etc/sshd_banner.txt

—-

> #Banner /some/path

root@root [/etc/ssh]# /etc/init.d/sshd restart

Stopping
sshd:
[ OK ]

Starting
sshd:
[ OK ]

Test your new configuration:

root@root [/etc/ssh]# ssh 127.0.0.1

This is a test
banner.

Your message/warning etc here.

root@127.0.0.1’s password: