One of the easiest way to alert unauthorized people that they are trespassing when they try to access the SSH of your server (maybe by mistake) is to add a warning message before and/or after login.
Of course, this does not mean that your server is secure when you add it, but at least they have been warned.
How to add a warning message before SSH login
First you will need to create a file with your message. For example:
vim /etc/ssh/sshd_banner
You can add the message you want, for example:
************************************************************* WARNING - PRIVATE ELECTRONIC DEVICE - ACCESS PROHIBITED This device is a private network device. Access to this device is not authorized. Any attempt for unauthorized access will be logged. *************************************************************
Add in the SSH daemon configuration the banner option:
vim /etc/ssh/sshd_config
Add at the end of the file:
Banner /etc/ssh/sshd_banner
How to add a warning message after SSH login
Just add the text you want in /etc/motd:
vim /etc/motd
You can add the message you want, for example:
__ __ .__ / \ / \ ____ | | ____ ____ _____ ____ \ \/\/ // __ \| | _/ ___\/ _ \ / \_/ __ \ \ /\ ___/| |_\ \__( <_> ) Y Y \ ___/ \__/\ / \___ >____/\___ >____/|__|_| /\___ > \/ \/ \/ \/ \/
Next time someone tries to log in it will look like this:
************************************************************* WARNING - PRIVATE ELECTRONIC DEVICE - ACCESS PROHIBITED This device is a private network device. Access to this device is not authorized. Any attempt for unauthorized access will be logged. ************************************************************* root@bubble's password: Last login: Thu Jan 30 08:08:45 2014 from 127.0.0.1 __ __ .__ / \ / \ ____ | | ____ ____ _____ ____ \ \/\/ // __ \| | _/ ___\/ _ \ / \_/ __ \ \ /\ ___/| |_\ \__( <_> ) Y Y \ ___/ \__/\ / \___ >____/\___ >____/|__|_| /\___ > \/ \/ \/ \/ \/