Disabling username/password logins

This is something that you'll only want to do after you've set up the key exchange with your clients. Otherwise, clients will be locked out of doing remote logins.

For both Ubuntu and CentOS machines, look for this line in the sshd_config file:

#PasswordAuthentication yes

Remove the comment symbol, change the parameter value to no, and restart the SSH daemon. The line should now look like this:

PasswordAuthentication no

Now, when the botnets scan your system, they'll see that doing a brute-force password attack would be useless. They'll then just go away and leave you alone.