The SSH protocol is a secure way to manage your linux / unix server.

It offers an encrypted method of remote command line connection.

This is probably the protocol you already use for server administration.

As with most protocols, username and password is used for authentication. While this is a common way to authenticate, it is still prone to brute force attacks.

A good way to mitigate the possibility of brute force attacks against your ssh daemon, is disabling password login and enabling certificate authentication.

The idea is simple, in order to login to the server, you will have to present a file containing a unique certificate. You may store this file on a dedicated hardware token, the hard drive or a simple flash disk.

Once this certificate validates, you will gain access.

A good article on setting this up is available on Security Focus.

http://www.securityfocus.com/infocus/1810