Also wanted to add that fail2ban (<a href="http://www.fail2ban.org/wiki/index.php/Main_Page" rel="nofollow">http://www.fail2ban.org/wiki/index.php/Main_Page</a>) is a fantastic tool -- it send you email reports of various issues like this, and show you a lot of information (registered through DNS) on the people that are trying to access your server (and failing)