Massive failed FTP attempts.

[Michael Nielson <safetytrick () gmail com>]

I run several small LAMP virtual servers, I've noticed a large amount of 
failed FTP login attempts, these all attempt to login with common FTP 
usernames like Administrator, or webmaster (the FTP server is proFTPd 
version 1.2.10).  The attacker will try from one IP address maybe 30 or 
40 times and then moving to a new IP address.  I have several questions, 
first what are they trying to do? Crack my password? Or exploit a bug 
with proftpd?  I've been more diligent about choosing a difficult to 
break password.  More important what can I do to limit the number of 
attempts on my server? 
Thanks tons!
Michael