BFD is a modular shell script for parsing applicable logs and checking for authentication failures. There is not much complexity or detail to BFD yet and likewise it is very straight-forward in its installation, configuration and usage. The reason behind BFD is very simple; the fact there is little to no authentication and brute force auditing programs in the linux community that work in conjunction with a firewall or real-time facility to place bans.
http://blog.webhosting.uk.com/2006/06/10/brute-force-detectionbfd-on-linux-server/