Blocking and preventing brute force attacks is one of the main things you want to do on your web server to add a layer of security.
Hackers can try to get into your system using a few different methods.
You can easily spot a brute force attempt by checking your servers log files. You will see a series of failed login attempts for the service they are trying to break into.
less /var/log/secure or
tail -f /var/log/secure
Check for failed login attemps such as:
Apr 11 19:02:10 fox proftpd: yourserver (usersip[usersip]) - USER theusername (Login failed): Incorrect password.
There are a few main ways to prevent the majority of brute force attempts.