Sou muito fã de fail2ban
Aqui está uma lista dos principais recursos disponíveis no Fail2ban.
Client/Server architecture.
Multi-threaded.
Highly configurable.
FAM/Gamin support.
Parses log files and looks for given patterns.
Executes commands when a pattern has been detected for the same IP address for more than X times. X can be changed.
After a given amount of time, executes another command in order to unban the IP address.
Uses Netfilter/Iptables by default but can also use TCP Wrapper (/etc/hosts.deny) and many other actions.
Handles log files rotation.
Can handle more than one service (sshd, apache, vsftpd, etc).
Resolves DNS hostname to IP address.