fail2ban

ban hosts that cause multiple authentication errors
Fail2ban monitors log files (e.g. /var/log/auth.log,
/var/log/apache/access.log) and temporarily or persistently bans
failure-prone addresses by updating existing firewall rules. Fail2ban allows
easy specification of different actions to be taken such as to ban an
IP using iptables or hostsdeny rules, or simply to send a
notification email.
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By default, it comes with filter expressions for various services
(sshd, apache, qmail, proftpd, sasl etc.) but configuration can be
easily extended for monitoring any other text file. All filters and
actions are given in the config files, thus fail2ban can be adopted
to be used with a variety of files and firewalls.