snort ids system
snort is a great tool to monitor your network, you can use mod-evasive perl mod for apache to deter ddos attacks
Description: Flexible Network Intrusion Detection System [MySQL]
Distribution of Snort with support for logging to a MySQL database.
Snort is a libpcap-based packet sniffer/logger which can be used as a lightweight network intrusion detection system. It features rules based logging and can perform content searching/matching in addition to being used to detect a variety of other attacks and probes, such as buffer overflows, stealth port scans, CGI attacks, SMB probes, and much more. Snort has a real-time alerting capability, with alerts being sent to syslog, a separate "alert" file, or even to a Windows computer via Samba.
Homepage: http://www.snort.org
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