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One feature that is commonly sought after in intrusion detection systems is the ability to automatically respond to an attack. Such responses for network traffic can take many forms against an attacker's perceived IP address, including the instantiation of firewall blocking rules, modification of routing tables, generation of ICMP port/host unreachable packets for UDP attacks, and use of TCP resets for attacks that take place over TCP connections. In this chapter, we'll explore the features, configuration, and implementation of the active response capabilities offered by psad.