A system for defending against a distributed denial-of-service attack includes an intrusion detection system, an active security management system and an active security node. The intrusion detection system generates alert data if a denial-of-service attack is detected. The active security management system manages a domain, analyzes the alert data, generates and transmits a backtracking sensor in a case of the distributed denial-of-service attack, transmits mobile sensors to a host backtracked by the backtracking sensor to remove a master or an agent program within the host; and generates and transmits a backtracking sensor by using an IP address of a host that has transmitted a packet to the removed master or agent program. The active security node executes the transmitted backtracking sensor to backtrack an attacking host of the distributed denial-of-service attack and, if the backtracked host is determined as a real attacker, intercepts a traffic generated from the real attacker.