Increased sophistication of present day worms leading to large scale Denial of Service attacks on vital installations has been a source of concern. Intrusion Detection Systems and Intrusion Prevention Systems provide a limited solution to a concurrent problem. Proactive preemptive techniques employed by Intrusion Prevention Systems are more effective than the reactive techniques employed by Intrusion Detection Systems. However these techniques are not adequate to counter a large scale distributed denial of service attacks.

In this paper, we propose a countermeasure which uses dynamic agents to mitigate Denial of Service attacks. These Dynamics Agents are called Gnipper vaccine. Gnipper vaccine provides an effective technique capable of providing an enterprise-wide response. Gnipper vaccine is an agent which resides on the host computer capable of dropping any malicious packets. The vaccine propagates one hop at a time towards the source of the attacker.