A low rate DoS attack detection algorithm is used, which relies on a characteristic of the low rate DoS attack in introducing high rate traffic for short periods, and then uses a proactive test based differentiation technique to filter the attack packets. The proactive test defends against DDoS attacks and low rate DoS attacks which tend to ignore the normal operation of network protocols, but it also differentiates legitimate traffic from low rate DoS attack traffic instigated by botnets. It leverages on the conformity of legitimate flows, which obey the network protocols. It also differentiates legitimate connections by checking their responses to the proactive tests which include puzzles for distinguishing botnets from human users.