i ABSTRACT OF THE MASTER'S THESIS

This document describes an implementation and the testing of an automatic defense system that uses rate-limiting to mitigate Denial of Service attacks. Denial of Service attacks – and particularly the distributed ones- are amongst the latest and most problematic trends in network security threats. Currently, a few effective defense methods exist against them. In this document, the proposal is to jointly use the capabilities of attack detection (via Intrusion Detection Systems) and Quality of Service to rate-limit these attacks. As an automatic reaction, rate-limiting has an advantage over blocking: it preserves the legitimate traffic that is mis-identified as belonging to an attack. This document describes in detail an already specified Rate-Limiting System. This system selects traffic into legitimate and attack aggregates thanks to an attack detection module. Based on this selection, routers direct the traffic aggregates into different queues. Attack queues are managed by a new Active Queue Management mechanism that enforces ratelimiting by randomly discarding packets. This thesis presents mainly an implementation of the Rate-Limiting System in a Linux environment and its testing. It appeared from the tests that HTTP and FTP-downloadin