Since the first denial of Service (DoS) attack appears, DoS attacks cause significant financial damage

every year. The majority researchers focus on detection and respond mechanism of DoS, whereas limited progress has

been made in the details of DoS attacks. In this paper, we introduce Custom Audit scr ipting Language (CASL) applied

to write simulation program for DoS attacks. Then we take a deep research in the methodology of DoS attacks and

simulate two typical DoS attacks in CASL. Additionally, recent trends of DoS attacks and problems associated with

defending against DoS attacks are also discussed. In short, our simulation attack programs can help examine the nature

of DoS attacks and find the corresponding countermeasures