Distributed Denial Of Service Attacks Analyzed

With the development of network technology, the security of network has become a very important subject. As a Internet attacking method, the Denianl of Service attack(DoS), has greatly endangered Internet recently. Distributed Denial of Service(DDoS) Attack is an newly developed attack type, which is the extension of Denial of Service(DoS) Attack. Due to its distributed characteristic, DDoS atacks possess more atack resources and have more destroying power. So, it is very difficult to keep them away. DDoS attacks bring much great threats to Internet security and research on them become a hotspot in network security fields.This paper makes a brief review about the history and present situation of network security, the present menace on the network and those security problems existing in the TCP/IP protocols. Especially about the cheating and attacking methods that utilize the weaknesses of TCP/IP. Second, it analyzes the attack mechanism of DDoS attacks detail and a thorough study and gives the classification of DDoS atacks means according to the different attack methods. Then, the research, comparisons and estimations of the counter measures in existence are made in detail.At last, it analyse SYN flood attack which harms all kinds of TCP Service, and introduce an defensing method - SYN cookies in detail. In the process of research, we get to know that SYN flood make resource consuming as attack means and under the current protocol condition, no matter whether operation systems have leaks or not, attack can always be implemented. So we can not realize the defence for them through patching operation system. In addition, the tradition detecting and defensing tactics require more artificial action, and therefore we can not respond the attacks timely. And facing the default of SYN cookies, design a mechamism for automatic opening and closing SYN cookies. Only when DDoS atacks happen, SYN cookies is opened. When the atacks are over, it is closed timely. Thus, this reduces the disadvantageous effect on the normal service availably.