The paper introduces the security content of computer network information system,and detailedly analyses the problems of the network information system security and defense strategy,emphatically introduces the new secu- rity threats and combinative defense strategy---dynamic defense to threats system and defense and response system.

According to the feature of modern air defense operation and by analyzing the factors of threat assessment, a new method of threat assessment is given, which is aimed at synthetic threat combining static threat and dynamic threat, and the factors of static threat and dynamic threat are analyzed, and the corresponding criteria are quantified.

Research and implementation of distributed worm detection and active defense system;

As for the automatic defending system,it includes two technology links such as browser monitoring and registry monitoring,and uses hook and thread to realize its goal.

Moreover,a dynamic defense.

Dynamic defense of network security is a researchful hotpot in the field of network security.

A dynamic defense algorithm is presented to defend a server against distributed denial-of-service (DDoS) attacks.