I. Introduction
A botnet is a network of compromised computers (or bots) which are controlled by the intruder (or bot master) [1], [2]. The bot master communicates and controls the bots by command and control (C&C) channels. That is, the bots will be connected to the C&C server to get the instructions from the bot master, and change their behavior according to those instructions as shown in Fig. 1. Botnets have become one of most serious threats in cyber security [3] since the bot master can trigger a powerful cyber-attack to a victim by manipulating huge amounts of bots. Those cyber-attacks include distributed denial-of-service (DDOS), identification theft, spamming etc. With the tremendous attack power, botnets not only used for acquiring economic profits but also to take advantage from organizations or governments.