DNS tunnel is a typical Internet covert channel used by attackers or bots to evade the malicious activities detection. The stolen information is encoded and encapsulated into the DNS packets to transfer. Since DNS traffic is common, most of the firewalls directly allow it to pass and IDS does not trigger an alarm with it. The popular signature-based detection methods and threshold-based methods are not flexible and make high false alarms. The approaches based on characters distribution features also do not perform well, because attackers can modify the encoding method to disturb the characters distributions. In this paper, we propose an effective and applicable DNS tunnel detection mechanism. The prototype system is deployed at the Recursive DNS for tunnel identification. We use four kinds of features including time-interval features, request packet size features, record type features and subdomain entropy features. We evaluate the performance of our proposal with Support Vector Machine, Decision Tree and Logistical Regression. The experiments show that the method can achieve high detection accuracy of 99.96%.
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