The Great Firewall’s active probing circumvention technique with port knocking and SDN

Abstract

This thesis observes the phenomenon of online censorship, both from blocking and mitigation perspectives. It enumerates and characterizes typical methods and types of Internet censorship, as well as effective circumvention solutions and strategies. Additionally, the study provides detailed observations of the Great Firewall, the ultimate weapon of a Chinese censor, and the Tor anonymity network, the broadly recognized anti-censorship and anti-surveillance tool. Furthermore, it illuminates the Tor network blocking and the firewall's scanning engine (active probing), which is used to detect mitigating servers. The results of the study indicate that 1) The Tor network is heavily suppressed in China; 2) Active probing technique still contributes to blocking decisions; and 3) The Great Firewall successfully engages blocking against obfs4 Tor bridges. Finally, the work suggests a solution for bypassing the Great Firewall using a traffic engineering approach, i.e., software-defined networking and the well-known port knocking technique.