| Summary: | Tunnels, a key technology of traffic obfuscation, are increasingly being used to evade censorship. While providing convenience to users, tunnel technology poses a hidden danger to cybersecurity due to its concealment and camouflage capabilities. In contrast to previous studies of encrypted traffic detection, we perform the first measurement study of tunnel traffic and its unique characteristics and focus on the challenges and solutions in detecting tunnel traffic among traditional and machine learning techniques. This study covers an almost twenty-year research period from 2003 to 2022. First, we present the concepts of two types of tunnels, broad and narrow tunnels, respectively, as well as a framework for major tunnel applications, such as Tor (the second-generation onion router), proxy, VPN, and their relationships. Second, we analyze state-of-the-art methods from traditional to machine learning applications to systematize tunnel traffic detection, including HTTP, HTTPS, DNS, SSH, TCP, ICMP and IPSec. A quantitative evaluation is presented with five crucial indicators applied to the detection methods and reviews. We further discuss the research work based on datasets, feature engineering, and challenges that have are solved, partly solved and unsolved. Finally, by providing open questions and the potential directions, we hope to inspire future work in this area.
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