Wireless ad hoc networks are vulnerable to Sybil attacks due to the broadcast nature of the wireless medium and an absence of central authority. In a Sybil attack, an adversary illegitimately claims to have multiple imaginary identities, called Sybil nodes. This attack is capable of vitiating various operations of these networks such as data aggregation, voting-based mechanisms, fair resource allocation schemes, misbehavior detection, and routing mechanisms. In this paper, we present a survey on the most promising techniques offered thus far to defend the three classes of ad hoc networks, i.e., Mobile Ad hoc Networks, Wireless Sensor Networks, and Wireless Mesh Networks, from the Sybil attack. The techniques include symmetric cryptography using a central authority, random key pre-distribution (key pool, single-space pairwise, and multi-space pairwise), radio resource testing, received signal strength indicator, time difference of arrival, neighborhood data, passive ad hoc Sybil identity detection, passive ad hoc Sybil identity with group detection, and energy trust-based system. Specifically, we analyze various approaches to mitigate the Sybil attack, together with their advantages and disadvantages. We emphasize making all of the concepts very clear and easily understandable to learners and researchers who are interested or planning to work in this area. To achieve this clarity, we illustrate all of the Sybil defense mechanisms with suitable examples and diagrams. In addition, we discuss the challenging research issues and future directions for mitigating the Sybil attack in wireless ad hoc networks.
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