This document discusses mechanisms to defend against Sybil attacks in mobile ad hoc networks. It introduces Sybil attacks and their impact on ad hoc networks. It then summarizes several detection models for Sybil attacks, including PKI-based protocols, threshold-based protocols, reputation schemes, and watchdog mechanisms. It also proposes a multifactor authentication scheme that combines cryptographic techniques with physical attribute verification. Finally, it outlines a passive identity detection scheme that leverages the mobility of nodes to identify Sybil identities that must move together.