Distance vector routing protocols (e.g., RIP) have been widely used on the Internet, and are being adapted to emerging wireless ad hoc networks. However, it is well-known that existing distance vector routing protocols are insecure due to: 1) the lack of strong authentication and authorization mechanisms; 2) the difficulty, if not impossibility, of validating routing updates which are aggregated results of other routers. In this paper, we introduce a secure routing protocol, namely S-RIP, based on a distance vector approach. In S-RIP, a router confirms the consistency of an advertised route with those nodes that have propogated that route. A reputation-based framework is proposed for determining how many nodes should be consulted, flexibly balancing security and efficiency. Our threat analysis and simulation results show that in S-RIP, a well-behaved node can uncover inconsistent routing information in a network with many misbehaving nodes assuming (in the present work) no two of them are in collusion, with relatively low extra routing overhead.