A Mobile Ad-hoc Network (MANET) is an independent and self-configurable network without any fixed infrastructure. Compared to traditional network, MANETs have unique characteristics such as wireless shared radio medium, limited communication range, highly dynamic topology, power constraints and lack of trusted centralized authority. When a node wishes to communicate with other node in the network, it invites all available nodes to participate in the routing mechanism; moreover, conventional routing protocols assume trusted co-operative environment. As a result, network layer of MANET is prone to various security attacks and therefore, finding a secure and efficient route becomes a major challenge. In this paper, we briefly introduce routing protocols, examine the behavior of various Denial-of-Service (DoS) attacks at the network layer of MANET and provide comprehensive survey of the existing defense approaches to these attacks.