In this era of big data, of quantified self and of smart cities, wireless sensor networks are meant to be used every day, for all sort of applications. Made of tiny sensors, they collect data and communicate through wireless technologies. Because they may take part in sensitive or military applications, security is an essential matter in such networks. Confidentiality and authenticity can be ensured by the use of dedicated mechanisms. Focusing on availability, we propose here a new practical approach to protect the network against denial of service attacks thanks to the use of traffic monitoring agents called cNodes. The approach uses a fair election process of cNodes in accordance with classical criteria related to residual energies and the presence of compromised nodes which may have greedy or jamming behaviors. Results obtained from simulations show that this method is effective both in terms of detection and of energy conservation.