In mobile Ad hoc networks, data is transmitted in airspace between mobile entities without any centralized control. This environment offers many and very important services for mobile users. However, the level of security in this kind of network has not yet reached a significant level of maturity against the ferocious attacks. In this work, we present a new model to trust routing protocols communications in MANETs against DoS attacks. We have used the box plot theory to define accurately the threshold to detect a DoS attack and to reduce the number of false positives.