With emergence of a wide range of wireless devices, protecting mobile ad hoc networks became an increasingly important and also a more difficult task. Intrusion detection system (IDS) can be deployed as a second line of defense in a Mobile Ad hoc Network (MANET). But it may be impossible to make every wireless node the monitoring nodes due to the resource limitation. So it is important to investigate distributed monitoring mechanism appropriate for ad hoc networks. A distributed scheme based on the concepts of connected weighted dominating set is proposed for selecting the monitoring nodes to detect network packets in ad hoc networks. The proposed scheme is aimed to minimize costs of network monitoring and IDS system. The performance of the proposed scheme is also investigated.