Internet of Things (IoT) commonly identifies the upcoming network society where all connectable devices will be able to communicate with one another. In addition, IoT devices are supposed to be directly connected to the Internet, and many of them are likely to be battery powered. Hence, they are particularly vulnerable to Denial of Service (DoS) attacks specifically aimed at quickly draining battery and severely reducing device lifetime. In this paper, we propose SMACK, a security service which efficiently identifies invalid messages early after their reception, by checking a short and lightweight Message Authentication Code (MAC). So doing, further useless processing on invalid messages can be avoided, thus reducing the impact of DoS attacks and preserving battery life. In particular, we provide an adaptation of SMACK for the standard Constrained Application Protocol (CoAP). Finally, we experimentally evaluate SMACK performance through our prototype implementation for the resource constrained CC2538 platform. Our results show that SMACK is efficient and affordable in terms of memory requirements, computing time, and energy consumption.