Emerging network technologies have complex network interfaces that have renewed concerns about network reliability. In this paper, we present an effective lowoverhead failure detection technique, which is based on a software watchdog timer that detects network processor hangs and a self-testing scheme that detects interface failures other than processor hangs. The proposed adaptive and concurrent self-testing scheme achieves failure detection by periodically directing the control flow to go through only active software modules in order to detect errors that affect instructions in the local memory of the network interface. The paper shows how this technique can be made to minimize the performance impact on the host system and be completely transparent to the user.