The OpenFlow protocol, as a fundamental element for Software Defined Networking (SDN) architecture, only supports for packet forwarding across switches in general networks. In this paper, the authors propose Software-Defined Function (SDF) which entitles administrators to manage the Internet of Things (IoT) devices and services through abstraction of the underlying infrastructure. The authors further present OpenFunction, a secure communications protocol stemmed from OpenFlow, which enables the IoT devices to be upgraded or reprogrammed remotely and securely. Finally, the authors implement a preliminary SDF system and evaluate its performance. The experimental results demonstrate that the SDF and OpenFunction can grant programmability, flexibility, centralization and security to the IoT.