More and more networking services create overlay network using physical links between geo-distributed data centers and many kinds of tunnel technologies, to make full use of link resources between data centers. To achieve efficient construction of overlay network and flexible traffic engineering between different tunnels in overlay network, this paper presents the design Software Overlay Router (SOR), which is used to relay packets. This paper also presents a stateful and a stateless algorithms based on OpenFlow protocol to establish a tunnel without the requirement of global IP address in a simple and scalable manner. The stateful algorithm modifies incoming packets according to a database of four tuples mapping relationship, then forwards modified packets based on kernel routing table through established tunnel, and finally reconstructs received packets and transfers to the original destination. While stateless algorithm benefits from using IP transition algorithm without the need of maintaining four tuples mapping relationship, which largely reduces the number of OpenFlow rules need to be updated. The experiment shows the SOR and the algorithms proposed in this paper can effectively improve utilization of network link resources.