With the recent advances in software-defined networking (SDN), the multi-tenant data centers provide more efficient and flexible cloud platform to their subscribers. However, as the number, scale, and diversity of distributed denial-of-service (DDoS) attack is dramatically escalated in recent years, the availability of those platforms is still under risk. We note that the state-of-art DDoS protection architectures did not fully utilize the potential of SDN and network function virtualization (NFV) to mitigate the impact of attack traffic on data center network. Therefore, in this paper, we exploit the flexibility of SDN and NFV to propose FlexProtect, a flexible distributed DDoS protection architecture for multi-tenant data centers. In FlexProtect, the detection virtual network functions (VNFs) are placed near the service provider and the defense VNFs are placed near the edge routers for effectively detection and avoid internal bandwidth consumption, respectively. Based on the architecture, we then propose FP-SYN, an anti-spoofing SYN flood protection mechanism. The emulation and simulation results with real-world data demonstrates that, compared with the traditional approach, the proposed architecture can significantly reduce 46% of the additional routing path and save 60% internal bandwidth consumption. Moreover, the proposed detection mechanism for anti-spoofing can achieve 98% accuracy.