Recently, several large-scale software defined networking (SDN) testbeds have been designed and developed. These SDN testbeds have spurred numerous network researchers to run their prototypes and experiments, as well as to set up novel architectures for the future Internet. Based on these efforts, SDN testbeds are actively contributing to future network research, bringing forth the attention of both academia and industry. In addition, researchers envisage these large-scale SDN testbeds to be the future Internet. In this paper, we present a comprehensive survey and research challenges for large-scale SDN testbeds. We first introduce the related work and background knowledge. Then, an overview of SDN testbeds is presented. In addition, five typical implementations of large-scale SDN testbeds around the world are described in detail, including design objectives, key technologies, deployment, and experiments. Moreover, an in-depth comparison of SDN testbeds is given. Finally, challenges and future works of SDN testbeds are discussed.