Software-Defined Networking (SDN) has emerged as a promising direction for next-generation network design. Due to its clean-slate and highly flexible design, it is believed to be the foundational principle for designing network architectures and improving their flexibility, resilience, reliability, and security. As the technology matures, research in both industry and academia has designed a considerable number of tools to scale software-defined networks, in preparation for the wide deployment in wide-area networks. In this paper, we survey the mechanisms that can be used to address the scalability issues in software-defined wide-area networks. Starting from a successful distributed system, the Domain Name System, we discuss the essential elements to make a large scale network infrastructure scalable. Then, the existing technologies proposed in the literature are reviewed in three categories: scaling out/up the data plane and scaling the control plane. We conclude with possible research directions towards scaling software-defined wide-area networks.