Content Delivery Networks (CDNs) carry an increasing traffic share due to the performance benefits enabled by a global deployment of proxy servers and a request-routing mechanism mapping clients to the closest CDN proxy. The standard approach to map clients/proxies is based on Over the Top (OTT) information such as delay measurements. As this approach is challenging with respect to finding the best network path, more recent Anycast CDN (A-CDN) architectures adopt Internet Protocol (IP) anycast routing. IP anycast gives the control of proxy selection completely to the network, while the standard approach gives the full control to the application. There is no middle ground enabling the use of both sets of information. Consequently, this work proposes Software Defined Anycast (SoDA) to fill the gap. SoDA enables anycast routing based on a thin layer of Software Defined Networking (SDN) functionality. Our approach allows CDN providers to take influence on anycast routing and enables collaborative load balancing at a constant SDN rule count. SoDA is shown to offer more fair resource usage at a better performance for Internet Service Providers (ISPs) and CDN providers by using a number of fine grained data sets from a large European tier-1 ISP for evaluation.