Distribution of live multimedia streams to a high number of end users through an ISP network may be efficiently implemented through IP multicasting. Multicast traffic engineering with traditional decentralized approaches, however, is still a challenging problem for network operators. In this article we present a centralized approach to management of source-specific multicast traffic flows relying on software defined networking. Our approach is suitable for hybrid IP/SDN networks, where network nodes behave as traditional IP routers by default, but whose packet forwarding behavior may be customized for specific flows thanks to an OpenFlow-enabled control plane. We also describe a traffic management system that we implemented as a proof of concept, which can be used by an ISP to compute multicast distribution trees for a given set of channels to be distributed to their customers.