Security-typed languages (STLs) are powerful tools for provably implementing policy in applications. The programmer maps policy onto programs by annotating types with information flow labels, and the STL compiler guarantees that data always obeys its label as it flows within an application. As data flows into or out of an application, however, a runtime system is needed to mediate between the information flow world within the application and the non-information flow world of the operating system. In the few existing STL applications, this problem has been handled in ad hoc ways that hindered software engineering and security analysis. In this paper, we present a principled approach to STL runtime system development along with policy infrastructure and class abstractions for the STL, Jif, that implement these principles. We demonstrate the effectiveness of our approach by using our infrastructure to develop a firewall application, FLOWWALL, that provably enforces its policy.