BSD Socket API for Simulator enables untouched Real World Application(RWA) binaries to run on the powerful modern general-purpose network simulators. It is simulator independent, so it can make use of the powerful functionality and versatile tools provided by modern general-purpose simulators such as NS-2. It is fully compatible with BSD Socket API, so RWA can run on it without re-linking and re-compiling. It is transparent to RWA, so RWAs are run on BSD Socket API for Simulator as they are on normal operating systems.BSD Socket API for Simulator is built on the concept of message redirecting. It has two critical parts: shared library and customized simulator application. The shared library is loaded into the address space of RWA. On one hand, messages sent by RWA are captured by the shared library and redirected to the customized simulator application. On the other hand, messages from simulator are redirected by the customized simulator application to the shared library.