A Remote Data Services (RDS) handler is a simple COM object that is required to expose the IDataFactoryHandler interface. The Microsoft Data Access Components (MDAC) 2.x software development kit (SDK) comes with the source code (both the Visual Basic—VB—and the Visual C++—VC++—versions) of the default handler, MSDFMAP.Handler. You can use this code as a reference for implementing new handlers. If you plan to use VB to develop a custom handler, you'll be happy to know that the SDK bundles a ready-made type library (msdfhdl.tlb) that makes implementing the necessary interface straightforward.
You need to register your custom handler as a COM object and as an RDS handler. To register your handler as an RDS handler, create a new Registry subkey in the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\DataFactory\HandlerInfo\Safe HandlerList subkey. The new subkey's name will be the same as the handler's programmatic identifier (ProgID), which you assign.