Opnums

Opnums, short for operation numbers, are values used in certain remote procedure call (RPC) protocols to identify individual procedures within an interface. In DCE/RPC and Microsoft RPC implementations, each interface exposes a set of procedures, and every procedure is assigned a distinct opnum. The opnum is included in the RPC request header and serves as the dispatcher key that selects the correct procedure on the server.

In practice, a server maintains a dispatch table or virtual table that maps opnums to server-side functions. When a request arrives, the runtime reads the opnum, looks up the corresponding handler, and invokes it. If the opnum is invalid or not implemented, the server typically returns an error such as an unknown opcode or service not available.

Opnums are generally defined by the interface’s description language, such as DCE/RPC Interface Definition Language or the Microsoft IDL. A compiler or code generator creates the dispatch table and stubs that use the opnum to call the appropriate implementation. Opnums are intended to be stable across releases of the same interface, with new procedures assigned new opnums. Deleting or changing a procedure’s opnum can break existing clients, so versioning and compatibility practices are important.

Security and compatibility considerations include validating opnums to avoid unintended dispatch, logging unknown opnums for auditing, and ensuring that changes to the interface do not invalidate clients. Opnums are an internal protocol detail rather than a user-facing feature, but they are essential for the correct routing of remote procedure calls.