My answer, every time, was simple: "you have to ask the user for it."
His counter, every time: "that's inconvenient, can you get it automatically from the web browser?"
Of course, like most caricatures of management advice, there is a grain of truth: technologists sometimes *do* sandbag and declare something impossible, just cause they don't want to do it. But that's hardly the default. And what a toxic relationship.