Forgive me for bumping an old thread, but I too am interested in an answer to this question. As things stand I'm going to commit to using out-of-date APIs in order to accomplish this. But it'd be nice if there were an officially supported and modern way to make socket connections and HTTP / HTTPS requests using a specific network interface.

Thank you Christian!

https://bugreports.qt.io/browse/QTBUG-88063 describes exactly what I've been seeing.

From the C++ side, QNetworkReply is not local in the sense that you get a pointer to a heap allocated object that lifetime you must handle yourself while outFile would a stack allocated object that would be destroyed at the end of the function.