Would also like to know this as I am facing the same issue and there's really no troubleshooting I can do myself as a third-party owns the RDS instance. When testing the connection it just loads for a while and then says "socket closed". From my understanding, this implies that the TCP session establishes (networking is fine) but immediately is closed by Oracle DB for whatever reason. I've gone through all the possibilities: we are using NNE, we checked and server-side NNE is at "REQUESTED" level (should be higher than "ACCEPTED"), and we checked the service name. Not sure what else to do.