r-nat is created to allow services inside a private network be accessed from internet. By changing the client connection address, no other change is required.
It has been tested in ubuntu 16.04 lts, windows 7/8.1/10,
debian 8(raspberry pi 3, orange pi zero, odroid c2, they are all using dietpi!).
source code is available here: https://github.com/ywjheart/r-nat