Disable Port removing on rewrite

[Schütrumpf, Niklas]

Hello,
during the conversion of my web servers at home I ran into some problems.
We are running more than one web servers at home and a few ports are open in the router for them.
So the standard ports (80, 443) are blocked for other server in our network.
One NGINX Server listens on port 80, and our router routes port 8081 from outside to port 80 locally.
On any rewrite NGINX strips the port from the URL. This is the server because I get a connection to the server and then I got rewritten.
Is there any config entry to disable the port remove on rewrite? I already tried “port_in_redirect off”, without success.
I already posted this question on SO and the NGINX forum, but I think in the mailing list i have more success.
Link to StackOverflow Question: http://stackoverflow.com/questions/39519136/prevent-nginx-to-remove-the-port
Link to NGINX forum: https://forum.nginx.org/read.php?11,269603

Currently i am using almost the default config, view SO for the config.

Thank you in anticipation!
- greetings from Germany
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