Redirect TCP connections from machine on internet to a machine behind a NAT

Question

I would like to find a tool that allows me to do the following:
I have machine A behind a NAT\Router with a web application, then I have server B accessible on the internet.
Using some application all TCP requesting incoming to server B on port N would be redirected to machine A on port 80. This way the web application would be exposed on the internet.
I would like to achieve this without changing any configuration on the router of machine A's network.
There would be some client tool running on machine A that would keep the connection open with server B and allow for this redirection.
In the future other machines like A, but on other local networks would have a similar mechanism where they would be reachable from the server B by usşing other ports.
Can anyone tell me what kind of tool I'm looking for? I keep reading about HTTP Tunnel and VPN, but I'm not sure that's exactly what I need.

Edit: Some missing information, A and B are running Windows, the NAT is usually done by a normal ADSL router, A and B are not on the same subnet nor behind the same NAT, A is behind the NAT, B is connected directly to internet.

Answer 1

Sounds to me like your asking to set-up a reverse proxy?

So this would be a server that listens on a port on machine B, which you could set-up/configure to pass requests coming in on that port to server B as long as B can talk to A.

So hxproxy possibly or nginx apache mod_proxy that type of thing. Have a quick read on Reverse proxies and see if that's the kitty for you

Answer 2

Some more information would help: Which operating systems are A, B and the NAT/Router running? Is B also behind the same NAT/Router as A? Are A and B on the same subnet?

Answer 3

One of the many solution is to use a ssh tunnel on A :

ssh -R <B IP>:<B port>:<A ip>:<A port>

With this option ssh will open the specified port/IP on B and redirect every connection made on this port on the specified port/IP on A.

You should enable the GatewayPorts option in sshd config on server B (otherwise you could only open a port on 127.0.0.1) and you have to be root as usual if you want to open a privileged port (below 1024).

Answer 4

I was able to do what I wanted with a TeamViewer VPN and a TCP Tunnel tool.
I opened a VPN connection to the machine I need with TeamViewer, and I redirect tcp connection on port X to the VPN ip on the port 80.
In the future we are planning to build a client/server tool that can do this tcp tunneling. I thought it would be easier to find a tool already doing this. But this requires a way to keep a connection open with the machine behind the NAT without any reconfiguration on the NAT, which is what Teamviewer does.