Can’t connect from internet, get error 51

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    I  try to use NoMachine free to connect to my machine from the internet. It works pretty well on local network, even when I connect through the public IP/port combo. However it doesn’t work as soon as I am outside of the local network, on the internet: I get error 51.

    My machine is on Ubuntu 20.04, ethernet with CPL connection to an ISP router with UPNP enabled. I can see the rule generated to forward public 23868 to local machine 4000.

    I have also manually opened 23868 on the router, but it doesn’t change anything. I have tried different things found on this page but nothing works.

    Please could you help me ?


    server: Ubuntu 20.04.2, Gnome 3.36.8, NoMachine free 7.6.2

    client: iPad air 4, iPadOS 14.6, don’t know where to find version number of NoMachine client

    router: bbox Fast5330b-r1, UPNP enabled, connection to machine over CPL


    You have a double nat maybe? Check this topic:

    My last comment in particular in that topic was “UPnP doesn’t support the double NAT scenario, you would need to enable port-forwarding on both routers.” Have you tried that?

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