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How would someone at tech support for a product (assume for now any generic ethernet/wifi connected device) remotely access the device needing troubleshooting when the device is connected to a local network behind NAT without VPN or port forwarding? The device does sent monitoring data to a remote server though. Nmap shows all ports on the device are either filtered or closed, using the "Discovery Scan".

Thanks in advance for any insight into how this is done.

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  • Some devices have the remote management or remote access built into them... Would need to know more specific information to answer this question. From the limited information you are giving, I would say "they don't" but without more details of exactly what you are referring to, it is impossible to answer this question as is.
    – acejavelin
    Feb 23 at 16:58
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    Could you give a more concrete example? It's usually the device that creates the connection, not the technician.
    – harrymc
    Feb 23 at 16:59
  • It's easy if the device runs software that connects out to a server on the internet. Consider screen sharing software like Zoom or Teams - they don't require port forwarding or VPN. There are tons of different remote management tools
    – Cpt.Whale
    Feb 23 at 17:02
  • So in this, I am specifically thinking of a SolarEdge Inverter sending data to the monitoring platform. But tech support can access the settings on the inverter and configure it remotely if needed even though the inverter is connected via Wi-Fi through a local network. How do they access the inverter to configure it?
    – David_M16
    Feb 23 at 19:50
  • As everyone is saying, the most likely cause is a reverse-tunnel connection, but one less likely possibility is upnp. UPNP allows applications inside the network to dynamically create and manage port forwarding rules on a compatible Internet access router. Less likely, but possible especially if its an older device. en.wikipedia.org/wiki/Universal_Plug_and_Play#NAT_traversal Also Its normal that nmap won't see open listening ports in a reverse tunnel scenario, because the listeners are on the remote application server, not on the device. Feb 23 at 23:04

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In most cases, the internal device behind the NAT initiate the connection to a remote server on the internet. Then the person who need to administrate the device connect to this server and use the existing link in order to connect to your device.

An example could be remote management of a computer using TeamViewer. You don't need to forward any port on your router when you 'r using TeamViewer. It's because your device using TeamViewer connect from inside to a remote Master server to open the link.

When the other person use the TeamViewer Client to connect to your computer, it actually connects to the Master server which is used as a bounce server, then it connects to your computer from the server using the opening link initiate by your computer itself.

This redirection process, which relies on a central server, is typically hidden from the user.

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  • So how could they connect to the device if all it is doing is sending monitoring data. I am specifically thinking of a solar inverter in this question. The manufacturer's tech department can connect to the device and adjust settings if necessary. How is this possible if it is connected on a local network.
    – David_M16
    Feb 23 at 19:52
  • The device on the local network open a link to an external server an wait for someone using the link to manage it. it's a widely used method to manage device in local network
    – S. Brottes
    Feb 23 at 21:40
  • @David_M16, that implies that the connections coming out of that device are doing more than just providing telemetry. The device is almost certianly opening up a connection that can be used as a reverse tunnel for their support applications. Feb 23 at 22:51

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