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Port forwarding will not remain open

Synopsis:
Forwarding of port 81 to a static IP address configured in a Hyper-V Ubuntu server will not remain open.

Setup:
A Windows 10 Pro (build 18362.175) hosts a Hyper-V (100.0.18362.1) Ubuntu 18.04 server running Apache/2.4.39 listening on port 81. A Hyper-V virtual switch is configured as a wireless network adapter on the host Windows system. The Ubuntu server’s Ethernet
is configured with a static IP address of 192.168.222. A Netgear R6250 router is configured to forward port 81 to IP address 192.168.222:81. Appropriate server settings are set so that the host can use a web browser to view the site on the VM at 192.168.222:81.

A No-IP free ddns account is configured to forward projectmana.ddns.net http requests to the public IP address reported by its service running on the Windows system. The status of port 81 for the public IP address is checked with No-IP’s port check tool (http://portchecktool.com).

Observations:
The port will not remain open. The public IP has remained constant for more than 48 hours. It is also true that no periodic refreshing (either by the router or No-IP’s Windows service) is running. In short there is no evident mechanism that causes the port
to be blocked. If the port is reported as blocked it can be opened by either turning on the installed No-IP Windows service or by clicking on Apply in the router’s DDNS screen. Leaving either of these services on or off has no effect on whether the port becomes
blocked. This suggests that neither the router or the No-IP service are responsible for the port becoming blocked while the public IP address remains constant.

The problem also occurs with port 80.

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