HttpListener in UWP application leads to "An attempt was made to access a socket in a way forbidden by its access permission"s

[llfab]

I have created an UWP application and wanted to run our REST server that contains a HttpListener. While starting HttpListener the following exception happened: "An attempt was made to access a socket in a way forbidden by its access permissions"

I had the REST server already running in WPF-based .net6.0 applications. For these to be able to listen on the specified URL, you need to add the URI to the URL access control list via netsh http add urlacl .... Other than that I had not seen any problems.

I also found this helpful information that UWP apps cannot receive networking from the loopback. You will find how to make that work here: https://social.msdn.microsoft.com/Forums/en-US/d7e12fe4-cddd-4365-abda-5d5ada33410d/are-any-special-step-required-for-uwp-httplistener?forum=wpdevelop

While looking through the web for others having the same issue, I was not able to find anything relevant to my specific problem. Trying with different URIs using both http and https I was finally able to detect the root cause:

• I used the very same URIs that I had used previously for my .net6.0 WPF application but just with a different sub-uri-path
• When I switched between http and https the error was still there
• However, when I changed (finally!) the port number, the HttpListener started successfully
• After trying some more I finally clarified that base URIs (including portnumber) cannot be listened to in UWP with HttpListener if they are registered in systems URLACL.
• Hence, either you remove the entry from the URLACL or you choose a different base URI (port number)
• Note: the problem was NOT, that the .net6.0 apps with the same URI base where running. The problem was only that these base URIs where registered in the URLACL

In URLACL the URI is regeistered and access-allowed for a user. In my case "Everyone". Even if I do not fully understand why this would enable .net6.0 apps to successfully start a HttpListener on these URI's while it would fail UWP apps for the same, I now recognize that I will need different base URIs for my .net6.0 and UWP apps.

Maybe you guys can attach more details to this discussion allowing others to not fall into the same trap