c# - Can't see localhost from UWP app

Question

I’m working on UWP app on my laptop. On a previous laptop with a pre-release Windows 10, I was able to get my app to see my web API service on localhost, but on this laptop, no matter what I try, I always get this error using HTTP client:

A connection with the server could not be established

var response = await client.GetAsync(BuildGetRequest()).AsTask(source.Token);

If I point to my published service on Azure, everything works fine. Here’s what I have tried:

• Allowed local network loopback is set in the App’s project properties. Confirmed that my app package name is checked in the AppContainer Loopback Exemption Utility in Fiddler.
• Set Internet (Client), Internet (Client & Server), Private Networks capabilities in Package.AppManifest
• Tried manually setting c:>checknetisolation loopbackexempt -a -n= from the command line
• UWP Enable local network loopback
• Completely disabled the firewall in an act of pure desperation.
• Run my Web API in full IIS rather than IIS Express.

Nothing makes any difference.

I have noticed that I’m having a similar problem in Edge, despite allowing Localhost loopback in about:flags and several other suggestions I found at:

• https://social.technet.microsoft.com/Forums/en-US/0face535-3c7a-4658-be34-6c376322ca34/microsoft-edge-cant-open-local-domains

My Web API project runs fine in every browser except Edge, so I’m guessing as Edge is a universal app, the problem is linked. When I debug it in Edge, I get this error:

Can’t connect to the proxy server

If I go to Settings and switch off the manual proxy server, run it again I get:

Hmm, we can’t reach this page

I notice though that for some reason the Manual Proxy server setting keeps switching back on during subsequent runs.

Could anyone suggest anything else I could try to get my UWP app to see my service on localhost?

Answer 1

It's not a bug it's a feature, called network isolation. It was introduced in Windows 8 (where Metro apps were called Windows Runtime apps).

For security reasons, a UWP app that is installed in the standard manner is not allowed to make network calls to the device it is installed on.

More details here and here.

The feature can be disabled in Visual Studio debug settings, also the article How to allow loopback for Windows Runtime apps talks about using the CheckNetIsolation Windows tool to enable loopback access per application:

CheckNetIsolation.exe LoopbackExempt -s

CheckNetIsolation.exe LoopbackExempt –a –p=S-1-15-2-4125766819-3228448775-2449327860-2490758337-1264241865-3581724871-2122349299

There are also GUI tools such as Enable Loopback Utility and Loopback Exemption Manager which make this task easier: