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Communicating with Localhost
paulmon
paulmon
08/28/2017
article
Learn how to create a TCP/IP connection with two processes by enabling localhost loopback.
windows iot, localhost, loopback, UWP, visual studio

Communicating with localhost (loopback)

On Windows IoT Core, if you want to create a TCP/IP connection between 2 processes running on the same device and one of them is a UWP app you must enable localhost loopback.

Loopback and the debugger 

By default, running under the Visual Studio debugger enables outbound loopback automatically for that debug session only.  You shouldn’t have to do anything as long as the loopback checkbox is checked in the debugger settings for your startup project.  If you want to implement a socket listener the you must enable localhost loopback for inbound connections (see below).

Enabling the inbound loopback policy

The localhost inbound loopback policy for Windows IoT Core must be enabled for UWP apps that implement servers. This policy is controlled by the following registry key:

    [HKEY_LOCAL_MACHINE\system\currentcontrolset\services\mpssvc\parameters]
        "IoTInboundLoopbackPolicy"=dword:00000001

This IoTInboundLoopbackPolicy registry key value must be set to dword:00000001 to enable. If you change the IoTInboundLoopbackPolicy registry value you must reboot for the change to take effect.  The localhost loopback policy should be enabled by default on Windows IoT Core

To verify that the value is set execute the following command on the Windows IoT Core device:

    reg query hklm\system\currentcontrolset\services\mpssvc\parameters /v IoTInboundLoopbackPolicy

To enable the policy execute the following command on the Windows IoT Core device:

    reg add hklm\system\currentcontrolset\services\mpssvc\parameters /v IoTInboundLoopbackPolicy /t REG_DWORD /d 1

Enabling loopback for a UWP application

Before you can enable loopback for an application you will need the package family name. You can find the package family name for an installed application by running iotstartup list. If the iotstartup list entry for the application is IoTCoreDefaultApp_1w720vyc4ccym!App then the package family name is IoTCoreDefaultApp_1w720vyc4ccym

To enable loopback for client connections use CheckNetIsolation.exe LoopbackExempt -a -n=<AppContainer or Package Family>. CheckNetIsolation.exe will configure loopback for the application and exit. This will enable the application to make outbound connections to a server.

Example: CheckNetIsolation.exe LoopbackExempt -a -n=IoTCoreDefaultApp_1w720vyc4ccym

To enable a server application to receive inbound connections use CheckNetIsolation.exe LoopbackExempt -is -n=<AppContainer or Package Family>. Unlike outbound connection configuration, inbound connections require CheckNetIsolation.exe to run continuously while the server application is receiving connections.  This requires an OS build newer than 10.0.14393.

Example: CheckNetIsolation.exe LoopbackExempt -is -n=IoTCoreDefaultApp_1w720vyc4ccym

The best way to run CheckNetIsolation.exe automatically on startup is to use schtasks.exe: schtasks /create /tn MyTask /f /sc onstart /ru system /tr "checknetisolation LoopbackExempt -is -n=IoTCoreDefaultApp_1w720vyc4ccym"

Upon rebooting you should be able to verify that checknetisolation.exe is running by using tlist.exe or Windows Device Portal