I understand this is an often asked question, however after days of research I've not found an answer to this particular problem.

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I have a new ASP.NET 5 (Core 1.0) MVC template that I'm trying to add a package to however each and every time I get in the Package Manager output:

Response status code does not indicate success: 407 (Proxy Authentication Required). 

and the following at the top of solution explorer:

my settings look fine and I am able to browse packages in the 'Manage Packages for Solution' screen.

I understand I'm being requested to supply credentials so where do I enter them? On the other hand all other applications on my machine have validated internet access, so why should I?

Any ideas / pointers are very welcome.

Visual Studio 2015 - 14.0.24720.00 Update 1
NuGet Package Manager for Visual Studio 2015 - 3.3.0.167

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Top section of Package Manager output:

Installing NuGet package AutoMapper.4.2.1.
Successfully installed 'AutoMapper 4.2.1' to CustomerPortal
========== Finished ==========
PATH=.\node_modules.bin;%PATH%;C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\Extensions\Microsoft\Web Tools\External;C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\Extensions\Microsoft\Web Tools\External\git
C:\Users\medmondson.dnx\runtimes\dnx-clr-win-x86.1.0.0-rc1-update1\bin\dnx.exe "C:\Users\medmondson.dnx\runtimes\dnx-clr-win-x86.1.0.0-rc1-update1\bin\lib\Microsoft.Dnx.Tooling\Microsoft.Dnx.Tooling.dll" restore "M:\visual studio 2015\Projects\CustomerPortal\src\CustomerPortal" -f "C:\Program Files (x86)\Microsoft Web Tools\DNU"
Microsoft .NET Development Utility Clr-x86-1.0.0-rc1-16231
GET https://www.nuget.org/api/v2/
GET http://packages.nuget.org/v1/FeedService.svc/
GET https://www.postsharp.net/nuget/packages/
Restoring packages for M:\visual studio 2015\Projects\CustomerPortal\src\CustomerPortal\project.json
GET https://www.nuget.org/api/v2/FindPackagesById()?id='AutoMapper'
GET http://packages.nuget.org/v1/FeedService.svc/FindPackagesById()?id='AutoMapper'
GET https://www.postsharp.net/nuget/packages/FindPackagesById()?id='AutoMapper'
Warning: FindPackagesById: AutoMapper
An error occurred while sending the request.
GET https://www.nuget.org/api/v2/FindPackagesById()?id='AutoMapper'
GET https://www.nuget.org/api/v2/FindPackagesById()?id='System.Collections.Immutable'
GET http://packages.nuget.org/v1/FeedService.svc/FindPackagesById()?id='System.Collections.Immutable'
GET https://www.postsharp.net/nuget/packages/FindPackagesById()?id='System.Collections.Immutable'
Warning: FindPackagesById: AutoMapper
An error occurred while sending the request.
GET https://www.postsharp.net/nuget/packages/FindPackagesById()?id='AutoMapper'
Warning: FindPackagesById: AutoMapper
Response status code does not indicate success: 407 (Proxy Authentication Required).

Update

I've managed to capture a non-working request (returning 407) and a working request with the only difference being the content of the Proxy-Authorization header which is longer. This would suggest the proxy authentication does exist but it's using a different protocol.

I had to modify the 32 bit machine.config (assumption being VS runs in 32 bit) to add the <system.net> section but omitting anything within the <defaultProxy> tag:

<system.net>
    <defaultProxy useDefaultCredentials="True" enabled="True" />
</system.net>

According to MSDN:

If the defaultProxy element is empty, the proxy settings from Internet Explorer will be used.

This is perfect for me as every other application on my machine works - including IE.

manchine.config location (Win 7): %SYSTEMROOT%\Microsoft.NET\Framework\v4.0.30319\Config\machine.config

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Note

I resolved this thanks to @user3063127 pointing me in the right direction (you have an upvote). As far as I can tell this only affects package restore on DNX projects and may well be fixed when RC2 is released.