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We have a private NuGet server (v2.6) that uses HTTPS and Windows authentication and is running on IIS 7.5. I have some issues connecting to this server.

Often the first time I pull up Manage NuGet packages it works, it shows me the packages on our private server. The next time it very quickly displays "Could not connect to the feed specified at... . Please verify that the package source (located in the Package Manager Settings) is valid and ensure your network connectivity."

If I run up Fiddler, it seems that the NuGet server is issuing a ton of 401s (Not Authorised), but somehow the first time it retries until it gets it right. The subsequent attempts it just tries once (with no authentication at all in the header) and then gives up (hence the quick display of the error message above).

I get the same issue on my work PC and when I'm working from home.

One workaround is to restart visual studio and it starts working again, briefly. However, that is not a long term solution.

Colin Mackay
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Windows Authentication uses 401 Challenges in this manner, and my guess would be your NuGet.Server is having issues with the negotiation header the client is sending back. This could be all sorts of things, but it's usually related to IIS being unable to authenticate your credentials due to its service principal name settings.

If you're setting up your own NuGet server, you may want to consider ProGet - it's free, and can use a built-in Web Server instead of IIS that may bypass some of these authentication challenges.

Karl Harnagy
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