Whilst trying to restore nuget packages I am getting the following:-
Error 1 The underlying connection was closed: An unexpected error occurred on a send.
Error 2 The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.
Error 3 The command ""D:\root\.nuget\NuGet.exe" install "D:\root\packages.config" -source "" -NonInteractive -RequireConsent -solutionDir "D:\root\"" exited with code 1.
Error 4 Metadata file 'D:\root\project.Website.dll' could not be found D:\root\project.UnitTests\CSC
I tried the solutions provided on: Visual Studio 2010 nuget error: The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel
I tried the solutions provided on: https://nuget.codeplex.com/discussions/275537
None of them work, I was wondering if anyone else is currently having this issue? If so has anyone come up with a solution?
It seemingly, may be a waiting game, for the SSL certificates to be fixed.
After some further debugging with Fiddler I got the following response:
fiddler.network.https> HTTPS handshake to az320820.vo.msecnd.net failed. System.IO.IOException Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host. < An existing connection was forcibly closed by the remote host
If I am correct, it seems like I am being rejected but the server, not sure why and I am hopeful this will soon resolve it self.
For now I have managed to get copies of packages from other developers who hadn't cleared their package cache and continue developing this way.
Someone has kindly raised the issue on code plex so hopefully we get somewhere soon: https://nuget.codeplex.com/workitem/4372
Update
As of 17th November, this seems to be working again. NuGet correctly telling me it's having issues getting an older version of Ninject.Web.Common.3.2.0.0 but we have the updated edition which is fine.
I was having the same problem until I invoked my network admins. We found that our IPS flagged the domain as malware and we blocked it at our internal network level.
Once we removed the block, everything is now working again.
We also had problems getting images and css from codeplex project sites and this fixed that issue as well.
I'm seeing this issue behind a corporate proxy using Visual Studio 2015 which utilizes the new v3 API (https://api.nuget.org/v3/index.json). In my instance, it has to do with an SSL intercepting corporate proxy (BlueCoat) that doesn't know how to handle a newer cipher protocol (eg: TLS 1.2 plus some newer key exchange cipher) being negotiated by that API endpoint I believe.
A quick fix for Visual Studio 2015 users is to change the URL from https to http. I have a feeling that this may not work if the package has a https license file.
Alternatively, switch to Visual Studio 2013 for package installation and updates.
Check if your internet connection is working and your proxy is setup and enabled in Internet Explorer LAN settings. In my case it was trying to download it from internet and failing, as my proxy was disabled! Hope this helps...
Type this command
On Linux Execute the following in the command line before executing the Git command:
export GIT_TRACE_PACKET=1;export GIT_TRACE=1;export GIT_CURL_VERBOSE=1
On Windows Execute the following in the command line before executing the Git command:
set GIT_TRACE_PACKET=1 set GIT_TRACE=1 set GIT_CURL_VERBOSE=1
来源：https://stackoverflow.com/questions/26928191/the-underlying-connection-was-closed-an-unexpected-error-occurred-on-a-send