On Windows Server 2008 R2 core I use dism.exe and the featurename: 'NetFx3-ServerCore' this feature is not available on Server 2012 (Windows Server 8).
Using the standard 3.51 Installer on Server 2012 Core tells me: "You must use the Role Management Tool to install or configure Microsoft .NET Framework 3.5"
In the GUI version (of Server 2012) I can do this and it downloads a package from Microsoft.com, but how do I do this on Core?
You can use Powershell to show the available Windows Features:
Because the data is a long and wide it is better to pipe it into a file and review it there.
it shows that '.NET Framework 3.5 Features' are available, but '.NET Framework 3.5 (includes .NET 2.0 and 3.0)' has been removed.
However, the files are still on the DVD, you can use:
or the PowerShell 'Add-WindowsFeature' commandlet to install it:
Edit: On Server 2012 R2 with Update 1 installed (KB2919355) this doesn't work if you are using the original DVD/ISO. You need a DVD or ISO which includes that update. For new installations, install .NET 3.5 before you apply KB2919355.
Where
x:
is the drive letter of the installation media or mapped network share that contains a copy of the installation files.A Windows update may be causing the problem: KB2966827 i.e. An update for 3.5 was installed even though the feature wasn't installed
Read here at the very bottom in the comments: http://social.technet.microsoft.com/Forums/windowsserver/en-US/5c16b88a-0f19-4aea-ad65-38f0bdb59b9c/install-net-framework-35-on-windows-server-2012-behind-the-firewall-does-not-recognize-sources?forum=winserver8gen