Jacob Marks: Solving Installation Problems with .NET 3.5 on Windows Server 2012 R2

While newer versions of Windows Server (2022/2025) have improved, installing .NET 3.5 still frequently fails because the payload is not part of the local Side-by-Side (SxS) store. The easiest way to solve this is to point the installer to the original installation media.

2026 Update: This method remains the gold standard for Windows Server 2022 and 2025. If you are in an air-gapped or WSUS-managed environment, the GUI will fail unless you provide this alternate path.

Option 1: The Fast Way (DISM)

Mount your Windows Server ISO (usually as drive D: or E:) and run this from an elevated Command Prompt. This forces the server to use the media instead of trying to reach Windows Update.

    dism /online /enable-feature /featurename:NetFx3 /all /Source:D:\sources\sxs /LimitAccess

Option 2: The PowerShell Way

If you prefer PowerShell, use the following command. The -Source parameter is key here.

    Install-WindowsFeature -Name NET-Framework-Core -Source D:\sources\sxs

Option 3: No Installation Media?

If you don't have the ISO, you can temporarily bypass WSUS to grab the files directly from Microsoft by changing a Group Policy setting:

Run gpedit.msc and go to: Computer Configuration > Administrative Templates > System.
Enable: "Specify settings for optional component installation and component repair".
Check the box: "Download repair content and optional features directly from Windows Update...".

Tip: If you are using a WIM file instead of a mounted drive, you can use the source format WIM:C:\install.wim:2 where "2" is the index of your server edition.