If you do not have the full ISO or are experiencing issues with the mounted drive, you can extract the .cab file.

Why It Matters

Installing on Windows Server 2012 R2 often fails because the source files aren't kept on the local disk to save space. The standard way to fix this is by using the microsoft-windows-netfx3-ondemand-package.cab file, which is found in the \sources\sxs folder of the original Windows Server 2012 R2 installation media. Option 1: The Quick Command Line Fix

Once the microsoft-windows-netfx3-ondemand-package.cab source is available or Windows Update routing is corrected, use one of the execution vectors below to deploy the component. Option 1: Command Line Interface (DISM) Installing .Net 3.5 on Windows 2012 R2 - Server Fault

To obtain a "new" copy today (even if the file date is old, the cryptographic hash remains valid), you have two legitimate options:

dism /online /enable-feature /featurename:NetFX3 /All /Source:D:\sources\sxs /LimitAccess Use code with caution. Note: Replace D: with your actual CD/DVD drive letter .

dism /online /enable-feature /featurename:NetFx3 /all /limitaccess /source:D:\sources\sxs Using PowerShell You can achieve the same result via PowerShell

Copy the file to a simple path on your target server, e.g., C:\Temp\NetFx3.cab . Avoid spaces in the path to prevent command-line parsing errors.

In highly secure enterprise environments, local feature installation might be blocked by Group Policy (GPO). If your installation still fails, check the following policy:

Navigate to: Computer Configuration > Administrative Templates > System .