Install-WindowsFeature Web-Net-Ext failed. Source files could not be found.

Error:

Trying to install .NET Framework 3.5 Features on a Windows Server 2016, either through GUI or PowerShell, returns below error (truncated):

“The request to add or remove features on the specified server failed.
Installation of one or more roles, role services, or features failed.
The source files could not be found.”

Cause:

.Net Framework 3.5 sources file are not pre-installed so we need to explicitly specify where “installer” can get the source files from.

Solution:

As per Microsoft’s official docs, there are 2 main solutions to make it work.

  1. Through GUI (Server Manager), when asked during the wizard, you need to specify the path to “Media:\Sources\SxS” folder.
  2. Through PowerShell, Install-WindowsFeature -Name Web-Net-Ext -source D:\Sources\SxS

Still, Microsoft says that even if this defined source is not found, installer should go to Windows Update to download required files. Let’s exclude those many possibilities that Windows Update is not accessible due to various reasons (no internet connection, GPOs disabling something etc.).

Anyway, since neither one of the above methods worked for me, I kept reading posts from 2nd+ google pages. This led me to not-so-old post from Michael Niehaus: https://blogs.technet.microsoft.com/mniehaus/2015/08/31/adding-features-including-net-3-5-to-windows-10/

Not sure what’s different with calling DISM directly (since Server Manager and Powershell probably does the same), but below solution worked for me.

Run this from PowerShell (including tilde characters): DISM.exe /Online /Add-Capability /CapabilityName:NetFx3~~~~ /Source:D:\Sources\SxS where D was the drive with my mounted Server 2016 ISO.

When above command is done, run this: Add-WindowsCapability –Online -Name NetFx3~~~~ –Source D:\Sources\SxS

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