When attempting to install .NET Framework 3.5 on Windows Server 2012 through Server Manager or DISM, many administrators encounter the frustrating error "The source files could not be found." This typically occurs when the system cannot locate installation files either locally or from Windows Update.

Microsoft's Windows Update servers for legacy frameworks have become increasingly unreliable. The installation process may fail because:

• Windows Update servers redirect to newer endpoints
• Corporate firewalls block legacy update channels
• Microsoft has deprecated certain authentication methods

Method 1: Forced Installation via DISM with Fallback

This hybrid approach combines DISM with Windows Update fallback:

dism.exe /online /enable-feature /featurename:NetFX3 /All /LimitAccess /Source:https://go.microsoft.com/fwlink/?LinkID=746924

Method 2: Using Windows Update MiniTool

When standard methods fail, third-party tools can help:

1. Download Windows Update MiniTool
2. Search for "KB2966828" (the .NET 3.5 update package)
3. Manually install the standalone package

Method 3: Registry Workaround for WSUS Environments

For domain-joined servers using WSUS:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Servicing]
"RepairContentServerSource"=dword:00000002
"UseWindowsUpdate"=dword:00000001

After attempting installation, verify with PowerShell:

Get-WindowsFeature NET-Framework-Core | Select Installed

Consider these last-resort options:

• Mount a Windows Server 2012 ISO and specify the SXS path
• Install via Group Policy for domain environments
• Use the offline installer package from Microsoft's catalog

Remember that .NET 3.5 installation issues often stem from network policies or update server availability rather than technical limitations. The methods above provide multiple pathways to successful installation without physical media.

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Many Windows Server 2012 administrators hit this roadblock when enabling .NET Framework 3.5 through Server Manager. The error message "The source files could not be downloaded" appears despite having internet connectivity. Let's explore why this happens and how to bypass it.

The default Windows Update-based installation often fails because:

• WSUS settings may block the download
• Corporate firewalls might intercept the request
• Microsoft's legacy download endpoints occasionally time out

When the DISM method with local sources isn't available, try these approaches:

Method 1: Forced Online Installation

Run this elevated PowerShell command to bypass typical restrictions:

Enable-WindowsOptionalFeature -Online -FeatureName "NetFx3" -All -LimitAccess -NoRestart

Method 2: Registry-Based Workaround

Create this registry key before attempting installation:

New-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Servicing" 
-Name "RepairContentServerSource" -Value 2 -PropertyType DWORD -Force

If you encounter error code 0x800F0906, it typically indicates either:

1. Windows Update services aren't running properly
2. The server can't reach Microsoft's legacy update endpoints

For corporate environments with restricted internet access, consider:

# Check Windows Update service status
Get-Service -Name wuauserv | Start-Service -PassThru

# Verify network connectivity to Microsoft
Test-NetConnection -ComputerName download.windowsupdate.com -Port 443

For environments with strict security policies, prepare offline installation files:

# Export required files
Export-WindowsCapabilitySource -Path C:\Temp\DotNet35 -Name NetFX3~3.1.0.0

# Install from export
Add-WindowsCapability -Online -Name NetFX3~3.1.0.0 -Source C:\Temp\DotNet35

Remember to validate the installation afterward:

Get-WindowsFeature | Where-Object {$_.Name -like "*NET*3.5*"}