Jtdylktuy

You can download the Windows Server 2012 trial version to get an ISO with the required source files.

Mount the ISO with the inbox ISO mounting and run DISM to install .Net 3.5:

dism.exe /online /enable-feature /featurename:NetFX3 /Source:d:sourcessxs /LimitAccess

If it fails, also try to run this Update before trying to activate .Net 3.5:

Update for the .NET Framework 3.5 on Windows 8, Windows 8.1, Windows Server 2012, and Windows Server 2012 R2

http://support2.microsoft.com/kb/3005628/en-us

This removes 2 security updates which may prevent setup of .Net 3.5

This answer combines information from multiple answers and also adds some extra steps not previously mentioned.

I managed to do it in the following way:

1. Download the ISO image for Windows Server 2012 R2 (Note! has to be R2) from https://www.microsoft.com/en-us/evalcenter/evaluate-windows-server-2012-r2 . It needs a windows live id registration and you need to agree to follow-up emails.


2. Once you've downloaded it to some location, transfer to the machine/VM you wish to install it to.


3. Copy it to another location within the target machine. This is to prevent "sorry, there was a problem mounting the file" error as described here, which relates to some transfer managers writing the file as sparse files. If you actually copied it already during the transfer in the previous step and not just mapped it, this step might not be needed: for me it was.


4. Right click the file in explorer and select "Mount". It should be mounted, I'm assuming to D: drive. If you get an error, revert to previous link.


5. Open cmd prompt as administrator and run the following commands (switches are explained in this other answer in a similar thread):

.

dism.exe /online /enable-feature /featurename:NetFX3ServerFeatures /all /Source:d:sourcessxs /LimitAccess

dism.exe /online /enable-feature /featurename:NetFX3 /all /Source:d:sourcessxs /LimitAccess

Both seem to be needed and in the order I've given. (source)

If you get an error "source files not found", reconfirm you have R2 image in use. That was the problem for me - the actual .NET 3.5 was missing from my image, which was plain 2012.

If both commands are successful, congratulations! You now have it installed. In case you're in doubt, you can confirm the version using steps outlined here.

Here's how i did it.

Download the ISO image for Windows Server 2012 R2 from https://www.microsoft.com/en-us/evalcenter/evaluate-windows-server-2012-r2 .

Put that Disc image on the computer that needs .Net Framwork 3.5

Right Click on the file and mount. This should create a D drive. Go to Server Manager, click Manage > Add roles and Features.

Click through to the Features selection and choose the .Net Framework 3.5 entry.

Keep clicking Next until you get to the final page.

Don't click Install yet- Click the small link at the bottom to "Specify Alternate Data Source."
Change the path to be D:SourcesSxS

Then install

For Windows 2012R2, the command should be

dism.exe /online /enable-feature /featurename:NetFX3ServerFeatures /Source:d:sourcessxs /LimitAccess

Get a windows iso image 2008 server or higher mount the image to the inbuilt image drive of windows server 2012 r2, proceed to add roles and features and select the dotnet framework 3.5 feature, click alternatives source to show the path name eg e:sourcessxs and click install. This should work.

Or run windows update and follow this https://support.microsoft.com/en-us/kb/3005628 for solution.

I specified C:WindowsWinSXS as the alternate source path. This will fail half the time on the first go round, but it seems to eventually work.

I did it this morning with no issues and it completed on the first try.

If mounting a https://www.microsoft.com/en-us/evalcenter/evaluate-windows-server-2012-r2 still fails, We determined a minor tweak to Eis's SUPERB INSTRUCTIONS March 19th/2016 was the last inch for success.

Downloading and mounting 2012 Server R2 install ISO is key, but we had to switch to the mounted drive lettersources directory and load the NEWER R2 v9600 version of the DISM executable. Just mounting the ISO and specifying the /source location failed to complete or even initiate any progress of installation since Windows was using the version 9200 of DISM that apparently hates the /source switch but only reported as much inside the %systemroot%LogsDISMdism.log.
The error explicitly described /sources as and UNKNOWN switch and thus was doomed to silent failure without properly mounting the R2 source ISO.

It was necessary to hard specify the newer version of DISM.exe 9600 inside the mounted ISO by changing directories to the mounted ISO path of dism.exe.

cd /d d:sources && .dism.exe /online  /enable-feature    featurename:NetFX3ServerFeatures **/Source**:d:sourcessxs /LimitAccess

cd /d d:sources && .dism.exe /online  /enable-feature /featurename:NetFX3 **/Source**:d:sourcessxs /LimitAccess

The command processor output will immediately specify whether you're executing the required version of dism.exe

*Deployment Image Servicing and Management tool



**Version: 6.3.9600.17031**



Image Version: 6.2.9200.16384



Enabling feature(s)



[==========================100.0%==========================]



The operation completed successfully.



Restart Windows to complete this operation.



Do you want to restart the computer now? (Y/N) n*



This was executed on a freshly turned up 2012 Server with only the Windows Update Agent v2 installed:



Windows Update Agent 7.8.9200.16927

Windows Update Agent 7.8.9200.16924





Windows Update Agent 7.8.9200.16927

Installation date: ‎01‎31‎2017 1304PM

Installation status: Succeeded

Windows Update Agent 7.8.9200.16927

Installation date: ‎01‎31‎2017 1304PM

Installation status: Succeeded