There are several blog posts, technet articles and Server Fault posts about installing .net 3.5 on Windows 8 and Server 2012/R2. The gist of it, is that you use the command line tool dism, with the /Source switch to specify the path to the SXS folder from the installation media used. Here is an example of the dism command:

dism.exe /online /enable-feature /featurename:NetFx3 /norestart /LimitAccess /Source:"\\FilleServer\Software\WindowsUpdates\Server2012R2-Update-SXS" /All

I have run into a couple of issues when running this on Azure VMs created using the latest versions of the Windows 2012 R2 Datacenter gallery image which I wanted to share.

1. The install media change between RTM and Update 1.

When this happened, I had to download the ISO that had update 1 slipstreamed an extract the SXS folder. You’ll notice in my example command above, the path contains \Server2012R2-Update-SXS. I have a corresponding folder that has the RTM bits.

2. Security Updates 2966827 or 2966828, fixed by 3005628.

The two security updates, 2966827 and 2966828, broke the ability to install .Net 3.5. Those two updates have been fixed so that they are only installed if .Net 3.5 has already been installed. Update 3005628 can be used to fix the issues if you have already installed one of the two problem updates, as was the case with the latest Azure gallery image.

I ended up using the MSU package (Download the Windows8.1-KB2966828-x64.msu package now), as I figured that the gallery images met the criteria: “Customers who are aware that the DISM “/StartComponentCleanup” and “/ResetBase” option was used on their system…”