I had already fixed this issue once , but this time the method I used first time didn’t work. I Think there should be lot of admins who faced this issue.
I tried to install dotnet framework 3.5 on windows server 2012 and it was failed. Below is the error message I got
The source files could not be downloaded.
Use the “source” option to specify the location of the files that are required to restore the feature. For more information about specifying a source location, see http://go.microsoft.com/fwlink/?LinkId=243077.
The DISM log file can be found at C:\Windows\Logs\DISM\dism.log
Windows couldn’t complete the requested changes.
In one occastion I followed the instructions at https://support.microsoft.com/en-us/kb/3002547
After you install security update 2966827 or 2966828 (described in Microsoft Security Bulletin MS14-046) for the Microsoft .NET Framework 3.5, and then you try to enable the optional Microsoft .NET Framework 3.5 feature in Windows Features for the first time, the feature does not install. You may notice this failure if you “stage” the installation before you add the Microsoft .NET Framework 3.5 feature.
I manually uninstalled these updates and installed dotnet 3.5 features.
On another occasion I followed below steps.
Mounted the Windows Media from an ISO image
Then Ran the command “DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:F:\Win2012\sxs”
Interestingly on another situation I had to install the update KB2966828. I know this is against what microsft mentioned on first method. But it worked for me