Had an issue where MDT was refusing to apply the .NET Framework 3.5. Looking at the DISM UI logs showed this error:
Error: 0x800f081f The source files could not be found. Use the "Source" option to specify the location of the files that are required to restore the feature. For more information on specifying a source location, see http://go.microsoft.com/fwlink/?LinkId=243077.
And some extracts from the DISM log:
2015-04-21 09:37:24, Info DISM DISM.EXE: Executing command line: C:windowssystem32DISM.exe /Online /Enable-Feature /FeatureName:"NetFx3" /Source:"C:MININTsourcesX64" /LimitAccess /All /NoRestart /logpath:C:MININTSMSOSDOSDLOGSZTIOSRole_Dism.log 2015-04-21 09:37:48, Error DISM DISM Package Manager: PID=3708 TID=3756 The source files could not be found; their location must be specified using the /source option to restore the feature. - GetCbsErrorMsg
MDT has been capable of installing Roles and Features in 8.x since MDT 2012 Update 1, and my setup had previously worked.
After much searching around, it turns out there’s an issue if some security updates have been installed before the .NET Framework 3.5 is installed. Microsoft have released this update to resolve the issue. It simply uninstalls the security updates, thereby allowing the .NET Framework to install successfully.
I added the hotfix into MDT as an application, and inserted it into the task sequence just prior to the Install Roles and Features step. Problem solved.