.NET Framework 4.6.1 and Exchange 2013 / 2016

Currently with Exchange 2013 CU12 and Exchange 2016 CU1, it is not supported to install .NET Framework 4.6.1 as it causes issues databases unexpectedly dismount or failover to alternative servers within a DAG cluster.  This issue is documented on the following KB article:

https://support.microsoft.com/en-us/kb/3095369

To ensure .NET Framework 4.6.1 does not install on your Exchange Servers, make sure you put the following registry key in place on your Exchange Servers:

1. Back up the registry.
2. Start Registry Editor. To do this, click Start, type regedit in the Start Search box, and then press Enter.
3. Locate and click the following subkey:
   
   HKEY_LOCAL_MACHINE\Software\Microsoft\NET Framework Setup\NDP
4. After you select this subkey, point to New on the Edit menu, and then click Key.
5. Type WU, and then press Enter.
6. Right-click WU, point to New, and then click DWORD Value.
7. Type BlockNetFramework461, and then press Enter.
8. Right-click BlockNetFramework461, and then click Modify.
9. In the Value data box, type 1, and then click OK.
10. On the File menu, click Exit to exit Registry Editor.

What if I have already installed .NET Framework 4.6.1 on my Exchange Servers?  If so use the following procedure:

1. If the server has already automatically updated to 4.6.1 and has not rebooted yet, do so now to allow the installation to complete
2. Stop all running services related to Exchange.  You can run the following cmdlet from Exchange Management Shell to accomplish this:  (Test-ServiceHealth).ServicesRunning | %{Stop-Service $_ -Force}
3. Go to add/remove programs, select view installed updates, and find the entry for KB3102467.  Uninstall the update.  Reboot when prompted.
4. Check the version of the .NET Framework and verify that it is showing 4.5.2.  If it shows a version prior to 4.5.2 go to windows update, check for updates, and install .NET 4.5.2 via the KB2934520 update.  Do NOT select 4.6.1/KB3102467.  Reboot when prompted.  If it shows 4.5.2 proceed to step 5.
5. Stop services using the command from step 2.  Run a repair of .NET 4.5.2 by downloading the offline installer, running setup, and choosing the repair option.  Reboot when setup is complete.
6. Apply the February security updates for .NET 4.5.2 by going to Windows update, checking for updates, and installing KB3122654 and KB3127226.  Do NOT select KB3102467.  Reboot after installation.
7. After reboot verify that the .NET Framework version is 4.5.2 and that security updates KB3122654 and KB3127226 are installed.
8. Follow the steps here to block future automatic installations of .NET 4.6.1.

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