Securing RD Gateway with MFA using the new NPS Extension for Azure MFA!

Introduction
Back in 2014 I co-authored an article together with Kristin Griffin on how to secure RD Gateway with Azure MFA. This article was based on putting an Azure MFA Server (previously Phone Factor) in place in your on-premises environment (or Azure IaaS) to act as the MFA Server and enforce Multifactor Authentication for all session coming through RD Gateway. You can get the article here: Step By Step – Using Windows Server 2012 R2 RD Gateway with Azure Multifactor Authentication. Although this is a great solution and I have successfully implemented this for various customers, the big downside has always been the mandatory MFA Server. As part of the setup, a server in your Active Directory had to be installed running the Azure MFA Server component to be able to inject Azure MFA into the login sequence. Not only did you have to install and maintain this MFA Server, synching and managing users (and in most cases you would set up 2 servers to be HA), the other downside was that this was yet another MFA Provider for your end user. MFA server comes with a self-service portal to allow users to do their own enrollment and it can leverage the same Azure Authenticator App. However, if your end users used Azure MFA to secure e.g. Office365 or other SaaS services, that would be a different MFA provider, with a different Self Service signup sequence etc.

Introducing the NPS Extension for Azure MFA
So what has changed? A few days ago Microsoft announced the availability of the Azure MFA Extension for NPS (preview)! Read about the announcement where Alex Simons, Director of Program Management of the Microsoft Identity Division and Yossi Banai, Program Manager on the Azure Active Directory team talk about this new (preview) feature here:

Azure AD News: Azure MFA cloud based protection for on-premises VPNs is now in public preview!

Although the article specifically talks about securing a VPN, I figured the same would apply to secure Remote Desktop Gateway. And it turned out it does! In my lab I was able to successfully secure RD Gateway with Azure MFA using this new Extension for NPS! In this article I want to take you through the setup process and show the end result.

Prerequisites
There are a few prerequisites to use the NPS extension for Azure MFA, these are:

- License
For this to work you obviously need a license for Azure MFA. This is included with Azure AD Premium, EM+S, or it can be based on an Azure MFA subscription

- NPS Server
A Server is needed where the NPS role is installed. This needs to be at least Windows Server 2008 R2 SP1 and can be combined with other roles, however it cannot be combined with the RD Gateway role itself.

- Libraries
The below two libraries are needed on the NPS server, although Microsoft Guidance says the NPS Extension installer performs those installations if they are not in place, it doesn’t. Be sure to download and install these components prior to installing the NPS Extension.

1 Microsoft Visual Studio 2013 C++ Redistributable (X64)
2 Microsoft Azure Active Directory Module for Windows PowerShell version 1.1.166

- Azure Active Directory

Obviously Azure Active Directory has to be in place and users who need access, need to have been enabled to use MFA.

Installing
As mentioned in the introduction, I have written an article on securing RD Gateway with Azure MFA Server before. As you read though the installation & configuration process, you’ll see similarities with this article. That is not a coincidence, the same basic principles of RD Gateway, RD CAP, Radius Client, Remote Radius Servers et cetera all also apply on this setup.

Installing and configuring AAD & AAD Sync
Note, if you already have AAD & AAD Sync in place you can obviously skip this paragraph.
First things first, you need Azure Active Directory as a prerequisite. I won’t go over the entire process of setting up ADDS and AAD because there are many guides out there that explain this process very well. Basically you create a new AAD using the Azure Classic portal (or PowerShell), similar to below.


With AAD in place, you can then start to sync your users from an on premises ADDS (or like in my case one that is running on Azure IaaS). To manage the AAD you can already use the New Azure Portal as shown below, although do be aware that this feature is still in preview in this portal. You can also use this portal to get a link to the most recent version or Microsoft Azure Active Directory Connect that you need to be able to sync users from ADDS to AAD.


Again, I won’t go into great detail explaining the installation & best practices of installing AAD Connect, if you need detailed guidance on that part, check Connect Active Directory with Azure Active Directory. Basically what you do is run the installer on a server that is part of your ADDS domain and the only thing you will have to provide are the credentials of an AAD account and an ADDS connect with the appropriate permissions to access both domains.


Once you have successfully finished the setup of AAD Connect and the initial synchronization took place, the portal will reflect this as shown below.


With the initial synchronization complete, you can now start to assign Azure MFA to your users. To do this, open the All Users section in the Azure Portal and click on the Multi-Factor Authentication link.


That will take you to the Azure MFA Management Portal. In the screenshot below you can see the steps to enable and enforce Azure MFA for my test user called rdstestmfa.


Installing and configuring the NPS Extension for Azure MFA
Now that we have AAD and AAD Sync in place, lets drill down into the actual installation of the NPS Extension for Azure MFA! The first step is to download the latest version of the installer, which can be found here: NPS Extension for Azure MFA.

The NPS Extension needs to be installed on a (virtual) server that is part of the ADDS domain and that is able to reach the RD Gateway. In my case I used an ou-of-the-box Windows Server 2016 VM in Azure IaaS, but it can be anything from Windows Server 2008 R2 SP1 or above. Before installing the Extension, 3 other requirements need to be place.

1. The NPS Server role needs to be installed. Open Server Manager and add the role called Network Policy and Access Services.


2. The library Microsoft Visual Studio 2013 C++ Redistributable (X64) needs to be installed. Microsoft documentation says this library is installed automatically as part of the NPS Extension installer, the current Preview version 0.9.0.1 does however not do this yet. You can get the download here

3. The Microsoft Azure Active Directory Module for Windows PowerShell version 1.1.166 needs to be installed. Again, Microsoft documentation says this module is installed automatically as part of the NPS Extension installer, but the current Preview version 0.9.0.1 does not do this yet. You can get that download here

Now that we have the prerequisites in place, we can start the NPS Extension installer. The setup is very straight forward, just hit Install and wait for the process to finish.


After the installation is finished, the Extension components are placed in the folder C:\Program Files\Microsoft\AzureMfa\

Now open a new PowerShell Prompt (with elevated permissions) and change the directory to C:\Program Files\Microsoft\AzureMfa\Config and run the PowerShell script called AzureMfaNpsExtnConfigSetup.ps1. The output should look similar to below.


While the PowerShell Script runs it will prompt you for the ID of your Azure AD tenant, you can find that in the Azure Portal, in the properties of your AAD domain.


The PowerShell script will prompt you to authenticate to AAD with appropriate permissions. The PowerShell script then performs the following actions (source).

- Create a self-signed certificate.
- Associate the public key of the certificate to the service principal on Azure AD.
- Store the cert in the local machine cert store.
- Grant access to the certificate’s private key to Network User.
- Restart the NPS.

This completes the installation of the NPS Extension. The final step is to connect RD Gateway to this NPS Extension to get Azure MFA into the authentication process.

It’s important to realize that installing the NPS Extension causes all authentications processed by this NPS server to go through Azure MFA. There is no way to make exceptions for specific users.

Configuring RD Gateway
With the installation of the NPS Extension complete, it’s now time to configure RD Gateway. As mentioned before, this process is very similar to what Kristin Griffin and I explained here. The first step is to configure RD Gateway to use a Central Server running NPS. To do so, open RD Gateway Manager, right click the server name, and select Properties. Now select the RD CAP Store tab, select the Central Server running NPS option and enter the IP address of the NPS Server where you previously installed the NPS Extension. Also provide a shared key and store this somewhere safe.


Now open NPS on the RD Gateway Server (not on the NPS Server that contains the NPS Extension, we’ll do that later).

Open the Remote RADIUS Server Groups and open the TS GATEWAY SERVER GROUP. Enter the IP Address of the NPS Server running the extension as a RADIUS Server, edit it and make sure the timeout settings match what is shown below.


Now go to the RADIUS clients tab and create a new radius client with a friendly name, the IP address of the NPS Server running the Extension and enter the same shared secret you used before.


Next, we need to configure two Connection Request Policies in NPS, one to forward requests to the Remote RADIUS Server Group (which is set to forward to NPS server running the extension), and the other to receive requests coming from MFA server (to be handled locally).

The easiest way to do this is to use the existing policy that was created when you created an RD CAP in RD Gateway. In NPS, expand the Policies section in the left side of the screen and then select Connection Request Policies. You should see a policy already created there, called TS GATEWAY AUTHORIZATION POLICY. Copy that Policy twice and rename those copies to “MFA Server Request No Forward” and “MFA Server Request Forward”.

Now edit the MFA Server Request No Forward and set the following settings, where Client IPv4 Address is the IP Address of the NPS Server running the NPS Extension. Make sure you also enable this policy.


Now edit the MFA Server Request Forward and set the following settings, so that this rule forwards to the TS SERVER GATEWAY GROUP. Again, make sure you also enable this policy.

And lastly, disable existing TS GATEWAY AUTHORIZATION POLICY, and set the processing order of the rules as shown below.


Configuring NPS ServerIt’s now time to configure the NPS Server running the extension to make sure it can send and receive RADIUS requests too. Open NPS on the NPS Server (not on the RD Gateway Server we did that before).

Open the Remote RADIUS Server Groups and create a new group called RDGW. Enter the IP Address of the RD Gateway as a RADIUS Server, edit it and make sure the timeout settings match what is shown below.


Now go to the RADIUS clients tab and create a new radius client with a friendly name, the IP address of the RD Gateway Server and enter the shared secret you used before.


Go to the Connection Request Policies tab and create a new Policy called To RDGW and use the source Remote Desktop Gateway. Set the condition to Virtual (VPN) and configure it to forward requests to the Remote Radius Group called RDGW that we created before. Make sure the policy is enabled. Below is was the Policy should look like.


Create another Policy called From RDGW and again use the source Remote Desktop Gateway. Set the condition to Client IPv4 Address and enter the IP address of the RD Gateway server. Configure it to handle request locally. Make sure the policy is enabled. Below is was the Policy should look like.


Preparing the user account for Azure MFA
Since our test user called rdstestmfa@rdsgurus.com is new to the organization, we first need to make sure that our test user is successfully configured to use Azure MFA. If your users are already configured for Azure MFA, you can obviously skip this step.

An easy way to do this is to logon to portal.office.com and sign in with the account. Since our test account was enforced to use Azure MFA, the portal will prompt us to configure MFA before we can continue. Click Set it up now to start that process.

In this case I chose Mobile App as the authentication method, downloaded the Azure Authenticator App for iOS and used that to scan the QR image on the portal. The Azure Authenticator App is available for Android, iOS of Windows Phone.

Click Done. To complete the verification, Azure MFA will now send an MFA request to the configured phone number of the user account.


The user account is now ready to use for our RD Gateway setup! If you want more detailed information on the Azure MFA Service, you can find that here: What is Azure Multi-Factor Authentication?

Testing the functionality
It’s now finally time to take a look at the end result!

You can basically use any RDP Client that has support for RD Gateway. For this scenario we’ll use the RD Web Access page. We log on to RD Web Access with our rdstestmfa@rdsgurus.com account and open the Desktop. In this case we used a Full Desktop Scenario, but these could also have been RemoteApps. The RDP Client will be launched showing the state Initiating Remote Connections.

A few seconds later, the NPS Extension will be triggered to send Azure MFA a request to prompt our user for two-factor authentication.


After pressing Verify on the Phone and without the user having to interact with the computer, the status changes to Loading the virtual machine.


And the desktop is then launched.


The end result is a great and seamless experience for the end user. Similar to using Azure MFA Server, but this time NPS directly contacting Azure MFA! This is a great improvement!

Eventlogs
When troubleshooting this setup, there are several Eventlogs that could come in handy.

The RD Gateway role logs event in the location:
Application and Services Logs > Microsoft > Windows > Terminal Services Gateway
Below is an example of the event that shows that end user met the requirements of the RD CAP.


The NPS Service role logs event in the location:
Custom Views > Server Roles > Network Policy and Access Services
Below is an example of NPS Granting access to a user. You can also check the Windows Security log for Auditing events.


And finally, the NPS Extension role logs event in the location:
Application and Services Logs > Microsoft > AzureMfa


Additionally, you can also use the Azure MFA Portal to create reports on the usage of Azure MFA.


Conclusion
This article ended up to become >2500 words, but I hope you find it valuable. To reiterate on what is explained in the instruction; MFA Server is this is a great solution. The big downside however has always been the mandatory MFA Server, and in most cases you would set up 2 of them to be HA. The other downside is that this was yet another MFA Provider for your end user. With the introduction of the NPS Extension for Azure MFA these downsides are now gone! You can now secure your RDS environment with Azure MFA without the need for MFA Server and a separate MFA Provider. I really believe this is a game changer not only for this scenario, but also for all other scenarios like VPN’s, websites et cetera where Azure MFA Server is currently in place. Great job by Microsoft and looking forward to this Extension becoming GA!