Integrating Microsoft Azure Conditional Access with Windows 10 Always On VPN has several important benefits. The most important is that it allows administrators to improve their security posture by enforcing access polices that can be dynamically applied. For example, requiring multifactor authentication (MFA) for privileged users (e.g., administrators) or sign-ins that appear to be risky, the type of device they are connecting with, the health of the endpoint, and much more.
Authentication Failure
When configuring Always On VPN to support Azure Conditional Access, administrators may expeirence a failed authentication during preliminary testing. Specifically, an event ID 20227 from the RasClient source may be encountered with the following error message.
“The user <username> dialed a connection named <connection name> which has failed. The error code returned on failure is 812.”
Looking at the event logs on the Network Policy Server (NPS) server reveals an event ID 6273 from the Microsoft Windows security auditing source with Reason Code 258 and the following Reason.
“The revocation function was unable to check revocation for the certificate.”
Root Cause
When Azure Conditional Access is configured for Always On VPN, a short-lived certificate (1 hour lifetime) is provisioned by Azure. This certificate does not include revocation information because, by design, a short-lived certificate does not need to be revoked. However, by default NPS always checks revocation when client authentication certificates are used for authentication. Since the certificate does not include this information, certificate revocation fails.
Resolution
The way to resolve this issue is to disable certificate revocation checking for Protected Extensible Authentication Protocol (PEAP) authentication requests. To do this, open an elevated PowerShell window on the NPS server and run the following command.
New-ItemProperty -Path ‘HKLM:\SYSTEM\CurrentControlSet\Services\RasMan\PPP\EAP\13\’ -Name IgnoreNoRevocationCheck -PropertyType DWORD -Value 1 -Force
Once complete, restart the NPS server for the changes to take effect.
Additional Information
Windows 10 Always On VPN Network Policy Server (NPS) Load Balancing
Windows 10 Always On VPN Network Policy Server (NPS) Server 2019 Bug
Microsoft recently made available an update for Windows 10 2004 that includes many important fixes for outstanding issues with Windows 10 Always On VPN. 
