Recently I did some validation testing with Always On VPN on Windows 11, and I’m happy to report that everything seems to work without issue. However, a few readers have reported 853 errors when establishing an Always On VPN connection after upgrading to Windows 11.
Can’t Connect
After upgrading to Windows 11, an Always On VPN connection may fail with the following error message.
“The remote access connection completed, but authentication failed because the certificate that authenticates the client to the server is not valid. Ensure the certificate used for authentication is valid.”
Error 853
In addition, the Application event log records an event ID 20227 from the RasClient source that includes the following message.
“The user <username> dialed a connection name <connection name> which has failed. The error code returned on failure is 853.”
Server Identity
This error will occur when using Protected Extensible Authentication Protocol (PEAP) authentication. Specifically, it can happen when the option to verify NPS server validity by its certificate is selected, and an explicit list of NPS servers is defined, as shown here.
Case Sensitive
In this specific scenario, Windows 11 now appears to be case-sensitive when it compares the NPS server name entered in the NPS configuration to the Subject Name on the certificate returned by the server. For example, if the Subject Name (or Subject Alternative Name, if present) entry on the NPS server certificate is nps.lab.richardhicks.net, using NPS.lab.richardhicks.net will not match and return an 853 error.
Windows 11
Case matching when validating the NPS server certificate is a change in behavior from Windows 10. Before Windows 11, this comparison was case-insensitive, and any combination of case would match if the entire hostname matched. Going forward, it appears Microsoft has also decided to require case matching to validate the server certificate.
Recommendations
Administrators should look carefully at the server certificate issued to the NPS server and ensure their client configuration accurately reflects the hostname in a case-sensitive manner to ensure a smooth migration from Windows 10 to Windows 11.
Additional Information
Troubleshooting Windows 10 Always On VPN Error 853
Windows 10 Always On VPN Network Policy Server (NPS) Load Balancing