Unlike DirectAccess, Windows 10 Always On VPN settings are deployed to the individual user, not the device. As such, there is no support for logging on without cached credentials using the default configuration. To address this limitation, and to provide feature parity with DirectAccess, Microsoft later introduced the device tunnel option in Windows 10 1709.
Device Tunnel Use Cases
The device tunnel is designed to allow the client device to establish an Always On VPN connection before the user logs on. This enables important scenarios such as logging on without cached credentials. This feature is crucial for organizations who expect users to log on to devices the first time remotely. The device tunnel can also be helpful for remote support, allowing administrators to manage remotely connected Always On VPN clients without having a user logged on. In addition, the device tunnel can alleviate some of the pain caused by administrators resetting remote worker’s passwords, or by users initiating a Self-Service Password Reset (SSPR).
Device Tunnel Requirements
The device tunnel requires Windows 10 Enterprise edition 1709 or later, and the client device must be joined to the domain. The device tunnel must be provisioned in the context of the local system account. Guidance for configuring and deploying a Windows 10 Always On VPN device tunnel can be found here.
- Windows 10 Always On VPN Device Tunnel Configuration using PowerShell
- Windows 10 Always On VPN Device Tunnel Configuration using Microsoft Intune
- Windows 10 Always On VPN Device Tunnel with Azure VPN Gateway
Device Tunnel Authentication
The device tunnel is authenticated using a certificate issued to the client device, much the same as DirectAccess does. Authentication takes place on the Routing and Remote Access Service (RRAS) VPN server. It does not require a Network Policy Server (NPS) to perform authentication for the device tunnel.
CRL Checking
Eventually an administrator may need to deny access to a device configured with an Always On VPN device tunnel connection. In theory, revoking the client device’s certificate and terminating their IPsec Security Associations (SAs) on the VPN server would accomplish this. However, Windows Server RRAS does not perform certificate revocation checking for Windows 10 Always On VPN device tunnel connections by default. Thankfully an update is available to enable this functionality. See Always On VPN Device Tunnel and Certificate Revocation for more details.
Configuration Best Practices
As the device tunnel is designed only to support domain authentication for remote clients, it should be configured with limited access to the on-premises infrastructure. Below is a list of required and optional infrastructure services that should be reachable over the device tunnel connection.
Required
- All domain controllers
- Enterprise DNS servers (if DNS is running on servers other than domain controllers)
Optional
- All issuing certification authority (CA) servers
- All certificate services online HTTP responders
- All certificate services Online Certificate Status Protocol (OCSP) servers
- System Center Configuration Manager (SCCM) distribution point servers
- Windows Server Update Services (WSUS) servers
- Management workstations
Limiting Access
Limiting access over the Always On VPN device tunnel can be accomplished in one of the following two ways.
Traffic Filters
The administrator can configure traffic filters on the device tunnel to restrict access only to those IP addresses required. However, be advised that when a traffic filter is enabled on the device tunnel, all inbound access will be blocked. This effectively prevents any remote management of the device from an on-premises system over the device tunnel.
Host Routes
An alternative to using traffic filters to limit access over the device tunnel is using host routes. Host routes are configured with a /32 prefix size and define a route to a specific individual host. The following is an example of host route configuration in ProfileXML.
Note: A PowerShell script that enumerates all enterprise domain controllers and outputs their IP addresses in XML format for use in ProfileXML can be found here.
Caveats
Some organizations may have hundreds or even thousands of domain controllers, so creating individual host route entries for all domain controllers in profileXML may not be practical. In this scenario it is recommended to add host routes only for the domain controllers that belong to the Active Directory site where the VPN server resides.
Supportability
Do not use the <DomainNameInformation> element in ProfileXML or enable force tunneling for the device tunnel. Neither of these configurations are supported.
Tunnel Coexistence
The device tunnel can be safely deployed in conjunction with the user tunnel whenever its functionality is required.
DNS Registration
If the device tunnel and user tunnel are both deployed, it is recommended that only one of the tunnels be configured to register in DNS. If the device tunnel is configured to register its IP address in DNS, be advised that only those devices with routes configured in the device tunnel VPN profile will be able to connect remotely to Always On VPN clients.
Additional Information
Windows 10 Always On VPN Device Tunnel with Azure VPN Gateway
Windows 10 Always On VPN Device Tunnel and Certificate Revocation
Windows 10 Always On VPN Device Tunnel Configuration with Microsoft Intune
Windows 10 Always On VPN Device Tunnel Does Not Connect Automatically
Windows 10 Always On VPN Device Tunnel Missing in Windows 10 UI
Deleting a Windows 10 Always On VPN Device Tunnel
Windows 10 Always On VPN Device Tunnel Configuration using PowerShell
A while back I wrote about 
The Internet Key Exchange version 2 (IKEv2) VPN protocol is the protocol of choice when the highest level of security is required for Always On VPN connections. It uses IPsec and features configurable security parameters that allow administrators to adjust policies to meet their specific security requirements. 
