Always On VPN IPsec Root Certificate Configuration Issue

Always On VPN Device Tunnel Status IndicatorWhen configuring a Windows Routing and Remote Access Service (RRAS) server to support Internet Key Exchange version 2 (IKEv2) VPN connections, it is essential for the administrator to define the root certification authority for which to accept IPsec security associations (SAs). Without defining this setting, the VPN server will accept a device certificate issued by any root certification authority defined in the Trusted Root Certification Authorities store. Details about configuring IKEv2 security and defining the root certification authority can be found here.

Multiple Root Certificates

Administrators may find that when they try to define a specific root certification authority, the setting may not be implemented as expected. This commonly occurs when there is more than one root certificate in the Trusted Root Certification Authorities store for the same PKI.

Always On VPN IPsec Root Certificate Configuration Issue

Certificate Selection

When running the PowerShell command Set-VpnAuthProtocol to define the root certification authority, PowerShell may ignore the administrator-defined certificate and choose a different one, as shown here. This will result in failed IPsec VPN connections from Windows 10 Always On VPN clients using IKEv2.

Always On VPN IPsec Root Certificate Configuration Issue

Certificate Publishing

This issue can occur when root certification authority certificates are published using Active Directory group policy. It appears that Windows prefers Active Directory group policy published certificates over those published directly in the Certification Authorities Container in Active Directory. To resolve this issue, remove any group policy objects that are publishing root certification authority certificates and ensure those root certificates are published in the Certification Authorities container in Active Directory.

PowerShell Script

A PowerShell script to configure this setting that can be found in my Always On VPN GitHub repository here. I have updated this script to validate the defined root certification authority certificate and warn the user if it does not match.

Additional Information

Set-Ikev2VpnRootCertificate.ps1 PowerShell script on GitHub

Windows 10 Always On VPN IKEv2 Security Configuration

Windows 10 Always On VPN IKEv2 Load Balancing and NAT

Windows 10 Always On VPN IKEv2 Features and Limitations

Windows 10 Always On VPN IKEv2 Fragmentation

Windows 10 Always On VPN IKEv2 Certificate Requirements

Always On VPN Device Tunnel Operation and Best Practices

Always On VPN Device Tunnel Operation and Best PracticesUnlike 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.

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.

Always On VPN Device Tunnel Operation and Best Practices

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.

Always On VPN Device Tunnel Operation and Best Practices

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

Microsoft Intune NDES Connector Error 0x80004003

Microsoft Intune NDES Connector Error 0x80004003To support certificate deployment for non-domain Windows 10 Always On VPN clients, a Windows Server with the Network Device Enrollment Service (NDES) role can be provisioned on-premises. In addition, the Microsoft Intune Connector must be installed and configured on the NDES server to allow Intune-managed clients to request and receive certificates from the on-premises Certification Authority (CA) server.

Connection Status Error

After installing the Microsoft Intune Connector, the administrator may encounter the following error message.

“An error occurred while connecting to the Intune Service. Error code is 0x80004003. The NDES Connector will retry the connection as soon as possible.”

 Microsoft Intune NDES Connector Error 0x80004003

IE Enhanced Security Configuration

This error can occur if Internet Explorer Enhanced Security Configuration (ESC) is enabled. To resolve this issue, disable ESC for administrators and users by opening the Server Manager on the NDES server and performing the following steps.

1. In the navigation pane click Local Server.
2. Click the On link next to IE Enhanced Security Configuration.
3. Click Off in the Administrators section.
4. Click Off in the Users section
5. Click Ok.

Microsoft Intune NDES Connector Error 0x80004003

Once complete, restart the NDES Connector service using the following PowerShell command.

Restart-Service NDESConnectorSvc -PassThru

Additional Configuration

Microsoft Intune NDES Connector Setup Wizard Ended Prematurely

Microsoft Intune NDES Connector Setup Wizard Ended Prematurely

Microsoft Intune NDES Connector Setup Wizard Ended PrematurelyA Windows Server with the Network Device Enrollment Service (NDES) role can be provisioned on-premises to support certificate deployment for non-domain Windows 10 Always On VPN clients. In addition, the Microsoft Intune Connector must be installed and configured on the NDES server to allow Intune-managed clients to request and receive certificates from the on-premises Certification Authority (CA) server.

Setup Wizard Ended Prematurely

When installing the Microsoft Intune Connector, the administrator may encounter a scenario where the setup wizard fails with the following error message.

“Microsoft Intune Connector Setup Wizard ended prematurely because of an error. Your system has not been modified. To install this program at a later time, run Setup Wizard again. Click the Finish button to exit the Setup Wizard.”

Microsoft Intune NDES Connector Setup Wizard Ended Prematurely

Cryptographic Service Provider

This error can occur if the NDES server certificate template is configured to use the Key Storage Provider cryptography service provider (CSP). When configuring the certificate template for the NDES server, the Legacy Cryptography Service Provider must be used, as shown here.

Microsoft Intune NDES Connector Setup Wizard Ended Prematurely

Additional Information

Deploying Windows 10 Always On VPN with Intune using Custom ProfileXML

Windows 10 Always On VPN Device Tunnel Configuration using Microsoft Intune

Deploying Windows 10 Always On VPN with Microsoft Intune

 

Troubleshooting Always On VPN Error Code 864

When configuring an Always On VPN connection, the administrator may encounter a scenario in which a VPN connection fails using either Internet Key Exchange version 2 (IKEv2) or Secure Socket Tunneling Protocol (SSTP). On the Windows 10 client the error message states the following.

“Can’t connect to [connection name]. The remote access connection completed, but authentication failed because a certificate that validates the server certificate was not found in the Trusted Root Certification Authorities certificate store.”

Troubleshooting Always On VPN Error Code 864

In addition, the Application event log records an error message with Event ID 20227 from the RasClient source. The error message states the following.

“The user [username] dialed a connection name [connection name] which has failed. The error code returned on failure is 864.”

Troubleshooting Always On VPN Error Code 864

NPS Server Certificate

Error code 864 is commonly caused by a missing or invalid server certificate on the Network Policy Server (NPS) performing authentication for VPN clients. The NPS server must have a certificate installed in its local computer certificate store from a trusted certification authority (CA) that includes the following.

Subject Name

The subject name must match the hostname defined in the EAP configuration for VPN clients. This may be the NPS server’s hostname but could also be an alias when NPS load balancing is configured.

Troubleshooting Always On VPN Error Code 864

Enhanced Key Usage

The NPS server certificate must include the Server Authentication Enhanced Key Usage (EKU).

Troubleshooting Always On VPN Error Code 864

NPS Policy Configuration

The NPS server certificate must also be selected in the network policy used for VPN client authentication. To confirm correct certificate configuration, open the properties for the Always On VPN network policy and follow the steps below.

1. Select the Constraints tab.
2. Highlight Authentication Methods.
3. Highlight Microsoft: Protected EAP (PEAP) in the EAP Types field.
4. Click Edit.
5. Select the NPS server certificate from the Certificate issued to drop-down list.

Troubleshooting Always On VPN Error Code 864

Ensure the NPS server certificate is also used for client certificate authentication by performing the following steps.

1. Highlight Smart Card or other certificate.
2. Click Edit.
3. Select the NPS server certificate from the Certificate issued to drop-down list.
4. Click Ok.

Troubleshooting Always On VPN Error Code 864

Additional Information

Windows 10 Always On VPN Network Policy Server (NPS) Load Balancing

Always On VPN Users Prompted for Certificate

Always On VPN Users Prompted for CertificateWhen deploying Windows 10 Always On VPN using Protected Extensible Authentication Protocol (PEAP) authentication with client certificates, administrators may find the VPN connection does not establish automatically. In this specific scenario the client is prompted to select a certificate to use to authenticate to the VPN server.

Always On VPN Users Prompted for Certificate

Multiple Certificates

This can occur when certificates from multiple Certification Authorities (CAs) are issued to the user that include the Client Authentication Enhanced Key Usage (EKU). When this happens, the user is forced to select the correct certificate to use for VPN authentication.

Clearly this is less than ideal, as it not only breaks the seamless and transparent nature of Always On VPN, the user may select the wrong certificate resulting in authentication failure. Ideally the client should be configured to select the correct certificate without user interaction.

Certificate Selection

Follow the steps below to configure automatic certificate selection for VPN authentication.

  1. On a VPN client, right-click the Always On VPN connection and choose Properties.
  2. Select the Security tab.
  3. In the Authentication section click Properties below Use Extensible Authentication Protocol (EAP).
  4. In the Select Authentication Method section click Configure.
  5. In the When connecting section click Advanced.
  6. Check the box next to Certificate Issuer.
  7. Select the root CA used to issue client authentication certificates for VPN authentication.
  8. Click Ok four times to save the configuration.

Always On VPN Users Prompted for Certificate

Once complete, export the EAP configuration to XML from the VPN client and paste the new settings in Intune or in your custom ProfileXML.

Certificate Purpose

By default, a client certificate requires only the Client Authentication EKU to establish a VPN connection. In some cases, this may not be desirable. For example, consider a deployment where Client Authentication certificates are issued to all users for Wi-Fi authentication. Depending on the Network Policy Server (NPS) configuration, these certificates may also be used to authenticate to the VPN.

VPN Specific Certificate

Follow the steps below to create a user authentication certificate template to be used exclusively for VPN authentication.

Certificate Template

  1. On the CA server, open the Certificate Templates management console (certtmpl.msc).
  2. Right-click the certificate template configured for VPN authentication and choose Properties.
  3. Select the Extension tab.
  4. Highlight Application Policies and click Edit.
  5. Click Add.
  6. Click New.
  7. Enter a descriptive name for the new application policy.
  8. Copy the Object identifier for later use and click Ok four times to save the configuration.

    Always On VPN Users Prompted for Certificate

  9. If certificate autoenrollment is configured and the certificate is already provisioned to users, right-click the certificate template and choose Reenroll All Certificate holders.

Client Configuration

  1. On the VPN client, follow the steps outlined previously to configure certificate selection.
  2. In addition to choosing a certificate issuer, select Extended Key Usage (EKU).
  3. Uncheck All Purpose.
  4. Select Client Authentication and the following EKUs.
  5. Click Add.
  6. Click Add once more.
  7. Enter the name of the custom EKU policy created previously.
  8. Enter the custom EKU object identifier copied previously from the custom policy.

    Always On VPN Users Prompted for Certificate

  9. Click Ok twice.
  10. Uncheck AnyPurpose and the following EKUs.
  11. Click Ok four times to save the configuration.

Always On VPN Users Prompted for Certificate

Once complete, export the EAP configuration to XML from the VPN client and paste the new settings in Intune or in your custom ProfileXML.

Additional Information

Windows 10 Always On VPN Clients Prompted for Authentication when Accessing Internal Resources

Get-EapConfiguration PowerShell Script on GitHub

Windows 10 Always On VPN Hands-On Training

Always On VPN Device Tunnel Configuration using Intune

Always On VPN Device Tunnel Configuration using IntuneA while back I described in detail how to configure a Windows 10 Always On VPN device tunnel connection using PowerShell. While using PowerShell is fine for local testing, it obviously doesn’t scale well. In theory you could deploy the PowerShell script and XML file using System Center Configuration Manager (SCCM), but using Microsoft Intune is the recommended and preferred deployment method. However, as of this writing Intune does not support device tunnel configuration natively. The administrator must create a ProfileXML manually and use Intune to deploy it.

Device Tunnel Prerequisites

I outlined the Always On VPN device tunnel prerequisites in my previous post here. To summarize, the client must be running Windows 10 Enterprise edition and be domain-joined. It must also have a certificate issued by the internal PKI with the Client Authentication EKU in the local computer certificate store.

ProfileXML

To begin, create a ProfileXML for the device tunnel that includes the required configuration settings and parameters for your deployment. You can find a sample Windows 10 Always On VPN device tunnel ProfileXML here.

Note: Be sure to define a custom IPsec policy in ProfileXML for the device tunnel. The default security settings for the IKEv2 protocol (required for the device tunnel) are quite poor. Details here.

Intune Deployment

Open the Intune management console and follow the steps below to deploy an Always On VPN device tunnel using Microsoft Intune.

Create Profile

1. Navigate to the Intune portal.
2. Click Device configuration.
3. Click Profiles.
4. Click Create profile.

Define Profile Settings

1. Enter a name for the VPN connection in the Name field.
2. Enter a description for the VPN connection in the Description field (optional).
3. Select Windows 10 and later from the Platform drop-down list.
4. Select Custom from the Profile type drop-down list.

Always On VPN Device Tunnel Configuration using Intune

Define Custom OMA-URI Settings

1. On the Custom OMA-URI Settings blade click Add.
2. Enter a name for the device tunnel in the Name field.
3. Enter a description for the VPN connection in the Description field (optional).
4. Enter the URI for the device tunnel in the OMA-URI field using the following syntax. If the profile name includes spaces they must be escaped, as shown here.

./Device/Vendor/MSFT/VPNv2/Example%20Profile%Name/ProfileXML

5. Select String (XML file) from the Data Type drop-down list.
6. Click the folder next to the Select a file field and chose the ProfileXML file created previously.
7. Click Ok twice and then click Create.

Always On VPN Device Tunnel Configuration using Intune

Assign Profile

Follow the steps below to assign the Always On VPN device tunnel profile to the appropriate device group.

1. Click Assignments.
2. Click Select groups to include.
3. Select the group that includes the Windows 10 client devices.
4. Click Select.
5. Click Save.

Always On VPN Device Tunnel Configuration using Intune

Demonstration Video

A video demonstration of the steps outlined above can be viewed here.

Additional Information

Windows 10 Always On VPN Device Tunnel Configuration using PowerShell

Windows 10 Always On VPN IKEv2 Security Configuration

Deleting a Windows 10 Always On VPN Device Tunnel

Windows 10 Always On VPN Device Tunnel Missing in the UI

Video: Deploying Windows 10 Always On VPN User Tunnel with Microsoft Intune

Always On VPN SSTP Connects then Disconnects

Always On VPN SSTP Connects then DisconnectsWhen Always On VPN clients are configured to use the Secure Socket Tunneling Protocol (SSTP) with Windows Server Routing and Remote Access Service (RRAS), administrators may encounter a scenario in which a client can establish a VPN connection using SSTP successfully, but is then disconnected immediately. The system event log contains an entry with Event ID 6 from the RasSstp source that includes the following error message.

“The SSTP-based VPN connection to the remote access server was terminated because of a security check failure. Security settings on the remote access server do not match settings on this computer. Contact the system administrator of the remote access server and relay the following information.”

Always On VPN Connect and Disconnect with SSTP

Common Causes

The two most common causes of this issue are when SSTP is configured for SSL offload, and when a VPN client is on a network where SSL inspection is taking place.

SSTP Offload

The most common cause of this issue is when SSL offload is configured for SSTP on an external load balancer or application delivery controller (ADC). To prevent interception from a Man-in-the-Middle attack, the VPN client sends the certificate hash of the SSL certificate used when the VPN connection was established. If this information does not match what is configured on the RRAS server, the connection is assumed to be compromised and the connection is immediately dropped.

SSL Inspection

Another scenario where this issue may occur is when a VPN client is behind a network device configured to perform SSL deep-packet inspection (DPI). SSTP VPN clients will be unable to connect to the VPN server in this scenario.

Resolution

When offloading SSL to another device, the RRAS server must be configured to know which SSL certificate is being presented to remote clients. This information is stored in the following registry key.

HKLM:\SYSTEM\CurrentControlSet\Services\SstpSvc\Parameters\SHA256CertificateHash

However, this registry entry requires a binary value, which makes it a challenge to configure manually. To resolve this problem, it is recommended that the same SSL certificate installed on the load balancer/ADC also be installed on the VPN server (even though SSL will be offloaded). To do this, first import the SSL certificate and private key in to the Local Computer certificate store, then open the RRAS management console and perform the following steps.

  1. Right-click the VPN server and choose Properties.
  2. Select the Security tab.
  3. Uncheck Use HTTP in the SSL Certificate Binding section.
  4. Select the appropriate SSL certificate from the Certificate drop-down list (click View to verify).
  5. Click Apply.

This will add the correct SSL certificate information to the registry. Next, re-enable HTTP for SSL offload by performing the following steps.

  1. Check Use HTTP in the SSL Certificate Binding section.
  2. Click Apply.

PowerShell Configuration

If the SSL certificate cannot be installed on the VPN server, or to automate this configuration across multiple servers remotely, download and run the Enable-SstpOffload PowerShell script from my GitHub repository here and run the following command.

Enable-SSTPOffload -CertificateHash [SHA256 Certificate Hash of Public SSL Certificate] -Restart

For example…

Enable-SSTPOffload -CertificateHash “C3AB8FF13720E8AD9047DD39466B3C8974E592C2FA383D4A3960714CAEF0C4F2” -Restart

Additional Information

Windows 10 Always On VPN Load Balancing and SSL Offload

Windows 10 Always On VPN SSTP Load Balancing with F5 BIG-IP

Windows 10 Always On VPN SSL Certificate Requirements for SSTP

Windows 10 Always On VPN Protocol Recommendations for Windows Server RRAS

 

Always On VPN SSTP Load Balancing and SSL Offload

SSL Certificate Considerations for DirectAccess IP-HTTPSThe Windows Server Routing and Remote Access Service (RRAS) is a popular choice for a VPN server to support Windows 10 Always On VPN deployments. One significant advantage RRAS provides is support for the Secure Socket Tunneling Protocol (SSTP). SSTP is a Microsoft proprietary VPN protocol that uses Transport Layer Security (TLS) to ensure privacy between the VPN client and server. The advantage to using a TLS-based transport is that it leverages the standard HTTPS TCP port 443, making it firewall friendly and ensuring ubiquitous remote access even behind highly restrictive firewalls.

Load Balancing SSTP

Load balancing SSTP can be accomplished in much the same way as a load balancing a common web server using HTTPS. The external load balancer is configured with a virtual IP address (VIP) and each VPN server is configured behind it. Session persistence should be configured to use SSL with source IP address persistence as a fallback.

SSL Offload for SSTP

In most cases, simply forwarding encrypted SSTP connections to the VPN server will be sufficient. However, offloading SSL/TLS processing to an Application Delivery Controller (ADC) or load balancer can be beneficial for the following reasons.

Resource Utilization

Enabling TLS offload for SSTP VPN connections can reduce CPU and memory utilization on the VPN server. However, this will likely only be necessary for very busy servers supporting many concurrent connections.

Security

In some cases, the administrator may not be able to install the public SSL certificate on the VPN server. For example, a security policy may exist that restricts SSL certificate installation to dedicated security devices using a Hardware Security Module (HSM). In some cases, it may be desirable to restrict access to high value certificates such as wildcard certificates.

Certificate Management

Often SSL certificates are implemented on load balancers to reduce certificate sprawl and to ease the management and administration burden in the enterprise. By having all enterprise certificates installed only on dedicated security devices, administrators can more effectively monitor and manage SSL certificate lifecycles.

SSTP Configuration for TLS Offload

Configuration changes must be made on the load balancer and the RRAS server to support TLS offload for SSTP.

Load Balancer

Install the public SSL certificate on the load balancer and configure it for TLS termination. Configure the load balancer to then use HTTP for backend server connections. Consult the load balancer vendor’s documentation for configuration guidance.

Load Balancing Always On VPN SSTP Load Balancing with F5 BIG-IP

RRAS Server

If the public SSL certificate is installed on the VPN server, enabling TLS offload for SSTP is simple and straightforward. Follow the steps below to enable TLS offload for SSTP VPN connections.

  1. Open the RRAS management console (rrasmgmt.msc).
  2. Right-click the VPN server and choose Properties.
  3. Select the Security tab.
  4. Check Use HTTP in the SSL Certificate Binding section.
  5. Click Ok and then Yes to restart the Remote Access service.

Always On VPN SSTP Load Balancing and SSL Offload

If the public SSL certificate is not or cannot be installed on the RRAS server, additional configuration will be required. Specifically, SSL offload for SSTP must be configured using the Enable-SSTPOffload PowerShell script, which can be downloaded here.

Once the script has been downloaded and imported, open an elevated PowerShell command window and enter the following command.

Enable-SSTPOffload -CertificateHash [SHA256 Certificate Hash of Public SSL Certificate] -Restart

For example…

Enable-SSTPOffload -CertificateHash “C3AB8FF13720E8AD9047DD39466B3C8974E592C2FA383D4A3960714CAEF0C4F2” -Restart

Re-Encryption

When offloading TLS for SSTP VPN connections, all traffic between the load balancer and the VPN server will be sent in the clear using HTTP. In some scenarios, TLS offload is required only for traffic inspection, not performance gain. When terminating TLS on the load balancer and re-encrypting connections to the VPN server is required, it is only supported if the same certificate is used on both the load balancer and the VPN server.

Additional Information

Windows 10 Always On VPN SSL Certificate Requirements for SSTP

Windows 10 Always On VPN SSL Load Balancing with F5 BIG-IP

Windows 10 Always On VPN IKEv2 and SSTP Fallback

Windows 10 Always On VPN Hands-On Training Classes for 2019

 

Always On VPN and IKEv2 Fragmentation

The IKEv2 protocol is a popular choice when designing an Always On VPN solution. When configured correctly it provides the best security compared to other protocols. The protocol is not without some unique challenges, however. IKEv2 is often blocked by firewalls, which can prevent connectivity. Another lesser know issue with IKEv2 is that of fragmentation. This can result in failed connectivity that can be difficult to troubleshoot.

IP Fragmentation

IKEv2 uses UDP for transport, and typically most packets are relatively small. The exception to this is when authentication takes place, especially when using client certificate authentication. The problem is further complicated by long certificate chains and by RSA keys, especially those that are greater than 2048 bit. If the payload exceeds 1500 bytes, the IP packet will have to be broken in to smaller fragments to be sent over the network. If an intermediary device in the path is configured to use a smaller Maximum Transmission Unit (MTU), that device may fragment the IP packets.

IP Fragmentation and Firewalls

Many routers and firewalls are configured to drop IP fragments by default. When this happens, IKEv2 communication may begin initially, but subsequently fail. This typically results in an error code 809 with a message stating the following.

“Can’t connect to [connection name]. The network connection between your computer and the VPN server could not be established because the remote server is not responding. This could be because one of the network devices (e.g. firewalls, NAT, routers, etc.) between your computer and the remote server is not configured to allow VPN connections. Please contact your Administrator or your service provider to determine which device may be causing the problem.”

Always On VPN and IKEv2 Fragmentation

Troubleshooting

When troubleshooting potential IKEv2 fragmentation-related connection failures, a network trace should be taken of the connection attempt on the client. Observe the packet sizes during the conversation, especially IKE_AUTH packets. Packet sizes exceeding the path MTU will have to be fragmented, as shown here.

Always On VPN and IKEv2 Fragmentation

Measuring Path MTU

Measuring the path MTU between the client and server can be helpful when troubleshooting fragmentation related issues. The mtupath.exe utility is an excellent and easy to use tool for this task. The tool can be downloaded here.

Always On VPN and IKEv2 Fragmentation

IKEv2 Fragmentation

To address the challenges with IP fragmentation and potential connectivity issues associated with network devices dropping fragmented packets, the IKEv2 protocol itself can be configured to perform fragmentation at the IKE layer. This eliminates the need for IP layer fragmentation, resulting in better reliability for IKEv2 VPN connections.

Both the server and the client must support IKEv2 fragmentation for this to occur. Many firewall and VPN vendors include support for IKEv2 fragmentation. Consult the vendor’s documentation for configuration guidance. For Windows Server Routing and Remote Access (RRAS) servers, the feature was first introduced in Windows Server 1803 and is supported in Windows Server 2019. Windows 10 clients support IKEv2 fragmentation beginning with Windows 10 1803.

Enabling IKEv2 Fragmentation

Windows 10 clients support IKEv2 fragmentation by default. However, it must be enabled on the server via the registry. The following PowerShell command will enable IKEv2 fragmentation support on Windows Server 1803 and later.

New-ItemProperty -Path “HKLM:\SYSTEM\CurrentControlSet\Services\RemoteAccess\Parameters\Ikev2\” -Name EnableServerFragmentation -PropertyType DWORD -Value 1 -Force

A PowerShell script to implement IKEv2 fragmentation can be found on my GitHub here.

Validation Testing

Once IKEv2 fragmentation is configured on the VPN server, a network capture will reveal the IKE_SA_INIT packet now includes the IKEV2_FRAGMENTATION_SUPPORTED notification message.

Always On VPN and IKEv2 Fragmentation

Additional Information

Windows 10 Always On VPN IKEv2 Security Configuration

RFC 7383 – IKEv2 Message Fragmentation

IEA Software MTU Path Scan Utility

Windows 10 Always On VPN Hands-On Training Classes

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