Always On VPN SSTP Load Balancing with Kemp LoadMaster

Always On VPN SSTP Load Balancing with Kemp LoadMaster The Windows Server Routing and Remote Access Service (RRAS) includes support for the Secure Socket Tunneling Protocol (SSTP), which is a Microsoft proprietary VPN protocol that uses SSL/TLS for security and privacy of VPN connections. The advantages of using SSTP for Always On VPN is that it is firewall friendly and ensures consistent remove connectivity even behind highly restrictive firewalls.

Load Balancing SSTP

In a recent post, I described some of the use cases and benefits of SSTP load balancing as well as the offloading of TLS for SSTP VPN connections. Using a load balancer for SSTP VPN connections increases scalability, and offloading TLS for SSTP reduces resource utilization and improves performance for VPN connections. There are positive security benefits too.

Note: A comprehensive reference with detailed, prescriptive guidance for configuring the Kemp LoadMaster for Always On VPN can be found in the Always On VPN Load Balancing Deployment Guide for Kemp Load Balancers. Download this free guide now!

Configuration

Enabling load balancing on the Kemp LoadMaster platform is fundamentally similar to load balancing HTTPS web servers. However, there are a few subtle but important differences.

Health Check

Using a standard TCP port check on the LoadMaster will not accurately reflect the health of the SSTP service running on the RRAS server. In addition, using a simple TCP port check could yield unexpected results. To ensure accurate service status monitoring, it is recommended that HTTP or HTTPS health checks be configured instead.

Real Server Check Method

Open the Kemp LoadMaster management console and follow the steps below to enable HTTP/HTTPS health checks for SSTP.

1. Expand Virtual Services in the navigation pane.
2. Click View/Modify Services.
3. Click Modify on the SSTP VPN virtual service.
4. Expand Real Servers.
5. Select HTTPS Protocol from the Real Server Check Method drop-down list. Alternatively, if TLS offload is enabled select HTTP Protocol.
6. In the URL field enter /sra_{BA195980-CD49-458b-9E23-C84EE0ADCD75}/ and click Set URL.
7. In the Status Codes field enter 401 and click Set Status Codes.
8. Check the box next to Use HTTP/1.1.
9. Select Head from the HTTP Method drop-down list.

Always On VPN SSTP Load Balancing with Kemp LoadMaster

TLS Offload

It is generally recommended that TLS offload not be enabled for SSTP VPN. However, if TLS offload is desired, it is configured in much the same way as a common HTTPS web server. Specific guidance for enabling TLS offload on the Kemp LoadMaster load balancer can be found in the Always On VPN Load Balancing Deployment Guide for Kemp Load Balancers. Details for configuring RRAS and SSTP to support TLS offload can be found here.

Certificates

When enabling TLS offload for SSTP VPN connections it is recommended that the public SSL certificate be installed on the RRAS server, even though TLS processing will be handled on the LoadMaster and HTTP will be used between the LoadMaster and the RRAS server. If installing the public SSL certificate on the RRAS server is not an option, additional configuration will be required. Specifically, TLS offload for SSTP must be configured using the Enable-SSTPOffload PowerShell script, which can be found here.

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

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

Example:

Enable-SSTPOffload -CertificateHash “C3AB8FF13720E8AD9047DD39466B3C8974E592C2FA383D4A3960714CAEF0C4F2” -Restart

Re-Encryption

When offloading TLS for SSTP VPN connections, all traffic between the LoadMaster and the RRAS server will be sent in the clear using HTTP. In some instances, TLS offload is required only for traffic inspection, not performance gain. In this scenario the LoadMaster will be configured to terminate and then re-encrypt connections to the RRAS server. When terminating TLS on the LoadMaster and re-encrypting connections to the RRAS server is required, the same certificate must be used on both the LoadMaster and the RRAS server. Using different certificates on the RRAS server and the load balancer is not supported.

Additional Information

Windows 10 Always On VPN Load Balancing Deployment Guide for Kemp Load Balancers

Windows 10 Always On VPN SSTP Load Balancing and SSL Offload

Windows 10 Always On VPN SSL Certificate Requirements for SSTP

Windows 10 Always On VPN ECDSA SSL Certificate Request for SSTP

Windows 10 Always On VPN SSTP Connects then Disconnects

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

Always On VPN SSTP Load Balancing with F5 BIG-IP

Always On VPN SSTP Load Balancing with F5 BIG-IP The Windows Server Routing and Remote Access Service (RRAS) includes support for the Secure Sockets Tunneling Protocol (SSTP), which is a Microsoft proprietary VPN protocol that uses SSL/TLS for security and privacy of VPN connections. The advantage of using SSTP for Always On VPN is that it is firewall friendly and ensures consistent remote connectivity even behind highly restrictive firewalls.

Load Balancing SSTP

In a recent post, I described some of the use cases and benefits of SSTP load balancing as well as the offloading of TLS for SSTP VPN connections. Using a load balancer for SSTP VPN connections increases scalability, and offloading TLS for SSTP reduces resource utilization and improves performance for VPN connections. There are positive security benefits too.

Configuration

Enabling load balancing for SSTP on the F5 BIG-IP platform is fundamentally similar to load balancing HTTPS web servers. However, there are a few subtle but important differences.

Default Monitor

The default HTTP and HTTPS monitors on the F5 will not accurately reflect the health of the SSTP service running on the RRAS server. In addition, using a simple TCP port monitor could yield unexpected results. To ensure accurate service status monitoring, a new custom monitor must be created to validate the health of the SSTP service.

Custom SSTP Monitor

Open the F5 BIG-IP management console and follow the steps below to create and assign a new custom monitor for SSTP.

Create Monitor

1. In the navigation tree highlight Local Traffic.
2. Click Monitors.
3. Click Create.

Always On VPN SSTP Load Balancing with F5 BIG-IP

4. Enter a descriptive name in the Name field and from the Type drop-down list choose HTTP if TLS offload is enabled, or HTTPS if it is not.
5. In the Send String field enter HEAD /sra_{BA195980-CD49-458b-9E23-C84EE0ADCD75}/ HTTP/1.1\r\nHost:r\nConnection: Close\r\n\r\n.
6. In the Receive String field enter HTTP/1.1 401.
7. Click Finished.

Always On VPN SSTP Load Balancing with F5 BIG-IP

Assign Monitor

1. Below Local Traffic click Pools.
2. Click on the SSTP VPN server pool.
3. In the Health Monitors section select the SSTP VPN health monitor from the Available list and make it Active.
4. Click Update.

Always On VPN SSTP Load Balancing with F5 BIG-IP

CLI Configuration

If you prefer to configure the SSTP VPN monitor using the F5’s Command Line Interface (CLI), you can download the monitor configuration from my GitHub here.

TLS Offload

It is generally recommended that TLS offload not be enabled for SSTP VPN. However, if TLS offload is desired, it is configured in much the same way as a common HTTPS web server. Specific guidance for enabling TLS offload on the F5 BIG-IP can be found here. Details for configuring RRAS and SSTP to support TLS offload can be found here.

Certificates

When enabling TLS offload for SSTP VPN connections it is recommended that the public SSL certificate be installed on the RRAS server, even though TLS processing will be handled on the F5 and HTTP will be used between the F5 and the RRAS server. If installing the public SSL certificate on the RRAS server is not an option, additional configuration will be required. Specifically, TLS offload for SSTP must be configured using the Enable-SSTPOffload PowerShell script, which can be found here.

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

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

Example:

Enable-SSTPOffload -CertificateHash “C3AB8FF13720E8AD9047DD39466B3C8974E592C2FA383D4A3960714CAEF0C4F2” -Restart

Re-Encryption

When offloading TLS for SSTP VPN connections, all traffic between the F5 and the RRAS server will be sent in the clear using HTTP. In some instances, TLS offload is required only for traffic inspection, not performance gain. In this scenario the F5 will be configured to terminate and then re-encrypt connections to the RRAS server. When terminating TLS on the F5 and re-encrypting connections to the RRAS server is required, the same certificate must be used on both the F5 and the RRAS server. Using different certificates on the RRAS server and the load balancer is not supported.

Additional Information

Windows 10 Always On VPN SSTP Load Balancing and SSL Offload

Windows 10 Always On VPN SSL Certificate Requirements for SSTP

Windows 10 Always On VPN ECDSA SSL Certificate Request for SSTP

Windows 10 Always On VPN SSTP Connects then Disconnects

Windows 10 Always On VPN Load Balancing Deployment Guide for Kemp Load Balancers

 

Troubleshooting Always On VPN Errors 691 and 812

Troubleshooting Always On VPN Errors 691 and 812When configuring Windows 10 Always On VPN using the Routing and Remote Access Service (RRAS) on Windows Server 2012 R2 and Extensible Authentication Protocol (EAP) authentication using client certificates, clients attempting to establish a VPN connection using Internet Key Exchange version 2 (IKEv2) may receive the following error.

“The connection was prevented because of a policy configured on your RAS/VPN server. Specifically, the authentication method used by the server to verify your username and password may not match the authentication method configured in your connection profile.”

Troubleshooting Always On VPN Errors 691 and 812

The event log on the client also records RasClient event ID 20227 stating “the error code returned on failure is 812”.

Troubleshooting Always On VPN Errors 691 and 812

Always On VPN clients using the Secure Socket Tunneling Protocol (SSTP) may receive the following error.

“The remote connection was denied because the user name and password combination you provided is not recognized, or the selected authentication protocol is not permitted on the remote access server.”

Troubleshooting Always On VPN Errors 691 and 812

The event log on the client also records RasClient event ID 20227 stating “the error code returned on failure is 691”.

Troubleshooting Always On VPN Errors 691 and 812

Resolution

These errors can occur when Transport Layer Security (TLS) 1.0 has been disabled on the RRAS server. To restore functionality, enable TLS 1.0 protocol support on the RRAS server. If disabling TLS 1.0 is required for compliance reasons, consider deploying RRAS on Windows Server 2016. TLS 1.0 can be safely disabled on Windows Server 2016 without breaking EAP client certificate authentication for Windows 10 Always On VPN clients.

Additional Information

Windows 10 Always On VPN Hands-On Training

What’s the Difference Between DirectAccess and Windows 10 Always On VPN?

5 Important Things DirectAccess Administrators Should Know About Windows 10 Always On VPN

3 Important Advantages of Windows 10 Always On VPN over DirectAccess 

Windows 10 Always On VPN and the Future of DirectAccess

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