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 load balancer 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

 

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

 

%d bloggers like this: