Join me this Thursday, April 9 at 10:00AM EDT for a Remote Access Q&A session hosted by Kemp Technologies. During this free live webinar, I’ll be answering all your questions as they relate to enterprise mobility, remote access, scalability and performance, security, and much more. Topics are not limited to Kemp products at all, so feel free to join and ask me anything you like! Register now and submit your questions!
Posted by Richard M. Hicks on April 7, 2020
https://directaccess.richardhicks.com/2020/04/07/remote-access-qa-webinar-hosted-by-kemp/
One of the many advantages of using Windows Server Routing and Remote Access Service (RRAS) as the VPN server to support Windows 10 Always On VPN connections is that it includes support for the Secure Socket Tunneling Protocol (SSTP). SSTP is a TLS-based VPN protocol that is easy to configure and deploy and is very firewall friendly. This ensures consistent and reliable connectivity even behind restrictive firewalls. The Citrix ADC (formerly NetScaler) is a popular platform for load balancing Always On VPN connections. In this article I’ll describe how to configure load balancing on the Citrix ADC for RRAS VPN connections using the SSTP VPN protocol.
Special Note: In December 2019 a serious security vulnerability was discovered on the Citrix ADC that gives an unauthenticated attacker the ability to arbitrarily execute code on the appliance. As of this writing a fix is not available (due end of January 2020) but a temporary workaround can be found here.
Previously I’ve written about some of the use cases and benefits of SSTP load balancing as well as the options for offloading TLS for SSTP VPN connections. Load balancing SSTP eliminates single points of failure and enables support for multiple RRAS VPN servers to increase scalability. It is generally recommended that the Citrix ADC be configured to pass through encrypted SSTP VPN connections. However, TLS offloading can be configured to improve performance and reduce resource utilization on VPN servers, if required.
Load balancing SSTP on the Citrix ADC is straightforward and not unlike load balancing a common HTTPS web server. Below are specific settings and parameters required to load balance SSTP using the Citrix ADC.
Note: This article is not a comprehensive configuration guide for the Citrix ADC. It assumes the administrator is familiar with basic load balancing concepts and has experience configuring the Citrix ADC.
The load balancing service for SSTP VPN should be configured to use TCP port 443 and the SSL_BRIDGE protocol. If TLS offload is required, TCP port 80 and the HTTP protocol can be configured. Additional configuration is required on the RRAS server when TLS offload is enabled, however. Detailed information for configuring RRAS and SSTP for TLS offload can be found here.
The virtual server is configured to use TCP port 443. It is recommended to use SSLSESSION persistence.
The LEASTCONNECTION load balancing method is the recommend option for load balancing method.
Using the default TCP monitor (tcp-default) is not recommended for monitoring SSTP, as a simple TCP port check does not accurately reflect the health of the SSTP service running on the RRAS server. To more precisely monitor the SSTP service status, a new custom monitor must be created and bound to the load balancing services. Follow the steps below to configure a custom SSTP VPN monitor on the Citrix ADC.
Once complete, bind the new service monitor to the load balancing services or service groups accordingly.
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 Citrix ADC can be found here. Details for configuring RRAS and SSTP to support TLS offload can be found here.
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 Citrix ADC and HTTP will be used between the Citrix ADC 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.ps1 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.ps1 -CertificateHash [SHA256 Certificate Hash of Public SSL Certificate] -Restart
Example:
.\Enable-SSTPOffload.ps1 -CertificateHash ‘C3AB8FF13720E8AD9047DD39466B3C8974E592C2FA383D4A3960714CAEF0C4F2’ -Restart
When offloading TLS for SSTP VPN connections, all traffic between the Citrix ADC 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 Citrix ADC will be configured to terminate and then re-encrypt connections to the RRAS server. When terminating TLS on the Citrix ADC and re-encrypting connections to the RRAS server is required, the same certificate must be used on both the Citrix ADC and the RRAS server. Using different certificates on the RRAS server and the load balancer is not supported.
Windows 10 Always On VPN Load Balancing and SSL Offload
SSL Offload Configuration for Citrix ADC (NetScaler)
Windows 10 Always On VPN SSTP Load Balancing with Kemp LoadMaster
Windows 10 Always On VPN SSTP Load Balancing with F5 BIG-IP
Windows 10 Always On VPN Connects then Disconnects
Windows 10 Always On VPN SSL Certificate Requirements for SSTP
Posted by Richard M. Hicks on January 13, 2020
https://directaccess.richardhicks.com/2020/01/13/always-on-vpn-sstp-load-balancing-with-citrix-netscaler-adc/
A recent update to the Kemp LoadMaster load balancer may cause failed connections for Always On VPN connections using IKEv2. SSTP VPN connections are unaffected.
When using the Kemp LoadMaster load balancer to load balance IKEv2, custom configuration is required to ensure proper operation. Specifically, the virtual service must be configured to use “port following” to ensure both the initial request on UDP port 500 and the subsequent request on UDP port 4500 are sent to the same real server. This requires the virtual service to be configured to operate at layer 7. Detailed configuration guidance for load balancing IKEv2 on the Kemp LoadMaster load balancer can be found here.
A recent release of the Load Master Operating System (LMOS) v7.2.48.0 introduced a bug that affects UDP services configured to operate at layer 7, which includes IKEv2. This bug breaks Always On VPN connections using IKEv2, resulting in failed connections. When this occurs, the administrator may encounter an error 809 message for device tunnel or user tunnel.
Administrators who use the Kemp LoadMaster load balancer to load balance Always On VPN IKEv2 connections and have updated to LMOS 7.2.48.0 are encouraged to update to LMOS 7.2.48.1 immediately. This latest update includes a fix that resolves broken IKEv2 load balancing for Always On VPN. Once the LoadMaster has been updated to 7.2.48.1, Always On VPN connections using IKEv2 should complete successfully.
Windows 10 Always On VPN IKEv2 Load Balancing and NAT
Windows 10 Always On VPN IKEv2 Load Balancing with Kemp LoadMaster Load Balancer
Windows 10 Always On VPN SSTP Load Balancing with Kemp LoadMaster Load Balancer
Windows 10 Always On VPN Load Balancing with Kemp LoadMaster in Azure
Windows 10 Always On VPN Load Balancing Deployment Guide for Kemp Load Balancers
Posted by Richard M. Hicks on November 18, 2019
https://directaccess.richardhicks.com/2019/11/18/always-on-vpn-ikev2-load-balancing-issue-with-kemp-loadmaster/
Previously I wrote about Always On VPN options for Microsoft Azure deployments. In that post I indicated that running Windows Server with the Routing and Remote Access Service (RRAS) role for VPN was an option to be considered, even though it is not a formally supported workload. Despite the lack of support by Microsoft, deploying RRAS in Azure works well and is quite popular. In fact, I recently published some configuration guidance for RRAS in Azure.
Multiple RRAS servers can be deployed in Azure to provide failover/redundancy or to increase capacity. While Windows Network Load Balancing (NLB) can be used on-premises for RRAS load balancing, NLB is not supported and doesn’t work in Azure. With that, there are several options for load balancing RRAS in Azure. They include DNS round robin, Azure Traffic Manager, the native Azure load balancer, Azure Application Gateway, or a dedicated load balancing virtual appliance.
The easiest way to provide load balancing for RRAS in Azure is to use round robin DNS. However, using this method has some serious limitations. Simple DNS round robin can lead to connection attempts to a server that is offline. In addition, this method doesn’t accurately balance the load and often results in uneven distribution of client connections.
Using Azure Traffic Manager is another alternative for load balancing RRAS in Azure. In this scenario each VPN server will have its own public IP address and FQDN for which Azure Traffic Manager will intelligently distribute traffic. Details on configuring Azure Traffic Manager for Always On VPN can be found here.
The native Azure load balancer can be configured to provide load balancing for RRAS in Azure. However, it has some serious limitations. Consider the following.
More information about the Azure Load Balancer can be found here.
The Azure Application Gateway can be used for load balancing RRAS SSTP VPN connections where advanced capabilities such as enhanced health checks and TLS offload are required. More information about the Azure Application Gateway can be found here.
Using a dedicated Application Delivery Controller (ADC), or load balancer is a very effective way to eliminate single points of failure for Always On VPN deployments hosted in Azure. ADCs provide many advanced features and capabilities to ensure full support for all RRAS VPN protocols. In addition, ADCs offer much better visibility and granular control over VPN connections. There are many solutions available as virtual appliances in the Azure marketplace that can be deployed to provide RRAS load balancing in Azure.
Deploying Windows Server RRAS in Azure for Always On VPN can be a cost-effective solution for many organizations. Although not a formally supported workload, I’ve deployed it numerous times and it works quite well. Consider using a dedicated ADC to increase scalability or provide failover and redundancy for RRAS in Azure whenever possible.
Windows 10 Always On VPN Options for Azure Deployments
Posted by Richard M. Hicks on October 28, 2019
https://directaccess.richardhicks.com/2019/10/28/always-on-vpn-load-balancing-for-rras-in-azure/
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.
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!
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.
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.
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.
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.
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
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.
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
Posted by Richard M. Hicks on July 8, 2019
https://directaccess.richardhicks.com/2019/07/08/always-on-vpn-sstp-load-balancing-with-kemp-loadmaster/
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.
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.
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.
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.
Open the F5 BIG-IP management console and follow the steps below to create and assign a new custom monitor for SSTP.
1. In the navigation tree highlight Local Traffic.
2. Click Monitors.
3. Click Create.
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.
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.
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.
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.
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
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.
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
Posted by Richard M. Hicks on June 17, 2019
https://directaccess.richardhicks.com/2019/06/17/always-on-vpn-sstp-load-balancing-with-f5-big-ip/
I’m pleased announce that Kemp has released their Load Balancing Deployment Guide for Windows 10 Always On VPN. Authored by yours truly, this guide provides detailed, prescriptive guidance for configuring the Kemp LoadMaster load balancer to provide important scalability and eliminate critical points of failure in Always On VPN deployments.
Included in the guide are configuration steps for load balancing VPN servers using IKEv2 and SSTP using Kemp LoadMaster. Crucial details for IKEv2 load balancing as well as SSL offload for SSTP are covered in detail. In addition, the guide includes information about load balancing important supporting infrastructure services such as the Network Policy Server (NPS). Finally, guidance is included for enabling active/passive or active/active load balancing as well as geographic load balancing for multisite Always On VPN deployments.
You can download the Windows 10 Always On VPN load balancing deployment guide for Kemp LoadMaster load balancers here.
Windows 10 Always On VPN Load Balancing Deployment Guide for Kemp LoadMaster Load Balancers
Windows 10 Always On VPN IKEv2 Load Balancing with the Kemp LoadMaster Load Balancer
Posted by Richard M. Hicks on May 13, 2019
https://directaccess.richardhicks.com/2019/05/13/always-on-vpn-load-balancing-deployment-guide-for-kemp-load-balancers/
The Internet Key Exchange version 2 (IKEv2) VPN protocol is a popular choice for Windows 10 Always On VPN deployments. IKEv2 is a standards-based IPsec VPN protocol with customizable security parameters that allows administrators to provide the highest level of protection for remote clients. In addition, it provides important interoperability with a variety of VPN devices, including Microsoft Windows Server Routing and Remote Access Service (RRAS) and non-Microsoft platforms such as Cisco, Checkpoint, Palo Alto, and others.
IKEv2 is clearly the protocol of choice in terms of security. It supports modern cryptography and is highly resistant to interception. It’s not without some operational challenges, however. Consider the following.
IKEv2 uses UDP ports 500 and 4500 for communication. Unfortunately, these ports are not always open. Often, they are blocked by network administrators to prevent users from bypassing security controls or attackers from exfiltrating data.
IKEv2 packets can become quite large at times, especially when using client certificate authentication with the Protected Extensible Authentication Protocol (PEAP). This can result in fragmentation occurring at the network layer. Unfortunately, many firewalls and network devices are configured to block IP fragments by default. This can result in failed connection attempts from some locations but not others.
Load balancing IKEv2 connections is not entirely straightforward. Without special configuration, load balancers can cause intermittent connectivity issues for Always On VPN connections. Guidance for configuring IKEv2 load balancing on the Kemp LoadMaster and the F5 BIG-IP can be found here:
IKEv2 fragmentation can be enabled to avoid IP fragmentation and restore reliable connectivity. IKEv2 fragmentation is supported in Windows 10 and Windows Server beginning with v1803. Guidance for enabling IKEv2 fragmentation on Windows Server RRAS can be found here. Support for IKEv2 fragmentation on non-Microsoft firewall/VPN devices is vendor-specific. Consult with your device manufacturer for more information.
Be advised that the default security settings for IKEv2 on Windows Server RRAS are very poor. The minimum recommended security settings and guidelines for implementing them can be found here.
IKEv2 is recommend for deployments where the highest level of security and protection is required for remote connections. In these scenarios, the sacrifice of ubiquitous availability in favor of ultimate security might be desired.
SSTP or another TLS-based VPN protocol is recommended if reliable operation and connectivity are desired. SSTP and TLS VPNs can be configured to provide very good security by following the security and implementation guidelines found here.
In theory, preferring IKEv2 and falling back to the Secure Socket Tunneling Protocol (SSTP) or another TLS-based VPN protocol when IKEv2 is unavailable would seem like a logical choice. This would ensure the highest level of protection, while still providing reliable connectivity. Unfortunately, the Windows VPN client doesn’t work this way in practice. Details here.
Windows 10 Always On VPN IKEv2 Load Balancing with F5 BIG-IP
Windows 10 Always On VPN IKEv2 Load Balancing with Kemp LoadMaster
Windows 10 Always On VPN IKEv2 Fragmentation
Windows 10 Always On VPN IKEv2 and SSTP Fallback
Windows 10 Always On VPN IKEv2 Security Configuration
Windows 10 Always On VPN Certificate Requirements for IKEv2
Windows 10 Always On VPN Protocol Recommendations for Windows Server RRAS
Posted by Richard M. Hicks on April 15, 2019
https://directaccess.richardhicks.com/2019/04/15/always-on-vpn-ikev2-features-and-limitations/
The Internet Key Exchange version 2 (IKEv2) is the protocol of choice for Always On VPN deployments where the highest level of security is required. Implementing Always On VPN at scale often requires multiple VPN servers to provide sufficient capacity and to provide redundancy. Commonly an Application Delivery Controller (ADC) or load balancer is configured in front of the VPN servers to provide scalability and high availability for Always On VPN.
In a recent post I described some of the unique challenges load balancing IKEv2 poses, and I demonstrated how to configure the Kemp LoadMaster load balancer to properly load balance IKEv2 VPN connections. In this post I’ll outline how to configure IKEv2 VPN load balancing on the F5 BIG-IP load balancer.
Note: This article assumes the administrator is familiar with basic F5 BIG-IP load balancer configuration, such as creating nodes, pools, virtual servers, etc.
Follow the steps below to create a virtual server on the F5 BIG-IP to load balance IKEv2 VPN connections.
To begin, create two pools on the load balancer. The first pool will be configured to use UDP port 500, and the second pool will be configured to use UDP port 4500. Each pool is configured with the VPN servers defined as the individual nodes.
Next create two virtual servers, the first configured to use UDP port 500 and the second to use UDP port 4500.
To ensure reliable connectivity for IKEv2 connections it is necessary for the VPN server to see the client’s original source IP address. When configuring virtual server, select None from the Source Address Translation drop-down list.
To ensure that both IKEv2 UDP 500 and 4500 packets are delivered to the same node, follow the steps below to create and assign a Persistence Profile.
1. Expand Local Traffic > Profiles and click Persistence.
2. Click Create.
3. Enter a descriptive name for the profile in the Name field.
4. Select Source Address Affinity from the Persistence Type drop-down list.
5. Click the Custom check box.
6. Select the option to Match Across Services.
7. Click Finished.
Assign the new persistence profile to both UDP 500 and 4500 virtual servers. Navigate to the Resources tab on each virtual server and select the new persistence profile from the Default Persistence Profile drop-down list. Be sure to do this for both virtual servers.
Windows 10 Always On VPN IKEv2 Load Balancing and NAT
Windows 10 Always On VPN IKEv2 Load Balancing with Kemp LoadMaster Load Balancer
Windows 10 Always On VPN IKEv2 Security Configuration
Windows 10 Always On VPN and IKEv2 Fragmentation
Windows 10 Always On VPN Certificate Requirements for IKEv2
Video: Windows 10 Always On VPN Load Balancing with the Kemp LoadMaster Load Balancer
Posted by Richard M. Hicks on March 11, 2019
https://directaccess.richardhicks.com/2019/03/11/always-on-vpn-ikev2-load-balancing-with-f5-big-ip/
The 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 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.
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.
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.
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.
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.
Configuration changes must be made on the load balancer and the RRAS server to support TLS offload for SSTP.
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
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.
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
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.
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
Posted by Richard M. Hicks on February 18, 2019
https://directaccess.richardhicks.com/2019/02/18/always-on-vpn-sstp-load-balancing-and-ssl-offload/
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