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All posts tagged load balancing
Remote Access Questions and Answers Webinar Hosted by Kemp
Posted by Richard M. Hicks on April 7, 2020
https://directaccess.richardhicks.com/2020/04/07/remote-access-qa-webinar-hosted-by-kemp/
Always On VPN SSTP Load Balancing with Citrix NetScaler ADC
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.
Load Balancing SSTP
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.
Configuration
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.
Service Settings
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.
Virtual Server Settings
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.
Service Monitoring
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.
- Open the Citrix ADC management console and expand Traffic Management.
- Select Monitors.
- Click Add.
- Enter a descriptive name in the Name field.
- Select HTTP form the Type drop-down list and click Select.
- Adjust the Interval and Response Time-out values according to your requirements.
- Enter 401 in the Response Codes field and click the “+” button.
- In the Response Codes field click the “x” next to 200.
- In the HTTP Request field enter HEAD /sra_{BA195980-CD49-458b-9E23-C84EE0ADCD75}/.
- Check the box next to Secure (not required if TLS offload is enabled).
- Select ns_default_ssl_profile_backend from the SSL profile drop-down list (not required if TLS offload is enabled).
- Click Create.
Once complete, bind the new service monitor to the load balancing services or service groups accordingly.
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 Citrix ADC 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 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
Re-Encryption
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.
Additional Information
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/
Always On VPN IKEv2 Load Balancing Issue with Kemp LoadMaster
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.
Load Balancing IKEv2
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.
Issues with LMOS 7.2.48.0
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.
Update Available
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.
Additional Information
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/