Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Always On VPN SSTP Load Balancing with Citrix NetScaler ADCThe Internet Key Exchange version 2 (IKEv2) VPN protocol is the protocol of choice when the highest level of security is required for Always On VPN connections. It uses IPsec and features configurable security parameters that allow administrators to adjust policies to meet their specific security requirements. IKEv2 is not without some important limitations, but organizations may insist on the use of IKEv2 to provide the greatest protection possible for remote connected clients. Due to complexities of the IKEv2 transport, special configuration on the Citrix ADC is required when load balancing this workload.

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 IKEv2

When an Always On VPN client establishes a connection using IKEv2, communication begins on UDP port 500, but switches to UDP port 4500 if Network Address Translation (NAT) is detected in the communication path between the client and the server. Because UDP is connectionless, custom configuration is required to ensure that VPN clients maintain connectivity to the same backend VPN server during this transition.

Initial Configuration

Load balancing IKEv2 using the Citrix ADC is similar to other workloads. Below are specific settings and parameters required to load balance IKEv2 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 services for IKEv2 VPN will use UDP ports 500 and 4500. Create the service group and assign group members for UDP 500 as follows.

Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Repeat the steps above to create the service group for UDP port 4500.

Virtual Server Settings

Two virtual servers are required, one for UDP port 500 and one for UDP port 4500. Ensure that the service group using UDP port 500 is bound to the virtual server using the same port.

Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Repeat the steps above to create the virtual service for UDP port 4500.

Service Monitoring

Since IKEv2 uses the UDP protocol, the only option for service monitoring is to use PING, which is configured by default. Ensure that the firewall on the VPN server allows inbound ICMPv4 and ICMPv6 Echo Request. The default PING monitor on the Citrix ADC will ping the resource every 5 seconds. If a different interval is required, the administrator can edit the PING monitor and bind that to the service or service group as necessary.

Persistency Group

A Persistency Group on the Citrix ADC will be configured to ensure that IKEv2 VPN client requests from the same client are always routed to the same backend server. Follow the steps below to create a Persistency Group and assign it to both IKEv2 virtual servers created previously.

  1. In the Citrix ADC management console expand Traffic Management > Load Balancing > Persistency Groups.
  2. Click Add.
  3. Enter a descriptive name for the Persistency Group.
  4. Select SOURCEIP from the Persistence drop-down list.
  5. Next to the Virtual Server Name section click the Add button.
  6. Add both previously configured IKEv2 virtual servers for UDP 500 and 4500.
  7. Click Create.

Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Use Client IP

To ensure reliable connectivity for IKEv2 VPN connections it is necessary for the VPN server to see the client’s original source IP address. Follow the steps below to configure the Service Group to forward the client’s IP address to the VPN server.

  1. In the Citrix ADC management console expand System, click Settings, and then click Configure Modes.
  2. Select Use Subnet IP.
  3. Click Ok.Always On VPN IKEv2 Load Balancing and NAT
  4. Expand Traffic Management, click Load Balancing, and then click Service Groups.
  5. Select the IKEv2 UDP 500 Service Group.
  6. Click Edit in the Settings section.
  7. Select Use Client IP.
  8. Repeat these steps on the IKEv2 UDP 4500 Service Group.Always On VPN IKEv2 Load Balancing and NAT

Note: Making the above changes will require configuring the VPN server to use the Citrix ADC as its default gateway.

Additional Information

Windows 10 Always On VPN IKEv2 Load Balancing and NAT

Windows 10 Always On VPN SSTP Load Balancing with Citrix NetScaler ADC

Windows 10 Always On VPN IKEv2 Features and Limitations

Windows 10 AlWAYS On VPN and IKEv2 Fragmentation

Windows 10 Always On VPN IKEv2 Security Configuration

Windows 10 Always On VPN Certificate Requirements for IKEv2

Always On VPN SSTP Load Balancing with Citrix NetScaler ADC

Always On VPN SSTP Load Balancing with Citrix NetScaler ADCOne 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 Application Delivery Controller (ADC), formerly known as 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.

Always On VPN SSTP Load Balancing with Citrix NetScaler ADC

Virtual Server Settings

The virtual server is configured to use TCP port 443. It is recommended to use SSLSESSION persistence.

Always On VPN SSTP Load Balancing with Citrix NetScaler ADC

The LEASTCONNECTION load balancing method is the recommend option for load balancing method.

Always On VPN SSTP Load Balancing with Citrix NetScaler ADC

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.

  1. Open the Citrix ADC management console and expand Traffic Management.
  2. Select Monitors.
  3. Click Add.
  4. Enter a descriptive name in the Name field.
  5. Select HTTP form the Type drop-down list and click Select.
  6. Adjust the Interval and Response Time-out values according to your requirements.
  7. Enter 401 in the Response Codes field and click the “+” button.
  8. In the Response Codes field click the “x” next to 200.
  9. In the HTTP Request field enter HEAD /sra_{BA195980-CD49-458b-9E23-C84EE0ADCD75}/.
  10. Check the box next to Secure (not required if TLS offload is enabled).
  11. Select ns_default_ssl_profile_backend from the SSL profile drop-down list (not required if TLS offload is enabled).
  12. Click Create.

Always On VPN SSTP Load Balancing with Citrix NetScaler ADC

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

Always On VPN Device Tunnel with Azure VPN Gateway

Always On VPN Device Tunnel with Azure VPN GatewayAlways On VPN is infrastructure independent, which allows for many different deployment scenarios including on-premises and cloud-based. In Microsoft Azure, the Azure VPN gateway can be configured to support Windows 10 Always On VPN client connections in some scenarios. Recently I wrote about using the Azure VPN gateway for Always On VPN user tunnels. In this post I’ll describe how to configure the Azure VPN gateway to support an Always On VPN device tunnel.

Limitations

There are a few crucial limitations that come with using the Azure VPN gateway for Always On VPN. Importantly, the Azure VPN gateway can support either user tunnels or device tunnels, not both at the same time. In addition, Azure supports only a single VPN gateway per VNet, so deploying an additional VPN gateway in the same VNet to support Always On VPN user tunnels is not an option.

Root CA Certificate

The Always On VPN device tunnel is authenticated using a machine certificate issued to domain-joined Windows 10 Enterprise edition clients by the organization’s internal Certification Authority (CA). The CA’s root certificate must be uploaded to Azure for the VPN gateway to authorize device tunnel connections. The root CA certificate can be exported using the Certification Authority management console (certsrv.msc) or via the command line.

Export Certificate – GUI

Follow the steps below to export a root CA certificate using the Certification Authority management console.

1. On the root CA server, open the Certification Authority management console.
2. Right-click the CA and choose Properties.
3. Select the CA server’s certificate and choose View Certificate.
4. Select the Details tab and click Copy to File.
5. Click Next.
6. Choose Base-64 encoded X.509 (.CER).

Always On VPN Device Tunnel with Azure VPN Gateway

7. Click Next.
8. Enter a location to save the file to.
9. Click Next, Finish, and Ok.

Export Certificate – Command Line

Follow the steps below to export a root CA certificate using the command line.

1. On the root CA server, open an elevated command window (not a PowerShell window).
2. Enter certutil.exe -ca.cert root_certificate.cer.
3. Enter certutil.exe -encode root.cer root_certificate_base64.cer.

Copy Public Key

1. Open the saved root certificate file using Notepad.
2. Copy the file contents between the BEGIN CERTIFICATE and END CERTIFICATE tags, as shown here. Use caution and don’t copy the carriage return at the end of the string.

Always On VPN Device Tunnel with Azure VPN Gateway

Point-to-Site Configuration

The Azure VPN gateway must be deployed as a Route-Based gateway to support point-to-site VPN connections. Detailed requirements for the gateway can be found here. Once the VPN gateway has been provisioned, follow the steps below to enable point-to-site configuration for Always On VPN device tunnels.

1. In the navigation pane of the Azure VPN gateway settings click Point-to-site configuration.
2. Click the Configure now link and specify an IPv4 address pool to be assigned to VPN clients. This IP address pool must be unique in the organization and must not overlap with an IP address ranges defined in the Azure virtual network.
3. From the Tunnel type drop-down list select IKEv2.
4. In the Root certificates section enter a descriptive name for the certificate in the Name field.
5. Copy and paste the Base64 encoded public key copied previously into the Public certificate data field.
6. Click Save to save the configuration.

Always On VPN Device Tunnel with Azure VPN Gateway

VPN Client Configuration

To support the Always On VPN device tunnel, the client must have a certificate issued by the internal CA with the Client Authentication Enhanced Key Usage (EKU). Detailed guidance for deploying a Windows 10 Always On VPN device tunnel can be found here.

Download VPN Configuration

1. Click Point-to-site configuration.
2. Click Download VPN client.
3. Click Save.
4. Open the downloaded zip file and extract the VpnSettings.xml file from the Generic folder.
5. Copy the FQDN in the VpnServer element in VpnSettings.xml. This is the FQDN that will be used in the template VPN connection and later in ProfileXML.

Create a Test VPN Connection

It is recommended to create a test VPN connection to perform validation testing of the Azure VPN gateway before provisioning an Always On VPN device tunnel broadly. On a domain-joined Windows 10 enterprise client, create a new VPN connection using IKEv2 with machine certificate authentication. Use the VPN server FQDN copied from the VpnSettings.xml file previously.

Always On VPN Device Tunnel with Azure VPN Gateway

Create an Always On VPN Connection

Once the VPN has been validated using the test profile created previously, an Always On VPN profile can be created and deployed using Intune, SCCM, or PowerShell. The following articles can be used for reference.

Deploy Always On VPN device tunnel using PowerShell

Deploy Always On VPN device tunnel using Intune

IKEv2 Security Configuration

The default IKEv2 security parameters used by the Azure VPN gateway are better than Windows Server, but the administrator will notice that a weak Diffie-Hellman (DH) key (Group 2 – 1024 bit) is used during IPsec phase 1 negotiation.

Always On VPN Device Tunnel with Azure VPN Gateway

Use the following PowerShell commands to update the default IKEv2 security parameters to recommended baseline defaults, including 2048-bit keys (DH group 14) and AES-128 for improved performance.

Connect-AzAccount
Select-AzSubscription -SubscriptionName [Azure Subscription Name]

$Gateway = [Gateway Name]
$ResourceGroup = [Resource Group Name]

$IPsecPolicy = New-AzVpnClientIpsecParameter -IpsecEncryption AES128 -IpsecIntegrity SHA256 -SALifeTime 28800 -SADataSize 102400000 -IkeEncryption AES128 -IkeIntegrity SHA256 -DhGroup DHGroup14 -PfsGroup PFS14

Set-AzVpnClientIpsecParameter -VirtualNetworkGatewayName $Gateway -ResourceGroupName $ResourceGroup -VpnClientIPsecParameter $IPsecPolicy

Note: Be sure to update the cryptography settings on the test VPN connection and in ProfileXML for Always On VPN connections to match the new VPN gateway settings. Failing to do so will result in an IPsec policy mismatch error.

Additional Information

Windows 10 Always On VPN User Tunnel with Azure VPN Gateway

Windows 10 Always On VPN IKEv2 Security Configuration

Windows 10 Always On VPN Device Tunnel Configuration using Microsoft Intune

Windows 10 Always On VPN Device Tunnel Configuration using PowerShell

Windows 10 Always On VPN Options for Azure Deployments

Windows 10 Always On VPN IKEv2 Features and Limitations

Always On VPN Hands On Training in 2020

The calendar of events for our Windows 10 Always On VPN hands-on training classes is now available!

2020 Training Calendar

This year we will be making the following stops.

  • March 3-5 – Denver, CO
  • April 14-16 – New York, NY
  • June 15-15 – Bern, Switzerland (hosted by RealStuff – register here!)
  • August 18-20 – Honolulu, HI
  • October 6-8 – Washington D.C.
  • December 1-3 – San Diego, CA

Always On VPN Hands-On Training

Comprehensive Training

My Windows 10 Always On VPN hands-on training classes will cover all aspects of designing, implementing, and supporting an Always On VPN solution in the enterprise. This three-day course will cover topics such as…

  • Windows 10 Always On VPN overview
  • Introduction to CSP
  • Infrastructure requirements
  • Planning and design considerations
  • Installation, configuration, and client provisioning

Advanced topics will include…

  • Redundancy and high availability
  • Cloud-based deployments
  • Third-party VPN infrastructure and client support
  • Multifactor authentication
  • Always On VPN migration strategies

Windows 10 Always On VPN Hands-On Training Classes for 2018

Register Today

Space is limited, so register now to reserve your spot in one of our Always On VPN hands-on training classes. Hope to see you there!

Always On VPN Hands-On Training

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