Always On VPN IKEv2 Load Balancing and NAT

Always On VPN IKEv2 Load Balancing and NATOver the last few weeks, I’ve worked with numerous organizations and individuals troubleshooting connectivity and performance issues associated with Windows 10 Always On VPN, and specifically connections using the Internet Key Exchange version 2 (IKEv2) VPN protocol. An issue that appears with some regularity is when Windows 10 clients fail to connect with error 809. In this scenario, the server will accept connections without issue for a period of time and then suddenly stop accepting requests. When this happens, existing connections continue to work without issue in most cases. Frequently this occurs with Windows Server Routing and Remote Access Service (RRAS) servers configured in a clustered array behind an External Load Balancer (ELB).

Network Address Translation

It is not uncommon to use Network Address Translation (NAT) when configuring Always On VPN. In fact, for most deployments the public IP address for the VPN server resides not on the VPN server, but on an edge firewall or load balancer connected directly to the Internet. The firewall/load balancer is then configured to translate the destination address to the private IP address assigned to the VPN server in the perimeter/DMZ or the internal network. This is known a Destination NAT (DNAT). Using this configuration, the client’s original source IP address is left intact. This configuration presents no issues for Always On VPN.

Source Address Translation

When troubleshooting these issues, the common denominator seems to be the use of Full NAT, which includes translating the source address in addition to the destination. This results in VPN client requests arriving at the VPN server as appearing not to come from the client’s original IP address, but the IP address of the network device (firewall or load balancer) that is translating the request. Full NAT may be explicitly configured by an administrator, or in the case of many load balancers, configured implicitly because the load balancer is effectively proxying the connection.

Known Issues

IKEv2 VPN connections use IPsec for encryption, and by default, Windows limits the number of IPsec Security Associations (SAs) coming from a single IP address. When a NAT device is performing destination/full NAT, the VPN server sees all inbound IKEv2 VPN requests as coming from the same IP address. When this happens, clients connecting using IKEv2 may fail to connect, most commonly when the server is under moderate to heavy load.

Resolution

The way to resolve this issue is to ensure that any load balancers or NAT devices are not translating the source address but are performing destination NAT only. The following is configuration guidance for F5, Citrix ADC (formerly NetScaler), and Kemp load balancers.

F5

On the F5 BIG-IP load balancer, navigate to the Properties > Configuration page of the IKEv2 UDP 500 virtual server and choose None from the Source Address Translation drop-down list. Repeat this step for the IKEv2 UDP 4500 virtual server.

Always On VPN IKEv2 Load Balancing and NAT

Citrix ADC

On the Citrix ADC load balancer, navigate to System > Settings > Configure Modes and check the option to Use Subnet IP.

Always On VPN IKEv2 Load Balancing and NAT

Next, navigate to Traffic Management > Load Balancing > Service Groups and select the IKEv2 UDP 500 service group. In the Settings section click edit and select Use Client IP. Repeat these steps for the IKEv2 UDP 4500 service group.

Always On VPN IKEv2 Load Balancing and NAT

Kemp

On the Kemp LoadMaster load balancer, navigate to Virtual Services > View/Modify Services and click Modify on the IKEv2 UDP 500 virtual service. Expand Standard Options and select Transparency. Repeat this step for the IKEv2 UDP 4500 virtual service.

Always On VPN IKEv2 Load Balancing and NAT

Caveat

Making the changes above may introduce routing issues in your environment. When configuring these settings, it may be necessary to configure the VPN server’s default gateway to use the load balancer to ensure proper routing. If this is not possible, consider implementing the workaround below.

Workaround

To fully resolve this issue the above changes should be made to ensure the VPN server can see the client’s original source IP address. If that’s not possible for any reason, the following registry key can be configured to increase the number of established SAs from a single IP address. Be advised this is only a partial workaround and may not fully eliminate failed IKEv2 connections. There are other settings in Windows that can prevent multiple connections from a single IP address which are not adjustable at this time.

To implement this registry change, open an elevated PowerShell command window on the RRAS server and run the following commands. Repeat these commands on all RRAS servers in the organization.

New-ItemProperty -Path ‘HKLM:SYSTEM\CurrentControlSet\Services\IKEEXT\Parameters\’ -Name IkeNumEstablishedForInitialQuery -PropertyType DWORD -Value 50000 -Force

Restart-Service IKEEXT -Force -PassThru

Additional Information

IPsec Traffic May Be Blocked When A Computer is Behind a Load Balancer

Windows 10 Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Windows 10 Always On VPN IKEv2 Load Balancing with F5 BIG-IP

Windows 10 Always On VPN IKEv2 Load Balancing with Kemp LoadMaster

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 IKEv2 Features and Limitations

Always On VPN IKEv2 Features and LimitationsThe 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 Limitations

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.

Firewalls

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.

Fragmentation

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

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

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.

IKEv2 Security and RRAS

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 or TLS?

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.

IKEv2 with TLS Fallback

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.

Additional Information

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

Always On VPN IKEv2 Load Balancing with F5 BIG-IP

Always On VPN IKEv2 Load Balancing with F5 BIG-IPThe 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.

Load Balancing IKEv2

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.

Initial Configuration

Follow the steps below to create a virtual server on the F5 BIG-IP to load balance IKEv2 VPN connections.

Pool Configuration

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.

Always On VPN IKEv2 Load Balancing with F5 BIG-IP

Virtual Server Configuration

Next create two virtual servers, the first configured to use UDP port 500 and the second to use UDP port 4500.

Always On VPN IKEv2 Load Balancing with F5 BIG-IP

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.

Always On VPN IKEv2 Load Balancing and NAT

Persistence Profile

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.

Always On VPN IKEv2 Load Balancing with F5 BIG-IP

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.

Always On VPN IKEv2 Load Balancing with F5 BIG-IP

Additional Resources

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

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