Always On VPN SSL Certificate Requirements for SSTP

Always On VPN Certificate Requirements for SSTPThe Windows Server 2016 Routing and Remote Access Service (RRAS) is commonly deployed as a VPN server for Windows 10 Always On VPN deployments. Using RRAS, Always On VPN administrators can take advantage of Microsoft’s proprietary Secure Socket Tunneling Protocol (SSTP) VPN protocol. SSTP is a Transport Layer Security (TLS) based VPN protocol that uses HTTPS over the standard TCP port 443 to encapsulate and encrypt communication between the Always On VPN client and the RRAS VPN server. SSTP is a firewall-friendly protocol that ensures ubiquitous remote network connectivity. Although IKEv2 is the protocol of choice when the highest level of security is required for VPN connections, SSTP can still provide very good security when implementation best practices are followed.

SSTP Certificate

Since SSTP uses HTTPS for transport, a common SSL certificate must be installed in the Local Computer/Personal/Certificates store on the RRAS VPN server. The certificate must include the Server Authentication Enhanced Key Usage (EKU) at a minimum. Often SSL certificates include both the Server Authentication and Client Authentication EKUs, but the Client Authentication EKU is not strictly required. The subject name on the certificate, or at least one of the Subject Alternative Name entries, must match the public hostname used by VPN clients to connect to the VPN server. Multi-SAN (sometimes referred to as UC certificates) and wildcard certificates are supported.

Always On VPN Certificate Requirements for SSTP

Certification Authority

It is recommended that the SSL certificate used for SSTP be issued by a public Certification Authority (CA). Public CAs typically have their Certificate Revocation Lists (CRLs) hosted on robust, highly available infrastructure. This reduces the chance of failed VPN connection attempts caused by the CRL being offline or unreachable.

Using an SSL certificate issued by an internal, private CA is supported if the CRL for the internal PKI is publicly available.

Key Type

RSA is the most common key type used for SSL certificates. However, Elliptic Curve Cryptography (ECC) keys offer better security and performance, so it is recommended that the SSTP SSL certificate be created using an ECC key instead.

Always On VPN Certificate Requirements for SSTP

To use an ECC key, be sure to specify the use of a Cryptographic Next Generation (CNG) key and select the ECDSA_P256 Microsoft Software Key Storage Provider (CSP) (or greater) when creating the Certificate Signing Request (CSR) for the SSTP SSL certificate.

Always On VPN Certificate Requirements for SSTP

Most public CAs will support certificate signing using ECC and Elliptic Curve Digital Signature Algorithm (ECDSA). If yours does not, find a better CA. 😉

Forward Secrecy

Forward secrecy (sometimes referred to as perfect forward secrecy, or PFS) ensures that session keys can’t be compromised even if the server’s private key is compromised. Using forward secrecy for SSTP is crucial to ensuring the highest levels of security for VPN connections.

To enforce the use of forward secrecy, the TLS configuration on the VPN server should be prioritized to prefer cipher suites with Elliptic Curve Diffie-Hellman Ephemeral (ECDHE) key exchange.

Authenticated Encryption

Authenticated encryption (AE) and authenticated encryption with associated data (AEAD) is a form of encryption that provides better data protection and integrity compared to older block or stream ciphers such as CBC or RC4.

To enforce the use of authenticated encryption, the TLS configuration on the VPN server should be prioritized to prefer cipher suites that support Galois/Counter Mode (GCM) block ciphers.

Important Note: In Windows Server 2016, GCM ciphers can be used with both RSA and ECC certificates. However, in Windows Server 2012 R2 GCM ciphers can only be used when an ECC certificate is used.

SSL Offload

Offloading SSL to a load balancer or application delivery controller (ADC) can be enabled to improve scalability and performance for SSTP VPN connections. I will cover SSL offload for SSTP in detail in a future post.

Summary

SSTP can provide good security for VPN connections when implementation and security best practices are followed. For optimum security, use an SSL certificate with an EC key and optimize the TLS configuration to use forward secrecy and authenticated cipher suites.

Additional Information

Always On VPN and Windows Server Routing and Remote Access Service (RRAS)

Always On VPN Protocol Recommendations for Windows Server RRAS

Always On VPN Certificate Requirements for IKEv2

3 Important Advantages of Always On VPN over DirectAccess

Microsoft SSTP Specification on MSDN

DirectAccess IP-HTTPS Performance Issues

DirectAccess IP-HTTPS Performance IssuesPerformance issues with DirectAccess are not uncommon. In fact, there are numerous threads on Microsoft and third-party forums where administrators frequently complain about slow download speeds, especially when using the IP-HTTPS IPv6 transition technology. Based on my experience the problem does not appear to be widespread but occurs with enough regularity that it is worthy of further investigation.

DirectAccess Design

The inherent design of DirectAccess is a major limiting factor for performance. DirectAccess uses a complex and heavy communication channel, with multiple layers of encapsulation, encryption, and translation. Fundamentally it is IPsec encrypted IPv6 traffic, encapsulated in HTTP, and then encrypted with Transport Layer Security (TLS) and routed over IPv4. It is then decrypted, decapsulated, decrypted again, then converted back to IPv4. The high protocol overhead incurred with multiple layers of encapsulation, encryption, and translation result in increased packet fragmentation, which further reduces performance.

DirectAccess Performance

Even under the best circumstances, DirectAccess performance is limited by many other factors, most notably the quality of the network connection between the client and the server. DirectAccess performs reasonably well over high bandwidth, low latency connections. However, network performance drops precipitously as latency increases and packet loss is encountered. This is to be expected given the design of the solution.

Intermediary Devices

It is not uncommon to find intermediary devices like firewalls, intrusion detection systems, malware scanners, and other security inspection devices limit the performance of DirectAccess clients. In addition, many security appliances have bandwidth caps enforced in software for licensing restrictions. Further, incorrect configuration of inline edge devices can contribute to increased fragmentation, which leads to poor performance as well.

Slow Downloads over IP-HTTPS

Many people report that download speeds seem to be artificially capped at 355Kbps. While this seems to be a display bug in the UI, there is plenty of evidence to indicate that, in some scenarios, DirectAccess is incapable of high throughput even over high-quality connections. Some who have deployed DirectAccess and VPN on the same server have reported that download speeds are only limited when using DirectAccess over IP-HTTPS and not with VPN using Secure Socket Tunneling Protocol (SSTP), which also uses TLS. This has led many to speculate that the issue is either a bug or a design flaw in the IP-HTTPS tunnel interface itself.

TCP Window Scaling Issues

In some of the network traces I’ve analyzed I’ve seen evidence that seems to support this theory. For example, a network trace taken when downloading a file over DirectAccess with IP-HTTPS showed the TCP window never scaled beyond 64K, which would seriously impede performance. Interestingly this doesn’t seem to happy when the client uploads files over IP-HTTPS. Clearly something unusual is happening.

Microsoft KB Article

Microsoft recently released a vaguely-worded KB article that appears to lend credence to some of these findings. The article seems to acknowledge the fact there are known issues with DirectAccess performance, but it lacks any specific details as to what the root cause is. Instead, it simply advises migrating to Windows 10 Always On VPN.

Summary

DirectAccess IP-HTTPS performance issues don’t appear to affect everyone, and the problem only seems to apply to file downloads and not to other types of traffic. However, there is mounting evidence of a systemic issue with DirectAccess performance especially over IP-HTTPS. Customers are advised to closely evaluate their uses cases for DirectAccess and if remote clients are frequently required to download large files over a DirectAccess connection, an alternative method of file transfer might be required. Optionally customers can consider evaluating alternative remote access solutions that offer better performance such as Windows 10 Always On VPN or third-party solutions such as NetMotion Mobility.

Additional Resources

Always On VPN and the Future of DirectAccess

What’s the Difference Between DirectAccess and Always On VPN?

NetMotion Mobility as an Alternative to Microsoft DirectAccess

What is the Difference Between DirectAccess and Always On VPN?

Always On VPN Device Tunnel Configuration Guidance Now AvailableDirectAccess has been around for many years, and with Microsoft now moving in the direction of Always On VPN, I’m often asked “What’s the difference between DirectAccess and Always On VPN?” Fundamentally they both provide seamless and transparent, always on remote access. However, Always On VPN has a number of advantages over DirectAccess in terms of security, authentication and management, performance, and supportability.

Security

DirectAccess provides full network connectivity when a client is connected remotely. It lacks any native features to control access on a granular basis. It is possible to restrict access to internal resources by placing a firewall between the DirectAccess server and the LAN, but the policy would apply to all connected clients.

Windows 10 Always On VPN includes support for granular traffic filtering. Where DirectAccess provides access to all internal resources when connected, Always On VPN allows administrators to restrict client access to internal resources in a variety of ways. In addition, traffic filter policies can be applied on a per-user or group basis. For example, users in accounting can be granted access only to their department servers. The same could be done for HR, finance, IT, and others.

Authentication and Management

DirectAccess includes support for strong user authentication with smart cards and one-time password (OTP) solutions. However, there is no provision to grant access based on device configuration or health, as that feature was removed in Windows Server 2016 and Windows 10. In addition, DirectAccess requires that clients and servers be joined to a domain, as all configuration settings are managed using Active Directory group policy.

Windows 10 Always On VPN includes support for modern authentication and management, which results in better overall security. Always On VPN clients can be joined to an Azure Active Directory and conditional access can also be enabled. Modern authentication support using Azure MFA and Windows Hello for Business is also supported. Always On VPN is managed using Mobile Device Management (MDM) solutions such as Microsoft Intune.

Performance

DirectAccess uses IPsec with IPv6, which must be encapsulated in TLS to be routed over the public IPv4 Internet. IPv6 traffic is then translated to IPv4 on the DirectAccess server. DirectAccess performance is often acceptable when clients have reliable, high quality Internet connections. However, if connection quality is fair to poor, the high protocol overhead of DirectAccess with its multiple layers of encapsulation and translation often yields poor performance.

The protocol of choice for Windows 10 Always On VPN deployments is IKEv2. It offers the best security and performance when compared to TLS-based protocols. In addition, Always On VPN does not rely exclusively on IPv6 as DirectAccess does. This reduces the many layers of encapsulation and eliminates the need for complex IPv6 transition and translation technologies, further improving performance over DirectAccess.

Supportability

DirectAccess is a Microsoft-proprietary solution that must be deployed using Windows Server and Active Directory. It also requires a Network Location Server (NLS) for clients to determine if they are inside or outside the network. NLS availability is crucial and ensuring that it is always reachable by internal clients can pose challenges, especially in very large organizations.

Windows 10 Always On VPN supporting infrastructure is much less complex than DirectAccess. There’s no requirement for a NLS, which means fewer servers to provision, manage, and monitor. In addition, Always On VPN is completely infrastructure independent and can be deployed using third-party VPN servers such as Cisco, Checkpoint, SonicWALL, Palo Alto, and more.

Summary

Windows 10 Always On VPN is the way of the future. It provides better overall security than DirectAccess, it performs better, and it is easier to manage and support.

Here’s a quick summary of some important aspects of VPN, DirectAccess, and Windows 10 Always On VPN.

Traditional VPN DirectAccess Always On VPN
Seamless and Transparent No Yes Yes
Automatic Connection Options None Always on Always on, app triggered
Protocol Support IPv4 and IPv6 IPv6 Only IPv4 and IPv6
Traffic Filtering No No Yes
Azure AD Integration No No Yes
Modern Management Yes No (group policy only) Yes (MDM)
Clients must be domain-joined? No Yes No
Requires Microsoft Infrastructure No Yes No
Supports Windows 7 Yes Yes Windows 10 only

Always On VPN Hands-On Training

If you are interested in learning more about Windows 10 Always On VPN, consider registering for one of my hands-on training classes. More details here.

Additional Resources

Always On VPN and the Future of Microsoft DirectAccess

5 Important Things DirectAccess Administrators Should Know about Windows 10 Always On VPN

3 Important Advantages of Windows 10 Always On VPN over DirectAccess

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