Always On VPN SSTP with Let’s Encrypt Certificates

Always On VPN SSTP Security Configuration

When configuring the Windows Server Routing and Remote Access Service (RRAS) to support Secure Socket Tunneling Protocol (SSTP) for Always On VPN user tunnel connections, administrators must install a Transport Layer Security (TLS) certificate on the VPN server. The best practice is to use a certificate issued by a public Certification Authority (CA). In addition, administrators should use a TLS certificate using Elliptic Curve Digital Signature Algorithm (ECDSA) for optimal security and performance.

Let’s Encrypt

Obtaining a public TLS certificate is not inherently difficult, nor is it expensive. However, Let’s Encrypt is a nonprofit public CA issues TLS certificates entirely for free. Always On VPN supports Let’s Encrypt TLS certificates, and installing a Let’s Encrypt certificate on the Always On VPN RRAS server is quite simple.

Pros and Cons

Using Let’s Encrypt certificates for Always On VPN has several significant advantages over traditional public CAs.

  • Cost – Let’s Encrypt certificates are free! No cost whatsoever.
  • Speed – Enrolling for a Let’s Encrypt certificate takes just a few minutes.
  • Trusted – Let’s Encrypt certificates are trusted by default in Windows 10 and Windows 11.

Let’s Encrypt is not without some drawbacks, however.

  • Lifetime – Let’s Encrypt certificates are only valid for 90 days.
  • Administration – Certificates must be redeployed frequently (every 90 days).
  • Security – PFX files (which include private keys) are left on disk by default.

It is possible to mitigate some of these drawbacks, though. For example, deleting PFX files after import can improve security. Alternatively, using a Certificate Signing Request (CSR) eliminates PFX files completely.

Also, it is possible to fully automate the Let’s Encrypt certificate enrollment and RRAS configuration process, which eases the administrative burden. And rotating certificates every 90 days could be considered an advantage from a security perspective! Enrolling new certificates (and specifically certificates with unique keys) is advantageous in that respect.

Certificate Enrollment

There are several different ways to enroll for Let’s Encrypt certificates. The preferred method is using PowerShell, as it works on both Windows Server with Desktop Experience (GUI) and Windows Server Core. Using PowerShell, administrators can also fully automate the enrollment and assignment of the certificate in RRAS.

PowerShell Module

To enroll for Let’s Encrypt TLS certificates on the VPN server, install the Posh-ACME PowerShell module. On the RRAS server, open an elevated PowerShell window and run the following command.

Install-Module Posh-ACME

Certificate Request

After installing the Posh-ACME PowerShell module, select a Let’s Encrypt environment by running the following command. Use LE_PROD for the production Let’s Encrypt server or LE_STAGE for the staging environment (used for testing).

Set-PAServer LE_PROD

Next, request a new certificate using the following command.

New-PACertificate -Domain vpn.example.net -Contact ‘[email protected]’ -CertKeyLength ec-256 -AcceptTOS -Install

The administrator is prompted to create a TXT record in public DNS to prove ownership of the domain. Using the example above, create a DNS record called _acme-challenge.vpn in the example.net DNS zone.

Once complete, the TLS certificate is automatically installed in the local computer certificate store on the VPN server and can be assigned in the RRAS management console, as shown here.

Note: R3 is a Let’s Encrypt issuing certification authority.

DNS Plugin

The Posh-ACME PowerShell module supports DNS plugins that allow administrators to automate the creation of the DNS TXT record used to authorize certificate enrollment. DNS plugins for many public DNS providers are available. Some of the more popular DNS providers are listed here.

  • Microsoft Azure
  • Amazon Route53
  • Cloudflare
  • Akamai
  • GoDaddy
  • Infoblox
  • Windows Server

A list of all supported DNS plugins for Posh-ACME can be found here.

Certificate Binding

Administrators can use the following PowerShell example code to automate the process of binding the new TLS certificate to the SSTP listener in RRAS.

$Thumbprint = <TLS certificate thumbprint>
$Cert = Get-ChildItem -Path Cert:\LocalMachine\My\$thumbprint
Set-RemoteAccess -SslCertificate $Cert
Restart-Service RemoteAccess -Passthru

Additional Information

Posh-ACME Tutorial

Windows 10 Always On VPN TLS Certificate Requirements for SSTP

Windows 10 Always On VPN SSTP Security Configuration

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