21 June 2021: Updated to resolve bug where wrong private key was passed to client_certs. This was exposed by service change.

There is a good guide to generating the necessary certificates and manually editing the openvpn config you can download from the portal in the official docs.

Being a sucker for punishment I wondered if I could automate the process (mainly because I always forget the openssl commands) and secondly it was to be run by someone else (not me) so I wanted it to be as simple as possible.

Warning: Please take time to understand the limitations of this certificate generation before production use (TF State file containing CA private keys) and review the limitations in the TLS provider for Terraform.

So how would this work? First choice is Terraform for the automation. Luckily that has a provider for generating certs! So the flow goes like this:

1. Create the Root CA
2. Use that to generate a client cert
3. Output the client cert for use in the openvpn config file
4. Inject the CA into the Azure VPN configuration and create it
5. Run a script to fetch the Azure VPN OpenVPN configuration file (as this contains the Pre-shared key we don’t set) then inject the client cert we outputted from the Terraform.
6. Connect

One gotcha – The TLS provider only outputs standard pems. Azure VPN requires the CA to be in a specific format outputted by the following command.

openssl x509 -in caCert.pem -outform der | base64 -w0 > caCert.der

As a result there is a null_resource and local_file resource to handle this translation.

So all you should need is Terraform, Bash, OpenSSL and Azure CLI – not perfect but it’s the best I could do! (This is currently a working draft – please use with caution).

	resource "random_string" "random" {
	length = 8
	special = false
	upper = false
	number = false
	}
	resource "azurerm_public_ip" "vpn_ip" {
	name = "vpn-ip"
	location = var.region
	resource_group_name = var.resource_group_name
	domain_name_label = random_string.random.result
	allocation_method = "Dynamic"
	tags = var.tags
	}
	resource "tls_private_key" "example" {
	algorithm = "RSA"
	rsa_bits = "2048"
	}
	# Create the root certificate
	resource "tls_self_signed_cert" "ca" {
	key_algorithm = tls_private_key.example.algorithm
	private_key_pem = tls_private_key.example.private_key_pem
	# Certificate expires after 1 year
	validity_period_hours = 8766
	# Generate a new certificate if Terraform is run within three
	# hours of the certificate's expiration time.
	early_renewal_hours = 200
	# Allow to be used as a CA
	is_ca_certificate = true
	allowed_uses = [
	"key_encipherment",
	"digital_signature",
	"server_auth",
	"client_auth",
	"cert_signing"
	]
	dns_names = [ azurerm_public_ip.vpn_ip.domain_name_label ]
	subject {
	common_name = "CAOpenVPN"
	organization = "dev env"
	}
	}
	resource "local_file" "ca_pem" {
	filename = "caCert.pem"
	content = tls_self_signed_cert.ca.cert_pem
	}
	resource "null_resource" "cert_encode" {
	provisioner "local-exec" {
	# Bootstrap script called with private_ip of each node in the clutser
	command = "openssl x509 -in caCert.pem -outform der | base64 -w0 > caCert.der"
	}
	depends_on = [ local_file.ca_pem ]
	}
	data "local_file" "ca_der" {
	filename = "caCert.der"
	depends_on = [
	null_resource.cert_encode
	]
	}
	resource "tls_private_key" "client_cert" {
	algorithm = "RSA"
	rsa_bits = "2048"
	}
	resource "tls_cert_request" "client_cert" {
	key_algorithm = tls_private_key.client_cert.algorithm
	private_key_pem = tls_private_key.client_cert.private_key_pem
	# dns_names = [ azurerm_public_ip.vpn_ip.domain_name_label ]
	subject {
	common_name = "ClientOpenVPN"
	organization = "dev env"
	}
	}
	resource "tls_locally_signed_cert" "client_cert" {
	cert_request_pem = tls_cert_request.client_cert.cert_request_pem
	ca_key_algorithm = tls_private_key.example.algorithm
	ca_private_key_pem = tls_private_key.example.private_key_pem
	ca_cert_pem = tls_self_signed_cert.ca.cert_pem
	validity_period_hours = 43800
	allowed_uses = [
	"key_encipherment",
	"digital_signature",
	"server_auth",
	"key_encipherment",
	"client_auth",
	]
	}
	resource "azurerm_virtual_network_gateway" "vpn-gateway" {
	name = "vpn-gateway"
	location = var.region
	resource_group_name = var.resource_group_name
	type = "Vpn"
	active_active = false
	enable_bgp = false
	sku = "VpnGw1"
	ip_configuration {
	name = "vnetGatewayConfig"
	public_ip_address_id = azurerm_public_ip.vpn_ip.id
	private_ip_address_allocation = "Dynamic"
	subnet_id = azurerm_subnet.yoursubnethere.id
	}
	vpn_client_configuration {
	address_space = ["10.1.0.0/16"]
	vpn_client_protocols = ["OpenVPN"]
	root_certificate {
	name = "terraformselfsignedder"
	public_cert_data = data.local_file.ca_der.content
	}
	}
	}
	output "client_cert" {
	value = tls_locally_signed_cert.client_cert.cert_pem
	}
	output "client_key" {
	value = tls_private_key.client_cert.private_key_pem
	}
	output "vpn_id" {
	value = azurerm_virtual_network_gateway.vpn-gateway.id
	}

view raw main.tf hosted with ❤ by GitHub

	#!/bin/bash
	set -e
	# Get vars from TF State
	VPN_ID=`terraform output vpn_id`
	VPN_CLIENT_CERT=`terraform output client_cert`
	VPN_CLIENT_KEY=`terraform output client_key`
	# Replace newlines with \n so sed doesn't break
	VPN_CLIENT_CERT="${VPN_CLIENT_CERT//$'\n'/\\n}"
	VPN_CLIENT_KEY="${VPN_CLIENT_KEY//$'\n'/\\n}"
	CONFIG_URL=`az network vnet-gateway vpn-client generate –ids $VPN_ID -o tsv`
	wget $CONFIG_URL -O "vpnconfig.zip"
	# Ignore complaint about backslash in filepaths
	unzip -o "vpnconfig.zip" -d "./vpnconftemp"|| true
	OPENVPN_CONFIG_FILE="./vpnconftemp/OpenVPN/vpnconfig.ovpn"
	echo "Updating file $OPENVPN_CONFIG_FILE"
	sed -i "s~\$CLIENTCERTIFICATE~$VPN_CLIENT_CERT~" $OPENVPN_CONFIG_FILE
	sed -i "s~\$PRIVATEKEY~$VPN_CLIENT_KEY~g" $OPENVPN_CONFIG_FILE
	cp $OPENVPN_CONFIG_FILE openvpn.ovpn
	rm -r ./vpnconftemp
	rm vpnconfig.zip

view raw openvpn_gen.sh hosted with ❤ by GitHub