Terraform, Azure VPN Gateway and OpenVPN Config

Recently I needed a quick way to spin-up a VPN Gateway and generate the openvpn config clients could use to connect.

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 = [
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 = [
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.client_cert.algorithm
ca_private_key_pem = tls_private_key.client_cert.private_key_pem
ca_cert_pem = tls_self_signed_cert.ca.cert_pem
validity_period_hours = 43800
allowed_uses = [
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 = [""]
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
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
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
echo "Updating file $OPENVPN_CONFIG_FILE"
cp $OPENVPN_CONFIG_FILE openvpn.ovpn
rm -r ./vpnconftemp
rm vpnconfig.zip
view raw openvpn_gen.sh hosted with ❤ by GitHub
#terraform, Coding, vscode

Terraform, Docker, Ubuntu 20.04, Go 1.14 and MemLock: Down the rabbit hole

I recently upgrade my machine and and installed the latest Ubuntu 20.04 as part of that.

Very smugly I fired it up the new install and, as I use devcontainers, looked forward to not installing lots of devtools as the Dockerfile in each project had all the tooling needed for VSCode to spin up and get going.

Sadly it wasn’t that smooth. After spinning up a project which uses terraform I found an odd message when running terraform plan

failed to retrieve schema from provider “random”: rpc error: code = Unavailable desc = connection error: desc = “transport: authentication handshake failed: EOF

error from terraform plan

Terraform has a provider model which uses GRPC to talk between the CLI and the individual providers. Random is one of the HashiCorp made providers so it’s a really odd one to see a bug in.

Initially I assumed that the downloaded provider was corrupted. Nope, clearing the download and retrying didn’t help.

So assuming I’d messed something up I:

  1. Tried changing the docker image using by the devcontainer. Nope. Same problem.
  2. Different versions of terraform. Nope. Same problem.
  3. Updated the Docker version I was using. Nope. Same problem.
  4. Restarted the machine. Nope. Same problem.

Now feeling quite frustrated I finally remembered a trick I’d used lots when building my own terraform providers. I enabled debug logging on the terraform CLI.

TF_LOG=DEBUG terraform plan

This is where it gets interesting…

Continue reading

Cleanup in Bash Scripts

[Brain dump so I don’t forget this one]

So you want your bash script to exit on an error but you’d like it to clean some stuff up before it closes after the error occurs.

No problem a TRAP can do this for you (read detailed docs for caveats).

In a very simple form it looks like this:

set -e
function cleanup()
echo -e "—-> Trap caught! Do cleanup here"
trap cleanup EXIT
# imagine some stuff happens here
# and it exists
exit 1
view raw trap.sh hosted with ❤ by GitHub
Using Trap to fire cleanup on exit

Learn more here: https://www.linuxjournal.com/content/bash-trap-command