How to Solve "admin_ssh_key is not a complete SSH2 Pub Key" in Terraform for Azure

When provisioning an Azure Linux Virtual Machine using Terraform, you might encounter the following error:

This issue occurs because Terraform expects a properly formatted SSH2 public key, but the provided key may not meet its expectations. Let's explore the cause and solution for this error.

Understanding the Problem

The error typically arises due to one of the following reasons:

1. Incorrect Key Format – The public key provided in var.ssh_public_key might be missing the correct SSH2 format, which usually follows this structure: ssh-rsa AAAAB3... user@hostname
2. Raw Key String Instead of a File – The key might be passed as a string variable inside Terraform instead of reading it from an actual key file.
3. Line Breaks or Encoding Issues – The key could contain unexpected line breaks, extra spaces, or encoding issues.

The Palliative Solution

A quick fix for this issue is to explicitly read the SSH public key file using the file() function:

resource "azurerm_linux_virtual_machine" "example" {
  name                = "example-vm"
  resource_group_name = azurerm_resource_group.example.name
  location            = azurerm_resource_group.example.location
  size               = "Standard_DS1_v2"
  admin_username      = "azureuser"

  admin_ssh_key {
    username   = "azureuser"
    public_key = file(var.ssh_public_key)
  }

  os_disk {
    caching              = "ReadWrite"
    storage_account_type = "Standard_LRS"
  }
}
        

Here’s what happens:

• file(var.ssh_public_key) ensures Terraform reads the actual key content from the specified file instead of treating it as a raw string.
• This avoids any formatting issues and ensures the key is correctly interpreted.

Better Long-Term Fix

While the above fix works, a more robust approach is to ensure the SSH key is properly formatted before using it in Terraform:

1. Verify the SSH Key Format

Run the following command on your Linux or Mac terminal to check the SSH public key format:

cat ~/.ssh/id_rsa.pub        

A valid key should look like this:

ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEA7e2b3... user@hostname        

2. Ensure the Key is Properly Loaded in Terraform

Instead of storing the key inside Terraform variables directly, use a variable to store the file path:

variable "ssh_public_key" {
  description = "Path to the SSH public key"
  default     = "~/.ssh/id_rsa.pub"
}

resource "azurerm_linux_virtual_machine" "example" {
  admin_ssh_key {
    username   = "azureuser"
    public_key = file(var.ssh_public_key)
  }
}
        

This ensures that Terraform always reads the correct key from the specified file.

Final Thoughts

The error "admin_ssh_key is not a complete SSH2 Public Key" is usually caused by incorrectly formatted SSH keys or improper handling within Terraform. By using file(var.ssh_public_key), you ensure Terraform correctly reads the SSH key, avoiding common formatting issues.

For a long-term fix:

? Verify your SSH key format

? Use file() to read the key from a file

? Avoid raw key strings in Terraform variables

By implementing these best practices, you’ll ensure a smooth and error-free experience when provisioning Azure Linux VMs with Terraform. ??

Do you often face Terraform errors like this?

Share your experience or ask any questions in the comments! Let’s troubleshoot together. ????