Recently I ran into a really interesting problem with Terraform. I wanted to be able to simultaneously specify the number of instances to be created using its count feature but I couldn't figure out how to give each instance a custom MAC address. That was until I spent an evening with Google before coming across the idea of using the length function to populate my count value. Here's the general idea...

In my homelab I use MAC address assignments in pfsense to map IP addresses to systems and as such I don't need to configure each system with a static IP, it gets a static IP via DHCP. I was creating an Openshift cluster and because this is Kubernetes underneath the concept of masters, workers and infra nodes comes to fore. Now, each of these class of nodes has several shared characteristics but also several custom ones, MAC address being the most important to begin with. Take the following instance definition:

resource "vsphere_virtual_machine" "masters311" {
  name             = "ocp311-m${count.index + 1}"
  resource_pool_id = "${data.vsphere_compute_cluster.cluster.resource_pool_id}"
  datastore_id     = "${}"
  folder           = "awesomo/ocp311"
  count            = length(var.macs_311_masters)

  num_cpus = 4
  memory   = 8192
  guest_id = "${data.vsphere_virtual_machine.template-rhel77.guest_id}"

  network_interface {
    network_id   = "${}"
    adapter_type = "${data.vsphere_virtual_machine.template-rhel77.network_interface_types[0]}"
    use_static_mac = true
    mac_address = "${var.macs_311_masters[count.index]}"

  disk {
    label            = "disk0"
    size             = "${data.vsphere_virtual_machine.template-rhel77.disks.0.size}"
    eagerly_scrub    = "${data.vsphere_virtual_machine.template-rhel77.disks.0.eagerly_scrub}"
    thin_provisioned = "${data.vsphere_virtual_machine.template-rhel77.disks.0.thin_provisioned}"

  clone {
    template_uuid = "${}"

    customize {
      linux_options {
        host_name = "ocp311-m${count.index + 1}"
        domain    = "ktz.lan"
      network_interface {}

Note under count the following count = length(var.macs_311_masters). This references a variable from thus

variable "macs_311_masters" {
    description = "311 master mac addresses"
    type        = list(string)
    default     = ["d6:18:04:15:3e:e4","9a:c0:31:6b:3a:b5","02:a3:17:8c:8d:d4"]

One of the most important features of Terraform is interpolation and in this instance the length calculation would return a value of 3 with a starting index of 0 (because in coding arrays always have an initial index of 0). This means I can use the value of 3 to populate the count variable but I can also reference each specific item in the list using the index.

This comes in useful in two places mac_address and host_name.

mac_address = "${var.macs_311_masters[count.index]}"

This example shows reference the mac address I want to access using [count.index] and is how each instance is assigned it's MAC.

host_name = "ocp311-m${count.index + 1}"

This example shows that you can not only dynamically populate variables using interpolation but you can also perform arithmetic operations on them. In this instance I perform {count.index + 1} because array indices always start at 0 and we don't want a hostname that reads ocp311-m0. Instead we want ocp311-m1 and our + 1 gets us there.

There you have it. A very neat and elegant solution that can be extended to many other more complex edge cases such as RAM, disk, CPU and more. The limit is your creativity.