Getting Started With Packer to Create vSphere Templates – Part 3 – Variables

Welcome back to part 3 of my Creating vSphere Templates using Packer series, if you missed part 1 or 2, you can find them here and here. In part 3 we will explore variables!

Why would we use variables? Variables allow you to specify customisations to your templating code without having to edit your actual build files. This can be useful when you are reusing code for multiple templates.

There are multiple types of variables that can be used, but we will talk about 2 types of input variables in this blog. They are what I will refer to as; User defined variables and Environment variables. We will talk about both during the blog post and the use cases for each.

Regardless of whether we use a user defined variable or an environment variable, we still need to declare them. This is done in a variable declaration file, so lets start with that!

Variable Declaration

Following the release of Packer version 1.7 the Hashicorp Configuration Language (HCL) format is now the preferred language over JSON. Everything you will see will be in HCL.

The variable declaration file is a pkr.hcl file used to declare any variables you will be using as part of your configuration, be it user defined or environment variables.

Lets take a look at a few of the variable types you can make use of as well as some of the options you can also set.

Variable Type

Here is a few common variable types, you don’t have to define a type at all, but you could then pass the wrong type of data into your config.

• String - E.g. The templates name or the name of a datastore.
• Boolean - E.g. A true or false value for whether you are using thin or thick provisioned disks.
• List - E.g. A list of DNS server IP addresses.

We will see examples of these later on.

Default Value

You can set default values for variables. These values will be used if no other variable value is found in either your pkrvar.hcl file or as an environment variable. Using default values can help reduce the amount of repeat configuration if you use a shared variable definition file.

Description

Another useful option is to be able to provide a description to a variable. This can be useful if you need to add any additional information about the variable or why a particular default has to be set.

Sensitive

You can also mark variables as sensitive for values such as keys, password or usernames etc, however you can mark any variable as sensitive if you have a need to. When a variable is marked as sensitive, it will not be displayed in any of Packers output.

User Defined Variables

Lets take a look at a few examples of declared variables in the variables.pkr.hcl file as well as any values then set for those variables in the user variables file. You will see a couple of examples of variables that have default, type and sensitive options set to give you an idea of some of the use cases.

Lets start with a basic user defined variable:

So in this example, we are declaring that we are going to use variables called ‘vsphere_datastore’ and ‘vsphere_portgroup_name’. We then have values defined for these variables in our pkrvar.hcl file. This can be any data type for the value, as no type has been defined.

In this example we have declared a variable with the type ‘String’, and also provided a default value. The configuration will use this default if no other value is defined either via a user variable or environment variable, but will be overridden should a variable value be set.

Here is an example again using a type and default values, but also providing a description to provide some additional information. Like the previous example, not providing a variable value either in the pkrvar.hcl file or in the terminal session as an environment variable, would result in the default value being used.

In this final example we are using the sensitive option. This will stop the value being displayed in any Packer output. Again, it’s using a default value, so you do not need to define a value in the pkrvar.hcl file unless you want to use a different value to this default.

Environmental Variables

Now let’s take a look at environment variables. These are especially useful if you want to use Packer as part of a workflow or automation pipeline, or to pass in secrets (passwords or keys) into the workflow from a secret management tool.

You still declare all your variables in your variables.pkr.hcl file as you would for user defined variables, but instead of providing a value in your pkrvar.hcl file, you create environment variables in your terminal session, in this case, PowerShell.

Packer will look for variables in the session with the prefix of PKR_VAR_. If Packer finds any variables with this prefix, it knows they are for its use.

You do not need to add this prefix anywhere in your configuration as Packer knows to ignore the prefix when matching the variable name.

For example lets set the vSphere connection password in the PowerShell session we are using. This can be done by running the following to set the variable:

1$env:PKR_VAR_vsphere_password = "VMware123!"

This example will match up to the variable declaration:

1variable "vsphere_password" {}

You do not need to provide a value in your pkrvar.hcl file as Packer will read the value from the ‘PKR_VAR_vsphere_password’ environment variable.

NOTE: If you also provide a user defined variable in pkrvar.hcl, this will take precedence over the environment variable.

You can find HashiCorps documentation on variables here, have a read to discover even more options.

Referencing a Variable from Build Blocks

Now we have taken a brief look at some of the ways to declare and define variables, lets now take a look at how you use them in your source block!

Here are some examples:

1  username       = var.vsphere_user
2  vcenter_server = var.vsphere_server
3  vm_name        = var.vm_name
4  vm_version     = var.vm_version

There are two components here. Firstly, ‘var.’ this defines that a variable is being referenced. Secondly, the name of the variable you wish to reference. Each variable referenced will need to exist in variables.pkr.hcl and either a default value specified or a user or environment variable set. It doesn’t matter whether you are using environment or user defined variables, this syntax is the same. Remember that you do not need to include ‘PKR_VAR_’ in the variable name in the source block when you are referencing an environment variable, it’s only needed as a prefix when actually setting the variable.

That concludes my brief overview of user defined and environment variables. Do checkout the link to HashiCorp’s official documentation above and you can also find an example of a variable declaration file here, and a pkrvar.hcl file here on my GitHub.

In Part 4 we will put all the blocks and files together to complete the configuration before moving onto the final part of the series, where we will deploy some templates!

Thanks for reading!

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