How can I get terraforms extern to execute `ssh-keygen -y -f ~/.ssh/id_rsa`?

Question

I seem to be confused about how external works. I tried:

data "external" "local_key" {
  program = [
    "ssh-keygen", "-y", "-f ~/.ssh/id_rsa"
  ]
}

This gives me:

failed to execute "ssh-keygen": ~/.ssh/id_rsa: No such file or directory

Which presumably happens because ~ expansion doesn't. ssh-keygen -y -f ~/.ssh/id_rsa works normally. So instead I tried invoking bash like:

data "external" "local_key" {
  program = [
    "bash"
  ]

  query {
    "-c" = "ssh-keygen -y -f ~/.ssh/id_rsa"
  }
}

I'm still getting the same issue. I realize I can just pass the output as a var to terraform from the outside, but I'm still curious what the solution is.

Answer 1

The best way to achieve what you're trying to do (generate an openssh public key from an id_rsa private key) is using the tls_public_key data provider built-in to terraform. Using the external provider should be considered an option of last resort because it introduces dependencies on the local OS which may not be portable. It's like an escape hatch.

You can use it like this:

data "tls_public_key" "example" {
  private_key_pem = "${file("~/.ssh/id_rsa")}"
}

Then to get the public key suitable for openssh, you can access

"${data.tls_public_key.example.public_key_openssh}" 

Lastly, if you are open to generating a new SSH key pair, you can use the terraform-tls-ssh-key-pair module which will write the keys to a new file and exposes a public_key output with the openssh public key.

Answer 2

So what happens in first case is, as Dan's already said, there's no shell used and as such nothing to expand the ~. Quoting the documentation about program:

Terraform does not execute the program through a shell

On the second case, bash receive in stdin something like this:

{ "-c": "ssh-keygen -y -f ~/.ssh/id_rsa" }

And this looks like a command block for bash, but -c is not a valid command.

What could work could be this kind of program (assuming no specific input):

jq -n --arg pubkey "$(ssh-keygen -y -f ~/.ssh/id_rsa)" '{"pubkey":$pubkey}'

So something like this should work to get the key in local_key["pubkey"] if I understand the documentation properly:

data "external" "local_key" {
  program = [
    "bash", "-c jq -n --arg pubkey \"$(ssh-keygen -y -f ~/.ssh/id_rsa)\" '{\"pubkey\":$pubkey}'"
  ]
}

There's a need to use bash for a one liner to take advantage of command subsitution. You can also do a .sh script like:

#!/bin/sh
jq -n --arg pubkey "$(ssh-keygen -y -f ~/.ssh/id_rsa)" '{\"pubkey\":$pubkey}'

And call this script in the program parameter.

Answer 3

Note: this answer is based solely on documentation, I didn't try it.

From External Data Source (emphasis mine):

The following arguments are supported:

• program - (Required) A list of strings, whose first element is the program to run and whose subsequent elements are optional command line arguments to the program. Terraform does not execute the program through a shell, so it is not necessary to escape shell metacharacters nor add quotes around arguments containing spaces.

Expanding ~ to the home directory is a capability of the shell, not of ssh-keygen and since in the 1st example ssh-keygen is invoked, it literally attempts to open a file named ~/.ssh/id_rsa. Which fails because a ~ directory doesn't exist (I'm not sure if that's what you mean by presumably happens because ~ expansion doesn't).

In the 2nd example you're passing the ssh-keygen command as a query, not as arguments to bash. Probably what's executed is not what you're expecting.

I'd try:

data "external" "local_key" {
  program = [
    "bash", "-c", "ssh-keygen", "-y", "-f ~/.ssh/id_rsa"
  ]
}

But I'm not sure if bash itself (or ssh-keygen for that matter) follows the External Program Protocol required (also on the above referenced page). You may need to write your own script to wrap the cmd you desire while also providing the data according to the protocol. Invoke that script as the program instead of bash or ssh-keygen.

Finally, the entire external data source page doesn't appear to suggest that export would be designed for interactive programs and ssh-keygen -y -f ~/.ssh/id_rsa asks for the user passphrase interactively. YMMV.

Answer 4

Have you considered other options? e.g. A discussion my peers and I engaged recently was "time and place". We use mostly Ansible for deploying servers. Yet some of our team wanted to use Terraform and others argued Cloud Formation. Our discussion concluded why not let Terraform do what it does best and leave the rest to Ansible? In that within the Terraform user_data section you can call your Ansible Playbooks. My 'limited' understanding of Terraform is that its purpose is to deploy services to a limited extent and not be a full on automation drop-in replacement for services like Ansible, Chef, Puppet, etc. As a side note, Ansible has a module to handle the ssh keys generation and plenty more.