Connection blocks for SSH in Terraform - Step-by-Step Execution

Process Flow - Connection blocks for SSH

Define resource requiring SSH

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Add connection block with SSH details

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Terraform uses SSH to connect

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Run remote commands or provisioners

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Close SSH connection

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Resource provisioned

Terraform uses connection blocks to specify SSH details, enabling it to connect to remote machines and run commands during provisioning.

Execution Sample

Terraform

resource "null_resource" "example" {
  connection {
    type        = "ssh"
    host        = "192.168.1.10"
    user        = "ubuntu"
    private_key = file("~/.ssh/id_rsa")
  }

  provisioner "remote-exec" {
    inline = ["echo Hello from Terraform"]
  }
}

This Terraform code connects via SSH to a remote machine and runs a simple echo command.

Process Table

Step	Action	SSH Connection Details	Provisioner Command	Result
1	Start resource creation	Not connected yet	No command run	Waiting to connect
2	Read connection block	type=ssh, host=192.168.1.10, user=ubuntu, key=~/.ssh/id_rsa	No command run	Ready to connect
3	Establish SSH connection	Connected to 192.168.1.10 as ubuntu	No command run	SSH session open
4	Run provisioner command	SSH session active	echo Hello from Terraform	Command executed successfully
5	Close SSH connection	Disconnected	No command run	SSH session closed
6	Resource creation complete	No connection	No command run	Resource provisioned

💡 SSH connection closed after running provisioner commands; resource creation finished.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
connection.type	undefined	ssh	ssh	ssh	ssh
connection.host	undefined	192.168.1.10	192.168.1.10	192.168.1.10	192.168.1.10
connection.user	undefined	ubuntu	ubuntu	ubuntu	ubuntu
connection.private_key	undefined	~/.ssh/id_rsa	~/.ssh/id_rsa	~/.ssh/id_rsa	~/.ssh/id_rsa
ssh_session	closed	closed	open	open	closed
provisioner.command_status	not run	not run	not run	success	success

Key Moments - 3 Insights

Why does Terraform need the private_key in the connection block?

Terraform uses the private_key to authenticate the SSH connection securely, as shown in step 2 of the execution_table where the key is read before connecting.

What happens if the SSH connection fails at step 3?

If SSH connection fails, Terraform cannot run the provisioner commands and resource creation will stop or error out, as the SSH session must be open to proceed.

Why is the SSH session closed after running commands?

Terraform closes the SSH session after running provisioner commands to free resources and maintain security, as seen in step 5 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the SSH connection status at step 3?

A Not connected yet

B Disconnected

C Connected to 192.168.1.10 as ubuntu

D Connection failed

💡 Hint

Check the 'SSH Connection Details' column at step 3 in the execution_table.

At which step does Terraform run the remote command?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Provisioner Command' and 'Result' columns in the execution_table.

If the private_key path is incorrect, what will most likely happen?

A SSH connection will fail at step 3

B Provisioner command runs successfully

C SSH session closes immediately at step 2

D Resource creation completes without SSH

💡 Hint

Refer to the importance of private_key in the connection block and step 3 connection establishment.

Concept Snapshot

Terraform connection blocks specify how to connect to remote machines.
Use type = "ssh", host, user, and private_key to set SSH details.
Terraform opens SSH, runs provisioner commands, then closes connection.
This enables remote configuration during resource creation.

Full Transcript

Terraform uses connection blocks to connect to remote machines via SSH. The connection block includes the SSH type, host IP, username, and private key path. Terraform reads these details, opens an SSH session, runs any remote commands defined in provisioners, then closes the SSH connection. This process allows Terraform to configure resources on remote servers securely and automatically during deployment.