Applying a firewall rule to a specific Compute Engine instance in GCP can be achieved by using network tags. Network tags are used to identify instances when defining firewall rules. Here’s how to apply a firewall rule to a specific instance using GCP Console, gcloud CLI, and Terraform:

1. Using GCP Console:

a. Go to the GCP Console: https://console.cloud.google.com/

b. Navigate to Compute Engine > VM instances.

c. Click on the instance you want to apply the firewall rule to.

d. Click the “Edit” button at the top of the page.

e. In the “Network tags” section, add a new tag (e.g., “allow-ssh”).

f. Navigate to VPC network > Firewall.

g. Click on the “Create firewall rule” button.

h. Configure the firewall rule settings, and under “Target tags,” enter the tag you added to the instance (e.g., “allow-ssh”).

i. Click the “Create” button to create the firewall rule. The rule will now be applied to the instance with the specified tag.

2. Using gcloud CLI:

a. First, add a network tag to the instance:

gcloud compute instances add-tags INSTANCE_NAME --tags allow-ssh --zone ZONE

Replace INSTANCE_NAME and ZONE with appropriate values.

b. Create a firewall rule that targets the tag:

gcloud compute firewall-rules create RULE_NAME --allow PROTOCOL:PORT --target-tags allow-ssh

Replace RULE_NAME, PROTOCOL, and PORT with appropriate values.

3. Using Terraform:

a. Modify your main.tf file to include the following resources:

resource "google_compute_instance" "example" {
  # ... existing configuration ...

  tags = ["allow-ssh"]
}

resource "google_compute_firewall" "example" {
  name    = "example-firewall-rule"
  network = "default"

  allow {
    protocol = "PROTOCOL"
    ports    = ["PORT"]
  }

  target_tags = ["allow-ssh"]
}

Replace PROTOCOL and PORT with appropriate values.

b. Run the following commands to apply the changes:

terraform init
terraform plan
terraform apply

This will create a firewall rule targeting the instance with the “allow-ssh” tag. The rule will only be applied to instances that have the specified tag.