How to import an AWS EC2 Instance with existing Elastic Network Interfaces (ENIs)

Background

It is possible to import an existing AWS EC2 instance with existing attached Network Interfaces into Terraform's management, but there is a known issue that can cause unexpected plan output after running terraform import on the aws_instance and associated aws_network_interface resources. This results in a couple of possible issue scenarios:

Issue A. Configuring the network_interface block on the aws_instance to reference the associated aws_network_interface forces recreation:

Terraform will perform the following actions:

 # aws_instance.example must be replaced

-/+ resource "aws_instance" "example" {
...
 + network_interface { # forces replacement
 + delete_on_termination = false
 + device_index = 0
 + network_interface_id = "eni-0aadab1c2f7ec218d"
 }
...  

Issue B. Configuring the aws_network_interface_attachment resource to connect the existing EC2 instance and network interface resources will show the resource will be created in the plan output:

 # aws_network_interface_attachment.foo will be created
 + resource "aws_network_interface_attachment" "foo" {

But the apply will error:

│ Error: Error attaching network interface (eni-0aadab1c2f7ec218d) to instance (i-0ff957ed6b6cbbe6b), message: "Instance 'i-0ff957ed6b6cbbe6b' already has an interface attached at device index '0'.", code: "InvalidParameterValue"
│ 
│ with aws_network_interface_attachment.foo,
│ on main.tf line 41, in resource "aws_network_interface_attachment" "foo":
│ 41: resource "aws_network_interface_attachment" "foo" {
│ 

Solution

Instead of configuring an aws_network_interface_attachment resource or the network_interface block on aws_instance, use the attachment block on the aws_network_interface resource, e.g.:

resource "aws_network_interface" "test" {
...
 attachment {
 instance = aws_instance.example.id
 device_index = 0
 }
}

This will allow the apply to proceed without unexpected recreation or errors related to the network interface(s):

No changes. Your infrastructure matches the configuration.

Terraform has compared your real infrastructure against your configuration and found no differences, so no changes are needed.

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Additional Resources

If you are unfamiliar with running the terraform import command, please check out the following resources:

• Importing Infrastructure
• How to Import Resources into a Remote State Managed by Terraform Cloud
• aws_instance import specifics
• aws_network_interface import specifics