Terraform one EC2 instance with two subnets - amazon-web-services

Question

I need to create one EC2 and associate 2 subnets to it.
variables.tf
variable "aws_subnet_id_this" {
description = "Subnet ID"
default = ["subnet-09df122a4faee8882", "subnet-2fcc756f02ddb4b62"]
}
main.tf
resource "aws_instance" "test" {
ami = var.ami_id
instance_type = var.ec2_instance_type
subnet_id = var.aws_subnet_id_this
key_name = var.pki_name
vpc_security_group_ids = [aws_security_group.Allow_SSH_in.id]
}
Error:
Error: Incorrect attribute value type
on main_count_data.tf line 57, in resource "aws_instance" "test":
57: subnet_id = var.aws_subnet_id_this
|----------------
| var.aws_subnet_id_eks is tuple with 2 elements
Inappropriate value for attribute "subnet_id": string required.
So I tried this:
main.tf
resource "aws_instance" "prueba" {
ami = var.ami_id
instance_type = var.ec2_instance_type
#subnet_id = var.aws_subnet_id_this
count = 2
subnet_id = "${element(var.aws_subnet_id_this, count.index)}"
key_name = var.pki_name
vpc_security_group_ids = [aws_security_group.Allow_SSH_in.id]
}
}
but this last portion of code tries to create a new EC2 instance with the second subnet and this is not what I expect tough.
To sum up: I need 1 EC2 containing 2 subnets defined in the variables.tf file.
How can I do this?

Answer 1

Below is an example of how you can create one instance with two NICs in different subnets. The NICs must be in same AZ. So instance can have two NICs in different subnets as long as they are in same AZ:
variable "aws_subnet_id_this" {
description = "Subnet ID"
default = ["subnet-09df122a4faee8882", "subnet-2fcc756f02ddb4b62"]
}
resource "aws_network_interface" "nic1" {
subnet_id = var.aws_subnet_id_this[0]
}
resource "aws_network_interface" "nic2" {
subnet_id = var.aws_subnet_id_this[1]
}
resource "aws_instance" "prueba" {
ami = var.ami_id
instance_type = var.ec2_instance_type
key_name = var.pki_name
network_interface {
device_index = 0
network_interface_id = aws_network_interface.nic1.id
}
network_interface {
device_index = 1
network_interface_id = aws_network_interface.nic2.id
}
}