terraform aws_route_table always change

Terraform Version

Terraform v0.12.6

Q: Terraform aws_route_table always change?

main.tf

resource "aws_route_table" "route_to_nat" {
  vpc_id = "${aws_vpc.vpc.id}"
  route {
    cidr_block = "0.0.0.0/0"
    gateway_id = "${aws_nat_gateway.nat.id}"
  }
  tags = {
    "Name"  = "Route-To-Nat"
  }
}

Error

  # aws_lb_target_group_attachment.test must be replaced
-/+ resource "aws_lb_target_group_attachment" "test" {
      ~ id               = "arn:aws:elasticloadbalancing:us-east-1:xxxxxxxxxxxx:targetgroup/test-lb-tg-tf/583716da0369baf2-20190814081528833600000001" -> (known after apply)
        port             = 80
        target_group_arn = "arn:aws:elasticloadbalancing:us-east-1:xxxxxxxxxxxx:targetgroup/test-lb-tg-tf/583716da0369baf2"
      ~ target_id        = "i-075a770cca5ab74e4" -> (known after apply) # forces replacement
    }

  # aws_route_table.route_to_nat will be updated in-place
  ~ resource "aws_route_table" "route_to_nat" {
        id               = "rtb-06e975d7fb6e235e3"
        owner_id         = "xxxxxxxxxxxx"
        propagating_vgws = []
      ~ route            = [
          - {
              - cidr_block                = "0.0.0.0/0"
              - egress_only_gateway_id    = ""
              - gateway_id                = ""
              - instance_id               = ""
              - ipv6_cidr_block           = ""
              - nat_gateway_id            = "nat-02fb444b359ab0f52"
              - network_interface_id      = ""
              - transit_gateway_id        = ""
              - vpc_peering_connection_id = ""
            },
          + {
              + cidr_block                = "0.0.0.0/0"
              + egress_only_gateway_id    = ""
              + gateway_id                = "nat-02fb444b359ab0f52"
              + instance_id               = ""
              + ipv6_cidr_block           = ""
              + nat_gateway_id            = ""
              + network_interface_id      = ""
              + transit_gateway_id        = ""
              + vpc_peering_connection_id = ""
            },
        ]
        tags             = {
            "Name" = "Route-To-Nat"
        }
        vpc_id           = "vpc-0b5320622318ef4c2"
    }

Solution

Use nat_gateway_id instead of gateway_id.

• Data Source: aws route table
  • gateway_id - The Internet Gateway ID.
  • nat_gateway_id - The NAT Gateway ID.
• Resource: aws route table
  • gateway_id - (Optional) Identifier of a VPC internet gateway or a virtual private gateway.
  • nat_gateway_id - (Optional) Identifier of a VPC NAT gateway.

  NOTE on gatewayid and natgatewayid: The AWS API is very forgiving with these two attributes and the awsroutetable resource can be created with a NAT ID specified as a Gateway ID attribute.This will lead to a permanent diff between your configuration and statefile, as the API returns the correct parameters in the returned route table. If you're experiencing constant diffs in your awsroute_table resources, the first thing to check is whether or not you're specifying a NAT ID instead of a Gateway ID, or vice-versa.

Reference:

• routing table entries always changed but in reality they don't change
• awsroutetable always triggers modify

why terraform aws_instance always recreate

Terraform Version

Terraform v0.12.6

Q: Why terraform aws_instance always recreate?

main.tf

data "aws_ami" "linux" {
  owners = ["self"]

  filter {
    name      = "tag:Description"
    values    = ["${var.instance_ami.base}"]
  }
}

resource "aws_instance" "linux" {
  ami                         = "${data.aws_ami.linux.id}"
  instance_type               = "t2.micro"
  subnet_id                   = "${aws_subnet.public.id}"
  availability_zone           = "${var.availability_zone.us-east-1}"
  security_groups             = ["${aws_security_group.allow_ssh.id}"] 
  associate_public_ip_address = true
  key_name                    = "HelloWorld"

  tags = {
    Name = "Web"
  }
}

Error

Terraform will perform the following actions:

  # aws_instance.linux must be replaced
-/+ resource "aws_instance" "linux" {
        ami                          = "ami-0c7157d76b3a45a34"
      ~ arn                          = "arn:aws:ec2:us-east-1:xxxxxxxxxxxx:instance/i-075a770cca5ab74e4" -> (known after apply)
        associate_public_ip_address  = true
        availability_zone            = "us-east-1a"
      ~ cpu_core_count               = 1 -> (known after apply)
      ~ cpu_threads_per_core         = 1 -> (known after apply)
      - disable_api_termination      = false -> null
      - ebs_optimized                = false -> null
        get_password_data            = false
      + host_id                      = (known after apply)
      ~ id                           = "i-075a770cca5ab74e4" -> (known after apply)
      ~ instance_state               = "running" -> (known after apply)
        instance_type                = "t2.micro"
      ~ ipv6_address_count           = 0 -> (known after apply)
      ~ ipv6_addresses               = [] -> (known after apply)
        key_name                     = "devops"
      - monitoring                   = false -> null
      + network_interface_id         = (known after apply)
      + password_data                = (known after apply)
      + placement_group              = (known after apply)
      ~ primary_network_interface_id = "eni-00fefb01fc9fa12de" -> (known after apply)
      ~ private_dns                  = "ip-172-88-0-166.ec2.internal" -> (known after apply)
      ~ private_ip                   = "172.88.0.166" -> (known after apply)
      ~ public_dns                   = "ec2-xx-xx-xx-xx.compute-1.amazonaws.com" -> (known after apply)
      ~ public_ip                    = "xx.xx.xx.xx" -> (known after apply)
      ~ security_groups              = [ # forces replacement
          + "sg-025186720be2334e9",
        ]
        source_dest_check            = true
        subnet_id                    = "subnet-0007ef0e7a0bf8cc6"
        tags                         = {
            "Name" = "Test"
        }
      ~ tenancy                      = "default" -> (known after apply)
      ~ volume_tags                  = {} -> (known after apply)
      ~ vpc_security_group_ids       = [
          - "sg-025186720be2334e9",
        ] -> (known after apply)

      - credit_specification {
          - cpu_credits = "standard" -> null
        }

      + ebs_block_device {
          + delete_on_termination = (known after apply)
          + device_name           = (known after apply)
          + encrypted             = (known after apply)
          + iops                  = (known after apply)
          + kms_key_id            = (known after apply)
          + snapshot_id           = (known after apply)
          + volume_id             = (known after apply)
          + volume_size           = (known after apply)
          + volume_type           = (known after apply)
        }

      + ephemeral_block_device {
          + device_name  = (known after apply)
          + no_device    = (known after apply)
          + virtual_name = (known after apply)
        }

      + network_interface {
          + delete_on_termination = (known after apply)
          + device_index          = (known after apply)
          + network_interface_id  = (known after apply)
        }

      ~ root_block_device {
          ~ delete_on_termination = false -> (known after apply)
          ~ encrypted             = false -> (known after apply)
          ~ iops                  = 120 -> (known after apply)
          + kms_key_id            = (known after apply)
          ~ volume_id             = "vol-0f77b0b8696ac4379" -> (known after apply)
          ~ volume_size           = 40 -> (known after apply)
          ~ volume_type           = "gp2" -> (known after apply)
        }
    }

Solution

Use vpc_security_group_ids instead of security_groups.

• Terraform Data Source: aws_instance
  • vpc_security_group_ids - The associated security groups in a non-default VPC.
  • security_groups - The associated security groups.
• Terraform Resource: aws_instance
  • vpc_security_group_ids - (Optional, VPC only) A list of security group IDs to associate with.
  • security_groups - (Optional, EC2-Classic and default VPC only) A list of security group names (EC2-Classic) or IDs (default VPC) to associate with.

Reference:

• AWS instance is always recreated