Terraform: aws_route_table always triggers modify

[dene14]

Hello there!

resource "aws_route_table" "private_1" {
    vpc_id = "${var.vpc_id}"
    route {
        cidr_block = "0.0.0.0/0"
        gateway_id = "${aws_nat_gateway.gw_1.id}"
    }

    tags {
        Name = "uat_platform_private_1"
    }
}

Always triggers terraform to modify a resource:

aws_route_table.private_1: Modifying...
  route.1498634145.cidr_block:                "" => "0.0.0.0/0"
  route.1498634145.gateway_id:                "" => "nat-0509bae43356cedc1"
  route.1498634145.instance_id:               "" => ""
  route.1498634145.nat_gateway_id:            "" => ""
  route.1498634145.network_interface_id:      "" => ""
  route.1498634145.vpc_peering_connection_id: "" => ""
  route.1584724321.cidr_block:                "0.0.0.0/0" => ""
  route.1584724321.gateway_id:                "" => ""
  route.1584724321.instance_id:               "" => ""
  route.1584724321.nat_gateway_id:            "nat-0509bae43356cedc1" => ""
  route.1584724321.network_interface_id:      "" => ""
  route.1584724321.vpc_peering_connection_id: "" => ""

It doesn't happen to public subnet routes, however it has internat gateway attached instead of NAT gateway.

[stack72]

hi @dene14

Thanks for opening the issue here. This is a pretty simple one to solve. You are trying to connect a NAT gateway to the parameter for the Internet Gateway - this is what is causing the continual changes

the code should be:

resource "aws_route_table" "private_1" {
    vpc_id = "${var.vpc_id}"
    route {
        cidr_block = "0.0.0.0/0"
        nat_gateway_id = "${aws_nat_gateway.gw_1.id}"
    }

    tags {
        Name = "uat_platform_private_1"
    }
}

:)

[dene14]

@stack72 Thanks for blazing fast answer Paul! I wasn't careful enough to check documentation...
However, for my better understanding, has it been done like so on purpose? Isn't route declaration is a simple attachment where router (NAT or IGW) has no any difference?

[stack72]

Hi @dene14

the AWS API actually treats each of these differently:

opts := ec2.CreateRouteInput{
                RouteTableId:           aws.String(d.Id()),
                DestinationCidrBlock:   aws.String(m["cidr_block"].(string)),
                GatewayId:              aws.String(m["gateway_id"].(string)),
                InstanceId:             aws.String(m["instance_id"].(string)),
                VpcPeeringConnectionId: aws.String(m["vpc_peering_connection_id"].(string)),
                NetworkInterfaceId:     aws.String(m["network_interface_id"].(string)),
            }

            if m["nat_gateway_id"].(string) != "" {
                opts.NatGatewayId = aws.String(m["nat_gateway_id"].(string))
            }

So we need to be clear that when some means a nat_gateway, that is what we pass to the API. We thought about trying something like checking for string prefixes but that would have broken potential naming changes from AWS

P.

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