[BUG] aws_route with conditional target to use nat_gateway_id or transit_gateway_id

[Bobonium]

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Terraform Version

0.12.24

Affected Resource(s)

• aws_route

Terraform Configuration Files

resource "aws_route" "route" {
  count = length(var.private_subnets)

  route_table_id         = element(aws_route_table.private_rts.*.id, count.index)
  destination_cidr_block = "0.0.0.0/0"
  nat_gateway_id         = var.enable_nat_gateway ? element(aws_nat_gateway.nat_gateways.*.id, count.index) : null
  transit_gateway_id     = var.enable_nat_gateway ? null : var.tgw_id
}

Expected Behavior

if enable_nat_gateway is true it should create the route for destination 0.0.0.0/0 with the nat_gateway_id. If it is false it should use the supplied tgw_id instead.

Actual Behavior

The plan output correctly shows the intended changes.
Running apply will fail because of

Error: Error: more than 1 target specified. Only 1 of gateway_id, egress_only_gateway_id, nat_gateway_id, instance_id, network_interface_id or vpc_peering_connection_id is

In our case we switched an existing system from enable_nat_gateways = true, to enable_nat_gateways = false. Plan output was correct, but above error happened. We were able to solve the problem by manually changing the route in the UI, the next terraform plan was then empty, because of the refresh.

Steps to Reproduce

1. terraform apply

Important Factoids

none

References

none

[ewbankkit]

Similar:

• aws_route Error: more than 1 targe specified if you change the destination type #684
• Can not change target type on existing aws_route #12631
• [BUG] Cannot replace Virtual Private Gateway route with Transit Gateway route in aws_route resource #13302

[ewbankkit]

Verified that the same problem occurs with Terraform 0.11 (and "" instead of null for the missing gateway ID).

[ghost]

This has been released in version 3.34.0 of the Terraform AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template for triage. Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!