Terraform - Unable to get default route table ID

Question

I'm unable to fetch the default route table id associated with the private subnet.

I get the following error. This seems to be reported on github. I'm not sure how to get the route table ID for private subnets.

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One workaround I tried was to produce a list of VPC subnets using the aws_subnet_ids Data Source, and then discover all route table ids by using the aws_route_table Data Source with count and subnet_id as a parameter for each iteration, but this does not always work, because if there are one or more subnets that are (non-explictly) associated with the Main route table, TF will error as follows:

data.tf:

data "aws_route_table" "private_subnet_RT" { depends_on = [ aws_subnet.private_subnet ] count = length(var.availability_zones) # subnet_id = "${element(aws_subnet.private_subnet.*.id, count.index)}" subnet_id = "subnet-01bae78f452ca0000" }

subnets.tf

# Create Private Subnets resource "aws_subnet" "private_subnet" { count = length(var.availability_zones) vpc_id = var.vpc_id cidr_block = var.private_subnet_cidr[count.index] availability_zone = "${element(var.availability_zones, count.index)}" tags = merge({ Name = "Private_subnet_${count.index} - ${var.environment}" }, var.private_subnet_tags) }

Though the route table doesn't show that its associated with a subnet..

marcincuber · Accepted Answer · 2020-12-04 21:37:03Z

I have a feeling that a filter should do well for you:

data "aws_route_table" "selected" { vpc_id = var.vpc_id filter { name = "association.main" values = ["true"] } }

If you interested in seeing more filters please navigate to official aws resource -> https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeRouteTables.html

praveen · Accepted Answer · 2021-08-20 10:01:09Z

If this could help anyone.

 data "aws_route_table" "selected" { subnet_id = var.subnet_id }

this would only return route table id of subnet which has explicit association with the route table. Meaning if the subnet not associated with any route table and ends up being a part of the Main route table, will not be returned by this until you make that association explicit by associating that subnet with Main route table explicitly

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