Terraform Client VPN Routes With More CIDRs Than Subnets

A laptop connects to the Client VPN endpoint, and terraform apply still fails on a route resource. The configuration usually has more destination CIDRs than associated subnets, and the route block picks a target with an index. That assumes the two collections are parallel. They are not, and the failure surfaces at apply time rather than at review.

Name the object that fails

Three resources are involved and each answers a different question. The association places the endpoint's network interface in a subnet. The route says which destination prefix is delivered to a target subnet. The authorization rule says which users may reach which prefix. A route pointing at a subnet that was never associated fails even though both objects exist, because the target must belong to this endpoint.

List the associations first: aws ec2 describe-client-vpn-target-networks --client-vpn-endpoint-id cvpn-endpoint-0123456789abcdef0.

Remember the VPC local route

The first association adds a local route for the whole VPC, so every subnet in that VPC is already reachable without a route resource. Extra routes exist for peered VPCs, transit gateway attachments, and on-premises prefixes that the VPC does not own. This is why an endpoint with three associations can legitimately carry seven destination entries.

Count the destinations that fall outside the VPC before adding routes. If that count is smaller than the CIDR list, the list is describing subnets that need no route at all.

Replace the index with a map

Wrapping the index in a modulo operator silences the error and keeps the modelling mistake. A destination that cannot be reached through the chosen target still carries no traffic, no matter how cleanly the plan applies. Store the relationship instead, and let Terraform check it before creating anything. The block below is illustrative and uses the provider's endpoint route arguments.

variable "client_vpn_destinations" {
  description = "Destination CIDR mapped to an associated subnet key"
  type        = map(string)
}

resource "aws_ec2_client_vpn_route" "this" {
  for_each               = var.client_vpn_destinations
  client_vpn_endpoint_id = aws_ec2_client_vpn_endpoint.this.id
  destination_cidr_block = each.key
  target_vpc_subnet_id   = aws_subnet.private[each.value].id

  lifecycle {
    precondition {
      condition     = contains(keys(aws_subnet.private), each.value)
      error_message = "Route target must be a subnet key in the associated map."
    }
  }
}

With the map beside the association list, adding a destination forces a decision about which subnet carries it. That is the conversation the index was hiding.

A different failure produces nonsense values rather than an index error. Passing a low-level construct where a higher-level interface is expected leaves the fields the API reads unpopulated, and the resulting CIDR is unusable. Terraform shows the same shape when a string arrives where an object is expected. Check the provider schema for the argument type.

Prove it from a client

Run terraform validate and a plan against real state before applying. Afterwards, compare the endpoint's target-network listing with the map rather than trusting a successful apply.

The acceptance test is a connected client reaching each intended prefix.

The authorization rule is where many setups stop early. It is scoped to a destination CIDR and a group, and a route without a matching authorization produces a connected client that cannot reach the prefix. Check both objects for every prefix, because an apply that creates the route and forgets the rule looks complete. Test with one user inside the group and one outside it.

When one prefix fails, compare its route entry with the authorization rule. A missing authorization and a missing route look similar in the client log and are fixed in different resources.