aws_network_interface.private_ips and aws_network_interface.private_ips_count act in an unexpected way

[aschepp]

Terraform Version

Terraform v0.9.8

Affected Resource(s)

• aws_network_interface

Terraform Configuration Files

resource "aws_network_interface" "interface1" {
	private_ips = [ "${var.ip1}", "${var.ip2}" ]
}

resource "aws_network_interface" "interface2" {
	private_ips_count = 2
}

output "primaryIP" {
  value = "${aws_network_interface.interface2.private_ips[0]}"
}

output "secondaryIP" {
  value = "${aws_network_interface.interface2.private_ips[1]}"
}

Expected Behavior

• The interface 1 should have two IPs, with ip1 being the primary IP and ip2 being the secondary IP.
• The interface 2 should have 2 IPs total assigned to it.
• The output primaryIP should have the primary IP of the interface 2.
• The output secondaryIP should have the secondary IP of the interface 2.

Actual Behavior

• Interface 1 will have ip1 and ip2 assigned in an undetermined order. This is because private_ips is a set with string hash as sorting.
• Interface 2 will have 3 IP assigned to it, one primary and two secondary IPs. The parameter should be renamed to secondary_private_ips_count.
• The outputs primaryIP and secondaryIP return the IPs in an undetermined order, because again, private_ips is a set with string hash as sorting.

Workaround

As there is no way to reliable get the secondary IP assigned to the interface, you have to use something like this:

resource "aws_network_interface" "interface2" {
	private_ips_count = 1
}

output "primaryIP" {
  value = "${aws_network_interface.interface2.private_ip}"
}

output "secondaryIP" {
  value = "${replace(join(",",aws_network_interface.interface2.private_ips),"/(^|,)${aws_network_interface.interface2.private_ip}(,|$)/", "")}"
}

Possible Fixes

• Use a List instead of a Set in aws_network_interface.go to store private_ips.
• Or do not include the primary ip in private_ips, as it's available as private_ip and rename private_ips to secondary_private_ips.
• Rename private_ips_count to secondary_private_ips_count to make it less confusing.

References

Some of these issues have already been mentioned in #836, but I wanted to extend it.

[jpbuecken]

• Rename private_ips_count to secondary_private_ips_count to make it less confusing.

At least that has been documented now:
https://www.terraform.io/docs/providers/aws/r/network_interface.html#private_ips_count

private_ips_count - (Optional) Number of secondary private IPs to assign to the ENI. The total number of private IPs will be 1 + private_ips_count, as a primary private IP will be assiged to an ENI by default.

Maybe the behavior described changed, because of
#8353

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