generate user_data including "IP Address" while creating ec2 instance using terraform

Question

I am trying to spin 2 ec2 instances using terraform. Something like this

resource "aws_instance" "example" {
  count                       = "${var.number_of_instances}"
  ami                         = "${var.ami_name}"
  associate_public_ip_address = "${var.associate_public_ip_address}"
  instance_type               = "${var.instance_type}"
  key_name                    = "${var.keyname}"
  subnet_id                   = "${element(var.subnet_ids, count.index)}"
  user_data                   = "${element(data.template_file.example.*.rendered, count.index)}"
  vpc_security_group_ids      = ["${aws_security_group.example.id}","${var.extra_security_group_id}"]
  root_block_device {
    volume_size = "${var.root_volume_size}"
    volume_type = "${var.root_volume_type}"
    iops        = "${var.root_volume_iops}"
  }
  tags {
    Name      = "${var.prefix}${var.name}${format("%02d", count.index + 1)}"
  }
}

In template_file all I am trying to do is to generate a config file with IP Address of both the instances using user_data but this fails saying Cycle Error.

Is there any way to get the file to generate with IP Address while the ec2 instances are coming up

What you have here is a "chicken or egg" problem: user_data must be fixed when the instance is first created, and at that point the instance doesn't have an IP address yet (unless you assign it a static one). To solve this, you'll need to introduce something new. For example, you could create some DNS records at a predictable name which your instances can then read to find one another after boot, though you'd need to design it to be resilient to there being a delay after the instance is created before the DNS record becomes available. — Martin Atkins
@MartinAtkins I actually thought about it and tried the external DNSName thing. But I have some stupid logic here in my company where Security won't allow connecting to external DNSName on a port anything else 443. — devnull

Adil B Adil B · Accepted Answer · 2019-04-15T16:36:55

Make use of the AWS Instance Metadata endpoint in your userdata script to get each instance's IP address and put it into a config file. Here is a Powershell example of a userdata script:

<powershell>
$HostIp = (Invoke-RestMethod -URI 'http://169.254.169.254/latest/meta-data/local-ipv4' -UseBasicParsing)
Add-Content "C:\installer\config.txt" "HostIp:$HostIp"
</powershell>

You can also get the instance's public-ipv4 in this manner if that's desired instead.

generate user_data including "IP Address" while creating ec2 instance using terraform

1 Answers