Is there a function to transfer ip to cidr_block format in terraform

[hanks]

like "${ip2cidr(ec2_instance.private_ip, 32)}", "172.21.3.211" -> "172.21.3.211/32"

[David Maze]

On Friday, September 16, 2016 at 1:20:49 AM UTC-4, hanks wrote:

like "${ip2cidr(ec2_instance.private_ip, 32)}", "172.21.3.211" -> "172.21.3.211/32"

variable "addr" { default = "172.21.3.211" }

variable "bits" { default = "25" }

resource "template_file" "cidr" {

  template = "$${network}/${var.bits}"

  vars {

    network = "${cidrhost("${var.addr}/${var.bits}", 0)}"

  }

}

output "cidr" { value = "${template_file.cidr.rendered}" }

[David Adams]

Here's a shorter version that can work around the need for a template resource:

    variable "addr" { default = "172.21.3.211" }

    variable "bits" { default = "32" }

    output "cidr" {

      value = "${cidrhost("${var.addr}/${var.bits}", 0)}/${var.bits}"

    }

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[Martin Atkins]

If you know you always want it to be a /32 prefix then I would literally write "${aws_instance.foo.private_ip}/32" ... might as well keep it simple, no?

Terraform has a selection of functions for manipulting CIDR prefixes, but they are focused on extending the prefix (subnets/host numbering given a prefix) rather than truncating it (calculating a CIDR mask given a host address and a prefix length). As David noted, you can (ab)use the cidrhost function to do this sort of truncation by exploiting the fact that it will replace any bits that extend beyond the given prefix with the given host number, which can be zero... but perhaps we could make that more explicit by having a first-class function for doing this.

[Trevor Robinson]

And for lists, you can use formatlist to append the "/32":

${formatlist("%s/32", aws_eip.stuff.*.public_ip)}

Or for multiple lists:

${formatlist("%s/32", concat(aws_eip.stuff.*.public_ip, aws_eip.things.*.public_ip))}

This need arises when putting IPs into AWS security group rules, since they require CIDR notation. Since that's not uncommon, a predefined function might be warranted.

-Trevor