Creating EC2 and Route 53 resources with Terraform

Inspired by the great series of Terraform-related posts published on the Gruntwork blog, I've been experimenting with Terraform the last couple of days. So far, I like it a lot, and I think the point that Yevgeniy Brikman makes in the first post of the series, on why they chose Terraform over other tools (which are the usual suspects Chef, Puppet, Ansible), is a very valid point: Terraform is a declarative and client-only orchestration tool that allows you to manage immutable infrastructure. Read that post for more details about why this is a good thing.

In this short post I'll show how I am using Terraform in its Docker image incarnation to create an AWS ELB and a Route 53 CNAME record pointing to the name of the newly-created ELB.

I created a directory called terraform and created this Dockerfile inside it:

$ cat Dockerfile
FROM hashicorp/terraform:full
COPY data /data/
WORKDIR /data

My Terraform configuration files are under terraform/data locally. I have 2 files, one for variable declarations and one for the actual resource declarations.

Here is the variable declaration file:

$ cat data/vars.tf

variable "access_key" {}
variable "secret_key" {}
variable "region" {
  default = "us-west-2"
}

variable "exposed_http_port" {
  description = "The HTTP port exposed by the application"
  default = 8888
}

variable "security_group_id" {
  description = "The ID of the ELB security group"
  default = "sg-SOMEID"
}

variable "host1_id" {
  description = "EC2 Instance ID for ELB Host #1"
  default = "i-SOMEID1"
}

variable "host2_id" {
  description = "EC2 Instance ID for ELB Host #2"
  default = "i-SOMEID2"
}

variable "elb_cname" {
  description = "CNAME for the ELB"
}

variable "route53_zone_id" {
  description = "Zone ID for the Route 53 zone "
  default = "MY_ZONE_ID"
}

Note that I am making some assumptions, namely that the ELB will point to 2 EC2 instances. This is because I know beforehand which 2 instances I want to point it to. In my case, those instances are configured as Rancher hosts, and the ELB's purpose is to expose as port 80 to the world an internal Rancher load balancer port (say 8888).

Here is the resource declaration file:

$ cat data/main.tf


# --------------------------------------------------------
# CONFIGURE THE AWS CONNECTION
# --------------------------------------------------------

provider "aws" {
  access_key = "${var.access_key}"
  secret_key = "${var.secret_key}"
  region = "${var.region}"
}

# --------------------------------------------------------
# CREATE A NEW ELB
# --------------------------------------------------------

resource "aws_elb" "my-elb" {
  name = "MY-ELB"
  availability_zones = ["us-west-2a", "us-west-2b", "us-west-2c"]

  listener {
    instance_port = "${var.exposed_http_port}"
    instance_protocol = "http"
    lb_port = 80
    lb_protocol = "http"
  }

  health_check {
    healthy_threshold = 2
    unhealthy_threshold = 2
    timeout = 3
    target = "TCP:${var.exposed_http_port}"
    interval = 10
  }

  instances = ["${var.host1_id}", "${var.host2_id}"]
  security_groups = ["${var.security_group_id}"]
  cross_zone_load_balancing = true
  idle_timeout = 400
  connection_draining = true
  connection_draining_timeout = 400

  tags {
    Name = "MY-ELB"
  }
}

# --------------------------------------------------------
# CREATE A ROUTE 53 CNAME FOR THE ELB
# --------------------------------------------------------

resource "aws_route53_record" "my-elb-cname" {
  zone_id = "${var.route53_zone_id}"
  name = "${var.elb_cname}"
  type = "CNAME"
  ttl = "300"
  records = ["${aws_elb.my-elb.dns_name}"]
}

First I declare a provider of type aws, then I declare 2 resources, one of type aws_elb, and another one of type aws_route53_record. These types are all detailed in the very good Terraform documentation for the AWS provider.

The aws_elb resource defines an ELB named my-elb, which points to the 2 EC2 instances mentioned above. The instances are specified by their variable names from vars.tf, using the syntax ${var.VARIABLE_NAME}, e.g. ${var.host1_id}. For the other properties of the ELB resource, consult the Terraform aws_elb documentation.

The aws_route53_record defined a CNAME record in the given Route 53 zone file (specified via the route53_zone_id variable). An important thing to note here is that the CNAME points to the name of the ELB just created via the aws_elb.my-elb.dns_name variable. This is one of the powerful things you can do in Terraform - reference properties of resources in other resources.

Again, for more details on aws_route53_record, consult the Terraform documentation.

Given these files, I built a local Docker image:

$ docker build -t terraform:local

I can then run the Terraform 'plan' command to see what Terraform intends to do:

$ docker run -t --rm terraform:local plan \
-var "access_key=$TERRAFORM_AWS_ACCESS_KEY" \
-var "secret_key=$TERRAFORM_AWS_SECRET_KEY" \
-var "exposed_http_port=$LB_EXPOSED_HTTP_PORT" \
-var "elb_cname=$ELB_CNAME"

The nice thing about this is that I can run Terraform in exactly the same way via Jenkins. The variables above are defined in Jenkins either as credentials of type 'secret text' (the 2 AWS keys), or as build parameters of type string. In Jenkins, the Docker image name would be specified as an ECR image, something of the type ECR_ID.dkr.ecr.us-west-2.amazonaws.com/terraform.

After making sure that the plan corresponds to what I expected, I ran the Terraform apply command:

$ docker run -t --rm terraform:local apply \
-var "access_key=$TERRAFORM_AWS_ACCESS_KEY" \
-var "secret_key=$TERRAFORM_AWS_SECRET_KEY" \
-var "exposed_http_port=$LB_EXPOSED_HTTP_PORT" \
-var "elb_cname=$ELB_CNAME"

One thing to note is that the AWS credentials I used are for an IAM user that has only the privileges needed to create the resources I need. I tinkered with the IAM policy generator until I got it right. Terraform will emit various AWS errors when it's not able to make certain calls. Those errors help you add the required privileges to the IAM policies. In my case, here are some example of policies.

Allow all ELB operations:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "Stmt1476919435000",
            "Effect": "Allow",
            "Action": [ "elasticloadbalancing:*"
            ],
            "Resource": [
                "*"
            ]
        }
    ]
}

Allow the ec2:DescribeSecurityGroups operation:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "Stmt1476983196000",
            "Effect": "Allow",
            "Action": [
                "ec2:DescribeSecurityGroups"
            ],
            "Resource": [
                "*"
            ]
        }
    ]
}
Allow the route53:GetHostedZone and GetChange operations:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "Stmt1476986919000",
            "Effect": "Allow",
            "Action": [
                "route53:GetHostedZone",
                "route53:GetChange"
            ],
            "Resource": [
                "*"
            ]
        }
    ]
}

Allow the creation, changing and listing of Route 53 record sets in a given Route 53 zone file:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "Stmt1476987070000",
            "Effect": "Allow",
            "Action": [
                "route53:ChangeResourceRecordSets",
                "route53:ListResourceRecordSets"
            ],
            "Resource": [
                "arn:aws:route53:::hostedzone/MY_ZONE_ID"
            ]
        }
    ]
}

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