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Terraform Edit

Terraform lets you describe the infrastructure you want and automatically creates, deletes, and modifies your existing infrastructure to match. OPA makes it possible to write policies that test the changes Terraform is about to make before it makes them. Such tests help in different ways:

tests help individual developers sanity check their Terraform changes
tests can auto-approve run-of-the-mill infrastructure changes and reduce the burden of peer-review
tests can help catch problems that arise when applying Terraform to production after applying it to staging

Goals

In this tutorial, you’ll learn how to use OPA to implement unit tests for Terraform plans that create and delete auto-scaling groups and servers.

Prerequisites

This tutorial requires

(This tutorial should also work with the latest version of Terraform, but it is untested. Contributions welcome!)

Getting Started

Steps

1. Create and save a Terraform plan

Create a Terraform file that includes an auto-scaling group and a server on AWS. (You will need to modify the shared_credentials_file to point to your AWS credentials.)

cat >main.tf <<EOF
provider "aws" {
    region = "us-west-1"
}
resource "aws_instance" "web" {
  instance_type = "t2.micro"
  ami = "ami-09b4b74c"
}
resource "aws_autoscaling_group" "my_asg" {
  availability_zones        = ["us-west-1a"]
  name                      = "my_asg"
  max_size                  = 5
  min_size                  = 1
  health_check_grace_period = 300
  health_check_type         = "ELB"
  desired_capacity          = 4
  force_delete              = true
  launch_configuration      = "my_web_config"
}
resource "aws_launch_configuration" "my_web_config" {
    name = "my_web_config"
    image_id = "ami-09b4b74c"
    instance_type = "t2.micro"
}
EOF

Then initialize Terraform and ask it to calculate what changes it will make and store the output in plan.binary.

terraform init
terraform plan --out tfplan.binary

2. Convert the Terraform plan into JSON

Use the command terraform show to convert the Terraform plan into JSON so that OPA can read the plan.

terraform show -json tfplan.binary > tfplan.json

Here is the expected contents of tfplan.json.

{
  "format_version": "0.1",
  "terraform_version": "0.12.6",
  "planned_values": {
    "root_module": {
      "resources": [
        {
          "address": "aws_autoscaling_group.my_asg",
          "mode": "managed",
          "type": "aws_autoscaling_group",
          "name": "my_asg",
          "provider_name": "aws",
          "schema_version": 0,
          "values": {
            "availability_zones": [
              "us-west-1a"
            ],
            "desired_capacity": 4,
            "enabled_metrics": null,
            "force_delete": true,
            "health_check_grace_period": 300,
            "health_check_type": "ELB",
            "initial_lifecycle_hook": [],
            "launch_configuration": "my_web_config",
            "launch_template": [],
            "max_size": 5,
            "metrics_granularity": "1Minute",
            "min_elb_capacity": null,
            "min_size": 1,
            "mixed_instances_policy": [],
            "name": "my_asg",
            "name_prefix": null,
            "placement_group": null,
            "protect_from_scale_in": false,
            "suspended_processes": null,
            "tag": [],
            "tags": null,
            "termination_policies": null,
            "timeouts": null,
            "wait_for_capacity_timeout": "10m",
            "wait_for_elb_capacity": null
          }
        },
        {
          "address": "aws_instance.web",
          "mode": "managed",
          "type": "aws_instance",
          "name": "web",
          "provider_name": "aws",
          "schema_version": 1,
          "values": {
            "ami": "ami-09b4b74c",
            "credit_specification": [],
            "disable_api_termination": null,
            "ebs_optimized": null,
            "get_password_data": false,
            "iam_instance_profile": null,
            "instance_initiated_shutdown_behavior": null,
            "instance_type": "t2.micro",
            "monitoring": null,
            "source_dest_check": true,
            "tags": null,
            "timeouts": null,
            "user_data": null,
            "user_data_base64": null
          }
        },
        {
          "address": "aws_launch_configuration.my_web_config",
          "mode": "managed",
          "type": "aws_launch_configuration",
          "name": "my_web_config",
          "provider_name": "aws",
          "schema_version": 0,
          "values": {
            "associate_public_ip_address": false,
            "enable_monitoring": true,
            "ephemeral_block_device": [],
            "iam_instance_profile": null,
            "image_id": "ami-09b4b74c",
            "instance_type": "t2.micro",
            "name": "my_web_config",
            "name_prefix": null,
            "placement_tenancy": null,
            "security_groups": null,
            "spot_price": null,
            "user_data": null,
            "user_data_base64": null,
            "vpc_classic_link_id": null,
            "vpc_classic_link_security_groups": null
          }
        }
      ]
    }
  },
  "resource_changes": [
    {
      "address": "aws_autoscaling_group.my_asg",
      "mode": "managed",
      "type": "aws_autoscaling_group",
      "name": "my_asg",
      "provider_name": "aws",
      "change": {
        "actions": [
          "create"
        ],
        "before": null,
        "after": {
          "availability_zones": [
            "us-west-1a"
          ],
          "desired_capacity": 4,
          "enabled_metrics": null,
          "force_delete": true,
          "health_check_grace_period": 300,
          "health_check_type": "ELB",
          "initial_lifecycle_hook": [],
          "launch_configuration": "my_web_config",
          "launch_template": [],
          "max_size": 5,
          "metrics_granularity": "1Minute",
          "min_elb_capacity": null,
          "min_size": 1,
          "mixed_instances_policy": [],
          "name": "my_asg",
          "name_prefix": null,
          "placement_group": null,
          "protect_from_scale_in": false,
          "suspended_processes": null,
          "tag": [],
          "tags": null,
          "termination_policies": null,
          "timeouts": null,
          "wait_for_capacity_timeout": "10m",
          "wait_for_elb_capacity": null
        },
        "after_unknown": {
          "arn": true,
          "availability_zones": [
            false
          ],
          "default_cooldown": true,
          "id": true,
          "initial_lifecycle_hook": [],
          "launch_template": [],
          "load_balancers": true,
          "mixed_instances_policy": [],
          "service_linked_role_arn": true,
          "tag": [],
          "target_group_arns": true,
          "vpc_zone_identifier": true
        }
      }
    },
    {
      "address": "aws_instance.web",
      "mode": "managed",
      "type": "aws_instance",
      "name": "web",
      "provider_name": "aws",
      "change": {
        "actions": [
          "create"
        ],
        "before": null,
        "after": {
          "ami": "ami-09b4b74c",
          "credit_specification": [],
          "disable_api_termination": null,
          "ebs_optimized": null,
          "get_password_data": false,
          "iam_instance_profile": null,
          "instance_initiated_shutdown_behavior": null,
          "instance_type": "t2.micro",
          "monitoring": null,
          "source_dest_check": true,
          "tags": null,
          "timeouts": null,
          "user_data": null,
          "user_data_base64": null
        },
        "after_unknown": {
          "arn": true,
          "associate_public_ip_address": true,
          "availability_zone": true,
          "cpu_core_count": true,
          "cpu_threads_per_core": true,
          "credit_specification": [],
          "ebs_block_device": true,
          "ephemeral_block_device": true,
          "host_id": true,
          "id": true,
          "instance_state": true,
          "ipv6_address_count": true,
          "ipv6_addresses": true,
          "key_name": true,
          "network_interface": true,
          "network_interface_id": true,
          "password_data": true,
          "placement_group": true,
          "primary_network_interface_id": true,
          "private_dns": true,
          "private_ip": true,
          "public_dns": true,
          "public_ip": true,
          "root_block_device": true,
          "security_groups": true,
          "subnet_id": true,
          "tenancy": true,
          "volume_tags": true,
          "vpc_security_group_ids": true
        }
      }
    },
    {
      "address": "aws_launch_configuration.my_web_config",
      "mode": "managed",
      "type": "aws_launch_configuration",
      "name": "my_web_config",
      "provider_name": "aws",
      "change": {
        "actions": [
          "create"
        ],
        "before": null,
        "after": {
          "associate_public_ip_address": false,
          "enable_monitoring": true,
          "ephemeral_block_device": [],
          "iam_instance_profile": null,
          "image_id": "ami-09b4b74c",
          "instance_type": "t2.micro",
          "name": "my_web_config",
          "name_prefix": null,
          "placement_tenancy": null,
          "security_groups": null,
          "spot_price": null,
          "user_data": null,
          "user_data_base64": null,
          "vpc_classic_link_id": null,
          "vpc_classic_link_security_groups": null
        },
        "after_unknown": {
          "ebs_block_device": true,
          "ebs_optimized": true,
          "ephemeral_block_device": [],
          "id": true,
          "key_name": true,
          "root_block_device": true
        }
      }
    }
  ],
  "configuration": {
    "provider_config": {
      "aws": {
        "name": "aws",
        "expressions": {
          "region": {
            "constant_value": "us-west-1"
          }
        }
      }
    },
    "root_module": {
      "resources": [
        {
          "address": "aws_autoscaling_group.my_asg",
          "mode": "managed",
          "type": "aws_autoscaling_group",
          "name": "my_asg",
          "provider_config_key": "aws",
          "expressions": {
            "availability_zones": {
              "constant_value": [
                "us-west-1a"
              ]
            },
            "desired_capacity": {
              "constant_value": 4
            },
            "force_delete": {
              "constant_value": true
            },
            "health_check_grace_period": {
              "constant_value": 300
            },
            "health_check_type": {
              "constant_value": "ELB"
            },
            "launch_configuration": {
              "constant_value": "my_web_config"
            },
            "max_size": {
              "constant_value": 5
            },
            "min_size": {
              "constant_value": 1
            },
            "name": {
              "constant_value": "my_asg"
            }
          },
          "schema_version": 0
        },
        {
          "address": "aws_instance.web",
          "mode": "managed",
          "type": "aws_instance",
          "name": "web",
          "provider_config_key": "aws",
          "expressions": {
            "ami": {
              "constant_value": "ami-09b4b74c"
            },
            "instance_type": {
              "constant_value": "t2.micro"
            }
          },
          "schema_version": 1
        },
        {
          "address": "aws_launch_configuration.my_web_config",
          "mode": "managed",
          "type": "aws_launch_configuration",
          "name": "my_web_config",
          "provider_config_key": "aws",
          "expressions": {
            "image_id": {
              "constant_value": "ami-09b4b74c"
            },
            "instance_type": {
              "constant_value": "t2.micro"
            },
            "name": {
              "constant_value": "my_web_config"
            }
          },
          "schema_version": 0
        }
      ]
    }
  }
}

The json plan output produced by terraform contains a lot of information. For this tutorial, we will be interested by:

.resource_changes: array containing all the actions that terraform will apply on the infrastructure.
.resource_changes[].type: the type of resource (eg aws_instance , aws_iam …)
.resource_changes[].change.actions: array of actions applied on the resource (create, update, delete…)

For more information about the json plan representation, please check the terraform documentation

3. Write the OPA policy to check the plan

The policy computes a score for a Terraform that combines

The number of deletions of each resource type
The number of creations of each resource type
The number of modifications of each resource type

The policy authorizes the plan when the score for the plan is below a threshold and there are no changes made to any IAM resources. (For simplicity, the threshold in this tutorial is the same for everyone, but in practice you would vary the threshold depending on the user.)

policy/terraform.rego:

package terraform.analysis

import input as tfplan

########################
# Parameters for Policy
########################

# acceptable score for automated authorization
blast_radius := 30

# weights assigned for each operation on each resource-type
weights := {
    "aws_autoscaling_group": {"delete": 100, "create": 10, "modify": 1},
    "aws_instance": {"delete": 10, "create": 1, "modify": 1}
}

# Consider exactly these resource types in calculations
resource_types := {"aws_autoscaling_group", "aws_instance", "aws_iam", "aws_launch_configuration"}

#########
# Policy
#########

# Authorization holds if score for the plan is acceptable and no changes are made to IAM
default authz := false
authz {
    score < blast_radius
    not touches_iam
}

# Compute the score for a Terraform plan as the weighted sum of deletions, creations, modifications
score := s {
    all := [ x |
            some resource_type
            crud := weights[resource_type];
            del := crud["delete"] * num_deletes[resource_type];
            new := crud["create"] * num_creates[resource_type];
            mod := crud["modify"] * num_modifies[resource_type];
            x := del + new + mod
    ]
    s := sum(all)
}

# Whether there is any change to IAM
touches_iam {
    all := resources["aws_iam"]
    count(all) > 0
}

####################
# Terraform Library
####################

# list of all resources of a given type
resources[resource_type] := all {
    some resource_type
    resource_types[resource_type]
    all := [name |
        name:= tfplan.resource_changes[_]
        name.type == resource_type
    ]
}

# number of creations of resources of a given type
num_creates[resource_type] := num {
    some resource_type
    resource_types[resource_type]
    all := resources[resource_type]
    creates := [res |  res:= all[_]; res.change.actions[_] == "create"]
    num := count(creates)
}


# number of deletions of resources of a given type
num_deletes[resource_type] := num {
    some resource_type
    resource_types[resource_type]
    all := resources[resource_type]
    deletions := [res |  res:= all[_]; res.change.actions[_] == "delete"]
    num := count(deletions)
}

# number of modifications to resources of a given type
num_modifies[resource_type] := num {
    some resource_type
    resource_types[resource_type]
    all := resources[resource_type]
    modifies := [res |  res:= all[_]; res.change.actions[_] == "update"]
    num := count(modifies)
}

4. Evaluate the OPA policy on the Terraform plan

To evaluate the policy against that plan, you hand OPA the policy, the Terraform plan as input, and ask it to evaluate terraform/analysis/authz.

opa exec --decision terraform/analysis/authz --bundle policy/ tfplan.json

data.terraform.analysis.authz

true

If you’re curious, you can ask for the score that the policy used to make the authorization decision. In our example, it is 11 (10 for the creation of the auto-scaling group and 1 for the creation of the server).

opa exec --decision terraform/analysis/score --bundle policy/ tfplan.json

data.terraform.analysis.score

If as suggested in the previous step, you want to modify your policy to make an authorization decision based on both the user and the Terraform plan, the input you would give to OPA would take the form {"user": <user>, "plan": <plan>}, and your policy would reference the user with input.user and the plan with input.plan. You could even go so far as to provide the Terraform state file and the AWS EC2 data to OPA and write policy using all of that context.

5. Create a Large Terraform plan and Evaluate it

Create a Terraform plan that creates enough resources to exceed the blast-radius permitted by policy.

cat >main.tf <<EOF
provider "aws" {
    region = "us-west-1"
}
resource "aws_instance" "web" {
  instance_type = "t2.micro"
  ami = "ami-09b4b74c"
}
resource "aws_autoscaling_group" "my_asg" {
  availability_zones        = ["us-west-1a"]
  name                      = "my_asg"
  max_size                  = 5
  min_size                  = 1
  health_check_grace_period = 300
  health_check_type         = "ELB"
  desired_capacity          = 4
  force_delete              = true
  launch_configuration      = "my_web_config"
}
resource "aws_launch_configuration" "my_web_config" {
    name = "my_web_config"
    image_id = "ami-09b4b74c"
    instance_type = "t2.micro"
}
resource "aws_autoscaling_group" "my_asg2" {
  availability_zones        = ["us-west-2a"]
  name                      = "my_asg2"
  max_size                  = 6
  min_size                  = 1
  health_check_grace_period = 300
  health_check_type         = "ELB"
  desired_capacity          = 4
  force_delete              = true
  launch_configuration      = "my_web_config"
}
resource "aws_autoscaling_group" "my_asg3" {
  availability_zones        = ["us-west-2b"]
  name                      = "my_asg3"
  max_size                  = 7
  min_size                  = 1
  health_check_grace_period = 300
  health_check_type         = "ELB"
  desired_capacity          = 4
  force_delete              = true
  launch_configuration      = "my_web_config"
}
EOF

Generate the Terraform plan and convert it to JSON.

terraform init
terraform plan --out tfplan_large.binary
terraform show -json tfplan_large.binary > tfplan_large.json

Evaluate the policy to see that it fails the policy tests and check the score.

opa exec --decision terraform/analysis/authz --bundle policy/ tfplan_large.json
opa exec --decision terraform/analysis/score --bundle policy/ tfplan_large.json

6. (Optional) Run OPA using a remote policy bundle

In addition to loading policies from the local filesystem, opa exec can fetch policies from remote locations via Bundles. To see this in action, first build the policies into a bundle:

opa build policy/

Next, serve the bundle via nginx:

docker run --rm --name bundle_server -d -p 8888:80 -v ${PWD}:/usr/share/nginx/html:ro nginx:latest

Then run opa exec with bundles enabled:

opa exec --decision terraform/analysis/authz \
  --set services.bundle_server.url=http://localhost:8888 \
  --set bundles.tutorial.resource=bundle.tar.gz \
  tfplan_large.json

Wrap Up

Congratulations for finishing the tutorial!

You learned a number of things about Terraform Testing with OPA:

OPA gives you fine-grained policy control over Terraform plans.
You can use data other than the plan itself (e.g. the user) when writing authorization policies.

Keep in mind that it’s up to you to decide how to use OPA’s Terraform tests and authorization decision. Here are some ideas.

Add it as part of your Terraform wrapper to implement unit tests on Terraform plans
Use it to automatically approve run-of-the-mill Terraform changes to reduce the burden of peer-review
Embed it into your deployment system to catch problems that arise when applying Terraform to production after applying it to staging

If you’d like to explore an additional example that uses terraform modules please continue below.

Working with Modules

Module Steps

1. Create and save Terraform module plan

Create a new Terraform file that includes a security group and security group from a module. (This example uses the module from https://github.com/terraform-aws-modules/terraform-aws-security-group)

cat >main.tf <<EOF
provider "aws" {
  region = "us-east-1"
}

data "aws_vpc" "default" {
  default = true
}

module "http_sg" {
  source = "git::https://github.com/terraform-aws-modules/terraform-aws-security-group.git?ref=v3.10.0"

  name        = "http-sg"
  description = "Security group with HTTP ports open for everybody (IPv4 CIDR), egress ports are all world open"
  vpc_id      = data.aws_vpc.default.id

  ingress_cidr_blocks = ["0.0.0.0/0"]
}


resource "aws_security_group" "allow_tls" {
  name        = "allow_tls"
  description = "Allow TLS inbound traffic"
  vpc_id      = data.aws_vpc.default.id

  ingress {
    description = "TLS from VPC"
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = ["10.0.0.0/8"]
  }

  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }

  tags = {
    Name = "allow_tls"
  }
}
EOF

Then initialize Terraform and ask it to calculate what changes it will make and store the output in tfplan.binary.

terraform init
terraform plan --out tfplan.binary

2. Convert the new Terraform plan into JSON

Use the Terraform show command to produce the json representation of the terraform plan

terraform show -json tfplan.binary > tfplan2.json

3. Write the OPA policy to collect resources

The policy evaluates if a security group is valid based on the contents of it’s description:

Resources can be specified under the root module or in child modules
We want to evaluate against the combined group of these resources
This example is scoped to the planned changes section of the json representation

The policy uses the walk keyword to explore the json structure, and uses conditions to filter for the specific paths where resources would be found.

policy/terraform_module.rego:

package terraform.module

deny[msg] {
    desc := resources[r].values.description
    contains(desc, "HTTP")
    msg := sprintf("No security groups should be using HTTP. Resource in violation: %v", [r.address])
}

resources := { r |
    some path, value

    # Walk over the JSON tree and check if the node we are
    # currently on is a module (either root or child) resources
    # value.
    walk(input.planned_values, [path, value])

    # Look for resources in the current value based on path
    rs := module_resources(path, value)

    # Aggregate them into `resources`
    r := rs[_]
}

# Variant to match root_module resources
module_resources(path, value) := rs {
    # Expect something like:
    #
    #     {
    #     	"root_module": {
    #         	"resources": [...],
    #             ...
    #         }
    #         ...
    #     }
    #
    # Where the path is [..., "root_module", "resources"]

    reverse_index(path, 1) == "resources"
    reverse_index(path, 2) == "root_module"
    rs := value
}

# Variant to match child_modules resources
module_resources(path, value) := rs {
    # Expect something like:
    #
    #     {
    #     	...
    #         "child_modules": [
    #         	{
    #             	"resources": [...],
    #                 ...
    #             },
    #             ...
    #         ]
    #         ...
    #     }
    #
    # Where the path is [..., "child_modules", 0, "resources"]
    # Note that there will always be an index int between `child_modules`
    # and `resources`. We know that walk will only visit each one once,
    # so we shouldn't need to keep track of what the index is.

    reverse_index(path, 1) == "resources"
    reverse_index(path, 3) == "child_modules"
    rs := value
}

reverse_index(path, idx) := value {
	value := path[count(path) - idx]
}

4. Evaluate the OPA policy on the Terraform module plan

To evaluate the policy against that plan, you hand OPA the policy, the Terraform plan as input, and ask it to evaluate data.terraform.module.deny.

opa exec --decision terraform/module/deny --bundle policy/ tfplan2.json

This should return one of the two resources. The security group created by the module uses HTTP in its description and therefore fails the evaluation.

{
  "result": [
    {
      "path": "tfplan2.json",
      "result": [
        "No security groups should be using HTTP. Resource in violation: module.http_sg.aws_security_group.this_name_prefix[0]"
      ]
    }
  ]
}

Module Wrap Up

Congratulations on finishing the tutorial!

You learned OPA can be used to determine if a proposed configuration is authorized.

Additional use cases might include:

Ensuring all resources have tags before they are created
Making sure naming standards for resources are followed
Security or operational requirements

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