---
title: Terraform
---

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 check
the changes Terraform is about to make before it makes them. Such policy based checks help by:

- Giving individual developers a first pass review of their Terraform changes
- Option to auto-approve run-of-the-mill infrastructure changes and reduce the burden of
  peer-review
- Enforcing organizational rules around the changes to infrastructure state

Terraform is a popular integration case for OPA and there are already a number
of popular tools for running policy on HCL and plan JSON. Browse existing
<EcosystemFeatureLink feature="terraform"> tools using OPA and
Terraform</EcosystemFeatureLink> in the OPA Ecosystem.

:::info Policy Limitations with Terraform Plans

OPA policies operate on the JSON plan generated by Terraform, which has certain limitations. Some
information may not be available at plan time, particularly:

- **Unknown values**: Computed attributes that won't be determined until apply time ([Terraform docs](https://developer.hashicorp.com/terraform/language/expressions/references#values-not-yet-known))
- **Dynamic blocks**: Resource configurations that depend on runtime evaluation ([Terraform docs](https://developer.hashicorp.com/terraform/language/expressions/dynamic-blocks))
- **Function calls**: Results of Terraform functions that haven't been evaluated yet ([Terraform docs](https://developer.hashicorp.com/terraform/language/expressions/function-calls))

Before writing policies, confirm that the information you need is available in the plan JSON output.

:::

## 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

- [Terraform 0.12.6](https://releases.hashicorp.com/terraform/0.12.6/)
- [OPA](https://github.com/open-policy-agent/opa/releases)

(This tutorial _should_ also work with the
[latest version of Terraform](https://developer.hashicorp.com/terraform), but
it is untested. Contributions welcome!)

## Getting Started

### 1. Create and save a Terraform plan

Create a [Terraform](https://developer.hashicorp.com/terraform/docs) 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.)

```shell
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`.

```shell
terraform init
terraform plan --out tfplan.binary
```

### 2. Convert the Terraform plan into JSON

Use the command [terraform show](https://developer.hashicorp.com/terraform/cli/commands/show) to convert the Terraform plan into
JSON so that OPA can read the plan.

```shell
terraform show -json tfplan.binary > tfplan.json
```

Here is the expected contents of `tfplan.json`.

```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, the relevant fields are:

- `.resource_changes`: array containing all the actions that terraform will apply on the infrastructure.
- `.resource_changes[].type`: the type of resource (e.g. `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](https://www.terraform.io/docs/internals/json-format.html#plan-representation)

### 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**:

```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 if {
    score < blast_radius
    not touches_iam
}

# Compute the score for a Terraform plan as the weighted sum of deletions, creations, modifications
score := s if {
    all_resources := [x |
        some resource_type, crud in weights

        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_resources)
}

# Whether there is any change to IAM
touches_iam if {
    all_resources := resources.aws_iam
    count(all_resources) > 0
}

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

# list of all resources of a given type
resources[resource_type] := all_resources if {
    some resource_type, _ in resource_types

    all_resources := [name |
        some name in tfplan.resource_changes
        name.type == resource_type
    ]
}

# number of creations of resources of a given type
num_creates[resource_type] := num if {
    some resource_type, _ in resource_types

    all_resources := resources[resource_type]
    creates := [res |
        some res in all_resources
        "create" in res.change.actions
    ]
    num := count(creates)
}

# number of deletions of resources of a given type
num_deletes[resource_type] := num if {
    some resource_type, _ in resource_types

    all_resources := resources[resource_type]

    deletions := [res |
        some res in all_resources
        "delete" in res.change.actions
    ]
    num := count(deletions)
}

# number of modifications to resources of a given type
num_modifies[resource_type] := num if {
    some resource_type, _ in resource_types

    all_resources := resources[resource_type]

    modifies := [res |
        some res in all_resources
        "update" in res.change.actions
    ]
    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`.

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

```json
{
  "result": [
    {
      "path": "tfplan.json",
      "result": true
    }
  ]
}
```

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

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

```json
{
  "result": [
    {
      "path": "tfplan.json",
      "result": 11
    }
  ]
}
```

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.

```shell
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.

```shell
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.

```shell
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](./management-bundles). To see this in action, first build the policies into a bundle:

```shell
opa build policy/
```

Next, serve the bundle via Nginx:

```bash
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
```

## Summary

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

### 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
[terraform-aws-modules](https://github.com/terraform-aws-modules/terraform-aws-security-group))

```shell
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`.

```shell
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

```shell
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
- The policy evaluates 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**:

```rego
package terraform.module

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

resources contains r if {
    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
    some r in module_resources(path, value)
}

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

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

# Variant to match child_modules resources
module_resources(path, value) := value if {
    # 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"
}

reverse_index(path, idx) := 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`.

```shell
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.

```shell
{
  "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]"
      ]
    }
  ]
}
```

## Summary

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

## Ecosystem Projects

<EcosystemEmbed feature="terraform">
As further reading, you might be interested to review the Terraform integrations
from the OPA Ecosystem.
</EcosystemEmbed>
