Policy Engine¶

The policy engine enables custom policy enforcement using OPA (Open Policy Agent) and Rego.

Overview¶

The policy engine allows you to define and enforce organizational policies on your Terraform configurations using the powerful Rego policy language.

Usage¶

# Run policy checks
terratidy policy

# With custom policy directory
terratidy policy --policy-dir ./policies

# Show policy input (for debugging)
terratidy policy --show-input

Configuration¶

engines:
  policy:
    enabled: true
    config:
      policy_dirs:
        - ./policies
        - ~/.terratidy/policies
      policy_files:
        - ./custom-policy.rego

Writing Policies¶

Policies are written in Rego (v1 syntax) and evaluated against a JSON representation of your Terraform modules. TerraTidy uses OPA v1.15.0 with Rego v1, which requires the import rego.v1 statement and updated rule syntax.

Basic Policy Structure¶

package terraform

import rego.v1

# Deny rule - creates an error
deny contains msg if {
    some resource in input.resources
    resource.type == "aws_s3_bucket"
    not resource.versioning
    msg := {
        "msg": sprintf("S3 bucket %s must have versioning enabled", [resource.name]),
        "rule": "s3-versioning-required",
        "severity": "error",
        "file": resource._file
    }
}

# Warn rule - creates a warning
warn contains msg if {
    some resource in input.resources
    resource.type == "aws_instance"
    not resource.tags
    msg := {
        "msg": sprintf("EC2 instance %s should have tags", [resource.name]),
        "rule": "required-tags",
        "severity": "warning",
        "file": resource._file
    }
}

Key Rego v1 Syntax Changes¶

Add import rego.v1 at the top of every policy file
Use deny contains msg if { ... } instead of deny[msg] { ... }
Use some resource in input.resources instead of resource := input.resources[_]

Input Structure¶

The policy engine provides the following input structure:

{
  "resources": [...],
  "data": [...],
  "modules": [...],
  "variables": [...],
  "outputs": [...],
  "locals": [...],
  "providers": [...],
  "terraform": {...},
  "_files": [...]
}

Each resource/block includes:

type: The resource type (e.g., "aws_instance")
name: The resource name
_block_type: The HCL block type (resource, data, module, etc.)
_file: Source file path where the block is defined
_range: Location object with start_line, end_line, start_column, end_column
All attributes as key-value pairs (raw expression text)

The _range field is useful for precise error reporting in custom policies:

msg := {
    "msg": "Issue description",
    "rule": "my-rule",
    "severity": "error",
    "file": resource._file,
    "line": resource._range.start_line
}

Built-in Policies¶

TerraTidy includes several built-in policies:

Policy	Description
`required-terraform-block`	Terraform block must exist
`required-version`	`required_version` must be specified
`required-providers`	Providers must have version constraints
`no-public-ssh`	Security groups cannot allow SSH from 0.0.0.0/0
`no-public-s3`	S3 buckets cannot have public-read ACL
`no-public-rds`	RDS instances cannot be publicly accessible
`required-tags`	Resources should have tags
`module-version`	External modules should have version constraints

Example Policies¶

Require Encryption¶

package terraform

import rego.v1

deny contains msg if {
    some resource in input.resources
    resource.type == "aws_ebs_volume"
    resource.encrypted != "true"
    msg := {
        "msg": sprintf("EBS volume %s must be encrypted", [resource.name]),
        "rule": "ebs-encryption",
        "severity": "error",
        "file": resource._file
    }
}

Naming Convention¶

package terraform

import rego.v1

deny contains msg if {
    some resource in input.resources
    not regex.match("^[a-z][a-z0-9_]*$", resource.name)
    msg := {
        "msg": sprintf("Resource %s.%s must use snake_case naming", [resource.type, resource.name]),
        "rule": "naming-convention",
        "severity": "warning",
        "file": resource._file
    }
}

Cost Control¶

package terraform

import rego.v1

expensive_types := {"aws_instance", "aws_db_instance", "aws_elasticache_cluster"}

warn contains msg if {
    some resource in input.resources
    resource.type in expensive_types
    not resource.tags.CostCenter
    msg := {
        "msg": sprintf("%s %s should have a CostCenter tag", [resource.type, resource.name]),
        "rule": "cost-center-tag",
        "severity": "warning",
        "file": resource._file
    }
}

Debugging Policies¶

Use the --show-input flag to see the JSON input:

terratidy policy --show-input > input.json

Then test your policy with OPA directly:

opa eval --input input.json --data policies/ "data.terraform.deny"