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Implementing policy as code with Terraform

Learn about Implementing policy as code with Terraform as part of Terraform Infrastructure as Code Mastery

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Implementing Policy as Code with Terraform

Policy as Code (PaC) is a crucial practice for ensuring compliance, security, and consistency in your infrastructure deployments. When combined with Terraform, it allows you to define and enforce organizational policies directly within your Infrastructure as Code (IaC) workflows. This module explores how to implement PaC with Terraform, focusing on best practices and advanced techniques.

What is Policy as Code?

Policy as Code treats policies—rules, standards, and compliance requirements—as executable code. This means policies are version-controlled, tested, and automated, just like application code. By codifying policies, organizations can proactively prevent misconfigurations and ensure adherence to security and governance standards before infrastructure is provisioned.

Policy as Code automates compliance and security checks within IaC.

Instead of manual reviews, policies are written as code and integrated into the CI/CD pipeline. This ensures that infrastructure changes are compliant by default.

The core benefit of Policy as Code is the shift from reactive, manual compliance checks to proactive, automated enforcement. This integration into the IaC workflow means that any proposed infrastructure change is evaluated against predefined policies. If a change violates a policy, it can be automatically flagged, rejected, or remediated, preventing non-compliant infrastructure from being deployed.

Terraform and Policy Enforcement

Terraform itself provides mechanisms for enforcing policies. The most prominent tool for this is Sentinel, HashiCorp's policy-as-code framework. Sentinel allows you to write policies in a declarative language that can be integrated into Terraform runs.

Sentinel: The Policy as Code Framework

Sentinel policies are written in the Sentinel policy language. They are typically executed during the Terraform plan phase, allowing you to review and validate proposed changes before they are applied. Sentinel policies can enforce a wide range of rules, from security configurations to cost controls and naming conventions.

Sentinel policies run during the terraform plan phase, acting as a gatekeeper before any infrastructure changes are made.

Key Concepts in Sentinel

Sentinel policies are structured around rules that evaluate conditions based on the Terraform plan. These policies can be categorized into 'soft mandatory' (warnings) and 'hard mandatory' (blocking). The framework uses 'imports' to bring in data about the Terraform configuration and state.

What is the primary phase in a Terraform workflow where Sentinel policies are typically executed?

The terraform plan phase.

Writing Sentinel Policies

Sentinel policies are written in

code
.sentinel
files. They consist of imports, rules, and potentially functions. A common pattern is to define a
code
main
rule that orchestrates the evaluation of other rules. Policies can be organized into policy sets for better management.

A typical Sentinel policy structure involves importing data from the Terraform plan, defining rules that check specific attributes of resources (e.g., ensuring encryption is enabled for S3 buckets), and specifying whether violations should be warnings or errors. The main rule acts as the entry point for policy evaluation.

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Text-based content

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Example Sentinel Policy: Enforcing Encryption

Consider a policy to ensure all S3 buckets have server-side encryption enabled. This policy would inspect the

code
aws_s3_bucket
resource and check the
code
server_side_encryption_configuration
block.

Integrating Sentinel with Terraform

Sentinel integration is typically achieved through Terraform Cloud or Terraform Enterprise. These platforms provide built-in support for Sentinel policy checks. For self-hosted Terraform, you can use the Sentinel CLI or integrate it into your CI/CD pipelines.

Best Practices for Policy as Code with Terraform

To effectively implement Policy as Code, consider these best practices:

  • Start Simple: Begin with a few critical policies (e.g., security, compliance) and gradually expand.
  • Version Control Policies: Store your Sentinel policies in a separate Git repository for tracking and collaboration.
  • Test Policies Thoroughly: Write unit tests for your Sentinel policies to ensure they behave as expected.
  • Use Clear Policy Names and Descriptions: Make it easy for users to understand why a policy is failing.
  • Implement a Gradual Rollout: Start with 'soft mandatory' policies to alert users, then move to 'hard mandatory' once policies are well-understood and adopted.
  • Integrate into CI/CD: Ensure policies are checked automatically as part of your deployment pipeline.

Advanced Topics and Alternatives

Beyond Sentinel, other tools and approaches can be used for Policy as Code with Terraform. Open Policy Agent (OPA) is a popular general-purpose policy engine that can also be integrated with Terraform. Understanding these alternatives can provide flexibility in choosing the best solution for your organization.

FeatureSentinelOpen Policy Agent (OPA)
Primary Use CaseTerraform-specific policy enforcementGeneral-purpose policy engine for various domains
Policy LanguageSentinel Policy LanguageRego
Integration with TerraformNative (Terraform Cloud/Enterprise)Via Terraform provider or external tools
Learning CurveModerate (Terraform-focused)Moderate to High (Broader scope)

Mastering Policy as Code with Terraform is essential for building robust, secure, and compliant cloud infrastructure. By leveraging tools like Sentinel, you can automate governance and ensure that your infrastructure adheres to organizational standards.

Learning Resources

HashiCorp Sentinel Documentation(documentation)

Official documentation for HashiCorp Sentinel, covering its concepts, language, and integration with Terraform Cloud/Enterprise.

Sentinel Policy Language Overview(documentation)

A deep dive into the Sentinel policy language, explaining its syntax, rules, and capabilities for defining policies.

Writing Your First Sentinel Policy(tutorial)

A hands-on tutorial to guide you through writing and testing your initial Sentinel policies.

Terraform Cloud Policy Enforcement(documentation)

Explains how to configure and run Sentinel policies within the Terraform Cloud environment.

Open Policy Agent (OPA) Documentation(documentation)

Comprehensive documentation for Open Policy Agent, a versatile policy engine that can be used with Terraform.

Using OPA with Terraform(documentation)

Guidance on integrating Open Policy Agent with Terraform for policy enforcement.

Sentinel Best Practices(blog)

Insights and recommendations for effectively implementing and managing Sentinel policies.

Policy as Code: The Future of Infrastructure Management(blog)

A blog post discussing the importance and benefits of Policy as Code in modern infrastructure operations.

Sentinel Policy Examples(documentation)

A repository of example Sentinel policies for various use cases, providing practical templates.

Introduction to Policy as Code(video)

A video explaining the fundamental concepts of Policy as Code and its significance in DevOps.