Terraform Policy as Code with Sentinel
Welcome to this comprehensive, student-friendly guide on Terraform Policy as Code with Sentinel! 🎉 Whether you’re a beginner or have some experience with Terraform, this tutorial will help you understand how to use Sentinel to enforce policies in your infrastructure as code. Don’t worry if this seems complex at first; we’ll break it down step by step. Let’s dive in! 🚀
What You’ll Learn 📚
- Understanding the basics of Terraform and Sentinel
- Key terminology and concepts
- How to write and apply simple to complex policies
- Troubleshooting common issues
Introduction to Terraform and Sentinel
Terraform is an open-source tool that allows you to define and provision infrastructure using a high-level configuration language. It’s like a blueprint for your cloud resources. Sentinel is a policy-as-code framework that enables you to enforce rules and policies on your Terraform configurations. Think of it as a security guard ensuring everything is in order before changes are applied. 🛡️
Key Terminology
- Policy as Code: Writing policies in a programming language to automate compliance and governance.
- Enforcement Level: Determines how strictly a policy is applied (e.g., advisory, soft-mandatory, hard-mandatory).
- Sentinel Policy: A set of rules written in Sentinel language to enforce specific conditions.
Getting Started with a Simple Example
Example 1: Basic Sentinel Policy
Let’s start with a simple policy that ensures all resources have tags. Tags are key-value pairs that help organize resources.
# terraform.hcl
policy "require-tags" {
rule "main" {
all tfplan.resources as _, r {
all r.instances as _, i {
i.attributes.tags is not null
}
}
}
}This policy checks if every resource in the Terraform plan has tags. If any resource is missing tags, the policy will fail.
Expected Output: If all resources have tags, the policy passes. Otherwise, it fails with a message indicating missing tags.
Progressively Complex Examples
Example 2: Enforcing Specific Tag Keys
Now, let’s enforce specific tag keys, like ‘Environment’ and ‘Owner’.
# terraform.hcl
policy "require-specific-tags" {
rule "main" {
all tfplan.resources as _, r {
all r.instances as _, i {
all ["Environment", "Owner"] as tag_key {
i.attributes.tags[tag_key] is not null
}
}
}
}
}This policy ensures that each resource has both ‘Environment’ and ‘Owner’ tags. If any tag is missing, the policy fails.
Expected Output: Policy passes if all resources have the required tags; otherwise, it fails.
Example 3: Restricting Resource Types
Let’s create a policy that restricts the creation of certain resource types, like ‘aws_instance’.
# terraform.hcl
policy "restrict-resource-types" {
rule "main" {
all tfplan.resources as type, _ {
type != "aws_instance"
}
}
}This policy prevents the creation of ‘aws_instance’ resources. If the plan includes such resources, the policy fails.
Expected Output: Policy passes if no ‘aws_instance’ resources are in the plan; otherwise, it fails.
Example 4: Combining Multiple Policies
Finally, let’s combine multiple policies to enforce both tag requirements and resource restrictions.
# terraform.hcl
policy "combined-policy" {
rule "tag-rule" {
all tfplan.resources as _, r {
all r.instances as _, i {
all ["Environment", "Owner"] as tag_key {
i.attributes.tags[tag_key] is not null
}
}
}
}
rule "resource-type-rule" {
all tfplan.resources as type, _ {
type != "aws_instance"
}
}
}This policy combines tag enforcement and resource type restrictions, ensuring compliance with both rules.
Expected Output: Policy passes if all conditions are met; otherwise, it fails.
Common Questions and Answers
- What is Sentinel, and why use it?
Sentinel is a policy-as-code framework that helps enforce rules and governance in your infrastructure code. It’s used to automate compliance and ensure best practices.
- How do I write a Sentinel policy?
Sentinel policies are written in HCL (HashiCorp Configuration Language) and define rules to enforce specific conditions on your Terraform configurations.
- Can I use Sentinel with other tools besides Terraform?
Yes, Sentinel can be used with other HashiCorp tools like Consul and Vault, but it’s most commonly associated with Terraform.
- What happens if a policy fails?
If a policy fails, the Terraform plan or apply operation is halted, preventing non-compliant changes from being made.
- How do I test my Sentinel policies?
You can test Sentinel policies using the Sentinel CLI, which allows you to simulate policy evaluations without applying changes.
Troubleshooting Common Issues
If your policy isn’t working as expected, check for syntax errors or incorrect logic in your rules. Use the Sentinel CLI to debug and test policies.
Remember, practice makes perfect! Keep experimenting with different policies and scenarios to get comfortable with Sentinel. You got this! 💪
Practice Exercises
- Create a policy that ensures all S3 buckets are private.
- Write a policy that restricts the use of a specific AWS region.
- Combine multiple policies to enforce both security and cost-saving measures.
For more information, check out the Sentinel documentation and Terraform documentation.