policy-as-code-runner

policy-as-code-runner

Overview

Conftest is a CLI that uses OPA (opa-docs) to test structured configuration files (Kubernetes manifests, Terraform plans, Dockerfiles, etc.) against Rego policies. This skill covers Conftest invocation + Rego policy authoring as the custom policy layer alongside Checkov / tfsec / KICS (which carry built-in checks).

When to use

• The team has custom policies that off-the-shelf scanners (Checkov / tfsec / KICS) don't cover (e.g., "all production resources must have a cost-center tag").
• A Kubernetes admission policy is needed (OPA Gatekeeper).
• Dockerfiles need policy review (no :latest tags, no apt-get upgrade, etc.).
• A multi-language IaC stack needs unified policy enforcement.

Step 1 - Install Conftest

# macOS
brew install conftest

# Or download binary
curl -LO https://github.com/open-policy-agent/conftest/releases/latest/download/conftest_<version>_Linux_x86_64.tar.gz

Step 2 - Author a policy in Rego

# policies/kubernetes/required_labels.rego
package main

# All pods must have these labels
required_labels := {"app", "version", "owner"}

deny[msg] {
    input.kind == "Deployment"
    label := required_labels[_]
    not input.spec.template.metadata.labels[label]
    msg := sprintf("Deployment '%s' missing required label '%s'", [input.metadata.name, label])
}

# All deployments must specify resource limits
deny[msg] {
    input.kind == "Deployment"
    container := input.spec.template.spec.containers[_]
    not container.resources.limits.memory
    msg := sprintf("Container '%s' in deployment '%s' missing memory limit", [container.name, input.metadata.name])
}

deny[msg] {
    input.kind == "Deployment"
    container := input.spec.template.spec.containers[_]
    not container.resources.limits.cpu
    msg := sprintf("Container '%s' in deployment '%s' missing CPU limit", [container.name, input.metadata.name])
}

The deny rule pattern: each deny[msg] rule that holds adds a message to the deny set. If deny is non-empty, the test fails.

Step 3 - Run Conftest

# Test a single manifest
conftest test deployment.yaml

# Test all manifests
conftest test manifests/

# With specific policy directory
conftest test deployment.yaml --policy ./policies/

# JSON output for CI parsing
conftest test deployment.yaml --output json

Step 4 - Test against Helm chart output

helm template myrelease charts/mychart/ -f values.yaml | conftest test -

The chart renders to manifests; Conftest validates them.

Step 5 - Test against Terraform plan

terraform plan -out=plan.tfplan
terraform show -json plan.tfplan > plan.json
conftest test plan.json --policy policies/terraform/

# policies/terraform/cost_center_tag.rego
package main

deny[msg] {
    resource := input.resource_changes[_]
    resource.type == "aws_instance"
    not resource.change.after.tags.cost_center
    msg := sprintf("EC2 instance '%s' missing cost_center tag", [resource.address])
}

Step 6 - Test against Dockerfile

# policies/dockerfile/no_latest.rego
package main

deny[msg] {
    input[i].Cmd == "from"
    val := input[i].Value
    contains(val[0], ":latest")
    msg := sprintf("Avoid :latest tag in FROM (line %d)", [i])
}

deny[msg] {
    input[i].Cmd == "run"
    val := input[i].Value[0]
    contains(val, "apt-get upgrade")
    msg := sprintf("Avoid 'apt-get upgrade' (line %d) — pin specific package versions", [i])
}

conftest test Dockerfile

Step 7 - Author tests for policies

Policies themselves should be tested:

# policies/kubernetes/required_labels_test.rego
package main

test_deployment_missing_labels {
    deny[_] with input as {
        "kind": "Deployment",
        "metadata": {"name": "test"},
        "spec": {"template": {"metadata": {"labels": {}}}}
    }
}

test_deployment_with_all_labels {
    count(deny) == 0 with input as {
        "kind": "Deployment",
        "metadata": {"name": "test"},
        "spec": {"template": {"metadata": {"labels": {"app": "x", "version": "1.0", "owner": "team"}}}, "spec": {"containers": [{"resources": {"limits": {"memory": "512Mi", "cpu": "500m"}}}]}}
    }
}

opa test policies/

Step 8 - CI integration

jobs:
  policy:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v5
      - run: |
          curl -L https://github.com/open-policy-agent/conftest/releases/download/v0.50.0/conftest_0.50.0_Linux_x86_64.tar.gz | tar xz
          sudo mv conftest /usr/local/bin/
      - name: Test policies
        run: opa test policies/
      - name: Conftest manifests
        run: conftest test manifests/ --policy policies/
      - name: Conftest Helm
        run: helm template myrelease charts/mychart/ -f values.yaml | conftest test - --policy policies/
      - name: Conftest Terraform
        run: |
          terraform plan -out=plan.tfplan
          terraform show -json plan.tfplan > plan.json
          conftest test plan.json --policy policies/terraform/

Step 9 - Bundle policies for OPA Gatekeeper (Kubernetes admission)

For runtime enforcement (vs PR-time):

apiVersion: templates.gatekeeper.sh/v1
kind: ConstraintTemplate
metadata:
  name: requiredlabels
spec:
  crd:
    spec:
      names: { kind: RequiredLabels }
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        # Same Rego from Step 2

Cluster admins install Gatekeeper; the constraint blocks Deployments that violate the policy at admission time.

Anti-patterns

Anti-pattern	Why it fails	Fix
Untested policies	Policy bugs let bad config through; over-strict policies block good config.	opa test against test fixtures (Step 7).
Policies in production without PR-time gating	Surprises at admission time; deployments fail.	CI gate first (Step 8); Gatekeeper second.
One mega-policy file	Hard to reason about; merge conflicts.	Per-domain policy files.
Rego complexity for simple checks	Hard to maintain; Rego learning curve.	Use Checkov / tfsec for built-in checks; OPA for custom.
Skipping the negation tests	Policy may pass for the wrong reason.	Test both deny and not-deny cases (Step 7).

Limitations

• Rego learning curve. New language; per opa-docs it's "purpose-built" but takes time to master.
• Per-tool integration varies. Conftest wraps OPA cleanly; some tools have native OPA integration, others don't.
• Policy proliferation. Like step libraries (per bdd-step-library-curator), policies can multiply; periodic review needed.

References

• opa - OPA overview, Rego language, common applications, CNCF graduated status.
• Conftest at conftest.dev.
• OPA Gatekeeper at open-policy-agent.github.io/gatekeeper/.
• checkov-policy, tfsec-policy, kics-policy - sister scanners with built-in checks (use these first; OPA for custom).