Sunwinds/conftest

Write tests against structured configuration data using the Open Policy Agent Rego query language

Conftest

• Join us on the Open Policy Agent Slack in the #conftest channel

What

conftest is a utility to help you write tests against structured configuration data. For instance you could write tests for your Kubernetes configurations, or Tekton pipeline definitions, Terraform code, Serverless configs or any other structured data.

conftest relies on the Rego language from Open Policy Agent for writing the assertions. You can read more about Rego in How do I write policies in the Open Policy Agent documentation.

Usage

conftest allows you to write policies using Open Policy Agent/rego and apply them to one or more configuration files.

As of today conftest supports:

• YAML
• JSON
• INI
• TOML
• HCL
• CUE
• Dockerfile
• HCL2 (Experimental)

Policies by default should be placed in a directory called policy but this can be overridden.

For instance, save the following as policy/deployment.rego:

package main


deny[msg] {
  input.kind = "Deployment"
  not input.spec.template.spec.securityContext.runAsNonRoot = true
  msg = "Containers must not run as root"
}

deny[msg] {
  input.kind = "Deployment"
  not input.spec.selector.matchLabels.app
  msg = "Containers must provide app label for pod selectors"
}

By default Conftest looks for deny and warn rules in the main namespace. This can be altered by running --namespace or provided on the configuration file.

Assuming you have a Kubernetes deployment in deployment.yaml you can run conftest like so:

$ conftest test deployment.yaml
FAIL - deployment.yaml - Containers must not run as root
FAIL - deployment.yaml - Deployments are not allowed

conftest can also be used with stdin:

$ cat deployment.yaml | conftest test -
FAIL - Containers must not run as root
FAIL - Deployments are not allowed

Note that conftest isn't specific to Kubernetes. It will happily let you write tests for any configuration files.

--input flag

conftest normally detects input type with the file extension, but you can force to use a different one with --input or -i flag.

For the available parsers, take a look at: parsers. For instance:

$ conftest test -p examples/hcl2/policy examples/hcl2/terraform.tf -i hcl2
FAIL - examples/hcl2/terraform.tf - Application environment is should be `staging_environment`

--combine flag

Of note is the --combine flag that is sub flag for conftest test, ala conftest test --combine. This flag introduces BREAKING CHANGES in how conftest provides input to rego policies. However, you may find it useful to as you can now compare multiple values from different configurations simultaneously.

Let's try it

Save the following as policy/combine.rego:

package main


deny[msg] {
  input.kind = "Deployment"
  deployment := input["deployment.yaml"]["spec"]["selector"]["matchLabels"]["app"]
  service := input["service.yaml"]["spec"]["selector"]["app"]
  
  deployment != service

  msg = sprintf("Expected these values to be the same but received %v for deployment and %v for service, [deployment, service])
}

Then make a file in your root as service_two.yaml with the values below

apiVersion: v1
kind: Service
metadata:
  name: hello-kubernetes
spec:
  type: LoadBalancer
  ports:
  - port: 80
    targetPort: 8080
  selector:
    app: goodbye-kubernetes

The --combine flag combines files into one input data structure. The structure is a map where each indice is the file path of the file being evaluated.

Assuming you have a Kubernetes deployment in deployment.yaml and a kubernetes service definition in service.yaml you can run conftest like so:

$ conftest test --combine deployment.yaml service.yaml

FAIL - Combined-configs (multi-file) - Expected these values to be the same but received hello-kubernetes for deployment and goodbye-kubernetes for service

This is just the tip of the iceberg. Now you can ensure that duplicate values match across the entirety of your configuration files.

Configuring Output

The output of conftest can be configured using the --output flag (-o).

As of today conftest supports the following output types:

• Plaintext --output=stdout
• JSON: --output=json
• TAP: --output=tap

Example Output

Plaintext

$ conftest test -p examples/kubernetes/policy examples/kubernetes/deployment.yaml 
FAIL - examples/kubernetes/deployment.yaml - Containers must not run as root in Deployment hello-kubernetes
FAIL - examples/kubernetes/deployment.yaml - Deployment hello-kubernetes must provide app/release labels for pod selectors
FAIL - examples/kubernetes/deployment.yaml - hello-kubernetes must include Kubernetes recommended labels: https://kubernetes.io/docs/concepts/overview/working-with-objects/common-labels/#labels 

JSON

$ conftest test -o json -p examples/kubernetes/policy examples/kubernetes/deployment.yaml
[
        {
                "filename": "examples/kubernetes/deployment.yaml",
                "warnings": [],
                "failures": [
                        "hello-kubernetes must include Kubernetes recommended labels: https://kubernetes.io/docs/concepts/overview/working-with-objects/common-labels/#labels ",
                        "Containers must not run as root in Deployment hello-kubernetes",
                        "Deployment hello-kubernetes must provide app/release labels for pod selectors"
                ]
        }
]

TAP

$ conftest test -o tap -p examples/kubernetes/policy examples/kubernetes/deployment.yaml 
1..3
not ok 1 - examples/kubernetes/deployment.yaml - Containers must not run as root in Deployment hello-kubernetes
not ok 2 - examples/kubernetes/deployment.yaml - Deployment hello-kubernetes must provide app/release labels for pod selectors
not ok 3 - examples/kubernetes/deployment.yaml - hello-kubernetes must include Kubernetes recommended labels: https://kubernetes.io/docs/concepts/overview/working-with-objects/common-labels/#labels 

Examples

You can find examples using various other tools in the examples directory, including:

• CUE
• Kustomize
• Terraform
• Serverless Framework
• AWS SAM Framework
• INI
• TOML
• Dockerfile
• HCL2

Configuration and external policies

Policies are often reusable between different projects, and Conftest supports a mechanism to specify dependent policies and to download them. The format reuses the Bundle defined by Open Policy Agent.

You can download individual policies directly:

conftest pull instrumenta.azurecr.io/test

Policies are stored in OCI-compatible registries. You can read more about this idea in this post.

If you have a compatible OCI registry you can also push new policy bundles like so:

conftest push instrumenta.azurecr.io/test

Conftest also supports a simple configuration file which can be used to store the list of dependent bundles and download them in one go. Create a conftest.toml configuration file like the following:

# You can override the directory in which to store and look for policies
policy = "tests"

# You can override the namespace which to search for rules
namespace = "conftest"

# An array of individual policies to download. Only the repository
# key is required. If tag is omitted then latest will be used
[[policies]]
repository = "instrumenta.azurecr.io/test"
tag = "latest"

With that in place you can use the following command to download all specified policies:

conftest update

If you want to download the latest policies and run the tests in one go you can do so with:

conftest test --update <file-to-test>

Debugging queries

When working on more complex queries, or when learning rego, it's useful to see exactly how the policy is applied. For this purpose you can use the --trace flag. This will output a large trace from Open Policy Agent like the following:

Example of trace

$ conftest test --trace deployment.yaml
Enter data.main.deny = _
| Eval data.main.deny = _
| Index data.main.deny = _ (matched 2 rules)
| Enter deny[msg] { data.kubernetes.is_deployment; not input.spec.template.spec.securityContext.runAsNonRoot = true; __local3__ = data.main.name; sprintf("Containers must not run as root in Deployment %s", [__local3__], __local0__); msg = __local0__ }
| | Eval data.kubernetes.is_deployment
| | Index data.kubernetes.is_deployment (matched 1 rule)
| | Enter is_deployment = true { input.kind = "Deployment" }
| | | Eval input.kind = "Deployment"
| | | Exit is_deployment = true { input.kind = "Deployment" }
| | Eval not input.spec.template.spec.securityContext.runAsNonRoot = true
| | | Eval input.spec.template.spec.securityContext.runAsNonRoot = true
| | | Fail input.spec.template.spec.securityContext.runAsNonRoot = true
| | Eval __local3__ = data.main.name
| | Index __local3__ = data.main.name (matched 2 rules)
| | Enter name = __local1__ { true; __local1__ = input.metadata.name }
| | | Eval true
| | | Eval __local1__ = input.metadata.name
| | | Exit name = __local1__ { true; __local1__ = input.metadata.name }
| | Eval sprintf("Containers must not run as root in Deployment %s", [__local3__], __local0__)
| | Eval msg = __local0__
| | Exit deny[msg] { data.kubernetes.is_deployment; not input.spec.template.spec.securityContext.runAsNonRoot = true; __local3__ = data.main.name; sprintf("Containers must not run as root in Deployment %s", [__local3__], __local0__); msg = __local0__ }
| Redo deny[msg] { data.kubernetes.is_deployment; not input.spec.template.spec.securityContext.runAsNonRoot = true; __local3__ = data.main.name; sprintf("Containers must not run as root in Deployment %s", [__local3__], __local0__); msg = __local0__ }
| | Redo msg = __local0__
| | Redo sprintf("Containers must not run as root in Deployment %s", [__local3__], __local0__)
| | Redo __local3__ = data.main.name
| | Redo name = __local1__ { true; __local1__ = input.metadata.name }
| | | Redo __local1__ = input.metadata.name
| | | Redo true
| | Enter name = __local2__ { true; __local2__ = input.metadata.name }
| | | Eval true
| | | Eval __local2__ = input.metadata.name
| | | Exit name = __local2__ { true; __local2__ = input.metadata.name }
| | Redo name = __local2__ { true; __local2__ = input.metadata.name }
| | | Redo __local2__ = input.metadata.name
| | | Redo true
| | Redo data.kubernetes.is_deployment
| | Redo is_deployment = true { input.kind = "Deployment" }
| | | Redo input.kind = "Deployment"
| Enter deny[msg] { data.kubernetes.is_deployment; not data.main.labels; __local4__ = data.main.name; sprintf("Deployment %s must provide app/release labels for pod selectors", [__local4__], __local1__); msg = __local1__ }
| | Eval data.kubernetes.is_deployment
| | Index data.kubernetes.is_deployment (matched 1 rule)
| | Eval not data.main.labels
| | | Eval data.main.labels
| | | Index data.main.labels (matched 1 rule)
| | | Enter labels = true { input.spec.selector.matchLabels.app; input.spec.selector.matchLabels.release }
| | | | Eval input.spec.selector.matchLabels.app
| | | | Eval input.spec.selector.matchLabels.release
| | | | Fail input.spec.selector.matchLabels.release
| | | | Redo input.spec.selector.matchLabels.app
| | | Fail data.main.labels
| | Eval __local4__ = data.main.name
| | Index __local4__ = data.main.name (matched 2 rules)
| | Eval sprintf("Deployment %s must provide app/release labels for pod selectors", [__local4__], __local1__)
| | Eval msg = __local1__
| | Exit deny[msg] { data.kubernetes.is_deployment; not data.main.labels; __local4__ = data.main.name; sprintf("Deployment %s must provide app/release labels for pod selectors", [__local4__], __local1__); msg = __local1__ }
| Redo deny[msg] { data.kubernetes.is_deployment; not data.main.labels; __local4__ = data.main.name; sprintf("Deployment %s must provide app/release labels for pod selectors", [__local4__], __local1__); msg = __local1__ }
| | Redo msg = __local1__
| | Redo sprintf("Deployment %s must provide app/release labels for pod selectors", [__local4__], __local1__)
| | Redo __local4__ = data.main.name
| | Redo data.kubernetes.is_deployment
| Exit data.main.deny = _
Redo data.main.deny = _
| Redo data.main.deny = _
Enter data.main.warn = _
| Eval data.main.warn = _
| Index data.main.warn = _ (matched 1 rule)
| Enter warn[msg] { data.kubernetes.is_service; __local2__ = data.main.name; sprintf("Found service %s but services are not allowed", [__local2__], __local0__); msg = __local0__ }
| | Eval data.kubernetes.is_service
| | Index data.kubernetes.is_service (matched 0 rules)
| | Fail data.kubernetes.is_service
| Exit data.main.warn = _
Redo data.main.warn = _
| Redo data.main.warn = _
FAIL - deployment.yaml - Containers must not run as root in Deployment hello-kubernetes
FAIL - deployment.yaml - Deployment hello-kubernetes must provide app/release labels for pod selectors

Installation

conftest releases are available for Windows, macOS and Linux on the releases page. On Linux and macOS you can probably download as follows:

$ wget https://github.com/instrumenta/conftest/releases/download/v0.10.0/conftest_0.10.0_Linux_x86_64.tar.gz
$ tar xzf conftest_0.10.0_Linux_x86_64.tar.gz
$ sudo mv conftest /usr/local/bin

Brew

If you're on a Mac and using Homebrew you can use:

brew tap instrumenta/instrumenta
brew install conftest

Scoop

You can also install using Scoop on Windows:

scoop bucket add instrumenta https://github.com/instrumenta/scoop-instrumenta
scoop install conftest

Docker

Conftest is also able to be used via Docker. Simply mount your configuration and policy at /project and specify the relevant command like so:

$ docker run --rm -v $(pwd):/project instrumenta/conftest test deployment.yaml
FAIL - deployment.yaml - Containers must not run as root in Deployment hello-kubernetes

Inspiration

• kubetest was a similar project of mine, using Skylark
• Open Policy Agent and the Rego query language
• The helm-opa plugin from @eicnix helped with understanding the OPA Go packages
• Tools from the wider infrastructure as code community, in particular rspec-puppet. Lots of my thoughts in my talk from KubeCon 2017
• The code in pkg/auth is copied from Oras and will be removed once this issue is resolved