Unit Testing Configuration Files

Table of Contents

• Overview
• Getting Started
  • Dockerfile
  • Kubernetes
• Next Steps

Overview

The era of Infrastructure-as-Code (IaC) has unlocked tremendous developer productivity and agility features. Now, as an Engineer, we can declare our infrastructure and environments as structured data in configuration files, such as Terraform templates, Dockerfiles, and Kubernetes manifests.

However, this agility and speed of provisioning and configuring infrastructure comes with a high risk of bugs in the form of misconfigurations.

Fortunately, we can solve this problem just as we can solve for other bugs in our products, by writing unit tests.

One such tool that can help us unit test our configuration files is conftest. What is unique about conftest is that it uses Open-Policy-Agent (OPA) and a policy language, called Rego to accomplish this.

This might appear difficult at first, but it will start to make sense.

Let's explore 2 use-cases where we can test our configurations!

Getting Started

First, some prerequisites:

• conftest:
  • macOS: brew install instrumenta/instrumenta/conftest
• (Optional) opa:
  • macOS: brew install opa

Dockerfile

Let's say we want to prevent some images and/or tags (e.g. latest).

We need to create a simple Dockerfile:

FROM kalilinux/kali-linux-docker:latest

ENTRYPOINT ["echo"]

Now, we need to create our first unit test file, let's call it test.rego, and place it in a directory, let's call it policy (this is configurable).

package main

disallowed_tags := ["latest"]
disallowed_images := ["kalilinux/kali-linux-docker"]

deny[msg] {
        input[i].Cmd == "from"
        val := input[i].Value
        tag := split(val[i], ":")[1]
        contains(tag, disallowed_tags[_])

        msg = sprintf("[%s] tag is not allowed", [tag])
}

deny[msg] {
        input[i].Cmd == "from"
        val := input[i].Value
        image := split(val[i], ":")[0]
        contains(image, disallowed_images[_])

        msg = sprintf("[%s] image is not allowed", [image])
}

Assuming we are in the right directory, we can test our Dockerfile:

$ ls
Dockerfile      policy/

$ conftest test -i Dockerfile ./Dockerfile
FAIL - ./Dockerfile - [latest] tag is not allowed
FAIL - ./Dockerfile - [kalilinux/kali-linux-docker] image is not allowed

Just to be sure, let's change this Dockerfile to pass the test:

# FROM kalilinux/kali-linux-docker:latest
FROM debian:buster

ENTRYPOINT ["echo"]

$ ls
Dockerfile      policy/

$ conftest test -i Dockerfile ./Dockerfile
PASS - ./Dockerfile - data.main.deny

"It works! But I don't understand how," I hear you thinking to yourself.

Let's break the Rego syntax down:

• package main is a way for us to put some rules that belong together in a namespace. In this case, we named it main because conftest defaults to it, but we can easily do something like package docker and then run conftest test -i Dockerfile --namespace docker ./Dockerfile

• disallowed_tags & disallowed_images are just simple variables that hold an array of strings

• deny[msg] { ... } is the start of the deny rule and it means that the Dockerfile should be rejected and the user should be given an error message msg if the conditions in the body (i.e. { ... }) are true

• Expressions in the body of the deny rule are treated as logical AND. For example:

  1 == 1                    # IF 1 is equal to 1
  contains("foobar", "foo") # AND "foobar" contains "foo"
                            # This would trigger the deny rule

• input[i].Cmd == "from" checks if the Docker command is FROM. input[i] means we can have multiple Dockerfiles being tested at once. This will iterate over them

• The next 2 lines are assignments just to split a string and store some data in variables

• contains(tag, disallowed_tags[_]) will return true if the tag we obtained from the Dockerfile contains one of the disallowed_tags. array[_] syntax means iterate over values

• msg := sprinf(...) creates the message we want to tell our user if this deny rule is triggered

• The second deny[msg] rule checks that the image itself is not on the blocklist.

Kubernetes

Let's say we want to ensure that all pods are running as a non-root user.

We need to create our deployment

$ mkdir -p kubernetes
$ cat <<EOF >./kubernetes/deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
        - name: nginx
          image: nginx:1.7.9
          ports:
            - containerPort: 80
EOF

Now, we need to create our unit test:

$ mkdir -p ./kubernetejjjs/policy
$ cat <<EOF >./kubernetes/policy/test.rego
package main

name := input.metadata.name

deny[msg] {
  input.kind == "Deployment"
  not input.spec.template.spec.securityContext.runAsNonRoot

  msg = sprintf("Containers must run as non root in Deployment %s. See: https://kubernetes.io/docs/tasks/configure-pod-container/security-context/", [name])
}
EOF

And, let's run it:

conftest test -i yaml ./kubernetes/deployment.yaml
FAIL - ./kubernetes/deployment.yaml - Containers must run as non root in Deployment nginx-deployment. See: https://kubernetes.io/docs/tasks/configure-pod-container/security-context/

This is a bit more straightforward:

• Get the metadata.name from the input (which is the Kubernetes Deployment yaml file)
• Create a deny rule that is triggered if:
  • input.kind is Deployment and
  • securityContext.runAsNonRoot is not set
• Return an error message to the user that containers must run as non-root and point them to the docs.

Next Steps

So, where to go from here?

The Rego language is vast and it can take a bit to wrap your head around how it works. You can even send and receive HTTP requests inside Rego.

I recommend reading the docs to learn more about Rego's capabilities:

• Reference sheet
• Cheat sheet

I also barely scratched the surface with conftest in this blog post. The repository has a nice list of examples that you should peruse at your leisure. conftest even supports sharing policies via uploading OPA bundles to OCI-compliant registries, e.g. conftest push ..., conftest pull ....

Lastly, if you have any questions, the OPA community is friendly and welcoming. Feel free to join the #conftest channel in OPA Slack.

Happy coding!