Sergei Bulavintsev

DevOps, RHCA lvl II, CKA, CKS, CKAD

6 minute read

I’ve passed Certified Kubernetes Security Specialist Exam with score 84%: Score This also means that now I obtain all three Kubernetes Certifications from LinuxFoundation(CKAD, CKA and CKS).

Compared to my previous attempt on beta exam I was well prepared, but still could feel time pressure. Exam itself felt more polished and thoughtful compared to beta, but I was lucky enough to face a couple bugs(Like not matching names in question and in k8s cluster).

I didn’t use any courses or exam simulators, slowly going through theory and practice. I have to say that even though this way gives you a lot of low level knowledge, it’s so time consuming that I couldn’t recommend it to anyone. I think that the most straightforward way to pass CKS exam at the moment is to buy Kubernetes CKS 2021 Complete Course + Simulator on Udemy.

Most difficult thing for me was setting up my own Lab environment, especially considering how many components are involved into exam. I even wrote a few posts describing my CKS prep experience.

I will share more details how I was preparing to CKS exam below.

Theory

Important topics

They are the same as they were on beta:

  • Make sure you’re familiar with kube-bench tool and CIS k8s Benchmarks. You must know how to apply kube-bench recommendations to secure your kube-api, kubelet and other k8s components
  • Make sure you have a good knowledge of RBAC. How to map role to entity, how to minimize user/serviceAccount permissions.
  • Usage of Admission controllers is also a must.
  • NSP, PSP, OPA, Audit Policies
  • Other topics in CKS Curriculum

Materials

Again, most of the materials are the same one I used for beta exam:

Practice

I’ve used following resources for practice:

I’d suggest starting with creating your lab environment and solving questions from CKS Exam series.

If you feel brave enough, you can create your own practice exam, like I did. For example, my practice exam consists of 16 questions and is pretty close to the real one. Unfortunately I can’t share it due to NDA.

Exam tips

As CKS has the same format as CKA, all tips for CKA(which internet is full of), can be applied to CKS. But I’ll still share a few from my side:

  • Create bash aliases.

Spend 30 seconds to create bash aliases, which can save you much more time during the whole exam. My favourite one is kubens:

alias kubens='kubectl config set-context --current --namespace '
  • Know how to configure and use Vim.

I drop following lines to .vimrc to make work more comfortable:

:set ai et sw=2 sts=2 hidden

Those options make work with YAML less painful:

  • ai = autoindend - Copy indent from current line when starting a new line (typing in Insert mode or when using the "o" or "O" command).
  • et = expandtab - In Insert mode: Use the appropriate number of spaces to insert a .
  • sw = shiftwidth - Number of spaces to use for each step of (auto)indent.
  • sts = shifttabstop - Number of spaces that a counts for while performing editing operations, like inserting a or using .
  • hidden - lets you jump between vim buffers without save, useful for gf

Before pasting block of code, use :set paste command to preserve formatting.

One or multiple lines can be easily shifted left or right with << or >>. It’s works on visual blocks as well.

You can toggle built-in path auto completion with C-x, C-f.

Using gf it’s possible to open file under your cursor, and C-o can be used to jump back.

Finally, you can run something like k edit po mypod and then write yaml to local folder with :w mypod.yaml

  • Use bookmarks

One really useful link which I didn’t saw in similar posts is Website Content examples. It’s easy to find any resource example there, as they’re already in YAML and you can use this link on exam.

Setting up your own lab

Carefully crafting test questions, I was able to make all questions work together with one Kubernetes cluster and 2 worker nodes.

For provisioning I used Vagrant with following VMs:

  • master
    • Ubuntu 18.04(the same version as on exam)
    • 2CPU, 2GB RAM
    • all questions related to hardening k8s components, audit, admission controllers, Trivy, etc..
  • worker1
    • Ubuntu 18.04
    • 2CPU, 2GB RAM
    • Uses docker runtime and responsible for questions with AppArmor, Image security scanning, investigating Falco/Sysdig
  • worker2
    • Ubuntu 18.04
    • 2CPU, 2GB RAM
    • Uses containerd runtime and responsible for gVisor.

The trick here is to use labels to assign pods to correct nodes.

Testing your solutions

I think that it’s important to validate your solutions every time you pass your test exams, and here’s why:

  • CKS has strict time limits, so you have to speed up
  • When you’re trying to increase your speed, you start making little errors, like typos, creating Pods in different namespaces, etc..
  • Each typo can lead to whole question failed
  • Some questions have multiple bullet-ins and it’s easy to miss one of them

For testing, I was using Python’s pytest testing framework and it’s plugin kubetest

Pytest can be used when you need to check output of a command, or when kubetest doesn’t yet support required Kubernetes resource.

For example, here’s how you can check that ETCD is properly configured and passed kube-bench test:

def test_cks_question2_kube_bench_etcd(host):
    """
        Test that etcd passes kube-bench
    """
    cmd = host.run("/home/vagrant/kube-bench  --exit-code=99 --nosummary --noremediations --nototals --group 2")
    assert cmd.rc == 0

In this example I’m checking that result file exists and contains correct entries:

def test_cks_question7_result(host):
    """ Test that result file exists and contains logs """

    assert host.file("/home/vagrant/result/question7-logs.txt").exists
    result = host.file("/home/vagrant/result/question7-logs.txt").content_string

    assert '/docker-entrypoint.sh: 13: /docker-entrypoint.sh: cannot create /dev/null: Permission denied' in result

Kubetest is worth using for checking k8s resource state. For example, here’s how you can check that node has correct labels:

def test_cka_question7_check_node_labels(kube):
    """ Test that node has correct labels """

    nodes = kube.get_nodes()
    assert 'worker2' in nodes

    node = nodes['worker2']
    assert node.obj.metadata.labels.get("security") == "apparmor"

This can be done with pure pytest and parsing kubectl output, but with kubetest tests are cleaner and more flexible:

import pytest

@pytest.mark.namespace(create=False, name='production')
def test_cks_question11(kube):
    """ Test that pods with  vulnerabilities are deleted and others are in
        place
    """
    pods = kube.get_pods()
    assert 'secpod01' in pods
    assert 'secpod02' in pods
    assert 'secpod03' not in pods
    assert 'secpod04' not in pods

Finally, advanced features like parametrization can be used to make complicated testing easy like a breeze:

import pytest

@pytest.mark.parametrize('command, answer', [
  ("update pods", "no"),
  ("get pods", "yes"),
  ("get statefulsets", "no"),
  ("update statefulsets", "yes")
])
def test_cks_question3_perms(host, command, answer):
    """ Test that ServiceAccount has correct permissions """

    cmd_pre = "kubectl --as=system:serviceaccount:web:frontend-sa -n webauth can-i " + command
    cmd = host.run(cmd_pre)
    assert cmd.stdout.strip() == answer

Testing is a broad and very interesting topic. Personally I write test for all my exams and find them very useful.

Conclusion

CKS is a great exam, passing which you can be proud of. CKS topics are pretty specific and probably aren’t used by most of developers working with Kubernetes. But if you decide to go for it, make sure you’re well prepared.

Good luck with your own Certified Kubernetes Security Specialist!

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