CTF Playbook

kCTF is a Kubernetes-based infrastructure for CTF competitions

CTF Playbook

Now that your challenges are ready, let’s talk about how to set up your cluster to ensure that everything runs smoothly during the CTF event itself.

If you haven’t set up a cluster yet, please follow the google cloud walkthrough to do so.

Also, make sure that every challenge has a working healthcheck. This will allow kubernetes to automatically restart challenges that stop working.


You want to make sure that all the challenges are scaled depending on how much traffic they receive.

First of all, we need to make sure that the cluster has enough nodes (VMs) to run the challenges on. You can use kctf cluster resize to add a new node pool to your cluster and delete the old one. The parameters allow you to configure the minimum and maximum amount of nodes for automatic scaling as well as what kind of machines to use. For example:

kctf cluster resize --min-nodes 4 --max-nodes 16 --num-nodes 5 --machine-type n2-standard-4

| :warning: the maximum number of nodes may be limited by the cloud project quotas | | — |

After enabling scaling for the number of nodes, we also want to enable scaling for the challenges. You can do this by adding a horizontalAutoScaler spec and a resource request to the challenge.yaml. For available fields, see:

kubectl explain challenge.spec.horizontalPodAutoscalerSpec

An example spec can look like this:

apiVersion: kctf.dev/v1
kind: Challenge
  name: mychallenge
  # [...]
    maxReplicas: 8
    minReplicas: 2
    targetCPUUtilizationPercentage: 60
          - name: 'challenge'
                memory: "1000Mi"
                cpu: "500m"

Start the challenge:

kctf chal start

And you can confirm that the autoscaler was created with kubectl:

kubectl get horizontalPodAutoscaler

Proof of Work

If you notice that one of the challenges uses a high amount of resources, you can enable a proof of work on every connection. Simply set the powDifficultySeconds parameter and restart the challenge:

apiVersion: kctf.dev/v1
kind: Challenge
  name: mychallenge
  powDifficultySeconds: 60

Note that the proof of work doesn’t support web challenges. For those, you can include a captcha on your web endpoints, for example reCAPTCHA.


We all know IRC is the best way to get alerts about broken challenges :). But if you want to keep an eye on the challenges and catch potential issues early, there are a few ways to check the health.

First of all, you can list all challenges with kubectl, this will show you which are deemed healthy by the healthcheck and which are not. Remember, healthchecks are important!

$ kubectl get challenges
mychallenge  healthy     Running   true       true
$ cd mychallenge
$ kctf chal status

If any of the challenges are broken, you can check out our troubleshooting docs for some debugging tips.

Another option is to use the google cloud web UI, which shows you various information about your cluster. For example: