Quickstart guide to writing a challenge

The basic steps when preparing a challenge are:

• A Docker image is built from the challenge directory. For the simplest challenges, replacing challenge/chal.c is sufficient.
• Edit challenge/Dockerfile to change the commandline or the files you want to include.
• To try the challenge locally, you will need to
  • create a a local cluster with kctf cluster create --type kind --start $configname
  • build the challenge binary with make -C challenge
  • and then deploy the challenge with kctf chal start
• To access the challenge, create a port forward with kctf chal debug port-forward and connect to it via nc localhost PORT using the printed port.
• Check out kctf chal <tab> for more commands.

Directory layout

The following files/directories are available:

/challenge.yaml

challenge.yaml is the main configuration file. You can use it to change settings like the name and namespace of the challenge, the exposed ports, the proof-of-work difficulty etc. For documentation on the available fields, you can run kubectl explain challenge and kubectl explain challenge.spec.

/challenge

The challenge directory contains a Dockerfile that describes the challenge and any challenge files. This template comes with a Makefile to build the challenge, which is the recommended way for pwnables if the deployed binary matters, e.g. if you hand it out as an attachment for ROP gadgets. If the binary layout doesn't matter, you can build it using an intermediate container as part of the Dockerfile similar to how the chroot is created.

/healthcheck

The healthcheck directory is optional. If you don't want to write a healthcheck, feel free to delete it. However, we strongly recommend that you implement a healthcheck :).

We provide a basic healthcheck skeleton that uses pwntools to implement the healthcheck code. The only requirement is that the healthcheck replies to GET requests to http://$host:45281/healthz with either a success or an error status code.

In most cases, you will only have to modify healthcheck/healthcheck.py.

API contract

Ensure your setup fulfills the following requirements to ensure it works with kCTF:

• Verify kctf_setup is used as the first command in the CMD instruction of your challenge/Dockerfile.
• You can do pretty much whatever you want in the challenge directory but:
• We strongly recommend using nsjail in all challenges. While nsjail is already installed, you need to configure it in challenge/nsjail.cfg. For more information on nsjail, see the official website.
• Your challenge receives connections on port 1337. The port can be changed in challenge.yaml.
• The healthcheck directory is optional.
  • If it exists, the image should run a webserver on port 45281 and respond to /healthz requests.