Firewall Cracking using Kali-Linux

Total amateur here. I have created a firewall using iptables on a Linux virtual machine and now I need to crack it, I was thinking using of using DoS, brute force attacks or crafting packets in some way but I’m not really sure how to implement any of these.

Looking for anything in the way of advice or sign posting me to useful resources, thanks.

I built the firewall using these tutorials: (only used for port knocking)