VPN killswitch Linux


I have a question regarding a VPN killswitch. I’m on linux and I don’t have nftables installed. I have iptables but an empty ruleset. Whenever I activate the killswitch I have no internet without connecting to the VPN.

How is such a killswitch possible without firewall rules? The killswitch doesn’t add an extra interface nor does anything special appear whenever i execute “ip rules”.

What other ways are there to block internet traffic on linux?

I’m new to the linux kernel and therefore have no understanding about it and how the OS operates in general.