mjhjmtu

I created iptables rules like this:

iptables --flush
iptables --delete-chain
iptables -t nat --flush
iptables -t nat --delete-chain
iptables -P OUTPUT DROP

Does internet work in that case? I guess it shouldn't.

But recently I found a nice cafe in my town and used their WiFi. I was surprised that with their WiFi I have an Internet connection even with the rules above.

How that can be possible?

Thank you for answers.

P.S. I tried to connect to many other hot spots and nowhere that problem appeared, except this cafe's WiFi.

I suspect something else is running scripts post-connect to modify the firewall rules. You can use sudo iptables -L -v to figure out what the currently applied rules are. That being said, I've used several different methods to connect to the internet (first Network Manager, then Wicd, and now systemd-networkd and wpa_supplicant directly) and I've never had any of them modify the firewall rules after connection, so I'm not sure why that is happening. Worst case, you can start a system service which runs wpa_cli with an action file that re-applies the firewall rules you desire upon connection (see the wpa_supplicant documentation for more information along with a sample action file).

Obviously other iptables calls have been made afterwards, probably by the component which configured the WiFi connection.