By default Ubuntu doesn't have open ports (exceptions: Avahi and dhcp).
Assuming that my system is connected to the internet by direct way (no hardware firewall used): If I am sending a UDP packet to for example 22.214.171.124:53. To be able to get an answer a program has to say 'If within 30 seconds (timeout) a UDP packet from 126.96.36.199 (with source port 53) is coming, it's welcome. All other packets I will drop.'
Can somebody tell me which program/module handles this on Ubuntu by default?
Thank you very much!