Conntrack is pretty clever in the way it classifies "connections", it has heuristics to determine what is a "related" connection (with many manually written rul

Conntrack is pretty clever in the way it classifies "connections", it has heuristics to determine what is a "related" connection (with many manually written rules for many protocols). For UDP it decides that if some packets go one way with UDP, then packets that arrive the exact opposite way not long after are the same connection.