I'm learning how to use iptables on ubuntu server.
Can you please explain to me what "Allowing Established Sessions" means and why should I include it in rules?
sudo iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
I understand concept of allowing specific ports and blocking others
sudo iptables -A INPUT -p tcp --dport ssh -j ACCEPT
sudo iptables -A INPUT -j DROP
But I don't get the concept of allowing established session.