iptables as firewall

• Setup iptables as a firewall.
• Drop packages from INPUT chain by default.
• However, we have to allow response packages and requests to local DNS cache.

iptables -P INPUT DROP
iptables -I INPUT 1 -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -I INPUT 2 -p icmp -j ACCEPT
iptables -I INPUT 3 -p udp --dport 53 -j ACCEPT