Firewall start for `iptables`

iptables.sh

# Flush rules
iptables -X
iptables -F
iptables -t nat -X
iptables -t nat -F
iptables -t mangle -X
iptables -t mangle -F
iptables -t raw -X
iptables -t raw -F
iptables -t security -X
iptables -t security -F

# Drop all packets by default
iptables -P INPUT DROP
iptables -P OUTPUT DROP
iptables -P FORWARD DROP

# All answer flows are allowed if they are ESTABLISHED or RELATED
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Masquerade outgoing connections
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE