Miscellany

iptables -P INPUT DROP
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -p tcp ! --syn -j REJECT --reject-with tcp-reset
iptables -A INPUT -m state --state INVALID -j DROP
iptables -P OUTPUT DROP
iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A OUTPUT -p tcp ! --syn -j REJECT --reject-with tcp-reset
iptables -A OUTPUT -m state --state INVALID -j DROP
iptables -P FORWARD DROP
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -p tcp ! --syn -j REJECT --reject-with tcp-reset
iptables -A FORWARD -m state --state INVALID -j DROP
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
iptables -A FORWARD -i lo -o lo -j ACCEPT
iptables -A INPUT -p tcp --syn -j REJECT --reject-with icmp-port-unreachable

iptables -N syn-flood
iptables -A syn-flood -m limit --limit 100/second --limit-burst 150 -j RETURN
iptables -A syn-flood -j LOG --log-prefix "SYN flood: "
iptables -A syn-flood -j DROP

iptables -A INPUT -p tcp -m tcp --tcp-flags SYN,RST,ACK SYN -m limit --limit 1/sec -j ACCEPT
iptables -A INPUT -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK RST -m limit --limit 1/sec -j ACCEPT