iptables -N syn-flood
iptables -N icmp-flood

iptables -A INPUT -i ppp0 -p icmp -m icmp --icmp-type 8 -j icmp-flood 
iptables -A INPUT -i ppp0 -p tcp -m tcp --tcp-flags SYN,RST,ACK SYN -j syn-flood 
iptables -A INPUT -i ppp0 -p tcp -m tcp --dport 22 -j ACCEPT 
iptables -A INPUT -i ppp0 -p icmp -j ACCEPT 
iptables -A INPUT -i ppp0 -m state --state INVALID,NEW -j DROP 

iptables -A icmp-flood -m limit --limit 1/sec -j RETURN
iptables -A icmp-flood -j DROP 
iptables -A syn-flood -m limit --limit 5/sec --limit-burst 4 -j RETURN 
iptables -A syn-flood -j DROP