A.iptables -t filter -A FORWARD -i eth0-o eth1–p tcp --ack -j ACCEPT
B.iptables -t filter -A INPUT -i eth0-m state --state RELATED，ESTABLISHED -j ACCEPT
C.iptables -t nat -A INPUT -i eth0–o eth1-m state --state RELATED，ESTABLISHED -j ACCEPT
D.iptables -t filter -A FORWARD -i eth0-o eth1-m state --state RELATED，ESTABLISHED -j ACCEPT