A.iptables -t filter -L FORWARDB.iptables -t filter -F FORWARDC.iptables -t filter -A FORWARDD.iptables -t filter -P FORWARD
A.POSTROUTING--FORWARD--PREROUTINGB.PREROUTING—FORWARD--POSTROUTINGC.INPUT—OUTPUT--OUTPUTD.OUTPUT—OUTPUT--INPUT
A.处理路由选择后的数据包B.处理路由选择前的数据包C.目标地址转换D.源地址转换