这个是我原来的cat /etc/sysconfig/iptablse
# Generated by iptables-save v1.3.5 on Tue Feb 2 16:40:22 2010
*nat
REROUTING ACCEPT [498:53290]

OSTROUTING ACCEPT [24:1536]

:OUTPUT ACCEPT [51:3331]
-A POSTROUTING -o eth0 -j MASQUERADE
-A POSTROUTING -s 172.16.0.0/255.255.0.0 -j SNAT --to-source 10.19.20.19
COMMIT
# Completed on Tue Feb 2 16:40:22 2010
# Generated by iptables-save v1.3.5 on Tue Feb 2 16:40:22 2010
*filter
:INPUT ACCEPT [3402:1692910]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [1842:1553030]
-A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -s 172.16.0.2 -j ACCEPT
COMMIT
# Completed on Tue Feb 2 16:40:22 2010
# Generated by iptables-save v1.3.5 on Tue Feb 2 16:40:22 2010
*mangle
REROUTING ACCEPT [3881:1769032]

:INPUT ACCEPT [3424:1721290]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [1864:1581410]
OSTROUTING ACCEPT [1907:1588047]

COMMIT
====
那个大侠说的需要-d 172.16.0.2,我觉得应该是这个原因!!
那我是不是我再加上一句iptables -A FORWARD -d 172.16.0.2 -j ACCEPT 就可以实现只有172.16.0.2可以上网,而别的所有地址全部被默认DROP掉?