先找一台可以上网的centos主机。假设其连接公网的网口是eth0，与客户机都属于192.168.0.0/16，使用nat方式上网，则进行如下这番配置：

vim /etc/sysconfig/iptables

*nat

:PREROUTING ACCEPT [0:0]

:POSTROUTING ACCEPT [0:0]

:OUTPUT ACCEPT [0:0]

-A POSTROUTING -s 192.168.0.0/16 -o eth0 -j MASQUERADE

COMMIT

*filter

:INPUT ACCEPT [0:0]

:FORWARD ACCEPT [0:0]

:OUTPUT ACCEPT [0:0]

-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

-A INPUT -p icmp -j ACCEPT

-A INPUT -i lo -j ACCEPT

-A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT

-A FORWARD -s 192.168.0.0/16 -j ACCEPT

-A FORWARD -d 192.168.0.0/16 -j ACCEPT

-A INPUT -j REJECT --reject-with icmp-host-prohibited

-A FORWARD -j REJECT --reject-with icmp-host-prohibited

COMMIT

保存退出后重启iptables服务，别忘了开启转发功能，

vim /etc/sysctl.conf

net.ipv4.ip_forward = 1

保存退出再执行

echo "1" >/proc/sys/net/ipv4/ip_forward

客户机只要能顺利与这台centos主机通信即可。假如这台centos主机是两块网卡一公网一内网，我个人猜想-A FORWARD -d后面要加的就是那个公网ip了。客户机网关则统一填写内网ip即可。