1. 首先在/etc/network/interfaces里加入下面的内容；这里eth0与外网连，eth1与内网连。
auto lo eth0 eth1
iface lo inet loopback
iface eth0 inet dhcp
iface eth1 inet static
address 192.168.0.1
netmask 255.255.255.0
2. echo "1" > /proc/sys/net/ipv4/ip_forward
或修改/etc/sysctl.conf，加上
net.ipv4.ip_forward = 1
3. iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
4. /etc/init.d/inetd restart 或者重新启动计算机。双网卡就可以用了
