本楼宽带用户接自同一交换机，交换机对各家用户限定固定大小的带宽。于是大家都在一个大局域网内，测试了一下，发现大部分用户的网关服务器IP皆为192.168.0.x，所以让公司一部分人通过其它网关服务器访问外网，偷点带宽。

　　服务器上两块网卡，eth0接入外网交换机，eth1为内网网口，内网偷带宽的机器IP段为192.168.1.x。

iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -o eth0 -j MASQUERADE

以下脚本由crontab每隔10分钟调用一次

#!/bin/sh

pid_file=/tmp/network_debianl.tmp

[[ -f $pid_file ]] && echo "Another instance with this program is running!" && exit

touch $pid_file

trap "script_exit" 2 3

function script_exit ()
{
    unlink $pid_file
    exit
}

ping -c 1 210.52.149.2
[[ $? = 0 ]] && echo "Connected internet" && unlink $pid_file && exit

if [ -f /etc/gateways ]; then
    for LOOP in `cat /etc/gateways`
    do
        ping -c 1 $LOOP
        if [ $? = 0 ]; then
            route del default
            route add default gw $LOOP
            sed '$d' /etc/resolv.conf > /etc/resolv.conf.bak
            mv /etc/resolv.conf.bak /etc/resolv.conf
            echo "nameserver $LOOP" >> /etc/resolv.conf
            ping -c 1 210.52.149.2
            [[ $? = 0 ]] && echo "Connected internet" && unlink $pid_file && exit
        fi
    done
fi

LOOP=1
while [ $LOOP -lt 250 ]
do
    [[ -f /etc/gateways ]] && grep 192.168.0.$LOOP /etc/gateways > /dev/null &&
      [[ $? = 0 ]] && LOOP=`expr $LOOP + 1` && continue
    ping -c 1 192.168.0.$LOOP
    if [ $? = 0 ]; then
        echo 192.168.0.$LOOP >> /etc/gateways
        route del default
        route add default gw 192.168.0.$LOOP
        sed '$d' /etc/resolv.conf > /etc/resolv.conf.bak
        mv /etc/resolv.conf.bak /etc/resolv.conf
        echo "nameserver 192.168.0.$LOOP" >> /etc/resolv.conf
        ping -c 1 210.52.149.2
        [[ $? = 0 ]] && echo "Connected internet" && unlink $pid_file && exit
    fi
    LOOP=`expr $LOOP + 1`
done

echo "Can't connect internet"

unlink $pid_file