通过将不再使用的安卓手机用作旁路网关，无需购买软路由，即可实现家中所有设备的科学上网

#!/system/bin/sh

tun='tun0' #虚拟接口名称
dev='wlan0' #物理接口名称，eth0、wlan0
interval=3 #检测网络状态间隔(秒)
pref=18000 #路由策略优先级

# 开启IP转发功能
sysctl -w net.ipv4.ip_forward=1

# 清除filter表转发链规则
iptables -F FORWARD

# 添加NAT转换，部分第三方VPN需要此设置否则无法上网，若要关闭请注释掉
iptables -t nat -A POSTROUTING -o $tun -j MASQUERADE

# 添加路由策略
ip rule add from all table main pref $pref
ip rule add from all iif $dev table $tun pref $(expr $pref - 1)

contain="from all iif $dev lookup $tun"

while true ;do
    if [[ $(ip rule) != *$contain* ]]; then
            if [[ $(ip ad|grep 'state UP') != *$dev* ]]; then
                echo -e "[$(date "+%H:%M:%S")]dev has been lost."
            else
                ip rule add from all iif $dev table $tun pref $(expr $pref - 1)
                echo -e "[$(date "+%H:%M:%S")]network changed, reset the routing policy."
            fi
    fi
    sleep $interval
done