linux nameserver自動切換腳本

通過dig指令探測多家公共nameserver的解析時間，選取解析最快的nameserver為首選dns伺服器

#!/bin/bash

dns_now=`cat /etc/resolv.conf |head -1 |awk '{print $2}'`
dns_tx='119.29.29.29'
dns_bd='180.76.76.76'
dns_114='114.114.114.114'
buff_time=30

dns_now_time=$(expr `dig [email protected]${dns_now} |grep 'Query time' |awk '{print $4}'` - ${buff_time})
dns_tx_time=`dig [email protected] |grep 'Query time' |awk '{print $4}'`
dns_bd_time=`dig [email protected] |grep 'Query time' |awk '{print $4}'`
dns_114_time=`dig [email protected] |grep 'Query time' |awk '{print $4}'`

if [ ${dns_tx_time} -lt ${dns_bd_time} ];then
    dns_first=${dns_tx}
    dns_first_time=${dns_tx_time}
    if [ ${dns_tx_time} -gt ${dns_114_time} ];then
        dns_first=${dns_114}
        dns_first_time=${dns_114_time}
        dns_second=${dns_tx}
        dns_third=${dns_bd}
    else
        if [ ${dns_bd_time} -lt ${dns_114_time} ];then
            dns_second=${dns_bd}
            dns_third=${dns_114}
        else
            dns_second=${dns_114}
            dns_third=${dns_bd}
        fi
    fi    
else
   dns_first=${dns_bd}
   dns_first_time=${dns_bd_time}
   if [ ${dns_bd_time} -gt ${dns_114_time} ];then
        dns_first=${dns_114}
        dns_first_time=${dns_114_time}
        dns_second=${dns_bd}
        dns_third=${dns_tx}
    else
        if [ ${dns_tx_time} -lt ${dns_114_time} ];then
            dns_second=${dns_tx}
            dns_third=${dns_114}
        else
            dns_second=${dns_114}
            dns_third=${dns_tx}
        fi
    fi     
fi

if [ ${dns_now_time} -gt ${dns_first_time} ] && [ ${dns_now} != ${dns_first} ];then
cat >/etc/resolv.conf <<EOF
nameserver ${dns_first}
nameserver ${dns_second}
nameserver ${dns_third}
EOF
    #sed -i "1c nameserver ${dns_first}" /etc/resolv.conf
    #sed -i "2c nameserver ${dns_second}" /etc/resolv.conf
    #sed -i "3c nameserver ${dns_third}" /etc/resolv.conf
fi