真实DNS服务器IP:
查看 /etc/resolvconf/resolv.conf.d/base 文件
查看53端口监听程序：
netstat -nap | grep 53
/lib/systemd/systemd-resolved
查看53端口实时流量:
tcpdump -nnn port 53
nslookup
www.baidu.com
停用systemd-resolvd服务：
systemctl stop systemd-resolvd
systemctl disable systemd-resolvd
rm -f /etc/systemd/system/multi-user.target.wants/systemd-resolved.service
参考：
https://www.feiqy.com/ubuntu-dns/
https://askubuntu.com/questions/1012641/dns-set-to-systemds-127-0-0-53-how-to-change-permanently
