Centos7.5 系统，保存退出后自动生效
vi /etc/resolv.conf
#阿里云DNS nameserver 223.5.5.5
nameserver 223.6.6.6
#百度DNS
nameserver 180.76.76.76
#腾讯DNS
nameserver 119.29.29.29
#google DNS
nameserver 8.8.8.8
#114 DNS
nameserver 114.114.114.114
nameserver 114.114.115.115