ping: www.baidu.com: 未知的名称或服务

/etc/resolv.conf加上以下配置即可

[root@CentOS7 ~]# vi /etc/resolv.conf

# Generated by NetworkManager                                               
  search localdomain                                                          
  nameserver 8.8.8.8                                                          
  nameserver 8.8.4.4