打开 /etc/systemd/resolved.conf，设置变量DNS:
$ sudo gedit /etc/systemd/resolved.conf
2. 单击"save" 按钮，保存设置
3. 输入一下命令，使设置生效
$ systemctl restart systemd-resolved //重启systemd-resolved服务
$ systemctl enable systemd-resolved//使能systemd-resolved服务
$ mv /etc/resolv.conf /etc/resolv.conf.bak//备份修改前的resolv.conf，这一步可以省略。
$ ln -s /run/systemd/resolve/resolv.conf /etc/ //这一步必须执行，否则DNS设置不能有效