By default the DNS Server of Ubuntu 16.04 LTS will be 127.0.0.1:53, i.e., localhost.
Therefore, system will forward DNS query to the localhost first, and forward to the DNS server which obtained from the DHCP server afterward.
Actually, I didn't prefer this method, so that I would like to turn off this mechanism via steps were listed below:
1. Open /etc/NetworkManager/NetworkManager.conf
2. To disable dnsmasq
3. Restart NetworkManager
4. Examine /etc/resolv.conf