Ubuntu - cannot resolve domain names

One day, one of my Ubuntu pcs suddenly cannot access internet at all. To be precise, my browser cannot access any wet pages, even Google!
Somebody mentioned that maybe it is a typical ipv6 problem. One way to disable ipv6 is to use about:config at fixefox. Then input ipv6 in the filter. When the option for disableipv6 is shown, right click on it to toggle it as "true". But this trick does not work for me.
After some more searching, I found the right answer: set up the DNS infor in /etc/resolv.conf.
Just add "nameserver 192.168.0.1", where 192.168.0.1 is also the gateway of my pc.

To enable resizing of Ubuntu Window, follow the tips in https://linuxfan.info/virtualbox-guest-additions

When uinsing Virtualbox, to enable guest os accessing network in the same way as host, follow 【VirtualBox】NATネットワークでゲストPC同士を通信させる