Linux systems which use a GUI often have a network manager running, which uses a dnsmasq instance running on a loopback address such as 127.0.0.1 or 127.0.1.1 to cache DNS requests, and adds this entry to /etc/resolv.conf. The dnsmasq service speeds up DNS look-ups and also provides DHCP services
sudo cat /run/resolvconf/resolv.conf
sudo cat /run/dnsmasq/resolv.conf
sudo cat /etc/systemd/resolved.conf
Disable the local DNS cache
/etc/NetworkManager/NetworkManager.conf
#dns=dnsmasq
systemctl restart network-manager
sudo systemctl disable dnsmasq
ubuntu 20.04
双网卡或局域网加无线网主机，路由指向通一个路由器
ls -l /etc/resolv.conf
/etc/resolv.conf -> /run/systemd/resolve/stub-resolv.conf
unlink /etc/resolv.conf
ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
sudo ip route del default dev wlp2s0
https://www.tecmint.com/set-permanent-dns-nameservers-in-ubuntu-debian/
更新IP地址
To renew or release an IP address for the eth0 interface, enter:
sudo dhclient -r eth0
sudo dhclient eth0