clear DNS cache:
sudo resolvectl flush-caches
problem to installing packages:
sudo apt update
sudo apt-get update
if net not working for getting apt packages :
sudo nano /etc/resolv.conf
and add these lines before nameserver ... :
nameserver 8.8.8.8
nameserver 8.8.4.4
solve apt update network issue:
echo "nameserver 8.8.8.8" | sudo tee /etc/resolv.conf > /dev/null