====== Ubuntu ======

===== Upgrade =====
<sxh [bash]>
sudo apt update
sudo apt upgrade
sudo apt dist-upgrade
sudo apt autoremove
sudo apt install update-manager-core
sudo do-release-upgrade
</sxh>

===== System checkup =====
  * Check cpu: ''lscpu''
  * Check memory: ''sudo dmidecode --type memory | less''

===== DNS bad config =====
If ''nslookup google.com'' gives error, there is problem. If ''nslookup google.com 8.8.8.8'' gives no error, it means your default DNS is not working but things work when you specify a DNS. So specify a DNS permanently. Do the following.

<sxh [bash]>
sudo vi /etc/systemd/resolved.conf # open the systemd config file file
DNS=8.8.8.8 8.8.4.4 1.1.1.1 # add this line at the bottom in nano
sudo service systemd-resolved restart # restart systemd-resolve
cat /run/systemd/resolve/resolv.conf # check if the DNS's are there
sudo rm /etc/resolv.conf # delete this symlink
sudo ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf # create new symlink
sudo service systemd-resolved restart # restart systemd-resolved again
</sxh>

And the problem should be fixed as a new ''nslookup google.com'' should show.