Further reading

• netstat – The easy tutorial: https://openmaniak.com/netstat.php
• Four ways to find which process is listening on a specific port: https://www.putorius.net/process-listening-on-port.html
• The official Nmap website: https://nmap.org/
• The GNU GRUB manual: https://www.gnu.org/software/grub/manual/grub/grub.html
• How to boot Ubuntu 18.04 into emergency and rescue mode: https://linuxconfig.org/how-to-boot-ubuntu-18-04-into-emergency-and-rescue-mode
• How to see the GRUB boot menu on Ubuntu 18.04: https://askubuntu.com/questions/16042/how-to-get-to-the-grub-menu-at-boot-time