🎯 Mission Briefing: Port 80 Is Taken

Since this morning the company website shows "Apache2 Debian Default Page — It works!" instead of the shop. Nobody touched nginx's configuration. Yesterday, though, a colleague installed something for a small PHP script…

Your objective

Find out why nginx isn't serving the site, find which program is holding the web port, and give the port back to nginx — for good.

Investigation toolkit

curl -s http://localhost | grep -i title # What does the server answer?
systemctl status nginx                   # Is nginx running? Why not?
sudo ss -tlnp                            # Which program listens on which port?
sudo lsof -i :80                         # The same question, asked of one port
tail /var/log/apt/history.log            # What was installed yesterday?

Fix and verify

sudo systemctl stop apache2              # Free port 80
sudo systemctl disable apache2           # …and keep it free after a reboot
sudo systemctl start nginx
curl -I http://localhost                 # Server: nginx?

💡 Only one program can listen on a port. Address already in use means someone else got there first — ss -tlnp tells you who. Without sudo you see the port but not the program.