Nginx 'conflicting server name "example.com" on 0.0.0.0:80, ignored' or 'a duplicate default server for 0.0.0.0:80'. Which site does nginx actually use, and how do I fix the config?

Cause: nginx picks the server block for a request by the listen address/port and then the Host header against server_name. The warning 'conflicting server name' means two server blocks listen on the same address and port with the same server_name: nginx uses the first one and ignores the other, so requests may go to the wrong site. 'duplicate default server' (an error that stops nginx from starting) means two server blocks on the same address and port both have default_server on their listen line. Common causes: a leftover default site (sites-enabled/default or conf.d/default.conf), the same site enabled twice (a copy plus a symlink), or tools like certbot adding an extra server block. Fix: 1. Find the duplicates: sudo nginx -T prints the full merged configuration; search it for the server_name or default_server named in the message, and note which files they come from. 2. Remove or disable the duplicate: delete the stale file or symlink (e.g. sites-enabled/default), or merge the two server blocks into one. 3. Keep exactly one default_server per address:port (it handles requests whose Host matches no server_name); the nginx docs explain that without default_server the first server block for that port is the default. 4. For HTTP and HTTPS, separate blocks are fine as long as they listen on different ports (80 vs 443 ssl). 5. Test and reload: sudo nginx -t, then sudo systemctl reload nginx (or nginx -s reload). Verify: nginx -t reports the configuration is OK with no conflicting-name warnings, and curl -H 'Host: example.com' http://<server>/ returns the intended site. Sources: https://nginx.org/en/docs/http/server_names.html https://nginx.org/en/docs/http/request_processing.html