nginx index directive does not work

Question

In the following configuration snippet for nginx:

server {
        listen 443;
        listen [::]:433;

        ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;

        server_name example.com;

        root /var/www/example.com;
        index index.html;
        error_page 502 = @fallback;
        error_page 404 = @not_found;

        location / {
            #try_files /index.html $uri =404; # A
            try_files $uri =404;              # B
        }

        location /service/ {
            rewrite ^/service/(.*)$ /$1 break;

            proxy_set_header X-Load-Balancer "a";
            proxy_set_header X-Real-IP $remote_addr;
            proxy_pass http://127.0.0.100:42424;
        }

        location @fallback {
            try_files /502.html =500;
        }

        location @not_found {
            try_files /404.html =500;
        }
}

Why does A correctly serve index.html (which is located in /var/www/example.com/) but B doesn't and results in the 404.html page being served instead?

Answer 1

The try_files directive processes each term in order, until it finds a file that exists.

If the file /var/www/example.com/index.html exists, the first term in case (A) will always be successful, and the URI will be internally changed to /index.html. The index directive is not involved. Also, case (A) will always return the /index.html file.

In case (B), try_files is looking for files only, and not directories. If you want try_files to match a directory and process it with the value of the index directive, use a term with a trailing /. For example:

try_files $uri $uri/ =404;

See this document for details.