How to handle 400 errors in Nginx when redirecting HTTP to HTTPS

I have a website like example.com over HTTP. Considering the secure stuff, I now want to change HTTP to HTTPS. And I hope that all old clients will still be able to visit my site, even if they are using example.com, which will be redirected to https via Nginx.

Of course I googled a lot, then my solution is:

upstream www {
  server 127.0.0.1:4000;
}
server {
  listen      80;
  listen      443 ssl;
  server_name localhost www example.com;

  ssl on;
  ssl_certificate  /usr/local/etc/docs/example.crt;
  ssl_certificate_key  /usr/local/etc/docs/example.key;

  if ($ssl_protocol = "") {
    rewrite ^ https://$host$request_uri? permanent;
  }

  # below are some other stuff
  # ...
}

      

        
        
        
      

    

But when I visit example.com I got:

400 Bad Request A simple HTTP request was sent on the HTTPS port

Then I change nginx.conf after reading Redirect to nginx and set the error_page to 497:

upstream www {
  server 127.0.0.1:4000;
}
server {
  listen      80;
  listen      443 ssl;
  server_name localhost www example.com;

  ssl on;
  ssl_certificate  /usr/local/etc/docs/example.crt;
  ssl_certificate_key  /usr/local/etc/docs/example.key;

  error_page 497  https://$host$request_uri;

  # below are some other stuff
  # ...
}

      

        
        
        
      

    

Then it works, it's okay. But I just don't know why, and the error_page solution just seems insane. So after reading Working with nginx 400 "Simple HTTP request was sent to HTTPS port" error , I add the default and remove the ssl.

upstream www {
  server 127.0.0.1:4000;
}
server {
  listen      80;
  listen      443 default ssl;
  server_name localhost www example.com;

  ssl on;
  ssl_certificate  /usr/local/etc/docs/example.crt;
  ssl_certificate_key  /usr/local/etc/docs/example.key;

  if ($ssl_protocol = "") {
    rewrite ^ https://$host$request_uri? permanent;
  }

  # below are some other stuff
  # ...
}

      

        
        
        
      

    

Fine! It works again. But I'm not sure:

• Which solution is correct?
• If both are correct, which is more SEO friendly?