nginx.conf

# The following recipe works with upstream rails proxy for custom 404s and 500s.
# Errors are usually handled via rails except if proxy is really down, in which case
# nginx needs a bit more configration.

server {
  # ...
  
  location / {
    error_page 404 = @rails; # let rails show a page with suggestions
    try_files maintenance.html @rails;
  }
  
  location @rails {
    proxy_pass ...

    proxy_intercept_errors on;  # Important!
  }
  
  error_page 500 502 503 @error_page;

  # this handles 50x errors that happen via upstream proxy,
  # when the normal error_page directive alone is not enough.
  
  location @error_page {
    root /var/nginx/app/htdocs; # location of 500.html file
    internal;
    rewrite ^ /500.html;
    break;
  }
  
}