How to distinguish a real 503 from Drupal maintenance page in Nginx

gistfile1.txt

## This relies on using Lua. In particular the header_filter_by_lua. The solution is based 
## on the presence of the Etag header for the drupal maintenance page (theme). For a real 503
## there's no Etag. We set the header X-Maintenance to YES in that case. Now you can distinguish
## between a real 503 and the Drupal maintenance page.

error_page 503 = @503;

location @503 {
    header_filter_by_lua '
        if ngx.header.etag ~= "" then
            ngx.header.X_Maintenance = "YES"
        end';
    return 503 "Site under maintenance. Well be back sthortly.";
}