nginx: redirect everything from http to https, except one url-pattern

If the iframe pages are always in the same directory, simple prefix locations could be used.

server {
    listen 443;

    location /l/ {  # redirect https iframe requests to http server
        return 301 http://$server_name$request_uri;
    }
    # ...
}

server {
    listen 80;

    location / {  # the default location redirects to https
        return 301 https://$server_name$request_uri;
    }

    location /l/ {}  # do not redirect requests for iframe location
    # ...
}