Actually the Host header is for HTTP/1.1. They are sending HTTP/1.0 requests. Since most modern browsers send HTTP/1.1 requests, they may be able to filter by

Actually the Host header is for HTTP/1.1. They are sending HTTP/1.0 requests. Since most modern browsers send HTTP/1.1 requests, they may be able to filter by ignoring HTTP/1.0 traffic in general.