// Clean the host, in case it arrives with unexpected stuff in it.
	host := r.Host
	if host == "" {
		if r.URL == nil {
			return errMissingHost
		}
		host = r.URL.Host
	}
	host, err = httpguts.PunycodeHostPort(host)
	if err != nil {
		return err
	}
	// Validate that the Host header is a valid header in general,
	// but don't validate the host itself. This is sufficient to avoid