code: func Host(hostname string) (string, error) {
  	if hostname == "" {
  		return "", trace.BadParameter("missing parameter hostname")
  	}
  	if !strings.Contains(hostname, ":") {
  		return hostname, nil
  	}
  	host, _, err := SplitHostPort(hostname)
  	return host, err
  }
docs: // Host extracts host from host:port string