code: func GetClientAddr(r *http.Request) string {
  	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
  		return xff + "," + r.RemoteAddr
  	}
  	return r.RemoteAddr
  }
docs: // Comma-separated list of HTTP clients involved in making this
  // request, starting with the original requestor and ending with the
  // remote end of our TCP connection (which is typically our own
  // proxy).