code: func (s *Server) findNode(addr Addr, targetID int160, callback func(krpc.Msg, error)) (err error) {
  	return s.query(addr, "find_node", &krpc.MsgArgs{
  		Target: targetID.AsByteArray(),
  		Want:   []krpc.Want{krpc.WantNodes, krpc.WantNodes6},
  	}, func(m krpc.Msg, err error) {
  		// Scrape peers from the response to put in the server's table before
  		// handing the response back to the caller.
  		s.mu.Lock()
  		s.addResponseNodes(m)
  		s.mu.Unlock()
  		callback(m, err)
  	})
  }
docs: // Sends a find_node query to addr. targetID is the node we're looking for.