code: func LookupPTR(addr string, server string, c *dns.Client, m *dns.Msg) (name []string, err error) {
  
  	reverse, err := dns.ReverseAddr(addr)
  	if err != nil {
  		return nil, err
  	}
  
  	m.SetQuestion(reverse, dns.TypePTR)
  
  	r, _, err := c.Exchange(m, net.JoinHostPort(server, "53"))
  	if err != nil {
  		return nil, err
  	}
  
  	for _, ans := range r.Answer {
  		name = append(name, ans.(*dns.PTR).Ptr)
  	}
  
  	return
  }
docs: // PTR record lookup