code: func lookupNameservers(fqdn string) ([]string, error) {
  	var authoritativeNss []string
  
  	zone, err := FindZoneByFqdn(fqdn)
  	if err != nil {
  		return nil, fmt.Errorf("could not determine the zone: %v", err)
  	}
  
  	r, err := dnsQuery(zone, dns.TypeNS, recursiveNameservers, true)
  	if err != nil {
  		return nil, err
  	}
  
  	for _, rr := range r.Answer {
  		if ns, ok := rr.(*dns.NS); ok {
  			authoritativeNss = append(authoritativeNss, strings.ToLower(ns.Ns))
  		}
  	}
  
  	if len(authoritativeNss) > 0 {
  		return authoritativeNss, nil
  	}
  	return nil, fmt.Errorf("could not determine authoritative nameservers")
  }
docs: // lookupNameservers returns the authoritative nameservers for the given fqdn.