reverse_dns_lookup_with_goroutine.go

// Do reverse dns lookup for list of ipv4 concurrently.
func (bs *BotSpecifier) ReverseDNSLookup(ctx context.Context, ipv4s []string) error {
	bs.Logger.WithFields(logrus.Fields{
		"package":  "specifier",
		"function": "ReverseDNSLookup",
	}).Debugf("Start Reverse DNS Lookup about %d ipv4s.", len(ipv4s))
	for _, ipv4 := range ipv4s {
		bs.WaitGroup.Add(1)
		go func(ipv4 string) {
			defer bs.WaitGroup.Done()
			addrs, err := bs.Resolver.LookupAddr(ctx, ipv4)
			if err != nil {
				// Most of error is "not found: 3 (NXDOMAIN)"
				// We don't care because what we want is to find well-known bot(spider) or crawler domains.
			} else {
				host := addrs[0]
				bs.Mutex.Lock()
				bs.DnsBook = append(bs.DnsBook, domain.DNS{IPv4: ipv4, Domain: host})
				bs.Mutex.Unlock()
			}
		}(ipv4)
	}
	bs.WaitGroup.Wait()
	bs.Logger.WithFields(logrus.Fields{
		"package":  "specifier",
		"function": "ReverseDNSLookup",
	}).Debugf("ReverseDNSLookup is Finished. We Looked up %d addresses.", len(bs.DnsBook))
	return nil
}