feat: explicitly query fallback servers for NS records

Fixes #29

Author: agaffney

internal/dns/dns.go

@@ -48,28 +48,18 @@ func handleQuery(w dns.ResponseWriter, r *dns.Msg) {
 		}
 	}
 
-	// Split query name into labels and lookup each domain and parent until we get a hit
-	queryLabels := dns.SplitDomainName(r.Question[0].Name)
-	for startLabelIdx := 0; startLabelIdx < len(queryLabels); startLabelIdx++ {
-		lookupDomainName := strings.Join(queryLabels[startLabelIdx:], ".")
-		nameServers, err := state.GetState().LookupDomain(lookupDomainName)
-		if err != nil {
-			logger.Errorf("failed to lookup domain: %s", err)
-		}
-		if nameServers == nil {
-			continue
-		}
+	nameserverDomain, nameservers, err := findNameserversForDomain(r.Question[0].Name)
+	if err != nil {
+		logger.Errorf("failed to lookup nameservers for %s: %s", r.Question[0].Name, err)
+	}
+	if nameservers != nil {
 		// Assemble response
 		m.SetReply(r)
 		if cfg.Dns.RecursionEnabled {
 			// Pick random nameserver for domain
-			tmpNameservers := []string{}
-			for nameserver := range nameServers {
-				tmpNameservers = append(tmpNameservers, nameserver)
-			}
-			tmpNameserver := nameServers[tmpNameservers[rand.Intn(len(tmpNameservers))]]
+			tmpNameserver := randomNameserverAddress(nameservers)
 			// Query the random domain nameserver we picked above
-			resp, err := queryServer(r, tmpNameserver)
+			resp, err := queryServer(r, tmpNameserver.String())
 			if err != nil {
 				// Send failure response
 				m.SetRcode(r, dns.RcodeServerFailure)
@@ -87,31 +77,30 @@ func handleQuery(w dns.ResponseWriter, r *dns.Msg) {
 				return
 			}
 		} else {
-			for nameserver, ipAddress := range nameServers {
-				// Add trailing dot to make everybody happy
-				nameserver = nameserver + `.`
+			for nameserver, addresses := range nameservers {
 				// NS record
 				ns := &dns.NS{
-					Hdr: dns.RR_Header{Name: (lookupDomainName + `.`), Rrtype: dns.TypeNS, Class: dns.ClassINET, Ttl: 999},
+					Hdr: dns.RR_Header{Name: (nameserverDomain), Rrtype: dns.TypeNS, Class: dns.ClassINET, Ttl: 999},
 					Ns:  nameserver,
 				}
 				m.Ns = append(m.Ns, ns)
-				// A or AAAA record
-				ipAddr := net.ParseIP(ipAddress)
-				if ipAddr.To4() != nil {
-					// IPv4
-					a := &dns.A{
-						Hdr: dns.RR_Header{Name: nameserver, Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 999},
-						A:   ipAddr,
-					}
-					m.Extra = append(m.Extra, a)
-				} else {
-					// IPv6
-					aaaa := &dns.AAAA{
-						Hdr:  dns.RR_Header{Name: nameserver, Rrtype: dns.TypeAAAA, Class: dns.ClassINET, Ttl: 999},
-						AAAA: ipAddr,
+				for _, address := range addresses {
+					// A or AAAA record
+					if address.To4() != nil {
+						// IPv4
+						a := &dns.A{
+							Hdr: dns.RR_Header{Name: nameserver, Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 999},
+							A:   address,
+						}
+						m.Extra = append(m.Extra, a)
+					} else {
+						// IPv6
+						aaaa := &dns.AAAA{
+							Hdr:  dns.RR_Header{Name: nameserver, Rrtype: dns.TypeAAAA, Class: dns.ClassINET, Ttl: 999},
+							AAAA: address,
+						}
+						m.Extra = append(m.Extra, aaaa)
 					}
-					m.Extra = append(m.Extra, aaaa)
 				}
 			}
 		}
@@ -178,3 +167,96 @@ func queryServer(req *dns.Msg, nameserver string) (*dns.Msg, error) {
 	in, err := dns.Exchange(m, fmt.Sprintf("%s:53", nameserver))
 	return in, err
 }
+
+func randomNameserverAddress(nameservers map[string][]net.IP) net.IP {
+	// Put all namserver addresses in single list
+	tmpNameservers := []net.IP{}
+	for _, addresses := range nameservers {
+		for _, address := range addresses {
+			tmpNameservers = append(tmpNameservers, address)
+		}
+	}
+	tmpNameserver := tmpNameservers[rand.Intn(len(tmpNameservers))]
+	return tmpNameserver
+}
+
+func doQuery(msg *dns.Msg, address string) (*dns.Msg, error) {
+	logger := logging.GetLogger()
+	logger.Debugf("querying %s: %s %s", address, dns.Type(msg.Question[0].Qtype).String(), msg.Question[0].Name)
+	resp, err := dns.Exchange(msg, address)
+	return resp, err
+}
+
+func findNameserversForDomain(recordName string) (string, map[string][]net.IP, error) {
+	cfg := config.GetConfig()
+
+	// Split record name into labels and lookup each domain and parent until we get a hit
+	queryLabels := dns.SplitDomainName(recordName)
+
+	// Check on-chain domains first
+	for startLabelIdx := 0; startLabelIdx < len(queryLabels); startLabelIdx++ {
+		lookupDomainName := strings.Join(queryLabels[startLabelIdx:], ".")
+		nameservers, err := state.GetState().LookupDomain(lookupDomainName)
+		if err != nil {
+			return "", nil, err
+		}
+		if nameservers != nil {
+			ret := map[string][]net.IP{}
+			for k, v := range nameservers {
+				k = k + `.`
+				ret[k] = append(ret[k], net.ParseIP(v))
+			}
+			return dns.Fqdn(lookupDomainName), ret, nil
+		}
+	}
+
+	// Query fallback servers, if configured
+	if len(cfg.Dns.FallbackServers) > 0 {
+		// Pick random fallback server
+		fallbackServer := cfg.Dns.FallbackServers[rand.Intn(len(cfg.Dns.FallbackServers))]
+		serverWithPort := fmt.Sprintf("%s:53", fallbackServer)
+		for startLabelIdx := 0; startLabelIdx < len(queryLabels); startLabelIdx++ {
+			lookupDomainName := dns.Fqdn(strings.Join(queryLabels[startLabelIdx:], "."))
+			m := new(dns.Msg)
+			m.SetQuestion(lookupDomainName, dns.TypeNS)
+			m.RecursionDesired = false
+			in, err := doQuery(m, serverWithPort)
+			if err != nil {
+				return "", nil, err
+			}
+			if in.Rcode == dns.RcodeSuccess {
+				if len(in.Answer) > 0 {
+					ret := map[string][]net.IP{}
+					for _, answer := range in.Answer {
+						switch v := answer.(type) {
+						case *dns.NS:
+							ns := v.Ns
+							ret[ns] = make([]net.IP, 0)
+							// Query for matching A/AAAA records
+							m2 := new(dns.Msg)
+							m2.SetQuestion(ns, dns.TypeA)
+							m2.RecursionDesired = false
+							in2, err := doQuery(m2, serverWithPort)
+							if err != nil {
+								return "", nil, err
+							}
+							for _, answer2 := range in2.Answer {
+								switch v := answer2.(type) {
+								case *dns.A:
+									ret[ns] = append(ret[ns], v.A)
+								case *dns.AAAA:
+									ret[ns] = append(ret[ns], v.AAAA)
+								}
+							}
+						}
+					}
+					if len(ret) > 0 {
+						return lookupDomainName, ret, nil
+					}
+				}
+			}
+		}
+	}
+
+	return "", nil, nil
+}