Merge pull request #48 from blinklabs-io/feat/rework-recursion

feat: rework recursion support

Author: agaffney

internal/dns/dns.go

@@ -59,7 +59,7 @@ func handleQuery(w dns.ResponseWriter, r *dns.Msg) {
 			// Pick random nameserver for domain
 			tmpNameserver := randomNameserverAddress(nameservers)
 			// Query the random domain nameserver we picked above
-			resp, err := queryServer(r, tmpNameserver.String())
+			resp, err := doQuery(r, tmpNameserver.String(), true)
 			if err != nil {
 				// Send failure response
 				m.SetRcode(r, dns.RcodeServerFailure)
@@ -112,30 +112,6 @@ func handleQuery(w dns.ResponseWriter, r *dns.Msg) {
 		return
 	}
 
-	// Query fallback servers if recursion is enabled
-	if cfg.Dns.RecursionEnabled {
-		// Pick random fallback server
-		fallbackServer := cfg.Dns.FallbackServers[rand.Intn(len(cfg.Dns.FallbackServers))]
-		// Query chosen server
-		fallbackResp, err := queryServer(r, fallbackServer)
-		if err != nil {
-			// Send failure response
-			m.SetRcode(r, dns.RcodeServerFailure)
-			if err := w.WriteMsg(m); err != nil {
-				logger.Errorf("failed to write response: %s", err)
-			}
-			logger.Errorf("failed to query fallback server: %s", err)
-			return
-		} else {
-			copyResponse(r, fallbackResp, m)
-			// Send response
-			if err := w.WriteMsg(m); err != nil {
-				logger.Errorf("failed to write response: %s", err)
-			}
-			return
-		}
-	}
-
 	// Return NXDOMAIN if we have no information about the requested domain or any of its parents
 	m.SetRcode(r, dns.RcodeNameError)
 	if err := w.WriteMsg(m); err != nil {
@@ -159,30 +135,64 @@ func copyResponse(req *dns.Msg, srcResp *dns.Msg, destResp *dns.Msg) {
 	}
 }
 
-func queryServer(req *dns.Msg, nameserver string) (*dns.Msg, error) {
-	m := new(dns.Msg)
-	m.Id = dns.Id()
-	m.RecursionDesired = req.RecursionDesired
-	m.Question = append(m.Question, req.Question...)
-	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 {
 		tmpNameservers = append(tmpNameservers, addresses...)
 	}
-	tmpNameserver := tmpNameservers[rand.Intn(len(tmpNameservers))]
-	return tmpNameserver
+	if len(tmpNameservers) > 0 {
+		tmpNameserver := tmpNameservers[rand.Intn(len(tmpNameservers))]
+		return tmpNameserver
+	}
+	return nil
 }
 
-func doQuery(msg *dns.Msg, address string) (*dns.Msg, error) {
+func doQuery(msg *dns.Msg, address string, recursive bool) (*dns.Msg, error) {
 	logger := logging.GetLogger()
-	logger.Debugf("querying %s: %s %s", address, dns.Type(msg.Question[0].Qtype).String(), msg.Question[0].Name)
+	// Default to a random fallback server if no address is specified
+	if address == "" {
+		address = randomFallbackServer()
+	}
+	// Add default port to address if there is none
+	if !strings.Contains(address, ":") {
+		address = address + `:53`
+	}
+	logger.Debugf("querying %s: %s", address, formatMessageQuestionSection(msg.Question))
 	resp, err := dns.Exchange(msg, address)
-	return resp, err
+	if err != nil {
+		return nil, err
+	}
+	logger.Debugf("response: rcode=%s, authoritative=%v, authority=%s, answer=%s, extra=%s", dns.RcodeToString[resp.Rcode], resp.Authoritative, formatMessageAnswerSection(resp.Ns), formatMessageAnswerSection(resp.Answer), formatMessageAnswerSection(resp.Extra))
+	// Immediately return authoritative response
+	if resp.Authoritative {
+		return resp, nil
+	}
+	if recursive {
+		if len(resp.Ns) > 0 {
+			nameservers := getNameserversFromResponse(resp)
+			randNsName, randNsAddress := randomNameserver(nameservers)
+			if randNsAddress == "" {
+				m := createQuery(randNsName, dns.TypeA)
+				// XXX: should this query the fallback servers or the server that gave us the NS response?
+				resp, err := doQuery(m, "", false)
+				if err != nil {
+					return nil, err
+				}
+				randNsAddress = getAddressForNameFromResponse(resp, randNsName)
+				if randNsAddress == "" {
+					// Return the current response if we couldn't get an address for the nameserver
+					return resp, nil
+				}
+			}
+			// Perform recursive query
+			return doQuery(msg, randNsAddress, true)
+		} else {
+			// Return the current response if there is no authority information
+			return resp, nil
+		}
+	}
+	return resp, nil
 }
 
 func findNameserversForDomain(recordName string) (string, map[string][]net.IP, error) {
@@ -211,14 +221,11 @@ func findNameserversForDomain(recordName string) (string, map[string][]net.IP, e
 	// 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)
+		fallbackServer := randomFallbackServer()
 		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)
+			m := createQuery(lookupDomainName, dns.TypeNS)
+			in, err := doQuery(m, fallbackServer, false)
 			if err != nil {
 				return "", nil, err
 			}
@@ -231,10 +238,8 @@ func findNameserversForDomain(recordName string) (string, map[string][]net.IP, e
 							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)
+							m2 := createQuery(ns, dns.TypeA)
+							in2, err := doQuery(m2, fallbackServer, false)
 							if err != nil {
 								return "", nil, err
 							}
@@ -258,3 +263,138 @@ func findNameserversForDomain(recordName string) (string, map[string][]net.IP, e
 
 	return "", nil, nil
 }
+
+func getNameserversFromResponse(msg *dns.Msg) map[string][]net.IP {
+	if len(msg.Ns) == 0 {
+		return nil
+	}
+	ret := map[string][]net.IP{}
+	for _, ns := range msg.Ns {
+		// TODO: handle SOA
+		switch v := ns.(type) {
+		case *dns.NS:
+			nsName := v.Ns
+			ret[nsName] = []net.IP{}
+			for _, extra := range msg.Extra {
+				if extra.Header().Name != nsName {
+					continue
+				}
+				switch v := extra.(type) {
+				case *dns.A:
+					ret[nsName] = append(
+						ret[nsName],
+						v.A,
+					)
+				case *dns.AAAA:
+					ret[nsName] = append(
+						ret[nsName],
+						v.AAAA,
+					)
+				}
+			}
+		}
+	}
+	return ret
+}
+
+func getAddressForNameFromResponse(msg *dns.Msg, recordName string) string {
+	var retRR dns.RR
+	for _, answer := range msg.Answer {
+		if answer.Header().Name == recordName {
+			retRR = answer
+			break
+		}
+	}
+	if retRR == nil {
+		for _, extra := range msg.Extra {
+			if extra.Header().Name == recordName {
+				retRR = extra
+				break
+			}
+		}
+	}
+	if retRR == nil {
+		return ""
+	}
+	switch v := retRR.(type) {
+	case *dns.A:
+		return v.A.String()
+	case *dns.AAAA:
+		return v.AAAA.String()
+	}
+	return ""
+}
+
+func randomNameserver(nameservers map[string][]net.IP) (string, string) {
+	mapKeys := []string{}
+	for k := range nameservers {
+		mapKeys = append(mapKeys, k)
+	}
+	if len(mapKeys) > 0 {
+		randNsName := mapKeys[rand.Intn(len(mapKeys))]
+		randNsAddresses := nameservers[randNsName]
+		randNsAddress := randNsAddresses[rand.Intn(len(randNsAddresses))].String()
+		return randNsName, randNsAddress
+	}
+	return "", ""
+}
+
+func createQuery(recordName string, recordType uint16) *dns.Msg {
+	m := new(dns.Msg)
+	m.SetQuestion(recordName, recordType)
+	m.RecursionDesired = false
+	return m
+}
+
+func randomFallbackServer() string {
+	cfg := config.GetConfig()
+	return cfg.Dns.FallbackServers[rand.Intn(
+		len(cfg.Dns.FallbackServers),
+	)]
+}
+
+func formatMessageAnswerSection(section []dns.RR) string {
+	ret := "[ "
+	for idx, rr := range section {
+		ret += fmt.Sprintf(
+			"< %s >",
+			strings.ReplaceAll(
+				strings.TrimPrefix(
+					rr.String(),
+					";",
+				),
+				"\t",
+				" ",
+			),
+		)
+		if idx != len(section)-1 {
+			ret += `,`
+		}
+		ret += ` `
+	}
+	ret += "]"
+	return ret
+}
+
+func formatMessageQuestionSection(section []dns.Question) string {
+	ret := "[ "
+	for idx, question := range section {
+		ret += fmt.Sprintf(
+			"< %s >",
+			strings.ReplaceAll(
+				strings.TrimPrefix(
+					question.String(),
+					";",
+				),
+				"\t",
+				" ",
+			),
+		)
+		if idx != len(section)-1 {
+			ret += `,`
+		}
+		ret += ` `
+	}
+	ret += "]"
+	return ret
+}