Resolve TXT records via CNAME (#3557)

* Add test case for TXT lookup via CNAME
Signed-off-by: Jonathan Nagy <nagytech@users.noreply.github.com>

* Return HostType of explicit TXT records
Signed-off-by: Jonathan Nagy <nagytech@users.noreply.github.com>

* Adapt TXT method lookup to allow lookup via CNAME
Signed-off-by: Jonathan Nagy <nagytech@users.noreply.github.com>

* Implement lookup of TXT records via CNAME
Signed-off-by: Jonathan Nagy <nagytech@users.noreply.github.com>

plugin/backend_lookup.go

@@ -325,15 +325,69 @@ func CNAME(ctx context.Context, b ServiceBackend, zone string, state request.Req
 }
 
 // TXT returns TXT records from Backend or an error.
-func TXT(ctx context.Context, b ServiceBackend, zone string, state request.Request, opt Options) (records []dns.RR, err error) {
-	services, err := b.Services(ctx, state, false, opt)
+func TXT(ctx context.Context, b ServiceBackend, zone string, state request.Request, previousRecords []dns.RR, opt Options) (records []dns.RR, err error) {
+
+	services, err := b.Services(ctx, state, true, opt)
 	if err != nil {
 		return nil, err
 	}
 
+	dup := make(map[string]struct{})
+
 	for _, serv := range services {
-		records = append(records, serv.NewTXT(state.QName()))
+
+		what, _ := serv.HostType()
+
+		switch what {
+		case dns.TypeCNAME:
+			if Name(state.Name()).Matches(dns.Fqdn(serv.Host)) {
+				// x CNAME x is a direct loop, don't add those
+				continue
+			}
+
+			newRecord := serv.NewCNAME(state.QName(), serv.Host)
+			if len(previousRecords) > 7 {
+				// don't add it, and just continue
+				continue
+			}
+			if dnsutil.DuplicateCNAME(newRecord, previousRecords) {
+				continue
+			}
+			if dns.IsSubDomain(zone, dns.Fqdn(serv.Host)) {
+				state1 := state.NewWithQuestion(serv.Host, state.QType())
+				state1.Zone = zone
+				nextRecords, err := TXT(ctx, b, zone, state1, append(previousRecords, newRecord), opt)
+
+				if err == nil {
+					// Not only have we found something we should add the CNAME and the IP addresses.
+					if len(nextRecords) > 0 {
+						records = append(records, newRecord)
+						records = append(records, nextRecords...)
+					}
+				}
+				continue
+			}
+			// This means we can not complete the CNAME, try to look else where.
+			target := newRecord.Target
+			// Lookup
+			m1, e1 := b.Lookup(ctx, state, target, state.QType())
+			if e1 != nil {
+				continue
+			}
+			// Len(m1.Answer) > 0 here is well?
+			records = append(records, newRecord)
+			records = append(records, m1.Answer...)
+			continue
+
+		case dns.TypeTXT:
+			if _, ok := dup[serv.Host]; !ok {
+				dup[serv.Host] = struct{}{}
+				return append(records, serv.NewTXT(state.QName())), nil
+			}
+
+		}
 	}
+
 	return records, nil
 }
 

plugin/etcd/cname_test.go

@@ -58,6 +58,9 @@ var servicesCname = []*msg.Service{
 	{Host: "cname6.region2.skydns.test", Key: "cname5.region2.skydns.test."},
 	{Host: "endpoint.region2.skydns.test", Key: "cname6.region2.skydns.test."},
 	{Host: "mainendpoint.region2.skydns.test", Key: "region2.skydns.test."},
+	{Host: "cname2.region3.skydns.test", Key: "cname3.region3.skydns.test."},
+	{Host: "cname1.region3.skydns.test", Key: "cname2.region3.skydns.test."},
+	{Host: "region3.skydns.test", Key: "cname1.region3.skydns.test."},
 	{Host: "", Key: "region3.skydns.test.", Text: "SOME-RECORD-TEXT"},
 	{Host: "10.240.0.1", Key: "endpoint.region2.skydns.test."},
 }
@@ -91,4 +94,13 @@ var dnsTestCasesCname = []test.Case{
 			test.SOA("skydns.test.	303	IN	SOA	ns.dns.skydns.test. hostmaster.skydns.test. 1546424605 7200 1800 86400 30"),
 		},
 	},
+	{
+		Qname: "cname3.region3.skydns.test.", Qtype: dns.TypeTXT,
+		Answer: []dns.RR{
+			test.CNAME("cname3.region3.skydns.test.	300	IN	CNAME	cname2.region3.skydns.test."),
+			test.CNAME("cname2.region3.skydns.test.	300	IN	CNAME	cname1.region3.skydns.test."),
+			test.CNAME("cname1.region3.skydns.test.	300	IN	CNAME	region3.skydns.test."),
+			test.TXT("region3.skydns.test. 300 IN TXT \"SOME-RECORD-TEXT\""),
+		},
+	},
 }

plugin/etcd/etcd.go

@@ -177,11 +177,9 @@ func (e *Etcd) TTL(kv *mvccpb.KeyValue, serv *msg.Service) uint32 {
 }
 
 // shouldInclude returns true if the service should be included in a list of records, given the qType. For all the
-// currently supported lookup types, the only one to allow for an empty Host field in the service are TXT records.
-// Similarly, the TXT record in turn requires the Text field to be set.
+// currently supported lookup types, the only one to allow for an empty Host field in the service are TXT records
+// which resolve directly.  If a TXT record is being resolved by CNAME, then we expect the Host field to have a 
+// value while the TXT field will be empty.
 func shouldInclude(serv *msg.Service, qType uint16) bool {
-	if qType == dns.TypeTXT {
-		return serv.Text != ""
-	}
-	return serv.Host != ""
+	return (qType == dns.TypeTXT && serv.Text != "") || serv.Host != ""
 }

plugin/etcd/handler.go

@@ -30,7 +30,7 @@ func (e *Etcd) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (
 	case dns.TypeAAAA:
 		records, err = plugin.AAAA(ctx, e, zone, state, nil, opt)
 	case dns.TypeTXT:
-		records, err = plugin.TXT(ctx, e, zone, state, opt)
+		records, err = plugin.TXT(ctx, e, zone, state, nil, opt)
 	case dns.TypeCNAME:
 		records, err = plugin.CNAME(ctx, e, zone, state, opt)
 	case dns.TypePTR:

plugin/etcd/msg/type.go

@@ -19,8 +19,12 @@ func (s *Service) HostType() (what uint16, normalized net.IP) {
 	ip := net.ParseIP(s.Host)
 
 	switch {
+
 	case ip == nil:
-		return dns.TypeCNAME, nil
+		if len(s.Text) == 0 {
+			return dns.TypeCNAME, nil
+		}
+		return dns.TypeTXT, nil
 
 	case ip.To4() != nil:
 		return dns.TypeA, ip.To4()

plugin/kubernetes/handler.go

@@ -35,7 +35,7 @@ func (k Kubernetes) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.M
 	case dns.TypeAAAA:
 		records, err = plugin.AAAA(ctx, &k, zone, state, nil, plugin.Options{})
 	case dns.TypeTXT:
-		records, err = plugin.TXT(ctx, &k, zone, state, plugin.Options{})
+		records, err = plugin.TXT(ctx, &k, zone, state, nil, plugin.Options{})
 	case dns.TypeCNAME:
 		records, err = plugin.CNAME(ctx, &k, zone, state, plugin.Options{})
 	case dns.TypePTR: