plugin/etcd: Filter empty host field by qtype (#2499)

When a query, different from a TXT lookup is performed, all services
with a missing `Host` field should be filtered out, as these otherwize
cause a line in the answer section with a single dot (`.`) as the
result. This behavior manifests for example when a TXT record is present
on a domain, eg. an A or SRV lookup is performed on said domain.

If there are no services containing a `Host` field, a `NODATA` response
should be given. If there are other Services, these alone should be
returned for the query.

Filter any service that has an empty Host field from all lookup types
other than TXT to solve this issue. At the same time the check for empty
`Text` fields in TXT queries are also moved to the same check in the
etcd ServiceBackend.

plugin/backend_lookup.go

@@ -331,9 +331,6 @@ func TXT(b ServiceBackend, zone string, state request.Request, opt Options) (rec
 	}
 
 	for _, serv := range services {
-		if serv.Text == "" {
-			continue
-		}
 		records = append(records, serv.NewTXT(state.QName()))
 	}
 	return records, nil

plugin/etcd/cname_test.go

@@ -58,6 +58,7 @@ 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: "", Key: "region3.skydns.test.", Text: "SOME-RECORD-TEXT"},
 	{Host: "10.240.0.1", Key: "endpoint.region2.skydns.test."},
 }
 
@@ -83,4 +84,11 @@ var dnsTestCasesCname = []test.Case{
 			test.CNAME("region2.skydns.test.	300	IN	CNAME	mainendpoint.region2.skydns.test."),
 		},
 	},
+	{
+		Qname: "region3.skydns.test.", Qtype: dns.TypeCNAME,
+		Rcode: dns.RcodeSuccess,
+		Ns: []dns.RR{
+			test.SOA("skydns.test.	303	IN	SOA	ns.dns.skydns.test. hostmaster.skydns.test. 1546424605 7200 1800 86400 30"),
+		},
+	},
 }

plugin/etcd/etcd.go

@@ -78,7 +78,7 @@ func (e *Etcd) Records(state request.Request, exact bool) ([]msg.Service, error)
 		return nil, err
 	}
 	segments := strings.Split(msg.Path(name, e.PathPrefix), "/")
-	return e.loopNodes(r.Kvs, segments, star)
+	return e.loopNodes(r.Kvs, segments, star, state.QType())
 }
 
 func (e *Etcd) get(path string, recursive bool) (*etcdcv3.GetResponse, error) {
@@ -115,7 +115,7 @@ func (e *Etcd) get(path string, recursive bool) (*etcdcv3.GetResponse, error) {
 	return r, nil
 }
 
-func (e *Etcd) loopNodes(kv []*mvccpb.KeyValue, nameParts []string, star bool) (sx []msg.Service, err error) {
+func (e *Etcd) loopNodes(kv []*mvccpb.KeyValue, nameParts []string, star bool, qType uint16) (sx []msg.Service, err error) {
 	bx := make(map[msg.Service]struct{})
 Nodes:
 	for _, n := range kv {
@@ -149,7 +149,10 @@ Nodes:
 		if serv.Priority == 0 {
 			serv.Priority = priority
 		}
-		sx = append(sx, *serv)
+
+		if shouldInclude(serv, qType) {
+			sx = append(sx, *serv)
+		}
 	}
 	return sx, nil
 }
@@ -173,3 +176,13 @@ func (e *Etcd) TTL(kv *mvccpb.KeyValue, serv *msg.Service) uint32 {
 	}
 	return serv.TTL
 }
+
+// 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.
+func shouldInclude(serv *msg.Service, qType uint16) bool {
+	if qType == dns.TypeTXT {
+		return serv.Text != ""
+	}
+	return serv.Host != ""
+}
\ No newline at end of file

plugin/etcd/lookup_test.go

@@ -27,6 +27,8 @@ var services = []*msg.Service{
 	{Host: "10.0.0.1", Port: 8080, Key: "a.server1.prod.region1.skydns.test."},
 	{Host: "10.0.0.2", Port: 8080, Key: "b.server1.prod.region1.skydns.test."},
 	{Host: "::1", Port: 8080, Key: "b.server6.prod.region1.skydns.test."},
+	// TXT record in server1.
+	{Host: "", Port: 8080, Text: "sometext", Key: "txt.server1.prod.region1.skydns.test."},
 	// Unresolvable internal name.
 	{Host: "unresolvable.skydns.test", Key: "cname.prod.region1.skydns.test."},
 	// Priority.
@@ -127,6 +129,13 @@ var dnsTestCases = []test.Case{
 		Qname: "cname.prod.region1.skydns.test.", Qtype: dns.TypeA,
 		Ns: []dns.RR{test.SOA("skydns.test. 30 SOA ns.dns.skydns.test. hostmaster.skydns.test. 0 0 0 0 0")},
 	},
+	// TXT Test
+	{
+		Qname: "server1.prod.region1.skydns.test.", Qtype: dns.TypeTXT,
+		Answer: []dns.RR{
+			test.TXT("server1.prod.region1.skydns.test. 303 IN TXT sometext"),
+		},
+	},
 	// Wildcard Test
 	{
 		Qname: "*.region1.skydns.test.", Qtype: dns.TypeSRV,