plugin/k8s_external: fix SRV queries doesn't work with AWS ELB/NLB (#4929)

* fix for issue #4927
Signed-off-by: denis-tingaikin <denis.tingajkin@xored.com>

* apply review comments
Signed-off-by: denis-tingaikin <denis.tingajkin@xored.com>

* apply review comments
Signed-off-by: denis-tingaikin <denis.tingajkin@xored.com>

plugin/k8s_external/external.go

@@ -97,7 +97,7 @@ func (e *External) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
 	case dns.TypeAAAA:
 		m.Answer = e.aaaa(ctx, svc, state)
 	case dns.TypeSRV:
-		m.Answer, m.Extra = e.srv(svc, state)
+		m.Answer, m.Extra = e.srv(ctx, svc, state)
 	default:
 		m.Ns = []dns.RR{e.soa(state)}
 	}

plugin/k8s_external/external_test.go

@@ -153,6 +153,12 @@ var tests = []test.Case{
 			test.A("svc11.testns.example.com.	5	IN	A	1.2.3.4"),
 		},
 	},
+	{
+		Qname: "_http._tcp.svc12.testns.example.com.", Qtype: dns.TypeSRV, Rcode: dns.RcodeSuccess,
+		Answer: []dns.RR{
+			test.SRV("_http._tcp.svc12.testns.example.com.	5	IN	SRV	0 100 80 dummy.hostname."),
+		},
+	},
 	{
 		Qname: "svc12.testns.example.com.", Qtype: dns.TypeA, Rcode: dns.RcodeSuccess,
 		Answer: []dns.RR{

plugin/k8s_external/msg_to_dns.go

@@ -22,9 +22,7 @@ func (e *External) a(ctx context.Context, services []msg.Service, state request.
 			rr := s.NewCNAME(state.QName(), s.Host)
 			records = append(records, rr)
 			if resp, err := e.upstream.Lookup(ctx, state, dns.Fqdn(s.Host), dns.TypeA); err == nil {
-				for _, rr := range resp.Answer {
-					records = append(records, rr)
-				}
+				records = append(records, resp.Answer...)
 			}
 
 		case dns.TypeA:
@@ -54,9 +52,7 @@ func (e *External) aaaa(ctx context.Context, services []msg.Service, state reque
 			rr := s.NewCNAME(state.QName(), s.Host)
 			records = append(records, rr)
 			if resp, err := e.upstream.Lookup(ctx, state, dns.Fqdn(s.Host), dns.TypeAAAA); err == nil {
-				for _, rr := range resp.Answer {
-					records = append(records, rr)
-				}
+				records = append(records, resp.Answer...)
 			}
 
 		case dns.TypeA:
@@ -74,7 +70,7 @@ func (e *External) aaaa(ctx context.Context, services []msg.Service, state reque
 	return records
 }
 
-func (e *External) srv(services []msg.Service, state request.Request) (records, extra []dns.RR) {
+func (e *External) srv(ctx context.Context, services []msg.Service, state request.Request) (records, extra []dns.RR) {
 	dup := make(map[item]struct{})
 
 	// Looping twice to get the right weight vs priority. This might break because we may drop duplicate SRV records latter on.
@@ -111,9 +107,21 @@ func (e *External) srv(services []msg.Service, state request.Request) (records,
 		what, ip := s.HostType()
 
 		switch what {
-		case dns.TypeCNAME:
-			// can't happen
 
+		case dns.TypeCNAME:
+			addr := dns.Fqdn(s.Host)
+			srv := s.NewSRV(state.QName(), weight)
+			if ok := isDuplicate(dup, srv.Target, "", srv.Port); !ok {
+				records = append(records, srv)
+			}
+			if ok := isDuplicate(dup, srv.Target, addr, 0); !ok {
+				if resp, err := e.upstream.Lookup(ctx, state, addr, dns.TypeA); err == nil {
+					extra = append(extra, resp.Answer...)
+				}
+				if resp, err := e.upstream.Lookup(ctx, state, addr, dns.TypeAAAA); err == nil {
+					extra = append(extra, resp.Answer...)
+				}
+			}
 		case dns.TypeA, dns.TypeAAAA:
 			addr := s.Host
 			s.Host = msg.Domain(s.Key)