plugin/k8s_external: Add support for PTR requests (#5435)

* Exclude External IP addresses from being added to the existing kubernetes' plugin IP->Service index
* Add support for PTR requests on External IPs of Services to the k8s_external plugin

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/k8s_external/README.md

@@ -10,7 +10,7 @@ This plugin allows an additional zone to resolve the external IP address(es) of
 service. This plugin is only useful if the *kubernetes* plugin is also loaded.
 
 The plugin uses an external zone to resolve in-cluster IP addresses. It only handles queries for A,
-AAAA and SRV records; all others result in NODATA responses. To make it a proper DNS zone, it handles
+AAAA, SRV, and PTR records; all others result in NODATA responses. To make it a proper DNS zone, it handles
 SOA and NS queries for the apex of the zone.
 
 By default the apex of the zone will look like the following (assuming the zone used is `example.org`):
@@ -101,6 +101,3 @@ zone transfers.  Notifies are not supported.
 For some background see [resolve external IP address](https://github.com/kubernetes/dns/issues/242).
 And [A records for services with Load Balancer IP](https://github.com/coredns/coredns/issues/1851).
 
-# Bugs
-
-PTR queries for the reverse zone is not supported.

plugin/k8s_external/external.go

@@ -105,6 +105,8 @@ func (e *External) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
 		m.Answer, m.Truncated = e.aaaa(ctx, svc, state)
 	case dns.TypeSRV:
 		m.Answer, m.Extra = e.srv(ctx, svc, state)
+	case dns.TypePTR:
+		m.Answer = e.ptr(svc, state)
 	default:
 		m.Ns = []dns.RR{e.soa(state)}
 	}

plugin/k8s_external/external_test.go

@@ -20,7 +20,7 @@ func TestExternal(t *testing.T) {
 	k.APIConn = &external{}
 
 	e := New()
-	e.Zones = []string{"example.com."}
+	e.Zones = []string{"example.com.", "in-addr.arpa."}
 	e.Next = test.NextHandler(dns.RcodeSuccess, nil)
 	e.externalFunc = k.External
 	e.externalAddrFunc = externalAddress  // internal test function
@@ -49,12 +49,33 @@ func TestExternal(t *testing.T) {
 			t.Error("Expected authoritative answer")
 		}
 		if err = test.SortAndCheck(resp, tc); err != nil {
-			t.Error(err)
+			t.Errorf("Test %d: %v", i, err)
 		}
 	}
 }
 
 var tests = []test.Case{
+	// PTR reverse lookup
+	{
+		Qname: "4.3.2.1.in-addr.arpa.", Qtype: dns.TypePTR, Rcode: dns.RcodeSuccess,
+		Answer: []dns.RR{
+			test.PTR("4.3.2.1.in-addr.arpa. 5 IN PTR svc1.testns.example.com."),
+		},
+	},
+	// Bad PTR reverse lookup using existing service name
+	{
+		Qname: "svc1.testns.example.com.", Qtype: dns.TypePTR, Rcode: dns.RcodeSuccess,
+		Ns: []dns.RR{
+			test.SOA("example.com.	5	IN	SOA	ns1.dns.example.com. hostmaster.example.com. 1499347823 7200 1800 86400 5"),
+		},
+	},
+	// Bad PTR reverse lookup using non-existing service name
+	{
+		Qname: "not-existing.testns.example.com.", Qtype: dns.TypePTR, Rcode: dns.RcodeNameError,
+		Ns: []dns.RR{
+			test.SOA("example.com.	5	IN	SOA	ns1.dns.example.com. hostmaster.example.com. 1499347823 7200 1800 86400 5"),
+		},
+	},
 	// A Service
 	{
 		Qname: "svc1.testns.example.com.", Qtype: dns.TypeA, Rcode: dns.RcodeSuccess,
@@ -155,7 +176,7 @@ var tests = []test.Case{
 	{
 		Qname: "svc11.testns.example.com.", Qtype: dns.TypeA, Rcode: dns.RcodeSuccess,
 		Answer: []dns.RR{
-			test.A("svc11.testns.example.com.	5	IN	A	1.2.3.4"),
+			test.A("svc11.testns.example.com.	5	IN	A	2.3.4.5"),
 		},
 	},
 	{
@@ -164,7 +185,7 @@ var tests = []test.Case{
 			test.SRV("_http._tcp.svc11.testns.example.com.	5	IN	SRV	0 100 80 svc11.testns.example.com."),
 		},
 		Extra: []dns.RR{
-			test.A("svc11.testns.example.com.	5	IN	A	1.2.3.4"),
+			test.A("svc11.testns.example.com.	5	IN	A	2.3.4.5"),
 		},
 	},
 	{
@@ -173,7 +194,7 @@ var tests = []test.Case{
 			test.SRV("svc11.testns.example.com.	5	IN	SRV	0 100 80 svc11.testns.example.com."),
 		},
 		Extra: []dns.RR{
-			test.A("svc11.testns.example.com.	5	IN	A	1.2.3.4"),
+			test.A("svc11.testns.example.com.	5	IN	A	2.3.4.5"),
 		},
 	},
 	// svc12
@@ -211,6 +232,20 @@ func (external) GetNodeByName(ctx context.Context, name string) (*api.Node, erro
 func (external) SvcIndex(s string) []*object.Service                               { return svcIndexExternal[s] }
 func (external) PodIndex(string) []*object.Pod                                     { return nil }
 
+func (external) SvcExtIndexReverse(ip string) (result []*object.Service) {
+	for _, svcs := range svcIndexExternal {
+		for _, svc := range svcs {
+			for _, exIp := range svc.ExternalIPs {
+				if exIp != ip {
+					continue
+				}
+				result = append(result, svc)
+			}
+		}
+	}
+	return result
+}
+
 func (external) GetNamespaceByName(name string) (*object.Namespace, error) {
 	return &object.Namespace{
 		Name: name,
@@ -243,7 +278,7 @@ var svcIndexExternal = map[string][]*object.Service{
 			Name:        "svc11",
 			Namespace:   "testns",
 			Type:        api.ServiceTypeLoadBalancer,
-			ExternalIPs: []string{"1.2.3.4"},
+			ExternalIPs: []string{"2.3.4.5"},
 			Ports:       []api.ServicePort{{Name: "http", Protocol: "tcp", Port: 80}},
 		},
 	},

plugin/k8s_external/msg_to_dns.go

@@ -5,6 +5,7 @@ import (
 	"math"
 
 	"github.com/coredns/coredns/plugin/etcd/msg"
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
 	"github.com/coredns/coredns/request"
 
 	"github.com/miekg/dns"
@@ -76,6 +77,19 @@ func (e *External) aaaa(ctx context.Context, services []msg.Service, state reque
 	return records, truncated
 }
 
+func (e *External) ptr(services []msg.Service, state request.Request) (records []dns.RR) {
+	dup := make(map[string]struct{})
+	for _, s := range services {
+		if _, ok := dup[s.Host]; !ok {
+			dup[s.Host] = struct{}{}
+			rr := s.NewPTR(state.QName(), dnsutil.Join(s.Host, e.Zones[0]))
+			rr.Hdr.Ttl = e.ttl
+			records = append(records, rr)
+		}
+	}
+	return records
+}
+
 func (e *External) srv(ctx context.Context, services []msg.Service, state request.Request) (records, extra []dns.RR) {
 	dup := make(map[item]struct{})
 

plugin/k8s_external/transfer_test.go

@@ -59,6 +59,11 @@ func TestTransferAXFR(t *testing.T) {
 			if ans.Header().Rrtype == dns.TypeTXT {
 				continue
 			}
+
+			// Exclude PTR records
+			if ans.Header().Rrtype == dns.TypePTR {
+				continue
+			}
 			expect = append(expect, ans)
 		}
 	}