Add new plugin: external - resolve k8s ingress and LB address with external names (#2379)

* Add new plugin: external

This plugin works in conjunction with the kubernetes plugin and exports
ingress and LB addresses as DNS records. It bypasses backend.go and
backend_lookup.go flow because it is not needed.

README, tests are implemented. The tests only exercise the unit tests,
this has not been tested in any ci.

Signed-off-by: Miek Gieben <miek@miek.nl>

* Rename to k8s_external

Signed-off-by: Miek Gieben <miek@miek.nl>

* go gen

Signed-off-by: Miek Gieben <miek@miek.nl>

core/dnsserver/zdirectives.go

@@ -34,6 +34,7 @@ var Directives = []string{
 	"hosts",
 	"route53",
 	"federation",
+	"k8s_external",
 	"kubernetes",
 	"file",
 	"auto",

core/plugin/zplugin.go

@@ -20,6 +20,7 @@ import (
 	_ "github.com/coredns/coredns/plugin/forward"
 	_ "github.com/coredns/coredns/plugin/health"
 	_ "github.com/coredns/coredns/plugin/hosts"
+	_ "github.com/coredns/coredns/plugin/k8s_external"
 	_ "github.com/coredns/coredns/plugin/kubernetes"
 	_ "github.com/coredns/coredns/plugin/loadbalance"
 	_ "github.com/coredns/coredns/plugin/log"

plugin.cfg

@@ -43,6 +43,7 @@ template:template
 hosts:hosts
 route53:route53
 federation:federation
+k8s_external:k8s_external
 kubernetes:kubernetes
 file:file
 auto:auto

plugin/k8s_external/OWNERS

@@ -0,0 +1,4 @@
+reviewers:
+  - miekg
+approvers:
+  - miekg

plugin/k8s_external/README.md

@@ -0,0 +1,78 @@
+# k8s_external
+
+## Name
+
+*k8s_external* - resolve load balancer and external IPs from outside kubernetes clusters.
+
+## Description
+
+This plugin allows an additional zone to resolve the external IP address(es) of a Kubernetes
+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
+SOA and NS queries for the apex of the zone.
+
+By default the apex of the zone will look like (assuming the zone used is `example.org`):
+
+~~~ dns
+example.org.	5 IN	SOA ns1.dns.example.org. hostmaster.example.org. (
+				12345      ; serial
+				14400      ; refresh (4 hours)
+				3600       ; retry (1 hour)
+				604800     ; expire (1 week)
+				5          ; minimum (4 hours)
+				)
+example.org		5 IN	NS ns1.dns.example.org.
+
+ns1.dns.example.org.  5 IN  A    ....
+ns1.dns.example.org.  5 IN  AAAA ....
+~~~
+
+Note we use the `dns` subdomain to place the records the DNS needs (see the `apex` directive). Also
+note the SOA's serial number is static. The IP addresses of the nameserver records are those of the
+CoreDNS service.
+
+The *k8s_external* plugin handles the subdomain `dns` and the apex of the zone by itself, all other
+queries are resolved to addresses in the cluster.
+
+## Syntax
+
+~~~
+k8s_external [ZONE...]
+~~~
+
+* **ZONES** zones *k8s_external* should be authoritative for.
+
+If you want to change the apex domain or use a different TTL for the return records you can use
+this extended syntax.
+
+~~~
+k8s_external [ZONE...] {
+    apex APEX
+    ttl TTL
+}
+~~~
+
+* **APEX** is the name (DNS label) to use the apex records, defaults to `dns`.
+* `ttl` allows you to set a custom **TTL** for responses. The default is 5 (seconds).
+
+# Examples
+
+Enable names under `example.org` to be resolved to in cluster DNS addresses.
+
+~~~
+. {
+   kubernetes cluster.local
+   k8s_external example.org
+}
+~~~
+
+# Also See
+
+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/apex.go

@@ -0,0 +1,110 @@
+package external
+
+import (
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// serveApex serves request that hit the zone' apex. A reply is written back to the client.
+func (e *External) serveApex(state request.Request) (int, error) {
+	m := new(dns.Msg)
+	m.SetReply(state.Req)
+	switch state.QType() {
+	case dns.TypeSOA:
+		m.Answer = []dns.RR{e.soa(state)}
+	case dns.TypeNS:
+		m.Answer = []dns.RR{e.ns(state)}
+
+		addr := e.externalAddrFunc(state)
+		for _, rr := range addr {
+			rr.Header().Ttl = e.ttl
+			rr.Header().Name = state.QName()
+			m.Extra = append(m.Extra, rr)
+		}
+	default:
+		m.Ns = []dns.RR{e.soa(state)}
+	}
+
+	state.W.WriteMsg(m)
+	return 0, nil
+}
+
+// serveSubApex serves requests that hit the zones fake 'dns' subdomain where our nameservers live.
+func (e *External) serveSubApex(state request.Request) (int, error) {
+	base, _ := dnsutil.TrimZone(state.Name(), state.Zone)
+
+	m := new(dns.Msg)
+	m.SetReply(state.Req)
+
+	// base is either dns. of ns1.dns (or another name), if it's longer return nxdomain
+	switch labels := dns.CountLabel(base); labels {
+	default:
+		m.SetRcode(m, dns.RcodeNameError)
+		m.Ns = []dns.RR{e.soa(state)}
+		state.W.WriteMsg(m)
+		return 0, nil
+	case 2:
+		nl, _ := dns.NextLabel(base, 0)
+		ns := base[:nl]
+		if ns != "ns1." {
+			// nxdomain
+			m.SetRcode(m, dns.RcodeNameError)
+			m.Ns = []dns.RR{e.soa(state)}
+			state.W.WriteMsg(m)
+			return 0, nil
+		}
+
+		addr := e.externalAddrFunc(state)
+		for _, rr := range addr {
+			rr.Header().Ttl = e.ttl
+			rr.Header().Name = state.QName()
+			switch state.QType() {
+			case dns.TypeA:
+				if rr.Header().Rrtype == dns.TypeA {
+					m.Answer = append(m.Answer, rr)
+				}
+			case dns.TypeAAAA:
+				if rr.Header().Rrtype == dns.TypeAAAA {
+					m.Answer = append(m.Answer, rr)
+				}
+			}
+		}
+
+		if len(m.Answer) == 0 {
+			m.Ns = []dns.RR{e.soa(state)}
+		}
+
+		state.W.WriteMsg(m)
+		return 0, nil
+
+	case 1:
+		// nodata for the dns empty non-terminal
+		m.Ns = []dns.RR{e.soa(state)}
+		state.W.WriteMsg(m)
+		return 0, nil
+	}
+}
+
+func (e *External) soa(state request.Request) *dns.SOA {
+	header := dns.RR_Header{Name: state.Zone, Rrtype: dns.TypeSOA, Ttl: e.ttl, Class: dns.ClassINET}
+
+	soa := &dns.SOA{Hdr: header,
+		Mbox:    dnsutil.Join(e.hostmaster, e.apex, state.Zone),
+		Ns:      dnsutil.Join("ns1", e.apex, state.Zone),
+		Serial:  12345, // Also dynamic?
+		Refresh: 7200,
+		Retry:   1800,
+		Expire:  86400,
+		Minttl:  e.ttl,
+	}
+	return soa
+}
+
+func (e *External) ns(state request.Request) *dns.NS {
+	header := dns.RR_Header{Name: state.Zone, Rrtype: dns.TypeNS, Ttl: e.ttl, Class: dns.ClassINET}
+	ns := &dns.NS{Hdr: header, Ns: dnsutil.Join("ns1", e.apex, state.Zone)}
+
+	return ns
+}

plugin/k8s_external/apex_test.go

@@ -0,0 +1,105 @@
+package external
+
+import (
+	"context"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/kubernetes"
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+func TestApex(t *testing.T) {
+	k := kubernetes.New([]string{"cluster.local."})
+	k.Namespaces = map[string]struct{}{"testns": struct{}{}}
+	k.APIConn = &external{}
+
+	e := New()
+	e.Zones = []string{"example.com."}
+	e.Next = test.NextHandler(dns.RcodeSuccess, nil)
+	e.externalFunc = k.External
+	e.externalAddrFunc = externalAddress // internal test function
+
+	ctx := context.TODO()
+	for i, tc := range testsApex {
+		r := tc.Msg()
+		w := dnstest.NewRecorder(&test.ResponseWriter{})
+
+		_, err := e.ServeDNS(ctx, w, r)
+		if err != tc.Error {
+			t.Errorf("Test %d expected no error, got %v", i, err)
+			return
+		}
+		if tc.Error != nil {
+			continue
+		}
+
+		resp := w.Msg
+		if resp == nil {
+			t.Fatalf("Test %d, got nil message and no error for %q", i, r.Question[0].Name)
+		}
+		test.SortAndCheck(t, resp, tc)
+	}
+}
+
+var testsApex = []test.Case{
+	{
+		Qname: "example.com.", Qtype: dns.TypeSOA, Rcode: dns.RcodeSuccess,
+		Answer: []dns.RR{
+			test.SOA("example.com.	5	IN	SOA	ns1.dns.example.com. hostmaster.example.com. 1499347823 7200 1800 86400 5"),
+		},
+	},
+	{
+		Qname: "example.com.", Qtype: dns.TypeNS, Rcode: dns.RcodeSuccess,
+		Answer: []dns.RR{
+			test.NS("example.com.	5	IN	NS	ns1.dns.example.com."),
+		},
+		Extra: []dns.RR{
+			test.A("example.com.	5	IN	A	127.0.0.1"),
+		},
+	},
+	{
+		Qname: "example.com.", Qtype: dns.TypeSRV, 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"),
+		},
+	},
+	{
+		Qname: "dns.example.com.", Qtype: dns.TypeSRV, 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"),
+		},
+	},
+	{
+		Qname: "dns.example.com.", Qtype: dns.TypeNS, 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"),
+		},
+	},
+	{
+		Qname: "ns1.dns.example.com.", Qtype: dns.TypeSRV, 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"),
+		},
+	},
+	{
+		Qname: "ns1.dns.example.com.", Qtype: dns.TypeNS, 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"),
+		},
+	},
+	{
+		Qname: "ns1.dns.example.com.", Qtype: dns.TypeAAAA, 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"),
+		},
+	},
+	{
+		Qname: "ns1.dns.example.com.", Qtype: dns.TypeA, Rcode: dns.RcodeSuccess,
+		Answer: []dns.RR{
+			test.A("ns1.dns.example.com.	5	IN	A	127.0.0.1"),
+		},
+	},
+}

plugin/k8s_external/external.go

@@ -0,0 +1,112 @@
+/*
+Package external implements external names for kubernetes clusters.
+
+This plugin only handles three qtypes (except the apex queries, because those are handled
+differently). We support A, AAAA and SRV request, for all other types we return NODATA or
+NXDOMAIN depending on the state of the cluster.
+
+A plugin willing to provide these services must implement the Externaler interface, although it
+likely only makes sense for the *kubernetes* plugin.
+
+*/
+package external
+
+import (
+	"context"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/etcd/msg"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// Externaler defines the interface that a plugin should implement in order to be used by External.
+type Externaler interface {
+	// External returns a slice of msg.Services that are looked up in the backend and match
+	// the request.
+	External(request.Request) ([]msg.Service, int)
+	// ExternalAddress should return a string slice of addresses for the nameserving endpoint.
+	ExternalAddress(state request.Request) []dns.RR
+}
+
+// External resolves Ingress and Loadbalance IPs from kubernetes clusters.
+type External struct {
+	Next  plugin.Handler
+	Zones []string
+
+	hostmaster string
+	apex       string
+	ttl        uint32
+
+	externalFunc     func(request.Request) ([]msg.Service, int)
+	externalAddrFunc func(request.Request) []dns.RR
+}
+
+// New returns a new and initialized *External.
+func New() *External {
+	e := &External{hostmaster: "hostmaster", ttl: 5, apex: "dns"}
+	return e
+}
+
+// ServeDNS implements the plugin.Handle interface.
+func (e *External) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	state := request.Request{W: w, Req: r}
+
+	zone := plugin.Zones(e.Zones).Matches(state.Name())
+	if zone == "" {
+		return plugin.NextOrFailure(e.Name(), e.Next, ctx, w, r)
+	}
+
+	if e.externalFunc == nil {
+		return plugin.NextOrFailure(e.Name(), e.Next, ctx, w, r)
+	}
+
+	state.Zone = zone
+	for _, z := range e.Zones {
+		// TODO(miek): save this in the External struct.
+		if state.Name() == z { // apex query
+			ret, err := e.serveApex(state)
+			return ret, err
+		}
+		if dns.IsSubDomain(e.apex+"."+z, state.Name()) {
+			// dns subdomain test for ns. and dns. queries
+			ret, err := e.serveSubApex(state)
+			return ret, err
+		}
+	}
+
+	svc, rcode := e.externalFunc(state)
+
+	m := new(dns.Msg)
+	m.SetReply(state.Req)
+
+	if len(svc) == 0 {
+		m.Rcode = rcode
+		m.Ns = []dns.RR{e.soa(state)}
+		w.WriteMsg(m)
+		return 0, nil
+	}
+
+	switch state.QType() {
+	case dns.TypeA:
+		m.Answer = e.a(svc, state)
+	case dns.TypeAAAA:
+		m.Answer = e.aaaa(svc, state)
+	case dns.TypeSRV:
+		m.Answer, m.Extra = e.srv(svc, state)
+	default:
+		m.Ns = []dns.RR{e.soa(state)}
+	}
+
+	// If we did have records, but queried for the wrong qtype return a nodata response.
+	if len(m.Answer) == 0 {
+		m.Ns = []dns.RR{e.soa(state)}
+	}
+
+	w.WriteMsg(m)
+	return 0, nil
+}
+
+// Name implements the Handler interface.
+func (e *External) Name() string { return "k8s_external" }

plugin/k8s_external/external_test.go

@@ -0,0 +1,210 @@
+package external
+
+import (
+	"context"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/kubernetes"
+	"github.com/coredns/coredns/plugin/kubernetes/object"
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/pkg/watch"
+	"github.com/coredns/coredns/plugin/test"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+	api "k8s.io/api/core/v1"
+	meta "k8s.io/apimachinery/pkg/apis/meta/v1"
+)
+
+func TestExternal(t *testing.T) {
+	k := kubernetes.New([]string{"cluster.local."})
+	k.Namespaces = map[string]struct{}{"testns": struct{}{}}
+	k.APIConn = &external{}
+
+	e := New()
+	e.Zones = []string{"example.com."}
+	e.Next = test.NextHandler(dns.RcodeSuccess, nil)
+	e.externalFunc = k.External
+	e.externalAddrFunc = externalAddress // internal test function
+
+	ctx := context.TODO()
+	for i, tc := range tests {
+		r := tc.Msg()
+		w := dnstest.NewRecorder(&test.ResponseWriter{})
+
+		_, err := e.ServeDNS(ctx, w, r)
+		if err != tc.Error {
+			t.Errorf("Test %d expected no error, got %v", i, err)
+			return
+		}
+		if tc.Error != nil {
+			continue
+		}
+
+		resp := w.Msg
+		if resp == nil {
+			t.Fatalf("Test %d, got nil message and no error for %q", i, r.Question[0].Name)
+		}
+		test.SortAndCheck(t, resp, tc)
+	}
+}
+
+var tests = []test.Case{
+	// A Service
+	{
+		Qname: "svc1.testns.example.com.", Qtype: dns.TypeA, Rcode: dns.RcodeSuccess,
+		Answer: []dns.RR{
+			test.A("svc1.testns.example.com.	5	IN	A	1.2.3.4"),
+		},
+	},
+	{
+		Qname: "svc1.testns.example.com.", Qtype: dns.TypeSRV, Rcode: dns.RcodeSuccess,
+		Answer: []dns.RR{test.SRV("svc1.testns.example.com.	5	IN	SRV	0 100 80 svc1.testns.example.com.")},
+		Extra: []dns.RR{test.A("svc1.testns.example.com.  5       IN      A       1.2.3.4")},
+	},
+	// SRV Service Not udp/tcp
+	{
+		Qname: "*._not-udp-or-tcp.svc1.testns.example.com.", Qtype: dns.TypeSRV, 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"),
+		},
+	},
+	// SRV Service
+	{
+		Qname: "_http._tcp.svc1.testns.example.com.", Qtype: dns.TypeSRV, Rcode: dns.RcodeSuccess,
+		Answer: []dns.RR{
+			test.SRV("_http._tcp.svc1.testns.example.com.	5	IN	SRV	0 100 80 svc1.testns.example.com."),
+		},
+		Extra: []dns.RR{
+			test.A("svc1.testns.example.com.	5	IN	A	1.2.3.4"),
+		},
+	},
+	// AAAA Service (with an existing A record, but no AAAA record)
+	{
+		Qname: "svc1.testns.example.com.", Qtype: dns.TypeAAAA, 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"),
+		},
+	},
+	// AAAA Service (non-existing service)
+	{
+		Qname: "svc0.testns.example.com.", Qtype: dns.TypeAAAA, 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 (non-existing service)
+	{
+		Qname: "svc0.testns.example.com.", Qtype: dns.TypeA, 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 (non-existing namespace)
+	{
+		Qname: "svc0.svc-nons.example.com.", Qtype: dns.TypeA, 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"),
+		},
+	},
+	// AAAA Service
+	{
+		Qname: "svc6.testns.example.com.", Qtype: dns.TypeAAAA, Rcode: dns.RcodeSuccess,
+		Answer: []dns.RR{
+			test.AAAA("svc6.testns.example.com.	5	IN	AAAA	1:2::5"),
+		},
+	},
+	// SRV
+	{
+		Qname: "_http._tcp.svc6.testns.example.com.", Qtype: dns.TypeSRV, Rcode: dns.RcodeSuccess,
+		Answer: []dns.RR{
+			test.SRV("_http._tcp.svc6.testns.example.com.	5	IN	SRV	0 100 80 svc6.testns.example.com."),
+		},
+		Extra: []dns.RR{
+			test.AAAA("svc6.testns.example.com.	5	IN	AAAA	1:2::5"),
+		},
+	},
+	// SRV
+	{
+		Qname: "svc6.testns.example.com.", Qtype: dns.TypeSRV, Rcode: dns.RcodeSuccess,
+		Answer: []dns.RR{
+			test.SRV("svc6.testns.example.com.	5	IN	SRV	0 100 80 svc6.testns.example.com."),
+		},
+		Extra: []dns.RR{
+			test.AAAA("svc6.testns.example.com.	5	IN	AAAA	1:2::5"),
+		},
+	},
+	{
+		Qname: "testns.example.com.", Qtype: dns.TypeA, 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"),
+		},
+	},
+	{
+		Qname: "testns.example.com.", Qtype: dns.TypeSOA, 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"),
+		},
+	},
+}
+
+type external struct{}
+
+func (external) HasSynced() bool                              { return true }
+func (external) Run()                                         { return }
+func (external) Stop() error                                  { return nil }
+func (external) EpIndexReverse(string) []*object.Endpoints    { return nil }
+func (external) SvcIndexReverse(string) []*object.Service     { return nil }
+func (external) Modified() int64                              { return 0 }
+func (external) SetWatchChan(watch.Chan)                      {}
+func (external) Watch(string) error                           { return nil }
+func (external) StopWatching(string)                          {}
+func (external) EpIndex(s string) []*object.Endpoints         { return nil }
+func (external) EndpointsList() []*object.Endpoints           { return nil }
+func (external) GetNodeByName(name string) (*api.Node, error) { return nil, nil }
+func (external) SvcIndex(s string) []*object.Service          { return svcIndexExternal[s] }
+func (external) PodIndex(string) []*object.Pod                { return nil }
+
+func (external) GetNamespaceByName(name string) (*api.Namespace, error) {
+	return &api.Namespace{
+		ObjectMeta: meta.ObjectMeta{
+			Name: name,
+		},
+	}, nil
+}
+
+var svcIndexExternal = map[string][]*object.Service{
+	"svc1.testns": {
+		{
+			Name:        "svc1",
+			Namespace:   "testns",
+			Type:        api.ServiceTypeClusterIP,
+			ClusterIP:   "10.0.0.1",
+			ExternalIPs: []string{"1.2.3.4"},
+			Ports:       []api.ServicePort{{Name: "http", Protocol: "tcp", Port: 80}},
+		},
+	},
+	"svc6.testns": {
+		{
+			Name:        "svc6",
+			Namespace:   "testns",
+			Type:        api.ServiceTypeClusterIP,
+			ClusterIP:   "10.0.0.3",
+			ExternalIPs: []string{"1:2::5"},
+			Ports:       []api.ServicePort{{Name: "http", Protocol: "tcp", Port: 80}},
+		},
+	},
+}
+
+func (external) ServiceList() []*object.Service {
+	var svcs []*object.Service
+	for _, svc := range svcIndexExternal {
+		svcs = append(svcs, svc...)
+	}
+	return svcs
+}
+
+func externalAddress(state request.Request) []dns.RR {
+	a := test.A("example.org. IN A 127.0.0.1")
+	return []dns.RR{a}
+}

plugin/k8s_external/msg_to_dns.go

@@ -0,0 +1,148 @@
+package external
+
+import (
+	"math"
+
+	"github.com/coredns/coredns/plugin/etcd/msg"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+func (e *External) a(services []msg.Service, state request.Request) (records []dns.RR) {
+	dup := make(map[string]struct{})
+
+	for _, s := range services {
+
+		what, ip := s.HostType()
+
+		switch what {
+		case dns.TypeCNAME:
+			// can't happen
+
+		case dns.TypeA:
+			if _, ok := dup[s.Host]; !ok {
+				dup[s.Host] = struct{}{}
+				rr := s.NewA(state.QName(), ip)
+				rr.Hdr.Ttl = e.ttl
+				records = append(records, rr)
+			}
+
+		case dns.TypeAAAA:
+			// nada
+		}
+	}
+	return records
+}
+
+func (e *External) aaaa(services []msg.Service, state request.Request) (records []dns.RR) {
+	dup := make(map[string]struct{})
+
+	for _, s := range services {
+
+		what, ip := s.HostType()
+
+		switch what {
+		case dns.TypeCNAME:
+			// can't happen
+
+		case dns.TypeA:
+			// nada
+
+		case dns.TypeAAAA:
+			if _, ok := dup[s.Host]; !ok {
+				dup[s.Host] = struct{}{}
+				rr := s.NewAAAA(state.QName(), ip)
+				rr.Hdr.Ttl = e.ttl
+				records = append(records, rr)
+			}
+		}
+	}
+	return records
+}
+
+func (e *External) srv(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.
+	w := make(map[int]int)
+	for _, s := range services {
+		weight := 100
+		if s.Weight != 0 {
+			weight = s.Weight
+		}
+		if _, ok := w[s.Priority]; !ok {
+			w[s.Priority] = weight
+			continue
+		}
+		w[s.Priority] += weight
+	}
+	for _, s := range services {
+		// Don't add the entry if the port is -1 (invalid). The kubernetes plugin uses port -1 when a service/endpoint
+		// does not have any declared ports.
+		if s.Port == -1 {
+			continue
+		}
+		w1 := 100.0 / float64(w[s.Priority])
+		if s.Weight == 0 {
+			w1 *= 100
+		} else {
+			w1 *= float64(s.Weight)
+		}
+		weight := uint16(math.Floor(w1))
+
+		what, ip := s.HostType()
+
+		switch what {
+		case dns.TypeCNAME:
+			// can't happen
+
+		case dns.TypeA, dns.TypeAAAA:
+			addr := s.Host
+			s.Host = msg.Domain(s.Key)
+			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 {
+				hdr := dns.RR_Header{Name: srv.Target, Rrtype: what, Class: dns.ClassINET, Ttl: e.ttl}
+
+				switch what {
+				case dns.TypeA:
+					extra = append(extra, &dns.A{Hdr: hdr, A: ip})
+				case dns.TypeAAAA:
+					extra = append(extra, &dns.AAAA{Hdr: hdr, AAAA: ip})
+				}
+			}
+		}
+	}
+	return records, extra
+}
+
+// not sure if this is even needed.
+
+// item holds records.
+type item struct {
+	name string // name of the record (either owner or something else unique).
+	port uint16 // port of the record (used for address records, A and AAAA).
+	addr string // address of the record (A and AAAA).
+}
+
+// isDuplicate uses m to see if the combo (name, addr, port) already exists. If it does
+// not exist already IsDuplicate will also add the record to the map.
+func isDuplicate(m map[item]struct{}, name, addr string, port uint16) bool {
+	if addr != "" {
+		_, ok := m[item{name, 0, addr}]
+		if !ok {
+			m[item{name, 0, addr}] = struct{}{}
+		}
+		return ok
+	}
+	_, ok := m[item{name, port, ""}]
+	if !ok {
+		m[item{name, port, ""}] = struct{}{}
+	}
+	return ok
+}

plugin/k8s_external/setup.go

@@ -0,0 +1,86 @@
+package external
+
+import (
+	"strconv"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("k8s_external", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	e, err := parse(c)
+	if err != nil {
+		return plugin.Error("k8s_external", err)
+	}
+
+	// Do this in OnStartup, so all plugins have been initialized.
+	c.OnStartup(func() error {
+		m := dnsserver.GetConfig(c).Handler("kubernetes")
+		if m == nil {
+			return nil
+		}
+		if x, ok := m.(Externaler); ok {
+			e.externalFunc = x.External
+			e.externalAddrFunc = x.ExternalAddress
+		}
+		return nil
+	})
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		e.Next = next
+		return e
+	})
+
+	return nil
+}
+
+func parse(c *caddy.Controller) (*External, error) {
+	e := New()
+
+	for c.Next() { // external
+		zones := c.RemainingArgs()
+		e.Zones = zones
+		if len(zones) == 0 {
+			e.Zones = make([]string, len(c.ServerBlockKeys))
+			copy(e.Zones, c.ServerBlockKeys)
+		}
+		for i, str := range e.Zones {
+			e.Zones[i] = plugin.Host(str).Normalize()
+		}
+		for c.NextBlock() {
+			switch c.Val() {
+			case "ttl":
+				args := c.RemainingArgs()
+				if len(args) == 0 {
+					return nil, c.ArgErr()
+				}
+				t, err := strconv.Atoi(args[0])
+				if err != nil {
+					return nil, err
+				}
+				if t < 0 || t > 3600 {
+					return nil, c.Errf("ttl must be in range [0, 3600]: %d", t)
+				}
+				e.ttl = uint32(t)
+			case "apex":
+				args := c.RemainingArgs()
+				if len(args) == 0 {
+					return nil, c.ArgErr()
+				}
+				e.apex = args[0]
+			default:
+				return nil, c.Errf("unknown property '%s'", c.Val())
+			}
+		}
+	}
+	return e, nil
+}

plugin/k8s_external/setup_test.go

@@ -0,0 +1,48 @@
+package external
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestSetup(t *testing.T) {
+	tests := []struct {
+		input        string
+		shouldErr    bool
+		expectedZone string
+		expectedApex string
+	}{
+		{`k8s_external`, false, "", "dns"},
+		{`k8s_external example.org`, false, "example.org.", "dns"},
+		{`k8s_external example.org {
+			apex testdns
+}`, false, "example.org.", "testdns"},
+	}
+
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.input)
+		e, err := parse(c)
+
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %d: Expected error but found %s for input %s", i, err, test.input)
+		}
+
+		if err != nil {
+			if !test.shouldErr {
+				t.Errorf("Test %d: Expected no error but found one for input %s. Error was: %v", i, test.input, err)
+			}
+		}
+
+		if !test.shouldErr && test.expectedZone != "" {
+			if test.expectedZone != e.Zones[0] {
+				t.Errorf("Test %d, expected zone %q for input %s, got: %q", i, test.expectedZone, test.input, e.Zones[0])
+			}
+		}
+		if !test.shouldErr {
+			if test.expectedApex != e.apex {
+				t.Errorf("Test %d, expected apex %q for input %s, got: %q", i, test.expectedApex, test.input, e.apex)
+			}
+		}
+	}
+}

plugin/kubernetes/controller.go

@@ -172,7 +172,11 @@ func svcIPIndexFunc(obj interface{}) ([]string, error) {
 	if !ok {
 		return nil, errObj
 	}
-	return []string{svc.ClusterIP}, nil
+	if len(svc.ExternalIPs) == 0 {
+		return []string{svc.ClusterIP}, nil
+	}
+
+	return append([]string{svc.ClusterIP}, svc.ExternalIPs...), nil
 }
 
 func svcNameNamespaceIndexFunc(obj interface{}) ([]string, error) {

plugin/kubernetes/external.go

@@ -0,0 +1,92 @@
+package kubernetes
+
+import (
+	"strings"
+
+	"github.com/coredns/coredns/plugin/etcd/msg"
+	"github.com/coredns/coredns/plugin/kubernetes/object"
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// External implements the ExternalFunc call from the external plugin.
+// It returns any services matching in the services' ExternalIPs.
+func (k *Kubernetes) External(state request.Request) ([]msg.Service, int) {
+	base, _ := dnsutil.TrimZone(state.Name(), state.Zone)
+
+	segs := dns.SplitDomainName(base)
+	last := len(segs) - 1
+	if last < 0 {
+		return nil, dns.RcodeServerFailure
+	}
+	// We dealing with a fairly normal domain name here, but; we still need to have the service
+	// and the namespace:
+	// service.namespace.<base>
+	//
+	// for service (and SRV) you can also say _tcp, and port (i.e. _http), we need those be picked
+	// up, unless they are not specified, then we use an internal wildcard.
+	port := "*"
+	protocol := "*"
+	namespace := segs[last]
+	if !k.namespaceExposed(namespace) || !k.namespace(namespace) {
+		return nil, dns.RcodeNameError
+	}
+
+	last--
+	if last < 0 {
+		return nil, dns.RcodeSuccess
+	}
+
+	service := segs[last]
+	last--
+	if last == 1 {
+		protocol = stripUnderscore(segs[last])
+		port = stripUnderscore(segs[last-1])
+		last -= 2
+	}
+
+	if last != -1 {
+		// too long
+		return nil, dns.RcodeNameError
+	}
+
+	idx := object.ServiceKey(service, namespace)
+	serviceList := k.APIConn.SvcIndex(idx)
+
+	services := []msg.Service{}
+	zonePath := msg.Path(state.Zone, coredns)
+	rcode := dns.RcodeNameError
+
+	for _, svc := range serviceList {
+		if namespace != svc.Namespace {
+			continue
+		}
+		if service != svc.Name {
+			continue
+		}
+
+		for _, ip := range svc.ExternalIPs {
+			for _, p := range svc.Ports {
+				if !(match(port, p.Name) && match(protocol, string(p.Protocol))) {
+					continue
+				}
+				rcode = dns.RcodeSuccess
+				s := msg.Service{Host: ip, Port: int(p.Port), TTL: k.ttl}
+				s.Key = strings.Join([]string{zonePath, svc.Namespace, svc.Name}, "/")
+
+				services = append(services, s)
+			}
+		}
+	}
+	return services, rcode
+}
+
+// ExternalAddress returns the external service address(es) for the CoreDNS service.
+func (k *Kubernetes) ExternalAddress(state request.Request) []dns.RR {
+	// This is probably wrong, because of all the fallback behavior of k.nsAddr, i.e. can get
+	// an address that isn't reacheable from outside the cluster.
+	rrs := []dns.RR{k.nsAddr()}
+	return rrs
+}

plugin/kubernetes/external_test.go

@@ -0,0 +1,139 @@
+package kubernetes
+
+import (
+	"testing"
+
+	"github.com/coredns/coredns/plugin/etcd/msg"
+	"github.com/coredns/coredns/plugin/kubernetes/object"
+	"github.com/coredns/coredns/plugin/pkg/watch"
+	"github.com/coredns/coredns/plugin/test"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+	api "k8s.io/api/core/v1"
+	meta "k8s.io/apimachinery/pkg/apis/meta/v1"
+)
+
+var extCases = []struct {
+	Qname string
+	Qtype uint16
+	Msg   []msg.Service
+	Rcode int
+}{
+	{
+		Qname: "svc1.testns.example.org.", Rcode: dns.RcodeSuccess,
+		Msg: []msg.Service{
+			msg.Service{Host: "1.2.3.4", Port: 80, TTL: 5, Key: "/c/org/example/testns/svc1"},
+		},
+	},
+	{
+		Qname: "svc6.testns.example.org.", Rcode: dns.RcodeSuccess,
+		Msg: []msg.Service{
+			msg.Service{Host: "1:2::5", Port: 80, TTL: 5, Key: "/c/org/example/testns/svc1"},
+		},
+	},
+	{
+		Qname: "*._not-udp-or-tcp.svc1.testns.example.com.", Rcode: dns.RcodeSuccess,
+	},
+	{
+		Qname: "_http._tcp.svc1.testns.example.com.", Rcode: dns.RcodeSuccess,
+		Msg: []msg.Service{
+			msg.Service{Host: "1.2.3.4", Port: 80, TTL: 5, Key: "/c/org/example/testns/svc1"},
+		},
+	},
+	{
+		Qname: "svc0.testns.example.com.", Rcode: dns.RcodeNameError,
+	},
+	{
+		Qname: "svc0.svc-nons.example.com.", Rcode: dns.RcodeNameError,
+	},
+}
+
+func TestExternal(t *testing.T) {
+	k := New([]string{"cluster.local."})
+	k.APIConn = &external{}
+	k.Next = test.NextHandler(dns.RcodeSuccess, nil)
+	k.Namespaces = map[string]struct{}{"testns": struct{}{}}
+
+	for i, tc := range extCases {
+		state := testRequest(tc.Qname)
+
+		svc, rcode := k.External(state)
+
+		if x := tc.Rcode; x != rcode {
+			t.Errorf("Test %d, expected rcode %d, got %d\n", i, x, rcode)
+		}
+
+		if len(svc) != len(tc.Msg) {
+			t.Errorf("Test %d, expected %d for messages, got %d\n", i, len(tc.Msg), len(svc))
+		}
+
+		for j, s := range svc {
+			if x := tc.Msg[j].Key; x != s.Key {
+				t.Errorf("Test %d, expected key %s, got %s\n", i, x, s.Key)
+			}
+			return
+		}
+	}
+}
+
+type external struct{}
+
+func (external) HasSynced() bool                              { return true }
+func (external) Run()                                         { return }
+func (external) Stop() error                                  { return nil }
+func (external) EpIndexReverse(string) []*object.Endpoints    { return nil }
+func (external) SvcIndexReverse(string) []*object.Service     { return nil }
+func (external) Modified() int64                              { return 0 }
+func (external) SetWatchChan(watch.Chan)                      {}
+func (external) Watch(string) error                           { return nil }
+func (external) StopWatching(string)                          {}
+func (external) EpIndex(s string) []*object.Endpoints         { return nil }
+func (external) EndpointsList() []*object.Endpoints           { return nil }
+func (external) GetNodeByName(name string) (*api.Node, error) { return nil, nil }
+func (external) SvcIndex(s string) []*object.Service          { return svcIndexExternal[s] }
+func (external) PodIndex(string) []*object.Pod                { return nil }
+
+func (external) GetNamespaceByName(name string) (*api.Namespace, error) {
+	return &api.Namespace{
+		ObjectMeta: meta.ObjectMeta{
+			Name: name,
+		},
+	}, nil
+}
+
+var svcIndexExternal = map[string][]*object.Service{
+	"svc1.testns": {
+		{
+			Name:        "svc1",
+			Namespace:   "testns",
+			Type:        api.ServiceTypeClusterIP,
+			ClusterIP:   "10.0.0.1",
+			ExternalIPs: []string{"1.2.3.4"},
+			Ports:       []api.ServicePort{{Name: "http", Protocol: "tcp", Port: 80}},
+		},
+	},
+	"svc6.testns": {
+		{
+			Name:        "svc6",
+			Namespace:   "testns",
+			Type:        api.ServiceTypeClusterIP,
+			ClusterIP:   "10.0.0.3",
+			ExternalIPs: []string{"1:2::5"},
+			Ports:       []api.ServicePort{{Name: "http", Protocol: "tcp", Port: 80}},
+		},
+	},
+}
+
+func (external) ServiceList() []*object.Service {
+	var svcs []*object.Service
+	for _, svc := range svcIndexExternal {
+		svcs = append(svcs, svc...)
+	}
+	return svcs
+}
+
+func testRequest(name string) request.Request {
+	m := new(dns.Msg).SetQuestion(name, dns.TypeA)
+	return request.Request{W: &test.ResponseWriter{}, Req: m, Zone: "example.org."}
+}

plugin/kubernetes/ns.go

@@ -12,6 +12,10 @@ func isDefaultNS(name, zone string) bool {
 	return strings.Index(name, defaultNSName) == 0 && strings.Index(name, zone) == len(defaultNSName)
 }
 
+// nsAddr return the A record for the CoreDNS service in the cluster. If it fails that it fallsback
+// on the local address of the machine we're running on.
+//
+// This function is rather expensive to run.
 func (k *Kubernetes) nsAddr() *dns.A {
 	var (
 		svcName      string

plugin/kubernetes/object/service.go

@@ -16,6 +16,9 @@ type Service struct {
 	ExternalName string
 	Ports        []api.ServicePort
 
+	// ExternalIPs we may want to export.
+	ExternalIPs []string
+
 	*Empty
 }
 
@@ -37,6 +40,8 @@ func ToService(obj interface{}) interface{} {
 		ClusterIP:    svc.Spec.ClusterIP,
 		Type:         svc.Spec.Type,
 		ExternalName: svc.Spec.ExternalName,
+
+		ExternalIPs: make([]string, len(svc.Status.LoadBalancer.Ingress)+len(svc.Spec.ExternalIPs)),
 	}
 
 	if len(svc.Spec.Ports) == 0 {
@@ -47,6 +52,11 @@ func ToService(obj interface{}) interface{} {
 		copy(s.Ports, svc.Spec.Ports)
 	}
 
+	li := copy(s.ExternalIPs, svc.Spec.ExternalIPs)
+	for i, lb := range svc.Status.LoadBalancer.Ingress {
+		s.ExternalIPs[li+i] = lb.IP
+	}
+
 	*svc = api.Service{}
 
 	return s
@@ -65,8 +75,10 @@ func (s *Service) DeepCopyObject() runtime.Object {
 		Type:         s.Type,
 		ExternalName: s.ExternalName,
 		Ports:        make([]api.ServicePort, len(s.Ports)),
+		ExternalIPs:  make([]string, len(s.ExternalIPs)),
 	}
 	copy(s1.Ports, s.Ports)
+	copy(s1.ExternalIPs, s.ExternalIPs)
 	return s1
 }