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>

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
+}