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/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
+}