plugin/kubernetes: Handle multiple local IPs and bind (#3208)

* use all local IPs

* mult/bind ips

* gofmt + boundIPs fix

* fix no matching endpoint case

* don't duplicate NS records in answer

* fix answer dedup

* fix comment

* add multi local ip test case

plugin/kubernetes/local.go

@@ -2,41 +2,54 @@ package kubernetes
 
 import (
 	"net"
+
+	"github.com/caddyserver/caddy"
+	"github.com/coredns/coredns/core/dnsserver"
 )
 
-func localPodIP() net.IP {
-	addrs, err := net.InterfaceAddrs()
-	if err != nil {
-		return nil
+// boundIPs returns the list of non-loopback IPs that CoreDNS is bound to
+func boundIPs(c *caddy.Controller) (ips []net.IP) {
+	conf := dnsserver.GetConfig(c)
+	hosts := conf.ListenHosts
+	if hosts == nil || hosts[0] == "" {
+		hosts = nil
+		addrs, err := net.InterfaceAddrs()
+		if err != nil {
+			return nil
+		}
+		for _, addr := range addrs {
+			hosts = append(hosts, addr.String())
+		}
 	}
-
-	for _, addr := range addrs {
-		ip, _, _ := net.ParseCIDR(addr.String())
+	for _, host := range hosts {
+		ip, _, _ := net.ParseCIDR(host)
 		ip4 := ip.To4()
 		if ip4 != nil && !ip4.IsLoopback() {
-			return ip4
+			ips = append(ips, ip4)
+			continue
 		}
 		ip6 := ip.To16()
 		if ip6 != nil && !ip6.IsLoopback() {
-			return ip6
+			ips = append(ips, ip6)
 		}
 	}
-	return nil
+	return ips
 }
 
 // LocalNodeName is exclusively used in federation plugin, will be deprecated later.
 func (k *Kubernetes) LocalNodeName() string {
-	localIP := k.interfaceAddrsFunc()
-	if localIP == nil {
+	if len(k.localIPs) == 0 {
 		return ""
 	}
 
-	// Find endpoint matching localIP
-	for _, ep := range k.APIConn.EpIndexReverse(localIP.String()) {
-		for _, eps := range ep.Subsets {
-			for _, addr := range eps.Addresses {
-				if localIP.Equal(net.ParseIP(addr.IP)) {
-					return addr.NodeName
+	// Find fist endpoint matching any localIP
+	for _, localIP := range k.localIPs {
+		for _, ep := range k.APIConn.EpIndexReverse(localIP.String()) {
+			for _, eps := range ep.Subsets {
+				for _, addr := range eps.Addresses {
+					if localIP.Equal(net.ParseIP(addr.IP)) {
+						return addr.NodeName
+					}
 				}
 			}
 		}