Add flag to support ClusterIP exposed CoreDNS (#1788)

Some environments (Calico bare-metal, etc) may allow direct client
reachability to the Service CIDR, bypassing the need to assign and use
LoadBalancerIPs.  This PR adds logic to determine if the coredns service
is of type ClusterIP or LoadBalancer and returns the respective IP addresses.

---------

Signed-off-by: Brandon Ewing <brandon.ewing@imc.com>

controllers/providers/assistant/assistant.go

@@ -21,10 +21,13 @@ Generated by GoLic, for more details see: https://github.com/AbsaOSS/golic
 import (
 	"time"
 
+	corev1 "k8s.io/api/core/v1"
 	externaldns "sigs.k8s.io/external-dns/endpoint"
 )
 
 type Assistant interface {
+	// GetCoreDNSService returns the CoreDNS Service
+	GetCoreDNSService() (*corev1.Service, error)
 	// CoreDNSExposedIPs retrieves list of exposed IP by CoreDNS
 	CoreDNSExposedIPs() ([]string, error)
 	// GetExternalTargets retrieves slice of targets from external clusters

controllers/providers/assistant/gslb.go

@@ -58,8 +58,8 @@ func NewGslbAssistant(client client.Client, k8gbNamespace string, edgeDNSServers
 	}
 }
 
-// CoreDNSExposedIPs retrieves list of IP's exposed by CoreDNS
-func (r *Gslb) CoreDNSExposedIPs() ([]string, error) {
+// GetCoreDNSService returns the CoreDNS Service
+func (r *Gslb) GetCoreDNSService() (*corev1.Service, error) {
 	serviceList := &corev1.ServiceList{}
 	sel, err := labels.Parse(coreDNSServiceLabel)
 	if err != nil {
@@ -88,7 +88,27 @@ func (r *Gslb) CoreDNSExposedIPs() ([]string, error) {
 		return nil, err
 	}
 	coreDNSService := &serviceList.Items[0]
+	return coreDNSService, nil
+}
 
+// CoreDNSExposedIPs retrieves list of IP's exposed by CoreDNS
+func (r *Gslb) CoreDNSExposedIPs() ([]string, error) {
+	coreDNSService, err := r.GetCoreDNSService()
+	if err != nil {
+		return nil, err
+	}
+	if coreDNSService.Spec.Type == "ClusterIP" {
+		if len(coreDNSService.Spec.ClusterIPs) == 0 {
+			errMessage := "no ClusterIPs found"
+			log.Warn().
+				Str("serviceName", coreDNSService.Name).
+				Msg(errMessage)
+			err := coreerrors.New(errMessage)
+			return nil, err
+		}
+		return coreDNSService.Spec.ClusterIPs, nil
+	}
+	// LoadBalancer / ExternalName / NodePort service
 	var lb corev1.LoadBalancerIngress
 	if len(coreDNSService.Status.LoadBalancer.Ingress) == 0 {
 		errMessage := "no LoadBalancer ExternalIPs are found"