Moves some functions from Coredns operator that could be reused

utils/utils.go

@@ -0,0 +1,81 @@
+package utils
+
+import (
+	"context"
+	"github.com/pkg/errors"
+	"strings"
+
+	corev1 "k8s.io/api/core/v1"
+	apierrors "k8s.io/apimachinery/pkg/api/errors"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"sigs.k8s.io/controller-runtime/pkg/client"
+	"k8s.io/klog"
+	"net"
+)
+
+const (
+	coreDNSDomain = "cluster.local"
+	coreDNSIP     = "10.96.0.10"
+)
+
+// getCoreDNSService fetches the CoreDNS Service
+func getCoreDNSService(ctx context.Context, c client.Client) (*corev1.Service, error) {
+	kubernetesService := &corev1.Service{}
+	id := client.ObjectKey{Namespace: metav1.NamespaceDefault, Name: "kubernetes"}
+
+	// Get the CoreDNS Service
+	err := c.Get(ctx, id, kubernetesService)
+
+	return kubernetesService, err
+}
+
+
+// FindDNSClusterIP tries to find the Cluster IP to be used by the DNS service
+// It is usually the 10th address to the Kubernetes Service Cluster IP
+// If the Kubernetes Service Cluster IP is not found, we default it to be "10.96.0.10"
+func FindDNSClusterIP(ctx context.Context, c client.Client) (string, error) {
+	kubernetesService, err := getCoreDNSService(ctx, c)
+	if err != nil && !apierrors.IsNotFound(err) {
+		return "", err
+	}
+
+	if apierrors.IsNotFound(err) {
+		// If it cannot determine the Cluster IP, we default it to "10.96.0.10"
+		return coreDNSIP, nil
+	}
+
+	ip := net.ParseIP(kubernetesService.Spec.ClusterIP)
+	if ip == nil {
+		return "", errors.Errorf("cannot parse kubernetes ClusterIP %q", kubernetesService.Spec.ClusterIP)
+	}
+
+	// The kubernetes Service ClusterIP is the 1st IP in the Service Subnet.
+	// Increment the right-most byte by 9 to get to the 10th address, canonically used for CoreDNS.
+	// This works for both IPV4, IPV6, and 16-byte IPV4 addresses.
+	ip[len(ip)-1] += 9
+
+	result := ip.String()
+	klog.Infof("determined ClusterIP for cluster should be %q", result)
+	return result, nil
+}
+
+// GetDNSDomain returns Kubernetes DNS cluster domain
+// If it cannot determine the domain, we default it to "cluster.local"
+// TODO (rajansandeep): find a better way to implement this?
+func GetDNSDomain() string {
+	svc := "kubernetes.default.svc"
+
+	cname, err := net.LookupCNAME(svc)
+	if err != nil {
+		// If it cannot determine the domain, we default it to "cluster.local"
+		klog.Infof("determined DNS Domain for cluster should be %q", coreDNSDomain)
+		return coreDNSDomain
+	}
+
+	domain := strings.TrimPrefix(cname, svc)
+	domain = strings.TrimSuffix(coreDNSDomain, ".")
+
+	klog.Infof("determined DNS Domain for CoreDNS should be %q", domain)
+
+	return domain
+}