Merge pull request #51990 from madhukar32/get_dns_ip

Automatic merge from submit-queue (batch tested with PRs 55798, 49579, 54862, 55188, 51990). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

kubeadm: Create dnsIP by selecting the tenth IP from k8s svc CIDR

**What this PR does / why we need it**:

Creates dnsIP by selecting the ninth IP from k8s svc cluster IP, instead of appending 0 to the k8s svcIP string. 

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #51997

**Special notes for your reviewer**:
This is helpful when we have service cluster range CIDR as 10.87.116.64/26 (for example), previously this would have failed while parsing the dnsIP, as we used to append a 0 to the k8s svc clusterIP string. This will get the same dnsIP 10.96.0.10 for very widely used service cluster range CIDR 10.96.0.0/12


**Release note**:
```release-note
None
```

cmd/kubeadm/app/cmd/phases/addons.go

@@ -116,6 +116,7 @@ func getAddonsSubCommands() []*cobra.Command {
 
 		if properties.use == "all" || properties.use == "kube-dns" {
 			cmd.Flags().StringVar(&cfg.Networking.DNSDomain, "service-dns-domain", cfg.Networking.DNSDomain, `Use alternative domain for services, e.g. "myorg.internal.`)
+			cmd.Flags().StringVar(&cfg.Networking.ServiceSubnet, "service-cidr", cfg.Networking.ServiceSubnet, `Use alternative range of IP address for service VIPs`)
 		}
 		subCmds = append(subCmds, cmd)
 	}

cmd/kubeadm/app/cmd/phases/addons_test.go

@@ -47,6 +47,7 @@ func TestAddonsSubCommandsHasFlags(t *testing.T) {
 				"apiserver-bind-port",
 				"pod-network-cidr",
 				"service-dns-domain",
+				"service-cidr",
 			},
 		},
 		{
@@ -61,6 +62,7 @@ func TestAddonsSubCommandsHasFlags(t *testing.T) {
 			command: "kube-dns",
 			additionalFlags: []string{
 				"service-dns-domain",
+				"service-cidr",
 			},
 		},
 	}

cmd/kubeadm/app/phases/addons/dns/BUILD

@@ -41,6 +41,7 @@ go_library(
         "//cmd/kubeadm/app/util:go_default_library",
         "//cmd/kubeadm/app/util/apiclient:go_default_library",
         "//pkg/api/legacyscheme:go_default_library",
+        "//pkg/registry/core/service/ipallocator:go_default_library",
         "//pkg/util/version:go_default_library",
         "//vendor/k8s.io/api/apps/v1beta2:go_default_library",
         "//vendor/k8s.io/api/core/v1:go_default_library",

cmd/kubeadm/app/phases/addons/dns/dns.go

@@ -34,6 +34,7 @@ import (
 	kubeadmutil "k8s.io/kubernetes/cmd/kubeadm/app/util"
 	"k8s.io/kubernetes/cmd/kubeadm/app/util/apiclient"
 	"k8s.io/kubernetes/pkg/api/legacyscheme"
+	"k8s.io/kubernetes/pkg/registry/core/service/ipallocator"
 	"k8s.io/kubernetes/pkg/util/version"
 )
 
@@ -59,7 +60,7 @@ func kubeDNSAddon(cfg *kubeadmapi.MasterConfiguration, client clientset.Interfac
 		return err
 	}
 
-	dnsip, err := getDNSIP(client)
+	dnsip, err := GetDNSIP(cfg.Networking.ServiceSubnet)
 	if err != nil {
 		return err
 	}
@@ -148,7 +149,7 @@ func coreDNSAddon(cfg *kubeadmapi.MasterConfiguration, client clientset.Interfac
 		return fmt.Errorf("error when parsing CoreDNS configMap template: %v", err)
 	}
 
-	dnsip, err := getDNSIP(client)
+	dnsip, err := GetDNSIP(cfg.Networking.ServiceSubnet)
 	if err != nil {
 		return err
 	}
@@ -244,21 +245,20 @@ func createDNSService(dnsService *v1.Service, serviceBytes []byte, client client
 	return nil
 }
 
-// getDNSIP fetches the kubernetes service's ClusterIP and appends a "0" to it in order to get the DNS IP
-func getDNSIP(client clientset.Interface) (net.IP, error) {
-	k8ssvc, err := client.CoreV1().Services(metav1.NamespaceDefault).Get("kubernetes", metav1.GetOptions{})
+// GetDNSIP returns a dnsIP, which is 10th IP in svcSubnet CIDR range
+func GetDNSIP(svcSubnet string) (net.IP, error) {
+
+	// Get the service subnet CIDR
+	_, svcSubnetCIDR, err := net.ParseCIDR(svcSubnet)
 	if err != nil {
-		return nil, fmt.Errorf("couldn't fetch information about the kubernetes service: %v", err)
+		return nil, fmt.Errorf("couldn't parse service subnet CIDR %q: %v", svcSubnet, err)
 	}
 
-	if len(k8ssvc.Spec.ClusterIP) == 0 {
-		return nil, fmt.Errorf("couldn't fetch a valid clusterIP from the kubernetes service")
+	// Selects the 10th IP in service subnet CIDR range as dnsIP
+	dnsIP, err := ipallocator.GetIndexedIP(svcSubnetCIDR, 10)
+	if err != nil {
+		return nil, fmt.Errorf("unable to get tenth IP address from service subnet CIDR %s: %v", svcSubnetCIDR.String(), err)
 	}
 
-	// Build an IP by taking the kubernetes service's clusterIP and appending a "0" and checking that it's valid
-	dnsIP := net.ParseIP(fmt.Sprintf("%s0", k8ssvc.Spec.ClusterIP))
-	if dnsIP == nil {
-		return nil, fmt.Errorf("could not parse dns ip %q: %v", dnsIP, err)
-	}
 	return dnsIP, nil
 }

cmd/kubeadm/app/phases/addons/dns/dns_test.go

@@ -144,3 +144,33 @@ func TestCompileManifests(t *testing.T) {
 		}
 	}
 }
+
+func TestGetDNSIP(t *testing.T) {
+	var tests = []struct {
+		svcSubnet, expectedDNSIP string
+	}{
+		{
+			svcSubnet:     "10.96.0.0/12",
+			expectedDNSIP: "10.96.0.10",
+		},
+		{
+			svcSubnet:     "10.87.116.64/26",
+			expectedDNSIP: "10.87.116.74",
+		},
+	}
+	for _, rt := range tests {
+		dnsIP, err := GetDNSIP(rt.svcSubnet)
+		if err != nil {
+			t.Fatalf("couldn't get dnsIP : %v", err)
+		}
+
+		actualDNSIP := dnsIP.String()
+		if actualDNSIP != rt.expectedDNSIP {
+			t.Errorf(
+				"failed GetDNSIP\n\texpected: %s\n\t  actual: %s",
+				rt.expectedDNSIP,
+				actualDNSIP,
+			)
+		}
+	}
+}