127.0.0.1-as-advertise-address

cmd/kubeadm/app/util/config/common.go

@@ -123,6 +123,14 @@ func VerifyAPIServerBindAddress(address string) error {
 	if ip == nil {
 		return errors.Errorf("cannot parse IP address: %s", address)
 	}
+	// There are users with network setups where default routes are present, but network interfaces
+	// use only link-local addresses (e.g. as described in RFC5549).
+	// In many cases that matching global unicast IP address can be found on loopback interface,
+	// so kubeadm allows users to specify address=Loopback for handling supporting the scenario above.
+	// Nb. SetAPIEndpointDynamicDefaults will try to translate loopback to a valid address afterwards
+	if ip.IsLoopback() {
+		return nil
+	}
 	if !ip.IsGlobalUnicast() {
 		return errors.Errorf("cannot use %q as the bind address for the API Server", address)
 	}

cmd/kubeadm/app/util/config/common_test.go

@@ -161,13 +161,13 @@ func TestVerifyAPIServerBindAddress(t *testing.T) {
 			address: "2001:db8:85a3::8a2e:370:7334",
 		},
 		{
-			name:          "invalid address: not a global unicast 0.0.0.0",
-			address:       "0.0.0.0",
-			expectedError: true,
+			name:          "valid address 127.0.0.1",
+			address:       "127.0.0.1",
+			expectedError: false,
 		},
 		{
-			name:          "invalid address: not a global unicast 127.0.0.1",
-			address:       "127.0.0.1",
+			name:          "invalid address: not a global unicast 0.0.0.0",
+			address:       "0.0.0.0",
 			expectedError: true,
 		},
 		{

cmd/kubeadm/app/util/config/initconfiguration.go

@@ -26,9 +26,10 @@ import (
 	"github.com/pkg/errors"
 	"k8s.io/klog"
 
-	"k8s.io/api/core/v1"
+	v1 "k8s.io/api/core/v1"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
+	netutil "k8s.io/apimachinery/pkg/util/net"
 	bootstraputil "k8s.io/cluster-bootstrap/token/util"
 	kubeadmapi "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm"
 	kubeadmscheme "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/scheme"
@@ -112,6 +113,22 @@ func SetAPIEndpointDynamicDefaults(cfg *kubeadmapi.APIEndpoint) error {
 	if addressIP == nil && cfg.AdvertiseAddress != "" {
 		return errors.Errorf("couldn't use \"%s\" as \"apiserver-advertise-address\", must be ipv4 or ipv6 address", cfg.AdvertiseAddress)
 	}
+
+	// kubeadm allows users to specify address=Loopback as a selector for global unicast IP address that can be found on loopback interface.
+	// e.g. This is required for network setups where default routes are present, but network interfaces use only link-local addresses (e.g. as described in RFC5549).
+	if addressIP.IsLoopback() {
+		loopbackIP, err := netutil.ChooseBindAddressForInterface(netutil.LoopbackInterfaceName)
+		if err != nil {
+			return err
+		}
+		if loopbackIP != nil {
+			klog.V(4).Infof("Found active IP %v on loopback interface", loopbackIP.String())
+			cfg.AdvertiseAddress = loopbackIP.String()
+			return nil
+		}
+		return errors.New("unable to resolve link-local addresses")
+	}
+
 	// This is the same logic as the API Server uses, except that if no interface is found the address is set to 0.0.0.0, which is invalid and cannot be used
 	// for bootstrapping a cluster.
 	ip, err := ChooseAPIServerBindAddress(addressIP)

staging/src/k8s.io/apimachinery/pkg/util/net/interface.go

@@ -36,6 +36,11 @@ const (
 	familyIPv6 AddressFamily = 6
 )
 
+const (
+	// LoopbackInterfaceName is the default name of the loopback interface
+	LoopbackInterfaceName = "lo"
+)
+
 const (
 	ipv4RouteFile = "/proc/net/route"
 	ipv6RouteFile = "/proc/net/ipv6_route"
@@ -414,3 +419,21 @@ func ChooseBindAddress(bindAddress net.IP) (net.IP, error) {
 	}
 	return bindAddress, nil
 }
+
+// ChooseBindAddressForInterface choose a global IP for a specific interface, with priority given to IPv4.
+// This is required in case of network setups where default routes are present, but network
+// interfaces use only link-local addresses (e.g. as described in RFC5549).
+// e.g when using BGP to announce a host IP over link-local ip addresses and this ip address is attached to the lo interface.
+func ChooseBindAddressForInterface(intfName string) (net.IP, error) {
+	var nw networkInterfacer = networkInterface{}
+	for _, family := range []AddressFamily{familyIPv4, familyIPv6} {
+		ip, err := getIPFromInterface(intfName, family, nw)
+		if err != nil {
+			return nil, err
+		}
+		if ip != nil {
+			return ip, nil
+		}
+	}
+	return nil, fmt.Errorf("unable to select an IP from %s network interface", intfName)
+}