autodetect global addresses on loopback interfaces

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

@@ -266,6 +266,36 @@ func getIPFromInterface(intfName string, forFamily AddressFamily, nw networkInte
 	return nil, nil
 }
 
+// getIPFromLoopbackInterface gets the IPs on a loopback interface and returns a global unicast address, if any.
+// The loopback interface must be up, the IP must in the family requested, and the IP must be a global unicast address.
+func getIPFromLoopbackInterface(forFamily AddressFamily, nw networkInterfacer) (net.IP, error) {
+	intfs, err := nw.Interfaces()
+	if err != nil {
+		return nil, err
+	}
+	for _, intf := range intfs {
+		if !isInterfaceUp(&intf) {
+			continue
+		}
+		if intf.Flags&(net.FlagLoopback) != 0 {
+			addrs, err := nw.Addrs(&intf)
+			if err != nil {
+				return nil, err
+			}
+			klog.V(4).Infof("Interface %q has %d addresses :%v.", intf.Name, len(addrs), addrs)
+			matchingIP, err := getMatchingGlobalIP(addrs, forFamily)
+			if err != nil {
+				return nil, err
+			}
+			if matchingIP != nil {
+				klog.V(4).Infof("Found valid IPv%d address %v for interface %q.", int(forFamily), matchingIP, intf.Name)
+				return matchingIP, nil
+			}
+		}
+	}
+	return nil, nil
+}
+
 // memberOf tells if the IP is of the desired family. Used for checking interface addresses.
 func memberOf(ip net.IP, family AddressFamily) bool {
 	if ip.To4() != nil {
@@ -393,8 +423,9 @@ func getAllDefaultRoutes() ([]Route, error) {
 }
 
 // chooseHostInterfaceFromRoute cycles through each default route provided, looking for a
-// global IP address from the interface for the route. addressFamilies determines whether it
-// prefers IPv4 or IPv6
+// global IP address from the interface for the route. If there are routes but no global
+// address is obtained from the interfaces, it checks if the loopback interface has a global address.
+// addressFamilies determines whether it prefers IPv4 or IPv6
 func chooseHostInterfaceFromRoute(routes []Route, nw networkInterfacer, addressFamilies AddressFamilyPreference) (net.IP, error) {
 	for _, family := range addressFamilies {
 		klog.V(4).Infof("Looking for default routes with IPv%d addresses", uint(family))
@@ -411,6 +442,17 @@ func chooseHostInterfaceFromRoute(routes []Route, nw networkInterfacer, addressF
 				klog.V(4).Infof("Found active IP %v ", finalIP)
 				return finalIP, nil
 			}
+			// In case of network setups where default routes are present, but network
+			// interfaces use only link-local addresses (e.g. as described in RFC5549).
+			// the global IP is assigned to the loopback interface, and we should use it
+			loopbackIP, err := getIPFromLoopbackInterface(family, nw)
+			if err != nil {
+				return nil, err
+			}
+			if loopbackIP != nil {
+				klog.V(4).Infof("Found active IP %v on Loopback interface", loopbackIP)
+				return loopbackIP, nil
+			}
 		}
 	}
 	klog.V(4).Infof("No active IP found by looking at default routes")

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

@@ -453,6 +453,55 @@ func (_ p2pNetworkInterface) Interfaces() ([]net.Interface, error) {
 	return []net.Interface{p2pIntf}, nil
 }
 
+// Interface with link locals and loopback interface with global addresses
+type linkLocalLoopbackNetworkInterface struct {
+}
+
+func (_ linkLocalLoopbackNetworkInterface) InterfaceByName(intfName string) (*net.Interface, error) {
+	if intfName == LoopbackInterfaceName {
+		return &loopbackIntf, nil
+	}
+	return &upIntf, nil
+}
+func (_ linkLocalLoopbackNetworkInterface) Addrs(intf *net.Interface) ([]net.Addr, error) {
+	var ifat []net.Addr
+	ifat = []net.Addr{addrStruct{val: "169.254.162.166/16"}, addrStruct{val: "fe80::200/10"}}
+	if intf.Name == LoopbackInterfaceName {
+		ifat = []net.Addr{addrStruct{val: "::1/128"}, addrStruct{val: "127.0.0.1/8"},
+			// global addresses on loopback interface
+			addrStruct{val: "10.1.1.1/32"}, addrStruct{val: "fd00:1:1::1/128"}}
+	}
+	return ifat, nil
+}
+func (_ linkLocalLoopbackNetworkInterface) Interfaces() ([]net.Interface, error) {
+	return []net.Interface{upIntf, loopbackIntf}, nil
+}
+
+// Interface and loopback interface with global addresses
+type globalsNetworkInterface struct {
+}
+
+func (_ globalsNetworkInterface) InterfaceByName(intfName string) (*net.Interface, error) {
+	if intfName == LoopbackInterfaceName {
+		return &loopbackIntf, nil
+	}
+	return &upIntf, nil
+}
+func (_ globalsNetworkInterface) Addrs(intf *net.Interface) ([]net.Addr, error) {
+	var ifat []net.Addr
+	ifat = []net.Addr{addrStruct{val: "169.254.162.166/16"}, addrStruct{val: "fe80::200/10"},
+		addrStruct{val: "192.168.1.1/31"}, addrStruct{val: "fd00::200/127"}}
+	if intf.Name == LoopbackInterfaceName {
+		ifat = []net.Addr{addrStruct{val: "::1/128"}, addrStruct{val: "127.0.0.1/8"},
+			// global addresses on loopback interface
+			addrStruct{val: "10.1.1.1/32"}, addrStruct{val: "fd00:1:1::1/128"}}
+	}
+	return ifat, nil
+}
+func (_ globalsNetworkInterface) Interfaces() ([]net.Interface, error) {
+	return []net.Interface{upIntf, loopbackIntf}, nil
+}
+
 // Unable to get IP addresses for interface
 type networkInterfaceFailGetAddrs struct {
 }
@@ -530,6 +579,33 @@ func TestGetIPFromInterface(t *testing.T) {
 	}
 }
 
+func TestGetIPFromLoopbackInterface(t *testing.T) {
+	testCases := []struct {
+		tcase      string
+		family     AddressFamily
+		nw         networkInterfacer
+		expected   net.IP
+		errStrFrag string
+	}{
+		{"ipv4", familyIPv4, linkLocalLoopbackNetworkInterface{}, net.ParseIP("10.1.1.1"), ""},
+		{"ipv6", familyIPv6, linkLocalLoopbackNetworkInterface{}, net.ParseIP("fd00:1:1::1"), ""},
+		{"no global ipv4", familyIPv4, loopbackNetworkInterface{}, nil, ""},
+		{"no global ipv6", familyIPv6, loopbackNetworkInterface{}, nil, ""},
+	}
+	for _, tc := range testCases {
+		ip, err := getIPFromLoopbackInterface(tc.family, tc.nw)
+		if err != nil {
+			if !strings.Contains(err.Error(), tc.errStrFrag) {
+				t.Errorf("case[%s]: Error string %q does not contain %q", tc.tcase, err, tc.errStrFrag)
+			}
+		} else if tc.errStrFrag != "" {
+			t.Errorf("case[%s]: Error %q expected, but seen %v", tc.tcase, tc.errStrFrag, err)
+		} else if !ip.Equal(tc.expected) {
+			t.Errorf("case[%v]: expected %v, got %+v .err : %v", tc.tcase, tc.expected, ip, err)
+		}
+	}
+}
+
 func TestChooseHostInterfaceFromRoute(t *testing.T) {
 	testCases := []struct {
 		tcase    string
@@ -544,6 +620,16 @@ func TestChooseHostInterfaceFromRoute(t *testing.T) {
 		{"single-stack ipv6, prefer v6", routeV6, ipv6NetworkInterface{}, preferIPv6, net.ParseIP("2001::200")},
 		{"dual stack", bothRoutes, v4v6NetworkInterface{}, preferIPv4, net.ParseIP("10.254.71.145")},
 		{"dual stack, prefer v6", bothRoutes, v4v6NetworkInterface{}, preferIPv6, net.ParseIP("2001::10")},
+		{"LLA and loopback with global, IPv4", routeV4, linkLocalLoopbackNetworkInterface{}, preferIPv4, net.ParseIP("10.1.1.1")},
+		{"LLA and loopback with global, IPv6", routeV6, linkLocalLoopbackNetworkInterface{}, preferIPv6, net.ParseIP("fd00:1:1::1")},
+		{"LLA and loopback with global, dual stack prefer IPv4", bothRoutes, linkLocalLoopbackNetworkInterface{}, preferIPv4, net.ParseIP("10.1.1.1")},
+		{"LLA and loopback with global, dual stack prefer IPv6", bothRoutes, linkLocalLoopbackNetworkInterface{}, preferIPv6, net.ParseIP("fd00:1:1::1")},
+		{"LLA and loopback with global, no routes", noRoutes, linkLocalLoopbackNetworkInterface{}, preferIPv6, nil},
+		{"interface and loopback with global, IPv4", routeV4, globalsNetworkInterface{}, preferIPv4, net.ParseIP("192.168.1.1")},
+		{"interface and loopback with global, IPv6", routeV6, globalsNetworkInterface{}, preferIPv6, net.ParseIP("fd00::200")},
+		{"interface and loopback with global, dual stack prefer IPv4", bothRoutes, globalsNetworkInterface{}, preferIPv4, net.ParseIP("192.168.1.1")},
+		{"interface and loopback with global, dual stack prefer IPv6", bothRoutes, globalsNetworkInterface{}, preferIPv6, net.ParseIP("fd00::200")},
+		{"interface and loopback with global, no routes", noRoutes, globalsNetworkInterface{}, preferIPv6, nil},
 		{"all LLA", routeV4, networkInterfaceWithOnlyLinkLocals{}, preferIPv4, nil},
 		{"no routes", noRoutes, validNetworkInterface{}, preferIPv4, nil},
 		{"fail get IP", routeV4, networkInterfaceFailGetAddrs{}, preferIPv4, nil},