Add status.podIP as a valid downward API target

Getting the public IP a container is supposed to use is O(hard),
and usually involves ugly gyrations in python or with interfaces.
Using the downward API means that the IP Kube is announcing to
other endpoints is also visible inside the container for pods to
identify themselves.

pkg/api/v1/conversion.go

@@ -44,6 +44,7 @@ func addConversionFuncs() {
 			case "metadata.name",
 				"metadata.namespace",
 				"status.phase",
+				"status.podIP",
 				"spec.nodeName":
 				return label, value, nil
 				// This is for backwards compatibility with old v1 clients which send spec.host

pkg/api/validation/validation.go

@@ -680,7 +680,7 @@ func validateEnvVarValueFrom(ev api.EnvVar) errs.ValidationErrorList {
 	return allErrs
 }
 
-var validFieldPathExpressions = util.NewStringSet("metadata.name", "metadata.namespace")
+var validFieldPathExpressions = util.NewStringSet("metadata.name", "metadata.namespace", "status.podIP")
 
 func validateObjectFieldSelector(fs *api.ObjectFieldSelector) errs.ValidationErrorList {
 	allErrs := errs.ValidationErrorList{}

pkg/api/validation/validation_test.go

@@ -667,7 +667,7 @@ func TestValidateEnv(t *testing.T) {
 					},
 				},
 			}},
-			expectedError: "[0].valueFrom.fieldRef.fieldPath: unsupported value 'status.phase', Details: supported values: metadata.name, metadata.namespace",
+			expectedError: "[0].valueFrom.fieldRef.fieldPath: unsupported value 'status.phase', Details: supported values: metadata.name, metadata.namespace, status.podIP",
 		},
 	}
 	for _, tc := range errorCases {

pkg/kubelet/kubelet.go

@@ -1051,7 +1051,10 @@ func (kl *Kubelet) podFieldSelectorRuntimeValue(fs *api.ObjectFieldSelector, pod
 	if err != nil {
 		return "", err
 	}
-
+	switch internalFieldPath {
+	case "status.podIP":
+		return pod.Status.PodIP, nil
+	}
 	return fieldpath.ExtractFieldPathAsString(pod, internalFieldPath)
 }
 

pkg/kubelet/kubelet_test.go

@@ -1187,6 +1187,15 @@ func TestMakeEnvironmentVariables(t *testing.T) {
 							},
 						},
 					},
+					{
+						Name: "POD_IP",
+						ValueFrom: &api.EnvVarSource{
+							FieldRef: &api.ObjectFieldSelector{
+								APIVersion: testapi.Version(),
+								FieldPath:  "status.podIP",
+							},
+						},
+					},
 				},
 			},
 			masterServiceNs: "nothing",
@@ -1194,6 +1203,7 @@ func TestMakeEnvironmentVariables(t *testing.T) {
 			expectedEnvs: []kubecontainer.EnvVar{
 				{Name: "POD_NAME", Value: "dapi-test-pod-name"},
 				{Name: "POD_NAMESPACE", Value: "downward-api"},
+				{Name: "POD_IP", Value: "1.2.3.4"},
 			},
 		},
 		{
@@ -1345,6 +1355,7 @@ func TestMakeEnvironmentVariables(t *testing.T) {
 				Name:      "dapi-test-pod-name",
 			},
 		}
+		testPod.Status.PodIP = "1.2.3.4"
 
 		result, err := kl.makeEnvironmentVariables(testPod, tc.container)
 		if err != nil {