update docker's resolv.conf file with options ndots:5

pkg/kubelet/dockertools/manager.go

@@ -60,6 +60,10 @@ const (
 
 	kubernetesPodLabel       = "io.kubernetes.pod.data"
 	kubernetesContainerLabel = "io.kubernetes.container.name"
+	// ndots specifies the minimum number of dots that a domain name must contain for the resolver to consider it as FQDN (fully-qualified)
+	// we want to able to consider SRV lookup names like _dns._udp.kube-dns.default.svc to be considered relative.
+	// hence, setting ndots to be 5.
+	ndotsDNSOption = "options ndots:5\n"
 )
 
 // DockerManager implements the Runtime interface.
@@ -1221,6 +1225,13 @@ func (dm *DockerManager) runContainerInPod(pod *api.Pod, container *api.Containe
 	}
 	if container.Name == PodInfraContainerName {
 		util.ApplyOomScoreAdj(containerInfo.State.Pid, podOomScoreAdj)
+		// currently, Docker does not have a flag by which the ndots option can be passed.
+		// (A seperate issue has been filed with Docker to add a ndots flag)
+		// The addNDotsOption call appends the ndots option to the resolv.conf file generated by docker.
+		// This resolv.conf file is shared by all containers of the same pod, and needs to be modified only once per pod.
+		// we modify it when the pause container is created since it is the first container created in the pod since it holds
+		// the networking namespace.
+		err = addNDotsOption(containerInfo.ResolvConfPath)
 	} else {
 		// Children processes of docker daemon will inheritant the OOM score from docker
 		// daemon process. We explicitly apply OOM score 0 by default to the user
@@ -1231,6 +1242,36 @@ func (dm *DockerManager) runContainerInPod(pod *api.Pod, container *api.Containe
 	return kubeletTypes.DockerID(id), err
 }
 
+func addNDotsOption(resolvFilePath string) error {
+	if len(resolvFilePath) == 0 {
+		glog.Errorf("DNS ResolvConfPath is empty.")
+		return nil
+	}
+
+	if _, err := os.Stat(resolvFilePath); os.IsNotExist(err) {
+		return fmt.Errorf("DNS ResolvConfPath specified but does not exist. It could not be updated: %s", resolvFilePath)
+	}
+
+	glog.V(4).Infof("DNS ResolvConfPath exists: %s. Will attempt to add ndots option: %s", resolvFilePath, ndotsDNSOption)
+
+	if err := appendToFile(resolvFilePath, ndotsDNSOption); err != nil {
+		glog.Errorf("resolv.conf could not be updated. err:%v", err)
+		return err
+	}
+	return nil
+}
+
+func appendToFile(filePath, stringToAppend string) error {
+	f, err := os.OpenFile(filePath, os.O_APPEND|os.O_WRONLY, 0644)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	_, err = f.WriteString(stringToAppend)
+	return err
+}
+
 // createPodInfraContainer starts the pod infra container for a pod. Returns the docker container ID of the newly created container.
 func (dm *DockerManager) createPodInfraContainer(pod *api.Pod) (kubeletTypes.DockerID, error) {
 	start := time.Now()