checks.go

/*
Copyright 2016 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package preflight

import (
	"bufio"
	"bytes"
	"encoding/json"
	"fmt"
	"io"
	"io/ioutil"
	"net"
	"net/http"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"time"

	"crypto/tls"
	"crypto/x509"

	"github.com/PuerkitoBio/purell"
	"github.com/blang/semver"
	"github.com/spf13/pflag"

	"net/url"

	apiservoptions "k8s.io/kubernetes/cmd/kube-apiserver/app/options"
	cmoptions "k8s.io/kubernetes/cmd/kube-controller-manager/app/options"
	kubeadmapi "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm"
	kubeadmconstants "k8s.io/kubernetes/cmd/kubeadm/app/constants"
	"k8s.io/kubernetes/pkg/api/validation"
	authzmodes "k8s.io/kubernetes/pkg/kubeapiserver/authorizer/modes"
	"k8s.io/kubernetes/pkg/util/initsystem"
	versionutil "k8s.io/kubernetes/pkg/util/version"
	kubeadmversion "k8s.io/kubernetes/pkg/version"
	schoptions "k8s.io/kubernetes/plugin/cmd/kube-scheduler/app/options"
	"k8s.io/kubernetes/test/e2e_node/system"
)

const (
	bridgenf                    = "/proc/sys/net/bridge/bridge-nf-call-iptables"
	externalEtcdRequestTimeout  = time.Duration(10 * time.Second)
	externalEtcdRequestRetries  = 3
	externalEtcdRequestInterval = time.Duration(5 * time.Second)
)

var (
	minExternalEtcdVersion = semver.MustParse(kubeadmconstants.MinExternalEtcdVersion)
)

type Error struct {
	Msg string
}

func (e *Error) Error() string {
	return fmt.Sprintf("[preflight] Some fatal errors occurred:\n%s%s", e.Msg, "[preflight] If you know what you are doing, you can skip pre-flight checks with `--skip-preflight-checks`")
}

// Checker validates the state of the system to ensure kubeadm will be
// successful as often as possilble.
type Checker interface {
	Check() (warnings, errors []error)
}

// ServiceCheck verifies that the given service is enabled and active. If we do not
// detect a supported init system however, all checks are skipped and a warning is
// returned.
type ServiceCheck struct {
	Service       string
	CheckIfActive bool
}

func (sc ServiceCheck) Check() (warnings, errors []error) {
	initSystem, err := initsystem.GetInitSystem()
	if err != nil {
		return []error{err}, nil
	}

	warnings = []error{}

	if !initSystem.ServiceExists(sc.Service) {
		warnings = append(warnings, fmt.Errorf("%s service does not exist", sc.Service))
		return warnings, nil
	}

	if !initSystem.ServiceIsEnabled(sc.Service) {
		warnings = append(warnings,
			fmt.Errorf("%s service is not enabled, please run 'systemctl enable %s.service'",
				sc.Service, sc.Service))
	}

	if sc.CheckIfActive && !initSystem.ServiceIsActive(sc.Service) {
		errors = append(errors,
			fmt.Errorf("%s service is not active, please run 'systemctl start %s.service'",
				sc.Service, sc.Service))
	}

	return warnings, errors
}

// FirewalldCheck checks if firewalld is enabled or active, and if so outputs a warning.
type FirewalldCheck struct {
	ports []int
}

func (fc FirewalldCheck) Check() (warnings, errors []error) {
	initSystem, err := initsystem.GetInitSystem()
	if err != nil {
		return []error{err}, nil
	}

	warnings = []error{}

	if !initSystem.ServiceExists("firewalld") {
		return nil, nil
	}

	if initSystem.ServiceIsActive("firewalld") {
		warnings = append(warnings,
			fmt.Errorf("firewalld is active, please ensure ports %v are open or your cluster may not function correctly",
				fc.ports))
	}

	return warnings, errors
}

// PortOpenCheck ensures the given port is available for use.
type PortOpenCheck struct {
	port int
}

func (poc PortOpenCheck) Check() (warnings, errors []error) {
	errors = []error{}
	ln, err := net.Listen("tcp", fmt.Sprintf(":%d", poc.port))
	if err != nil {
		errors = append(errors, fmt.Errorf("Port %d is in use", poc.port))
	}
	if ln != nil {
		ln.Close()
	}

	return nil, errors
}

// IsRootCheck verifies user is root
type IsRootCheck struct{}

func (irc IsRootCheck) Check() (warnings, errors []error) {
	errors = []error{}
	if os.Getuid() != 0 {
		errors = append(errors, fmt.Errorf("user is not running as root"))
	}

	return nil, errors
}

// DirAvailableCheck checks if the given directory either does not exist, or is empty.
type DirAvailableCheck struct {
	Path string
}

func (dac DirAvailableCheck) Check() (warnings, errors []error) {
	errors = []error{}
	// If it doesn't exist we are good:
	if _, err := os.Stat(dac.Path); os.IsNotExist(err) {
		return nil, nil
	}

	f, err := os.Open(dac.Path)
	if err != nil {
		errors = append(errors, fmt.Errorf("unable to check if %s is empty: %s", dac.Path, err))
		return nil, errors
	}
	defer f.Close()

	_, err = f.Readdirnames(1)
	if err != io.EOF {
		errors = append(errors, fmt.Errorf("%s is not empty", dac.Path))
	}

	return nil, errors
}

// FileAvailableCheck checks that the given file does not already exist.
type FileAvailableCheck struct {
	Path string
}

func (fac FileAvailableCheck) Check() (warnings, errors []error) {
	errors = []error{}
	if _, err := os.Stat(fac.Path); err == nil {
		errors = append(errors, fmt.Errorf("%s already exists", fac.Path))
	}
	return nil, errors
}

// FileExistingCheck checks that the given file does not already exist.
type FileExistingCheck struct {
	Path string
}

func (fac FileExistingCheck) Check() (warnings, errors []error) {
	errors = []error{}
	if _, err := os.Stat(fac.Path); err != nil {
		errors = append(errors, fmt.Errorf("%s doesn't exist", fac.Path))
	}
	return nil, errors
}

// FileContentCheck checks that the given file contains the string Content.
type FileContentCheck struct {
	Path    string
	Content []byte
}

func (fcc FileContentCheck) Check() (warnings, errors []error) {
	f, err := os.Open(fcc.Path)
	if err != nil {
		return nil, []error{fmt.Errorf("%s does not exist", fcc.Path)}
	}

	lr := io.LimitReader(f, int64(len(fcc.Content)))
	defer f.Close()

	buf := &bytes.Buffer{}
	_, err = io.Copy(buf, lr)
	if err != nil {
		return nil, []error{fmt.Errorf("%s could not be read", fcc.Path)}
	}

	if !bytes.Equal(buf.Bytes(), fcc.Content) {
		return nil, []error{fmt.Errorf("%s contents are not set to %s", fcc.Path, fcc.Content)}
	}
	return nil, []error{}

}

// InPathCheck checks if the given executable is present in the path
type InPathCheck struct {
	executable string
	mandatory  bool
}

func (ipc InPathCheck) Check() (warnings, errors []error) {
	_, err := exec.LookPath(ipc.executable)
	if err != nil {
		if ipc.mandatory {
			// Return as an error:
			return nil, []error{fmt.Errorf("%s not found in system path", ipc.executable)}
		}
		// Return as a warning:
		return []error{fmt.Errorf("%s not found in system path", ipc.executable)}, nil
	}
	return nil, nil
}

// HostnameCheck checks if hostname match dns sub domain regex.
// If hostname doesn't match this regex, kubelet will not launch static pods like kube-apiserver/kube-controller-manager and so on.
type HostnameCheck struct {
	nodeName string
}

func (hc HostnameCheck) Check() (warnings, errors []error) {
	errors = []error{}
	warnings = []error{}
	for _, msg := range validation.ValidateNodeName(hc.nodeName, false) {
		errors = append(errors, fmt.Errorf("hostname \"%s\" %s", hc.nodeName, msg))
	}
	addr, err := net.LookupHost(hc.nodeName)
	if addr == nil {
		warnings = append(warnings, fmt.Errorf("hostname \"%s\" could not be reached", hc.nodeName))
	}
	if err != nil {
		warnings = append(warnings, fmt.Errorf("hostname \"%s\" %s", hc.nodeName, err))
	}
	return warnings, errors
}

// HTTPProxyCheck checks if https connection to specific host is going
// to be done directly or over proxy. If proxy detected, it will return warning.
type HTTPProxyCheck struct {
	Proto string
	Host  string
	Port  int
}

func (hst HTTPProxyCheck) Check() (warnings, errors []error) {

	url := fmt.Sprintf("%s://%s:%d", hst.Proto, hst.Host, hst.Port)

	req, err := http.NewRequest("GET", url, nil)
	if err != nil {
		return nil, []error{err}
	}

	proxy, err := http.DefaultTransport.(*http.Transport).Proxy(req)
	if err != nil {
		return nil, []error{err}
	}
	if proxy != nil {
		return []error{fmt.Errorf("Connection to %q uses proxy %q. If that is not intended, adjust your proxy settings", url, proxy)}, nil
	}
	return nil, nil
}

// ExtraArgsCheck checks if arguments are valid.
type ExtraArgsCheck struct {
	APIServerExtraArgs         map[string]string
	ControllerManagerExtraArgs map[string]string
	SchedulerExtraArgs         map[string]string
}

func (eac ExtraArgsCheck) Check() (warnings, errors []error) {
	argsCheck := func(name string, args map[string]string, f *pflag.FlagSet) []error {
		errs := []error{}
		for k, v := range args {
			if err := f.Set(k, v); err != nil {
				errs = append(errs, fmt.Errorf("%s: failed to parse extra argument --%s=%s", name, k, v))
			}
		}
		return errs
	}

	warnings = []error{}
	if len(eac.APIServerExtraArgs) > 0 {
		flags := pflag.NewFlagSet("", pflag.ContinueOnError)
		s := apiservoptions.NewServerRunOptions()
		s.AddFlags(flags)
		warnings = append(warnings, argsCheck("kube-apiserver", eac.APIServerExtraArgs, flags)...)
	}
	if len(eac.ControllerManagerExtraArgs) > 0 {
		flags := pflag.NewFlagSet("", pflag.ContinueOnError)
		s := cmoptions.NewCMServer()
		s.AddFlags(flags, []string{}, []string{})
		warnings = append(warnings, argsCheck("kube-controller-manager", eac.ControllerManagerExtraArgs, flags)...)
	}
	if len(eac.SchedulerExtraArgs) > 0 {
		flags := pflag.NewFlagSet("", pflag.ContinueOnError)
		s := schoptions.NewSchedulerServer()
		s.AddFlags(flags)
		warnings = append(warnings, argsCheck("kube-scheduler", eac.SchedulerExtraArgs, flags)...)
	}
	return warnings, nil
}

type SystemVerificationCheck struct{}

func (sysver SystemVerificationCheck) Check() (warnings, errors []error) {
	// Create a buffered writer and choose a quite large value (1M) and suppose the output from the system verification test won't exceed the limit
	// Run the system verification check, but write to out buffered writer instead of stdout
	bufw := bufio.NewWriterSize(os.Stdout, 1*1024*1024)
	reporter := &system.StreamReporter{WriteStream: bufw}

	var errs []error
	var warns []error
	// All the validators we'd like to run:
	var validators = []system.Validator{
		&system.OSValidator{Reporter: reporter},
		&system.KernelValidator{Reporter: reporter},
		&system.CgroupsValidator{Reporter: reporter},
		&system.DockerValidator{Reporter: reporter},
	}

	// Run all validators
	for _, v := range validators {
		warn, err := v.Validate(system.DefaultSysSpec)
		if err != nil {
			errs = append(errs, err)
		}
		if warn != nil {
			warns = append(warns, warn)
		}
	}

	if len(errs) != 0 {
		// Only print the output from the system verification check if the check failed
		fmt.Println("[preflight] The system verification failed. Printing the output from the verification:")
		bufw.Flush()
		return warns, errs
	}
	return warns, nil
}

type KubernetesVersionCheck struct {
	KubeadmVersion    string
	KubernetesVersion string
}

func (kubever KubernetesVersionCheck) Check() (warnings, errors []error) {

	// Skip this check for "super-custom builds", where apimachinery/the overall codebase version is not set.
	if strings.HasPrefix(kubever.KubeadmVersion, "v0.0.0") {
		return nil, nil
	}

	kadmVersion, err := versionutil.ParseSemantic(kubever.KubeadmVersion)
	if err != nil {
		return nil, []error{fmt.Errorf("couldn't parse kubeadm version %q: %v", kubever.KubeadmVersion, err)}
	}

	k8sVersion, err := versionutil.ParseSemantic(kubever.KubernetesVersion)
	if err != nil {
		return nil, []error{fmt.Errorf("couldn't parse kubernetes version %q: %v", kubever.KubernetesVersion, err)}
	}

	// Checks if k8sVersion greater or equal than the first unsupported versions by current version of kubeadm,
	// that is major.minor+1 (all patch and pre-releases versions included)
	// NB. in semver patches number is a numeric, while prerelease is a string where numeric identifiers always have lower precedence than non-numeric identifiers.
	//     thus setting the value to x.y.0-0 we are defining the very first patch - prereleases within x.y minor release.
	firstUnsupportedVersion := versionutil.MustParseSemantic(fmt.Sprintf("%d.%d.%s", kadmVersion.Major(), kadmVersion.Minor()+1, "0-0"))
	if k8sVersion.AtLeast(firstUnsupportedVersion) {
		return []error{fmt.Errorf("kubernetes version is greater than kubeadm version. Please consider to upgrade kubeadm. kubernetes version: %s. Kubeadm version: %d.%d.x", k8sVersion, kadmVersion.Components()[0], kadmVersion.Components()[1])}, nil
	}

	return nil, nil
}

// SwapCheck warns if swap is enabled
type SwapCheck struct{}

func (swc SwapCheck) Check() (warnings, errors []error) {
	f, err := os.Open("/proc/swaps")
	if err != nil {
		// /proc/swaps not available, thus no reasons to warn
		return nil, nil
	}
	defer f.Close()
	var buf []string
	scanner := bufio.NewScanner(f)
	for scanner.Scan() {
		buf = append(buf, scanner.Text())
	}
	if err := scanner.Err(); err != nil {
		return nil, []error{fmt.Errorf("error parsing /proc/swaps: %v", err)}
	}

	if len(buf) > 1 {
		return []error{fmt.Errorf("Running with swap on is not supported. Please disable swap or set kubelet's --fail-swap-on flag to false.")}, nil
	}

	return nil, nil
}

type etcdVersionResponse struct {
	Etcdserver  string `json:"etcdserver"`
	Etcdcluster string `json:"etcdcluster"`
}

// ExternalEtcdVersionCheck checks if version of external etcd meets the demand of kubeadm
type ExternalEtcdVersionCheck struct {
	Etcd kubeadmapi.Etcd
}

func (evc ExternalEtcdVersionCheck) Check() (warnings, errors []error) {
	var config *tls.Config
	var err error
	if config, err = evc.configRootCAs(config); err != nil {
		errors = append(errors, err)
		return nil, errors
	}
	if config, err = evc.configCertAndKey(config); err != nil {
		errors = append(errors, err)
		return nil, errors
	}

	client := evc.getHTTPClient(config)
	for _, endpoint := range evc.Etcd.Endpoints {
		if _, err := url.Parse(endpoint); err != nil {
			errors = append(errors, fmt.Errorf("failed to parse external etcd endpoint %s : %v", endpoint, err))
			continue
		}
		resp := etcdVersionResponse{}
		var err error
		versionURL := fmt.Sprintf("%s/%s", endpoint, "version")
		if tmpVersionURL, err := purell.NormalizeURLString(versionURL, purell.FlagRemoveDuplicateSlashes); err != nil {
			errors = append(errors, fmt.Errorf("failed to normalize external etcd version url %s : %v", versionURL, err))
			continue
		} else {
			versionURL = tmpVersionURL
		}
		if err = getEtcdVersionResponse(client, versionURL, &resp); err != nil {
			errors = append(errors, err)
			continue
		}

		etcdVersion, err := semver.Parse(resp.Etcdserver)
		if err != nil {
			errors = append(errors, fmt.Errorf("couldn't parse external etcd version %q: %v", resp.Etcdserver, err))
			continue
		}
		if etcdVersion.LT(minExternalEtcdVersion) {
			errors = append(errors, fmt.Errorf("this version of kubeadm only supports external etcd version >= %s. Current version: %s", kubeadmconstants.MinExternalEtcdVersion, resp.Etcdserver))
			continue
		}
	}

	return nil, errors
}

// configRootCAs configures and returns a reference to tls.Config instance if CAFile is provided
func (evc ExternalEtcdVersionCheck) configRootCAs(config *tls.Config) (*tls.Config, error) {
	var CACertPool *x509.CertPool
	if evc.Etcd.CAFile != "" {
		CACert, err := ioutil.ReadFile(evc.Etcd.CAFile)
		if err != nil {
			return nil, fmt.Errorf("couldn't load external etcd's server certificate %s: %v", evc.Etcd.CAFile, err)
		}
		CACertPool = x509.NewCertPool()
		CACertPool.AppendCertsFromPEM(CACert)
	}
	if CACertPool != nil {
		if config == nil {
			config = &tls.Config{}
		}
		config.RootCAs = CACertPool
	}
	return config, nil
}

// configCertAndKey configures and returns a reference to tls.Config instance if CertFile and KeyFile pair is provided
func (evc ExternalEtcdVersionCheck) configCertAndKey(config *tls.Config) (*tls.Config, error) {
	var cert tls.Certificate
	if evc.Etcd.CertFile != "" && evc.Etcd.KeyFile != "" {
		var err error
		cert, err = tls.LoadX509KeyPair(evc.Etcd.CertFile, evc.Etcd.KeyFile)
		if err != nil {
			return nil, fmt.Errorf("couldn't load external etcd's certificate and key pair %s, %s: %v", evc.Etcd.CertFile, evc.Etcd.KeyFile, err)
		}
		if config == nil {
			config = &tls.Config{}
		}
		config.Certificates = []tls.Certificate{cert}
	}
	return config, nil
}
func (evc ExternalEtcdVersionCheck) getHTTPClient(config *tls.Config) *http.Client {
	if config != nil {
		transport := &http.Transport{
			TLSClientConfig: config,
		}
		return &http.Client{
			Transport: transport,
			Timeout:   externalEtcdRequestTimeout,
		}
	}
	return &http.Client{Timeout: externalEtcdRequestTimeout}
}
func getEtcdVersionResponse(client *http.Client, url string, target interface{}) error {
	loopCount := externalEtcdRequestRetries + 1
	var err error
	var stopRetry bool
	for loopCount > 0 {
		if loopCount <= externalEtcdRequestRetries {
			time.Sleep(externalEtcdRequestInterval)
		}
		stopRetry, err = func() (stopRetry bool, err error) {
			r, err := client.Get(url)
			if err != nil {
				loopCount--
				return false, nil
			}
			defer r.Body.Close()

			if r != nil && r.StatusCode >= 500 && r.StatusCode <= 599 {
				loopCount--
				return false, nil
			}
			return true, json.NewDecoder(r.Body).Decode(target)

		}()
		if stopRetry {
			break
		}
	}
	return err
}
func RunInitMasterChecks(cfg *kubeadmapi.MasterConfiguration) error {
	// First, check if we're root separately from the other preflight checks and fail fast
	if err := RunRootCheckOnly(); err != nil {
		return err
	}

	checks := []Checker{
		KubernetesVersionCheck{KubernetesVersion: cfg.KubernetesVersion, KubeadmVersion: kubeadmversion.Get().GitVersion},
		SystemVerificationCheck{},
		IsRootCheck{},
		HostnameCheck{nodeName: cfg.NodeName},
		ServiceCheck{Service: "kubelet", CheckIfActive: false},
		ServiceCheck{Service: "docker", CheckIfActive: true},
		FirewalldCheck{ports: []int{int(cfg.API.BindPort), 10250}},
		PortOpenCheck{port: int(cfg.API.BindPort)},
		PortOpenCheck{port: 10250},
		PortOpenCheck{port: 10251},
		PortOpenCheck{port: 10252},
		HTTPProxyCheck{Proto: "https", Host: cfg.API.AdvertiseAddress, Port: int(cfg.API.BindPort)},
		DirAvailableCheck{Path: filepath.Join(kubeadmconstants.KubernetesDir, kubeadmconstants.ManifestsSubDirName)},
		DirAvailableCheck{Path: "/var/lib/kubelet"},
		FileContentCheck{Path: bridgenf, Content: []byte{'1'}},
		SwapCheck{},
		InPathCheck{executable: "ip", mandatory: true},
		InPathCheck{executable: "iptables", mandatory: true},
		InPathCheck{executable: "mount", mandatory: true},
		InPathCheck{executable: "nsenter", mandatory: true},
		InPathCheck{executable: "ebtables", mandatory: false},
		InPathCheck{executable: "ethtool", mandatory: false},
		InPathCheck{executable: "socat", mandatory: false},
		InPathCheck{executable: "tc", mandatory: false},
		InPathCheck{executable: "touch", mandatory: false},
		ExtraArgsCheck{
			APIServerExtraArgs:         cfg.APIServerExtraArgs,
			ControllerManagerExtraArgs: cfg.ControllerManagerExtraArgs,
			SchedulerExtraArgs:         cfg.SchedulerExtraArgs,
		},
	}

	if len(cfg.Etcd.Endpoints) == 0 {
		// Only do etcd related checks when no external endpoints were specified
		checks = append(checks,
			PortOpenCheck{port: 2379},
			DirAvailableCheck{Path: cfg.Etcd.DataDir},
		)
	} else {
		// Only check etcd version when external endpoints are specified
		checks = append(checks,
			ExternalEtcdVersionCheck{Etcd: cfg.Etcd},
		)
	}

	// Check the config for authorization mode
	for _, authzMode := range cfg.AuthorizationModes {
		switch authzMode {
		case authzmodes.ModeABAC:
			checks = append(checks, FileExistingCheck{Path: kubeadmconstants.AuthorizationPolicyPath})
		case authzmodes.ModeWebhook:
			checks = append(checks, FileExistingCheck{Path: kubeadmconstants.AuthorizationWebhookConfigPath})
		}
	}

	return RunChecks(checks, os.Stderr)
}

func RunJoinNodeChecks(cfg *kubeadmapi.NodeConfiguration) error {
	// First, check if we're root separately from the other preflight checks and fail fast
	if err := RunRootCheckOnly(); err != nil {
		return err
	}

	checks := []Checker{
		SystemVerificationCheck{},
		IsRootCheck{},
		HostnameCheck{cfg.NodeName},
		ServiceCheck{Service: "kubelet", CheckIfActive: false},
		ServiceCheck{Service: "docker", CheckIfActive: true},
		PortOpenCheck{port: 10250},
		DirAvailableCheck{Path: filepath.Join(kubeadmconstants.KubernetesDir, kubeadmconstants.ManifestsSubDirName)},
		DirAvailableCheck{Path: "/var/lib/kubelet"},
		FileAvailableCheck{Path: cfg.CACertPath},
		FileAvailableCheck{Path: filepath.Join(kubeadmconstants.KubernetesDir, kubeadmconstants.KubeletKubeConfigFileName)},
		FileContentCheck{Path: bridgenf, Content: []byte{'1'}},
		SwapCheck{},
		InPathCheck{executable: "ip", mandatory: true},
		InPathCheck{executable: "iptables", mandatory: true},
		InPathCheck{executable: "mount", mandatory: true},
		InPathCheck{executable: "nsenter", mandatory: true},
		InPathCheck{executable: "ebtables", mandatory: false},
		InPathCheck{executable: "ethtool", mandatory: false},
		InPathCheck{executable: "socat", mandatory: false},
		InPathCheck{executable: "tc", mandatory: false},
		InPathCheck{executable: "touch", mandatory: false},
	}

	return RunChecks(checks, os.Stderr)
}

func RunRootCheckOnly() error {
	checks := []Checker{
		IsRootCheck{},
	}

	return RunChecks(checks, os.Stderr)
}

// RunChecks runs each check, displays it's warnings/errors, and once all
// are processed will exit if any errors occurred.
func RunChecks(checks []Checker, ww io.Writer) error {
	found := []error{}
	for _, c := range checks {
		warnings, errs := c.Check()
		for _, w := range warnings {
			io.WriteString(ww, fmt.Sprintf("[preflight] WARNING: %v\n", w))
		}
		found = append(found, errs...)
	}
	if len(found) > 0 {
		var errs bytes.Buffer
		for _, i := range found {
			errs.WriteString("\t" + i.Error() + "\n")
		}
		return &Error{Msg: errs.String()}
	}
	return nil
}

func TryStartKubelet() {
	// If we notice that the kubelet service is inactive, try to start it
	initSystem, err := initsystem.GetInitSystem()
	if err != nil {
		fmt.Println("[preflight] No supported init system detected, won't ensure kubelet is running.")
	} else if initSystem.ServiceExists("kubelet") && !initSystem.ServiceIsActive("kubelet") {

		fmt.Println("[preflight] Starting the kubelet service")
		if err := initSystem.ServiceStart("kubelet"); err != nil {
			fmt.Printf("[preflight] WARNING: Unable to start the kubelet service: [%v]\n", err)
			fmt.Println("[preflight] WARNING: Please ensure kubelet is running manually.")
		}
	}
}