Warn user if connections to API or Discovery are going to be over proxy

kubeadm during initialization of master and slave nodes need to make
several API calls directly to the node where it is running or master.
In environments with http/https proxies, user might accidentally
have configuration where connections to API would go over proxy instead
of directly.

User can re-run kubeadm with corrected NO_PROXY variable. Example:

  $ NO_PROXY=* kubeadm join ...

cmd/kubeadm/app/cmd/init.go

@@ -168,16 +168,6 @@ func NewInit(cfgPath string, cfg *kubeadmapi.MasterConfiguration, skipPreFlight
 		}
 	}
 
-	if !skipPreFlight {
-		fmt.Println("Running pre-flight checks")
-		err := preflight.RunInitMasterChecks(cfg)
-		if err != nil {
-			return nil, &preflight.PreFlightError{Msg: err.Error()}
-		}
-	} else {
-		fmt.Println("Skipping pre-flight checks")
-	}
-
 	// Auto-detect the IP
 	if len(cfg.API.AdvertiseAddresses) == 0 {
 		// TODO(phase1+) perhaps we could actually grab eth0 and eth1
@@ -188,6 +178,16 @@ func NewInit(cfgPath string, cfg *kubeadmapi.MasterConfiguration, skipPreFlight
 		cfg.API.AdvertiseAddresses = []string{ip.String()}
 	}
 
+	if !skipPreFlight {
+		fmt.Println("Running pre-flight checks")
+		err := preflight.RunInitMasterChecks(cfg)
+		if err != nil {
+			return nil, &preflight.PreFlightError{Msg: err.Error()}
+		}
+	} else {
+		fmt.Println("Skipping pre-flight checks")
+	}
+
 	// TODO(phase1+) create a custom flag
 	if cfg.CloudProvider != "" {
 		if cloudprovider.IsCloudProvider(cfg.CloudProvider) {

cmd/kubeadm/app/cmd/join.go

@@ -98,9 +98,15 @@ func NewJoin(cfgPath string, args []string, cfg *kubeadmapi.NodeConfiguration, s
 		}
 	}
 
+	// TODO(phase1+) this we are missing args from the help text, there should be a way to tell cobra about it
+	if len(args) == 0 && len(cfg.MasterAddresses) == 0 {
+		return nil, fmt.Errorf("must specify master IP address (see --help)")
+	}
+	cfg.MasterAddresses = append(cfg.MasterAddresses, args...)
+
 	if !skipPreFlight {
 		fmt.Println("Running pre-flight checks")
-		err := preflight.RunJoinNodeChecks()
+		err := preflight.RunJoinNodeChecks(cfg)
 		if err != nil {
 			return nil, &preflight.PreFlightError{Msg: err.Error()}
 		}
@@ -108,12 +114,6 @@ func NewJoin(cfgPath string, args []string, cfg *kubeadmapi.NodeConfiguration, s
 		fmt.Println("Skipping pre-flight checks")
 	}
 
-	// TODO(phase1+) this we are missing args from the help text, there should be a way to tell cobra about it
-	if len(args) == 0 && len(cfg.MasterAddresses) == 0 {
-		return nil, fmt.Errorf("must specify master IP address (see --help)")
-	}
-	cfg.MasterAddresses = append(cfg.MasterAddresses, args...)
-
 	ok, err := kubeadmutil.UseGivenTokenIfValid(&cfg.Secrets)
 	if !ok {
 		if err != nil {

cmd/kubeadm/app/preflight/checks.go

@@ -21,6 +21,7 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"net/http"
 	"os"
 	"os/exec"
 
@@ -185,6 +186,33 @@ func (hc HostnameCheck) Check() (warnings, errors []error) {
 	return nil, 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
+}
+
 func RunInitMasterChecks(cfg *kubeadmapi.MasterConfiguration) error {
 	// TODO: Some of these ports should come from kubeadm config eventually:
 	checks := []PreFlightCheck{
@@ -199,6 +227,7 @@ func RunInitMasterChecks(cfg *kubeadmapi.MasterConfiguration) error {
 		PortOpenCheck{port: 10250},
 		PortOpenCheck{port: 10251},
 		PortOpenCheck{port: 10252},
+		HttpProxyCheck{Proto: "https", Host: cfg.API.AdvertiseAddresses[0], Port: int(cfg.API.BindPort)},
 		DirAvailableCheck{Path: "/etc/kubernetes/manifests"},
 		DirAvailableCheck{Path: "/etc/kubernetes/pki"},
 		DirAvailableCheck{Path: "/var/lib/etcd"},
@@ -219,7 +248,7 @@ func RunInitMasterChecks(cfg *kubeadmapi.MasterConfiguration) error {
 	return runChecks(checks, os.Stderr)
 }
 
-func RunJoinNodeChecks() error {
+func RunJoinNodeChecks(cfg *kubeadmapi.NodeConfiguration) error {
 	// TODO: Some of these ports should come from kubeadm config eventually:
 	checks := []PreFlightCheck{
 		IsRootCheck{root: true},
@@ -227,6 +256,8 @@ func RunJoinNodeChecks() error {
 		ServiceCheck{Service: "docker"},
 		ServiceCheck{Service: "kubelet"},
 		PortOpenCheck{port: 10250},
+		HttpProxyCheck{Proto: "https", Host: cfg.MasterAddresses[0], Port: int(cfg.APIPort)},
+		HttpProxyCheck{Proto: "http", Host: cfg.MasterAddresses[0], Port: int(cfg.DiscoveryPort)},
 		DirAvailableCheck{Path: "/etc/kubernetes"},
 		DirAvailableCheck{Path: "/var/lib/kubelet"},
 		InPathCheck{executable: "ebtables", mandatory: true},