Add ssh-host command for getting the ssh host keys

This is similar to the existing minikube "ssh-key" command, but gets
the content of the host key instead of the path to the identity key.

The output of this command can be added to the ~/.ssh/known_hosts,
for strict host key authentication. For instance when using Docker.

cmd/minikube/cmd/root.go

@@ -221,6 +221,7 @@ func init() {
 			Message: translate.T("Troubleshooting Commands:"),
 			Commands: []*cobra.Command{
 				sshKeyCmd,
+				sshHostCmd,
 				ipCmd,
 				logsCmd,
 				updateCheckCmd,

cmd/minikube/cmd/ssh-host.go

@@ -0,0 +1,74 @@
+/*
+Copyright 2016 The Kubernetes Authors All rights reserved.
+
+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 cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+
+	"k8s.io/minikube/pkg/minikube/config"
+	"k8s.io/minikube/pkg/minikube/driver"
+	"k8s.io/minikube/pkg/minikube/exit"
+	"k8s.io/minikube/pkg/minikube/machine"
+	"k8s.io/minikube/pkg/minikube/mustload"
+	"k8s.io/minikube/pkg/minikube/node"
+	"k8s.io/minikube/pkg/minikube/out"
+	"k8s.io/minikube/pkg/minikube/reason"
+)
+
+// sshHostCmd represents the sshHostCmd command
+var sshHostCmd = &cobra.Command{
+	Use:   "ssh-host",
+	Short: "Retrieve the ssh host key of the specified node",
+	Long:  "Retrieve the ssh host key of the specified node.",
+	Run: func(cmd *cobra.Command, args []string) {
+		cname := ClusterFlagValue()
+		co := mustload.Running(cname)
+		if co.CP.Host.DriverName == driver.None {
+			exit.Message(reason.Usage, "'none' driver does not support 'minikube ssh-host' command")
+		}
+
+		var err error
+		var n *config.Node
+		if nodeName == "" {
+			n = co.CP.Node
+		} else {
+			n, _, err = node.Retrieve(*co.Config, nodeName)
+			if err != nil {
+				exit.Message(reason.GuestNodeRetrieve, "Node {{.nodeName}} does not exist.", out.V{"nodeName": nodeName})
+			}
+		}
+
+		scanArgs := []string{"-t", "rsa"}
+
+		keys, err := machine.RunSSHHostCommand(co.API, *co.Config, *n, "ssh-keyscan", scanArgs)
+		if err != nil {
+			// This is typically due to a non-zero exit code, so no need for flourish.
+			out.ErrLn("ssh-keyscan: %v", err)
+			// It'd be nice if we could pass up the correct error code here :(
+			os.Exit(1)
+		}
+
+		fmt.Printf("%s", keys)
+	},
+}
+
+func init() {
+	sshHostCmd.Flags().StringVarP(&nodeName, "node", "n", "", "The node to ssh into. Defaults to the primary control plane.")
+}

pkg/minikube/machine/ssh.go

@@ -17,29 +17,42 @@ limitations under the License.
 package machine
 
 import (
+	"fmt"
+	"os/exec"
+
 	"github.com/docker/machine/libmachine"
+	"github.com/docker/machine/libmachine/host"
 	"github.com/docker/machine/libmachine/ssh"
 	"github.com/docker/machine/libmachine/state"
 	"github.com/pkg/errors"
 	"k8s.io/minikube/pkg/minikube/config"
 	"k8s.io/minikube/pkg/minikube/driver"
 )
 
-// CreateSSHShell creates a new SSH shell / client
-func CreateSSHShell(api libmachine.API, cc config.ClusterConfig, n config.Node, args []string, native bool) error {
+func getHost(api libmachine.API, cc config.ClusterConfig, n config.Node) (*host.Host, error) {
 	machineName := driver.MachineName(cc, n)
 	host, err := LoadHost(api, machineName)
 	if err != nil {
-		return errors.Wrap(err, "host exists and load")
+		return nil, errors.Wrap(err, "host exists and load")
 	}
 
 	currentState, err := host.Driver.GetState()
 	if err != nil {
-		return errors.Wrap(err, "state")
+		return nil, errors.Wrap(err, "state")
 	}
 
 	if currentState != state.Running {
-		return errors.Errorf("%q is not running", machineName)
+		return nil, errors.Errorf("%q is not running", machineName)
+	}
+
+	return host, nil
+}
+
+// CreateSSHShell creates a new SSH shell / client
+func CreateSSHShell(api libmachine.API, cc config.ClusterConfig, n config.Node, args []string, native bool) error {
+	host, err := getHost(api, cc, n)
+	if err != nil {
+		return err
 	}
 
 	if native {
@@ -55,3 +68,35 @@ func CreateSSHShell(api libmachine.API, cc config.ClusterConfig, n config.Node,
 	}
 	return client.Shell(args...)
 }
+
+// RunSSHHostCommand runs a command to the SSH host
+func RunSSHHostCommand(api libmachine.API, cc config.ClusterConfig, n config.Node, command string, args []string) (string, error) {
+	host, err := getHost(api, cc, n)
+	if err != nil {
+		return "", err
+	}
+
+	cmdPath, err := exec.LookPath(command)
+	if err != nil {
+		return "", err
+	}
+
+	port, err := host.Driver.GetSSHPort()
+	if err != nil {
+		return "", err
+	}
+
+	args = append(args, "-p")
+	args = append(args, fmt.Sprintf("%d", port))
+
+	addr, err := host.Driver.GetSSHHostname()
+	if err != nil {
+		return "", err
+	}
+
+	args = append(args, addr)
+
+	cmd := exec.Command(cmdPath, args...)
+	output, err := cmd.Output()
+	return string(output), err
+}

site/content/en/docs/commands/ssh-host.md

@@ -0,0 +1,46 @@
+---
+title: "ssh-host"
+description: >
+  Retrieve the ssh host key of the specified node
+---
+
+
+## minikube ssh-host
+
+Retrieve the ssh host key of the specified node
+
+### Synopsis
+
+Retrieve the ssh host key of the specified node.
+
+```shell
+minikube ssh-host [flags]
+```
+
+### Options
+
+```
+  -n, --node string   The node to ssh into. Defaults to the primary control plane.
+```
+
+### Options inherited from parent commands
+
+```
+      --add_dir_header                   If true, adds the file directory to the header of the log messages
+      --alsologtostderr                  log to standard error as well as files
+  -b, --bootstrapper string              The name of the cluster bootstrapper that will set up the Kubernetes cluster. (default "kubeadm")
+  -h, --help                             
+      --log_backtrace_at traceLocation   when logging hits line file:N, emit a stack trace (default :0)
+      --log_dir string                   If non-empty, write log files in this directory
+      --log_file string                  If non-empty, use this log file
+      --log_file_max_size uint           Defines the maximum size a log file can grow to. Unit is megabytes. If the value is 0, the maximum file size is unlimited. (default 1800)
+      --logtostderr                      log to standard error instead of files
+      --one_output                       If true, only write logs to their native severity level (vs also writing to each lower severity level
+  -p, --profile string                   The name of the minikube VM being used. This can be set to allow having multiple instances of minikube independently. (default "minikube")
+      --skip_headers                     If true, avoid header prefixes in the log messages
+      --skip_log_headers                 If true, avoid headers when opening log files
+      --stderrthreshold severity         logs at or above this threshold go to stderr (default 2)
+  -v, --v Level                          number for the log level verbosity
+      --vmodule moduleSpec               comma-separated list of pattern=N settings for file-filtered logging
+```
+