cmd/tailscale/cli: add start of 'ssh' subcommand

Updates #3802

Change-Id: Iabc07c00c7e4f43944cfe7daec8d2b66ac002289
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -171,6 +171,7 @@ change in the future.
 			statusCmd,
 			pingCmd,
 			ncCmd,
+			sshCmd,
 			versionCmd,
 			webCmd,
 			fileCmd,

cmd/tailscale/cli/ssh.go

@@ -0,0 +1,97 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cli
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"os/user"
+	"runtime"
+	"strings"
+	"syscall"
+
+	"github.com/alessio/shellescape"
+	"github.com/peterbourgon/ff/v3/ffcli"
+	"tailscale.com/envknob"
+)
+
+var sshCmd = &ffcli.Command{
+	Name:       "ssh",
+	ShortUsage: "ssh [user@]<host> [args...]",
+	ShortHelp:  "SSH to a Tailscale machine",
+	Exec:       runSSH,
+}
+
+func runSSH(ctx context.Context, args []string) error {
+	if len(args) == 0 {
+		return errors.New("usage: ssh [user@]<host>")
+	}
+	arg, argRest := args[0], args[1:]
+	username, host, ok := strings.Cut(arg, "@")
+	if !ok {
+		host = arg
+		lu, err := user.Current()
+		if err != nil {
+			return nil
+		}
+		username = lu.Username
+	}
+	ssh, err := exec.LookPath("ssh")
+	if err != nil {
+		// TODO(bradfitz): use Go's crypto/ssh client instead
+		// of failing. But for now:
+		return fmt.Errorf("no system 'ssh' command found: %w", err)
+	}
+	tailscaleBin, err := os.Executable()
+	if err != nil {
+		return err
+	}
+	argv := append([]string{
+		ssh,
+
+		"-o", fmt.Sprintf("ProxyCommand %s --socket=%s nc %%h %%p",
+			shellescape.Quote(tailscaleBin),
+			shellescape.Quote(rootArgs.socket),
+		),
+
+		// Explicitly rebuild the user@host argument rather than
+		// passing it through.  In general, the use of OpenSSH's ssh
+		// binary is a crutch for now.  We don't want to be
+		// Hyrum-locked into passing through all OpenSSH flags to the
+		// OpenSSH client forever. We try to make our flags and args
+		// be compatible, but only a subset. The "tailscale ssh"
+		// command should be a simple and portable one. If they want
+		// to use a different one, we'll later be making stock ssh
+		// work well by default too. (doing things like automatically
+		// setting known_hosts, etc)
+		username + "@" + host,
+	}, argRest...)
+
+	if runtime.GOOS == "windows" {
+		// Don't use syscall.Exec on Windows.
+		cmd := exec.Command(ssh, argv[1:]...)
+		cmd.Stderr = os.Stderr
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		var ee *exec.ExitError
+		err := cmd.Run()
+		if errors.As(err, &ee) {
+			os.Exit(ee.ExitCode())
+		}
+		return err
+	}
+
+	if envknob.Bool("TS_DEBUG_SSH_EXEC") {
+		log.Printf("Running: %q, %q ...", ssh, argv)
+	}
+	if err := syscall.Exec(ssh, argv, os.Environ()); err != nil {
+		return err
+	}
+	return errors.New("unreachable")
+}

cmd/tailscale/depaware.txt

@@ -1,5 +1,6 @@
 tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/depaware)
 
+        github.com/alessio/shellescape                               from tailscale.com/cmd/tailscale/cli
    W 💣 github.com/alexbrainman/sspi                                 from github.com/alexbrainman/sspi/negotiate+
    W    github.com/alexbrainman/sspi/internal/common                 from github.com/alexbrainman/sspi/negotiate
    W 💣 github.com/alexbrainman/sspi/negotiate                       from tailscale.com/net/tshttpproxy
@@ -193,7 +194,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         os                                                           from crypto/rand+
         os/exec                                                      from github.com/toqueteos/webbrowser+
         os/signal                                                    from tailscale.com/cmd/tailscale/cli
-        os/user                                                      from tailscale.com/util/groupmember
+        os/user                                                      from tailscale.com/util/groupmember+
         path                                                         from html/template+
         path/filepath                                                from crypto/x509+
         reflect                                                      from crypto/x509+

go.mod

@@ -5,6 +5,7 @@ go 1.18
 require (
 	filippo.io/mkcert v1.4.3
 	github.com/akutz/memconn v0.1.0
+	github.com/alessio/shellescape v1.4.1
 	github.com/alexbrainman/sspi v0.0.0-20210105120005-909beea2cc74
 	github.com/aws/aws-sdk-go-v2 v1.11.2
 	github.com/aws/aws-sdk-go-v2/config v1.11.0

go.sum

@@ -103,6 +103,8 @@ github.com/alecthomas/template v0.0.0-20190718012654-fb15b899a751/go.mod h1:LOuy
 github.com/alecthomas/units v0.0.0-20151022065526-2efee857e7cf/go.mod h1:ybxpYRFXyAe+OPACYpWeL0wqObRcbAqCMya13uyzqw0=
 github.com/alecthomas/units v0.0.0-20190717042225-c3de453c63f4/go.mod h1:ybxpYRFXyAe+OPACYpWeL0wqObRcbAqCMya13uyzqw0=
 github.com/alecthomas/units v0.0.0-20190924025748-f65c72e2690d/go.mod h1:rBZYJk541a8SKzHPHnH3zbiI+7dagKZ0cgpgrD7Fyho=
+github.com/alessio/shellescape v1.4.1 h1:V7yhSDDn8LP4lc4jS8pFkt0zCnzVJlG5JXy9BVKJUX0=
+github.com/alessio/shellescape v1.4.1/go.mod h1:PZAiSCk0LJaZkiCSkPv8qIobYglO3FPpyFjDCtHLS30=
 github.com/alexbrainman/sspi v0.0.0-20210105120005-909beea2cc74 h1:Kk6a4nehpJ3UuJRqlA3JxYxBZEqCeOmATOvrbT4p9RA=
 github.com/alexbrainman/sspi v0.0.0-20210105120005-909beea2cc74/go.mod h1:cEWa1LVoE5KvSD9ONXsZrj0z6KqySlCCNKHlLzbqAt4=
 github.com/alexkohler/prealloc v1.0.0 h1:Hbq0/3fJPQhNkN0dR95AVrr6R7tou91y0uHG5pOcUuw=