ssh/tailssh: fall back to using su when no TTY available on Linux

This allows pam authentication to run for ssh sessions, triggering
automation like pam_mkhomedir.

Updates #11854

Signed-off-by: Percy Wegmann <percy@tailscale.com>

Makefile

@@ -115,10 +115,7 @@ sshintegrationtest: ## Run the SSH integration tests in various Docker container
 	echo "Testing on ubuntu:focal" && docker build --build-arg="BASE=ubuntu:focal" -t ssh-ubuntu-focal ssh/tailssh/testcontainers && \
 	echo "Testing on ubuntu:jammy" && docker build --build-arg="BASE=ubuntu:jammy" -t ssh-ubuntu-jammy ssh/tailssh/testcontainers && \
 	echo "Testing on ubuntu:mantic" && docker build --build-arg="BASE=ubuntu:mantic" -t ssh-ubuntu-mantic ssh/tailssh/testcontainers && \
-	echo "Testing on ubuntu:noble" && docker build --build-arg="BASE=ubuntu:noble" -t ssh-ubuntu-noble ssh/tailssh/testcontainers && \
+	echo "Testing on ubuntu:noble" && docker build --build-arg="BASE=ubuntu:noble" -t ssh-ubuntu-noble ssh/tailssh/testcontainers
-	echo "Testing on fedora:38" && docker build --build-arg="BASE=dokken/fedora-38" -t ssh-fedora-38 ssh/tailssh/testcontainers && \
-	echo "Testing on fedora:39" && docker build --build-arg="BASE=dokken/fedora-39" -t ssh-fedora-39 ssh/tailssh/testcontainers && \
-	echo "Testing on fedora:40" && docker build --build-arg="BASE=dokken/fedora-40" -t ssh-fedora-40 ssh/tailssh/testcontainers
 
 help: ## Show this help
 	@echo "\nSpecify a command. The choices are:\n"

go.mod

@@ -14,6 +14,7 @@ require (
 	github.com/aws/aws-sdk-go-v2/feature/s3/manager v1.11.64
 	github.com/aws/aws-sdk-go-v2/service/s3 v1.33.0
 	github.com/aws/aws-sdk-go-v2/service/ssm v1.44.7
+	github.com/bramvdbogaerde/go-scp v1.4.0
 	github.com/coreos/go-iptables v0.7.1-0.20240112124308-65c67c9f46e6
 	github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf
 	github.com/creack/pty v1.1.21

go.sum

@@ -177,6 +177,8 @@ github.com/blizzy78/varnamelen v0.8.0 h1:oqSblyuQvFsW1hbBHh1zfwrKe3kcSj0rnXkKzsQ
 github.com/blizzy78/varnamelen v0.8.0/go.mod h1:V9TzQZ4fLJ1DSrjVDfl89H7aMnTvKkApdHeyESmyR7k=
 github.com/bombsimon/wsl/v3 v3.4.0 h1:RkSxjT3tmlptwfgEgTgU+KYKLI35p/tviNXNXiL2aNU=
 github.com/bombsimon/wsl/v3 v3.4.0/go.mod h1:KkIB+TXkqy6MvK9BDZVbZxKNYsE1/oLRJbIFtf14qqo=
+github.com/bramvdbogaerde/go-scp v1.4.0 h1:jKMwpwCbcX1KyvDbm/PDJuXcMuNVlLGi0Q0reuzjyKY=
+github.com/bramvdbogaerde/go-scp v1.4.0/go.mod h1:on2aH5AxaFb2G0N5Vsdy6B0Ml7k9HuHSwfo1y0QzAbQ=
 github.com/breml/bidichk v0.2.4 h1:i3yedFWWQ7YzjdZJHnPo9d/xURinSq3OM+gyM43K4/8=
 github.com/breml/bidichk v0.2.4/go.mod h1:7Zk0kRFt1LIZxtQdl9W9JwGAcLTTkOs+tN7wuEYGJ3s=
 github.com/breml/errchkjson v0.3.1 h1:hlIeXuspTyt8Y/UmP5qy1JocGNR00KQHgfaNtRAjoxQ=

ssh/tailssh/incubator.go

@@ -36,6 +36,7 @@
 	"golang.org/x/sys/unix"
 	"tailscale.com/cmd/tailscaled/childproc"
 	"tailscale.com/hostinfo"
+	"tailscale.com/tailcfg"
 	"tailscale.com/tempfork/gliderlabs/ssh"
 	"tailscale.com/types/logger"
 	"tailscale.com/version/distro"
@@ -43,18 +44,22 @@
 
 func init() {
 	childproc.Add("ssh", beIncubator)
+	childproc.Add("sftp", beSFTP)
 }
 
 var ptyName = func(f *os.File) (string, error) {
 	return "", fmt.Errorf("unimplemented")
 }
 
-// maybeStartLoginSession starts a new login session for the specified UID.
+// maybeStartLoginSession informs the system that we are about to log someone
-// On success, it may return a non-nil close func which must be closed to
+// in. On success, it may return a non-nil close func which must be closed to
 // release the session.
+// We can only do this if we are running as root.
+// This is best effort to still allow running on machines where
+// we don't support starting sessions, e.g. darwin.
 // See maybeStartLoginSessionLinux.
-var maybeStartLoginSession = func(logf logger.Logf, ia incubatorArgs) (close func() error, err error) {
+var maybeStartLoginSession = func(dlogf logger.Logf, ia incubatorArgs) (close func() error) {
-	return nil, nil
+	return nil
 }
 
 // newIncubatorCommand returns a new exec.Cmd configured with
@@ -64,40 +69,39 @@ func init() {
 // exec.CommandContext.
 //
 // The returned Cmd.Env is guaranteed to be nil; the caller populates it.
-func (ss *sshSession) newIncubatorCommand() (cmd *exec.Cmd) {
+func (ss *sshSession) newIncubatorCommand(logf logger.Logf) (cmd *exec.Cmd, err error) {
 	defer func() {
 		if cmd.Env != nil {
 			panic("internal error")
 		}
 	}()
-	var (
+
-		name    string
+	var isSFTP, isShell bool
-		args    []string
-		isSFTP  bool
-		isShell bool
-	)
 	switch ss.Subsystem() {
 	case "sftp":
 		isSFTP = true
 	case "":
-		name = ss.conn.localUser.LoginShell()
+		isShell = ss.RawCommand() == ""
-		if rawCmd := ss.RawCommand(); rawCmd != "" {
-			args = append(args, "-c", rawCmd)
-		} else {
-			isShell = true
-			args = append(args, "-l") // login shell
-		}
 	default:
 		panic(fmt.Sprintf("unexpected subsystem: %v", ss.Subsystem()))
 	}
 
 	if ss.conn.srv.tailscaledPath == "" {
-		// TODO(maisem): this doesn't work with sftp
+		if isSFTP {
-		return exec.CommandContext(ss.ctx, name, args...)
+			// SFTP relies on the embedded Go-based SFTP server in tailscaled,
+			// so without tailscaled, we can't serve SFTP.
+			return nil, errors.New("no tailscaled found on path, can't serve SFTP")
 		}
+
+		loginShell := ss.conn.localUser.LoginShell()
+		args := shellArgs(isShell, ss.RawCommand())
+		logf("directly running %s %q", loginShell, args)
+		return exec.CommandContext(ss.ctx, loginShell, args...), nil
+	}
+
 	lu := ss.conn.localUser
 	ci := ss.conn.info
-	gids := strings.Join(ss.conn.userGroupIDs, ",")
+	groups := strings.Join(ss.conn.userGroupIDs, ",")
 	remoteUser := ci.uprof.LoginName
 	if ci.node.IsTagged() {
 		remoteUser = strings.Join(ci.node.Tags().AsSlice(), ",")
@@ -106,9 +110,10 @@ func (ss *sshSession) newIncubatorCommand() (cmd *exec.Cmd) {
 	incubatorArgs := []string{
 		"be-child",
 		"ssh",
+		"--login-shell=" + lu.LoginShell(),
 		"--uid=" + lu.Uid,
 		"--gid=" + lu.Gid,
-		"--groups=" + gids,
+		"--groups=" + groups,
 		"--local-user=" + lu.Username,
 		"--remote-user=" + remoteUser,
 		"--remote-ip=" + ci.src.Addr().String(),
@@ -116,39 +121,31 @@ func (ss *sshSession) newIncubatorCommand() (cmd *exec.Cmd) {
 		"--tty-name=",     // updated in-place by startWithPTY
 	}
 
+	forceV1Behavior := ss.conn.srv.lb.NetMap().HasCap(tailcfg.NodeAttrSSHBehaviorV1)
+	if forceV1Behavior {
+		incubatorArgs = append(incubatorArgs, "--force-v1-behavior")
+	}
+
 	if debugTest.Load() {
 		incubatorArgs = append(incubatorArgs, "--debug-test")
 	}
 
-	if isSFTP {
+	switch {
-		incubatorArgs = append(incubatorArgs, "--sftp")
+	case isSFTP:
-	} else {
+		// Note that we include both the `--sftp` flag and a command to launch
-		if isShell {
+		// tailscaled as `be-child sftp`. If login or su is available, and
+		// we're not running with tailcfg.NodeAttrSSHBehaviorV1, this will
+		// result in serving SFTP within a login shell, with full PAM
+		// integration. Otherwise, we'll serve SFTP in the incubator process
+		// with no PAM integration.
+		incubatorArgs = append(incubatorArgs, "--sftp", fmt.Sprintf("--cmd=%s be-child sftp", ss.conn.srv.tailscaledPath))
+	case isShell:
 		incubatorArgs = append(incubatorArgs, "--shell")
+	default:
+		incubatorArgs = append(incubatorArgs, "--cmd="+ss.RawCommand())
 	}
-		// Only the macOS version of the login command supports executing a
+
-		// command, all other versions only support launching a shell
+	return exec.CommandContext(ss.ctx, ss.conn.srv.tailscaledPath, incubatorArgs...), nil
-		// without taking any arguments.
-		shouldUseLoginCmd := isShell || runtime.GOOS == "darwin"
-		if hostinfo.IsSELinuxEnforcing() {
-			// If we're running on a SELinux-enabled system, the login
-			// command will be unable to set the correct context for the
-			// shell. Fall back to using the incubator to launch the shell.
-			// See http://github.com/tailscale/tailscale/issues/4908.
-			shouldUseLoginCmd = false
-		}
-		if shouldUseLoginCmd {
-			if lp, err := exec.LookPath("login"); err == nil {
-				incubatorArgs = append(incubatorArgs, "--login-cmd="+lp)
-			}
-		}
-		incubatorArgs = append(incubatorArgs, "--cmd="+name)
-		if len(args) > 0 {
-			incubatorArgs = append(incubatorArgs, "--")
-			incubatorArgs = append(incubatorArgs, args...)
-		}
-	}
-	return exec.CommandContext(ss.ctx, ss.conn.srv.tailscaledPath, incubatorArgs...)
 }
 
 var debugIncubator bool
@@ -170,51 +167,60 @@ func (stdRWC) Close() error {
 }
 
 type incubatorArgs struct {
+	loginShell      string
 	uid             int
 	gid             int
-	groups       string
+	gids            []int
 	localUser       string
 	remoteUser      string
 	remoteIP        string
 	ttyName         string
 	hasTTY          bool
-	cmdName      string
+	cmd             string
 	isSFTP          bool
 	isShell         bool
-	loginCmdPath string
+	forceV1Behavior bool
-	cmdArgs      []string
 	debugTest       bool
 }
 
-func parseIncubatorArgs(args []string) (a incubatorArgs) {
+func parseIncubatorArgs(args []string) (incubatorArgs, error) {
+	var ia incubatorArgs
+	var groups string
+
 	flags := flag.NewFlagSet("", flag.ExitOnError)
-	flags.IntVar(&a.uid, "uid", 0, "the uid of local-user")
+	flags.StringVar(&ia.loginShell, "login-shell", "", "path to the user's preferred login shell")
-	flags.IntVar(&a.gid, "gid", 0, "the gid of local-user")
+	flags.IntVar(&ia.uid, "uid", 0, "the uid of local-user")
-	flags.StringVar(&a.groups, "groups", "", "comma-separated list of gids of local-user")
+	flags.IntVar(&ia.gid, "gid", 0, "the gid of local-user")
-	flags.StringVar(&a.localUser, "local-user", "", "the user to run as")
+	flags.StringVar(&groups, "groups", "", "comma-separated list of gids of local-user")
-	flags.StringVar(&a.remoteUser, "remote-user", "", "the remote user/tags")
+	flags.StringVar(&ia.localUser, "local-user", "", "the user to run as")
-	flags.StringVar(&a.remoteIP, "remote-ip", "", "the remote Tailscale IP")
+	flags.StringVar(&ia.remoteUser, "remote-user", "", "the remote user/tags")
-	flags.StringVar(&a.ttyName, "tty-name", "", "the tty name (pts/3)")
+	flags.StringVar(&ia.remoteIP, "remote-ip", "", "the remote Tailscale IP")
-	flags.BoolVar(&a.hasTTY, "has-tty", false, "is the output attached to a tty")
+	flags.StringVar(&ia.ttyName, "tty-name", "", "the tty name (pts/3)")
-	flags.StringVar(&a.cmdName, "cmd", "", "the cmd to launch (ignored in sftp mode)")
+	flags.BoolVar(&ia.hasTTY, "has-tty", false, "is the output attached to a tty")
-	flags.BoolVar(&a.isShell, "shell", false, "is launching a shell (with no cmds)")
+	flags.StringVar(&ia.cmd, "cmd", "", "the cmd to launch, including all arguments (ignored in sftp mode)")
-	flags.BoolVar(&a.isSFTP, "sftp", false, "run sftp server (cmd is ignored)")
+	flags.BoolVar(&ia.isShell, "shell", false, "is launching a shell (with no cmds)")
-	flags.StringVar(&a.loginCmdPath, "login-cmd", "", "the path to `login` cmd")
+	flags.BoolVar(&ia.isSFTP, "sftp", false, "run sftp server (cmd is ignored)")
-	flags.BoolVar(&a.debugTest, "debug-test", false, "should debug in test mode")
+	flags.BoolVar(&ia.forceV1Behavior, "force-v1-behavior", false, "allow falling back to the su command if login is unavailable")
+	flags.BoolVar(&ia.debugTest, "debug-test", false, "should debug in test mode")
 	flags.Parse(args)
-	a.cmdArgs = flags.Args()
+
-	return a
+	for _, g := range strings.Split(groups, ",") {
+		gid, err := strconv.Atoi(g)
+		if err != nil {
+			return ia, fmt.Errorf("unable to parse group id %q: %w", g, err)
+		}
+		ia.gids = append(ia.gids, gid)
+	}
+
+	return ia, nil
 }
 
 // beIncubator is the entrypoint to the `tailscaled be-child ssh` subcommand.
-// It is responsible for informing the system of a new login session for the user.
+// It is responsible for informing the system of a new login session for the
-// This is sometimes necessary for mounting home directories and decrypting file
+// user. This is sometimes necessary for mounting home directories and
-// systems.
+// decrypting file systems.
 //
-// Tailscaled launches the incubator as the same user as it was
+// Tailscaled launches the incubator as the same user as it was launched as.
-// launched as.  The incubator then registers a new session with the
-// OS, sets its UID and groups to the specified `--uid`, `--gid` and
-// `--groups` and then launches the requested `--cmd`.
 func beIncubator(args []string) error {
 	// To defend against issues like https://golang.org/issue/1435,
 	// defensively lock our current goroutine's thread to the current
@@ -226,22 +232,25 @@ func beIncubator(args []string) error {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
-	ia := parseIncubatorArgs(args)
+	ia, err := parseIncubatorArgs(args)
+	if err != nil {
+		return err
+	}
 	if ia.isSFTP && ia.isShell {
 		return fmt.Errorf("--sftp and --shell are mutually exclusive")
 	}
 
-	logf := logger.Discard
+	dlogf := logger.Discard
 	if debugIncubator {
 		// We don't own stdout or stderr, so the only place we can log is syslog.
 		if sl, err := syslog.New(syslog.LOG_INFO|syslog.LOG_DAEMON, "tailscaled-ssh"); err == nil {
-			logf = log.New(sl, "", 0).Printf
+			dlogf = log.New(sl, "", 0).Printf
 		}
 	} else if ia.debugTest {
-		// In testing, we don't always have syslog, log to a temp file
+		// In testing, we don't always have syslog, so log to a temp file.
 		if logFile, err := os.OpenFile("/tmp/tailscalessh.log", os.O_APPEND|os.O_WRONLY, 0666); err == nil {
 			lf := log.New(logFile, "", 0)
-			logf = func(msg string, args ...any) {
+			dlogf = func(msg string, args ...any) {
 				lf.Printf(msg, args...)
 				logFile.Sync()
 			}
@@ -249,41 +258,55 @@ func beIncubator(args []string) error {
 		}
 	}
 
-	euid := os.Geteuid()
+	if !shouldAttemptLoginShell(dlogf, ia) {
-	runningAsRoot := euid == 0
+		dlogf("not attempting login shell")
-	if runningAsRoot && ia.loginCmdPath != "" {
+		return handleInProcess(dlogf, ia)
-		// Check if we can exec into the login command instead of trying to
-		// incubate ourselves.
-		if la := ia.loginArgs(); la != nil {
-			return unix.Exec(ia.loginCmdPath, la, os.Environ())
-		}
 	}
 
-	// Inform the system that we are about to log someone in.
+	// First try the login command
-	// We can only do this if we are running as root.
+	if err := tryExecLogin(dlogf, ia); err != nil {
-	// This is best effort to still allow running on machines where
+		return err
-	// we don't support starting sessions, e.g. darwin.
+	}
-	sessionCloser, err := maybeStartLoginSession(logf, ia)
+
-	if err == nil && sessionCloser != nil {
+	// If we got here, we weren't able to use login (because tryExecLogin
+	// returned without replacing the running process), maybe we can use
+	// su.
+	if handled, err := trySU(dlogf, ia); handled {
+		return err
+	} else {
+		dlogf("not attempting su")
+		return handleInProcess(dlogf, ia)
+	}
+}
+
+func handleInProcess(dlogf logger.Logf, ia incubatorArgs) error {
+	if ia.isSFTP {
+		return handleSFTPInProcess(dlogf, ia)
+	}
+	return handleSSHInProcess(dlogf, ia)
+}
+
+func handleSFTPInProcess(dlogf logger.Logf, ia incubatorArgs) error {
+	dlogf("handling sftp")
+
+	sessionCloser := maybeStartLoginSession(dlogf, ia)
+	if sessionCloser != nil {
 		defer sessionCloser()
 	}
 
-	var groupIDs []int
+	if err := dropPrivileges(dlogf, ia); err != nil {
-	for _, g := range strings.Split(ia.groups, ",") {
-		gid, err := strconv.ParseInt(g, 10, 32)
-		if err != nil {
-			return err
-		}
-		groupIDs = append(groupIDs, int(gid))
-	}
-
-	if err := dropPrivileges(logf, ia.uid, ia.gid, groupIDs); err != nil {
 		return err
 	}
 
-	if ia.isSFTP {
+	return serveSFTP()
-		logf("handling sftp")
+}
 
+// beSFTP serves SFTP in-process.
+func beSFTP(args []string) error {
+	return serveSFTP()
+}
+
+func serveSFTP() error {
 	server, err := sftp.NewServer(stdRWC{})
 	if err != nil {
 		return err
@@ -294,27 +317,174 @@ func beIncubator(args []string) error {
 		return err
 	}
 	return nil
+}
+
+// shouldAttemptLoginShell decides whether we should attempt to get a full
+// login shell with the login or su commands. We will attempt a login shell
+// if all of the following conditions are met.
+//
+// - We are running as root
+// - This is not an SELinuxEnforcing host
+//
+// The last condition exists because if we're running on a SELinux-enabled
+// system, neiher login nor su will be able to set the correct context for the
+// shell. So, we don't bother trying to run them and instead fall back to using
+// the incubator to launch the shell.
+// See http://github.com/tailscale/tailscale/issues/4908.
+func shouldAttemptLoginShell(dlogf logger.Logf, ia incubatorArgs) bool {
+	if ia.forceV1Behavior && ia.isSFTP {
+		// v1 behavior did not run SFTP within a login shell.
+		dlogf("Forcing v1 behavior, won't use login shell for SFTP")
+		return false
 	}
 
-	cmd := exec.Command(ia.cmdName, ia.cmdArgs...)
+	return runningAsRoot() && !hostinfo.IsSELinuxEnforcing()
-	cmd.Stdin = os.Stdin
+}
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
-	cmd.Env = os.Environ()
 
-	if ia.hasTTY {
+func runningAsRoot() bool {
-		// If we were launched with a tty then we should
+	euid := os.Geteuid()
-		// mark that as the ctty of the child. However,
+	return euid == 0
-		// as the ctty is being passed from the parent
+}
-		// we set the child to foreground instead which
+
-		// also passes the ctty.
+// tryExecLogin attempts to handle the ssh session by creating a full login
-		// However, we can not do this if never had a tty to
+// shell using the login command. If it never tried, it returns nil. If it
-		// begin with.
+// failed to do so, it returns an error.
-		cmd.SysProcAttr = &syscall.SysProcAttr{
+//
-			Foreground: true,
+// Creating a login shell in this way allows us to register the remote IP of
+// the login session, trigger PAM authentication, and get the "remote" PAM
+// profile.
+//
+// However, login is subject to some limitations.
+//
+// 1. login cannot be used to execute commands except on macOS.
+// 2. On Linux and BSD, login requires a TTY to keep running.
+//
+// In these cases, tryExecLogin returns (false, nil) to indicate that processing
+// should fall through to other methods, such as using the su command.
+//
+// Note that this uses unix.Exec to replace the current process, so in cases
+// where we actually do run login, no subsequent Go code will execute.
+func tryExecLogin(dlogf logger.Logf, ia incubatorArgs) error {
+	// Only the macOS version of the login command supports executing a
+	// command, all other versions only support launching a shell without
+	// taking any arguments.
+	if !ia.isShell && runtime.GOOS != "darwin" {
+		dlogf("won't use login because we're not in a shell or on macOS")
+		return nil
+	}
+
+	switch runtime.GOOS {
+	case "linux", "freebsd", "openbsd":
+		if !ia.hasTTY {
+			dlogf("can't use login because of missing TTY")
+			// We can only use the login command if a shell was requested with
+			// a TTY. If there is no TTY, login exits immediately, which
+			// breaks things like mosh and VSCode.
+			return nil
 		}
 	}
-	err = cmd.Run()
+
+	loginCmdPath, err := exec.LookPath("login")
+	if err != nil {
+		dlogf("failed to get login args: %s", err)
+		return nil
+	}
+	loginArgs := ia.loginArgs(loginCmdPath)
+	dlogf("logging in with %s %+v", loginCmdPath, loginArgs)
+	// replace the running process
+	return unix.Exec(loginCmdPath, loginArgs, os.Environ())
+}
+
+// trySU attempts to start a login shell using su. If su is available and
+// supports the necessary arguments, this returns true, plus the result of
+// executing su. Otherwise, it returns (false, nil).
+//
+// Creating a login shell in this way allows us to trigger PAM authentication
+// and get the "login" PAM profile.
+//
+// Unlike login, su often does not require a TTY, so on Linux hosts that have
+// an su command which accepts the right flags, we'll use su instead of login
+// when no TTY is available.
+func trySU(dlogf logger.Logf, ia incubatorArgs) (handled bool, err error) {
+	if ia.forceV1Behavior {
+		// v1 behavior did not use su.
+		dlogf("Forcing v1 behavior, won't use su")
+		return false, nil
+	}
+
+	su := findSU(dlogf, ia)
+	if su == "" {
+		return false, nil
+	}
+
+	sessionCloser := maybeStartLoginSession(dlogf, ia)
+	if sessionCloser != nil {
+		defer sessionCloser()
+	}
+
+	loginArgs := []string{"-l", ia.localUser}
+	if ia.cmd != "" {
+		// Note - unlike the login command, su allows using both -l and -c.
+		loginArgs = append(loginArgs, "-c", ia.cmd)
+	}
+
+	dlogf("logging in with %s %q", su, loginArgs)
+	cmd := newCommand(ia.hasTTY, su, loginArgs)
+	return true, cmd.Run()
+}
+
+// findSU attempts to find an su command which supports the -l and -c flags.
+// This actually calls the su command, which can cause side effects like
+// triggering pam_mkhomedir. If a suitable su is not available, this returns
+// "".
+func findSU(dlogf logger.Logf, ia incubatorArgs) string {
+	// Currently, we only support falling back to su on Linux. This
+	// potentially could work on BSDs as well, but requires testing.
+	if runtime.GOOS != "linux" {
+		return ""
+	}
+
+	// gokrazy doesn't include su. And, if someone installs a breakglass/
+	// debugging package on gokrazy, we don't want to use its su.
+	if distro.Get() == distro.Gokrazy {
+		return ""
+	}
+
+	su, err := exec.LookPath("su")
+	if err != nil {
+		dlogf("can't find su command: %v", err)
+		return ""
+	}
+
+	// First try to execute su -l <user> -c true to make sure su supports the
+	// necessary arguments.
+	err = exec.Command(su, "-l", ia.localUser, "-c", "true").Run()
+	if err != nil {
+		dlogf("su check failed: %s", err)
+		return ""
+	}
+
+	return su
+}
+
+// handleSSHInProcess is a last resort if we couldn't use login or su. It
+// registers a new session with the OS, sets its UID, GID and groups to the
+// specified values, and then launches the requested `--cmd` in the user's
+// login shell.
+func handleSSHInProcess(dlogf logger.Logf, ia incubatorArgs) error {
+	sessionCloser := maybeStartLoginSession(dlogf, ia)
+	if sessionCloser != nil {
+		defer sessionCloser()
+	}
+
+	if err := dropPrivileges(dlogf, ia); err != nil {
+		return err
+	}
+
+	args := shellArgs(ia.isShell, ia.cmd)
+	dlogf("running %s %q", ia.loginShell, args)
+	cmd := newCommand(ia.hasTTY, ia.loginShell, args)
+	err := cmd.Run()
 	if ee, ok := err.(*exec.ExitError); ok {
 		ps := ee.ProcessState
 		code := ps.ExitCode()
@@ -330,6 +500,26 @@ func beIncubator(args []string) error {
 	return err
 }
 
+func newCommand(hasTTY bool, cmdPath string, cmdArgs []string) *exec.Cmd {
+	cmd := exec.Command(cmdPath, cmdArgs...)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.Env = os.Environ()
+
+	if hasTTY {
+		// If we were launched with a tty then we should mark that as the ctty
+		// of the child. However, as the ctty is being passed from the parent
+		// we set the child to foreground instead which also passes the ctty.
+		// However, we can not do this if never had a tty to begin with.
+		cmd.SysProcAttr = &syscall.SysProcAttr{
+			Foreground: true,
+		}
+	}
+
+	return cmd
+}
+
 const (
 	// This controls whether we assert that our privileges were dropped
 	// using geteuid/getegid; it's a const and not an envknob because the
@@ -344,19 +534,26 @@ func beIncubator(args []string) error {
 	assertPrivilegesWereDroppedByAttemptingToUnDrop = false
 )
 
-// dropPrivileges contains all the logic for dropping privileges to a different
+// dropPrivileges calls doDropPrivileges with uid, gid, and gids from the given
+// incubatorArgs.
+func dropPrivileges(dlogf logger.Logf, ia incubatorArgs) error {
+	return doDropPrivileges(dlogf, ia.uid, ia.gid, ia.gids)
+}
+
+// doDropPrivileges contains all the logic for dropping privileges to a different
 // UID, GID, and set of supplementary groups. This function is
 // security-sensitive and ordering-dependent; please be very cautious if/when
 // refactoring.
 //
-// WARNING: if you change this function, you *MUST* run the TestDropPrivileges
+// WARNING: if you change this function, you *MUST* run the TestDoDropPrivileges
 // test in this package as root on at least Linux, FreeBSD and Darwin. This can
 // be done by running:
 //
-//	go test -c ./ssh/tailssh/ && sudo ./tailssh.test -test.v -test.run TestDropPrivileges
+//	go test -c ./ssh/tailssh/ && sudo ./tailssh.test -test.v -test.run TestDoDropPrivileges
-func dropPrivileges(logf logger.Logf, wantUid, wantGid int, supplementaryGroups []int) error {
+func doDropPrivileges(dlogf logger.Logf, wantUid, wantGid int, supplementaryGroups []int) error {
+	dlogf("dropping privileges")
 	fatalf := func(format string, args ...any) {
-		logf("[unexpected] error dropping privileges: "+format, args...)
+		dlogf("[unexpected] error dropping privileges: "+format, args...)
 		os.Exit(1)
 	}
 
@@ -448,7 +645,11 @@ func dropPrivileges(logf logger.Logf, wantUid, wantGid int, supplementaryGroups
 //
 // It sets ss.cmd, stdin, stdout, and stderr.
 func (ss *sshSession) launchProcess() error {
-	ss.cmd = ss.newIncubatorCommand()
+	var err error
+	ss.cmd, err = ss.newIncubatorCommand(ss.logf)
+	if err != nil {
+		return err
+	}
 
 	cmd := ss.cmd
 	homeDir := ss.conn.localUser.HomeDir
@@ -749,18 +950,11 @@ func fileExists(path string) bool {
 }
 
 // loginArgs returns the arguments to use to exec the login binary.
-// It returns nil if the login binary should not be used.
+func (ia *incubatorArgs) loginArgs(loginCmdPath string) []string {
-// The login binary is only used:
-//   - on darwin, if the client is requesting a shell or a command.
-//   - on linux and BSD, if the client is requesting a shell with a TTY.
-func (ia *incubatorArgs) loginArgs() []string {
-	if ia.isSFTP {
-		return nil
-	}
 	switch runtime.GOOS {
 	case "darwin":
 		args := []string{
-			ia.loginCmdPath,
+			loginCmdPath,
 			"-f", // already authenticated
 
 			// login typically discards the previous environment, but we want to
@@ -773,39 +967,35 @@ func (ia *incubatorArgs) loginArgs() []string {
 		if !ia.hasTTY {
 			args[2] = "-pq" // -q is "quiet" which suppresses the login banner
 		}
-		if ia.cmdName != "" {
+		if ia.cmd != "" {
-			args = append(args, ia.cmdName)
+			args = append(args, ia.loginShell, "-c", ia.cmd)
-			args = append(args, ia.cmdArgs...)
 		}
+
 		return args
 	case "linux":
-		if !ia.isShell || !ia.hasTTY {
-			// We can only use login command if a shell was requested with a TTY. If
-			// there is no TTY, login exits immediately, which breaks things likes
-			// mosh and VSCode.
-			return nil
-		}
 		if distro.Get() == distro.Arch && !fileExists("/etc/pam.d/remote") {
 			// See https://github.com/tailscale/tailscale/issues/4924
 			//
 			// Arch uses a different login binary that makes the -h flag set the PAM
 			// service to "remote". So if they don't have that configured, don't
 			// pass -h.
-			return []string{ia.loginCmdPath, "-f", ia.localUser, "-p"}
+			return []string{loginCmdPath, "-f", ia.localUser, "-p"}
 		}
-		return []string{ia.loginCmdPath, "-f", ia.localUser, "-h", ia.remoteIP, "-p"}
+		return []string{loginCmdPath, "-f", ia.localUser, "-h", ia.remoteIP, "-p"}
 	case "freebsd", "openbsd":
-		if !ia.isShell || !ia.hasTTY {
+		return []string{loginCmdPath, "-fp", "-h", ia.remoteIP, ia.localUser}
-			// We can only use login command if a shell was requested with a TTY. If
-			// there is no TTY, login exits immediately, which breaks things likes
-			// mosh and VSCode.
-			return nil
-		}
-		return []string{ia.loginCmdPath, "-fp", "-h", ia.remoteIP, ia.localUser}
 	}
 	panic("unimplemented")
 }
 
+func shellArgs(isShell bool, cmd string) []string {
+	if isShell {
+		return []string{"-l"}
+	} else {
+		return []string{"-c", cmd}
+	}
+}
+
 func setGroups(groupIDs []int) error {
 	if runtime.GOOS == "darwin" && len(groupIDs) > 16 {
 		// darwin returns "invalid argument" if more than 16 groups are passed to syscall.Setgroups

ssh/tailssh/incubator_linux.go

@@ -146,11 +146,11 @@ func releaseSession(sessionID string) error {
 }
 
 // maybeStartLoginSessionLinux is the linux implementation of maybeStartLoginSession.
-func maybeStartLoginSessionLinux(logf logger.Logf, ia incubatorArgs) (func() error, error) {
+func maybeStartLoginSessionLinux(dlogf logger.Logf, ia incubatorArgs) func() error {
 	if os.Geteuid() != 0 {
-		return nil, nil
+		return nil
 	}
-	logf("starting session for user %d", ia.uid)
+	dlogf("starting session for user %d", ia.uid)
 	// The only way we can actually start a new session is if we are
 	// running outside one and are root, which is typically the case
 	// for systemd managed tailscaled.
@@ -160,14 +160,14 @@ func maybeStartLoginSessionLinux(logf logger.Logf, ia incubatorArgs) (func() err
 		// We can look at the DBus GetSessionByPID API.
 		// https://www.freedesktop.org/software/systemd/man/org.freedesktop.login1.html
 		// For now best effort is fine.
-		logf("ssh: failed to CreateSession for user %q (%d) %v", ia.localUser, ia.uid, err)
+		dlogf("ssh: failed to CreateSession for user %q (%d) %v", ia.localUser, ia.uid, err)
-		return nil, nil
+		return nil
 	}
 	os.Setenv("DBUS_SESSION_BUS_ADDRESS", fmt.Sprintf("unix:path=%v/bus", resp.runtimePath))
 	if !resp.existing {
 		return func() error {
 			return releaseSession(resp.sessionID)
-		}, nil
 		}
-	return nil, nil
+	}
+	return nil
 }

ssh/tailssh/privs_test.go

@@ -23,7 +23,7 @@
 	"tailscale.com/types/logger"
 )
 
-func TestDropPrivileges(t *testing.T) {
+func TestDoDropPrivileges(t *testing.T) {
 	type SubprocInput struct {
 		UID              int
 		GID              int
@@ -49,7 +49,7 @@ type SubprocOutput struct {
 		f := os.NewFile(3, "out.json")
 
 		// We're in our subprocess; actually drop privileges now.
-		dropPrivileges(t.Logf, input.UID, input.GID, input.AdditionalGroups)
+		doDropPrivileges(t.Logf, input.UID, input.GID, input.AdditionalGroups)
 
 		additional, _ := syscall.Getgroups()
 

ssh/tailssh/tailssh_integration_test.go

@@ -8,6 +8,7 @@
 
 import (
 	"bufio"
+	"context"
 	"crypto/ecdsa"
 	"crypto/ed25519"
 	"crypto/elliptic"
@@ -28,6 +29,7 @@
 	"testing"
 	"time"
 
+	"github.com/bramvdbogaerde/go-scp"
 	"github.com/google/go-cmp/cmp"
 	"github.com/pkg/sftp"
 	gossh "github.com/tailscale/golang-x-crypto/ssh"
@@ -36,6 +38,7 @@
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
 	"tailscale.com/types/netmap"
+	"tailscale.com/util/set"
 )
 
 // This file contains integration tests of the SSH functionality. These tests
@@ -58,7 +61,7 @@ func TestMain(m *testing.M) {
 	file.Close()
 
 	// Tail our log file.
-	cmd := exec.Command("tail", "-f", "/tmp/tailscalessh.log")
+	cmd := exec.Command("tail", "-F", "/tmp/tailscalessh.log")
 
 	r, err := cmd.StdoutPipe()
 	if err != nil {
@@ -77,6 +80,12 @@ func TestMain(m *testing.M) {
 	if err != nil {
 		return
 	}
+	defer func() {
+		// tail -f has a default sleep interval of 1 second, so it takes a
+		// moment for it to finish reading our log file after we've terminated.
+		// So, wait a bit to let it catch up.
+		time.Sleep(2 * time.Second)
+	}()
 
 	m.Run()
 }
@@ -95,18 +104,38 @@ func TestIntegrationSSH(t *testing.T) {
 	tests := []struct {
 		cmd             string
 		want            []string
+		forceV1Behavior bool
+		skip            bool
 	}{
 		{
 			cmd:             "id",
 			want:            []string{"testuser", "groupone", "grouptwo"},
+			forceV1Behavior: false,
+		},
+		{
+			cmd:             "id",
+			want:            []string{"testuser", "groupone", "grouptwo"},
+			forceV1Behavior: true,
 		},
 		{
 			cmd:             "pwd",
 			want:            []string{homeDir},
+			skip:            !fallbackToSUAvailable(),
+			forceV1Behavior: false,
+		},
+		{
+			cmd:             "echo 'hello'",
+			want:            []string{"hello"},
+			skip:            !fallbackToSUAvailable(),
+			forceV1Behavior: false,
 		},
 	}
 
 	for _, test := range tests {
+		if test.skip {
+			continue
+		}
+
 		// run every test both without and with a shell
 		for _, shell := range []bool{false, true} {
 			shellQualifier := "no_shell"
@@ -114,8 +143,13 @@ func TestIntegrationSSH(t *testing.T) {
 				shellQualifier = "shell"
 			}
 
-			t.Run(fmt.Sprintf("%s_%s", test.cmd, shellQualifier), func(t *testing.T) {
+			versionQualifier := "v2"
-				s := testSession(t)
+			if test.forceV1Behavior {
+				versionQualifier = "v1"
+			}
+
+			t.Run(fmt.Sprintf("%s_%s_%s", test.cmd, shellQualifier, versionQualifier), func(t *testing.T) {
+				s := testSession(t, test.forceV1Behavior)
 
 				if shell {
 					err := s.RequestPty("xterm", 40, 80, ssh.TerminalModes{
@@ -124,11 +158,19 @@ func TestIntegrationSSH(t *testing.T) {
 						ssh.TTY_OP_OSPEED: 14400,
 					})
 					if err != nil {
-						t.Fatalf("unable to request shell: %s", err)
+						t.Fatalf("unable to request PTY: %s", err)
-					}
 					}
 
-				got := s.run(t, test.cmd)
+					err = s.Shell()
+					if err != nil {
+						t.Fatalf("unable to request shell: %s", err)
+					}
+
+					// Read the shell prompt
+					s.read()
+				}
+
+				got := s.run(t, test.cmd, shell)
 				for _, want := range test.want {
 					if !strings.Contains(got, want) {
 						t.Errorf("%q does not contain %q", got, want)
@@ -145,10 +187,19 @@ func TestIntegrationSFTP(t *testing.T) {
 		debugTest.Store(false)
 	})
 
-	filePath := "/tmp/sftptest.dat"
+	for _, forceV1Behavior := range []bool{false, true} {
+		name := "v2"
+		if forceV1Behavior {
+			name = "v1"
+		}
+		t.Run(name, func(t *testing.T) {
+			filePath := "/home/testuser/sftptest.dat"
+			if forceV1Behavior || !fallbackToSUAvailable() {
+				filePath = "/tmp/sftptest.dat"
+			}
 			wantText := "hello world"
 
-	cl := testClient(t)
+			cl := testClient(t, forceV1Behavior)
 			scl, err := sftp.NewClient(cl)
 			if err != nil {
 				t.Fatalf("can't get sftp client: %s", err)
@@ -181,12 +232,88 @@ func TestIntegrationSFTP(t *testing.T) {
 			}
 
 			s := testSessionFor(t, cl)
-	got := s.run(t, "ls -l "+filePath)
+			got := s.run(t, "ls -l "+filePath, false)
 			if !strings.Contains(got, "testuser") {
 				t.Fatalf("unexpected file owner user: %s", got)
 			} else if !strings.Contains(got, "testuser") {
 				t.Fatalf("unexpected file owner group: %s", got)
 			}
+		})
+	}
+}
+
+func TestIntegrationSCP(t *testing.T) {
+	debugTest.Store(true)
+	t.Cleanup(func() {
+		debugTest.Store(false)
+	})
+
+	for _, forceV1Behavior := range []bool{false, true} {
+		name := "v2"
+		if forceV1Behavior {
+			name = "v1"
+		}
+		t.Run(name, func(t *testing.T) {
+			filePath := "/home/testuser/scptest.dat"
+			if !fallbackToSUAvailable() {
+				filePath = "/tmp/scptest.dat"
+			}
+			wantText := "hello world"
+
+			cl := testClient(t, forceV1Behavior)
+			scl, err := scp.NewClientBySSH(cl)
+			if err != nil {
+				t.Fatalf("can't get sftp client: %s", err)
+			}
+
+			err = scl.Copy(context.Background(), strings.NewReader(wantText), filePath, "0644", int64(len(wantText)))
+			if err != nil {
+				t.Fatalf("can't create file: %s", err)
+			}
+
+			outfile, err := os.CreateTemp("", "")
+			if err != nil {
+				t.Fatalf("can't create temp file: %s", err)
+			}
+			err = scl.CopyFromRemote(context.Background(), outfile, filePath)
+			if err != nil {
+				t.Fatalf("can't copy file from remote: %s", err)
+			}
+			outfile.Close()
+
+			gotText, err := os.ReadFile(outfile.Name())
+			if err != nil {
+				t.Fatalf("can't read file: %s", err)
+			}
+			if diff := cmp.Diff(string(gotText), wantText); diff != "" {
+				t.Fatalf("unexpected file contents (-got +want):\n%s", diff)
+			}
+
+			s := testSessionFor(t, cl)
+			got := s.run(t, "ls -l "+filePath, false)
+			if !strings.Contains(got, "testuser") {
+				t.Fatalf("unexpected file owner user: %s", got)
+			} else if !strings.Contains(got, "testuser") {
+				t.Fatalf("unexpected file owner group: %s", got)
+			}
+		})
+	}
+}
+
+func fallbackToSUAvailable() bool {
+	if runtime.GOOS != "linux" {
+		return false
+	}
+
+	_, err := exec.LookPath("su")
+	if err != nil {
+		return false
+	}
+
+	// Some operating systems like Fedora seem to require login to be present
+	// in order for su to work.
+	_, err = exec.LookPath("login")
+	return err == nil
 }
 
 type session struct {
@@ -197,14 +324,25 @@ type session struct {
 	stderr io.ReadCloser
 }
 
-func (s *session) run(t *testing.T, cmdString string) string {
+func (s *session) run(t *testing.T, cmdString string, shell bool) string {
 	t.Helper()
 
+	if shell {
+		_, err := s.stdin.Write([]byte(fmt.Sprintf("%s\n", cmdString)))
+		if err != nil {
+			t.Fatalf("unable to send command to shell: %s", err)
+		}
+	} else {
 		err := s.Start(cmdString)
 		if err != nil {
 			t.Fatalf("unable to start command: %s", err)
 		}
+	}
 
+	return s.read()
+}
+
+func (s *session) read() string {
 	ch := make(chan []byte)
 	go func() {
 		for {
@@ -228,7 +366,7 @@ func (s *session) run(t *testing.T, cmdString string) string {
 		select {
 		case b := <-ch:
 			_got = append(_got, b...)
-		case <-time.After(25 * time.Millisecond):
+		case <-time.After(1 * time.Second):
 			break readLoop
 		}
 	}
@@ -236,12 +374,12 @@ func (s *session) run(t *testing.T, cmdString string) string {
 	return string(_got)
 }
 
-func testClient(t *testing.T) *ssh.Client {
+func testClient(t *testing.T, forceV1Behavior bool) *ssh.Client {
 	t.Helper()
 
 	username := "testuser"
 	srv := &server{
-		lb:             &testBackend{localUser: username},
+		lb:             &testBackend{localUser: username, forceV1Behavior: forceV1Behavior},
 		logf:           log.Printf,
 		tailscaledPath: os.Getenv("TAILSCALED_PATH"),
 		timeNow:        time.Now,
@@ -271,8 +409,8 @@ func testClient(t *testing.T) *ssh.Client {
 	return cl
 }
 
-func testSession(t *testing.T) *session {
+func testSession(t *testing.T, forceV1Behavior bool) *session {
-	cl := testClient(t)
+	cl := testClient(t, forceV1Behavior)
 	return testSessionFor(t, cl)
 }
 
@@ -300,6 +438,7 @@ func testSessionFor(t *testing.T, cl *ssh.Client) *session {
 // testBackend implements ipnLocalBackend
 type testBackend struct {
 	localUser       string
+	forceV1Behavior bool
 }
 
 func (tb *testBackend) GetSSH_HostKeys() ([]gossh.Signer, error) {
@@ -339,16 +478,21 @@ func (tb *testBackend) ShouldRunSSH() bool {
 }
 
 func (tb *testBackend) NetMap() *netmap.NetworkMap {
+	capMap := make(set.Set[tailcfg.NodeCapability])
+	if tb.forceV1Behavior {
+		capMap[tailcfg.NodeAttrSSHBehaviorV1] = struct{}{}
+	}
 	return &netmap.NetworkMap{
 		SSHPolicy: &tailcfg.SSHPolicy{
 			Rules: []*tailcfg.SSHRule{
-				&tailcfg.SSHRule{
+				{
 					Principals: []*tailcfg.SSHPrincipal{{Any: true}},
 					Action:     &tailcfg.SSHAction{Accept: true},
 					SSHUsers:   map[string]string{"*": tb.localUser},
 				},
 			},
 		},
+		AllCaps: capMap,
 	}
 }
 

ssh/tailssh/testcontainers/Dockerfile

@@ -1,18 +1,51 @@
 ARG BASE
 FROM ${BASE}
 
+RUN echo "Install openssh, needed for scp."
+RUN apt-get update -y && apt-get install -y openssh-client
+
 RUN groupadd -g 10000 groupone
 RUN groupadd -g 10001 grouptwo
-RUN useradd -g 10000 -G 10001 -u 10002 -m testuser
+# Note - we do not create the user's home directory, pam_mkhomedir will do that
-COPY . .
+# for us, and we want to test that PAM gets triggered by Tailscale SSH.
+RUN useradd -g 10000 -G 10001 -u 10002 testuser
 
-# First run tests normally.
+RUN echo "Set up pam_mkhomedir."
-RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.run TestIntegration
+RUN sed -i -e 's/Default: no/Default: yes/g' /usr/share/pam-configs/mkhomedir || echo "might not be ubuntu"
+RUN cat /usr/share/pam-configs/mkhomedir
+RUN pam-auth-update --enable mkhomedir
 
-# Then remove the login command and make sure tests still pass.
+COPY tailscaled .
-RUN rm `which login`
+COPY tailssh.test .
-RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.run TestIntegration
 
-# Then run tests as non-root user testuser.
+RUN chmod 755 tailscaled
+
+RUN echo "First run tests normally."
+RUN rm -Rf /home/testuser
+RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSFTP
+RUN rm -Rf /home/testuser
+RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSCP
+RUN rm -Rf /home/testuser
+RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSSH
+
+RUN echo "Then run tests as non-root user testuser and make sure tests still pass."
 RUN chown testuser:groupone /tmp/tailscalessh.log
-RUN TAILSCALED_PATH=`pwd`tailscaled su -m testuser -c "./tailssh.test -test.run TestIntegration"
+RUN TAILSCALED_PATH=`pwd`tailscaled su -m testuser -c "./tailssh.test -test.v -test.run TestIntegration TestDoDropPrivileges"
+
+RUN echo "Then remove the login command and make sure tests still pass."
+RUN chown root:root /tmp/tailscalessh.log
+RUN rm `which login`
+RUN rm -Rf /home/testuser
+RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSFTP
+RUN rm -Rf /home/testuser
+RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSCP
+RUN rm -Rf /home/testuser
+RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSSH
+
+RUN echo "Then remove the su command and make sure tests still pass."
+RUN chown root:root /tmp/tailscalessh.log
+RUN rm `which su`
+RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegration
+
+RUN echo "Test doDropPrivileges"
+RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestDoDropPrivileges

tailcfg/tailcfg.go

@@ -136,7 +136,8 @@
 //   - 93: 2024-05-06: added support for stateful firewalling.
 //   - 94: 2024-05-06: Client understands Node.IsJailed.
 //   - 95: 2024-05-06: Client uses NodeAttrUserDialUseRoutes to change DNS dialing behavior.
-const CurrentCapabilityVersion CapabilityVersion = 95
+//   - 96: 2024-05-29: Client understands NodeAttrSSHBehaviorV1
+const CurrentCapabilityVersion CapabilityVersion = 96
 
 type StableID string
 
@@ -2274,6 +2275,10 @@ type Oauth2Token struct {
 	// depending on the destination address and the configured routes. When present, it also makes
 	// the DNS forwarder use UserDial instead of SystemDial when dialing resolvers.
 	NodeAttrUserDialUseRoutes NodeCapability = "user-dial-routes"
+
+	// NodeAttrSSHBehaviorV1 forces SSH to use the V1 behavior (no su, run SFTP in-process)
+	// Added 2024-05-29 in Tailscale version 1.68.
+	NodeAttrSSHBehaviorV1 NodeCapability = "ssh-behavior-v1"
 )
 
 // SetDNSRequest is a request to add a DNS record.