implement permissions

cmd/root.go

@@ -21,6 +21,21 @@ var flag struct {
 	sshUnixSocket string
 	sshShell      string
 	sshUsers      []string
+
+	allowTcpipForward bool
+	allowDirectTcpip  bool
+	allowExecute      bool
+	allowSftp         bool
+}
+
+var allPermissionFlags = []struct {
+	name    string
+	flagPtr *bool
+}{
+	{name: "tcpip-forward", flagPtr: &flag.allowTcpipForward},
+	{name: "direct-tcpip", flagPtr: &flag.allowDirectTcpip},
+	{name: "execute", flagPtr: &flag.allowExecute},
+	{name: "sftp", flagPtr: &flag.allowSftp},
 }
 
 type sshUser struct {
@@ -38,6 +53,12 @@ func init() {
 	RootCmd.PersistentFlags().StringVarP(&flag.sshShell, "shell", "", "", "Shell")
 	//RootCmd.PersistentFlags().StringVar(&flag.dnsServer, "dns-server", "", "DNS server (e.g. 1.1.1.1:53)")
 	RootCmd.PersistentFlags().StringArrayVarP(&flag.sshUsers, "user", "", nil, `SSH user name (e.g. "john:mypassword")`)
+
+	// Permission flags
+	RootCmd.PersistentFlags().BoolVarP(&flag.allowTcpipForward, "allow-tcpip-forward", "", false, "client can use remote forwarding")
+	RootCmd.PersistentFlags().BoolVarP(&flag.allowDirectTcpip, "allow-direct-tcpip", "", false, "client can use local forwarding and SOCKS proxy")
+	RootCmd.PersistentFlags().BoolVarP(&flag.allowExecute, "allow-execute", "", false, "client can use shell/interactive shell")
+	RootCmd.PersistentFlags().BoolVarP(&flag.allowSftp, "allow-sftp", "", false, "client can use SFTP and SSHFS")
 }
 
 var RootCmd = &cobra.Command{
@@ -51,8 +72,26 @@ var RootCmd = &cobra.Command{
 			return nil
 		}
 		logger := slog.Default()
+
+		// Allow all permissions if all permission is not set
+		{
+			allPermissionFalse := true
+			for _, permissionFlag := range allPermissionFlags {
+				allPermissionFalse = allPermissionFalse && !*permissionFlag.flagPtr
+			}
+			if allPermissionFalse {
+				for _, permissionFlag := range allPermissionFlags {
+					*permissionFlag.flagPtr = true
+				}
+			}
+		}
+
 		sshServer := &handy_sshd.Server{
 			Logger:            logger,
+			AllowTcpipForward: flag.allowTcpipForward,
+			AllowDirectTcpip:  flag.allowDirectTcpip,
+			AllowExecute:      flag.allowExecute,
+			AllowSftp:         flag.allowSftp,
 		}
 		var sshUsers []sshUser
 		for _, u := range flag.sshUsers {
@@ -109,6 +148,9 @@ var RootCmd = &cobra.Command{
 			logger.Info(fmt.Sprintf("listening on %s...", flag.sshUnixSocket))
 		}
 		defer ln.Close()
+
+		showPermissions(logger)
+
 		for {
 			conn, err := ln.Accept()
 			if err != nil {
@@ -121,9 +163,29 @@ var RootCmd = &cobra.Command{
 				conn.Close()
 				continue
 			}
-			logger.Info("new SSH connection", "client_version", string(sshConn.ClientVersion()))
+			logger.Info("new SSH connection", "remote_address", sshConn.RemoteAddr(), "client_version", string(sshConn.ClientVersion()))
 			go sshServer.HandleGlobalRequests(sshConn, reqs)
 			go sshServer.HandleChannels(flag.sshShell, chans)
 		}
 	},
 }
+
+func showPermissions(logger *slog.Logger) {
+	var allowedList []string
+	var notAllowedList []string
+	for _, permissionFlag := range allPermissionFlags {
+		if *permissionFlag.flagPtr {
+			allowedList = append(allowedList, `"`+permissionFlag.name+`"`)
+		} else {
+			notAllowedList = append(notAllowedList, `"`+permissionFlag.name+`"`)
+		}
+	}
+	showList := func(l []string) string {
+		if len(l) == 0 {
+			return "none"
+		}
+		return strings.Join(l, ", ")
+	}
+	logger.Info(fmt.Sprintf("allowed: %s", showList(allowedList)))
+	logger.Info(fmt.Sprintf("NOT allowed: %s", showList(notAllowedList)))
+}

server.go

@@ -28,6 +28,13 @@ import (
 
 type Server struct {
 	Logger *slog.Logger
+
+	// Permissions
+	AllowTcpipForward bool
+	AllowDirectTcpip  bool
+	AllowExecute      bool // this should not be split into "allow-exec" and "allow-pty-req" for now because "pty-req" can be used not for shell execution.
+	AllowSftp         bool
+
 	// TODO: DNS server ?
 }
 
@@ -47,6 +54,10 @@ func (s *Server) handleChannel(shell string, newChannel ssh.NewChannel) {
 	case "session":
 		s.handleSession(shell, newChannel)
 	case "direct-tcpip":
+		if !s.AllowDirectTcpip {
+			newChannel.Reject(ssh.Prohibited, "direct-tcpip not allowed")
+			break
+		}
 		s.handleDirectTcpip(newChannel)
 	default:
 		newChannel.Reject(ssh.UnknownChannelType, fmt.Sprintf("unknown channel type: %s", newChannel.ChannelType()))
@@ -67,6 +78,11 @@ func (s *Server) handleSession(shell string, newChannel ssh.NewChannel) {
 	for req := range requests {
 		switch req.Type {
 		case "exec":
+			if !s.AllowExecute {
+				s.Logger.Info("execution not allowed (exec)")
+				req.Reply(false, nil)
+				break
+			}
 			s.handleExecRequest(req, connection)
 		case "shell":
 			// We only accept the default shell
@@ -75,6 +91,11 @@ func (s *Server) handleSession(shell string, newChannel ssh.NewChannel) {
 				req.Reply(true, nil)
 			}
 		case "pty-req":
+			if !s.AllowExecute {
+				s.Logger.Info("execution not allowed (pty-req)")
+				req.Reply(false, nil)
+				break
+			}
 			termLen := req.Payload[3]
 			w, h := parseDims(req.Payload[termLen+4:])
 			shf, err = s.createPty(shell, connection)
@@ -140,9 +161,17 @@ func (s *Server) handleExecRequest(req *ssh.Request, connection ssh.Channel) {
 
 func (s *Server) handleSessionSubSystem(req *ssh.Request, connection ssh.Channel) {
 	// https://github.com/pkg/sftp/blob/42e9800606febe03f9cdf1d1283719af4a5e6456/examples/go-sftp-server/main.go#L111
-	ok := string(req.Payload[4:]) == "sftp"
+	if string(req.Payload[4:]) != "sftp" {
-	req.Reply(ok, nil)
+		req.Reply(false, nil)
+		return
+	}
+	if !s.AllowSftp {
+		s.Logger.Info("sftp not allowed")
+		req.Reply(false, nil)
+		return
+	}
 
+	req.Reply(true, nil)
 	serverOptions := []sftp.ServerOption{
 		sftp.WithDebug(os.Stderr),
 	}
@@ -232,6 +261,11 @@ func (s *Server) HandleGlobalRequests(sshConn *ssh.ServerConn, reqs <-chan *ssh.
 	for req := range reqs {
 		switch req.Type {
 		case "tcpip-forward":
+			if !s.AllowTcpipForward {
+				s.Logger.Info("tcpip-forward not allowed")
+				req.Reply(false, nil)
+				break
+			}
 			s.handleTcpipForward(sshConn, req)
 		default:
 			// discard