ssh/tailssh: add support for unix sockets

Updates tailscale#6232
Signed-off-by: Samuel Corsi-House <chouse.samuel@gmail.com>

ssh/tailssh/tailssh.go

@@ -440,7 +440,8 @@ func (srv *server) newConn() (*conn, error) {
 	c := &conn{srv: srv}
 	now := srv.now()
 	c.connID = fmt.Sprintf("ssh-conn-%s-%02x", now.UTC().Format("20060102T150405"), randBytes(5))
-	fwdHandler := &ssh.ForwardedTCPHandler{}
+	fwdHandlerTCP := &ssh.ForwardedTCPHandler{}
+	fwdHandlerUnix := &ssh.ForwardedUnixHandler{}
 	c.Server = &ssh.Server{
 		Version:              "Tailscale",
 		ServerConfigCallback: c.ServerConfig,
@@ -452,18 +453,23 @@ func (srv *server) newConn() (*conn, error) {
 		Handler:                       c.handleSessionPostSSHAuth,
 		LocalPortForwardingCallback:   c.mayForwardLocalPortTo,
 		ReversePortForwardingCallback: c.mayReversePortForwardTo,
+		LocalUnixForwardingCallback:   c.mayForwardLocalUnixTo,
+		ReverseUnixForwardingCallback: c.mayReverseUnixForwardTo,
 		SubsystemHandlers: map[string]ssh.SubsystemHandler{
 			"sftp": c.handleSessionPostSSHAuth,
 		},
 		// Note: the direct-tcpip channel handler and LocalPortForwardingCallback
 		// only adds support for forwarding ports from the local machine.
 		// TODO(maisem/bradfitz): add remote port forwarding support.
 		ChannelHandlers: map[string]ssh.ChannelHandler{
-			"direct-tcpip": ssh.DirectTCPIPHandler,
+			"direct-tcpip":                   ssh.DirectTCPIPHandler,
+			"direct-streamlocal@openssh.com": ssh.DirectStreamLocalHandler,
 		},
 		RequestHandlers: map[string]ssh.RequestHandler{
-			"tcpip-forward":        fwdHandler.HandleSSHRequest,
-			"cancel-tcpip-forward": fwdHandler.HandleSSHRequest,
+			"tcpip-forward":                          fwdHandlerTCP.HandleSSHRequest,
+			"cancel-tcpip-forward":                   fwdHandlerTCP.HandleSSHRequest,
+			"streamlocal-forward@openssh.com":        fwdHandlerUnix.HandleSSHRequest,
+			"cancel-streamlocal-forward@openssh.com": fwdHandlerUnix.HandleSSHRequest,
 		},
 	}
 	ss := c.Server
@@ -514,6 +520,34 @@ func (c *conn) mayForwardLocalPortTo(ctx ssh.Context, destinationHost string, de
 	return false
 }
 
+// mayReverseUnixForwardTo reports whether the ctx should be allowed to unix forward
+// to the specified host.
+func (c *conn) mayReverseUnixForwardTo(ctx ssh.Context, socketPath string) bool {
+	if sshDisableForwarding() {
+		return false
+	}
+	if c.finalAction != nil && c.finalAction.AllowRemoteUnixForwarding {
+		metricRemoteUnixForward.Add(1)
+		return true
+	}
+	// TODO(Xenfo): undo
+	return true
+}
+
+// mayForwardLocalUnixTo reports whether the ctx should be allowed to unix forward
+// to the specified host.
+func (c *conn) mayForwardLocalUnixTo(ctx ssh.Context, socketPath string) bool {
+	if sshDisableForwarding() {
+		return false
+	}
+	if c.finalAction != nil && c.finalAction.AllowLocalUnixForwarding {
+		metricLocalUnixForward.Add(1)
+		return true
+	}
+	// TODO(Xenfo): undo
+	return true
+}
+
 // havePubKeyPolicy reports whether any policy rule may provide access by means
 // of a ssh.PublicKey.
 func (c *conn) havePubKeyPolicy() bool {
@@ -1928,6 +1962,8 @@ var (
 	metricSFTP                = clientmetric.NewCounter("ssh_sftp_sessions")
 	metricLocalPortForward    = clientmetric.NewCounter("ssh_local_port_forward_requests")
 	metricRemotePortForward   = clientmetric.NewCounter("ssh_remote_port_forward_requests")
+	metricLocalUnixForward    = clientmetric.NewCounter("ssh_local_unix_forward_requests")
+	metricRemoteUnixForward   = clientmetric.NewCounter("ssh_remote_unix_forward_requests")
 )
 
 // userVisibleError is a wrapper around an error that implements

tailcfg/tailcfg.go

@@ -136,7 +136,8 @@ type CapabilityVersion int
 //   - 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: 2023-06-08: Client understands SSHAction.AllowLocalUnixForwarding and SSHAction.AllowRemoteUnixForwarding.
+const CurrentCapabilityVersion CapabilityVersion = 96
 
 type StableID string
 
@@ -2456,6 +2457,14 @@ type SSHAction struct {
 	// to use remote port forwarding if requested.
 	AllowRemotePortForwarding bool `json:"allowRemotePortForwarding,omitempty"`
 
+	// AllowLocalUnixForwarding, if true, allows accepted connections
+	// to use local unix forwarding if requested.
+	AllowLocalUnixForwarding bool `json:"allowLocalUnixForwarding,omitempty"`
+
+	// AllowRemoteUnixForwarding, if true, allows accepted connections
+	// to use remote unix forwarding if requested.
+	AllowRemoteUnixForwarding bool `json:"allowRemoteUnixForwarding,omitempty"`
+
 	// Recorders defines the destinations of the SSH session recorders.
 	// The recording will be uploaded to http://addr:port/record.
 	Recorders []netip.AddrPort `json:"recorders,omitempty"`

tempfork/gliderlabs/ssh/options_test.go

@@ -51,7 +51,7 @@ func TestPasswordAuth(t *testing.T) {
 
 func TestPasswordAuthBadPass(t *testing.T) {
 	t.Parallel()
-	l := newLocalListener()
+	l := newLocalTCPListener()
 	srv := &Server{Handler: func(s Session) {}}
 	srv.SetOption(PasswordAuth(func(ctx Context, password string) bool {
 		return false

tempfork/gliderlabs/ssh/server.go

@@ -45,7 +45,9 @@ type Server struct {
 	PtyCallback                   PtyCallback                   // callback for allowing PTY sessions, allows all if nil
 	ConnCallback                  ConnCallback                  // optional callback for wrapping net.Conn before handling
 	LocalPortForwardingCallback   LocalPortForwardingCallback   // callback for allowing local port forwarding, denies all if nil
+	LocalUnixForwardingCallback   LocalUnixForwardingCallback   // callback for allowing local unix forwarding (direct-streamlocal@openssh.com), denies all if nil
 	ReversePortForwardingCallback ReversePortForwardingCallback // callback for allowing reverse port forwarding, denies all if nil
+	ReverseUnixForwardingCallback ReverseUnixForwardingCallback // callback for allowing reverse unix forwarding (streamlocal-forward@openssh.com), denies all if nil
 	ServerConfigCallback          ServerConfigCallback          // callback for configuring detailed SSH options
 	SessionRequestCallback        SessionRequestCallback        // callback for allowing or denying SSH sessions
 

tempfork/gliderlabs/ssh/server_test.go

@@ -31,7 +31,7 @@ func TestAddHostKey(t *testing.T) {
 }
 
 func TestServerShutdown(t *testing.T) {
-	l := newLocalListener()
+	l := newLocalTCPListener()
 	testBytes := []byte("Hello world\n")
 	s := &Server{
 		Handler: func(s Session) {
@@ -82,7 +82,7 @@ func TestServerShutdown(t *testing.T) {
 }
 
 func TestServerClose(t *testing.T) {
-	l := newLocalListener()
+	l := newLocalTCPListener()
 	s := &Server{
 		Handler: func(s Session) {
 			time.Sleep(5 * time.Second)

tempfork/gliderlabs/ssh/session_test.go

@@ -22,14 +22,25 @@ func (srv *Server) serveOnce(l net.Listener) error {
 		return e
 	}
 	srv.ChannelHandlers = map[string]ChannelHandler{
-		"session":      DefaultSessionHandler,
-		"direct-tcpip": DirectTCPIPHandler,
+		"session":                        DefaultSessionHandler,
+		"direct-tcpip":                   DirectTCPIPHandler,
+		"direct-streamlocal@openssh.com": DirectStreamLocalHandler,
 	}
+
+	forwardedTCPHandler := &ForwardedTCPHandler{}
+	forwardedUnixHandler := &ForwardedUnixHandler{}
+	srv.RequestHandlers = map[string]RequestHandler{
+		"tcpip-forward":                          forwardedTCPHandler.HandleSSHRequest,
+		"cancel-tcpip-forward":                   forwardedTCPHandler.HandleSSHRequest,
+		"streamlocal-forward@openssh.com":        forwardedUnixHandler.HandleSSHRequest,
+		"cancel-streamlocal-forward@openssh.com": forwardedUnixHandler.HandleSSHRequest,
+	}
+
 	srv.HandleConn(conn)
 	return nil
 }
 
-func newLocalListener() net.Listener {
+func newLocalTCPListener() net.Listener {
 	l, err := net.Listen("tcp", "127.0.0.1:0")
 	if err != nil {
 		if l, err = net.Listen("tcp6", "[::1]:0"); err != nil {
@@ -66,7 +77,7 @@ func newClientSession(t *testing.T, addr string, config *gossh.ClientConfig) (*g
 }
 
 func newTestSession(t *testing.T, srv *Server, cfg *gossh.ClientConfig) (*gossh.Session, *gossh.Client, func()) {
-	l := newLocalListener()
+	l := newLocalTCPListener()
 	go srv.serveOnce(l)
 	return newClientSession(t, l.Addr().String(), cfg)
 }

tempfork/gliderlabs/ssh/ssh.go

@@ -62,9 +62,17 @@ type ConnCallback func(ctx Context, conn net.Conn) net.Conn
 // LocalPortForwardingCallback is a hook for allowing port forwarding
 type LocalPortForwardingCallback func(ctx Context, destinationHost string, destinationPort uint32) bool
 
+// LocalUnixForwardingCallback is a hook for allowing unix forwarding
+// (direct-streamlocal@openssh.com)
+type LocalUnixForwardingCallback func(ctx Context, socketPath string) bool
+
 // ReversePortForwardingCallback is a hook for allowing reverse port forwarding
 type ReversePortForwardingCallback func(ctx Context, bindHost string, bindPort uint32) bool
 
+// ReverseUnixForwardingCallback is a hook for allowing reverse unix forwarding
+// (streamlocal-forward@openssh.com).
+type ReverseUnixForwardingCallback func(ctx Context, socketPath string) bool
+
 // ServerConfigCallback is a hook for creating custom default server configs
 type ServerConfigCallback func(ctx Context) *gossh.ServerConfig