ssh/tailssh: add support for remote/reverse port forwarding

This basically allows running services on the SSH client and reaching
them from the SSH server during the session.

Updates #6575

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ssh/tailssh/tailssh.go

@@ -422,6 +422,7 @@ 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{}
 	c.Server = &ssh.Server{
 		Version:              "Tailscale",
 		ServerConfigCallback: c.ServerConfig,
@@ -432,6 +433,7 @@ func (srv *server) newConn() (*conn, error) {
 
 		Handler:                       c.handleSessionPostSSHAuth,
 		LocalPortForwardingCallback:   c.mayForwardLocalPortTo,
+		ReversePortForwardingCallback: c.mayReversePortForwardTo,
 		SubsystemHandlers: map[string]ssh.SubsystemHandler{
 			"sftp": c.handleSessionPostSSHAuth,
 		},
@@ -441,7 +443,10 @@ func (srv *server) newConn() (*conn, error) {
 		ChannelHandlers: map[string]ssh.ChannelHandler{
 			"direct-tcpip": ssh.DirectTCPIPHandler,
 		},
-		RequestHandlers: map[string]ssh.RequestHandler{},
+		RequestHandlers: map[string]ssh.RequestHandler{
+			"tcpip-forward":        fwdHandler.HandleSSHRequest,
+			"cancel-tcpip-forward": fwdHandler.HandleSSHRequest,
+		},
 	}
 	ss := c.Server
 	for k, v := range ssh.DefaultRequestHandlers {
@@ -463,6 +468,17 @@ func (srv *server) newConn() (*conn, error) {
 	return c, nil
 }
 
+// mayReversePortPortForwardTo reports whether the ctx should be allowed to port forward
+// to the specified host and port.
+// TODO(bradfitz/maisem): should we have more checks on host/port?
+func (c *conn) mayReversePortForwardTo(ctx ssh.Context, destinationHost string, destinationPort uint32) bool {
+	if c.finalAction != nil && c.finalAction.AllowRemotePortForwarding {
+		metricRemotePortForward.Add(1)
+		return true
+	}
+	return false
+}
+
 // mayForwardLocalPortTo reports whether the ctx should be allowed to port forward
 // to the specified host and port.
 // TODO(bradfitz/maisem): should we have more checks on host/port?
@@ -1860,6 +1876,7 @@ var (
 	metricPolicyChangeKick     = clientmetric.NewCounter("ssh_policy_change_kick")
 	metricSFTP                 = clientmetric.NewCounter("ssh_sftp_requests")
 	metricLocalPortForward     = clientmetric.NewCounter("ssh_local_port_forward_requests")
+	metricRemotePortForward    = clientmetric.NewCounter("ssh_remote_port_forward_requests")
 )
 
 // userVisibleError is a wrapper around an error that implements

tailcfg/tailcfg.go

@@ -99,7 +99,8 @@ type CapabilityVersion int
 //   - 60: 2023-04-06: Client understands IsWireGuardOnly
 //   - 61: 2023-04-18: Client understand SSHAction.SSHRecorderFailureAction
 //   - 62: 2023-05-05: Client can notify control over noise for SSHEventNotificationRequest recording failure events
-const CurrentCapabilityVersion CapabilityVersion = 62
+//   - 63: 2023-06-08: Client understands SSHAction.AllowRemotePortForwarding.
+const CurrentCapabilityVersion CapabilityVersion = 63
 
 type StableID string
 
@@ -2048,6 +2049,10 @@ type SSHAction struct {
 	// to use local port forwarding if requested.
 	AllowLocalPortForwarding bool `json:"allowLocalPortForwarding,omitempty"`
 
+	// AllowRemotePortForwarding, if true, allows accepted connections
+	// to use remote port forwarding if requested.
+	AllowRemotePortForwarding bool `json:"allowRemotePortForwarding,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"`

tailcfg/tailcfg_clone.go

@@ -415,6 +415,7 @@ var _SSHActionCloneNeedsRegeneration = SSHAction(struct {
 	AllowAgentForwarding      bool
 	HoldAndDelegate           string
 	AllowLocalPortForwarding  bool
+	AllowRemotePortForwarding bool
 	Recorders                 []netip.AddrPort
 	OnRecordingFailure        *SSHRecorderFailureAction
 }{})

tailcfg/tailcfg_view.go

@@ -940,6 +940,7 @@ func (v SSHActionView) SessionDuration() time.Duration         { return v.ж.Ses
 func (v SSHActionView) AllowAgentForwarding() bool             { return v.ж.AllowAgentForwarding }
 func (v SSHActionView) HoldAndDelegate() string                { return v.ж.HoldAndDelegate }
 func (v SSHActionView) AllowLocalPortForwarding() bool         { return v.ж.AllowLocalPortForwarding }
+func (v SSHActionView) AllowRemotePortForwarding() bool        { return v.ж.AllowRemotePortForwarding }
 func (v SSHActionView) Recorders() views.Slice[netip.AddrPort] { return views.SliceOf(v.ж.Recorders) }
 func (v SSHActionView) OnRecordingFailure() *SSHRecorderFailureAction {
 	if v.ж.OnRecordingFailure == nil {
@@ -958,6 +959,7 @@ var _SSHActionViewNeedsRegeneration = SSHAction(struct {
 	AllowAgentForwarding      bool
 	HoldAndDelegate           string
 	AllowLocalPortForwarding  bool
+	AllowRemotePortForwarding bool
 	Recorders                 []netip.AddrPort
 	OnRecordingFailure        *SSHRecorderFailureAction
 }{})