ssh/tailssh: support placeholders in SSHAction.HoldAndDelegate URL

Updates #3802

Change-Id: I60f9827409d14fd4f4824d102ba11db49bf0d365
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/tailssh.go

@@ -214,48 +214,76 @@ func (srv *server) handleSSH(s ssh.Session) {
 		return
 	}
 
+	lu, err := user.Lookup(localUser)
+	if err != nil {
+		logf("ssh: user Lookup %q: %v", localUser, err)
+		s.Exit(1)
+		return
+	}
+
+	ss := srv.newSSHSession(s, ci, lu)
+	action, err = ss.resolveTerminalAction(action)
+	if err != nil {
+		logf("ssh: resolveTerminalAction: %v", err)
+		io.WriteString(s.Stderr(), "Access denied: failed to resolve SSHAction.\n")
+		s.Exit(1)
+		return
+	}
+	if action.Reject || !action.Accept {
+		logf("ssh: access denied for %q from %v", ci.uprof.LoginName, ci.src.IP())
+		s.Exit(1)
+		return
+	}
+
+	ss.action = action
+	ss.run()
+}
+
+// resolveTerminalAction either returns action (if it's Accept or Reject) or else
+// loops, fetching new SSHActions from the control plane.
+//
+// Any action with a Message in the chain will be printed to ss.
+//
+// The returned SSHAction will be either Reject or Accept.
+func (ss *sshSession) resolveTerminalAction(action *tailcfg.SSHAction) (*tailcfg.SSHAction, error) {
 	// Loop processing/fetching Actions until one reaches a
 	// terminal state (Accept, Reject, or invalid Action), or
 	// until fetchSSHAction times out due to the context being
 	// done (client disconnect) or its 30 minute timeout passes.
 	// (Which is a long time for somebody to see login
 	// instructions and go to a URL to do something.)
-ProcessAction:
 	for {
 		if action.Message != "" {
-			io.WriteString(s.Stderr(), strings.Replace(action.Message, "\n", "\r\n", -1))
+			io.WriteString(ss.Stderr(), strings.Replace(action.Message, "\n", "\r\n", -1))
 		}
-		if action.Reject {
-			logf("ssh: access denied for %q from %v", ci.uprof.LoginName, ci.src.IP())
-			s.Exit(1)
-			return
-		}
-		if action.Accept {
-			break ProcessAction
+		if action.Accept || action.Reject {
+			return action, nil
 		}
 		url := action.HoldAndDelegate
 		if url == "" {
-			logf("ssh: access denied; SSHAction has neither Reject, Accept, or next step URL")
-			s.Exit(1)
-			return
+			return nil, errors.New("reached Action that lacked Accept, Reject, and HoldAndDelegate")
 		}
-		action, err = srv.fetchSSHAction(s.Context(), url)
+		url = ss.expandDelegateURL(url)
+		var err error
+		action, err = ss.srv.fetchSSHAction(ss.Context(), url)
 		if err != nil {
-			logf("ssh: fetching SSAction from %s: %v", url, err)
-			s.Exit(1)
-			return
+			return nil, fmt.Errorf("fetching SSHAction from %s: %w", url, err)
 		}
 	}
-
-	lu, err := user.Lookup(localUser)
-	if err != nil {
-		logf("ssh: user Lookup %q: %v", localUser, err)
-		s.Exit(1)
-		return
 }
 
-	ss := srv.newSSHSession(s, ci, lu, action)
-	ss.run()
+func (ss *sshSession) expandDelegateURL(url string) string {
+	nm := ss.srv.lb.NetMap()
+	var dstNodeID string
+	if nm != nil {
+		dstNodeID = fmt.Sprint(int64(nm.SelfNode.ID))
+	}
+	return strings.NewReplacer(
+		"$SRC_NODE_ID", fmt.Sprint(int64(ss.connInfo.node.ID)),
+		"$DST_NODE_ID", dstNodeID,
+		"$SSH_USER", ss.connInfo.sshUser,
+		"$LOCAL_USER", ss.localUser.Username,
+	).Replace(url)
 }
 
 // sshSession is an accepted Tailscale SSH session.
@@ -284,7 +312,7 @@ type sshSession struct {
 	exitOnce sync.Once
 }
 
-func (srv *server) newSSHSession(s ssh.Session, ci *sshConnInfo, lu *user.User, action *tailcfg.SSHAction) *sshSession {
+func (srv *server) newSSHSession(s ssh.Session, ci *sshConnInfo, lu *user.User) *sshSession {
 	sharedID := fmt.Sprintf("%s-%02x", ci.now.UTC().Format("20060102T150405"), randBytes(5))
 	return &sshSession{
 		Session:   s,
@@ -292,7 +320,6 @@ func (srv *server) newSSHSession(s ssh.Session, ci *sshConnInfo, lu *user.User,
 		sharedID:  sharedID,
 		ctx:       newSSHContext(),
 		srv:       srv,
-		action:    action,
 		localUser: lu,
 		connInfo:  ci,
 		logf:      logger.WithPrefix(srv.logf, "ssh-session("+sharedID+"): "),
@@ -317,12 +344,20 @@ func (srv *server) fetchSSHAction(ctx context.Context, url string) (*tailcfg.SSH
 			continue
 		}
 		if res.StatusCode != 200 {
+			body, _ := io.ReadAll(res.Body)
 			res.Body.Close()
+			if len(body) > 1<<10 {
+				body = body[:1<<10]
+			}
+			srv.logf("fetch of %v: %s, %s", url, res.Status, body)
 			bo.BackOff(ctx, fmt.Errorf("unexpected status: %v", res.Status))
 			continue
 		}
 		a := new(tailcfg.SSHAction)
-		if err := json.NewDecoder(res.Body).Decode(a); err != nil {
+		err = json.NewDecoder(res.Body).Decode(a)
+		res.Body.Close()
+		if err != nil {
+			srv.logf("invalid next SSHAction JSON from %v: %v", url, err)
 			bo.BackOff(ctx, err)
 			continue
 		}

ssh/tailssh/tailssh_test.go

@@ -224,7 +224,8 @@ func TestSSH(t *testing.T) {
 	}
 
 	ss.Handler = func(s ssh.Session) {
-		ss := srv.newSSHSession(s, ci, u, &tailcfg.SSHAction{Accept: true})
+		ss := srv.newSSHSession(s, ci, u)
+		ss.action = &tailcfg.SSHAction{Accept: true}
 		ss.run()
 	}