diff --git a/tstest/mts/mts.go b/tstest/mts/mts.go
new file mode 100644
index 000000000..c10d69d8d
--- /dev/null
+++ b/tstest/mts/mts.go
@@ -0,0 +1,599 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build linux || darwin
+
+// The mts ("Multiple Tailscale") command runs multiple tailscaled instances for
+// development, managing their directories and sockets, and lets you easily direct
+// tailscale CLI commands to them.
+package main
+
+import (
+	"bufio"
+	"context"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"maps"
+	"net"
+	"net/http"
+	"net/url"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"slices"
+	"strings"
+	"sync"
+	"syscall"
+	"time"
+
+	"tailscale.com/client/local"
+	"tailscale.com/types/bools"
+	"tailscale.com/types/lazy"
+	"tailscale.com/util/mak"
+)
+
+func usage(args ...any) {
+	var format string
+	if len(args) > 0 {
+		format, args = args[0].(string), args[1:]
+	}
+	if format != "" {
+		format = strings.TrimSpace(format) + "\n\n"
+		fmt.Fprintf(os.Stderr, format, args...)
+	}
+	io.WriteString(os.Stderr, strings.TrimSpace(`
+usage:
+
+   mts server <subcommand>      # manage tailscaled instances
+   mts server run               # run the mts server (parent process of all tailscaled)
+   mts server list              # list all tailscaled and their state
+   mts server list <name>       # show details of named instance
+   mts server add <name>        # add+start new named tailscaled
+   mts server start <name>      # start a previously added tailscaled
+   mts server stop <name>       # stop & remove a named tailscaled
+   mts server rm <name>         # stop & remove a named tailscaled
+   mts server logs [-f] <name>  # get/follow tailscaled logs
+
+  mts <inst-name> [tailscale CLI args] # run Tailscale CLI against a named instance
+    e.g.
+      mts gmail1 up
+      mts github2 status --json
+	`)+"\n")
+	os.Exit(1)
+}
+
+func main() {
+	// Don't use flag.Parse here; we mostly just delegate through
+	// to the Tailscale CLI.
+
+	if len(os.Args) < 2 {
+		usage()
+	}
+	firstArg, args := os.Args[1], os.Args[2:]
+	if firstArg == "server" || firstArg == "s" {
+		if err := runMTSServer(args); err != nil {
+			log.Fatal(err)
+		}
+	} else {
+		var c Client
+		inst := firstArg
+		c.RunCommand(inst, args)
+	}
+}
+
+func runMTSServer(args []string) error {
+	if len(args) == 0 {
+		usage()
+	}
+	cmd, args := args[0], args[1:]
+	if cmd == "run" {
+		var s Server
+		return s.Run()
+	}
+
+	// Commands other than "run" all use the HTTP client to
+	// hit the mts server over its unix socket.
+	var c Client
+
+	switch cmd {
+	default:
+		usage("unknown mts server subcommand %q", cmd)
+	case "list", "ls":
+		list, err := c.List()
+		if err != nil {
+			return err
+		}
+		if len(args) == 0 {
+			names := slices.Sorted(maps.Keys(list.Instances))
+			for _, name := range names {
+				running := list.Instances[name].Running
+				fmt.Printf("%10s %s\n", bools.IfElse(running, "RUNNING", "stopped"), name)
+			}
+		} else {
+			for _, name := range args {
+				inst, ok := list.Instances[name]
+				if !ok {
+					return fmt.Errorf("no instance named %q", name)
+				}
+				je := json.NewEncoder(os.Stdout)
+				je.SetIndent("", "  ")
+				if err := je.Encode(inst); err != nil {
+					return err
+				}
+			}
+		}
+
+	case "rm":
+		if len(args) == 0 {
+			return fmt.Errorf("missing instance name(s) to remove")
+		}
+		log.SetFlags(0)
+		for _, name := range args {
+			ok, err := c.Remove(name)
+			if err != nil {
+				return err
+			}
+			if ok {
+				log.Printf("%s deleted.", name)
+			} else {
+				log.Printf("%s didn't exist.", name)
+			}
+		}
+	case "stop":
+		if len(args) == 0 {
+			return fmt.Errorf("missing instance name(s) to stop")
+		}
+		log.SetFlags(0)
+		for _, name := range args {
+			ok, err := c.Stop(name)
+			if err != nil {
+				return err
+			}
+			if ok {
+				log.Printf("%s stopped.", name)
+			} else {
+				log.Printf("%s didn't exist.", name)
+			}
+		}
+	case "start", "restart":
+		list, err := c.List()
+		if err != nil {
+			return err
+		}
+		shouldStop := cmd == "restart"
+		for _, arg := range args {
+			is, ok := list.Instances[arg]
+			if !ok {
+				return fmt.Errorf("no instance named %q", arg)
+			}
+			if is.Running {
+				if shouldStop {
+					if _, err := c.Stop(arg); err != nil {
+						return fmt.Errorf("stopping %q: %w", arg, err)
+					}
+				} else {
+					log.SetFlags(0)
+					log.Printf("%s already running.", arg)
+					continue
+				}
+			}
+			// Creating an existing one starts it up.
+			if err := c.Create(arg); err != nil {
+				return fmt.Errorf("starting %q: %w", arg, err)
+			}
+		}
+	case "add":
+		if len(args) == 0 {
+			return fmt.Errorf("missing instance name(s) to add")
+		}
+		for _, name := range args {
+			if err := c.Create(name); err != nil {
+				return fmt.Errorf("creating %q: %w", name, err)
+			}
+		}
+	case "logs":
+		fs := flag.NewFlagSet("logs", flag.ExitOnError)
+		fs.Usage = func() { usage() }
+		follow := fs.Bool("f", false, "follow logs")
+		fs.Parse(args)
+		log.Printf("Parsed; following=%v, args=%q", *follow, fs.Args())
+		if fs.NArg() != 1 {
+			usage()
+		}
+		cmd := bools.IfElse(*follow, "tail", "cat")
+		args := []string{cmd}
+		if *follow {
+			args = append(args, "-f")
+		}
+		path, err := exec.LookPath(cmd)
+		if err != nil {
+			return fmt.Errorf("looking up %q: %w", cmd, err)
+		}
+		args = append(args, instLogsFile(fs.Arg(0)))
+		log.Fatal(syscall.Exec(path, args, os.Environ()))
+	}
+	return nil
+}
+
+type Client struct {
+}
+
+func (c *Client) client() *http.Client {
+	return &http.Client{
+		Transport: &http.Transport{
+			DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+				return net.Dial("unix", mtsSock())
+			},
+		},
+	}
+}
+
+func getJSON[T any](res *http.Response, err error) (T, error) {
+	var ret T
+	if err != nil {
+		return ret, err
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		body, _ := io.ReadAll(res.Body)
+		return ret, fmt.Errorf("unexpected status: %v: %s", res.Status, body)
+	}
+	if err := json.NewDecoder(res.Body).Decode(&ret); err != nil {
+		return ret, err
+	}
+	return ret, nil
+}
+
+func (c *Client) List() (listResponse, error) {
+	return getJSON[listResponse](c.client().Get("http://mts/list"))
+}
+
+func (c *Client) Remove(name string) (found bool, err error) {
+	return getJSON[bool](c.client().PostForm("http://mts/rm", url.Values{
+		"name": []string{name},
+	}))
+}
+
+func (c *Client) Stop(name string) (found bool, err error) {
+	return getJSON[bool](c.client().PostForm("http://mts/stop", url.Values{
+		"name": []string{name},
+	}))
+}
+
+func (c *Client) Create(name string) error {
+	req, err := http.NewRequest("POST", "http://mts/create/"+name, nil)
+	if err != nil {
+		return err
+	}
+	resp, err := c.client().Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		body, _ := io.ReadAll(resp.Body)
+		return fmt.Errorf("unexpected status: %v: %s", resp.Status, body)
+	}
+	return nil
+}
+
+func (c *Client) RunCommand(name string, args []string) {
+	sock := instSock(name)
+	lc := &local.Client{
+		Socket:        sock,
+		UseSocketOnly: true,
+	}
+	probeCtx, cancel := context.WithTimeout(context.Background(), 250*time.Millisecond)
+	defer cancel()
+	if _, err := lc.StatusWithoutPeers(probeCtx); err != nil {
+		log.Fatalf("instance %q not running? start with 'mts server start %q'; got error: %v", name, name, err)
+	}
+	args = append([]string{"run", "tailscale.com/cmd/tailscale", "--socket=" + sock}, args...)
+	cmd := exec.Command("go", args...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.Stdin = os.Stdin
+	err := cmd.Run()
+	if err == nil {
+		os.Exit(0)
+	}
+	if exitErr, ok := err.(*exec.ExitError); ok {
+		os.Exit(exitErr.ExitCode())
+	}
+	panic(err)
+}
+
+type Server struct {
+	lazyTailscaled lazy.GValue[string]
+
+	mu   sync.Mutex
+	cmds map[string]*exec.Cmd // running tailscaled instances
+}
+
+func (s *Server) tailscaled() string {
+	v, err := s.lazyTailscaled.GetErr(func() (string, error) {
+		out, err := exec.Command("go", "list", "-f", "{{.Target}}", "tailscale.com/cmd/tailscaled").CombinedOutput()
+		if err != nil {
+			return "", err
+		}
+		return strings.TrimSpace(string(out)), nil
+	})
+	if err != nil {
+		panic(err)
+	}
+	return v
+}
+
+func (s *Server) Run() error {
+	if err := os.MkdirAll(mtsRoot(), 0700); err != nil {
+		return err
+	}
+	sock := mtsSock()
+	os.Remove(sock)
+	log.Printf("Multi-Tailscaled Server running; listening on %q ...", sock)
+	ln, err := net.Listen("unix", sock)
+	if err != nil {
+		return err
+	}
+	return http.Serve(ln, s)
+}
+
+var validNameRx = regexp.MustCompile(`^[a-zA-Z0-9_-]+$`)
+
+func validInstanceName(name string) bool {
+	return validNameRx.MatchString(name)
+}
+
+func (s *Server) InstanceRunning(name string) bool {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	_, ok := s.cmds[name]
+	return ok
+}
+
+func (s *Server) Stop(name string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if cmd, ok := s.cmds[name]; ok {
+		if err := cmd.Process.Kill(); err != nil {
+			log.Printf("error killing %q: %v", name, err)
+		}
+		delete(s.cmds, name)
+	}
+}
+
+func (s *Server) RunInstance(name string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if _, ok := s.cmds[name]; ok {
+		return fmt.Errorf("instance %q already running", name)
+	}
+
+	if !validInstanceName(name) {
+		return fmt.Errorf("invalid instance name %q", name)
+	}
+	dir := filepath.Join(mtsRoot(), name)
+	if err := os.MkdirAll(dir, 0700); err != nil {
+		return err
+	}
+
+	env := os.Environ()
+	env = append(env, "TS_DEBUG_LOG_RATE=all")
+	if ef, err := os.Open(instEnvFile(name)); err == nil {
+		defer ef.Close()
+		sc := bufio.NewScanner(ef)
+		for sc.Scan() {
+			t := strings.TrimSpace(sc.Text())
+			if strings.HasPrefix(t, "#") || !strings.Contains(t, "=") {
+				continue
+			}
+			env = append(env, t)
+		}
+	} else if os.IsNotExist(err) {
+		// Write an example one.
+		os.WriteFile(instEnvFile(name), fmt.Appendf(nil, "# Example mts env.txt file; uncomment/add stuff you want for %q\n\n#TS_DEBUG_MAP=1\n#TS_DEBUG_REGISTER=1\n#TS_NO_LOGS_NO_SUPPORT=1\n", name), 0600)
+	}
+
+	extraArgs := []string{"--verbose=1"}
+	if af, err := os.Open(instArgsFile(name)); err == nil {
+		extraArgs = nil // clear default args
+		defer af.Close()
+		sc := bufio.NewScanner(af)
+		for sc.Scan() {
+			t := strings.TrimSpace(sc.Text())
+			if strings.HasPrefix(t, "#") || t == "" {
+				continue
+			}
+			extraArgs = append(extraArgs, t)
+		}
+	} else if os.IsNotExist(err) {
+		// Write an example one.
+		os.WriteFile(instArgsFile(name), fmt.Appendf(nil, "# Example mts args.txt file for instance %q.\n# One line per extra arg to tailscaled; no magic string quoting\n\n--verbose=1\n#--socks5-server=127.0.0.1:5000\n", name), 0600)
+	}
+
+	log.Printf("Running Tailscale daemon %q in %q", name, dir)
+
+	args := []string{
+		"--tun=userspace-networking",
+		"--statedir=" + filepath.Join(dir),
+		"--socket=" + filepath.Join(dir, "tailscaled.sock"),
+	}
+	args = append(args, extraArgs...)
+
+	cmd := exec.Command(s.tailscaled(), args...)
+	cmd.Dir = dir
+	cmd.Env = env
+
+	out, err := cmd.StdoutPipe()
+	if err != nil {
+		return err
+	}
+	cmd.Stderr = cmd.Stdout
+
+	logs := instLogsFile(name)
+	logFile, err := os.OpenFile(logs, os.O_CREATE|os.O_WRONLY|os.O_APPEND|os.O_TRUNC, 0644)
+	if err != nil {
+		return fmt.Errorf("opening logs file: %w", err)
+	}
+
+	go func() {
+		bs := bufio.NewScanner(out)
+		for bs.Scan() {
+			// TODO(bradfitz): record in memory too, serve via HTTP
+			line := strings.TrimSpace(bs.Text())
+			fmt.Fprintf(logFile, "%s\n", line)
+			fmt.Printf("tailscaled[%s]: %s\n", name, line)
+		}
+	}()
+
+	if err := cmd.Start(); err != nil {
+		return err
+	}
+	go func() {
+		err := cmd.Wait()
+		logFile.Close()
+		log.Printf("Tailscale daemon %q exited: %v", name, err)
+		s.mu.Lock()
+		defer s.mu.Unlock()
+		delete(s.cmds, name)
+	}()
+
+	mak.Set(&s.cmds, name, cmd)
+	return nil
+}
+
+type listResponse struct {
+	// Instances maps instance name to its details.
+	Instances map[string]listResponseInstance `json:"instances"`
+}
+
+type listResponseInstance struct {
+	Name    string `json:"name"`
+	Dir     string `json:"dir"`
+	Sock    string `json:"sock"`
+	Running bool   `json:"running"`
+	Env     string `json:"env"`
+	Args    string `json:"args"`
+	Logs    string `json:"logs"`
+}
+
+func writeJSON(w http.ResponseWriter, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	e := json.NewEncoder(w)
+	e.SetIndent("", "  ")
+	e.Encode(v)
+}
+
+func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if r.URL.Path == "/list" {
+		var res listResponse
+		for _, name := range s.InstanceNames() {
+			mak.Set(&res.Instances, name, listResponseInstance{
+				Name:    name,
+				Dir:     instDir(name),
+				Sock:    instSock(name),
+				Running: s.InstanceRunning(name),
+				Env:     instEnvFile(name),
+				Args:    instArgsFile(name),
+				Logs:    instLogsFile(name),
+			})
+		}
+		writeJSON(w, res)
+		return
+	}
+	if r.URL.Path == "/rm" || r.URL.Path == "/stop" {
+		shouldRemove := r.URL.Path == "/rm"
+		if r.Method != "POST" {
+			http.Error(w, "POST required", http.StatusMethodNotAllowed)
+			return
+		}
+		target := r.FormValue("name")
+		var ok bool
+		for _, name := range s.InstanceNames() {
+			if name != target {
+				continue
+			}
+			ok = true
+			s.Stop(name)
+			if shouldRemove {
+				if err := os.RemoveAll(instDir(name)); err != nil {
+					http.Error(w, err.Error(), http.StatusInternalServerError)
+					return
+				}
+			}
+			break
+		}
+		writeJSON(w, ok)
+		return
+	}
+	if inst, ok := strings.CutPrefix(r.URL.Path, "/create/"); ok {
+		if !s.InstanceRunning(inst) {
+			if err := s.RunInstance(inst); err != nil {
+				http.Error(w, err.Error(), http.StatusInternalServerError)
+				return
+			}
+		}
+		fmt.Fprintf(w, "OK\n")
+		return
+	}
+	if r.URL.Path == "/" {
+		fmt.Fprintf(w, "This is mts, the multi-tailscaled server.\n")
+		return
+	}
+	http.NotFound(w, r)
+}
+
+func (s *Server) InstanceNames() []string {
+	var ret []string
+	des, err := os.ReadDir(mtsRoot())
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
+		panic(err)
+	}
+	for _, de := range des {
+		if !de.IsDir() {
+			continue
+		}
+		ret = append(ret, de.Name())
+	}
+	return ret
+}
+
+func mtsRoot() string {
+	dir, err := os.UserConfigDir()
+	if err != nil {
+		panic(err)
+	}
+	return filepath.Join(dir, "multi-tailscale-dev")
+}
+
+func instDir(name string) string {
+	return filepath.Join(mtsRoot(), name)
+}
+
+func instSock(name string) string {
+	return filepath.Join(instDir(name), "tailscaled.sock")
+}
+
+func instEnvFile(name string) string {
+	return filepath.Join(mtsRoot(), name, "env.txt")
+}
+
+func instArgsFile(name string) string {
+	return filepath.Join(mtsRoot(), name, "args.txt")
+}
+
+func instLogsFile(name string) string {
+	return filepath.Join(mtsRoot(), name, "logs.txt")
+}
+
+func mtsSock() string {
+	return filepath.Join(mtsRoot(), "mts.sock")
+}