derp, cmd/derper: add frameWatchConns, framePeerPresent for inter-DERP routing

This lets a trusted DERP client that knows a pre-shared key subscribe
to the connection list. Upon subscribing, they get the current set
of connected public keys, and then all changes over time.

This lets a set of DERP server peers within a region all stay connected to
each other and know which clients are connected to which nodes.

Updates #388

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

cmd/derper/derper.go

@@ -20,6 +20,7 @@ import (
 	"os"
 	"path/filepath"
 	"regexp"
+	"strings"
 	"time"
 
 	"github.com/tailscale/wireguard-go/wgcfg"
@@ -42,6 +43,8 @@ var (
 	hostname      = flag.String("hostname", "derp.tailscale.com", "LetsEncrypt host name, if addr's port is :443")
 	logCollection = flag.String("logcollection", "", "If non-empty, logtail collection to log to")
 	runSTUN       = flag.Bool("stun", false, "also run a STUN server")
+	meshPSKFile   = flag.String("mesh-psk-file", defaultMeshPSKFile(), "if non-empty, path to file containing the mesh pre-shared key file. It should contain some hex string; whitespace is trimmed.")
+	meshWith      = flag.String("mesh-with", "", "optional comma-separated list of hostnames to mesh with; the server's own hostname can be in the list, in which case it's ignored if its DNS resolves to an IP on the machine")
 )
 
 type config struct {
@@ -118,6 +121,23 @@ func main() {
 	letsEncrypt := tsweb.IsProd443(*addr)
 
 	s := derp.NewServer(key.Private(cfg.PrivateKey), log.Printf)
+
+	if *meshPSKFile != "" {
+		b, err := ioutil.ReadFile(*meshPSKFile)
+		if err != nil {
+			log.Fatal(err)
+		}
+		key := strings.TrimSpace(string(b))
+		if matched, _ := regexp.MatchString(`(?i)^[0-9a-f]{64,}$`, key); !matched {
+			log.Fatalf("key in %s must contain 64+ hex digits", *meshPSKFile)
+		}
+		s.SetMeshKey(key)
+		log.Printf("DERP mesh key configured")
+	}
+
+	// TODO(bradfitz): parse & use the *meshWith
+	_ = *meshWith
+
 	expvar.Publish("derp", s.ExpVar())
 
 	// Create our own mux so we don't expose /debug/ stuff to the world.
@@ -192,6 +212,7 @@ func debugHandler(s *derp.Server) http.Handler {
 `)
 		f("<li><b>Hostname:</b> %v</li>\n", *hostname)
 		f("<li><b>Uptime:</b> %v</li>\n", tsweb.Uptime())
+		f("<li><b>Mesh Key:</b> %v</li>\n", s.HasMeshKey())
 
 		f(`<li><a href="/debug/vars">/debug/vars</a> (Go)</li>
    <li><a href="/debug/varz">/debug/varz</a> (Prometheus)</li>
@@ -268,7 +289,7 @@ func serveSTUN() {
 	}
 }
 
-var validProdHostname = regexp.MustCompile(`^derp(\d+|\-\w+)?\.tailscale\.com\.?$`)
+var validProdHostname = regexp.MustCompile(`^derp([^.]*)\.tailscale\.com\.?$`)
 
 func prodAutocertHostPolicy(_ context.Context, host string) error {
 	if validProdHostname.MatchString(host) {
@@ -276,3 +297,11 @@ func prodAutocertHostPolicy(_ context.Context, host string) error {
 	}
 	return errors.New("invalid hostname")
 }
+
+func defaultMeshPSKFile() string {
+	const def = "/home/derp/keys/derp-mesh.key"
+	if _, err := os.Stat(def); err == nil {
+		return def
+	}
+	return ""
+}

cmd/derper/derper_test.go

@@ -17,10 +17,11 @@ func TestProdAutocertHostPolicy(t *testing.T) {
 		{"derp.tailscale.com", true},
 		{"derp.tailscale.com.", true},
 		{"derp1.tailscale.com", true},
+		{"derp1b.tailscale.com", true},
 		{"derp2.tailscale.com", true},
 		{"derp02.tailscale.com", true},
 		{"derp-nyc.tailscale.com", true},
-		{"derpfoo.tailscale.com", false},
+		{"derpfoo.tailscale.com", true},
 		{"derp02.bar.tailscale.com", false},
 		{"example.net", false},
 	}

derp/derp.go

@@ -33,6 +33,7 @@ const magic = "DERP🔑" // 8 bytes: 0x44 45 52 50 f0 9f 94 91
 
 const (
 	nonceLen       = 24
+	frameHeaderLen = 1 + 4 // frameType byte + 4 byte length
 	keyLen         = 32
 	maxInfoLen     = 1 << 20
 	keepAlive      = 60 * time.Second
@@ -81,6 +82,18 @@ const (
 	// framePeerGone to B so B can forget that a reverse path
 	// exists on that connection to get back to A.
 	framePeerGone = frameType(0x08) // 32B pub key of peer that's gone
+
+	// framePeerPresent is like framePeerGone, but for other
+	// members of the DERP region when they're meshed up together.
+	framePeerPresent = frameType(0x09) // 32B pub key of peer that's connected
+
+	// frameWatchConns is how one DERP node in a regional mesh
+	// subscribes to the others in the region.
+	// There's no payload. If the sender doesn't have permission, the connection
+	// is closed. Otherwise, the client is initially flooded with
+	// framePeerPresent for all connected nodes, and then a stream of
+	// framePeerPresent & framePeerGone has peers connect and disconnect.
+	frameWatchConns = frameType(0x10)
 )
 
 var bin = binary.BigEndian

derp/derp_client.go

@@ -27,6 +27,7 @@ type Client struct {
 	logf         logger.Logf
 	nc           Conn
 	br           *bufio.Reader
+	meshKey      string
 
 	wmu     sync.Mutex // hold while writing to bw
 	bw      *bufio.Writer
@@ -34,6 +35,15 @@ type Client struct {
 }
 
 func NewClient(privateKey key.Private, nc Conn, brw *bufio.ReadWriter, logf logger.Logf) (*Client, error) {
+	noMeshKey := ""
+	return NewMeshClient(privateKey, nc, brw, logf, noMeshKey)
+}
+
+// NewMeshClient is the Client constructor for trusted clients that
+// are a peer in a cluster mesh.
+//
+// An empty meshKey is equivalent to just using NewClient.
+func NewMeshClient(privateKey key.Private, nc Conn, brw *bufio.ReadWriter, logf logger.Logf, meshKey string) (*Client, error) {
 	c := &Client{
 		privateKey: privateKey,
 		publicKey:  privateKey.Public(),
@@ -41,8 +51,8 @@ func NewClient(privateKey key.Private, nc Conn, brw *bufio.ReadWriter, logf logg
 		nc:         nc,
 		br:         brw.Reader,
 		bw:         brw.Writer,
+		meshKey:    meshKey,
 	}
-
 	if err := c.recvServerKey(); err != nil {
 		return nil, fmt.Errorf("derp.Client: failed to receive server key: %v", err)
 	}
@@ -109,6 +119,12 @@ func (c *Client) recvServerInfo() (*serverInfo, error) {
 
 type clientInfo struct {
 	Version int // `json:"version,omitempty"`
+
+	// MeshKey optionally specifies a pre-shared key used by
+	// trusted clients.  It's required to subscribe to the
+	// connection list & forward packets. It's empty for regular
+	// users.
+	MeshKey string // `json:"meshKey,omitempty"`
 }
 
 func (c *Client) sendClientKey() error {
@@ -116,7 +132,10 @@ func (c *Client) sendClientKey() error {
 	if _, err := crand.Read(nonce[:]); err != nil {
 		return err
 	}
-	msg, err := json.Marshal(clientInfo{Version: protocolVersion})
+	msg, err := json.Marshal(clientInfo{
+		Version: protocolVersion,
+		MeshKey: c.meshKey,
+	})
 	if err != nil {
 		return err
 	}
@@ -186,6 +205,17 @@ func (c *Client) NotePreferred(preferred bool) (err error) {
 	return c.bw.Flush()
 }
 
+// WatchConnectionChanges sends a request to subscribe to the peer's connection list.
+// It's a fatal error if the client wasn't created with NewMeshClient.
+func (c *Client) WatchConnectionChanges() error {
+	c.wmu.Lock()
+	defer c.wmu.Unlock()
+	if err := writeFrameHeader(c.bw, frameWatchConns, 0); err != nil {
+		return err
+	}
+	return c.bw.Flush()
+}
+
 // ReceivedMessage represents a type returned by Client.Recv. Unless
 // otherwise documented, the returned message aliases the byte slice
 // provided to Recv and thus the message is only as good as that
@@ -211,11 +241,21 @@ type PeerGoneMessage key.Public
 
 func (PeerGoneMessage) msg() {}
 
+// PeerPresentMessage is a ReceivedMessage that indicates that the client
+// is connected to the server. (Only used by trusted mesh clients)
+type PeerPresentMessage key.Public
+
+func (PeerPresentMessage) msg() {}
+
 // Recv reads a message from the DERP server.
 // The provided buffer must be large enough to receive a complete packet,
 // which in practice are are 1.5-4 KB, but can be up to 64 KB.
 // Once Recv returns an error, the Client is dead forever.
 func (c *Client) Recv(b []byte) (m ReceivedMessage, err error) {
+	return c.recvTimeout(b, 120*time.Second)
+}
+
+func (c *Client) recvTimeout(b []byte, timeout time.Duration) (m ReceivedMessage, err error) {
 	if c.readErr != nil {
 		return nil, c.readErr
 	}
@@ -227,7 +267,7 @@ func (c *Client) Recv(b []byte) (m ReceivedMessage, err error) {
 	}()
 
 	for {
-		c.nc.SetReadDeadline(time.Now().Add(120 * time.Second))
+		c.nc.SetReadDeadline(time.Now().Add(timeout))
 		t, n, err := readFrame(c.br, 1<<20, b)
 		if err != nil {
 			return nil, err
@@ -248,6 +288,15 @@ func (c *Client) Recv(b []byte) (m ReceivedMessage, err error) {
 			copy(pg[:], b[:keyLen])
 			return pg, nil
 
+		case framePeerPresent:
+			if n < keyLen {
+				c.logf("[unexpected] dropping short peerPresent frame from DERP server")
+				continue
+			}
+			var pg PeerPresentMessage
+			copy(pg[:], b[:keyLen])
+			return pg, nil
+
 		case frameRecvPacket:
 			var rp ReceivedPacket
 			if c.protoVersion < protocolSrcAddrs {

derp/derp_server.go

@@ -47,6 +47,7 @@ type Server struct {
 	publicKey  key.Public
 	logf       logger.Logf
 	memSys0    uint64 // runtime.MemStats.Sys at start (or early-ish)
+	meshKey    string
 
 	// Counters:
 	packetsSent, bytesSent  expvar.Int
@@ -72,6 +73,7 @@ type Server struct {
 	netConns    map[Conn]chan struct{} // chan is closed when conn closes
 	clients     map[key.Public]*sclient
 	clientsEver map[key.Public]bool // never deleted from, for stats; fine for now
+	watchers    map[*sclient]bool   // mesh peer -> true
 }
 
 // Conn is the subset of the underlying net.Conn the DERP Server needs.
@@ -101,6 +103,7 @@ func NewServer(privateKey key.Private, logf logger.Logf) *Server {
 		clientsEver:          make(map[key.Public]bool),
 		netConns:             make(map[Conn]chan struct{}),
 		memSys0:              ms.Sys,
+		watchers:             map[*sclient]bool{},
 	}
 	s.packetsDroppedUnknown = s.packetsDroppedReason.Get("unknown_dest")
 	s.packetsDroppedGone = s.packetsDroppedReason.Get("gone")
@@ -110,6 +113,16 @@ func NewServer(privateKey key.Private, logf logger.Logf) *Server {
 	return s
 }
 
+// SetMesh sets the pre-shared key that regional DERP servers used to mesh
+// amongst themselves.
+//
+// It must be called before serving begins.
+func (s *Server) SetMeshKey(v string) {
+	s.meshKey = v
+}
+
+func (s *Server) HasMeshKey() bool { return s.meshKey != "" }
+
 // Close closes the server and waits for the connections to disconnect.
 func (s *Server) Close() error {
 	s.mu.Lock()
@@ -188,6 +201,19 @@ func (s *Server) registerClient(c *sclient) {
 	s.clients[c.key] = c
 	s.clientsEver[c.key] = true
 	s.curClients.Add(1)
+	s.broadcastPeerStateChangeLocked(c.key, true)
+}
+
+// broadcastPeerStateChangeLocked enqueues a message to all watchers
+// (other DERP nodes in the region, or trusted clients) that peer's
+// presence changed.
+//
+// s.mu must be held.
+func (s *Server) broadcastPeerStateChangeLocked(peer key.Public, present bool) {
+	for w := range s.watchers {
+		w.peerStateChange = append(w.peerStateChange, peerConnState{peer: peer, present: present})
+		go w.requestMeshUpdate()
+	}
 }
 
 // unregisterClient removes a client from the server.
@@ -199,6 +225,10 @@ func (s *Server) unregisterClient(c *sclient) {
 		c.logf("removing connection")
 		delete(s.clients, c.key)
 	}
+	if c.canMesh {
+		delete(s.watchers, c)
+	}
+	s.broadcastPeerStateChangeLocked(c.key, false)
 
 	s.curClients.Add(-1)
 	if c.preferred {
@@ -224,6 +254,26 @@ func (s *Server) unregisterClient(c *sclient) {
 	}
 }
 
+func (s *Server) addWatcher(c *sclient) {
+	if !c.canMesh {
+		panic("invariant: addWatcher called without permissions")
+	}
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	// Queue messages for each already-connected client.
+	for peer := range s.clients {
+		c.peerStateChange = append(c.peerStateChange, peerConnState{peer: peer, present: true})
+	}
+
+	// And enroll the watcher in future updates (of both
+	// connections & disconnections).
+	s.watchers[c] = true
+
+	go c.requestMeshUpdate()
+}
+
 func (s *Server) accept(nc Conn, brw *bufio.ReadWriter, remoteAddr string, connNum int64) error {
 	br, bw := brw.Reader, brw.Writer
 	nc.SetDeadline(time.Now().Add(10 * time.Second))
@@ -259,6 +309,10 @@ func (s *Server) accept(nc Conn, brw *bufio.ReadWriter, remoteAddr string, connN
 		sendQueue:   make(chan pkt, perClientSendQueueDepth),
 		peerGone:    make(chan key.Public),
 		sentTo:      make(map[key.Public]int64),
+		canMesh:     clientInfo.MeshKey != "" && clientInfo.MeshKey == s.meshKey,
+	}
+	if c.canMesh {
+		c.meshUpdate = make(chan struct{})
 	}
 	if clientInfo != nil {
 		c.info = *clientInfo
@@ -307,6 +361,8 @@ func (c *sclient) run(ctx context.Context) error {
 			err = c.handleFrameNotePreferred(ft, fl)
 		case frameSendPacket:
 			err = c.handleFrameSendPacket(ft, fl)
+		case frameWatchConns:
+			err = c.handleFrameWatchConns(ft, fl)
 		default:
 			err = c.handleUnknownFrame(ft, fl)
 		}
@@ -333,6 +389,17 @@ func (c *sclient) handleFrameNotePreferred(ft frameType, fl uint32) error {
 	return nil
 }
 
+func (c *sclient) handleFrameWatchConns(ft frameType, fl uint32) error {
+	if fl != 0 {
+		return fmt.Errorf("handleFrameWatchConns wrong size")
+	}
+	if !c.canMesh {
+		return fmt.Errorf("insufficient permissions")
+	}
+	c.s.addWatcher(c)
+	return nil
+}
+
 func (c *sclient) handleFrameSendPacket(ft frameType, fl uint32) error {
 	s := c.s
 
@@ -418,6 +485,16 @@ func (c *sclient) requestPeerGoneWrite(peer key.Public) {
 	}
 }
 
+func (c *sclient) requestMeshUpdate() {
+	if !c.canMesh {
+		panic("unexpected requestMeshUpdate")
+	}
+	select {
+	case c.meshUpdate <- struct{}{}:
+	case <-c.done:
+	}
+}
+
 func (s *Server) verifyClient(clientKey key.Public, info *clientInfo) error {
 	// TODO(crawshaw): implement policy constraints on who can use the DERP server
 	// TODO(bradfitz): ... and at what rate.
@@ -532,7 +609,9 @@ type sclient struct {
 	done       <-chan struct{} // closed when connection closes
 	remoteAddr string          // usually ip:port from net.Conn.RemoteAddr().String()
 	sendQueue  chan pkt        // packets queued to this client; never closed
-	peerGone   chan key.Public // write request that a previous sender has disconnected
+	peerGone   chan key.Public // write request that a previous sender has disconnected (not used by mesh peers)
+	meshUpdate chan struct{}   // write request to write peerStateChange
+	canMesh    bool            // clientInfo had correct mesh token for inter-region routing
 
 	// Owned by run, not thread-safe.
 	br          *bufio.Reader
@@ -547,6 +626,21 @@ type sclient struct {
 	// sentTo tracks all the peers this client has ever sent a packet to, and at which
 	// connection number.
 	sentTo map[key.Public]int64 // recipient => rcpt's latest sclient.connNum
+
+	// Guarded by s.mu
+	//
+	// peerStateChange is used by mesh peers (a set of regional
+	// DERP servers) and contains records that need to be sent to
+	// the client for them to update their map of who's connected
+	// to this node.
+	peerStateChange []peerConnState
+}
+
+// peerConnState represents whether a peer is connected to the server
+// or not.
+type peerConnState struct {
+	peer    key.Public
+	present bool
 }
 
 // pkt is a request to write a data frame to an sclient.
@@ -628,6 +722,9 @@ func (c *sclient) sendLoop(ctx context.Context) error {
 		case peer := <-c.peerGone:
 			werr = c.sendPeerGone(peer)
 			continue
+		case <-c.meshUpdate:
+			werr = c.sendMeshUpdates()
+			continue
 		case msg := <-c.sendQueue:
 			werr = c.sendPacket(msg.src, msg.bs)
 			continue
@@ -648,6 +745,9 @@ func (c *sclient) sendLoop(ctx context.Context) error {
 			return nil
 		case peer := <-c.peerGone:
 			werr = c.sendPeerGone(peer)
+		case <-c.meshUpdate:
+			werr = c.sendMeshUpdates()
+			continue
 		case msg := <-c.sendQueue:
 			werr = c.sendPacket(msg.src, msg.bs)
 		case <-keepAliveTick.C:
@@ -677,6 +777,59 @@ func (c *sclient) sendPeerGone(peer key.Public) error {
 	return err
 }
 
+// sendPeerPresent sends a peerPresent frame, without flushing.
+func (c *sclient) sendPeerPresent(peer key.Public) error {
+	c.setWriteDeadline()
+	if err := writeFrameHeader(c.bw, framePeerPresent, keyLen); err != nil {
+		return err
+	}
+	_, err := c.bw.Write(peer[:])
+	return err
+}
+
+// sendMeshUpdates drains as many mesh peerStateChange entries as
+// possible into the write buffer WITHOUT flushing or otherwise
+// blocking (as it holds c.s.mu while working). If it can't drain them
+// all, it schedules itself to be called again in the future.
+func (c *sclient) sendMeshUpdates() error {
+	c.s.mu.Lock()
+	defer c.s.mu.Unlock()
+
+	writes := 0
+	for _, pcs := range c.peerStateChange {
+		if c.bw.Available() <= frameHeaderLen+keyLen {
+			break
+		}
+		var err error
+		if pcs.present {
+			err = c.sendPeerPresent(pcs.peer)
+		} else {
+			err = c.sendPeerGone(pcs.peer)
+		}
+		if err != nil {
+			// Shouldn't happen, though, as we're writing
+			// into available buffer space, not the
+			// network.
+			return err
+		}
+		writes++
+	}
+
+	remain := copy(c.peerStateChange, c.peerStateChange[writes:])
+	c.peerStateChange = c.peerStateChange[:remain]
+
+	// Did we manage to write them all into the bufio buffer without flushing?
+	if len(c.peerStateChange) == 0 {
+		if cap(c.peerStateChange) > 16 {
+			c.peerStateChange = nil
+		}
+	} else {
+		// Didn't finish in the buffer space provided; schedule a future run.
+		go c.requestMeshUpdate()
+	}
+	return nil
+}
+
 // sendPacket writes contents to the client in a RecvPacket frame. If
 // srcKey.IsZero, uses the old DERPv1 framing format, otherwise uses
 // DERPv2. The bytes of contents are only valid until this function
@@ -729,6 +882,7 @@ func (s *Server) ExpVar() expvar.Var {
 	m := new(metrics.Set)
 	m.Set("counter_unique_clients_ever", s.expVarFunc(func() interface{} { return len(s.clientsEver) }))
 	m.Set("gauge_memstats_sys0", expvar.Func(func() interface{} { return int64(s.memSys0) }))
+	m.Set("gauge_watchers", s.expVarFunc(func() interface{} { return len(s.watchers) }))
 	m.Set("gauge_current_connnections", &s.curClients)
 	m.Set("gauge_current_home_connnections", &s.curHomeClients)
 	m.Set("accepts", &s.accepts)

derp/derp_test.go

@@ -13,11 +13,13 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"sync"
 	"testing"
 	"time"
 
 	"tailscale.com/net/nettest"
 	"tailscale.com/types/key"
+	"tailscale.com/types/logger"
 )
 
 func newPrivateKey(t *testing.T) (k key.Private) {
@@ -391,3 +393,229 @@ func TestSendFreeze(t *testing.T) {
 		}
 	}
 }
+
+type testServer struct {
+	s    *Server
+	ln   net.Listener
+	logf logger.Logf
+
+	mu      sync.Mutex
+	pubName map[key.Public]string
+	clients map[*testClient]bool
+}
+
+func (ts *testServer) addTestClient(c *testClient) {
+	ts.mu.Lock()
+	defer ts.mu.Unlock()
+	ts.clients[c] = true
+}
+
+func (ts *testServer) addKeyName(k key.Public, name string) {
+	ts.mu.Lock()
+	defer ts.mu.Unlock()
+	ts.pubName[k] = name
+	ts.logf("test adding named key %q for %x", name, k)
+}
+
+func (ts *testServer) keyName(k key.Public) string {
+	ts.mu.Lock()
+	defer ts.mu.Unlock()
+	if name, ok := ts.pubName[k]; ok {
+		return name
+	}
+	return k.ShortString()
+}
+
+func (ts *testServer) close(t *testing.T) error {
+	ts.ln.Close()
+	ts.s.Close()
+	for c := range ts.clients {
+		c.close(t)
+	}
+	return nil
+}
+
+func newTestServer(t *testing.T) *testServer {
+	t.Helper()
+	logf := logger.WithPrefix(t.Logf, "derp-server: ")
+	s := NewServer(newPrivateKey(t), logf)
+	s.SetMeshKey("mesh-key")
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	go func() {
+		i := 0
+		for {
+			i++
+			c, err := ln.Accept()
+			if err != nil {
+				return
+			}
+			// TODO: register c in ts so Close also closes it?
+			go func(i int) {
+				brwServer := bufio.NewReadWriter(bufio.NewReader(c), bufio.NewWriter(c))
+				go s.Accept(c, brwServer, fmt.Sprintf("test-client-%d", i))
+			}(i)
+		}
+	}()
+	return &testServer{
+		s:       s,
+		ln:      ln,
+		logf:    logf,
+		clients: map[*testClient]bool{},
+		pubName: map[key.Public]string{},
+	}
+}
+
+type testClient struct {
+	name   string
+	c      *Client
+	nc     net.Conn
+	pub    key.Public
+	ts     *testServer
+	closed bool
+}
+
+func newTestClient(t *testing.T, ts *testServer, name string, newClient func(net.Conn, key.Private, logger.Logf) (*Client, error)) *testClient {
+	t.Helper()
+	nc, err := net.Dial("tcp", ts.ln.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	key := newPrivateKey(t)
+	ts.addKeyName(key.Public(), name)
+	c, err := newClient(nc, key, logger.WithPrefix(t.Logf, "client-"+name+": "))
+	if err != nil {
+		t.Fatal(err)
+	}
+	tc := &testClient{
+		name: name,
+		nc:   nc,
+		c:    c,
+		ts:   ts,
+		pub:  key.Public(),
+	}
+	ts.addTestClient(tc)
+	return tc
+}
+
+func newRegularClient(t *testing.T, ts *testServer, name string) *testClient {
+	return newTestClient(t, ts, name, func(nc net.Conn, priv key.Private, logf logger.Logf) (*Client, error) {
+		brw := bufio.NewReadWriter(bufio.NewReader(nc), bufio.NewWriter(nc))
+		return NewClient(priv, nc, brw, logf)
+	})
+}
+
+func newTestWatcher(t *testing.T, ts *testServer, name string) *testClient {
+	return newTestClient(t, ts, name, func(nc net.Conn, priv key.Private, logf logger.Logf) (*Client, error) {
+		brw := bufio.NewReadWriter(bufio.NewReader(nc), bufio.NewWriter(nc))
+		c, err := NewMeshClient(priv, nc, brw, logf, "mesh-key")
+		if err != nil {
+			return nil, err
+		}
+		if err := c.WatchConnectionChanges(); err != nil {
+			return nil, err
+		}
+		return c, nil
+	})
+}
+
+func (tc *testClient) wantPresent(t *testing.T, peers ...key.Public) {
+	t.Helper()
+	want := map[key.Public]bool{}
+	for _, k := range peers {
+		want[k] = true
+	}
+
+	var buf [64 << 10]byte
+	for {
+		m, err := tc.c.recvTimeout(buf[:], time.Second)
+		if err != nil {
+			t.Fatal(err)
+		}
+		switch m := m.(type) {
+		case PeerPresentMessage:
+			got := key.Public(m)
+			if !want[got] {
+				t.Fatalf("got peer present for %v; want present for %v", tc.ts.keyName(got), logger.ArgWriter(func(bw *bufio.Writer) {
+					for _, pub := range peers {
+						fmt.Fprintf(bw, "%s ", tc.ts.keyName(pub))
+					}
+				}))
+			}
+			delete(want, got)
+			if len(want) == 0 {
+				return
+			}
+		default:
+			t.Fatalf("unexpected message type %T", m)
+		}
+	}
+}
+
+func (tc *testClient) wantGone(t *testing.T, peer key.Public) {
+	t.Helper()
+	var buf [64 << 10]byte
+	m, err := tc.c.recvTimeout(buf[:], time.Second)
+	if err != nil {
+		t.Fatal(err)
+	}
+	switch m := m.(type) {
+	case PeerGoneMessage:
+		got := key.Public(m)
+		if peer != got {
+			t.Errorf("got gone message for %v; want gone for %v", tc.ts.keyName(got), tc.ts.keyName(peer))
+		}
+	default:
+		t.Fatalf("unexpected message type %T", m)
+	}
+}
+
+func (c *testClient) close(t *testing.T) {
+	t.Helper()
+	if c.closed {
+		return
+	}
+	c.closed = true
+	t.Logf("closing client %q (%x)", c.name, c.pub)
+	c.nc.Close()
+}
+
+// TestWatch tests the connection watcher mechanism used by regional
+// DERP nodes to mesh up with each other.
+func TestWatch(t *testing.T) {
+	ts := newTestServer(t)
+	defer ts.close(t)
+
+	w1 := newTestWatcher(t, ts, "w1")
+	w1.wantPresent(t, w1.pub)
+
+	c1 := newRegularClient(t, ts, "c1")
+	w1.wantPresent(t, c1.pub)
+
+	c2 := newRegularClient(t, ts, "c2")
+	w1.wantPresent(t, c2.pub)
+
+	w2 := newTestWatcher(t, ts, "w2")
+	w1.wantPresent(t, w2.pub)
+	w2.wantPresent(t, w1.pub, w2.pub, c1.pub, c2.pub)
+
+	c3 := newRegularClient(t, ts, "c3")
+	w1.wantPresent(t, c3.pub)
+	w2.wantPresent(t, c3.pub)
+
+	c2.close(t)
+	w1.wantGone(t, c2.pub)
+	w2.wantGone(t, c2.pub)
+
+	w3 := newTestWatcher(t, ts, "w3")
+	w1.wantPresent(t, w3.pub)
+	w2.wantPresent(t, w3.pub)
+	w3.wantPresent(t, c1.pub, c3.pub, w1.pub, w2.pub, w3.pub)
+
+	c1.close(t)
+	w1.wantGone(t, c1.pub)
+	w2.wantGone(t, c1.pub)
+	w3.wantGone(t, c1.pub)
+}

derp/derphttp/derphttp_client.go

@@ -43,6 +43,7 @@ import (
 type Client struct {
 	TLSConfig *tls.Config        // optional; nil means default
 	DNSCache  *dnscache.Resolver // optional; nil means no caching
+	MeshKey   string             // optional; for trusted clients
 
 	privateKey key.Private
 	logf       logger.Logf
@@ -272,7 +273,7 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 		return nil, fmt.Errorf("GET failed: %v: %s", err, b)
 	}
 
-	derpClient, err := derp.NewClient(c.privateKey, httpConn, brw, c.logf)
+	derpClient, err := derp.NewMeshClient(c.privateKey, httpConn, brw, c.logf, c.MeshKey)
 	if err != nil {
 		return nil, err
 	}
@@ -492,6 +493,18 @@ func (c *Client) NotePreferred(v bool) {
 	}
 }
 
+func (c *Client) WatchConnectionChanges() error {
+	client, err := c.connect(context.TODO(), "derphttp.Client.WatchConnectionChanges")
+	if err != nil {
+		return err
+	}
+	err = client.WatchConnectionChanges()
+	if err != nil {
+		c.closeForReconnect(client)
+	}
+	return err
+}
+
 func (c *Client) Recv(b []byte) (derp.ReceivedMessage, error) {
 	client, err := c.connect(context.TODO(), "derphttp.Client.Recv")
 	if err != nil {