derp{,/derphttp},magicsock: tell DERP server when ping acks can be expected

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

derp/derp_client.go

@@ -28,6 +28,7 @@ type Client struct {
 	nc          Conn
 	br          *bufio.Reader
 	meshKey     string
+	canAckPings bool
 
 	wmu sync.Mutex // hold while writing to bw
 	bw  *bufio.Writer
@@ -50,6 +51,7 @@ func (f clientOptFunc) update(o *clientOpt) { f(o) }
 type clientOpt struct {
 	MeshKey     string
 	ServerPub   key.Public
+	CanAckPings bool
 }
 
 // MeshKey returns a ClientOpt to pass to the DERP server during connect to get
@@ -64,6 +66,12 @@ func ServerPublicKey(key key.Public) ClientOpt {
 	return clientOptFunc(func(o *clientOpt) { o.ServerPub = key })
 }
 
+// CanAckPings returns a ClientOpt to set whether it advertises to the
+// server that it's capable of acknowledging ping requests.
+func CanAckPings(v bool) ClientOpt {
+	return clientOptFunc(func(o *clientOpt) { o.CanAckPings = v })
+}
+
 func NewClient(privateKey key.Private, nc Conn, brw *bufio.ReadWriter, logf logger.Logf, opts ...ClientOpt) (*Client, error) {
 	var opt clientOpt
 	for _, o := range opts {
@@ -84,6 +92,7 @@ func newClient(privateKey key.Private, nc Conn, brw *bufio.ReadWriter, logf logg
 		br:          brw.Reader,
 		bw:          brw.Writer,
 		meshKey:     opt.MeshKey,
+		canAckPings: opt.CanAckPings,
 	}
 	if opt.ServerPub.IsZero() {
 		if err := c.recvServerKey(); err != nil {
@@ -147,6 +156,10 @@ type clientInfo struct {
 	// connection list & forward packets. It's empty for regular
 	// users.
 	MeshKey string `json:"meshKey,omitempty"`
+
+	// CanAckPings is whether the client declares it's able to ack
+	// pings.
+	CanAckPings bool
 }
 
 func (c *Client) sendClientKey() error {
@@ -157,6 +170,7 @@ func (c *Client) sendClientKey() error {
 	msg, err := json.Marshal(clientInfo{
 		Version:     ProtocolVersion,
 		MeshKey:     c.meshKey,
+		CanAckPings: c.canAckPings,
 	})
 	if err != nil {
 		return err

derp/derphttp/derphttp_client.go

@@ -63,6 +63,7 @@ type Client struct {
 
 	mu           sync.Mutex
 	preferred    bool
+	canAckPings  bool
 	closed       bool
 	netConn      io.Closer
 	client       *derp.Client
@@ -333,7 +334,11 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 			return nil, 0, fmt.Errorf("GET failed: %v: %s", err, b)
 		}
 	}
-	derpClient, err = derp.NewClient(c.privateKey, httpConn, brw, c.logf, derp.MeshKey(c.MeshKey), derp.ServerPublicKey(serverPub))
+	derpClient, err = derp.NewClient(c.privateKey, httpConn, brw, c.logf,
+		derp.MeshKey(c.MeshKey),
+		derp.ServerPublicKey(serverPub),
+		derp.CanAckPings(c.canAckPings),
+	)
 	if err != nil {
 		return nil, 0, err
 	}
@@ -665,6 +670,15 @@ func (c *Client) SendPong(data [8]byte) error {
 	return dc.SendPong(data)
 }
 
+// SetCanAckPings sets whether this client will reply to ping requests from the server.
+//
+// This only affects future connections.
+func (c *Client) SetCanAckPings(v bool) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.canAckPings = v
+}
+
 // NotePreferred notes whether this Client is the caller's preferred
 // (home) DERP node. It's only used for stats.
 func (c *Client) NotePreferred(v bool) {

wgengine/magicsock/magicsock.go

@@ -1308,6 +1308,7 @@ func (c *Conn) derpWriteChanOfAddr(addr netaddr.IPPort, peer key.Public) chan<-
 		return c.derpMap.Regions[regionID]
 	})
 
+	dc.SetCanAckPings(true)
 	dc.NotePreferred(c.myDerp == regionID)
 	dc.DNSCache = dnscache.Get()