cmd/tailscale/cli: add ping subcommand

For example:

$ tailscale ping -h
USAGE
  ping <hostname-or-IP>

FLAGS
  -c 10                   max number of pings to send
  -stop-once-direct true  stop once a direct path is established
  -verbose false          verbose output

$ tailscale ping mon.ts.tailscale.com
pong from monitoring (100.88.178.64) via DERP(sfo) in 65ms
pong from monitoring (100.88.178.64) via DERP(sfo) in 252ms
pong from monitoring (100.88.178.64) via [2604:a880:2:d1::36:d001]:41641 in 33ms

Fixes #661

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

ipn/backend.go

@@ -62,6 +62,7 @@ type Notify struct {
 	Status        *ipnstate.Status          // full status
 	BrowseToURL   *string                   // UI should open a browser right now
 	BackendLogID  *string                   // public logtail id used by backend
+	PingResult    *ipnstate.PingResult
 
 	// LocalTCPPort, if non-nil, informs the UI frontend which
 	// (non-zero) localhost TCP port it's listening on.
@@ -156,4 +157,8 @@ type Backend interface {
 	// make sure they react properly with keys that are going to
 	// expire.
 	FakeExpireAfter(x time.Duration)
+	// Ping attempts to start connecting to the given IP and sends a Notify
+	// with its PingResult. If the host is down, there might never
+	// be a PingResult sent. The cmd/tailscale CLI client adds a timeout.
+	Ping(ip string)
 }

ipn/fake_test.go

@@ -90,3 +90,7 @@ func (b *FakeBackend) RequestStatus() {
 func (b *FakeBackend) FakeExpireAfter(x time.Duration) {
 	b.notify(Notify{NetMap: &controlclient.NetworkMap{}})
 }
+
+func (b *FakeBackend) Ping(ip string) {
+	b.notify(Notify{PingResult: &ipnstate.PingResult{}})
+}

ipn/ipnstate/ipnstate.go

@@ -322,3 +322,21 @@ func osEmoji(os string) string {
 	}
 	return "👽"
 }
+
+// PingResult contains response information for the "tailscale ping" subcommand,
+// saying how Tailscale can reach a Tailscale IP or subnet-routed IP.
+type PingResult struct {
+	IP       string // ping destination
+	NodeIP   string // Tailscale IP of node handling IP (different for subnet routers)
+	NodeName string // DNS name base or (possibly not unique) hostname
+
+	Err            string
+	LatencySeconds float64
+
+	Endpoint string // ip:port if direct UDP was used
+
+	DERPRegionID   int    // non-zero if DERP was used
+	DERPRegionCode string // three-letter airport/region code if DERP was used
+
+	// TODO(bradfitz): details like whether port mapping was used on either side? (Once supported)
+}

ipn/local.go

@@ -745,6 +745,17 @@ func (b *LocalBackend) FakeExpireAfter(x time.Duration) {
 	b.send(Notify{NetMap: b.netMap})
 }
 
+func (b *LocalBackend) Ping(ipStr string) {
+	ip, err := netaddr.ParseIP(ipStr)
+	if err != nil {
+		b.logf("ignoring Ping request to invalid IP %q", ipStr)
+		return
+	}
+	b.e.Ping(ip, func(pr *ipnstate.PingResult) {
+		b.send(Notify{PingResult: pr})
+	})
+}
+
 func (b *LocalBackend) parseWgStatus(s *wgengine.Status) (ret EngineStatus) {
 	var (
 		peerStats []string

ipn/message.go

@@ -33,6 +33,10 @@ type FakeExpireAfterArgs struct {
 	Duration time.Duration
 }
 
+type PingArgs struct {
+	IP string
+}
+
 // Command is a command message that is JSON encoded and sent by a
 // frontend to a backend.
 type Command struct {
@@ -56,6 +60,7 @@ type Command struct {
 	RequestEngineStatus   *NoArgs
 	RequestStatus         *NoArgs
 	FakeExpireAfter       *FakeExpireAfterArgs
+	Ping                  *PingArgs
 }
 
 type BackendServer struct {
@@ -148,6 +153,9 @@ func (bs *BackendServer) GotCommand(cmd *Command) error {
 	} else if c := cmd.FakeExpireAfter; c != nil {
 		bs.b.FakeExpireAfter(c.Duration)
 		return nil
+	} else if c := cmd.Ping; c != nil {
+		bs.b.Ping(c.IP)
+		return nil
 	} else {
 		return fmt.Errorf("BackendServer.Do: no command specified")
 	}
@@ -254,6 +262,10 @@ func (bc *BackendClient) FakeExpireAfter(x time.Duration) {
 	bc.send(Command{FakeExpireAfter: &FakeExpireAfterArgs{Duration: x}})
 }
 
+func (bc *BackendClient) Ping(ip string) {
+	bc.send(Command{Ping: &PingArgs{IP: ip}})
+}
+
 // MaxMessageSize is the maximum message size, in bytes.
 const MaxMessageSize = 10 << 20