cmd/tailscale: make "tailscale ping" also resolve names without DNS

This lets "tailscale ping $NAME" work even if MagicDNS is off, letting you
ping a name that shows up in "tailscale status".

More user friendly.

cmd/tailscale/cli/ping.go

@@ -64,11 +64,50 @@ func runPing(ctx context.Context, args []string) error {
 	c, bc, ctx, cancel := connect(ctx)
 	defer cancel()
 
-	if len(args) != 1 {
+	if len(args) != 1 || args[0] == "" {
 		return errors.New("usage: ping <hostname-or-IP>")
 	}
-	hostOrIP := args[0]
 	var ip string
+	prc := make(chan *ipnstate.PingResult, 1)
+	stc := make(chan *ipnstate.Status, 1)
+	bc.SetNotifyCallback(func(n ipn.Notify) {
+		if n.ErrMessage != nil {
+			log.Fatal(*n.ErrMessage)
+		}
+		if pr := n.PingResult; pr != nil && pr.IP == ip {
+			prc <- pr
+		}
+		if n.Status != nil {
+			stc <- n.Status
+		}
+	})
+	go pump(ctx, bc, c)
+
+	hostOrIP := args[0]
+
+	// If the argument is an IP address, use it directly without any resolution.
+	if net.ParseIP(hostOrIP) != nil {
+		ip = hostOrIP
+	}
+
+	// Otherwise, try to resolve it first from the network peer list.
+	if ip == "" {
+		bc.RequestStatus()
+		select {
+		case st := <-stc:
+			for _, ps := range st.Peer {
+				if hostOrIP == dnsOrQuoteHostname(st, ps) || hostOrIP == ps.DNSName {
+					ip = ps.TailAddr
+					break
+				}
+			}
+		case <-ctx.Done():
+			return ctx.Err()
+		}
+	}
+
+	// Finally, use DNS.
+	if ip == "" {
 		var res net.Resolver
 		if addrs, err := res.LookupHost(ctx, hostOrIP); err != nil {
 			return fmt.Errorf("error looking up IP of %q: %v", hostOrIP, err)
@@ -77,21 +116,11 @@ func runPing(ctx context.Context, args []string) error {
 		} else {
 			ip = addrs[0]
 		}
+	}
 	if pingArgs.verbose && ip != hostOrIP {
 		log.Printf("lookup %q => %q", hostOrIP, ip)
 	}
 
-	ch := make(chan *ipnstate.PingResult, 1)
-	bc.SetNotifyCallback(func(n ipn.Notify) {
-		if n.ErrMessage != nil {
-			log.Fatal(*n.ErrMessage)
-		}
-		if pr := n.PingResult; pr != nil && pr.IP == ip {
-			ch <- pr
-		}
-	})
-	go pump(ctx, bc, c)
-
 	n := 0
 	anyPong := false
 	for {
@@ -101,7 +130,7 @@ func runPing(ctx context.Context, args []string) error {
 		select {
 		case <-timer.C:
 			fmt.Printf("timeout waiting for ping reply\n")
-		case pr := <-ch:
+		case pr := <-prc:
 			timer.Stop()
 			if pr.Err != "" {
 				return errors.New(pr.Err)