cmd/tailscale: fix "tailscale ip $self-host-hostname"

And in the process, fix the related confusing error messages from
pinging your own IP or hostname.

Fixes #2803

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

cmd/tailscale/cli/file.go

@@ -91,7 +91,7 @@ func runCp(ctx context.Context, args []string) error {
 	} else if hadBrackets && (err != nil || !ip.Is6()) {
 		return errors.New("unexpected brackets around target")
 	}
-	ip, err := tailscaleIPFromArg(ctx, target)
+	ip, _, err := tailscaleIPFromArg(ctx, target)
 	if err != nil {
 		return err
 	}

cmd/tailscale/cli/ip.go

@@ -57,7 +57,7 @@ func runIP(ctx context.Context, args []string) error {
 	}
 	ips := st.TailscaleIPs
 	if of != "" {
-		ip, err := tailscaleIPFromArg(ctx, of)
+		ip, _, err := tailscaleIPFromArg(ctx, of)
 		if err != nil {
 			return err
 		}
@@ -101,5 +101,12 @@ func peerMatchingIP(st *ipnstate.Status, ipStr string) (ps *ipnstate.PeerStatus,
 			}
 		}
 	}
+	if ps := st.Self; ps != nil {
+		for _, pip := range ps.TailscaleIPs {
+			if ip == pip {
+				return ps, true
+			}
+		}
+	}
 	return nil, false
 }

cmd/tailscale/cli/ping.go

@@ -84,10 +84,14 @@ func runPing(ctx context.Context, args []string) error {
 	go func() { pumpErr <- pump(ctx, bc, c) }()
 
 	hostOrIP := args[0]
-	ip, err := tailscaleIPFromArg(ctx, hostOrIP)
+	ip, self, err := tailscaleIPFromArg(ctx, hostOrIP)
 	if err != nil {
 		return err
 	}
+	if self {
+		fmt.Printf("%v is local Tailscale IP\n", ip)
+		return nil
+	}
 
 	if pingArgs.verbose && ip != hostOrIP {
 		log.Printf("lookup %q => %q", hostOrIP, ip)
@@ -107,6 +111,10 @@ func runPing(ctx context.Context, args []string) error {
 		case pr := <-prc:
 			timer.Stop()
 			if pr.Err != "" {
+				if pr.IsLocalIP {
+					fmt.Println(pr.Err)
+					return nil
+				}
 				return errors.New(pr.Err)
 			}
 			latency := time.Duration(pr.LatencySeconds * float64(time.Second)).Round(time.Millisecond)
@@ -147,33 +155,39 @@ func runPing(ctx context.Context, args []string) error {
 	}
 }
 
-func tailscaleIPFromArg(ctx context.Context, hostOrIP string) (ip string, err error) {
+func tailscaleIPFromArg(ctx context.Context, hostOrIP string) (ip string, self bool, err error) {
 	// If the argument is an IP address, use it directly without any resolution.
 	if net.ParseIP(hostOrIP) != nil {
-		return hostOrIP, nil
+		return hostOrIP, false, nil
 	}
 
 	// Otherwise, try to resolve it first from the network peer list.
 	st, err := tailscale.Status(ctx)
 	if err != nil {
-		return "", err
+		return "", false, err
+	}
+	match := func(ps *ipnstate.PeerStatus) bool {
+		return strings.EqualFold(hostOrIP, dnsOrQuoteHostname(st, ps)) || hostOrIP == ps.DNSName
 	}
 	for _, ps := range st.Peer {
-		if hostOrIP == dnsOrQuoteHostname(st, ps) || hostOrIP == ps.DNSName {
+		if match(ps) {
 			if len(ps.TailscaleIPs) == 0 {
-				return "", errors.New("node found but lacks an IP")
+				return "", false, errors.New("node found but lacks an IP")
 			}
-			return ps.TailscaleIPs[0].String(), nil
+			return ps.TailscaleIPs[0].String(), false, nil
 		}
 	}
+	if match(st.Self) && len(st.Self.TailscaleIPs) > 0 {
+		return st.Self.TailscaleIPs[0].String(), true, nil
+	}
 
 	// Finally, use DNS.
 	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)
+		return "", false, fmt.Errorf("error looking up IP of %q: %v", hostOrIP, err)
 	} else if len(addrs) == 0 {
-		return "", fmt.Errorf("no IPs found for %q", hostOrIP)
+		return "", false, fmt.Errorf("no IPs found for %q", hostOrIP)
 	} else {
-		return addrs[0], nil
+		return addrs[0], false, nil
 	}
 }