cmd/netcheck: better DERP latency checking output

Fixes #206

Signed-off-by: Zijie Lu <zijie@tailscale.com>

cmd/tailscale/netcheck.go

@@ -51,6 +51,13 @@ func runNetcheck(ctx context.Context, args []string) error {
 	}
 	fmt.Printf("\t* MappingVariesByDestIP: %v\n", report.MappingVariesByDestIP)
 	fmt.Printf("\t* HairPinning: %v\n", report.HairPinning)
+
+	// When DERP latency checking failed,
+	// magicsock will try to pick the DERP server that
+	// most of your other nodes are also using
+	if len(report.DERPLatency) == 0 {
+		fmt.Printf("\t* Nearest DERP: unknown (no response to latency probes)\n")
+	} else {
 		fmt.Printf("\t* Nearest DERP: %v (%v)\n", report.PreferredDERP, c.DERP.LocationOfID(report.PreferredDERP))
 		fmt.Printf("\t* DERP latency:\n")
 		var ss []string
@@ -61,5 +68,6 @@ func runNetcheck(ctx context.Context, args []string) error {
 		for _, s := range ss {
 			fmt.Printf("\t\t- %s = %v\n", s, report.DERPLatency[s])
 		}
+	}
 	return nil
 }