cmd/stunc: add command for making debug stun requests

Signed-off-by: James Tucker <james@tailscale.com>

cmd/stunc/stunc.go

@@ -0,0 +1,58 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Command stunc makes a STUN request to a STUN server and prints the result.
+package main
+
+import (
+	"log"
+	"net"
+	"os"
+
+	"tailscale.com/net/stun"
+)
+
+func main() {
+	log.SetFlags(0)
+
+	if len(os.Args) != 2 {
+		log.Fatalf("usage: %s <hostname>", os.Args[0])
+	}
+	host := os.Args[1]
+
+	uaddr, err := net.ResolveUDPAddr("udp", host+":3478")
+	if err != nil {
+		log.Fatal(err)
+	}
+	c, err := net.ListenUDP("udp", nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	txID := stun.NewTxID()
+	req := stun.Request(txID)
+
+	_, err = c.WriteToUDP(req, uaddr)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	var buf [1024]byte
+	n, raddr, err := c.ReadFromUDPAddrPort(buf[:])
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	tid, saddr, err := stun.ParseResponse(buf[:n])
+	if err != nil {
+		log.Fatal(err)
+	}
+	if tid != txID {
+		log.Fatalf("txid mismatch: got %v, want %v", tid, txID)
+	}
+
+	log.Printf("sent addr: %v", uaddr)
+	log.Printf("from addr: %v", raddr)
+	log.Printf("stun addr: %v", saddr)
+}