cmd/{derper,stund},net/stunserver: add standalone stun server

Add a standalone server for STUN that can be hosted independently of the
derper, and factor that back into the derper.

Fixes #8434
Closes #8435
Closes #10745

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

cmd/stund/stund.go

@@ -0,0 +1,48 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// The stund binary is a standalone STUN server.
+package main
+
+import (
+	"context"
+	"flag"
+	"io"
+	"log"
+	"net/http"
+	"os/signal"
+	"syscall"
+
+	"tailscale.com/net/stunserver"
+	"tailscale.com/tsweb"
+)
+
+var (
+	stunAddr = flag.String("stun", ":3478", "UDP address on which to start the STUN server")
+	httpAddr = flag.String("http", ":3479", "address on which to start the debug http server")
+)
+
+func main() {
+	flag.Parse()
+
+	ctx, cancel := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer cancel()
+
+	log.Printf("HTTP server listening on %s", *httpAddr)
+	go http.ListenAndServe(*httpAddr, mux())
+
+	s := stunserver.New(ctx)
+	if err := s.ListenAndServe(*stunAddr); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func mux() *http.ServeMux {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		io.WriteString(w, "<h1>stund</h1><a href=/debug>/debug</a>")
+	})
+	debug := tsweb.Debugger(mux)
+	debug.KV("stun_addr", *stunAddr)
+	return mux
+}