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>

net/stunserver/stunserver_test.go

@@ -0,0 +1,88 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package stunserver
+
+import (
+	"context"
+	"net"
+	"sync"
+	"testing"
+	"time"
+
+	"tailscale.com/net/stun"
+	"tailscale.com/util/must"
+)
+
+func TestSTUNServer(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	s := New(ctx)
+	must.Do(s.Listen("localhost:0"))
+	var w sync.WaitGroup
+	w.Add(1)
+	var serveErr error
+	go func() {
+		defer w.Done()
+		serveErr = s.Serve()
+	}()
+
+	c := must.Get(net.DialUDP("udp", nil, s.LocalAddr().(*net.UDPAddr)))
+	defer c.Close()
+	c.SetDeadline(time.Now().Add(5 * time.Second))
+	txid := stun.NewTxID()
+	_, err := c.Write(stun.Request(txid))
+	if err != nil {
+		t.Fatalf("failed to write STUN request: %v", err)
+	}
+	var buf [64 << 10]byte
+	n, err := c.Read(buf[:])
+	if err != nil {
+		t.Fatalf("failed to read STUN response: %v", err)
+	}
+	if !stun.Is(buf[:n]) {
+		t.Fatalf("response is not STUN")
+	}
+	tid, _, err := stun.ParseResponse(buf[:n])
+	if err != nil {
+		t.Fatalf("failed to parse STUN response: %v", err)
+	}
+	if tid != txid {
+		t.Fatalf("STUN response has wrong transaction ID; got %d, want %d", tid, txid)
+	}
+
+	cancel()
+	w.Wait()
+	if serveErr != nil {
+		t.Fatalf("failed to listen and serve: %v", serveErr)
+	}
+}
+
+func BenchmarkServerSTUN(b *testing.B) {
+	b.ReportAllocs()
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	s := New(ctx)
+	s.Listen("localhost:0")
+	go s.Serve()
+	addr := s.LocalAddr().(*net.UDPAddr)
+
+	var resBuf [1500]byte
+	cc, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.ParseIP("127.0.0.1")})
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	tx := stun.NewTxID()
+	req := stun.Request(tx)
+	for i := 0; i < b.N; i++ {
+		if _, err := cc.WriteToUDP(req, addr); err != nil {
+			b.Fatal(err)
+		}
+		_, _, err := cc.ReadFromUDP(resBuf[:])
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}