tstest/natlab/vnet: add start of virtual network-based NAT Lab

Updates #13038

Change-Id: I3c74120d73149c1329288621f6474bbbcaa7e1a6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/vnet/run-krazy.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+
+echo "Type 'C-a c' to enter monitor; q to quit."
+
+set -eux
+qemu-system-x86_64 -M microvm,isa-serial=off \
+    -m 1G \
+    -nodefaults -no-user-config -nographic \
+    -kernel $HOME/src/github.com/tailscale/gokrazy-kernel/vmlinuz \
+    -append "console=hvc0 root=PARTUUID=60c24cc1-f3f9-427a-8199-dd02023b0001/PARTNROFF=1 ro init=/gokrazy/init panic=10 oops=panic pci=off nousb tsc=unstable clocksource=hpet tailscale-tta=1" \
+    -drive id=blk0,file=$HOME/src/tailscale.com/gokrazy/tsapp.img,format=raw \
+    -device virtio-blk-device,drive=blk0 \
+    -netdev stream,id=net0,addr.type=unix,addr.path=/tmp/qemu.sock \
+    -device virtio-serial-device \
+    -device virtio-net-device,netdev=net0,mac=52:cc:cc:cc:cc:00 \
+    -chardev stdio,id=virtiocon0,mux=on \
+    -device virtconsole,chardev=virtiocon0 \
+    -mon chardev=virtiocon0,mode=readline \
+    -audio none
+

cmd/vnet/vnet-main.go

@@ -0,0 +1,101 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// The vnet binary runs a virtual network stack in userspace for qemu instances
+// to connect to and simulate various network conditions.
+package main
+
+import (
+	"context"
+	"flag"
+	"log"
+	"net"
+	"os"
+	"time"
+
+	"tailscale.com/tstest/natlab/vnet"
+)
+
+var (
+	listen  = flag.String("listen", "/tmp/qemu.sock", "path to listen on")
+	nat     = flag.String("nat", "easy", "type of NAT to use")
+	portmap = flag.Bool("portmap", false, "enable portmapping")
+	dgram   = flag.Bool("dgram", false, "enable datagram mode; for use with macOS Hypervisor.Framework and VZFileHandleNetworkDeviceAttachment")
+)
+
+func main() {
+	flag.Parse()
+
+	if _, err := os.Stat(*listen); err == nil {
+		os.Remove(*listen)
+	}
+
+	var srv net.Listener
+	var err error
+	var conn *net.UnixConn
+	if *dgram {
+		addr, err := net.ResolveUnixAddr("unixgram", *listen)
+		if err != nil {
+			log.Fatalf("ResolveUnixAddr: %v", err)
+		}
+		conn, err = net.ListenUnixgram("unixgram", addr)
+		if err != nil {
+			log.Fatalf("ListenUnixgram: %v", err)
+		}
+		defer conn.Close()
+	} else {
+		srv, err = net.Listen("unix", *listen)
+	}
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	var c vnet.Config
+	node1 := c.AddNode(c.AddNetwork("2.1.1.1", "192.168.1.1/24", vnet.NAT(*nat)))
+	c.AddNode(c.AddNetwork("2.2.2.2", "10.2.0.1/16", vnet.NAT(*nat)))
+	if *portmap {
+		node1.Network().AddService(vnet.NATPMP)
+	}
+
+	s, err := vnet.New(&c)
+	if err != nil {
+		log.Fatalf("newServer: %v", err)
+	}
+
+	if err := s.PopulateDERPMapIPs(); err != nil {
+		log.Printf("warning: ignoring failure to populate DERP map: %v", err)
+	}
+
+	s.WriteStartingBanner(os.Stdout)
+
+	go func() {
+		getStatus := func() {
+			ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+			defer cancel()
+			st, err := s.NodeStatus(ctx, node1)
+			if err != nil {
+				log.Printf("NodeStatus: %v", err)
+				return
+			}
+			log.Printf("NodeStatus: %q", st)
+		}
+		for {
+			time.Sleep(5 * time.Second)
+			getStatus()
+		}
+	}()
+
+	if conn != nil {
+		s.ServeUnixConn(conn, vnet.ProtocolUnixDGRAM)
+		return
+	}
+
+	for {
+		c, err := srv.Accept()
+		if err != nil {
+			log.Printf("Accept: %v", err)
+			continue
+		}
+		go s.ServeUnixConn(c.(*net.UnixConn), vnet.ProtocolQEMU)
+	}
+}