tun/netstack: implement ICMP ping

Provide a PacketConn interface for netstack's ICMP endpoint; netstack
currently only provides EchoRequest/EchoResponse ICMP support, so this
code exposes only an interface for doing ping.

Signed-off-by: Thomas Ptacek <thomas@sockpuppet.org>
[Jason: rework structure, match std go interfaces, add example code]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tun/netstack/examples/ping_client.go

@@ -0,0 +1,76 @@
+//go:build ignore
+// +build ignore
+
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2019-2021 WireGuard LLC. All Rights Reserved.
+ */
+
+package main
+
+import (
+	"bytes"
+	"log"
+	"math/rand"
+	"time"
+
+	"golang.org/x/net/icmp"
+	"golang.org/x/net/ipv4"
+
+	"golang.zx2c4.com/go118/netip"
+	"golang.zx2c4.com/wireguard/conn"
+	"golang.zx2c4.com/wireguard/device"
+	"golang.zx2c4.com/wireguard/tun/netstack"
+)
+
+func main() {
+	tun, tnet, err := netstack.CreateNetTUN(
+		[]netip.Addr{netip.MustParseAddr("192.168.4.29")},
+		[]netip.Addr{netip.MustParseAddr("8.8.8.8")},
+		1420)
+	if err != nil {
+		log.Panic(err)
+	}
+	dev := device.NewDevice(tun, conn.NewDefaultBind(), device.NewLogger(device.LogLevelVerbose, ""))
+	dev.IpcSet(`private_key=a8dac1d8a70a751f0f699fb14ba1cff7b79cf4fbd8f09f44c6e6a90d0369604f
+public_key=25123c5dcd3328ff645e4f2a3fce0d754400d3887a0cb7c56f0267e20fbf3c5b
+endpoint=163.172.161.0:12912
+allowed_ip=0.0.0.0/0
+`)
+	err = dev.Up()
+	if err != nil {
+		log.Panic(err)
+	}
+
+	socket, err := tnet.Dial("ping4", "zx2c4.com")
+	if err != nil {
+		log.Panic(err)
+	}
+	requestPing := icmp.Echo{
+		Seq:  rand.Intn(1 << 16),
+		Data: []byte("gopher burrow"),
+	}
+	icmpBytes, _ := (&icmp.Message{Type: ipv4.ICMPTypeEcho, Code: 0, Body: &requestPing}).Marshal(nil)
+	socket.SetReadDeadline(time.Now().Add(time.Second * 10))
+	start := time.Now()
+	_, err = socket.Write(icmpBytes)
+	if err != nil {
+		log.Panic(err)
+	}
+	n, err := socket.Read(icmpBytes[:])
+	if err != nil {
+		log.Panic(err)
+	}
+	replyPacket, err := icmp.ParseMessage(1, icmpBytes[:n])
+	if err != nil {
+		log.Panic(err)
+	}
+	replyPing, ok := replyPacket.Body.(*icmp.Echo)
+	if !ok {
+		log.Panicf("invalid reply type: %v", replyPacket)
+	}
+	if !bytes.Equal(replyPing.Data, requestPing.Data) || replyPing.Seq != requestPing.Seq {
+		log.Panicf("invalid ping reply: %v", replyPing)
+	}
+	log.Printf("Ping latency: %v", time.Since(start))
+}