check inner packet srcIP against server-verified srcIP, Windows support

client/os.go

@@ -0,0 +1,61 @@
+//go:build linux || darwin
+
+package main
+
+import (
+	"os"
+	"os/exec"
+	"runtime"
+
+	"github.com/songgao/water"
+)
+
+var iface *water.Interface
+
+func isAdmin() bool {
+	return os.Geteuid() == 0
+}
+
+func setupInterface() {
+	config := water.Config{DeviceType: water.TUN}
+	if runtime.GOOS == "linux" {
+		config.Name = "baalvpn"
+	} else {
+		panic("TODO")
+	}
+
+	var err error
+	iface, err = water.New(config)
+	if err != nil {
+		panic(err)
+	}
+
+	if runtime.GOOS == "linux" {
+		if err := exec.Command("ip", "addr", "add", internalIP.String()+"/32", "dev", "baalvpn").Run(); err != nil {
+			panic(err)
+		}
+		if err := exec.Command("ip", "link", "set", "dev", "baalvpn", "up").Run(); err != nil {
+			panic(err)
+		}
+	} else {
+		if err := exec.Command("ifconfig", iface.Name(), "inet6", internalIP.String(), internalIP.String(), "prefixlen", "128", "up").Run(); err != nil {
+			panic(err)
+		}
+	}
+}
+
+func writePkt(pkt []byte) error {
+	_, err := iface.Write(pkt)
+	return err
+}
+
+func readPkt() ([]byte, error) {
+	pkt := make([]byte, 2000)
+	n, err := iface.Read(pkt)
+	if err != nil {
+		return nil, err
+	}
+	return pkt[:n], nil
+}
+
+func releasePkt(pkt []byte) {}