Beginning work on TUN interface

And outbound routing

I am not entirely convinced the use of net.IP is a good idea,
since the internal representation of net.IP is a byte slice
and all constructor functions in "net" return 16 byte slices
(padded for IPv4), while the use in this project uses 4 byte slices.
Which may be confusing.

src/tun_linux.go

@@ -0,0 +1,80 @@
+package main
+
+import (
+	"encoding/binary"
+	"errors"
+	"os"
+	"strings"
+	"syscall"
+	"unsafe"
+)
+
+/* Platform dependent functions for interacting with
+ * TUN devices on linux systems
+ *
+ */
+
+const CloneDevicePath = "/dev/net/tun"
+
+const (
+	IFF_NO_PI = 0x1000
+	IFF_TUN   = 0x1
+	IFNAMSIZ  = 0x10
+	TUNSETIFF = 0x400454CA
+)
+
+type NativeTun struct {
+	fd   *os.File
+	name string
+	mtu  uint
+}
+
+func (tun *NativeTun) Name() string {
+	return tun.name
+}
+
+func (tun *NativeTun) MTU() uint {
+	return tun.mtu
+}
+
+func (tun *NativeTun) Write(d []byte) (int, error) {
+	return tun.fd.Write(d)
+}
+
+func (tun *NativeTun) Read(d []byte) (int, error) {
+	return tun.fd.Read(d)
+}
+
+func CreateTUN(name string) (TUN, error) {
+	// Open clone device
+	fd, err := os.OpenFile(CloneDevicePath, os.O_RDWR, 0)
+	if err != nil {
+		return nil, err
+	}
+
+	// Prepare ifreq struct
+	var ifr [18]byte
+	var flags uint16 = IFF_TUN | IFF_NO_PI
+	nameBytes := []byte(name)
+	if len(nameBytes) >= IFNAMSIZ {
+		return nil, errors.New("Name size too long")
+	}
+	copy(ifr[:], nameBytes)
+	binary.LittleEndian.PutUint16(ifr[16:], flags)
+
+	// Create new device
+	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL,
+		uintptr(fd.Fd()), uintptr(TUNSETIFF),
+		uintptr(unsafe.Pointer(&ifr[0])))
+	if errno != 0 {
+		return nil, errors.New("Failed to create tun, ioctl call failed")
+	}
+
+	// Read name of interface
+	newName := string(ifr[:])
+	newName = newName[:strings.Index(newName, "\000")]
+	return &NativeTun{
+		fd:   fd,
+		name: newName,
+	}, nil
+}