Removed IFF_NO_PI from TUN linux

This change was needed for the Linux TUN status hack
to work properly (not increment the error counter).

This commit also updates the TUN interface to allow for
the construction / removal of the TUN info headers in-place.

src/tun_linux.go

@@ -7,6 +7,7 @@ import (
 	"encoding/binary"
 	"errors"
 	"fmt"
+	"golang.org/x/net/ipv6"
 	"golang.org/x/sys/unix"
 	"net"
 	"os"
@@ -249,16 +250,41 @@ func (tun *NativeTun) MTU() (int, error) {
 	return int(val), nil
 }
 
-func (tun *NativeTun) Write(d []byte) (int, error) {
-	return tun.fd.Write(d)
+func (tun *NativeTun) Write(buff []byte, offset int) (int, error) {
+
+	// reserve space for header
+
+	buff = buff[offset-4:]
+
+	// add packet information header
+
+	buff[0] = 0x00
+	buff[1] = 0x00
+
+	if buff[4] == ipv6.Version<<4 {
+		buff[2] = 0x86
+		buff[3] = 0xdd
+	} else {
+		buff[2] = 0x08
+		buff[3] = 0x00
+	}
+
+	// write
+
+	return tun.fd.Write(buff)
 }
 
-func (tun *NativeTun) Read(d []byte) (int, error) {
+func (tun *NativeTun) Read(buff []byte, offset int) (int, error) {
 	select {
 	case err := <-tun.errors:
 		return 0, err
 	default:
-		return tun.fd.Read(d)
+		buff := buff[offset-4:]
+		n, err := tun.fd.Read(buff[:])
+		if n < 4 {
+			return 0, err
+		}
+		return n - 4, err
 	}
 }
 
@@ -306,7 +332,7 @@ func CreateTUN(name string) (TUNDevice, error) {
 	// create new device
 
 	var ifr [IFReqSize]byte
-	var flags uint16 = unix.IFF_TUN | unix.IFF_NO_PI
+	var flags uint16 = unix.IFF_TUN // | unix.IFF_NO_PI
 	nameBytes := []byte(name)
 	if len(nameBytes) >= unix.IFNAMSIZ {
 		return nil, errors.New("Interface name too long")