Synnefo » snf-ganeti

import struct

import fcntl

# TUN/TAP driver constants, taken from <linux/if_tun.h>

# They are architecture-independent and already hardcoded in qemu-kvm source,

# so we can safely include them here.

TUNSETIFF = 0x400454ca

TUNGETIFF = 0x800454d2

TUNGETFEATURES = 0x800454cf

IFF_TAP = 0x0002

IFF_NO_PI = 0x1000

IFF_VNET_HDR = 0x4000

def _ProbeTapVnetHdr(fd):

  """Check whether to enable the IFF_VNET_HDR flag.

  To do this, _all_ of the following conditions must be met:

   1. TUNGETFEATURES ioctl() *must* be implemented

   2. TUNGETFEATURES ioctl() result *must* contain the IFF_VNET_HDR flag

   3. TUNGETIFF ioctl() *must* be implemented; reading the kernel code in

      drivers/net/tun.c there is no way to test this until after the tap device

      has been created using TUNSETIFF, and there is no way to change the

      IFF_VNET_HDR flag after creating the interface, catch-22! However both

      TUNGETIFF and TUNGETFEATURES were introduced in kernel version 2.6.27,

      thus we can expect TUNGETIFF to be present if TUNGETFEATURES is.

   @type fd: int

   @param fd: the file descriptor of /dev/net/tun

  """

  req = struct.pack("I", 0)

  try:

    res = fcntl.ioctl(fd, TUNGETFEATURES, req)

  except EnvironmentError:

    logging.warning("TUNGETFEATURES ioctl() not implemented")

    return False

  tunflags = struct.unpack("I", res)[0]

  if tunflags & IFF_VNET_HDR:

    return True

  else:

    logging.warning("Host does not support IFF_VNET_HDR, not enabling")

    return False

def _OpenTap(vnet_hdr=True):

  """Open a new tap device and return its file descriptor.

  This is intended to be used by a qemu-type hypervisor together with the -net

  tap,fd=<fd> command line parameter.

  @type vnet_hdr: boolean

  @param vnet_hdr: Enable the VNET Header

  @return: (ifname, tapfd)

  @rtype: tuple

  """

  try:

    tapfd = os.open("/dev/net/tun", os.O_RDWR)

  except EnvironmentError:

    raise errors.HypervisorError("Failed to open /dev/net/tun")

  flags = IFF_TAP | IFF_NO_PI

  if vnet_hdr and _ProbeTapVnetHdr(tapfd):

    flags |= IFF_VNET_HDR

  # The struct ifreq ioctl request (see netdevice(7))

  ifr = struct.pack("16sh", "", flags)

  try:

    res = fcntl.ioctl(tapfd, TUNSETIFF, ifr)

  except EnvironmentError:

    raise errors.HypervisorError("Failed to allocate a new TAP device")

  # Get the interface name from the ioctl

  ifname = struct.unpack("16sh", res)[0].strip("\x00")

  return (ifname, tapfd)