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XNB(4)			     Kernel Interfaces Manual			  XNB(4)

NAME
     xnb -- Xen Paravirtualized Backend Ethernet Driver

SYNOPSIS
     To compile this driver into the kernel, place the following lines in your
     kernel configuration file:

	   options XENHVM
	   device xenpci

DESCRIPTION
     The  xnb  driver provides the back half of a paravirtualized xen(4) network
     connection.  The netback and netfront drivers appear  to  their  respective
     operating	systems  as Ethernet devices linked by a crossover cable.  Typi-
     cally, xnb will run on Domain 0 and the netfront driver will run on a guest
     domain.  However, it is also possible to run xnb on a guest domain.  It may
     be bridged or routed to provide the netfront's domain access to other guest
     domains or to a physical network.

     In most respects, the xnb device appears to the OS as an other Ethernet de-
     vice.  It can be configured at runtime entirely with ifconfig(8) .  In par-
     ticular, it supports MAC changing, arbitrary MTU  sizes,  checksum  offload
     for  IP, UDP, and TCP for both receive and transmit, and TSO.  However, see
     "CAVEATS" before enabling txcsum, rxcsum, or tso.

SYSCTL VARIABLES
     The following read-only variables are available via sysctl(8):

     dev.xnb.%d.dump_rings
	     Displays information about the ring buffers used to  pass	requests
	     between the netfront and netback.	Mostly useful for debugging, but
	     can also be used to get traffic statistics.

     dev.xnb.%d.unit_test_results
	     Runs  a builtin suite of unit tests and displays the results.  Does
	     not affect the operation of the driver in any way.  Note  that  the
	     test  suite  simulates  error conditions; this will result in error
	     messages being printed to the system system log.

CAVEATS
     Packets sent through Xennet pass over shared memory, so  the  protocol  in-
     cludes  no form of link-layer checksum or CRC.  Furthermore, Xennet drivers
     always report to their hosts that they support receive and transmit  check-
     sum offloading.  They "offload" the checksum calculation by simply skipping
     it.   That works fine for packets that are exchanged between two domains on
     the same machine.	However, when a Xennet interface is bridged to a  physi-
     cal interface, a correct checksum must be attached to any packets bound for
     that  physical  interface.   Currently,  FreeBSD lacks any mechanism for an
     ethernet device to inform the OS that newly received packets are valid even
     though their checksums are not.  So if the netfront driver is configured to
     offload checksum calculations, it will pass non-checksumed packets to xnb ,
     which must then calculate the  checksum  in  software  before  passing  the
     packet to the OS.

     For  this reason, it is recommended that if xnb is bridged to a physcal in-
     terface, then transmit checksum offloading should be disabled on  the  net-
     front.   The Xennet protocol does not have any mechanism for the netback to
     request the netfront to do this; the operator must do it manually.

SEE ALSO
     arp(4), netintro(4), ng_ether(4), ifconfig(8), xen(4)

HISTORY
     The xnb device driver first appeared in FreeBSD 10.0

AUTHORS
     The xnb driver was written by Alan Somers <alans@spectralogic.com> and
     John Suykerbuyk <johns@spectralogic.com>.

BUGS
     The xnb driver does not properly checksum UDP datagrams that span more than
     one Ethernet frame.  Nor does  it	correctly  checksum  IPv6  packets.   To
     workaround  that  bug, disable transmit checksum offloading on the netfront
     driver.

FreeBSD 9.1			 January 6, 2012			  XNB(4)
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