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.  Typically, 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 device.  It can be configured  at
       runtime  entirely  with  ifconfig(8)  .   In  particular,  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  includes  no  form  of  link-layer
       checksum  or CRC.  Furthermore, Xennet drivers always report to their hosts that they support receive and
       transmit checksum 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 physical 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 interface, then transmit  checksum
       offloading  should  be disabled on the netfront.  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.

Debian                                           January 6, 2012                                          XNB(4)