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
       domain access to other guest domains or to a physical network.

       In most respects, the xnb device appears to the OS as any 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 log.

SEE ALSO

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

HISTORY

       The xnb device driver first appeared in FreeBSD 10.0.

AUTHORS

       The xnb driver was written by Alan Somers <asomers@FreeBSD.org> and John Suykerbuyk.

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 physical 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.

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                                            June 6, 2014                                            XNB(4)