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     xnb — Xen Paravirtualized Backend Ethernet Driver


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

           options XENHVM
           device xenpci


     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.


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

             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.

             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.


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


     The xnb device driver first appeared in FreeBSD 10.0.


     The xnb driver was written by Alan Somers <> and John Suykerbuyk.


     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.


     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.