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UNTITLED                             LOCAL                            UNTITLED

     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'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.

     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 system log.

     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

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

     The xnb device driver first appeared in FreeBSD 10.0

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

     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 11.0-PRERELEASE         January 6, 2012        FreeBSD 11.0-PRERELEASE


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