bionic (4) xnb.4freebsd.gz

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.