XNB(4) BSD Kernel Interfaces Manual XNB(4)
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 net-
front'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 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 <alans@spectralogic.com> and John Suykerbuyk <johns@spectralogic.com>.
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 inter-
face 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.
BSD
June 6, 2014 BSD