LO(4)                  FreeBSD Kernel Interfaces Manual                  LO(4)

NAME
     lo - software loopback network interface

SYNOPSIS
     device loop

DESCRIPTION
     The loop interface is a software loopback mechanism which may be used for
     performance analysis, software testing, and/or local communication.  As
     with other network interfaces, the loopback interface must have network
     addresses assigned for each address family with which it is to be used.
     These addresses may be set with the appropriate ioctl(2) commands for
     corresponding address families.  The loopback interface should be the
     last interface configured, as protocols may use the order of
     configuration as an indication of priority.  The loopback should never be
     configured first unless no hardware interfaces exist.

     If the transmit checksum offload capability flag is enabled on a loopback
     interface, checksums will not be generated by IP, UDP, or TCP for packets
     sent on the interface.

     If the receive checksum offload capability flag is enabled on a loopback
     interface, checksums will not be validated by IP, UDP, or TCP for packets
     received on the interface.

     By default, both receive and transmit checksum flags will be enabled, in
     order to avoid the overhead of checksumming for local communication where
     data corruption is unlikely.  If transmit checksum generation is
     disabled, then validation should also be disabled in order to avoid
     packets being dropped due to invalid checksums.

DIAGNOSTICS
     lo%d: can't handle af%d.  The interface was handed a message with
     addresses formatted in an unsuitable address family; the packet was
     dropped.

SEE ALSO
     inet(4), intro(4)

HISTORY
     The lo device appeared in 4.2BSD.  The current checksum generation and
     validation avoidance policy appeared in FreeBSD 8.0.

FreeBSD 13.1-RELEASE-p6        January 25, 2012        FreeBSD 13.1-RELEASE-p6