LO(4)                                                      BSD 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 or changed with the SIOCSIFADDR ioctl(2).  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 hard-
     ware 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.

BSD                                                               March 15, 2009                                                               BSD