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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 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.