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.

Debian                                           March 15, 2009                                            LO(4)