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LO(4)                    BSD Kernel Interfaces Manual                    LO(4)

     lo -- software loopback network interface

     pseudo-device loop

     The loop interface is a software loopback mechanism which may be used for
     performance analysis, software testing, and/or local communication.

     A loop interface can be created at runtime using the ifconfig loN create
     command or by setting up a hostname.if(5) configuration file for
     netstart(8).  The lo0 interface will always exist and cannot be destroyed
     using ifconfig(8).

     As with other network interfaces, the loopback interface must have net-
     work 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 config-
     ured, 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.

     lo0: can't handle afX.  The interface was handed a message with addresses
     formatted in an unsuitable address family; the packet was dropped.

     inet(4), inet6(4), netintro(4), hostname.if(5), ifconfig(8), netstart(8)

     The lo device appeared in 4.2BSD.

     Previous versions of the system enabled the loopback interface automati-
     cally, using a non-standard Internet address (127.1).  Use of that
     address is now discouraged; a reserved host address for the local network
     should be used instead.

BSD                              June 2, 2013                              BSD