LO(4)                     OpenBSD Programmer's Manual                    LO(4)

NAME
     lo - software loopback network interface

SYNOPSIS
     pseudo-device loop

DESCRIPTION
     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.

     Configuring a loopback interface for inet(4) with the link1 flag set will
     make the interface answer to the whole set of addresses identified as be-
     ing in super-net which is specified by the address and netmask.  Obvious-
     ly you should not set the link1 flag on interface lo0, but instead use
     another interface like lo1.

EXAMPLES
     # ifconfig lo1 create
     # ifconfig lo1 inet 192.168.1.1 netmask 255.255.255.0 link1

     is equivalent to:

     # ifconfig lo1 create
     # awk 'BEGIN {for(i=1;i<255;i++) \
             print "ifconfig lo1 inet 192.168.1."i" netmask 255.255.255.255 alias"}'| \
             sh

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

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

HISTORY
     The lo device appeared in 4.2BSD.

     The wildcard functionality first appeared in OpenBSD 2.3.

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

     Care should be taken when using NAT with interfaces that have the link1
     flag set, because it may believe the packets are coming from a loopback
     address.

OpenBSD 4.7                      May 31, 2007                                1
