.\"	$OpenBSD: lo.4,v 1.12 2000/11/06 22:16:33 matthieu Exp $
.\"	$NetBSD: lo.4,v 1.3 1994/11/30 16:22:23 jtc Exp $
.\"
.\" Copyright (c) 1983, 1991, 1993
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.\"     @(#)lo.4	8.1 (Berkeley) 6/5/93
.\"
.Dd June 5, 1993
.Dt LO 4
.Os
.Sh NAME
.Nm lo
.Nd software loopback network interface
.Sh SYNOPSIS
.Sy pseudo-device Nm loop Em <number>
.Sh DESCRIPTION
The
.Nm 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
.Dv SIOCSIFADDR
.Xr 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
.Em never
be configured first unless no hardware
interfaces exist.
.Pp
Configuring a loopback interface for
.Xr inet 4
with the
.Em link1
flag set will make the interface answer to the whole set of
addresses identified as being in super-net which is specified
by the address and netmask. Obviously you should not set the
.Em link1
flag on interface
.Nm lo0 Ns ,
but instead use another interface like
.Nm lo1 Ns .
.Sh DIAGNOSTICS
.Bl -diag
.It 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.
.El
.Sh EXAMPLES
ifconfig lo1 inet 192.168.1.1 netmask 255.255.255.0 link1
.Pp
is equivalent to:
.Pp
awk 'BEGIN {for(i=1;i<255;i++) \
print "ifconfig lo1 inet 192.168.1."i" netmask 255.255.255.255 alias"}'|sh
.Sh SEE ALSO
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr netintro 4 ,
.Xr ns 4 ,
.Xr ifconfig 8
.Sh HISTORY
The
.Nm
device appeared in
.Bx 4.2 .
.Pp
The wildcard functionality first appeared in
.Ox 2.3 .
.Sh BUGS
Previous versions of the system enabled the loopback interface
automatically, using a nonstandard Internet address (127.1).
Use of that address is now discouraged; a reserved host address
for the local network should be used instead.
.Pp
You should be careful using
.Xr ipnat 8
with
.Em link1
set on an interface, because it may believe the packets are coming
from a loopback address.