.\"	$OpenBSD: lo.4,v 1.26 2007/05/31 19:19:50 jmc Exp $
.\"	$NetBSD: lo.4,v 1.3 1994/11/30 16:22:23 jtc Exp $
.\"
.\" Copyright (c) 1983, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)lo.4	8.1 (Berkeley) 6/5/93
.\"
.Dd $Mdocdate: May 31 2007 $
.Dt LO 4
.Os
.Sh NAME
.Nm lo
.Nd software loopback network interface
.Sh SYNOPSIS
.Cd "pseudo-device loop"
.Sh DESCRIPTION
The
.Nm loop
interface is a software loopback mechanism which may be
used for performance analysis, software testing, and/or local
communication.
.Pp
A
.Nm loop
interface can be created at runtime using the
.Ic ifconfig lo Ns Ar N Ic create
command or by setting up a
.Xr hostname.if 5
configuration file for
.Xr netstart 8 .
The
.Nm lo0
interface will always exist and cannot be destroyed using
.Xr ifconfig 8 .
.Pp
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 ,
but instead use another interface like
.Nm lo1 .
.Sh EXAMPLES
.Bd -literal
# ifconfig lo1 create
# ifconfig lo1 inet 192.168.1.1 netmask 255.255.255.0 link1
.Ed
.Pp
is equivalent to:
.Bd -literal
# ifconfig lo1 create
# awk 'BEGIN {for(i=1;i<255;i++) \e
	print "ifconfig lo1 inet 192.168.1."i" netmask 255.255.255.255 alias"}'| \e
	sh
.Ed
.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 SEE ALSO
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr netintro 4 ,
.Xr hostname.if 5 ,
.Xr ifconfig 8 ,
.Xr netstart 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 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.
.Pp
Care should be taken when using NAT with interfaces that have the
.Em link1
flag set, because it may believe the packets are coming from a
loopback address.