1.\" $NetBSD: lo.4,v 1.10 2006/09/02 23:40:22 wiz Exp $ 2.\" 3.\" Copyright (c) 1983, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" @(#)lo.4 8.1 (Berkeley) 6/5/93 31.\" 32.Dd September 3, 2006 33.Dt LO 4 34.Os 35.Sh NAME 36.Nm lo 37.Nd software loopback network interface 38.Sh SYNOPSIS 39.Cd "pseudo-device loop" 40.Sh DESCRIPTION 41The 42.Nm loop 43interface is a software loopback mechanism which may be used for 44performance analysis, software testing, and/or local communication. 45As with other network interfaces, the loopback interface must have 46network addresses assigned for each address family with which it 47is to be used. 48These addresses may be set or changed with the 49.Dv SIOCSIFADDR 50.Xr ioctl 2 . 51The loopback interface should be the last interface configured, as 52protocols may use the order of configuration as an indication of 53priority. 54The loopback should 55.Em never 56be configured first unless no hardware interfaces exist. 57.Pp 58The loopback interface 59.Nm lo0 60is created at boottime, it always exists and cannot be destroyed 61with 62.Xr ifconfig 8 . 63Additional loopback interfaces can be created by using the 64.Xr ifconfig 8 65.Cm create 66command. 67.Sh DIAGNOSTICS 68.Bl -diag 69.It lo%d: can't handle af%d . 70The interface was handed a message with addresses formatted in an 71unsuitable address family; the packet was dropped. 72.El 73.Sh SEE ALSO 74.Xr inet 4 , 75.Xr intro 4 , 76.Xr ifconfig 8 77.Sh HISTORY 78The 79.Nm 80device appeared in 81.Bx 4.2 . 82.Sh BUGS 83Previous versions of the system enabled the loopback interface 84automatically, using a nonstandard Internet address (127.1). 85Use of that address is now discouraged; a reserved host address 86for the local network should be used instead. 87