1.\" $NetBSD: lo.4,v 1.8 2004/12/04 16:47:10 peter 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 June 5, 1993 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 44used for performance analysis, software testing, and/or local 45communication. 46As with other network interfaces, the loopback interface must have 47network addresses assigned for each address family with which it is to be used. 48These addresses 49may be set or changed with the 50.Dv SIOCSIFADDR 51.Xr ioctl 2 . 52The loopback interface should be the last interface configured, 53as protocols may use the order of configuration as an indication of priority. 54The loopback should 55.Em never 56be configured first unless no hardware 57interfaces exist. 58.Pp 59The loopback interface 60.Nm lo0 61is created at boottime, it always exists and cannot be destroyed with 62.Xr ifconfig 8 . 63Additional loopback interfaces can be created by using the 64.Xr ifconfig 8 65.Sq create 66command. 67.Sh DIAGNOSTICS 68.Bl -diag 69.It lo%d: can't handle af%d . 70The interface was handed 71a message with addresses formatted in an unsuitable address 72family; the packet was dropped. 73.El 74.Sh SEE ALSO 75.Xr inet 4 , 76.Xr intro 4 , 77.Xr ns 4 , 78.Xr ifconfig 8 79.Sh HISTORY 80The 81.Nm 82device appeared in 83.Bx 4.2 . 84.Sh BUGS 85Previous versions of the system enabled the loopback interface 86automatically, using a nonstandard Internet address (127.1). 87Use of that address is now discouraged; a reserved host address 88for the local network should be used instead. 89