1.\" $OpenBSD: lo.4,v 1.28 2013/06/02 20:23:33 tedu Exp $ 2.\" $NetBSD: lo.4,v 1.3 1994/11/30 16:22:23 jtc Exp $ 3.\" 4.\" Copyright (c) 1983, 1991, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of the University nor the names of its contributors 16.\" may be used to endorse or promote products derived from this software 17.\" without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 20.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 21.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 22.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 23.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 24.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 25.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29.\" SUCH DAMAGE. 30.\" 31.\" @(#)lo.4 8.1 (Berkeley) 6/5/93 32.\" 33.Dd $Mdocdate: June 2 2013 $ 34.Dt LO 4 35.Os 36.Sh NAME 37.Nm lo 38.Nd software loopback network interface 39.Sh SYNOPSIS 40.Cd "pseudo-device loop" 41.Sh DESCRIPTION 42The 43.Nm loop 44interface is a software loopback mechanism which may be 45used for performance analysis, software testing, and/or local 46communication. 47.Pp 48A 49.Nm loop 50interface can be created at runtime using the 51.Ic ifconfig lo Ns Ar N Ic create 52command or by setting up a 53.Xr hostname.if 5 54configuration file for 55.Xr netstart 8 . 56The 57.Nm lo0 58interface will always exist and cannot be destroyed using 59.Xr ifconfig 8 . 60.Pp 61As with other network interfaces, the loopback interface must have 62network addresses assigned for each address family with which it is to be used. 63These addresses 64may be set or changed with the 65.Dv SIOCSIFADDR 66.Xr ioctl 2 . 67The loopback interface should be the last interface configured, 68as protocols may use the order of configuration as an indication of priority. 69The loopback should 70.Em never 71be configured first unless no hardware 72interfaces exist. 73.Sh DIAGNOSTICS 74.Bl -diag 75.It lo0: can't handle afX. 76The interface was handed 77a message with addresses formatted in an unsuitable address 78family; the packet was dropped. 79.El 80.Sh SEE ALSO 81.Xr inet 4 , 82.Xr inet6 4 , 83.Xr netintro 4 , 84.Xr hostname.if 5 , 85.Xr ifconfig 8 , 86.Xr netstart 8 87.Sh HISTORY 88The 89.Nm 90device appeared in 91.Bx 4.2 . 92.Sh BUGS 93Previous versions of the system enabled the loopback interface 94automatically, using a non-standard Internet address (127.1). 95Use of that address is now discouraged; a reserved host address 96for the local network should be used instead. 97