1.\" Copyright (c) 1983, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)lo.4 8.1 (Berkeley) 6/5/93 29.\" $FreeBSD: src/share/man/man4/lo.4,v 1.7.2.1 2001/08/17 13:08:38 ru Exp $ 30.\" $DragonFly: src/share/man/man4/lo.4,v 1.2 2003/06/17 04:36:59 dillon Exp $ 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.Sh DIAGNOSTICS 59.Bl -diag 60.It lo%d: can't handle af%d. 61The interface was handed 62a message with addresses formatted in an unsuitable address 63family; the packet was dropped. 64.El 65.Sh SEE ALSO 66.Xr inet 4 , 67.Xr intro 4 68.\" .Xr ns 4 69.Sh HISTORY 70The 71.Nm 72device appeared in 73.Bx 4.2 . 74.Sh BUGS 75Previous versions of the system enabled the loopback interface 76automatically, using a nonstandard Internet address (127.1). 77Use of that address is now discouraged; a reserved host address 78for the local network should be used instead. 79