1.\" $NetBSD: lo.4,v 1.3 1994/11/30 16:22:23 jtc 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. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)lo.4 8.1 (Berkeley) 6/5/93 35.\" 36.Dd June 5, 1993 37.Dt LO 4 38.Os BSD 4.2 39.Sh NAME 40.Nm lo 41.Nd software loopback network interface 42.Sh SYNOPSIS 43.Sy pseudo-device 44.Nm loop 45.Sh DESCRIPTION 46The 47.Nm loop 48interface is a software loopback mechanism which may be 49used for performance analysis, software testing, and/or local 50communication. 51As with other network interfaces, the loopback interface must have 52network addresses assigned for each address family with which it is to be used. 53These addresses 54may be set or changed with the 55.Dv SIOCSIFADDR 56.Xr ioctl 2 . 57The loopback interface should be the last interface configured, 58as protocols may use the order of configuration as an indication of priority. 59The loopback should 60.Em never 61be configured first unless no hardware 62interfaces exist. 63.Sh DIAGNOSTICS 64.Bl -diag 65.It lo%d: can't handle af%d. 66The interface was handed 67a message with addresses formatted in an unsuitable address 68family; the packet was dropped. 69.El 70.Sh SEE ALSO 71.Xr intro 4 , 72.Xr inet 4 , 73.Xr ns 4 74.Sh HISTORY 75The 76.Nm 77device appeared in 78.Bx 4.2 . 79.Sh BUGS 80Previous versions of the system enabled the loopback interface 81automatically, using a nonstandard Internet address (127.1). 82Use of that address is now discouraged; a reserved host address 83for the local network should be used instead. 84