1.\" Copyright (c) 1983, 1991 The Regents of the University of California. 2.\" 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. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" $Id$ 33.\" 34.Dd May 28, 1994 35.Dt LO 4 36.Os BSD 4.2 37.Sh NAME 38.Nm lo 39.Nd software loopback network interface 40.Sh SYNOPSIS 41.Nm loop . 42.Sh DESCRIPTION 43The 44.Nm loop 45interface is a software loopback mechanism which may be 46used for performance analysis, software testing, and/or local 47communication. 48As with other network interfaces, the loopback interface must have 49network addresses assigned for each address family with which it is to be used. 50These addresses 51may be set or changed with the 52.Dv SIOCSIFADDR 53.Xr ioctl 2 . 54The loopback interface should be the last interface configured, 55as protocols may use the order of configuration as an indication of priority. 56The loopback should 57.Em never 58be configured first unless no hardware 59interfaces exist. 60.Sh DIAGNOSTICS 61.Bl -diag 62.It lo%d: can't handle af%d. 63The interface was handed 64a message with addresses formatted in an unsuitable address 65family; the packet was dropped. 66.El 67.Sh SEE ALSO 68.Xr intro 4 , 69.Xr inet 4 , 70.Xr ns 4 71.Sh HISTORY 72The 73.Nm 74device appeared in 75.Bx 4.2 . 76.Sh BUGS 77Previous versions of the system enabled the loopback interface 78automatically, using a nonstandard Internet address (127.1). 79Use of that address is now discouraged; a reserved host address 80for the local network should be used instead. 81