1.\" $OpenBSD: lo.4,v 1.12 2000/11/06 22:16:33 matthieu 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. All advertising materials mentioning features or use of this software 16.\" must display the following acknowledgement: 17.\" This product includes software developed by the University of 18.\" California, Berkeley and its contributors. 19.\" 4. Neither the name of the University nor the names of its contributors 20.\" may be used to endorse or promote products derived from this software 21.\" without specific prior written permission. 22.\" 23.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 24.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 25.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 26.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 27.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 28.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 29.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 30.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 31.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 32.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 33.\" SUCH DAMAGE. 34.\" 35.\" @(#)lo.4 8.1 (Berkeley) 6/5/93 36.\" 37.Dd June 5, 1993 38.Dt LO 4 39.Os 40.Sh NAME 41.Nm lo 42.Nd software loopback network interface 43.Sh SYNOPSIS 44.Sy pseudo-device Nm loop Em <number> 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.Pp 64Configuring a loopback interface for 65.Xr inet 4 66with the 67.Em link1 68flag set will make the interface answer to the whole set of 69addresses identified as being in super-net which is specified 70by the address and netmask. Obviously you should not set the 71.Em link1 72flag on interface 73.Nm lo0 Ns , 74but instead use another interface like 75.Nm lo1 Ns . 76.Sh DIAGNOSTICS 77.Bl -diag 78.It lo%d: can't handle af%d. 79The interface was handed 80a message with addresses formatted in an unsuitable address 81family; the packet was dropped. 82.El 83.Sh EXAMPLES 84ifconfig lo1 inet 192.168.1.1 netmask 255.255.255.0 link1 85.Pp 86is equivalent to: 87.Pp 88awk 'BEGIN {for(i=1;i<255;i++) \ 89print "ifconfig lo1 inet 192.168.1."i" netmask 255.255.255.255 alias"}'|sh 90.Sh SEE ALSO 91.Xr inet 4 , 92.Xr inet6 4 , 93.Xr netintro 4 , 94.Xr ns 4 , 95.Xr ifconfig 8 96.Sh HISTORY 97The 98.Nm 99device appeared in 100.Bx 4.2 . 101.Pp 102The wildcard functionality first appeared in 103.Ox 2.3 . 104.Sh BUGS 105Previous versions of the system enabled the loopback interface 106automatically, using a nonstandard Internet address (127.1). 107Use of that address is now discouraged; a reserved host address 108for the local network should be used instead. 109.Pp 110You should be careful using 111.Xr ipnat 8 112with 113.Em link1 114set on an interface, because it may believe the packets are coming 115from a loopback address. 116