1.\" $OpenBSD: lo.4,v 1.26 2007/05/31 19:19:50 jmc 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: May 31 2007 $ 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.Pp 74Configuring a loopback interface for 75.Xr inet 4 76with the 77.Em link1 78flag set will make the interface answer to the whole set of 79addresses identified as being in super-net which is specified 80by the address and netmask. 81Obviously you should not set the 82.Em link1 83flag on interface 84.Nm lo0 , 85but instead use another interface like 86.Nm lo1 . 87.Sh EXAMPLES 88.Bd -literal 89# ifconfig lo1 create 90# ifconfig lo1 inet 192.168.1.1 netmask 255.255.255.0 link1 91.Ed 92.Pp 93is equivalent to: 94.Bd -literal 95# ifconfig lo1 create 96# awk 'BEGIN {for(i=1;i<255;i++) \e 97 print "ifconfig lo1 inet 192.168.1."i" netmask 255.255.255.255 alias"}'| \e 98 sh 99.Ed 100.Sh DIAGNOSTICS 101.Bl -diag 102.It lo%d: can't handle af%d. 103The interface was handed 104a message with addresses formatted in an unsuitable address 105family; the packet was dropped. 106.El 107.Sh SEE ALSO 108.Xr inet 4 , 109.Xr inet6 4 , 110.Xr netintro 4 , 111.Xr hostname.if 5 , 112.Xr ifconfig 8 , 113.Xr netstart 8 114.Sh HISTORY 115The 116.Nm 117device appeared in 118.Bx 4.2 . 119.Pp 120The wildcard functionality first appeared in 121.Ox 2.3 . 122.Sh BUGS 123Previous versions of the system enabled the loopback interface 124automatically, using a non-standard Internet address (127.1). 125Use of that address is now discouraged; a reserved host address 126for the local network should be used instead. 127.Pp 128Care should be taken when using NAT with interfaces that have the 129.Em link1 130flag set, because it may believe the packets are coming from a 131loopback address. 132