1.\" Copyright (c) 1983, 1991, 1993 2.\" The Regents of the University of California. 3.\" Copyright (c) 2009 Robert N. M. Watson 4.\" 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. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" @(#)lo.4 8.1 (Berkeley) 6/5/93 31.\" 32.Dd January 25, 2012 33.Dt LO 4 34.Os 35.Sh NAME 36.Nm lo 37.Nd software loopback network interface 38.Sh SYNOPSIS 39.Cd "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 with the appropriate 50.Xr ioctl 2 51commands for corresponding address families. 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.Pp 59If the transmit checksum offload capability flag is enabled on a loopback 60interface, checksums will not be generated by IP, UDP, or TCP for packets 61sent on the interface. 62.Pp 63If the receive checksum offload capability flag is enabled on a loopback 64interface, checksums will not be validated by IP, UDP, or TCP for packets 65received on the interface. 66.Pp 67By default, both receive and transmit checksum flags will be enabled, in 68order to avoid the overhead of checksumming for local communication where 69data corruption is unlikely. 70If transmit checksum generation is disabled, then validation should also be 71disabled in order to avoid packets being dropped due to invalid checksums. 72.Sh DIAGNOSTICS 73.Bl -diag 74.It lo%d: can't handle af%d. 75The interface was handed 76a message with addresses formatted in an unsuitable address 77family; the packet was dropped. 78.El 79.Sh SEE ALSO 80.Xr inet 4 , 81.Xr intro 4 82.\" .Xr ns 4 83.Sh HISTORY 84The 85.Nm 86device appeared in 87.Bx 4.2 . 88The current checksum generation and validation avoidance policy appeared in 89.Fx 8.0 . 90