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.\" $FreeBSD$ 32.\" 33.Dd January 25, 2012 34.Dt LO 4 35.Os 36.Sh NAME 37.Nm lo 38.Nd software loopback network interface 39.Sh SYNOPSIS 40.Cd "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. 47As with other network interfaces, the loopback interface must have 48network addresses assigned for each address family with which it is to be used. 49These addresses 50may be set with the appropriate 51.Xr ioctl 2 52commands for corresponding address families. 53The loopback interface should be the last interface configured, 54as protocols may use the order of configuration as an indication of priority. 55The loopback should 56.Em never 57be configured first unless no hardware 58interfaces exist. 59.Pp 60If the transmit checksum offload capability flag is enabled on a loopback 61interface, checksums will not be generated by IP, UDP, or TCP for packets 62sent on the interface. 63.Pp 64If the receive checksum offload capability flag is enabled on a loopback 65interface, checksums will not be validated by IP, UDP, or TCP for packets 66received on the interface. 67.Pp 68By default, both receive and transmit checksum flags will be enabled, in 69order to avoid the overhead of checksumming for local communication where 70data corruption is unlikely. 71If transmit checksum generation is disabled, then validation should also be 72disabled in order to avoid packets being dropped due to invalid checksums. 73.Sh DIAGNOSTICS 74.Bl -diag 75.It lo%d: can't handle af%d. 76The interface was handed 77a message with addresses formatted in an unsuitable address 78family; the packet was dropped. 79.El 80.Sh SEE ALSO 81.Xr inet 4 , 82.Xr intro 4 83.\" .Xr ns 4 84.Sh HISTORY 85The 86.Nm 87device appeared in 88.Bx 4.2 . 89The current checksum generation and validation avoidance policy appeared in 90.Fx 8.0 . 91