1.\" Copyright (c) 1983, 1991, 1993 2.\" The Regents of the University of California. 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. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)lo.4 8.1 (Berkeley) 6/5/93 29.\" $FreeBSD: src/share/man/man4/lo.4,v 1.7.2.1 2001/08/17 13:08:38 ru Exp $ 30.\" 31.Dd February 8, 2024 32.Dt LO 4 33.Os 34.Sh NAME 35.Nm lo 36.Nd software loopback network interface 37.Sh SYNOPSIS 38.Cd "pseudo-device loop" 39.Sh DESCRIPTION 40The 41.Nm loop 42interface is a software loopback mechanism which may be 43used for performance analysis, software testing, and/or local 44communication. 45As with other network interfaces, the loopback interface must have 46network addresses assigned for each address family with which it is to be used. 47These addresses 48may be set or changed with the appropriate 49.Xr ioctl 2 50commands for corresponding address families. 51The loopback interface should be the last interface configured, 52as protocols may use the order of configuration as an indication of priority. 53The loopback should 54.Em never 55be configured first unless no hardware 56interfaces exist. 57.Pp 58If the transmit checksum offload capability flag is enabled on a loopback 59interface, checksums will not be generated by IP, UDP, or TCP for packets 60sent on the interface. 61.Pp 62If the receive checksum offload capability flag is enabled on a loopback 63interface, checksums will not be validated by IP, UDP, or TCP for packets 64received on the interface. 65.Pp 66By default, both receive and transmit checksum flags will be enabled, in 67order to avoid the overhead of checksumming for local communication where 68data corruption is unlikely. 69If transmit checksum generation is disabled, then validation should also be 70disabled in order to avoid packets being dropped due to invalid checksums. 71.Sh DIAGNOSTICS 72.Bl -diag 73.It lo%d: can't handle af%d. 74The interface was handed 75a message with addresses formatted in an unsuitable address 76family; the packet was dropped. 77.El 78.Sh SEE ALSO 79.Xr inet 4 , 80.Xr intro 4 81.\" .Xr ns 4 82.Sh HISTORY 83The 84.Nm 85device appeared in 86.Bx 4.2 . 87The checksum generation and validation avoidance policy appeared in 88.Dx 2.2 . 89