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.Dd June 23, 2024 31.Dt LO 4 32.Os 33.Sh NAME 34.Nm lo 35.Nd software loopback network interface 36.Sh SYNOPSIS 37.Cd "device loop" 38.Sh DESCRIPTION 39The 40.Nm loop 41interface is a software loopback mechanism which may be 42used for performance analysis, software testing, and/or local 43communication. 44As with other network interfaces, the loopback interface must have 45network addresses assigned for each address family with which it is to be used. 46These addresses 47may be set with the appropriate 48.Xr ioctl 2 49commands for corresponding address families. 50The loopback interface should be the last interface configured, 51as protocols may use the order of configuration as an indication of priority. 52The loopback should 53.Em never 54be configured first unless no hardware 55interfaces exist. 56.Pp 57If the transmit checksum offload capability flag is enabled on a loopback 58interface, checksums will not be generated by IP, UDP, TCP, or SCTP for packets 59sent on the interface. 60.Pp 61If the receive checksum offload capability flag is enabled on a loopback 62interface, checksums will not be validated by IP, UDP, TCP, or SCTP for packets 63received on the interface. 64.Pp 65By default, both receive and transmit checksum flags will be enabled, in 66order to avoid the overhead of checksumming for local communication where 67data corruption is unlikely. 68If transmit checksum generation is disabled, then validation should also be 69disabled in order to avoid packets being dropped due to invalid checksums. 70.Sh DIAGNOSTICS 71.Bl -diag 72.It lo%d: can't handle af%d. 73The interface was handed 74a message with addresses formatted in an unsuitable address 75family; the packet was dropped. 76.El 77.Sh SEE ALSO 78.Xr inet 4 , 79.Xr intro 4 80.\" .Xr ns 4 81.Sh HISTORY 82The 83.Nm 84device appeared in 85.Bx 4.2 . 86The current checksum generation and validation avoidance policy appeared in 87.Fx 8.0 . 88