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 28, 2026 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 is enabled on a loopback interface, 58checksums will not be generated by IP, UDP, TCP, or SCTP for packets sent on the 59interface. 60By default, this capability is enabled to avoid the overhead of checksumming for 61local communication where data corruption is unlikely. 62.Pp 63If the receive checksum offload capability is disabled on a loopback interface, 64the flags that indicate a valid checksum will be unset if set previously. 65By default, this capability is enabled to avoid another checksum validation if 66it has already been validated. 67.Sh DIAGNOSTICS 68.Bl -diag 69.It lo%d: can't handle af%d. 70The interface was handed 71a message with addresses formatted in an unsuitable address 72family; the packet was dropped. 73.El 74.Sh SEE ALSO 75.Xr inet 4 , 76.Xr intro 4 77.\" .Xr ns 4 78.Sh HISTORY 79The 80.Nm 81device appeared in 82.Bx 4.2 . 83The current checksum generation and validation avoidance policy appeared in 84.Fx 8.0 . 85