xref: /freebsd/share/man/man4/lo.4 (revision edf8578117e8844e02c0121147f45e4609b30680)
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