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+.\" Copyright (c) 1983, 1991, 1993
+.\"	The Regents of the University of California.
+.\" Copyright (c) 2009 Robert N. M. Watson
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"     @(#)lo.4	8.1 (Berkeley) 6/5/93
+.\" $FreeBSD$
+.\"
+.Dd March 15, 2009
+.Dt LO 4
+.Os
+.Sh NAME
+.Nm lo
+.Nd software loopback network interface
+.Sh SYNOPSIS
+.Cd "device loop"
+.Sh DESCRIPTION
+The
+.Nm loop
+interface is a software loopback mechanism which may be
+used for performance analysis, software testing, and/or local
+communication.
+As with other network interfaces, the loopback interface must have
+network addresses assigned for each address family with which it is to be used.
+These addresses
+may be set or changed with the
+.Dv SIOCSIFADDR
+.Xr ioctl 2 .
+The loopback interface should be the last interface configured,
+as protocols may use the order of configuration as an indication of priority.
+The loopback should
+.Em never
+be configured first unless no hardware
+interfaces exist.
+.Pp
+If the transmit checksum offload capability flag is enabled on a loopback
+interface, checksums will not be generated by IP, UDP, or TCP for packets
+sent on the interface.
+.Pp
+If the receive checksum offload capability flag is enabled on a loopback
+interface, checksums will not be validated by IP, UDP, or TCP for packets
+received on the interface.
+.Pp
+By default, both receive and transmit checksum flags will be enabled, in
+order to avoid the overhead of checksumming for local communication where
+data corruption is unlikely.
+If transmit checksum generation is disabled, then validation should also be
+disabled in order to avoid packets being dropped due to invalid checksums.
+.Sh DIAGNOSTICS
+.Bl -diag
+.It lo%d: can't handle af%d.
+The interface was handed
+a message with addresses formatted in an unsuitable address
+family; the packet was dropped.
+.El
+.Sh SEE ALSO
+.Xr inet 4 ,
+.Xr intro 4
+.\" .Xr ns 4
+.Sh HISTORY
+The
+.Nm
+device appeared in
+.Bx 4.2 .
+The current checksum generation and validation avoidance policy appeared in
+.Fx 8.0 .