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.\" Copyright (c) 2009 Robert N. M. Watson
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.\"     @(#)lo.4	8.1 (Berkeley) 6/5/93
.\"
.Dd June 23, 2024
.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 with the appropriate
.Xr ioctl 2
commands for corresponding address families.
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, TCP, or SCTP 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, TCP, or SCTP 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 .