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.\"     @(#)lo.4	8.1 (Berkeley) 6/5/93
.\"
.Dd September 3, 2006
.Dt LO 4
.Os
.Sh NAME
.Nm lo
.Nd software loopback network interface
.Sh SYNOPSIS
.Cd "pseudo-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
The loopback interface
.Nm lo0
is created at boottime, it always exists and cannot be destroyed
with
.Xr ifconfig 8 .
Additional loopback interfaces can be created by using the
.Xr ifconfig 8
.Cm create
command.
.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 ifconfig 8
.Sh HISTORY
The
.Nm
device appeared in
.Bx 4.2 .
.Sh BUGS
Previous versions of the system enabled the loopback interface
automatically, using a nonstandard Internet address (127.1).
Use of that address is now discouraged; a reserved host address
for the local network should be used instead.