'\"macro stdmacro
.if n .pH g7.lo @(#)lo	40.2 of 10/10/89
.\" Copyright 1989 AT&T
.nr X
.if \nX=0 .ds x} lo 7 "TCP/IP" "\&"
.if \nX=1 .ds x} lo 7 "TCP/IP"
.if \nX=2 .ds x} lo 7 "" "\&"
.if \nX=3 .ds x} lo "" "" "\&"
.TH \*(x}
.SH NAME
\f4lo\f1 \- software loopback network interface
.SH SYNOPSIS
\f4d = open ("/dev/loop", \s-1O_RDWR\s0);\f1
.SH DESCRIPTION
.LP
The
\f4loopback\f1
device is a software datalink provider (interface driver)
that returns all packets it receives to their source without involving
any hardware devices.
It is a
.SM STREAMS
device conforming to the datalink 
provider interface (\s-1DLPI\s0).
See
\f4if\f1(7)
for a general description of network interfaces.
.LP
The
\f4loopback\f1
interface is used to access Internet services on the local machine.
Because it is available on all machines,
including those with no hardware network interfaces,
programs can use it for guaranteed access to local servers.
A typical application is the
\f4comsat\f1(1M)
server which accepts notification of mail delivery
from a local client.
The loopback interface is also used for performance analysis and testing.
.LP
By convention, the name of the loopback interface is \f4lo0\f1, and 
it is
configured with Internet address 127.0.0.1.
This address may be changed with the
\f4SIOCSIFADDR\f1
\f4ioctl(\|)\f1.
.SH SEE ALSO
\f4comsat\f1(1M),
\f4if\f1(7),
\f4inet\f1(7).
