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Loopback (
loop-back) describes ways of routing electronic signals, digital data streams, or flows of items from their originating facility back to the receiving end of the source without intentional processing or modification. This is primarily a means of testing the transmission or transportation infrastructure.
Example applications:
- Performing transmission tests of access lines from the serving switching center, which usually does not require the assistance of personnel at the served terminal.
- Testing between stations (not necessarily adjacent) where in two lines are used, with the test being done at one station and the two lines interconnected at the distant station. Commonly called loop around when the interconnecting circuit is accessed by dialing.
- A patch cable, applied manually or automatically, remotely or locally, that facilitates a loop-back test.
- A communication channel with only one endpoint. Any message transmitted through such a channel is immediately received by the same channel.
Virtual network interface In
TCP/IP a
loopback device is a
virtual network interface implemented in software only and not connected to any hardware, but which is fully integrated into the computer system's internal network infrastructure. Any traffic that a computer program sends to the loopback interface is immediately received on the same interface.
Correspondingly, the
Internet Protocol (IP) specifies a
loopback network. In
IPv4 this is the network with the
CIDR prefix 127.0.0.0/8 (
RFC 3330). The most commonly used
IP address on the loopback device is 127.0.0.1 for
IPv4, although any address in the range 127.0.0.1 to 127.255.255.254 is mapped to it.
IPv6 designates only a single address for this function, 0:0:0:0:0:0:0:1 (also written as ::1), having the ::1/128 prefix (
RFC 3513). The standard, officially reserved,
domain name for these addresses is
localhost (
RFC 2606).
On
Unix-like systems, the loopback interface usually has the device name lo or lo0.
A loopback interface has several uses. It may be used by network client software on a computer to communicate with server software on the same computer, namely on a computer running a
web server, pointing a
web browser to the
URLs http://127.0.0.1/ or
http://localhost/ will access that computer's own
web site. This works without any actual network connection–so it is useful for testing services without exposing them to security risks from remote network access. Likewise,
pinging the loopback interface is a basic test of the functionality of the IP stack in the operating system.
Packets sent in an IP network with a source address belonging to the loopback interface can cause a number of problems for older or buggy network software. Such packets are known as
‘martian packets’ [1] and the source addresses of these are collectively known as
‘bogons’. The Internet Protocol specification dictates that such packets must not be transmitted outside of a host, and must be dropped if received on a network interface (cf.
RFC 1700,
RFC 2893).
One notable exception to the use of the loopback network addresses (127/8) is their use in
Multiprotocol Label Switching (MPLS) traceroute error detection techniques (
RFC 4379) in which their property of not being routable provides a convenient means to avoid delivery of faulty packets to end users.
Network equipment Some network equipment uses the term
loopback for a virtual interface used for management purposes. Unlike a proper loopback interface, this
loopback device is not used to talk with itself.
Such an interface is assigned an address that can be accessed from management equipment over a network but is not assigned to any of the real interfaces on the device. This loopback address is also used for management
datagrams, such as alarms, originating from the equipment. The property that makes this virtual interface special is that applications that use it will send or receive traffic using the address assigned to the virtual interface as opposed to the address on the physical interface through which the traffic passes.
Serial interfaces A serial communications
transceiver can use loopback for testing its functionality. For example, a device's
transmit pin connected to its
receive pin will result in the device receiving exactly what it transmits. Moving this looping connection to the remote end of a cable adds the cable to this test. Moving it to the far end of a modem link extends the test further. This is a common troubleshooting technique and is often combined with a specialized test device that sends specific patterns and counts any errors that come back (see
Bit Error Rate Test). Some devices include built-in loopback capability.
A simple serial interface loopback test, called
paperclip test, is sometimes used to identify serial ports of a computer and verify operation. It utilizes a terminal emulator application to send characters, with flow control set to off, to the serial port and receive the same back. For this purpose, a paperclip is used to short pin 2 to pin 3 (the receive and transmit pins) on a standard
RS-232 interface using
D-subminiature DE-9 or DB-25 connectors.
Telecommunications In
telecommunications, loopback (short
loop) is a hardware or software method which feeds a received signal or data back to the sender. It is used as an aid in debugging physical connection problems. As a test, many data communication devices can be configured to send specific patterns (such as
all ones) on an interface and can detect the reception of this signal on the same port. This is called a loopback test and can be performed within a
modem or
transceiver by connecting its output to its own input. A circuit between two points in different locations may be tested by applying a test signal on the
circuit in one location, and having the network device at the other location send a signal back through the circuit. If this device receives its own signal back, this proves that the circuit is functioning.
A
hardware loop is a simple device that physically connects the
receive channel to the
transmit channel. In the case of a network termination connector such as
X.21, this is typically done by simply connecting the pins together in the connector. Media such as
optical fiber or
coaxial cable, which have separate transmit and receive connectors, can simply be looped together with a single strand of the appropriate medium.
A
modem can be configured to loop incoming signals from either the remote modem or the local
terminal. This is referred to as loopback or software loop.
Other applications A
named pipe can also be seen as a loopback on file system level: While
Unix domain sockets and
network sockets are using two connected files for
Inter-process communication, a named pipe consists of only one file.
The audio systems
OSS,
ALSA and
PulseAudio have also loopback modules for recording the audio output of applications and testing purposes. Unlike physical loopbacks, there is no double analogue/digital conversion and no disruption due to hardware malfunctions.
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