Loopback
Encyclopedia
Loopback describes ways of routing electronic signals, digital data streams, or flows of items from their originating facility back to the source without intentional processing or modification. This is primarily a means of testing the transmission or transportation infrastructure.
Example applications:
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 127/8 (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 127.0.0.0 to 127.255.255.255 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
ing 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 packet
s’ http://www.catb.org/~esr/jargon/html/M/martian.html 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.
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.
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.
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.
Example applications:
- Performing transmissionTransmission (telecommunications)Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
tests of accessAccess networkAn access network is that part of a telecommunications network which connects subscribers to their immediate service provider. It is contrasted with the core network, which connects local providers to each other...
lines from the serving switching center, which usually does not require the assistance of personnel at the served terminalTerminal (telecommunication)In the context of telecommunications, a terminal is a device which is capable of communicating over a line. Examples of terminals are telephones, fax machines, and network devices - printers and workstations....
. - 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 aroundLoop aroundA loop line or loop around is a telephone company test circuit. The circuit has two associated phone numbers. When one side of the loop is called , the caller receives a test tone of approximately 1000 Hz...
when the interconnecting circuit is accessed by dialing. - A patch cablePatch cableA patch cable or patch cord is an electrical or optical cable used to connect one electronic or optical device to another for signal routing. Devices of different types are connected with patch cords...
, applied manually or automatically, remotely or locally, that facilitates a loop-back test. - A communication channel with only one endpointCommunication endpointA communication endpoint is an interface exposed by a communicating party or by a communication channel. An example of the latter type of a communication endpoint is a publish-subscribe topic or a group in group communication systems....
. Any message transmitted through such a channelChannel (communications)In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
is immediately received by the same channel.
Virtual network interface
In TCP/IP a loopback device is a virtual network interfaceVirtual Interface
A Virtual Interface or Virtual Network Interface is an abstract virtualized representation of a computer network interface that may or may not correspond directly to a physical 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
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP) specifies a loopback network. In IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
this is the network with the CIDR prefix 127/8 (RFC 3330). The most commonly used IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
on the loopback device is 127.0.0.1 for IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
, although any address in the range 127.0.0.0 to 127.255.255.255 is mapped to it. IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
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
Domain name
A domain name is an identification string that defines a realm of administrative autonomy, authority, or control in the Internet. Domain names are formed by the rules and procedures of the Domain Name System ....
for these addresses is localhost
Localhost
In computer networking, localhost is the standard hostname given to the address of the loopback network interface. The name is also a reserved top-level domain name In computer networking, localhost (meaning this computer) is the standard hostname given to the address of the loopback network...
(RFC 2606).
On Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
-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
Web server
Web server can refer to either the hardware or the software that helps to deliver content that can be accessed through the Internet....
, pointing a web browser
Web browser
A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...
to the URLs
Uniform Resource Locator
In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
http://127.0.0.1/ http://localhost/ Ping
Ping is a computer network administration utility used to test the reachability of a host on an Internet Protocol network and to measure the round-trip time for messages sent from the originating host to a destination computer...
ing 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 packet
Martian packet
A Martian packet is an IP packet which specifies a source or destination address that is reserved for special-use by Internet Assigned Numbers Authority and cannot actually originate as claimed or be delivered....
s’ http://www.catb.org/~esr/jargon/html/M/martian.html and the source addresses of these are collectively known as ‘bogons
Bogon filtering
A bogon is a bogus IP address, and an informal name for an IP packet on the public Internet that claims to be from an area of the IP address space reserved, but not yet allocated or delegated by the Internet Assigned Numbers Authority or a delegated Regional Internet Registry...
’. 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
Multiprotocol Label Switching
Multiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...
(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 transceiverTransceiver
A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s...
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
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
interface using D-subminiature
D-subminiature
The D-subminiature or D-sub is a common type of electrical connector. They are named for their characteristic D-shaped metal shield. When they were introduced, D-subs were among the smaller connectors used on computer systems....
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 modemModem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
or transceiver
Transceiver
A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s...
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
Telecommunication circuit
A telecommunication circuit is any line, conductor, or other conduit by which information is transmitted.A dedicated circuit, private circuit, or leased line is a line that is dedicated to only one use...
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
X.21
X.21 is an interface specification for differential communications introduced in the mid 1970s by the ITU-T. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications between carriers and customers' equipment...
, this is typically done by simply connecting the pins together in the connector. Media such as optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
or coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
, which have separate transmit and receive connectors, can simply be looped together with a single strand of the appropriate medium.
A modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
can be configured to loop incoming signals from either the remote modem or the local terminal
Terminal (telecommunication)
In the context of telecommunications, a terminal is a device which is capable of communicating over a line. Examples of terminals are telephones, fax machines, and network devices - printers and workstations....
. This is referred to as loopback or software loop.