Loopback
Encyclopedia : L : LO : LOO : Loopback
A loopback is a communications channel with only one endpoint. Any message transmitted through such a channel is immediately received by the selfsame channel.
An RS-232 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 Tester (BERT) ). Some devices include built-in loopback capability.
TCP/IP specifies a loopback network. Under IPv4, this has the CIDR address of 127.0.0.0/8. Most IP implementations support a loopback interface, which represents the loopback facility. Any traffic that a computer program sends on the loopback network is addressed to the same computer. The most commonly used IP address on the loopback network is 127.0.0.1 for IPv4 and ::1 for IPv6. The standard domain name for this address is localhost. A loopback interface is a type of 'circuitless IP address' or 'virtual IP' address, as the IP address is not associated with any one particular interface (or circuit) on the host or router.
On Unix-like systems, the loopback interface is commonly called 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–viz., on a computer running a web server, pointing a web browser at the URL http://127.0.0.1/ will access that computer's own web site. This can be done without the computer being connected to any network–so it is useful for testing services without exposing them to remote network access. Likewise, to ping the loopback interface is a basic test that one's IP stack is working properly.
Loopback addresses are the subject of a reasonably common Internet prank: directing an inexperienced user to connect to (or attack) his own loopback interface as if it were a remote server [link]. Note, however, that contrary to popular belief a computer will normally not crash if it flood-pings the loopback address. The only effect is a busy processor.
Packets sent on a "real" IP network with a source address on the loopback interface can cause a number of problems for older or buggy network software. Such packets are known as "martian packets". [link]
Some network equipment use the term loopback for a virtual interface used for management purposes. Such an interface is assigned an address which 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 often used for management datagrams, such as alarms, originating from the equipment.
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