Loopback

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A loopback or a loop circuit is a message or information channel with only one end point, so that the sender and receiver are identical.

In communication technology , loopbacks are usually used to check the reachability of a destination. In this way, both the message channel to the destination and the destination itself are checked. The communication channel usually consists of several transmission sections lying one behind the other. By making the loop switching one after the other at all end points of the transmission sections, the entire route to the destination can be checked and a possible interruption can be found. In general, there are different types of loopbacks:

  • Loopbacks, in which the remote station both loops the received signal back to the transmitter and forwards it unchanged to the destination.
  • Loopbacks, in which the remote station loops the received signal back to the transmitter and feeds an alarm indication signal to the target instead of this signal .

Loopback in the Internet Protocol

The Internet Protocol (IP) specifies a loopback network. Specially reserved IP addresses are the address space from to for IPv4 , which is mostly used, and the address for IPv6 Most IP implementations support a loopback loop, whereby all packets sent by a computer program to these addresses are addressed to the same computer become. The standard for domains of these addresses is localhost . 127.0.0.1127.255.255.254127.0.0.1::1

On Unix- like systems, the loopback interface is generally called loor lo0.

The loopback interface is used, among other things, by client software to communicate with a server on the same computer.

An example: A web server is running on a computer. If now the browser as a URL http://127.0.0.1/or http://[::1]/indicating this will access the website of their own computer. Since no network connection is required for this, loopback is quite useful for testing various services without making them accessible remotely.

It is also a popular test method to ping the loopback interface in order to verify the correct functioning of the Internet protocol implementation.

Loopback addresses can be used on the Internet to fool newbies. An inexperienced user who thinks he is accessing (or attacking) a third-party server carries out these actions on his own computer. Many users are aware that stands 127.0.0.1for their own computer, but not that (e.g.) 127.34.123.43refers to themselves in the same way.

Packets with a loopback interface as the sender address that are sent to an existing IP network can pose a greater number of problems for older or faulty network software.

127.0.53.53was introduced by ICANN as a warning signal for the reassignment of top-level domains (TLD), where name overlaps between an internal namespace and the public domain name system (DNS) can occur. For example, if you weber.example.comonly communicate with a server by name weber, the answer came from the Internet that the domain does not exist and the host name used in the local network was used. After the introduction of the TLD .weber, the request goes to the responsible external DNS server. This can lead to undesirable information leaks. This is indicated by a response for 90 days from registration 127.0.53.53, which leads to an error message and prompts administrators to enter the full name ( FQHN ) for their host.

Individual evidence

  1. ↑ RFC 6890 : Special-Purpose IP Address Registries
  2. ↑ RFC 4291 : IP Version 6 Addressing Architecture
  3. ↑ Carsten Strotmann: Strange DNS answer 127.0.53.53. In: Heise.de - Tips and Tricks. February 5, 2016, accessed February 14, 2016 .

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