Telnet (sometimes stylized TELNET It is a protocol for bidirectional 8-bit communications. Its main goal was to connect terminal devices and terminal-oriented processes. Telnet's use for this purpose has waned significantly in favor of SSH. Some extensions to Telnet which would provide encryption have been proposed. The Telnet protocol abstracts any terminal as a Network Virtual Terminal (NVT). The client must simulate a NVT using the NVT codes when messaging the server.

Telnet predated UDP/IP and originally ran over Network Control Protocol (NCP). The telnet service is best understood in the context of a user with a simple terminal using the local Telnet program (known as the client program) to run a logon session on a remote computer where the user's communications needs are handled by a Telnet server program.

Telnet service

A Telnet service is an application providing services over the Telnet protocol. Most operating systems provide a service that can be installed or enabled to provide Telnet services to clients.

Name

The official specification stylizes the name as TELNET, which is not used as an acronym or abbreviation.

In his 2015 book WHOIS Running the Internet: Protocol, Policy, and Privacy, Internet researcher Garth O. Bruen claims that Telnet was originally short for "Teletype Over Network Protocol".

History

Telnet was originally developed for ARPANET in 1969. Initially, it was an ad hoc protocol with no formal specification, but after extensive work in the 1970s, including numerous RFCs, it was officially formalized in and , which together form Internet standard 8.

Since then, many additional RFCs have updated or extended the Telnet specification, both to address issues in the original standard and to add new capabilities.

Telnet services can be exploited to leak information about the server (such as hostnames, IP addresses, and brand) by packet sniffing the banner. This information can then be searched to determine if a Telnet service accepts a connection without authentication. Telnet is frequently exploited by malware due to being improperly configured.

  • Telnet, by default, does not encrypt any data sent over the connection (including passwords), and so it is often feasible to eavesdrop on the communications and use the password later for malicious purposes; anybody who has access to a router, switch, hub or gateway located on the network between the two hosts where Telnet is being used can intercept the packets passing by and obtain login, password and whatever else is typed with a packet analyzer. However, most Telnet implementations do not support these extensions; and they do not address other vulnerabilities such as parsing the banner information.

Uses

Historical

thumb|upright=1.3|[[BusyBox runs under the Microsoft Telnet Client from a router.|alt=Screenshot of a black screen with the output of the help command and a # prompt.]]

Historically, Telnet provided access to a command-line interface on a remote host. However, because of serious security concerns when using Telnet over an open network such as the Internet, its use for this purpose has waned significantly in favor of SSH. The usage of Telnet for remote management has declined rapidly, especially on the public Internet, in favor of the Secure Shell (SSH) protocol. SSH provides much of the functionality of telnet, with the addition of strong encryption to prevent sensitive data such as passwords from being intercepted, and public key authentication, to ensure that the remote computer is actually who it claims to be.

Modern

The Telnet protocol is mainly used for legacy equipment that does not support more modern communication mechanisms. For example, many industrial and scientific devices only have Telnet available as a communication option. Some are built with only a standard RS-232 port and use a serial server hardware appliance to provide the translation between the TCP/Telnet data and the RS-232 serial data. In such cases, SSH is not an option unless the interface appliance can be configured for SSH (or is replaced with one supporting SSH).

Telnet support has become highly unusual in new applications, though amateur radio operators and multi-user dungeons do continue to utilize it.

Security researchers estimated that 7,096,465 exposed systems on the Internet continue to use Telnet as of 2021. However, estimates of this number have varied significantly, depending on the number of ports scanned beyond the default TCP port 23.

Technical details

The technical details of Telnet are defined by a variety of specifications including . but nothing is known about its actual software support.

|-

|1

|Echo

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|2

|Reconnection

|NIC 15391 of 1973

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|3

|Suppress Go Ahead

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|The "Go Ahead" command code (249) in the original Telnet protocol is used to notify to the other end that the other end could start sending back messages. This was used in "half duplex" communication, as some terminals could send messages and receive messages, but not simultaneously.

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|4

|Approx Message Size Negotiation

|NIC 15393 of 1973

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|5

|Status

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|6

|Timing Mark

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|7

|Remote Controlled Trans and Echo

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|8

|Output Line Width

|NIC 20196 of August 1978

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|9

|Output Page Size

|NIC 20197 of August 1978

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|10

|Output Carriage-Return Disposition

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|11

|Output Horizontal Tab Stops

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|12

|Output Horizontal Tab Disposition

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|13

|Output Formfeed Disposition

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|14

|Output Vertical Tabstops

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|15

|Output Vertical Tab Disposition

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|16

|Output Linefeed Disposition

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|17

|Extended ASCII

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|18

|Logout

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|19

|Byte Macro

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|20

|Data Entry Terminal

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|21

|SUPDUP

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|22

|SUPDUP Output

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|23

|Send Location

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|24

|Terminal Type

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|25

|End of Record

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|26

|TACACS User Identification

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|27

|Output Marking

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|28

|Terminal Location Number

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|29

|Telnet 3270 Regime

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|30

|X.3 PAD

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|31

|Negotiate About Window Size

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|32

|Terminal Speed

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|33

|Remote Flow Control

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|34

|Linemode

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|35

|X Display Location

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|36

|Environment Option

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|37

|Authentication Option

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|38

|Encryption Option

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|39

|New Environment Option

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|40

|TN3270E

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|<small>See IBM 3270</small>

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|41

|XAUTH

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|42

|CHARSET

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|43

|Telnet Remote Serial Port (RSP)

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|44

|Com Port Control Option

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|45

|Telnet Suppress Local Echo

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|46

|Telnet Start TLS

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|47

|KERMIT

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|<small>See Kermit (protocol)</small>

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|48

|SEND-URL

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|49

|FORWARD_X

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|50-137

|Unassigned

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|138

|TELOPT PRAGMA LOGON

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|139

|TELOPT SSPI LOGON

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|140

|TELOPT PRAGMA HEARTBEAT

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|141-254

|Unassigned

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|255

|Extended-Options-List

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| colspan="4" |

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Client applications

  • AbsoluteTelnet is a telnet client for Windows. It also supports SSH and SFTP.
  • cURL is a multi protocol transfer tool and library
  • Inetutils includes a telnet client and server and is installed by default on many Linux distributions.
  • Line Mode Browser, a command line web browser
  • NCSA Telnet
  • PuTTY and plink command line are a free, open-source SSH, Telnet, rlogin, and raw TCP client for Windows, Linux, and Unix.
  • Rtelnet is a SOCKS client version of Telnet, providing similar functionality of telnet to those hosts which are behind firewall and NAT.
  • RUMBA
  • SecureCRT from Van Dyke Software
  • SyncTERM is a BBS terminal program supporting Telnet, SSHv2, RLogin, Serial, Windows, *nix, and Mac OS X platforms, X/Y/ZMODEM and various BBS terminal emulations
  • telnet.exe command line utility included in default installation of many versions of Microsoft Windows.
  • TeraTerm
  • ZOC Terminal

Star Wars: Episode IV – A New Hope from 1977 has been recreated as a text art movie served through Telnet.

See also

  • Banner grabbing
  • HyTelnet
  • Kermit
  • List of terminal emulators
  • Reverse telnet
  • SSH
  • Virtual terminal

References

Further reading

Internet standards

  • , Telnet Protocol Specification
  • , Telnet Option Specifications
  • , Telnet Binary Transmission
  • , Telnet Echo Option
  • , Telnet Suppress Go Ahead Option
  • , Telnet Status Option
  • , Telnet Timing Mark Option
  • , Telnet Extended Options: List Option

Proposed standards

  • , Telnet End of Record Option
  • , Telnet Window Size Option
  • , Telnet Terminal Speed Option
  • , Telnet Terminal-Type Option
  • , Telnet X Display Location Option
  • , Requirements for Internet Hosts - Application and Support
  • , Telnet Linemode Option
  • , Telnet Remote Flow Control Option
  • , Telnet Environment Option
  • , Telnet Authentication Option
  • , Telnet Authentication: Kerberos Version 5
  • , TELNET Authentication Using DSA
  • , Telnet Authentication: SRP
  • , Telnet Data Encryption Option
  • , The telnet URI Scheme

Informational/experimental

  • , The Q Method of Implementing TELNET Option Negotiation
  • , Telnet Environment Option Interoperability Issues
  • , TELNET CHARSET Option

Other RFCs

  • , TELNET RANDOMLY-LOSE Option (April Fool's Day RFC joke)
  • , Telnet 3270 Regime Option
  • , TELNET SUBLIMINAL-MESSAGE Option (April Fool's Day RFC joke)
  • , 5250 Telnet Interface
  • , TN3270 Enhancements
  • , Telnet Com Port Control Option
  • , IBM's iSeries Telnet Enhancements
  • Telnet Options—The official list of assigned option numbers at iana.org
  • Telnet Interactions Described as a Sequence Diagram
  • Telnet protocol description, with NVT reference
  • Microsoft TechNet:Telnet commands
  • TELNET: The Mother of All (Application) Protocols
  • Contains a list of telnet addresses and list of telnet clients