Telnet consists of two components: (1) the protocol itself which specifies how two parties are to communicate and (2) the software application that provides the service. User data is interspersed in-band with Telnet control information in an 8-bit byte oriented data connection over the Transmission Control Protocol (TCP). Telnet was developed as secret technology in 1969 beginning with RFC15, extended in RFC855, and standardized as Internet Engineering Task Force (IETF) Internet Standard STD 8, one of the first Internet standards.[4][5] Telnet transmits all information including usernames and passwords in plaintext so it is not recommended for security-sensitive applications such as remote management of routers.[2][6] Telnet's use for this purpose has waned significantly in favor of SSH.[7] Some extensions to Telnet which would provide encryption have been proposed.[8]
Telnet consists of two components: (1) the protocol itself and (2) the service component. The telnet protocol is a client-server protocol, based on a reliableconnection-oriented transport.[2] This protocol is used to establish a connection to Transmission Control Protocol (TCP) port number 23 or 2323, where a Telnet server application is listening.[3][9][10] 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).[11] 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 Protocol
History
Even though Telnet was an ad hoc protocol with no official definition until March 5, 1973,[12] the name actually referred to Teletype Over Network Protocol as the RFC206 (NIC 7176) on Telnet makes the connection clear:[13]
The TELNET protocol is based upon the notion of a virtual teletype, employing a 7-bit ASCII character set. The primary function of a User TELNET, then, is to provide the means by which its users can 'hit' all the keys on that virtual teletype.[14]
Essentially, it used an 8-bit channel to exchange 7-bit ASCII data. Any byte with the high bit set was a special Telnet character. On March 5, 1973, a Telnet protocol standard was defined at UCLA[15] with the publication of two NIC documents: Telnet Protocol Specification, NIC 15372, and Telnet Option Specifications, NIC 15373.
Extensions
Many extensions were made for Telnet because of its negotiable options protocol architecture. Some of these extensions have been adopted as Internet standards, IETF documents STD 27 through STD 32. Some extensions have been widely implemented and others are proposed standards on the IETF standards track (see below).
Telnet service
The Telnet service is the 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.[16]
Security vulnerabilities
Telnet is vulnerable to network-based cyberattacks, such as packet sniffing sensitive information including passwords and fingerprinting.[6][17] Telnet services can also 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 also frequently exploited by malware due to being improperly configured.[10] In fact, Telnet is targeted by attackers more frequently than other common protocols, especially when compared to UPnP, CoAP, MQTT, AMQP and XMPP[citation needed]. Common devices targeted are Internet of things devices, routers and modems.
The SANS Institute recommends that the use of Telnet for remote logins should be discontinued under normal circumstances for the following reasons:[18]
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.[17]
Most Telnet implementations lack authentication. An estimated 22,887 Telnet-enabled devices found by security researchers not only lacked authentication but also provided unrestricted access to the system.[10]
Extensions to Telnet provide Transport Layer Security (TLS) security and Simple Authentication and Security Layer (SASL) authentication that address the above concerns.[8] However, most Telnet implementations do not support these extensions; and they do not address other vulnerabilities such as parsing the banner information.[17]
IBM 5250 or 3270 workstation emulation is supported via custom telnet clients, TN5250/TN3270, and IBM i systems. Clients and servers designed to pass IBM 5250 data streams over Telnet generally do support SSL encryption, as SSH does not include 5250 emulation. Under IBM i (also known as OS/400), port 992 is the default port for secured telnet.[19]
Uses
BusyBox running under the Microsoft Telnet Client from a router.
Historical uses
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.[20] The usage of Telnet for remote management has declined rapidly, especially on the public Internet, in favor of the Secure Shell (SSH) protocol.[2][21] 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 day uses
The Telnet client may be used in debugging network services such as SMTP, IRC, HTTP, FTP or POP3, to issue commands to a server and examine the responses.[16] For example, Telnet client applications can establish an interactive TCP session to a port other than the Telnet server port. However, communication with such ports does not involve the Telnet protocol, because these services merely use a transparent 8-bit TCP connection, because most elements of the telnet protocol were designed around the idea of accessing a command line interface and none of these options or mechanisms is employed in most other internet service connections.
For example, a command line telnet client could make an HTTP request to a web server on TCP port 80 as follows:[22]
$ telnetwww.example.com80GET /path/to/file.html HTTP/1.1Host: www.example.comConnection: close
The older protocol is used these days only in rare cases to access decades-old legacy equipment that does not support more modern protocols.[23] For example, a large number of 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 is commonly used by amateur radio operators for providing public information.[24]
Despite recommendation against 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.[10]
Technical details
The technical details of Telnet are defined by a variety of specifications including RFC854.[3]
Telnet commands consist of at least two bytes.[3] The first byte is the IAC escape character (typically byte 255) followed by the byte code for a given command:
Name
Byte code
Explanation
Notes
SE (Subnegotiation end)
240
End of negotiation (or data block) of a sub-service of a protocol mechanism
NOP (No operation)
241
Data packet that does nothing
Data Mark
242
Break
243
Interrupt Process
244
Request that other party ends current process
Abort output
245
Request that other party stops sending output
Are you there?
246
Erase character
247
Erase Line
248
Go ahead
249
SB (Subnegotiation begin)
250
Initiate the negotiation of a sub-service of a protocol mechanism
WILL
251
Informs other party that this party will use a protocol mechanism
WON'T
252
Informs other party that this party will not use a protocol mechanism
DO
253
Instruct other party to use a protocol mechanism
DON'T
254
Instruct other party to not use a protocol mechanism
All data octets except 0xff are transmitted over Telnet as is.
(0xff, or 255 in decimal, is the IAC byte (Interpret As Command) which signals that the next byte is a telnet command. The command to insert 0xff into the stream is 0xff, so 0xff must be escaped by doubling it when sending data over the telnet protocol.)[3]
Telnet options
Telnet also has a variety of options that terminals implementing Telnet should support.
The 8-bit mode (so named binary option) is intended to transmit binary data, not ASCII characters. The standard suggests the interpretation of codes 0000–0176 as ASCII, but does not offer any meaning for high-bit-set data octets. There was an attempt to introduce a switchable character encoding support like HTTP has,[25] but nothing is known about its actual software support.
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.
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.
^Crocker, Stephen D.; Heafner, John F.; Metcalfe, Robert M.; Postel, Jonathan B. (1971). "Function-oriented protocols for the ARPA computer network". Proceedings of the November 16-18, 1971, fall joint computer conference on - AFIPS '71 (Fall). Association for Computing Machinery. pp. 271–279. doi:10.1145/1478873.1478908. ISBN9781450379090.
^Wheen, Andrew (2011). Dot-dash to Dot.Com: How Modern Telecommunications Evolved from the Telegraph to the Internet. Springer. p. 132. ISBN9781441967596.
^Meinel, Christoph; Sack, Harald (2013). Internetworking: Technological Foundations and Applications. X.media.publishing. p. 57. ISBN978-3642353918.
^Ylonen, Tatu. "History of the SSH Protocol". SSH home page. SSH Communications Security, Inc. Archived from the original on 25 July 2018. Retrieved 14 June 2017.
