The name "Telnet" refers to two things: a protocol itself specifying how two parties are to communicate and a software application that implements the protocol as a service.[2] 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 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][3] Telnet's use for this purpose has waned significantly in favor of SSH.[4] Some extensions to Telnet which would provide encryption have been proposed.[5]
This section needs expansionwith: more details about the protocol itself. You can help by adding to it. (talk)(April 2025)
The telnet protocol is a client-server protocol that runs on a reliableconnection-oriented transport.[citation needed] Most often, a telnet client connects over TCP to port 23 or 2323, where a Telnet server application is listening.[1][6][7] 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).[8] 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.[citation needed]
Name
While the official specification stylizes the name as TELNET, it is not defined therein as an acronym or abbreviation.[1]
In a 1972 paper, when discussing one of the early forms of the protocol, Stephen Crocker et al. used "TELNET" explicitly as an abbreviation of "telecommunications network."[9]
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."[10]
History
Telnet was originally developed for ARPANET in 1969.[11] Initially, it was an ad hoc protocol with no formal specification,[12] but after extensive work in the 1970s, including numerous RFCs, it was officially formalized in RFC854 and RFC855, which together form internet standard 8.[11][13]
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.[11] Some of these extensions have also been adopted as Internet standards, particularly standards 27 through 32 (see below).
Security vulnerabilities
Telnet is vulnerable to network-based cyberattacks, such as packet sniffing sensitive information including passwords and fingerprinting.[3][14][15] 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.[7] 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:[16]
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.[15]
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.[7]
Extensions to Telnet provide Transport Layer Security (TLS) security and Simple Authentication and Security Layer (SASL) authentication that address the above concerns.[5] However, most Telnet implementations do not support these extensions; and they do not address other vulnerabilities such as parsing the banner information.[15] Telnet over VPN is a viable option if SSHv2 is not supported, or a VPN is already used to securely tunnel other application data to the remote network the Telnet server is present in. However, precautions must be taken: ideally the VPN should terminate on the Telnet server itself, unless the LAN has additional security measures against eavesdropping and modification by other devices such as additional encryption and/or VLANs. This is because Telnet traffic leaves the VPN server in its insecure plaintext form after it is decrypted. The VPN software should be a trusted one that is heavily audited (e.g. OpenVPN, WireGuard, IPsec), using preferably certificate-based/public key mutual authentication.
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 TelnetS (Telnet over SSL/TLS).[17]
There is also an open-source telnet client implementation that supports TLS over port 992 available at Github page for Telnet-TLS with binaries for Mac and Windows. It can be built from source code on Linux also.
Uses
Historical uses
BusyBox running under the Microsoft Telnet Client from a router.
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.[18] The usage of Telnet for remote management has declined rapidly, especially on the public Internet, in favor of the Secure Shell (SSH) protocol.[2][19] 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 protocol is mainly used for legacy equipment that does not support more modern communication mechanisms.[20] 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).[citation needed]
Telnet is commonly used by amateur radio operators for providing public information.[21]
Despite recommendations 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.[7]
The Telnet client may be used in debugging network services such as SMTP, IRC, or HTTP servers, to issue commands to the server and examine the responses. In this case, when the Telnet client establishes a TCP connection to a port other than the standard Telnet server port, it does not use the Telnet protocol, and can be used instead to send and receive data over the TCP connection directly.[22][bettersourceneeded]
Technical details
The technical details of Telnet are defined by a variety of specifications including RFC854.[1]
Telnet commands
Telnet commands consist of at least two bytes.[1] The first byte is the IAC escape character (byte 255) followed by the byte code for a given command:
Name
Byte Value (dec)
Byte value (hex)
Explanation
SE (Subnegotiation end)
240
0xf0
End of negotiation (or data block) of a sub-service of a protocol mechanism
NOP (No operation)
241
0xf1
Data packet that does nothing
Data Mark
242
0xf2
Break
243
0xf3
Interrupt Process
244
0xf4
Request that other party ends current process
Abort output
245
0xf5
Request that other party stops sending output
Are you there?
246
0xf6
Erase character
247
0xf7
Erase Line
248
0xf8
Go ahead
249
0xf9
SB (Subnegotiation begin)
250
0xfa
Initiate the negotiation of a sub-service of a protocol mechanism
WILL
251
0xfb
Informs other party that this party will use a protocol mechanism
WON'T
252
0xfc
Informs other party that this party will not use a protocol mechanism
DO
253
0xfd
Instruct other party to use a protocol mechanism
DON'T
254
0xfe
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.)[1]
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,[23] 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.
1 2 Daş, Resul; Karabade, Abubakar; Tuna, Gurkan (2015). "Common network attack types and defense mechanisms". 2015 23nd Signal Processing and Communications Applications Conference (SIU). pp.2658–2661. doi:10.1109/SIU.2015.7130435. ISBN978-1-4673-7386-9. S2CID11256038.
↑ Todorov, Dobromir (2007). Mechanics of user identification and authentication: fundamentals of identity management. Boca Raton: Auerbach Publications. ISBN978-1-4200-5220-6. OCLC263353270.
↑ 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.
↑ Bruen, Garth O. (2015). WHOIS Running the Internet: Protocol, Policy, and Privacy (1sted.). Wiley. p.25. ISBN9781118679555.
↑ Todorov, Dobromir (2007). Mechanics of user identification and authentication: fundamentals of identity management. Boca Raton: Auerbach Publications. ISBN978-1-4200-5220-6. OCLC263353270.
↑ Poulsen, Kevin (2 April 2007). "Telnet, dead at 35...RIP". Wired. p.24. Archived from the original on 21 December 2016. Retrieved 14 June 2017.
↑ 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.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
