Telnet

Not to be confused with Telenet.

Telnet (short for "telecommunications network") ^[1] is a client/server application protocol that provides access to virtual terminals of remote systems on local area networks or the Internet. ^[2] It is a protocol for bidirectional 8-bit communications. Its main goal was to connect terminal devices and terminal-oriented processes. ^[3]

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 RFC   15, extended in RFC   855, 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]

Components

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 reliable connection-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 RFC   206 (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]
• Most Telnet authentication mechanisms are vulnerable to being intercepted by Man-in-the-middle attacks. ^[17]

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 RFC   854. ^[3]

USASCII control codes

Name	Byte code	Explanation	Notes
NULL	0
Line feed	10
Carriage return	13
Bell	7
Backspace	8
Horizontal tab	9
Vertical tab	11
Form feed	12
Source: J. Postel and Reynolds (1983) [3]

Telnet commands

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
IAC	255	Sequence Initializer/Escape Character
Source: J. Postel and Reynolds (1983) [3]

Interpret As Command

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.

Telnet Options

Code	Name	Spec	Notes
0	Binary Transmission	RFC   856	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.
1	Echo	RFC   857
2	Reconnection	NIC 15391 of 1973
3	Suppress Go Ahead	RFC   858	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.
4	Approx Message Size Negotiation	NIC 15393 of 1973
5	Status	RFC   859
6	Timing Mark	RFC   860
7	Remote Controlled Trans and Echo	RFC   726
8	Output Line Width	NIC 20196 of August 1978
9	Output Page Size	NIC 20197 of August 1978
10	Output Carriage-Return Disposition	RFC   652
11	Output Horizontal Tab Stops	RFC   653
12	Output Horizontal Tab Disposition	RFC   654
13	Output Formfeed Disposition	RFC   655
14	Output Vertical Tabstops	RFC   656
15	Output Vertical Tab Disposition	RFC   657
16	Output Linefeed Disposition	RFC   658
17	Extended ASCII	RFC   698
18	Logout	RFC   727
19	Byte Macro	RFC   735
20	Data Entry Terminal	RFC   1043 RFC   732
21	SUPDUP	RFC   736 RFC   734
22	SUPDUP Output	RFC   749
23	Send Location	RFC   779
24	Terminal Type	RFC   1091
25	End of Record	RFC   885
26	TACACS User Identification	RFC   927
27	Output Marking	RFC   933
28	Terminal Location Number	RFC   946
29	Telnet 3270 Regime	RFC   1041
30	X.3 PAD	RFC   1053
31	Negotiate About Window Size	RFC   1073
32	Terminal Speed	RFC   1079
33	Remote Flow Control	RFC   1372
34	Linemode	RFC   1184
35	X Display Location	RFC   1096
36	Environment Option	RFC   1408
37	Authentication Option	RFC   2941
38	Encryption Option	RFC   2946
39	New Environment Option	RFC   1572
40	TN3270E	RFC   2355	See IBM 3270
41	XAUTH
42	CHARSET	RFC   2066
43	Telnet Remote Serial Port (RSP)
44	Com Port Control Option	RFC   2217
45	Telnet Suppress Local Echo
46	Telnet Start TLS
47	KERMIT	RFC   2840	See Kermit (protocol)
48	SEND-URL
49	FORWARD_X
50-137	Unassigned
138	TELOPT PRAGMA LOGON
139	TELOPT SSPI LOGON
140	TELOPT PRAGMA HEARTBEAT
141-254	Unassigned
255	Extended-Options-List	RFC   861
Source: Internet Assigned Numbers Authority (n.d.) [26]

Telnet clients

• AbsoluteTelnet is a telnet client for Windows. It also supports SSH and SFTP.
• 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 (Terminal Emulator)
• 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

In popular culture

Star Wars: Episode IV – A New Hope from 1977 has been recreated as a text art movie served through Telnet. ^[27]

See also

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

References

1. 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. ISBN   9781450379090.
2. Valenčić, D.; Mateljan, V. (2019). "Implementation of NETCONF Protocol". 2019 42nd International Convention on Information and Communication Technology, Electronics and Microelectronics (MIPRO). pp. 421–430. doi:10.23919/MIPRO.2019.8756925. ISBN   978-953-233-098-4. S2CID   195883872.
3. Postel, J.; Reynolds, J. K. (1983). "Telnet Protocol Specification". Network Working Group. doi: 10.17487/RFC0854 . ISSN   2070-1721. RFC 854.
4. Wheen, Andrew (2011). Dot-dash to Dot.Com: How Modern Telecommunications Evolved from the Telegraph to the Internet. Springer. p. 132. ISBN   9781441967596.
5. Meinel, Christoph; Sack, Harald (2013). Internetworking: Technological Foundations and Applications. X.media.publishing. p. 57. ISBN   978-3642353918.
6. 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. ISBN   978-1-4673-7386-9. S2CID   11256038.
7. Todorov, Dobromir (2007). Mechanics of user identification and authentication : fundamentals of identity management. Boca Raton: Auerbach Publications. ISBN   978-1-4200-5220-6. OCLC   263353270.
8. Mahmood, H.B. (2003). "Transport layer security protocol in Telnet". 9th Asia-Pacific Conference on Communications (IEEE Cat. No.03EX732). Vol. 3. pp. 1033–1037 Vol.3. doi:10.1109/APCC.2003.1274255. ISBN   0-7803-8114-9. S2CID   56798078.
9. "Service Name and Transport Protocol Port Number Registry". www.iana.org. Retrieved 2023-01-12.
10. Srinivasa, Shreyas; Pedersen, Jens Myrup; Vasilomanolakis, Emmanouil (2021-11-02). "Open for hire". Proceedings of the 21st ACM Internet Measurement Conference. IMC '21. New York, NY, USA: Association for Computing Machinery. pp. 195–215. doi:10.1145/3487552.3487833. ISBN   978-1-4503-9129-0. S2CID   240357287.
11. Postel, J. (1981). "NCP/TCP transition plan". Network Working Group. doi: 10.17487/RFC0801 . ISSN   2070-1721. RFC 801.
12. Telnet Protocol. April 3, 1972. doi: 10.17487/RFC0318 . RFC 318.
13. Bruen, Garth O. (2015). WHOIS Running the Internet: Protocol, Policy, and Privacy (1st ed.). Wiley. p. 25. ISBN   9781118679555.
14. J. White (9 August 1971). A User TELNET - Description of an Initial Implementation. Computer Research Lab, UCSB. doi: 10.17487/RFC0206 . RFC 206.
15. TELNET Protocol Specification. 1 May 1973. doi: 10.17487/RFC0495 . RFC 495.
16. Shimonski, Robert J.; Eaton, Wally; Khan, Umer; Gordienko, Yuri (2002-01-01), Shimonski, Robert J.; Eaton, Wally; Khan, Umer; Gordienko, Yuri (eds.), "Chapter 11 - Detecting and Performing Security Breaches with Sniffer Pro", Sniffer Pro Network Optimization and Troubleshooting Handbook, Burlington: Syngress, pp. 513–565, doi:10.1016/B978-193183657-9/50015-0, ISBN   978-1-931836-57-9 , retrieved 2023-01-12
17. Samtani, Sagar; Yu, Shuo; Zhu, Hongyi; Patton, Mark; Chen, Hsinchun (2016). "Identifying SCADA vulnerabilities using passive and active vulnerability assessment techniques". 2016 IEEE Conference on Intelligence and Security Informatics (ISI). pp. 25–30. doi:10.1109/ISI.2016.7745438. ISBN   978-1-5090-3865-7. S2CID   11741873.
18. Kirk, Jeremy (2007-02-12). "Zero-day flaw in Solaris allows remote attacks". Network World. Retrieved 2023-01-12.
19. "TCP/IP Ports Required for IBM i Access and Related Functions". IBM Support. Archived from the original on 2016-09-18. Retrieved 2016-09-07.
20. Todorov, Dobromir (2007). Mechanics of user identification and authentication : fundamentals of identity management. Boca Raton: Auerbach Publications. ISBN   978-1-4200-5220-6. OCLC   263353270.
21. 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.
22. "SysAdmin MD". www.sysadmin.md. Retrieved 2023-01-12.
23. 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.
24. "Telnet | PDF | Networking Standards | Internet Standards". Scribd. Retrieved 2023-01-12.
25. TELNET CHARSET Option. doi: 10.17487/RFC2066 . RFC 2066.
26. "Telnet Options". www.iana.org. Retrieved 2023-01-12.
27. "The Lost Worlds of Telnet". The New Stack. 10 March 2019. Retrieved 5 June 2022.

Further reading

Internet Standards

• RFC   854, Telnet Protocol Specification
• RFC   855, Telnet Option Specifications
• RFC   856, Telnet Binary Transmission
• RFC   857, Telnet Echo Option
• RFC   858, Telnet Suppress Go Ahead Option
• RFC   859, Telnet Status Option
• RFC   860, Telnet Timing Mark Option
• RFC   861, Telnet Extended Options: List Option

Proposed Standards

• RFC   885, Telnet End of Record Option
• RFC   1073, Telnet Window Size Option
• RFC   1079, Telnet Terminal Speed Option
• RFC   1091, Telnet Terminal-Type Option
• RFC   1096, Telnet X Display Location Option
• RFC   1123, Requirements for Internet Hosts - Application and Support
• RFC   1184, Telnet Linemode Option
• RFC   1372, Telnet Remote Flow Control Option
• RFC   1572, Telnet Environment Option
• RFC   2941, Telnet Authentication Option
• RFC   2942, Telnet Authentication: Kerberos Version 5
• RFC   2943, TELNET Authentication Using DSA
• RFC   2944, Telnet Authentication: SRP
• RFC   2946, Telnet Data Encryption Option
• RFC   4248, The telnet URI Scheme

Informational/experimental

• RFC   1143, The Q Method of Implementing TELNET Option Negotiation
• RFC   1571, Telnet Environment Option Interoperability Issues
• RFC   2066, TELNET CHARSET Option

Other RFCs

• RFC   748, TELNET RANDOMLY-LOSE Option (April Fool's Day RFC joke)
• RFC   1041, Telnet 3270 Regime Option
• RFC   1097, TELNET SUBLIMINAL-MESSAGE Option (April Fool's Day RFC joke)
• RFC   1205, 5250 Telnet Interface
• RFC   2355, TN3270 Enhancements
• RFC   2217, Telnet Com Port Control Option
• RFC   4777, IBM's iSeries Telnet Enhancements

External links

• 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
• "telnet.org—information about telnet". telnet.org. Retrieved 2020-01-07. Contains a list of telnet addresses and list of telnet clients