Telnet

Description

The telnet protocol is a client-server protocol that runs on a reliable connection-oriented transport. Most often, a telnet client connects over TCP to port 23 or 2323, where a Telnet server application is listening. 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 as an acronym or abbreviation.

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".

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.{{cite book |access-date=5 April 2025 |access-date=2025-04-06

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. 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. 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 is targeted by attackers more frequently than other common protocols, especially when compared to UPnP, CoAP, MQTT, AMQP, and XMPP. 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:

• 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.
• Many Telnet implementations lack authentication.
• Most Telnet authentication mechanisms are vulnerable to being intercepted by Man-in-the-middle attacks. Extensions to Telnet provide Transport Layer Security (TLS) security and Simple Authentication and Security Layer (SASL) authentication that address the above concerns. However, most Telnet implementations do not support these extensions; and they do not address other vulnerabilities such as parsing the banner information. 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).

Uses

Historical

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.

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.

Technical details

The technical details of Telnet are defined by a variety of specifications including .

Commands

Telnet commands consist of at least two bytes. The first byte is the IAC escape character (byte 255) followed by the byte code for a given command:

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.)

Options

Telnet has a variety of options that terminals implementing Telnet should support.

Client applications

• 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.

References

References

1. (2019). "2019 42nd International Convention on Information and Communication Technology, Electronics and Microelectronics (MIPRO)".
2. (2015). "2015 23nd Signal Processing and Communications Applications Conference (SIU)".
3. Todorov, Dobromir. (2007). "Mechanics of user identification and authentication : fundamentals of identity management". Auerbach Publications.
4. (1983). "Telnet Protocol Specification". Network Working Group.
5. "Service Name and Transport Protocol Port Number Registry".
6. (2021-11-02). "Proceedings of the 21st ACM Internet Measurement Conference". Association for Computing Machinery.
7. Postel, J.. (1981). "NCP/TCP transition plan". Network Working Group.
8. (1971). "Proceedings of the November 16-18, 1971, fall joint computer conference on - AFIPS '71 (Fall)". Association for Computing Machinery.
9. Bruen, Garth O.. (2015). "WHOIS Running the Internet: Protocol, Policy, and Privacy". Wiley.
10. (April 3, 1972). "Telnet Protocol".
11. (2002-01-01). "Chapter 11 - Detecting and Performing Security Breaches with Sniffer Pro". Syngress.
12. (2016). "2016 IEEE Conference on Intelligence and Security Informatics (ISI)".
13. Kirk, Jeremy. (2007-02-12). "Zero-day flaw in Solaris allows remote attacks".
14. Mahmood, H.B.. (2003). "9th Asia-Pacific Conference on Communications (IEEE Cat. No.03EX732)".
15. "TCP/IP Ports Required for IBM i Access and Related Functions". IBM Support.
16. Todorov, Dobromir. (2007). "Mechanics of user identification and authentication : fundamentals of identity management". Auerbach Publications.
17. (2 April 2007). "Telnet, dead at 35...RIP".
18. "History of the SSH Protocol". SSH Communications Security, Inc..
19. "What is Telnet? {{!}} PDF {{!}} Networking Standards {{!}} Internet Standards".
20. Bartle, Richard. (2003). "[[Designing Virtual Worlds]]". New Riders.
21. "Debugging server services with telnet".
22. "TELNET CHARSET Option".
23. "Telnet Options".
24. (10 March 2019). "The Lost Worlds of Telnet".