TELNET (TELecommunication NETwork) is a network protocol used on the Internet or local area network (LAN) connections. It was developed in 1969 beginning with RFC 15 and standardized as IETF STD 8, one of the first Internet standards.
The term telnet also refers to software which implements the client part of the protocol. TELNET clients have been available on most Unix systems for many years and are available for virtually all platforms. Most network equipment and OSs with a TCP/IP stack support some kind of TELNET service server for their remote configuration (including ones based on Windows NT). Because of security issues with TELNET, its use has waned as it is replaced by the use of SSH for remote access.
Most often, a user will be telneting to a Unix-like server system or a simple network device such as a switch. For example, a user might "telnet in from home to check his mail at school". In doing so, he would be using a telnet client to connect from his computer to one of his servers. Once the connection is established, he would then log in with his account information and execute operating system commands remotely on that computer, such as ls or cd.
On many systems, the client may also be used to make interactive raw-TCP sessions, even when that option is not available, telnet sessions are equivalent to raw TCP as long as byte 255 never appears in the data.
The protocol has many extensions, some of which have been adopted as Internet standards. IETF standards STD 27 through STD 32 define various extensions, most of which are extremely common. Other extensions are on the IETF standards track as proposed standards.
When TELNET was initially developed in 1969, most users of networked computers were in the computer departments of academic institutions, or at large private and government research facilities. In this environment, security was not nearly as much of a concern as it became after the bandwidth explosion of the 1990s. The rise in the number of people with access to the Internet, and by extension, the number of people attempting to crack other people's servers made encrypted alternatives much more necessary.
Experts in computer security, such as SANS Institute, and the members of the comp.os.linux.security newsgroup recommend that the use of TELNET for remote logins should be discontinued under all 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 practical 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 and password information (and whatever else is typed) with any of several common utilities like tcpdump and Wireshark.
- Most implementations of TELNET lack an authentication scheme that makes it possible to ensure that communication is carried out between the two desired hosts, and not intercepted in the middle.
- Commonly used TELNET daemons have several vulnerabilities discovered over the years.
These security-related shortcomings have seen the usage of the TELNET protocol drop rapidly, especially on the public Internet, in favor of the ssh protocol, first released in 1995. 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.
As has happened with other early Internet protocols, extensions to the TELNET protocol provide TLS security and SASL authentication that address the above issues. However, most TELNET implementations do not support these extensions; and there has been relatively little interest in implementing these as SSH is adequate for most purposes. The main advantage of TLS-TELNET would be the ability to use certificate-authority signed server certificates to authenticate a server host to a client that does not yet have the server key stored. In SSH, there is a weakness in that the user must trust the first session to a host when it has not yet acquired the server key.
IBM 5250 or 3270 workstation emulation is supported via custom TELNET clients, TN5250/TN3270, and IBM servers. 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 OS/400, Port 992 is the default port for Secured TELNET.
As of the mid-2000s, while the TELNET protocol itself has been mostly superseded, TELNET clients are still used, often when diagnosing problems, to manually "talk" to other services without specialized client software. For example, it is sometimes used in debugging network services such as an SMTP, IRC, HTTP, FTP or POP3 server, by serving as a simple way to send commands to the server and examine the responses.
However, other software such as nc (netcat) or socat on Unix (or PuTTY on Windows) are finding greater favor with some system administrators for testing purposes, as they can be called with arguments not to send any terminal control handshaking data. Also netcat does not distort the \377 octet, which allows raw access to TCP socket, unlike any standard-compliant TELNET software.
In the 2007 Microsoft Windows release, Windows Vista, Telnet.exe is no longer installed by default, but is still included as an installable feature.