Telnet - Current status

As of the mid-2000s, while the Telnet protocol itself has been mostly superseded for remote login, Telnet clients are still used, often when diagnosing problems, to manually "talk" to other services without specialized client software.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.



This approach has limitations as Telnet clients speak is close to, but not equivalent to, raw mode (due to terminal control handshaking and the special rules regarding \377 and \15). Thus, 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.



Telnet is popular with:

* enterprise networks to access host applications, e.g. on IBM Mainframes.
* administration of network elements, e.g., in commissioning, integration and maintenance of core network elements in mobile communication networks.
* MUD games played over the Internet, as well as talkers, MUSHes, MUCKs, MOOes, and the resurgent BBS community.
* embedded systems

Telnet - protocol details

Telnet is a client-server protocol, based on a reliable connection-oriented transport. Typically this protocol used to establish a connection to TCP port 23, where a getty-equivalent program (telnetd) is listening, although Telnet predates TCP/IP and was originally run on NCP.

Initially, Telnet was an ad-hoc protocol with no official definition. 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 5th, 1973, a meeting was held at UCLA where "New Telnet" was defined in two NIC documents: Telnet Protocol Specification, NIC #15372, and Telnet Option Specifications, NIC #15373.

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.

Telnet 5250

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.