Telnet

For the packet switched network, see Telenet.

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.

"To telnet" is also used as a verb meaning to establish or use a TELNET or other interactive TCP connection, as in, "To change your password, telnet to the server and run the passwd command".

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. It is commonly believed that a telnet session which does not include an IAC (character 255) is functionally identical. This is not the case however due to special NVT (Network Virtual Terminal) rules such as the requirement for a bare CR (ASCII 13) to be followed by a NULL (ASCII 0).

Protocol details

TELNET is a client-server protocol, based on a reliable connection-oriented transport. Typically this is TCP port 23, 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.

Security

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 have no authentication 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.

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.

Current status

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.

TELNET is still very popular in enterprise networks to access host applications, e.g. on IBM Mainframes.

TELNET is still widely used for administration of network elements, e.g., in commissioning, integration and maintenance of core network elements in mobile communication networks.

TELNET is also heavily used for MUD games played over the Internet, as well as talkers, MUSHes, MUCKs, MOOes, and the resurgent BBS community.

In the Microsoft Windows Vista environment, Telnet is not enabled by default but can be freely enabled by the user via the control panel. This can be done by accessing the Control Panel->Programs->Turn Windows Features On or Off Dialog. More Vista support literature for TELNET may be found online at http://search.microsoft.com/ and by searching "vista telnet" or something similar.

See also

• Reverse telnet
• Telnet 3270 (TN3270)
• HyTelnet
• Kermit
• NCSA Telnet

External links

• Telnet Options - The official list of assigned option numbers at iana.org
• telnet.org places List of publicly accessible TELNET servers
• BBS Corner - Telnet BBS Guide
• Telnet Interactions Described as a Sequence Diagram
• Telnet START-TLS Option draft-altman-telnet-starttls-02.txt 2006

Related RFCs

• RFC 748 TELNET randomly-lose option
• 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
• RFC 885 Telnet end of record option
• RFC 1041 Telnet 3270 regime 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 1097 Telnet subliminal-message option
• RFC 1184 Telnet linemode option
• RFC 1205 5250 Telnet interface
• RFC 1372 Telnet remote flow control option
• RFC 2217 Telnet Com Port Control 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
• RFC 4777 IBM's iSeries Telnet Enhancements

TELNET clients

Windows

• Windows comes with a built in telnet client, accessible from the command prompt.
• AbsoluteTelnet is a shareware client for all versions of Windows, and includes telnet, SSH1, SSH2, and file transfer using SFTP, ZModem, YModem, and XModem. Includes a tabbed interface.
• Hyperterminal Private Edition is another Windows telnet client.
• TeraTerm is a free telnet/SSH client for Windows that offers more features than the built-in telnet as well as offering a free SSH plug-in.
• TN3270 Plus is a shareware telnet/SSH client for Windows.

Macintosh

• tn3270 is a free TELNET client for Macintosh designed to work with IBM mainframe systems that use the TN3270 protocol.
• Terminal is a TELNET capable command line interface application that comes as part of all versions of Macintosh OS X.
• NiftyTelnet is a small Telnet client for older MacOS versions that also support SSH.
• dataComet is a full-featured Telnet & SSH application for the Macintosh.

Multiplatform

• PuTTY is a free SSH, TELNET, rlogin, and raw TCP client for Windows, Linux, and Unix.
• mTelnet is a free full-screen TELNET client for Windows & OS/2. Easy to use client with Zmodem download capability.
• Twisted Conch includes a telnet client/server implementation.
• IVT is a free multisession TELNET client for Windows & DOS. Also supports SSH and Kerberos (not free). Includes useful features like auto-login and scripting.
• SyncTERM is a free open-source TELNET/RLogin/SSH client/terminal for Windows and *nix platforms with windowed/full-screen and Zmodem support.

Free with Source Code

• dtelnet is a free TELNET client for Windows.
• Pueblo/UE is a free TELNET client for Windows with support for in-line HTML.
• Console telnet is a full screen (console) TELNET client for MS Win32 environments.

Non-free Source Code

• Whitehorn Secure Terminal free TELNET client for Windows.
• Simpterm is a Unicode savy TELNET client for MS Win32 environments that for example can be used for Chinese.
• SimpleMU is a shareware TELNET client.
• CRT and SecureCRT are TELNET clients with scripting capabilities in VBScript, Javascript and Perl.

TELNET servers

Free with Source Code

• telnetd is an embeddable TELNET daemon written in Java.
• Poor Woman's Telnet Server is a standalone java TELNET server which bases upon telnetd.
• Synchronet is a Win32 and Linux BBS Server that also supports http, nntp, pop3, ftp and other protocols.