Telnet

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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. Typically, telnet provides access to a command-line interface on a remote machine.

The term telnet also refers to software which implements the client part of the protocol. Telnet clients are available for virtually all platforms. Most network equipment and OSes 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 in favor 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 telnetting to a Unix-like
server system or a simple network device such as a router. 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).

[edit] Protocol details

Telnet is a client-server protocol, based on a reliable connection-oriented transport. Typically this protocol is 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.

Before March 5th, 1973, Telnet was an ad-hoc protocol with no official definition ^[1].
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 ^[2]
where "New Telnet" was defined in two NIC documents: Telnet Protocol
Specification, NIC #15372, and Telnet Option Specifications, NIC
#15373. This new protocol, and not the old telnet protocol is what
continues in use today.

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.

[edit] 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 of a necessity.

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 that would ensure 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.

[edit] 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.

[edit] 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 what 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

[edit] Related RFCs

[edit] Telnet clients

• PuTTY is a free, open source SSH, Telnet, rlogin, and raw TCP client for Windows, Linux, and Unix.
• NCSA Telnet

[edit] See also

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

[edit] References

[edit] 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