Telnet





Telnet is a network protocol used on the Internet. IETF document STD 8 (aka
RFC 854 and RFC 855) starts:

     The purpose of the TELNET Protocol is to provide a fairly general,
     bi-directional, eight-bit byte oriented communications facility.

It is typically used to provide user oriented command line login sessions
between hosts on the internet.

By extension, telnet is also the name of a program that a user can use to
invoke a Telnet session to a remote host; the telnet program provides the
client part of the protocol. Telnet clients have been available on most Unix
systems for many years, and are available for virtually all types of
computers.

"To telnet" is also used as a verb meaning to establish or use such a
connexion, as in, "If you need to change your password you need to telnet to
the server and run the passwd command".

Protocol details

Telnet is a client-server protocol, based on TCP, and clients generally
connect to port 23 on the host providing the service (though like many
protocols in use on the Internet which port to use is fairly easy to
change). Partly because of the design of the protocol and partly because of
the flexibility typically provided by telnet programs, it is also possible
to use a Telnet program to establish an interactive TCP connection to some
other service on an internet host. A classic use of this is telnetting to
port 25 (where typically an SMTP server is to be found) to debug a mail server.

The Telnet protocol can be divided into a core and a set of extensions. The
core protocol is described by IETF documents RFC 854 and RFC 855 which are
also collected together in STD 8. STD 8 defines fairly basic operating
characteristics of the protocol and a means of defining and implementing
extensions. There are many extensions, some of which have been adopted as
Internet Standards, some of which haven't. IETF STD document numbers from 27
through to 32 define various Telnet extensions (most of which are extremely
common). Of the remaining extensions the most useful ones are probably those
that are on the IETF standards track as proposed standards; details can be
found in STD 1.

Security

There are three main problems with Telnet, making it a bad choice for modern
systems from the point of view of security:

   * Commonly used telnet daemons have several vulnerabilities discovered
     over the years, and probably several more still exist.
   * Telnet does not encrypt any data send over the connection (including
     passwords), and so it is trivial to eavesdrop on the communications and
     use the password later for malicious purposes.
   * Telnet lacks 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.

In environments where security is important, such as on the public Internet,
telnet should not be used. Telnet sessions are unencrypted. This means that
anybody who has access to any router, switch, or gateway located on the
network between the two hosts where telnet is being used can intercept the
telnet packets passing by and easily obtain login and password information
(and whatever else is typed) with any of several common utilities like
tcpdump and Ethereal.

These flaws have seen the usage of the Telnet protocol drop rapidly in favor
of a more secure and functional protocol called SSH, released in 1998. SSH
provides all functionality present in 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.

Experts in Computer Security, such as SANS, 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.

When telnet was being developed in the early 1980s, 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. With the exponential rise in the number of
people with access to the Internet, and by extension, the number of people
attempting to crack into other people's servers, telnet should generally not
ever be used on networks with Internet connectivity.

Telnet clients are still occasionally used to manually "talk" to other
services. It is sometimes used in debugging network services such as an SMTP
server, by serving as a simple way to send commands to the server and
examine the responses.