Telnet (teletype network) is a network protocol used on the Internet or local
area networks to provide a bidirectional interactive communications facility.
Typically, telnet provides access to a command-line interface on a remote host
via a virtual terminal connection which consists of an 8-bit byte oriented data
connection over the Transmission Control Protocol (TCP). User data is
interspersed in-band with TELNET control information.
Telnet is also used as a verb. To
telnet means to establish a connection with the Telnet protocol, either with
command line client or with a programmatic interface. For example, a common
directive might be: "To change your password, telnet to the server, login and
run the passwd command." 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 recommend that the use of Telnet for remote logins
should be discontinued under all normal circumstances, for the following
reasons:
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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).
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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.
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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 Secure Shell (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 Transport Layer Security
(TLS) security and Simple Authentication and Security Layer (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.
Source: Wikipedia, the free encyclopedia