Telnet is a network protocol used on the Internet or local area
networks to provide a bidirectional interactive text-oriented
communication facility using a virtual terminal connection. User data
is interspersed in-band with Telnet control information in an 8-bit
byte oriented data connection over the Transmission Control Protocol
(TCP).
Telnet was developed in 1969 beginning with RFC 15, extended in RFC
854, and standardized as Internet Engineering Task Force (IETF)
Internet Standard STD 8, one of the first Internet standards.
Historically, Telnet provided access to a command-line interface
(usually, of an operating system) on a remote host. Most network
equipment and operating systems with a TCP/IP stack support a Telnet
service for remote configuration (including systems based on Windows
NT). Because of security issues with Telnet, its use for this purpose
has waned in favor of SSH.
The term telnet may also refer to the software that implements the
client part of the protocol. Telnet client applications are available
for virtually all computer platforms. 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, log in and run the passwd command." Most often, a user
will be telnetting to a Unix-like server system or a network device
(such as a router) and obtain a login prompt to a command line text
interface or a character-based full-screen manager.
Contents
1 History and standards
2 Security
3 Telnet 5250
4 Telnet data
5 Current status
6 Related RFCs
7 Telnet clients
8 See also
9 References
10 External links
History and standards
Telnet is a client-server protocol, based on a reliable
connection-oriented transport. Typically this protocol is used to
establish a connection to Transmission Control Protocol (TCP) port
number 23, where a Telnet server application (telnetd) is listening.
Telnet, however, predates TCP/IP and was originally run over Network
Control Program (NCP) protocols.
Before March 5, 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 5, 1973, a Telnet protocol standard was defined at
UCLA[2] with the publication of two NIC documents: Telnet Protocol
Specification, NIC #15372, and Telnet Option Specifications, NIC
#15373.
Because of negotiable options protocol architecture, many extensions
were made for it, some of which have been adopted as Internet
standards, IETF documents STD 27 through STD 32. Some extensions have
been widely implemented and others are proposed standards on the IETF
standards track (see below)
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 hack other people's servers made encrypted
alternatives much more of a necessity.
Experts in computer security, such as SANS Institute, 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,
password and whatever else is typed with a packet analyzer.
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 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.
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.[citation needed]
Telnet data
All data octets except \377 are transmitted over the TCP transport as
is. Therefore, a Telnet client application may also be used to
establish an interactive raw TCP session, and it is commonly believed
that such session which does not use the IAC (\377 character, or 255
in decimal) is functionally identical.[citation needed] This is not
the case, however, because there are other network virtual terminal
(NVT) rules, such as the requirement for a bare carriage return
character (CR, ASCII 13) to be followed by a NULL (ASCII 0) character,
that distinguish the telnet protocol from raw TCP sessions. On the
other hand, many systems now possess true raw TCP clients, such as
netcat or socat on UNIX and PuTTY on Windows, which also can be used
to manually "talk" to other services without specialized client
software. Nevertheless, Telnet is still sometimes used in debugging
network services such as SMTP, IRC, HTTP, FTP or POP3 servers, to
issue commands to a server and examine the responses, but of all these
protocols only FTP really uses Telnet data format.
Another difference of Telnet from a raw TCP session is that Telnet is
not 8-bit clean by default. 8-bit mode may be negotiated, but
high-bit-set octets may be garbled until this mode was requested, and
it obviously will not be requested in non-Telnet connection. The 8-bit
mode (so named binary option) is intended to transmit binary data, not
characters though. The standard suggests the interpretation of codes
\000–\176 as ASCII, but does not offer any meaning for high-bit-set
data octets. There was an attempt to introduce a switchable character
encoding support like HTTP has,[3] but nothing is known about its
actual software support.
Current status
As of mid-2010, the Telnet protocol itself has been mostly superseded
for remote login. Telnet is popular in various application areas:
Enterprise networks to access host applications, e.g., on IBM Mainframes.
Administration of network elements, e.g., in configuring routers
on a home network, in commissioning, integration and maintenance of
core network elements in mobile communication networks, and many
industrial control systems.
MUD games played over the Internet, as well as talkers, MUSHes,
MUCKs, MOOes.
Telnet Bulletin Board Systems.
Internet game clubs, like the Internet Chess Club, the Free
Internet Chess Server and the Internet Go server.
Connection to Amateur Radio DX clusters.
Embedded systems.
Mobile data collection applications where telnet runs over secure networks
Troubleshooting and testing of basic TCP functionality between IP
endpoints, often as a response to editing firewall rules, initial
endpoint configuration, or partial connectivity loss.
Related RFCs
RFC 137, TELNET protocol specification
RFC 139, TELNET protocol specification
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 1123, Requirements for Internet Hosts - Application and Support
RFC 1143, The Q Method of Implementing TELNET Option Negotiation
RFC 1184, Telnet linemode option
RFC 1205, 5250 Telnet interface
RFC 1372, Telnet remote flow control option
RFC 1572, Telnet Environment 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
PuTTY is a free, open source SSH, Telnet, rlogin, and raw TCP
client for Windows, Linux, and Unix.
AbsoluteTelnet is a telnet client for Windows. It also supports
SSH and SFTP,
Host Explorer part of Hummingbird Connectivity; it implements the
telnet, TN 3270, 5250, Ansi, and other protocols.
RUMBA (Terminal Emulator)
Line Mode Browser, a command line web browser
NCSA Telnet
TeraTerm
Passport from Zephyr Corp, available as both regular executables
and a web-based SSH/Telnet application
SecureCRT from Van Dyke Software
TeSSH from Zugg Software for Windows based upon the zMUD and CMUD
code base also supports SSH and SFTP
TN3270 Plus from SDI USA, Inc. available for Windows and supports
terminal emulation types TN3270, TN5250, VT100 and VT220. It also
supports SSH, TLS and SSL.
ZOC Terminal
SyncTERM BBS terminal program supporting Telnet, SSHv2, RLogin,
Serial, Windows, *nix, and Mac OS X platforms, X/Y/ZMODEM and various
BBS terminal emulations
PCMan and KKMan are often used by Telnet user in Taiwan (Republic
of China), where Telnet BBS are still popular.
PowerTerm InterConnect from Ericom available for Windows, Mac OS
X, Linux, Windows CE and supports 35 terminal emulation types
including TN3270, TN5250, VT420, Wyse and others with SSH and SSL.
Rtelnet is a SOCKS client version of Telnet, providing similar
functionality of telnet to those hosts which are behind firewall and
NAT.
Xshell is a terminal emulator that supports SSH, SFTP (SSH File
Transfer Protocol), Telnet, rlogin and SERIAL from NetSarang.
See also
Virtual terminal
Reverse telnet
HyTelnet
Kermit
SSH
References
^ RFC 318 - documentation of old ad-hoc telnet protocol
^ RFC 495 - Announcement of Telnet protocol
^ RFC 2066: TELNET CHARSET Option
networks to provide a bidirectional interactive text-oriented
communication facility using a virtual terminal connection. User data
is interspersed in-band with Telnet control information in an 8-bit
byte oriented data connection over the Transmission Control Protocol
(TCP).
Telnet was developed in 1969 beginning with RFC 15, extended in RFC
854, and standardized as Internet Engineering Task Force (IETF)
Internet Standard STD 8, one of the first Internet standards.
Historically, Telnet provided access to a command-line interface
(usually, of an operating system) on a remote host. Most network
equipment and operating systems with a TCP/IP stack support a Telnet
service for remote configuration (including systems based on Windows
NT). Because of security issues with Telnet, its use for this purpose
has waned in favor of SSH.
The term telnet may also refer to the software that implements the
client part of the protocol. Telnet client applications are available
for virtually all computer platforms. 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, log in and run the passwd command." Most often, a user
will be telnetting to a Unix-like server system or a network device
(such as a router) and obtain a login prompt to a command line text
interface or a character-based full-screen manager.
Contents
1 History and standards
2 Security
3 Telnet 5250
4 Telnet data
5 Current status
6 Related RFCs
7 Telnet clients
8 See also
9 References
10 External links
History and standards
Telnet is a client-server protocol, based on a reliable
connection-oriented transport. Typically this protocol is used to
establish a connection to Transmission Control Protocol (TCP) port
number 23, where a Telnet server application (telnetd) is listening.
Telnet, however, predates TCP/IP and was originally run over Network
Control Program (NCP) protocols.
Before March 5, 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 5, 1973, a Telnet protocol standard was defined at
UCLA[2] with the publication of two NIC documents: Telnet Protocol
Specification, NIC #15372, and Telnet Option Specifications, NIC
#15373.
Because of negotiable options protocol architecture, many extensions
were made for it, some of which have been adopted as Internet
standards, IETF documents STD 27 through STD 32. Some extensions have
been widely implemented and others are proposed standards on the IETF
standards track (see below)
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 hack other people's servers made encrypted
alternatives much more of a necessity.
Experts in computer security, such as SANS Institute, 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,
password and whatever else is typed with a packet analyzer.
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 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.
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.[citation needed]
Telnet data
All data octets except \377 are transmitted over the TCP transport as
is. Therefore, a Telnet client application may also be used to
establish an interactive raw TCP session, and it is commonly believed
that such session which does not use the IAC (\377 character, or 255
in decimal) is functionally identical.[citation needed] This is not
the case, however, because there are other network virtual terminal
(NVT) rules, such as the requirement for a bare carriage return
character (CR, ASCII 13) to be followed by a NULL (ASCII 0) character,
that distinguish the telnet protocol from raw TCP sessions. On the
other hand, many systems now possess true raw TCP clients, such as
netcat or socat on UNIX and PuTTY on Windows, which also can be used
to manually "talk" to other services without specialized client
software. Nevertheless, Telnet is still sometimes used in debugging
network services such as SMTP, IRC, HTTP, FTP or POP3 servers, to
issue commands to a server and examine the responses, but of all these
protocols only FTP really uses Telnet data format.
Another difference of Telnet from a raw TCP session is that Telnet is
not 8-bit clean by default. 8-bit mode may be negotiated, but
high-bit-set octets may be garbled until this mode was requested, and
it obviously will not be requested in non-Telnet connection. The 8-bit
mode (so named binary option) is intended to transmit binary data, not
characters though. The standard suggests the interpretation of codes
\000–\176 as ASCII, but does not offer any meaning for high-bit-set
data octets. There was an attempt to introduce a switchable character
encoding support like HTTP has,[3] but nothing is known about its
actual software support.
Current status
As of mid-2010, the Telnet protocol itself has been mostly superseded
for remote login. Telnet is popular in various application areas:
Enterprise networks to access host applications, e.g., on IBM Mainframes.
Administration of network elements, e.g., in configuring routers
on a home network, in commissioning, integration and maintenance of
core network elements in mobile communication networks, and many
industrial control systems.
MUD games played over the Internet, as well as talkers, MUSHes,
MUCKs, MOOes.
Telnet Bulletin Board Systems.
Internet game clubs, like the Internet Chess Club, the Free
Internet Chess Server and the Internet Go server.
Connection to Amateur Radio DX clusters.
Embedded systems.
Mobile data collection applications where telnet runs over secure networks
Troubleshooting and testing of basic TCP functionality between IP
endpoints, often as a response to editing firewall rules, initial
endpoint configuration, or partial connectivity loss.
Related RFCs
RFC 137, TELNET protocol specification
RFC 139, TELNET protocol specification
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 1123, Requirements for Internet Hosts - Application and Support
RFC 1143, The Q Method of Implementing TELNET Option Negotiation
RFC 1184, Telnet linemode option
RFC 1205, 5250 Telnet interface
RFC 1372, Telnet remote flow control option
RFC 1572, Telnet Environment 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
PuTTY is a free, open source SSH, Telnet, rlogin, and raw TCP
client for Windows, Linux, and Unix.
AbsoluteTelnet is a telnet client for Windows. It also supports
SSH and SFTP,
Host Explorer part of Hummingbird Connectivity; it implements the
telnet, TN 3270, 5250, Ansi, and other protocols.
RUMBA (Terminal Emulator)
Line Mode Browser, a command line web browser
NCSA Telnet
TeraTerm
Passport from Zephyr Corp, available as both regular executables
and a web-based SSH/Telnet application
SecureCRT from Van Dyke Software
TeSSH from Zugg Software for Windows based upon the zMUD and CMUD
code base also supports SSH and SFTP
TN3270 Plus from SDI USA, Inc. available for Windows and supports
terminal emulation types TN3270, TN5250, VT100 and VT220. It also
supports SSH, TLS and SSL.
ZOC Terminal
SyncTERM BBS terminal program supporting Telnet, SSHv2, RLogin,
Serial, Windows, *nix, and Mac OS X platforms, X/Y/ZMODEM and various
BBS terminal emulations
PCMan and KKMan are often used by Telnet user in Taiwan (Republic
of China), where Telnet BBS are still popular.
PowerTerm InterConnect from Ericom available for Windows, Mac OS
X, Linux, Windows CE and supports 35 terminal emulation types
including TN3270, TN5250, VT420, Wyse and others with SSH and SSL.
Rtelnet is a SOCKS client version of Telnet, providing similar
functionality of telnet to those hosts which are behind firewall and
NAT.
Xshell is a terminal emulator that supports SSH, SFTP (SSH File
Transfer Protocol), Telnet, rlogin and SERIAL from NetSarang.
See also
Virtual terminal
Reverse telnet
HyTelnet
Kermit
SSH
References
^ RFC 318 - documentation of old ad-hoc telnet protocol
^ RFC 495 - Announcement of Telnet protocol
^ RFC 2066: TELNET CHARSET Option
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