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Telnet
Telnet (short for "teletype network") is a client/server application protocol that provides access to virtual terminals of remote systems on local area networks or the Internet. It is a protocol for bidirectional 8-bit communications. Its main goal was to connect terminal devices and terminal-oriented processes.
Telnet consists of two components: (1) the protocol itself which specifies how two parties are to communicate and (2) the software application that provides the service. 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 855, and standardized as Internet Engineering Task Force (IETF) Internet Standard STD 8, one of the first Internet standards. Telnet transmits all information including usernames and passwords in plaintext so it is not recommended for security-sensitive applications such as remote management of routers. Telnet's use for this purpose has waned significantly in favor of SSH. Some extensions to Telnet which would provide encryption have been proposed.
Components
Telnet consists of two components: (1) the protocol itself and (2) the service component. The telnet protocol is a client-server protocol, based on a reliable connection-oriented transport. This protocol is used to establish a connection to Transmission Control Protocol (TCP) port number 23 or 2323, where a Telnet server application is listening. The Telnet protocol abstracts any terminal as a Network Virtual Terminal (NVT). The client must simulate a NVT using the NVT codes when messaging the server.
Telnet predated UDP/IP and originally ran over Network Control Protocol (NCP). The telnet service is best understood in the context of a user with a simple terminal using the local Telnet program (known as the client program) to run a logon session on a remote computer where the user's communications needs are handled by a Telnet server program.
Telnet Protocol
History
Even though Telnet was an ad hoc protocol with no official definition until March 5, 1973, the name actually referred to Teletype Over Network Protocol as the RFC 206 (NIC 7176) on Telnet makes the connection clear:
The TELNET protocol is based upon the notion of a virtual teletype, employing a 7-bit ASCII character set. The primary function of a User TELNET, then, is to provide the means by which its users can 'hit' all the keys on that virtual teletype.
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 with the publication of two NIC documents: Telnet Protocol Specification, NIC 15372, and Telnet Option Specifications, NIC 15373.
Extensions
Many extensions were made for Telnet because of its negotiable options protocol architecture. Some of these extensions 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).
Telnet service
The Telnet service is the application providing services over the Telnet protocol. Most operating systems provide a service that can be installed or enabled to provide Telnet services to clients.
Security vulnerabilities
Telnet is vulnerable to network-based cyberattacks, such as packet sniffing sensitive information including passwords and fingerprinting. Telnet services can also be exploited to leak information about the server (such as hostnames, IP addresses and brand) by packet sniffing the banner. This information can then be searched to determine if a Telnet service accepts a connection without authentication. Telnet is also frequently exploited by malware due to being improperly configured. In fact, Telnet is targeted by attackers more frequently than other common protocols, especially when compared to UPnP, CoAP, MQTT, AMQP and XMPP. Common devices targeted are Internet of things devices, routers and modems.
The SANS Institute recommends that the use of Telnet for remote logins should be discontinued under 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 feasible 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 Telnet implementations lack authentication. An estimated 22,887 Telnet-enabled devices found by security researchers not only lacked authentication but also provided unrestricted access to the system.
Most Telnet authentication mechanisms are vulnerable to being intercepted by Man-in-the-middle attacks.
Extensions to Telnet provide Transport Layer Security (TLS) security and Simple Authentication and Security Layer (SASL) authentication that address the above concerns. However, most Telnet implementations do not support these extensions; and they do not address other vulnerabilities such as parsing the banner information.
IBM 5250 or 3270 workstation emulation is supported via custom telnet clients, TN5250/TN3270, and IBM i systems. 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 IBM i (also known as OS/400), port 992 is the default port for secured telnet.
Uses
Historical uses
Historically, Telnet provided access to a command-line interface on a remote host. However, because of serious security concerns when using Telnet over an open network such as the Internet, its use for this purpose has waned significantly in favor of SSH. The usage of Telnet for remote management has declined rapidly, especially on the public Internet, in favor of the Secure Shell (SSH) protocol. 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.
Modern day uses
Telnet may be used in debugging network services such as SMTP, IRC, HTTP, FTP or POP3, to issue commands to a server and examine the responses. For example, Telnet client applications can establish an interactive TCP session to a port other than the Telnet server port. For example, a command line telnet client could make an HTTP request to a web server on TCP port 80 as follows:
The older protocol is used these days only in rare cases to access decades-old legacy equipment that does not support more modern protocols. For example, a large number of industrial and scientific devices only have Telnet available as a communication option. Some are built with only a standard RS-232 port and use a serial server hardware appliance to provide the translation between the TCP/Telnet data and the RS-232 serial data. In such cases, SSH is not an option unless the interface appliance can be configured for SSH (or is replaced with one supporting SSH).
Telnet is commonly used by amateur radio operators for providing public information.
Although recommended against, security researchers estimated that 7,096,465 devices connected to the Internet continue to use Telnet, however, much less are often estimated because most estimates only scan for TCP port 23.
Technical details
The technical details of Telnet are defined by a variety of specifications including RFC 854.
USASCII control codes
NULL
240
Line feed
241
Carriage return
242
Bell
243
Backspace
244
Horizontal tab
245
Vertical tab
246
Form feed
247
Telnet commands
Telnet commands consist of at least two bytes. The first byte is the IAC escape character (typically byte 255) followed by the byte code for a given command:
SE (Subnegotiation end)
240
NOP (No operation)
241
Data Mark
242
Break
243
Interrupt Process
244
Abort output
245
Are you there?
246
Erase character
247
Erase Line
248
Go ahead
249
SB (Subnegotiation begin)
250
WILL
251
WON'T
252
DO
253
DON'T
254
Interpret As Command
All data octets except 0xff are transmitted over Telnet as is. (0xff, or 255 in decimal, is the IAC byte (Interpret As Command) which signals that the next byte is a telnet command. The command to insert 0xff into the stream is 0xff, so 0xff must be escaped by doubling it when sending data over the telnet protocol.)
Telnet options
Telnet also has a variety of options that terminals implementing Telnet should support.
0
Binary Transmission
The 8-bit mode (so named binary option) is intended to transmit binary data, not ASCII characters. The standard suggests the interpretation of codes 0000–0176 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, but nothing is known about its actual software support.
1
Echo
2
Reconnection
NIC 15391 of 1973
3
Suppress Go Ahead
The "Go Ahead" command code (249) in the original Telnet protocol is used to notify to the other end that the other end could start sending back messages. This was used in "half duplex" communication, as some terminals could send messages and receive messages, but not simultaneously.
4
Approx Message Size Negotiation
NIC 15393 of 1973
5
Status
6
Timing Mark
7
Remote Controlled Trans and Echo
8
Output Line Width
NIC 20196 of August 1978
9
Output Page Size
NIC 20197 of August 1978
10
Output Carriage-Return Disposition
RFC 652
11
Output Horizontal Tab Stops
RFC 653
12
Output Horizontal Tab Disposition
RFC 654
13
Output Formfeed Disposition
RFC 655
14
Output Vertical Tabstops
RFC 656
15
Output Vertical Tab Disposition
RFC 657
16
Output Linefeed Disposition
RFC 658
17
Extended ASCII
RFC 698
18
Logout
RFC 727
19
Byte Macro
RFC 735
20
Data Entry Terminal
21
SUPDUP
22
SUPDUP Output
RFC 749
23
Send Location
RFC 779
24
Terminal Type
RFC 1091
25
End of Record
RFC 885
26
TACACS User Identification
RFC 927
27
Output Marking
RFC 933
28
Terminal Location Number
RFC 946
29
Telnet 3270 Regime
RFC 1041
30
X.3 PAD
RFC 1053
31
Negotiate About Window Size
RFC 1073
32
Terminal Speed
RFC 1079
33
Remote Flow Control
RFC 1372
34
Linemode
RFC 1184
35
X Display Location
RFC 1096
36
Environment Option
RFC 1408
37
Authentication Option
RFC 2941
38
Encryption Option
RFC 2946
39
New Environment Option
RFC 1572
40
TN3270E
RFC 2355
41
XAUTH
42
CHARSET
RFC 2066
43
Telnet Remote Serial Port (RSP)
44
Com Port Control Option
RFC 2217
45
Telnet Suppress Local Echo
46
Telnet Start TLS
47
KERMIT
RFC 2840
48
SEND-URL
49
FORWARD_X
50-137
Unassigned
138
TELOPT PRAGMA LOGON
139
TELOPT SSPI LOGON
140
TELOPT PRAGMA HEARTBEAT
141-254
Unassigned
255
Extended-Options-List
RFC 861
Internet Standards
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
Proposed Standards
RFC 885, Telnet End of Record 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 1184, Telnet Linemode Option
RFC 1372, Telnet Remote Flow Control Option
RFC 1572, Telnet Environment 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
Informational/experimental
RFC 1143, The Q Method of Implementing TELNET Option Negotiation
RFC 1571, Telnet Environment Option Interoperability Issues
Other RFCs
RFC 1041, Telnet 3270 Regime Option
RFC 2355, TN3270 Enhancements
RFC 1205, 5250 Telnet Interface
RFC 2217, Telnet Com Port Control Option
RFC 4777, IBM's iSeries Telnet Enhancements
Telnet clients
PuTTY and plink command line are 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,
RUMBA (Terminal Emulator)
Line Mode Browser, a command line web browser
NCSA Telnet
TeraTerm
SecureCRT from Van Dyke Software
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
Rtelnet is a SOCKS client version of Telnet, providing similar functionality of telnet to those hosts which are behind firewall and NAT.
Inetutils includes a telnet client and server and is installed by default on many Linux distributions.
telnet.exe command line utility included in default installation of many versions of Microsoft Windows.
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