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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:

$ telnet www.example.com 80
GET /path/to/file.html HTTP/1.1
Host: www.example.com
Connection: close

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

Name	Byte code	Explanation	Notes
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:

Name	Byte code	Explanation	Notes
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.

Code	Name	Spec	Notes
0	Binary Transmission	RFC 856	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	RFC 857
2	Reconnection	NIC 15391 of 1973
3	Suppress Go Ahead	RFC 858	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	RFC 859
6	Timing Mark	RFC 860
7	Remote Controlled Trans and Echo	RFC 726
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	RFC 1043 RFC 732
21	SUPDUP	RFC 736 RFC 734
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.