Telnet (sometimes stylizedTELNET[1]) is aclient-serverapplication protocol that provides access to virtualterminals ofremote systems onlocal area networks or theInternet.[2] It is a protocol for bidirectional 8-bit communications. Its main goal was to connect terminal devices and terminal-oriented processes.[1]
The name "Telnet" refers to two things: a protocol specifying how two parties are to communicate and a software application that implements the protocol as a service.[2] User data is interspersedin-band with Telnet control information in an 8-bitbyte oriented data connection over theTransmission Control Protocol (TCP). Telnet transmits all information including usernames and passwords in plaintext so it is not recommended for security-sensitive applications such asremote management of routers.[2][3] Telnet's use for this purpose has waned significantly in favor ofSSH.[4] Some extensions to Telnet which would provide encryption have been proposed.[5]
| Internet protocol suite |
|---|
| Application layer |
| Transport layer |
| Internet layer |
| Link layer |
![[icon]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aWiki_letter_w_cropped.svg) | This sectionneeds expansion with: more details about the protocol itself. You can help byadding missing information. (talk)(April 2025) |
The telnet protocol is aclient-server protocol that runs on areliableconnection-oriented transport.[citation needed] Most often, a telnet client connects overTCP toport 23 or 2323, where a Telnet server application is listening.[1][6][7] The Telnet protocol abstracts any terminal as a Network Virtual Terminal (NVT). The client must simulate a NVT using theNVT codes when messaging the server.
Telnet predated UDP/IP and originally ran overNetwork Control Protocol (NCP).[8] 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.
A Telnet service is an 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.[citation needed]
The official specification stylizes the name as TELNET, which is not as an acronym or abbreviation.[1]
In a 1972 paper, when discussing one of the early forms of the protocol,Stephen Crocker et al. used "TELNET" explicitly as an abbreviation of "telecommunications network".[9]
In his 2015 bookWHOIS Running the Internet: Protocol, Policy, and Privacy, Internet researcher Garth O. Bruen claims that Telnet was originally short for "Teletype Over Network Protocol".[10]
Telnet was originally developed forARPANET in 1969.[11] Initially, it was anad hoc protocol with no formal specification,[12] but after extensive work in the 1970s, including numerous RFCs, it was officially formalized inRFC 854 andRFC 855, which together form Internet standard 8.[11][13]
Since then, many additional RFCs have updated or extended the Telnet specification, both to address issues in the original standard and to add new capabilities.[11] Some of these extensions have also been adopted asInternet standards, particularly standards 27 through 32 (seebelow).
Telnet is vulnerable to network-based cyberattacks, such aspacket sniffing sensitive information including passwords andfingerprinting.[3][14][15] Telnet services can 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 withoutauthentication. Telnet is frequently exploited bymalware due to being improperly configured.[7] Telnet is targeted by attackers more frequently than other common protocols, especially when compared toUPnP,CoAP,MQTT,AMQP, andXMPP.[citation needed] Common devices targeted areInternet of things devices, routers, and modems.
TheSANS Institute recommends that the use of Telnet for remote logins should be discontinued under normal circumstances for the following reasons:[16]
Extensions to Telnet provideTransport Layer Security (TLS) security andSimple Authentication and Security Layer (SASL) authentication that address the above concerns.[5] However, most Telnet implementations do not support these extensions; and they do not address other vulnerabilities such as parsing the banner information.[15] Telnet overVPN is a viable option if SSHv2 is not supported, or a VPN is already used to securely tunnel other application data to the remote network the Telnet server is present in. However, precautions must be taken: ideally the VPN should terminate on the Telnet server itself, unless theLAN has additional security measures against eavesdropping and modification by other devices such as additional encryption and/orVLANs. This is because Telnet traffic leaves the VPN server in its insecure plaintext form after it is decrypted. The VPN software should be a trusted one that is heavily audited (e.g.OpenVPN,WireGuard,IPsec), using preferably certificate-based/public keymutual authentication.
IBM 5250 or 3270 workstation emulation is supported via custom telnet clients,TN5250/TN3270, andIBM i systems. Clients and servers designed to passIBM 5250 data streams over Telnet generally do supportSSL encryption, as SSH does not include 5250 emulation. UnderIBM i (also known as OS/400), port 992 is the default port for TelnetS (Telnet over SSL/TLS).[17]
Historically, Telnet provided access to acommand-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 ofSSH.[18] The usage of Telnet for remote management has declined rapidly, especially on the publicInternet, in favor of theSecure Shell (SSH) protocol.[2][19] SSH provides much of the functionality of telnet, with the addition of strong encryption to prevent sensitive data such as passwords from being intercepted, andpublic key authentication, to ensure that the remote computer is actually who it claims to be.
The Telnet protocol is mainly used for legacy equipment that does not support more modern communication mechanisms.[20] For example, many industrial and scientific devices only have Telnet available as a communication option. Some are built with only a standardRS-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).[citation needed]
Telnet support has become highly unusual in new applications, thoughamateur radio operators[21] andmulti-user dungeons[22] do continue to utilize it.
Security researchers estimated that 7,096,465 exposed systems on the Internet continue to use Telnet as of 2021. However, estimates of this number have varied significantly, depending on the number of ports scanned beyond the default TCP port 23.[7]
The Telnet client may be used indebugging network services such asSMTP,IRC, orHTTP servers, to issue commands to the server and examine the responses. In this case, when the Telnet client establishes a TCP connection to a port other than the standard Telnet server port, it does not use the Telnet protocol, and can be used instead to send and receive data over the TCP connection directly.[23][better source needed]
The technical details of Telnet are defined by a variety of specifications includingRFC 854.[1]
Telnet commands consist of at least two bytes.[1] The first byte is the IAC escape character (byte 255) followed by the byte code for a given command:
| Name | Byte Value (dec) | Byte value (hex) | Explanation |
|---|---|---|---|
| SE (Subnegotiation end) | 240 | 0xf0 | End of negotiation (or data block) of a sub-service of a protocol mechanism |
| NOP (No operation) | 241 | 0xf1 | Data packet that does nothing |
| Data Mark | 242 | 0xf2 | |
| Break | 243 | 0xf3 | |
| Interrupt Process | 244 | 0xf4 | Request that other party ends current process |
| Abort output | 245 | 0xf5 | Request that other party stops sending output |
| Are you there? | 246 | 0xf6 | |
| Erase character | 247 | 0xf7 | |
| Erase Line | 248 | 0xf8 | |
| Go ahead | 249 | 0xf9 | |
| SB (Subnegotiation begin) | 250 | 0xfa | Initiate the negotiation of a sub-service of a protocol mechanism |
| WILL | 251 | 0xfb | Informs other party that this party will use a protocol mechanism |
| WON'T | 252 | 0xfc | Informs other party that this party will not use a protocol mechanism |
| DO | 253 | 0xfd | Instruct other party to use a protocol mechanism |
| DON'T | 254 | 0xfe | Instruct other party to not use a protocol mechanism |
| IAC | 255 | 0xff | Sequence Initializer/Escape Character |
| Source: J. Postel and Reynolds (1983)[1] | |||
All dataoctets 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.)[1]
Telnet 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 namedbinary 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-setdata octets. There was an attempt to introduce a switchable character encoding support like HTTP has,[24] 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 | ||
| 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 | SeeIBM 3270 |
| 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 | SeeKermit (protocol) |
| 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 | |
Source: Internet Assigned Numbers Authority (n.d.)[25] | |||
Star Wars: Episode IV – A New Hope from 1977 has been recreated as atext art movie served through Telnet.[26]
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