Telnet (sometimes stylized TELNET) is a client-server application protocol that provides access to virtual terminals of remote systems on local area networks or the Internet.[1] It is a protocol for bidirectional 8-bit communications. Its main goal was to connect terminal devices and terminal-oriented processes.
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. 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 transmits all information including usernames and passwords in plaintext so it is not recommended for security-sensitive applications such as remote management of routers.[2] Telnet's use for this purpose has waned significantly in favor of SSH.[3] Some extensions to Telnet which would provide encryption have been proposed.
The telnet protocol is a client-server protocol that runs on a reliable connection-oriented transport. Most often, a telnet client connects over TCP to port 23 or 2323, where a Telnet server application is listening.[4] [5] [6] 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).[7] 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.
The official specification stylizes the name as TELNET, which is not as an acronym or abbreviation.
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".[8]
In his 2015 book WHOIS Running the Internet: Protocol, Policy, and Privacy, Internet researcher Garth O. Bruen claims that Telnet was originally short for "Teletype Over Network Protocol".[9]
Telnet was originally developed for ARPANET in 1969.[10] Initially, it was an ad hoc protocol with no formal specification,[11] but after extensive work in the 1970s, including numerous RFCs, it was officially formalized in and, which together form Internet standard 8.[12]
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. Some of these extensions have also been adopted as Internet standards, particularly standards 27 through 32 (see below).
Telnet is vulnerable to network-based cyberattacks, such as packet sniffing sensitive information including passwords and fingerprinting.[13] 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 without authentication. Telnet is frequently exploited by malware due to being improperly configured. 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:[14]
Extensions to Telnet provide Transport Layer Security (TLS) security and Simple Authentication and Security Layer (SASL) authentication that address the above concerns.[15] However, most Telnet implementations do not support these extensions; and they do not address other vulnerabilities such as parsing the banner information. Telnet over VPN 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 the LAN has additional security measures against eavesdropping and modification by other devices such as additional encryption and/or VLANs. 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 key mutual authentication.
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 TelnetS (Telnet over SSL/TLS).[16]
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.[17] The usage of Telnet for remote management has declined rapidly, especially on the public Internet, in favor of the Secure Shell (SSH) protocol.[18] 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.
The Telnet protocol is mainly used for legacy equipment that does not support more modern communication mechanisms.[19] For example, many 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 support has become highly unusual in new applications, though amateur radio operators[20] and multi-user dungeons[21] 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.
The Telnet client may be used in debugging network services such as SMTP, IRC, or HTTP 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.[22]
The technical details of Telnet are defined by a variety of specifications including .
Telnet commands consist of at least two bytes. The first byte is the IAC escape character (byte 255) followed by the byte code for a given command:
| 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) | ||||
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 has a variety of options that terminals implementing Telnet should support.
| 0 | Binary Transmission | |||
| 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 | |||
| 11 | Output Horizontal Tab Stops | |||
| 12 | Output Horizontal Tab Disposition | |||
| 13 | Output Formfeed Disposition | |||
| 14 | Output Vertical Tabstops | |||
| 15 | Output Vertical Tab Disposition | |||
| 16 | Output Linefeed Disposition | |||
| 17 | Extended ASCII | |||
| 18 | Logout | |||
| 19 | Byte Macro | |||
| 20 | Data Entry Terminal | |||
| 21 | SUPDUP | |||
| 22 | SUPDUP Output | |||
| 23 | Send Location | |||
| 24 | Terminal Type | |||
| 25 | End of Record | |||
| 26 | TACACS User Identification | |||
| 27 | Output Marking | |||
| 28 | Terminal Location Number | |||
| 29 | Telnet 3270 Regime | |||
| 30 | X.3 PAD | |||
| 31 | Negotiate About Window Size | |||
| 32 | Terminal Speed | |||
| 33 | Remote Flow Control | |||
| 34 | Linemode | |||
| 35 | X Display Location | |||
| 36 | Environment Option | |||
| 37 | Authentication Option | |||
| 38 | Encryption Option | |||
| 39 | New Environment Option | |||
| 40 | TN3270E | See IBM 3270 | ||
| 41 | XAUTH | |||
| 42 | CHARSET | |||
| 43 | Telnet Remote Serial Port (RSP) | |||
| 44 | Com Port Control Option | |||
| 45 | Telnet Suppress Local Echo | |||
| 46 | Telnet Start TLS | |||
| 47 | KERMIT | See Kermit (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 | |||
from 1977 has been recreated as a text art movie served through Telnet.[23]
. Richard Bartle . . New Riders . 2003 . 0-13-101816-7.
