|Family:||Internet protocol family|
|Operation area:||Remote control of computers|
|Port:||23 / TCP|
RFC 855 (STD 8)
Telnet ( Tel etype Net work ) is the name of the Internet widespread network protocol . This old and well-known client / server protocol is based on a character-oriented data exchange via a TCP connection. In addition to the protocol, the name Telnet is also used for the Telnet server services and especially for Telnet clients. Both services use the Telnet protocol for communication.
The Telnet protocol consists of a set of core functions and some extensions. The core protocol is described in the IETF documents RFC 854 and RFC 855 (STD 8). STD 8 describes some basic working methods of the protocol and expansion options.
There are numerous extensions to the protocol, some of which have been incorporated as Internet standards. IETF-STD documents 27-32 describe these extensions.
Telnet clients can be called up by default under the name telnet on all common operating systems such as Linux , Unix , macOS and on all network-compatible versions of Windows . Since macOS High Sierra , Telnet is no longer natively supported by the operating system, but can be installed via Homebrew and then used as usual. A well-known free open source client is PuTTY .
Since Vista, the Telnet client has to be activated under Microsoft Windows first. The service must first be installed and started on a Windows Server 2008 . For remote access, the user must also be added to the defined “TelnetClients” group and the firewall must be set so that the standard port 23 is not blocked.
Telnet was developed in 1969 as part of the ARPANET (Advanced Research Projects Agency Network) project, with the aim of being able to use expensive computing time, application programs and databases remotely. But it wasn't used for the first time until 1974.
Telnet is typically used for remote control of computers in the form of text-based inputs and outputs. To do this, the Telnet client establishes an unencrypted connection to a Telnet server. In this phase, a required password is transferred in clear text. After the connection has been established, the Telnet protocol is optionally initiated. In this way, programs can be operated without a graphical user interface. Usually it is a login console with full authority. Due to the high level of command and unencrypted transmission, this procedure is considered insecure and should be replaced by an SSH connection that also implements the Telnet protocol but transmits it in encrypted form.
The connection between a Telnet client and a Telnet server is often referred to as a Telnet service.
The controlling unit can be both a remote device and a program installed on a computer. The presentation of the transmitted information can vary depending on the device.
As soon as the connection between the Telnet client and the Telnet server has been established, the keyboard entries are sent from the controlling terminal to the remote computer, and from there, texts are in turn transmitted back to the terminal. The remote computer transmits e.g. B. the text-based outputs of a program, such as an interface for entering commands to the operating system. In this way, one computer can be used to remotely control another computer. This remote access can even take place over more than two levels.
Nowadays there are few essential areas of application for a Telnet client:
- Addressing a Telnet server and logging into the server system. As already mentioned, the password is transmitted in clear text. Therefore, the encrypted Secure Shell (SSH) service should preferably be used, if available.
- Connection test with any TCP / IP server, e.g. B. with HTTP , SMTP or IMAP servers. Usually only the handshake is checked and no safety-critical information is transmitted.
- Access to network-compatible firmware of various devices in the local network.
The Telnet service (interaction between client and server) is possibly a. used for the following areas of application:
- Access to a remote console (e.g. UNIX or DOS shell) with the option of using all commands and programs that run in text mode.
- Access to text-based applications on an application server.
- Database queries, such as searching in library catalogs.
- Remote configuration of devices and output of operating data.
Many of these applications are only known to an experienced specialist audience. For example, use Internet Chess Players often the Free Internet Chess Server and Gospieler the Internet Go Server, also Pandanet called. Their graphical user interface (GUI) interprets the text output of the console and shows the opponent's move on the chess or go board. Your own move is carried out with the mouse, but the information about it is again transmitted as text. Blind chess is then played without a GUI.
- The client program replaces many programs of services that one wants to use on the Internet.
- A connection is established between two computers, even if they are running on different operating systems.
- Access to all resources is possible if the authorization is given.
- No security functionalities - e.g. B. passwords are sent in clear text.
- Because of the full access, hackers can have an easy time.
- Tn3270 , a Telnet variant for communication between IBM 3270 terminals and mainframes
- Telnet 5250 , a Telnet variant for communication between IBM 5250 terminals and IBM System i with OS / 400
- PuTTY , a package with freely available Telnet and SSH clients
- Secure Shell (ssh)
- Remote shell (rsh)
- Kermit (software)
- RFC 15 - Network Subsystem for Time Sharing Hosts
- RFC 854 / STD 8 ** - Telnet Protocol Specification
- RFC 855 / STD 8 - Telnet Option Specifications
- List of Telnet options
- RFC 856 / STD 27 - Telnet Binary Transmission
- RFC 857 / STD 28 - Telnet Echo Option
- RFC 858 / STD 29 - Telnet Suppress Go Ahead Option
- RFC 859 / STD 30 - Telnet Status Option
- RFC 860 / STD 31 - Telnet Timing Mark Option
- RFC 861 / STD 32 - Telnet Extended Options: List Option
- 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 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