Telnet

Family:

Operation area:

Remote control of computers

Port:

23 / TCP

Telnet in the TCP / IP protocol stack :
application	Telnet
transport	TCP
Internet	IP ( IPv4 , IPv6 )
Network access	Ethernet	Token bus	Token ring	FDDI	...

Standards:

RFC 854

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.

history

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.

Intended use

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.

advantages

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.

disadvantage

No security functionalities - e.g. B. passwords are sent in clear text.
Because of the full access, hackers can have an easy time.

Individual evidence

↑ Origin of the name
↑ Activation of the Telnet client under Windows 7. Microsoft, accessed on July 30, 2017 .
↑ The Telnet Song by Guy L. Steele, Jr.

[1] Origin of the name

[2] Activation of the Telnet client under Windows 7. Microsoft, accessed on July 30, 2017 .

[3] The Telnet Song by Guy L. Steele, Jr.

Telnet

contents

history

Intended use

advantages

disadvantage

See also

Web links

Individual evidence