Telnet Protocol

Telnet is a network protocol used to access a remote computer system through a terminal session. The name is an abbreviation of "Telecommunication Network." Telnet works by establishing an interactive terminal connection between the client and the server, allowing the user to work on the remote system as if they were on a local terminal. Due to this capability, Telnet has been widely used in remote system management, network device configuration, and general access applications.

Telnet operates based on a client-server architecture. It resides in the application layer of the OSI model and uses the TCP (Transport Layer) protocol for data transmission. The most common port number used is 23. Despite its simple structure, Telnet has advanced control mechanisms and terminal emulation support.

Historical Development

Telnet was first tested in 1969 as the first application layer protocol on ARPANET. This early application was designed for remote access and interaction with central computer systems. The first official draft of the protocol was published in 1971 with RFC 97, and it was standardized in 1983 as Internet Standard #8. During its development, the concept of Network Virtual Terminal (NVT) enabled communication between different systems using a common terminal standard. Telnet was widely used in UNIX system access, library catalogs, information databases, remote management, and multi-user game systems during its early years.

Working Principle

When the Telnet client connects to a remote server, it gains access to the terminal service on the server. After the connection is established, the user authenticates within the session provided by the server and gains access to system resources. This session is generally executed with character-based input and response.

Telnet sessions are provided over the TCP protocol, ensuring connection-oriented, ordered, and reliable data transmission. Telnet follows the VT100 terminal emulation standards and provides the user with the impression of working on a terminal operating on the remote system. During a Telnet session, behaviors such as character-based transmission and echoing are mutually negotiated between the client and server.

Technical Features

The Telnet protocol is defined under specific standards. The following RFC documents define the basic protocol and optional functions:

• RFC 854: Telnet protocol definition
• RFC 855: Option structure and negotiation rules
• RFC 856-861: Additional functions like trained data transmission, echo option, timing, status reporting

Key technical features include:

• Uses a TCP connection (port 23)
• Operates with the ASCII character set (NVT standard)
• Symmetric client-server structure
• Echo, character mode, and control characters can be negotiated during the session
• Telnet commands are defined using the IAC (Interpret As Command) structure (WILL, DO, DON’T, etc.)
• Session control can be achieved using shortcuts like "Ctrl + ]"

Telnet transmits the data entered by the user on a character basis. This may cause slow response times on high-latency networks. For such cases, echo settings and flow control mechanisms can be negotiated. Telnet also includes commands like AYT (Are You There) to test if the other party is alive.

Areas of Use

During its development, Telnet was commonly used for accessing and managing networked devices. Prominent use cases include:

• Remote access to UNIX and Linux systems
• Configuration of network devices such as Cisco
• Access to library catalogs (e.g., METU, Bilkent, TÜBİTAK)
• Database searches and information banks
• Multi-user games and Telnet-based services (e.g., MUD, MOOs)

Some Telnet clients and applications have used specific port numbers (e.g., 23, 2000, 3000) for specialized services. These services offered functions such as information access, text-based conversations, or data viewing.

Security Vulnerabilities and Criticisms

Telnet transmits data in plaintext without encryption. This leads to significant security vulnerabilities, especially in environments where network traffic can be intercepted. Usernames, passwords, and session data are transmitted unencrypted and can be captured through techniques like "packet sniffing." This poses a serious threat, particularly in multi-user systems. Due to these security flaws, Telnet has been replaced by the SSH (Secure Shell) protocol, which provides encrypted data transmission. SSH, like Telnet, offers remote system management but prioritizes security with features like authentication, data integrity, and encryption. In many modern systems, the Telnet protocol has been disabled or completely removed.

Telnet has been one of the fundamental protocols developed in the early days of computer networks for interaction with remote systems. With its terminal emulation, TCP-based connection structure, and application layer features, it has been widely used for many years. However, due to its plaintext data transmission, it is considered a weak protocol in terms of security and has been replaced by encrypted communication protocols like SSH in modern systems. The architecture and development process of Telnet have influenced the formation of technical approaches toward application layer protocols in the historical development of network protocols.