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Telnet is an Application layer protocol providing reliable unencrypted communication allowing users to access the command line of a computer from a remote location and provides very little security.

  • Telnet is one of the earliest protocols developed
  • Telnet provides reliable communication via TCP
  • Telnet is an Application (operates at the OSI Model's Application Layer)
  • Telnet provides access to the command prompt remotely
  • Telnet utilizes TCP/IP to support communication
  • Information is communicated as ASCII Text
  • Telnet is carried inside the payload of TCP (encapsulated in TCP)
  • Commands
    • open
    • close
    • quit

Telnet was one of the first protocols developed for use over TCP/IP. Telnet is an application designed for reliable communication via a virtual terminal. It was intended to be a bi-directional byte-oriented communications protocol utilizing 7-bit ASCII for use in creating communication between terminals (Internet end points) or processes across the Internet.

Telnet is one of the oldest IP protocols and from it several other protocols were developed. A telnet server listens for connections on TCP port 23. When a connection is opened from a telnet client to a server, the client attempts to connect to the server machine using TCP on port 23. The client uses a local port above 1023.

The client and server will negotiate supported Telnet options and the connection will be established. The remote server will then provide services over that TCP connection. The client sends in ASCII text data and the server responds according to it's design. Telnet is the most basic of all TCP based protocols. When the client receives input from the user, it forwards that information to the telnet server.

The client normally will send in the user data one ASCII character at a time unless the NAGLE algorithm for TCP is in use. The Nagle algorithm changes the way TCP handles segments and can alter how data gets buffered before transmission to the other end.


Telnet Control Functions

  • Interrupt Process (IP)
  • Abort Output (AO)
  • Are You There (AYT)
  • Erase Character (EC)
  • Erase Line (EL)



Linux Telnet


Microsoft Telnet (Windows)

Commands may be abbreviated. Supported commands are: c - close close current connection d - display display operating parameters
o - open hostname [port] connect to hostname (default port 23).
q - quit exit telnet
set - set set options (type 'set ?' for a list)
sen - send send strings to server
st - status print status information
u - unset unset options (type 'unset ?' for a list)
?/h - help print help information Options for the set command
Microsoft Telnet> set ?
bsasdel Backspace will be sent as delete
crlf New line mode - Causes return key to send CR & LF
delasbs Delete will be sent as backspace
escape x x is an escape charater to enter telnet client prompt
localecho Turn on localecho.
logfile x x is current client log file
logging Turn on logging
mode x x is console or stream
ntlm Turn on NTLM authentication.
term x x is ansi, vt100, vt52, or vtnt Default Operating Parameters
Escape Character is 'CTRL+]'
Will auth(NTLM Authentication)
Local echo off
New line mode - Causes return key to send CR & LF
Current mode: Console
Will term type
Preferred term type is ANSI



The NAGLE algorithm makes telnet more efficient. Rather than wrap up every single character in a complete IP datagram, the whole input buffer of the keyboard or computer is sent at once or stored and sent as a group of characters once the return key is pressed on the keyboard (an end of line is detected on standard input by the telnet client).



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