The TELNET Protocol

Reference: RFC 854

TELNET vs. telnet

• TELNET is a protocol that provides “a general, bi-directional, eight-bit byte oriented communications facility”.
• telnet is a program that supports the TELNET protocol over TCP.
• Many application protocols are built upon the TELNET protocol.

The TELNET Protocol

• TCP connection
• data and control over the same connection.
• Network Virtual Terminal
• negotiated options

Network Virtual Terminal

• intermediate representation of a generic terminal.
• provides a standard language for communication of terminal control functions.

Network Virtual Terminal

Negotiated Options

• All NVTs support a minimal set of capabilities.
• Some terminals have more capabilites than the minimal set.
• The 2 endpoints negotiate a set of mutually acceptable options (character set, echo mode, etc).
• The protocol for requesting optional features is well defined and includes rules for eliminating possible negotiation “loops”.

The set of options is not part of the TELNET protocol, so that new terminal features can be incorporated without changing the TELNET protocol.

Option examples

• Line mode vs. character mode
• echo modes
• character set (EBCDIC vs. ASCII)
  

Control Functions

• TELNET includes support for a series of control functions commonly supported by servers.
• This provides a uniform mechanism for communication of (the supported) control functions.
• Interrupt Process (IP)
• suspend/abort process.
• Abort Output (AO)
• process can complete, but send no more output to user’s terminal.
• Are You There (AYT)
• check to see if system is still running.
• Erase Character (EC)
• delete last character sent
• typically used to edit keyboard input.
• Erase Line (EL)
• delete all current input in current line.

Command Structure

• All TELNET commands and data flow through the same TCP connection.
• Commands start with a special character called the Interpret as Command escape character (IAC).
• The IAC code is 255.
• If a 255 is sent as data - it must be followed by another 255.

Looking for Commands

• Each receiver must look at each byte that arrives and look for IAC.
• If IAC is found and the next byte is IAC - a single byte is presented to the application/terminal.
• If IAC is followed by any other code - the TELNET layer interprets this as a command.

Command Codes

• IP 243
• AO 244
• AYT 245
• EC 246
• EL 247
• WILL 251
• WON’T 252
• DO 253
• DON’T 254
• IAC 255