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Telnet

The Telnet Protocol

The Telnet protocol is often thought of as simply providing a facility for remote logins to computer via the Internet. This was its original purpose although it can be used for many other purposes. It is best understood in the context of a user with a simple terminal using the local telnet program (known as the client program) to run a login session on a remote computer where his communications needs are handled by a telnet server program. It should be emphasised that the telnet server can pass on the data it has received from the client to many other types of process including a remote login server. It is described in RFC854 and was first published in 1983.

Commands

The telnet protocol also specifies various commands that control the method and various details of the interaction between the client and server. These commands are incorporated within the data stream. The commands are distinguished by the use of various characters with the most significant bit set. Commands are always introduced by a character with the decimal code 255 known as an Interpret as command (IAC) character. The complete set of special characters is

Name	Decimal Code	Meaning
SE	240	End of subnegotiation parameters.
NOP	241	No operation
DM	242	Data mark. Indicates the position of a Synch event within the data stream. This should always be accompanied by a TCP urgent notification.
BRK	243	Break. Indicates that the "break" or "attention" key was hit.
IP	244	Suspend, interrupt or abort the process to which the NVT is connected.
AO	245	Abort output. Allows the current process to run to completion but do not send its output to the user.
AYT	246	Are you there. Send back to the NVT some visible evidence that the AYT was received.
EC	247	Erase character. The receiver should delete the last preceding undeleted character from the data stream.
EL	248	Erase line. Delete characters from the data stream back to but not including the previous CRLF.
GA	249	Go ahead. Used, under certain circumstances, to tell the other end that it can transmit.
SB	250	Subnegotiation of the indicated option follows.
WILL	251	Indicates the desire to begin performing, or confirmation that you are now performing, the indicated option.
WONT	252	Indicates the refusal to perform, or continue performing, the indicated option.
DO	253	Indicates the request that the other party perform, or confirmation that you are expecting the other party to perform, the indicated option.
DONT	254	Indicates the demand that the other party stop performing, or confirmation that you are no longer expecting the other party to perform, the indicated option.
IAC	255	Interpret as command

There are a variety of options that can be negotiated between a telnet client and server using commands at any stage during the connection. They are described in detail in separate RFCs. The following are the most important.

Decimal code	Name	RFC
1	echo	857
3	suppress go ahead	858
5	status	859
6	timing mark	860
24	terminal type	1091
31	window size	1073
32	terminal speed	1079
33	remote flow control	1372
34	linemode	1184
36	environment variables	1408

Options are agreed by a process of negotiation which results in the client and server having a common view of various extra capabilities that affect the interchange and the operation of applications.Either end of a telnet dialogue can enable or disable an option either locally or remotely. The initiator sends a 3 byte command of the form IAC,<type of operation>,<option>The response is of the same form.Operation is one of

Description	Decimal Code	Action
WILL	251	Sender wants to do something.
DO	252	Sender wants the other end to do something.
WONT	253	Sender doesn't want to do something.
DONT	254	Sender wants the other not to do something.

Associated with each of the these there are various possible responses

Sender Sent	Receiver Responds	Implication
WILL	DO	The sender would like to use a certain facility if the receiver can handle it. Option is now in effect
WILL	DONT	Receiver says it cannot support the option. Option is not in effect.
DO	WILL	The sender says it can handle traffic from the sender if the sender wishes to use a certain option. Option is now in effect.
DO	WONT	Receiver says it cannot support the option. Option is not in effect.
WONT	DONT	Option disabled. DONT is only valid response.
DONT	WONT	Option disabled. WONT is only valid response.

For example if the sender wants the other end to suppress go-ahead it would send the byte sequence255(IAC),251(WILL),3The final byte of the three byte sequence identifies the required action.For some of the negotiable options values need to be communicated once support of the option has been agreed. This is done using sub-option negotiation.

Values are communicated via an exchange of value query commands and responses in the following form. IAC,SB,<option code number>,1,IAC,SEandIAC,SB,<option code>,0,<value>,IAC,SE

For example if the client wishes to identify the terminal type to the server the following exchange might take place
Client 255(IAC),251(WILL),24
Server 255(IAC),253(DO),24
Server 255(IAC),250(SB),24,1,255(IAC),240(SE)
Client 255(IAC),250(SB),24,0,'V','T','2','2','0',255(IAC),240(SE)

The first exchange establishes that terminal type (option number 24) will be handled, the server then enquires of the client what value it wishes to associate with the terminal type. The sequence SB,24,1 implies sub-option negotiation for option type 24, value required (1). The IAC,SE sequence indicates the end of this request. The repsonse IAC,SB,24,0,'V'... implies sub-option negotiation for option type 24, value supplied (0), the IAC,SE sequence indicates the end of the response (and the supplied value).

The encoding of the value is specific to the option but a sequence of characters, as shown above, is common.

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