The Telnet Command

We have seen how we use Mac applications such as NiftyTelent or BetterTelnet PPC. To finish off this section we consider how we run simple command line Telnet clients.

On most Unix systems and DOS Prompts a telnet session can be initiated using the telnet command. Most users simply type:

telnet <remote host>

but if the user just types telnet then various options and subcommands are available which can be used to study the behaviour of the session.
Here's an exmaple of a telnet session from scitsc to ccub

bash$ telnet

telnet> toggle options

Will show option processing.

telnet> open ccub

Trying 134.220.1.20 ...

Connected to ccub.wlv.ac.uk.

Escape character is '^]'.

SENT do SUPPRESS GO AHEAD

SENT will TERMINAL TYPE (reply)

RCVD do TERMINAL TYPE (don't reply)

RCVD will SUPPRESS GO AHEAD (don't reply)

RCVD will ECHO (reply)

SENT do ECHO (reply)

RCVD do ECHO (reply)

SENT wont ECHO (reply)

UNIX(r) System V Release 4.0 (ccub)

RCVD dont ECHO (don't reply)

login: Login timed out after 60 seconds

Connection closed by foreign host. 

Telnet 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. The following are the most important:

Decimal code	Name
1	echo
3	suppress go ahead
5	status
6	timing mark
24	terminal type
31	window size
32	terminal speed
33	remote flow control
34	linemode
36	environment variables

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	Receiver	Implication
Sent	Responds
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 sequence

255(IAC),251(WILL),3

The 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,SE

and

IAC,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 above works as follows:

• 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 sequenceSB,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.