Telnet offers users the capability of running programs remotely and facilitates remote administration. Telnet is available for practically all operating systems and eases integration in heterogeneous networking environments.
Telnet 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 logon session on a remote computer where the user's communications needs are handled by a Telnet server program.
It should be emphasized that the Telnet server can pass on the data it has received from the client to many other types of processes including a remote logon server. This is described in RFC854 and was first published in 1983.
The Network Virtual Terminal
Communication is established using TCP/IP and is based on a Network Virtual Terminal (NVT). On the client, the Telnet program is responsible for translating incoming NVT codes to codes understood by the client's display device as well as for translating client-generated keyboard codes into outgoing NVT codes.
The NVT uses 7-bit codes for characters. The display device, referred to as a printer in the RFC, is only required to display the standard printing ASCII characters represented by 7-bit codes and to recognize and process certain control codes. The 7-bit characters are transmitted as 8-bit bytes with the most significant bit set to zero. An end-of-line is transmitted as a carriage return (CR) followed by a line feed (LF). If you want to transmit an actual carriage return, this is transmitted as a carriage return followed by a NUL (all bits zero) character.
NVT ASCII is used by many other Internet protocols like SMTP and FTP.
The following control codes are required to be understood by the NVT.
|Line Feed||LF||10||Moves the printer to the next print line, keeping the same horizontal position. |
|Carriage Return||CR||13||Moves the printer to the left margin of the current line. |
The following further control codes are optional but should have the indicated defined effect on the display.
|BELL||BEL||7||Produces an audible or visible signal (which does NOT move the print head. |
|Back Space||BS||8||Moves the print head one character position towards the left margin. (On a printing device, this mechanism was commonly used to form composite characters by printing two basic characters on top of each other.)|
|Horizontal Tab||HT||9||Moves the printer to the next horizontal tab stop. It remains unspecified how either party determines or establishes where such tab stops are located.|
|Vertical Tab||VT||11||Moves the printer to the next vertical tab stop. It remains unspecified how either party determines or establishes where such tab stops are located. |
|Form Feed||FF||12||Moves the printer to the top of the next page, keeping the same horizontal position. (On visual displays, this commonly clears the screen and moves the cursor to the top left corner.)|
The NVT keyboard is specified as being capable of generating all 128 ASCII codes by using keys, key combinations, or key sequences.
The Telnet protocol uses various commands to control the client-server connection. These commands are transmitted within the data stream. The commands are distinguished from the data by setting the most significant bit to 1. (Remember that data is transmitted as 7-bits with the eighth bit set to 0) Commands are always introduced by the Interpret as command (IAC) character.
Here is the complete set of commands:
|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 hi. |
|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 does 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|| Under certain circumstances used to tell the other end that it can transmit.|
|SB||250||Subnegotiation||Subnegotiation of the indicated option follows.|
|WILL||251||will||Indicates the desire to begin performing, or confirmation that you are now performing, the indicated option.|
|WONT||252||wont||Indicates the refusal to perform, or continue performing, the indicated option.|
|DO||253||do||Indicates the request that the other party perform, or confirmation that you are expecting the other party to perform, the indicated option.|
|DONT||254||dont||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||Interpret as a command|
Options give the client and server a common view of the connection. They can be negotiated at any time during the connection by the use of commands. They are described in separate RFCs.
The following are examples of common options:
|3||suppress go ahead||858|
|33||remote flow control||1372|
Either end of a Telnet conversation can locally or remotely enable or disable an option. 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:
|WILL||251||Sender wants to do something.|
|WONT||252||Sender doesn't want to do something.|
|DO||253||Sender wants the other end to do something.|
|DONT||254||Sender wants the other not to do something.|
Associated with each of the these commands 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 sequence:
The final byte of the 3-byte sequence identifies the required action.
Some option's values need to be communicated after support of the option has been agreed. This is done using sub-option negotiation. Values are negotiated using value query commands and responses in the following form:
For example, if the client wants to identify the terminal type to the server, the following exchange might take place:
The first exchange establishes that terminal type (option number 24) is 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 response 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.
Descriptions of Telnet Options
Many of those listed are self-evident, but some call for more information.
Suppress Go Ahead
The original Telnet implementation defaulted to half duplex operation. This means that data traffic could only go in one direction at a time and specific action is required to indicate the end of traffic in one direction and that traffic may now start in the other direction. [This similar to the use of "roger" and "over" by amateur and CB radio operators.] The specific action is the inclusion of a GA character in the data stream.
Modern links normally allow bi-directional operation and the "suppress go ahead" option is enabled.
The echo option is enabled, usually by the server, to indicate that the server echos every character it receives. A combination of "suppress go ahead" and "echo" is called character-at-a-time mode meaning that each character is separately transmitted and echoed.
There is an understanding known as kludge-line mode, which means that if either "suppress go ahead" or "echo" is enabled but not both, then Telnet operates in line-at-a-time mode meaning that complete lines are assembled at each end and transmitted in one "go".
This option replaces and supersedes the line mode kludge.
Remote Flow Control
This option controls where the special flow control effects of Ctrl+S or Ctrl+Q are implemented.
Telnet Control Functions
The Telnet protocol includes a number of control functions. These are initiated in response to conditions detected by the client (usually certain special keys or key combinations) or server. The detected condition causes a special character to be incorporated in the data stream.
This is used by the client to cause the suspension or termination of the server process. Typically, the user types Ctrl+C on the keyboard. An IP (244) character is included in the data stream.
This is used to suppress the transmission of remote process output. An AO (238) character is included in the data stream.
Are You There
This is used to trigger a visible response from the other end of the connection to confirm the operation of the link and the remote process. An AYT (246) character is incorporated in the data stream.
This is sent to the display to tell it to delete the immediately preceding character from the display. An EC (247) character is incorporated in the data stream.
This option causes the deletion of the current line of input. An EL (248) character is incorporated in the data stream.
Some control functions such as AO and IP require immediate action and this may cause difficulties if data is held in buffers awaiting input requests from a (possibly misbehaving) remote process. To work around this problem, a DM (242) character is sent in a TCP Urgent segment, this tells the receiver to examine the data stream for "interesting" characters such as IP, AO, and AYT. This is known as the Telnet synchronization mechanism.
A DM not in a TCP Urgent segment has no effect.
The Telnet Command
On Windows NT and most UNIX systems, a Telnet session can be initiated using the Telnet command. Most users simply type:
However, if the user just types telnet
, then various options and subcommands are available.
The following is an example of a Telnet session from sfuclnt to sfusrvr.
Microsoft (R) Windows NT (TM) Version 4.00 (Build 1381)
Welcome to Microsoft Telnet Client
Telnet Client Build 5.00.99034.1
Escape Character is 'CTRL+]'
Microsoft Telnet> open sfusrvr
**** The screen will clear and the following information is displayed:
Microsoft (R) Windows NT (TM) Version 4.00 (Build 1381)
Welcome to Microsoft Telnet Service
Telnet Server Build 5.00.99034.1
**** The screen will clear again and the following information is displayed:
Welcome to Microsoft Telnet Server.