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.
Communication is established using the TCP/IP protocols
and communication is based on a set of facilities
known as a Network Virtual Terminal (NVT).
At the user or client end the telnet client program
is responsible for mapping incoming NVT codes to the
actual codes needed to operate the user's display
device and is also responsible for mapping user generated
keyboard sequences into NVT sequences.
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 recognise
and process certain control codes. The 7 bit characters
are transmitted as 8 bit bytes with most significant
bit set to zero. An end-of-line is transmitted as
the character sequence CR (carriage return) followed
by LF (line feed). If it is desired 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.
The following control codes are required to be understood
by the Network Virtual Terminal.
| Name |
code |
Decimal Value |
Function |
| NULL
| NUL
| 0
| No operation
|
| 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.
| Name |
code |
Decimal Value |
Function |
| 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 devices
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.
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.
Common Telnet options:
| Decimal code |
Option Name |
RFC |
| 0
| Transmit Binary
| 856
|
| 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
|
All Telnet options:
| Decimal Code |
Option Name |
RFC |
| 0 |
Transmit Binary |
856 |
| 1 |
Echo |
857 |
| 2 |
Reconnection |
|
| 3 |
Suppress Go Ahead |
858 |
| 4 |
Approx Message Size Negotiation. |
|
| 5 |
Status |
859 |
| 6 |
Timing Mark |
860 |
| 7 |
Remote Controlled Trans and Echo |
563,
726 |
| 8 |
Output Line Width |
|
| 9 |
Output Page Size |
|
| 10 |
Negotiate About Output Carriage-Return Disposition |
652 |
| 11 |
Negotiate About Output Horizontal Tabstops |
653 |
| 12 |
NAOHTD, Negotiate About Output Horizontal Tab
Disposition |
654 |
| 13 |
Negotiate About Output Formfeed Disposition |
655 |
| 14 |
Negotiate About Vertical Tabstops |
656 |
| 15 |
Negotiate About Output Vertcial Tab Disposition |
657 |
| 16 |
Negotiate About Output Linefeed Disposition |
658 |
| 17 |
Extended ASCII. |
698 |
| 18 |
Logout. |
727 |
| 19 |
Byte Macro |
735 |
| 20 |
Data Entry Terminal |
732,1043 |
| 21 |
SUPDUP |
734,
736 |
| 22 |
SUPDUP Output |
749 |
| 23 |
Send Location |
779 |
| 24 |
Terminal Type |
1091 |
| 25 |
End of Record |
885 |
| 26 |
TACACS User Identification |
927 |
| 27 |
Output Marking |
933 |
| 28 |
TTYLOC, Terminal Location Number. |
946 |
| 29 |
Telnet 3270 Regime |
1041 |
| 30 |
X.3 PAD. |
1053 |
| 31 |
NAWS, Negotiate About Window Size. |
1073 |
| 32 |
Terminal Speed |
1079 |
| 33 |
Remote Flow Control |
1372 |
| 34 |
Linemode |
1184 |
| 35 |
X Display Location. |
1096 |
| 36 |
Environment |
1408 |
| 37 |
Authentication |
1416,
2941, 2942,
2943,2951 |
| 38 |
Encryption Option |
2946 |
| 39 |
New Environment |
1572 |
| 40 |
TN3270E |
2355 |
| 41 |
XAUTH |
|
| 42 |
CHARSET |
2066 |
| 43 |
RSP, Telnet Remote Serial Port |
|
| 44 |
Com Port Control |
2217 |
| 45 |
Telnet Suppress Local Echo |
|
| 46 |
Telnet Start TLS |
|
| 47 |
KERMIT |
2840 |
| 48 |
SEND-URL |
|
| 49 |
FORWARD_X |
|
50
-
137 |
|
|
| 138 |
TELOPT PRAGMA LOGON |
|
| 139 |
TELOPT SSPI LOGON |
|
| 140 |
TELOPT PRAGMA HEARTBEAT |
|
141
-
254 |
|
|
| 255 |
Extended-Options-List |
RFC 861 |
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.
|
| 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 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 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 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.
Telnet Negotiable Options
Many of those listed are self-evident, but some call
for more comments.
- 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.
- echo
The echo option is enabled, usually by the server,
to indicate that the server will echo 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".
- linemode
This option replaces and supersedes the line
mode kludge.
- remote flow control
This option controls where the special flow control
effects of Ctrl-S/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.
- Interrupt Process
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.
- Abort Output
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 to confirm the operation of the
link and the remote process. An AYT (246) character
is incorporated in the data stream.
- Erase character
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.
- Erase line
Causes the deletion of the current line of input.
An EL (248) character is incorporated in the data
stream.
- Data Mark
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 overcome 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 synch mechanism.
A DM not in a TCP Urgent segment has no effect.
For more information, see the
Telnet Protocol Revisited
For the server to find out if a client supports the
telnet
protocol at all, one good approach is to a simple,
commonly
used telnet command to the client. If the client reacts
conform to the protocol (or sends telnet commands
itself), the
server can continue to negotiate further options.
If the client
does not react, the server can safely refrain from
further
negotiations.
The following list is a more or less comprehensive
overview of
the telnet related RFCs (available for example on
https://www.faqs.org/rfcs):
RFC Titel rel. Code
495 TELNET Protocol Specification
513 Comments on the new TELNET specifications
559 Comments on the new TELNET Protocol and its Implem
595 Some Thoughts in Defense of the TELNET Go-Ahead
596 Second Thoughts on Telnet Go-Ahead
652 Telnet Output Carriage-Return Disposition Option
NAOCRD 10
653 Telnet Output Horizontal Tabstops Option NAOHTS
11
654 Telnet Output Horizontal Tab Disposition Option
NAOHTD 12
655 Telnet Output Formfeed Disposition Option NAOFFD
13
656 Telnet Output Vertical Tabstops Option NAOVTS
14
657 Telnet Output Vertical Tab Disposition Option
NAOVTD 15
658 Telnet Output Linefeed Disposition NAOLFD 16
698 Telnet Extended Ascii Option X-ASCII 17
727 Telnet Logout Option LOGOUT 18
728 A Minor Pitfall in the Telnet Protocol
735 Revised TELNET Byte Macro Option BM 19
749 Telnet SUPDUP-OUTPUT Option SUPDUP 22
764 Telnet Protocol Specification
779 Telnet SEND-LOCATION Option SENDLOC 23
818 The Remote User Telnet Service
854 Telnet Protocol Specification
855 Telnet Option Specifications
856 Telnet Binary Transmission BINARY 0
857 Telnet Echo Option ECHO 1
858 Telnet Suppress Go Ahead Option SGA 3
859 Telnet Status Option STATUS 5
860 Telnet Timing Mark Option TM 6
861 Telnet Extended Options - List Option EXOPL 255
884 Telnet Terminal Type Option TTYPE 24
885 Telnet End of Record Option EOR 25
930 Telnet Terminal Type Option TTYPE 24
933 Output Marking Telnet Option OUTMRK 27
946 Telnet Terminal Location Number Option TTYLOC
28
1043 Telnet Data Entry Terminal Option DODIIS Implement
DET 20
1053 Telnet X.3 PAD Option X.3-PAD 30
1073 Telnet Window Size Option NAWS 31
1079 Telnet Terminal Speed Option TSPEED 32
1080 Telnet Remote Flow Control Option FLOWCTRL 33
1091 Telnet Terminal-Type Option TTYPE 24
1096 Telnet X Display Location Option XDISPLOC 35
1116 Telnet Linemode Option LINEMODE 34
1143 The Q Method of Implementing TELNET Option Negotia
1184 Telnet Linemode Option LINEMODE 34
1372 Telnet Remote Flow Control Option FLOWCTRL 33
1408 Telnet Environment Option ENVIRON 36
1571 Telnet Environment Option Interoperability Issues
1572 Telnet Environment Option NEWENV 39
2066 Telnet Charset Option CHARSET 42
2217 Telnet Com Port Control Option COMPORT 44
2877 5250 Telnet Enhancements
All negotiations start with the special character
IAC which is
defined in /usr/include/arpa/telnet.h (or in
src/driver/telnet.h for 3.2(.1)) and has the decimal
value of
255. Negotiations are based on different telnetoptions
(their
values are defined in telnet.h too). Before a negotiation
can
start the client and the server have to agree that
they
support the option.
This works in the following way:
If a client wants to send something to the server
it has to
send 'IAC WILL option' (For terminaltype negotation
this would
be the 3 bytes 255,251,24; again, check telnet.h)
to confirm
that it is able to do that. If the server is supporting
that
option and wants to receive something it sends 'IAC
DO option'
(255,253,option)
If one side is receiving an 'IAC WILL option' and
has not yet
sent with DO or DONT it has to respond with either
'IAC DO
option' if it will support this negotiation or 'IAC
DONT
option' if it won't.
If one side is receiving an 'IAC DO option' and
has not yet
sent a WILL or WONT it has to reply with either 'IAC
WILL
option' if it supports the option or 'IAC WONT option'
if not.
A small example: Lets assume we want to negotiating
terminaltype. (TELOPT_TTYPE with value 24). client
is the
telnet executable on the playerside, the server is
the
gamedriver.
client server
IAC WILL TTYPE
IAC DO TTYPE
Or:
IAC DO TTYPE
IAC WILL TTYPE
After this we are ready to transfer the terminaltype
from the
client to the server as explained below.
Now we are ready to start the real negotiations.
I explain the
3 options I have currently implemented.
First TerminalType aka TTYPE aka 24 aka TELOPT_TTYPE
assuming
the client and the server have exchanged WILL/DO.
The server is now free to send 'IAC SB TELOPT_TTYPE
TELQUAL_SEND IAC SE' which will be replied with 'IAC
SB
TELOPT_TTYPE TELQUAL_IS terminaltype IAC SE' where
terminaltype is a non-zero terminated string (it's
terminated
by the IAC) (For values look up telnet.h) AND switch
the
client's terminalemulation to 'terminaltype'. terminaltype
is
case-insensitive. terminal-type may be UNKNOWN. The
server may
repeat the SEND request and the client will respond
with the
next preferred terminaltype. If this is the same as
the
previous received, it marks the end of the list of
terminaltypes. The next SEND request will start the
terminaltypes from the beginning.
Example: (we have exchanged WILL/DO already)
client server
IAC SB TTYPE SEND IAC SE
IAC SB TTYPE IS VT200 IAC SE
IAC SB TTYPE SEND IAC SE
IAC SB TTYPE IS VT100 IAC SE
IAC SB TTYPE SEND IAC SE
IAC SB TTYPE IS VT52 IAC SE
IAC SB TTYPE SEND IAC SE
IAC SB TTYPE IS VT52 IAC SE
/* this marks that we have all terminaltypes. We decide
to use the
* vt200 mode so we have to skip to VT200
*/
IAC SB TTYPE SEND IAC SE
IAC SB TTYPE IS VT200 IAC SE
Next important option is NAWS (31) or WindowSizeNegotiation.
This one is a bit easier than terminaltype. After
having
received a IAC DO NAWS from the server, the client
will reply
with IAC WILL NAWS and immediately after that send
IAC SB NAWS
columns_high columns_low lines_high lines_low IAC
SE where
xx_low refers to the lowbyte of xx and xx_high refers
to the
highbyte of xx. This will be automagically resent
at every
windowresize (when the client gets a SIGWINCH for
example) or
at your request with 'IAC SB NAWS SEND IAC SE'.
Example: (WILL/DO exchanged)
client server
IAC SB NAWS 0 80 0 24 IAC SE /* the standard vt100
windowsize */
/* no reply */
And, a bit less important but most complex, the
LINEMODE (34)
option. It was implemented it due to the fact, that
some weird DOS telnets would not work otherwise. Implemented
are only the absolute basic feature, which is the
actual
switching the telnet to linemode. After exchanging
WILL/DO the
server sends a modechange request to the client using
IAC SB
LINEMODE LM_MODE MODE_EDIT IAC SE, which should turn
on local
commandline-editing for the client. If a client supports
LINEMODE it HAS to support this modechange. The client
will
reply with IAC SB LINEMODE LM_MODE MODE_EDIT|MODE_ACK
IAC SE
(x|y is bitwise or). Thats it for linemode. (You will
perhaps
receive other IAC SB LINEMODEs with other LM_xxx ...
you may
ignore them. (At least IRIX 5.x sends IAC SB LINEMODE
LM_SLC
.... IAC SE which declares the local characterset.)).
Example: (WILL/DO negotiated)
client server
IAC SB LINEMODE LM_MODE
MODE_EDIT IAC SE
IAC SB LINEMODE LM_MODE
MODE_EDIT|MODE_ACK IAC SE
Further details on linemode can be found in RFC
1184.
