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Telnet Protocol Commands
(Page 3 of 3)
Telnet Protocol Command Codes
Table 281
lists the Telnet protocol commands in numerical byte value order, showing
for each its command code and name and describing its meaning and use.
Table 281: Telnet Protocol Commands
Command
Byte Value (Decimal)
|
Command
Code
|
Command
|
Description
|
240
|
SE
|
Subnegotiation
End
|
Marks the end of a Telnet option
subnegotiation, used with the SB code to specify more specific
option parameters. See
the topic on Telnet options for details.
|
241
|
NOP
|
No
Operation
|
Null command;
does nothing.
|
242
|
DM
|
Data Mark
|
Used to mark the end of a sequence
of data that the recipient should scan for urgent Telnet commands. See
the topic on Telnet interrupt handling for more information.
|
243
|
BRK
|
Break
|
Represents
the pressing of the break or attention key on
the terminal.
|
244
|
IP
|
Interrupt
Process
|
Tells the recipient to interrupt,
abort, suspend or terminate the process currently in use.
|
245
|
AO
|
Abort
Output
|
Instructs the
remote host to continue running the current process, but discard all
remaining output from it. This may be needed if a program starts to
send unexpectedly large amounts of data to the user.
|
246
|
AYT
|
Are You There
|
May be used to check that the
remote host is still alive. When this character is sent
the remote host returns some type of output to indicate that it is still
functioning.
|
247
|
EC
|
Erase
Character
|
Instructs the
recipient to delete the last undeleted character from the data stream.
Used to undo the sending of a character.
|
248
|
EL
|
Erase Line
|
Tells the recipient to delete
all characters from the data stream back to (but not including) the
last end of line (CR+LF) sequence.
|
249
|
GA
|
Go
Ahead
|
Used in Telnet
half-duplex mode to signal the other device that it may transmit.
|
250
|
SB
|
Subnegotiation
|
Marks the beginning of a Telnet
option subnegotiation, used when an option requires the client and server
to exchange parameters. See
the topic on Telnet options for a full description.
|
251
|
WILL
|
Will
Perform
|
In Telnet
option negotiation, indicates that the
device sending this code is willing to perform or continue performing
a particular option.
|
252
|
WONT
|
Wont
Perform
|
In Telnet
option negotiation, indicates that the
device sending this code is either not willing to perform a particular
option, or is now refusing to continue to perform it.
|
253
|
DO
|
Do
Perform
|
In Telnet
option negotiation, requests that the
other device perform a particular option or confirms the expectation
that the other device will perform that option.
|
254
|
DONT
|
Dont
Perform
|
In Telnet
option negotiation, specifies that the
other party not perform an option, or confirms a devices expectation
that the other party not perform an option.
|
255
|
IAC
|
Interpret
As Command
|
Precedes command
values 240 through 254 as described above. A pair of IAC bytes in a
row represents the data value 255.
|
Perhaps ironically, the
Telnet command are not used as much today as they were when Telnet was
in its early days, because many of the compatibility issues that we
discussed earlier no longer exist. ASCII has become the standard character
set of the computing world, so many of the functions such as aborting
output or interrupting a process no longer require the use of Telnet
commands. They are still widely used, however, for internal Telnet operations
such as option negotiation.
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Version 3.0 - Version Date: September 20, 2005
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