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Telnet Linemode Option

Option code: 34

See also: RFC 1184

The default behavior of a telnet connection is that user input is processed (e.g. deleting a char or inserting at a different position) locally and sent when the ENTER key has been hit. Effectively the client only send complete input lines, which is why this is called line mode

Some applications may require tighter control or want to get immediately informed on keystrokes. This RFC allows finetuning how clients process user input.

warning

It is very rare that this RFC is supported by clients. Usually only pure telnet clients implement it.

Handshake​

The server demands (WILL) to enable the LINEMODE feature. If the client supports that, it answers with DO, followed by a subnegotiation that informs the server about its capabilities.

This informational subnegotiation can get quite long.

note

You can usually ignose that information. Skip over it if you are not interested.

IAC SB LINEMODE SLC 
  1   0    0 
  3  98    3 
  4   2   15 
  5   0    0 
  7  98   28 
  8   2    4 
  9  66   26 
 10   2  127 
 11   2   21 
 12   2   23 
 13   2   18 
 14   2   22 
 15   2   17 
 16   2   19 
 17   0    0 
 18   0    0

which translates to:

  • Operation SYNCH is not supported
  • Operation IP is supported and uses character 0x3 - which can be changed - [VALUE, FLUSH_IN, FLUSH_OUT]
  • Operation AO is supported and uses character 0xf - which can be changed - [VALUE]
  • Operation AYT is not supported
  • Operation ABORT is supported and uses character 0x1c - which can be changed - [VALUE, FLUSH_IN, FLUSH_OUT]
  • Operation EOF is supported and uses character 0x4 - which can be changed - [VALUE]
  • Operation SUSP is supported and uses character 0x1a - which can be changed - [VALUE, FLUSH_IN]
  • Operation EC is supported and uses character 0x7f - which can be changed - [VALUE]
  • Operation EL is supported and uses character 0x15 - which can be changed - [VALUE]
  • Operation EW is supported and uses character 0x17 - which can be changed - [VALUE]
  • Operation RP is supported and uses character 0x12 - which can be changed - [VALUE]
  • Operation LNEXT is supported and uses character 0x16 - which can be changed - [VALUE]
  • Operation XON is supported and uses character 0x11 - which can be changed - [VALUE]
  • Operation XOFF is supported and uses character 0x13 - which can be changed - [VALUE]
  • Operation FORW1 is not supported
  • Operation FORW2 is not supported

To translate that even further, it means

  • If someone presses CTRL-C, the client does not send a Telnet Interrrupt Process (IP), but an ASCII 0x03 (ETX)
  • Instead of an Telnet AO Abort Output an ASCII 0xf (SI) is sent.
  • Instead of IAC ABORT (255 238) the ASCII control code 0x1C (FS File Seperator) is sent
  • Instead of IAC EOF (255 236) the ASCII control code 0x4 (EOT End of Transmission) is sent
  • Instead of IAC SUSP (255 237) the ASCII control code 0x1a (SUB - Substitute) is sent
  • and so on. Also if you want, to can reconfigure each setting.

Entering character-a-time mode​

With the MODE command, the server can control the client behaviour. It does that by sending a bitvector for the following data

FLAGValueMeaning
EDIT0Character-a-time mode
1Line mode
TRAPSIG0Send signals as ASCII values
2Send signals as Telnet equivalents
MODE_ACK4This is an ACK for the previous command
SOFT_TAB0TAB should be send as ASCII 0x9
8TAB should be expanded into spaces
LIT_ECHO0Non-printable characters can be treated at will
16Show Non-printable characters as literals

So, if you want to enable character-a-time mode, the EDIT bit must not be set.

Example: Clearing EDIT (and other) and setting TRAPSIG flag

Switching back to line mode​

Example: Setting EDIT and TRAPSIG flag

What else​

If you are willing to experiment a bit further, you can have a look at the FORWARDMASK command. In normal linemode that client flushes its input buffer when the ENTER key is pressed - this can be changed. The FORWARDMASK gets a 32 octet (16, if your telnet connection does not transmit binary) parameter, resulting in a 256 (or 128) bit vector. Each bit in this vector represents an ASCII code that should flush the input buffer.