TELNET Commands and Options
The TELNET protocol has different commands for interaction between TELNET client and server. Normally, these commands are not visible to the end user. TELNET commands and options are exchanged between TELNET client and TELNET server software. TELNET commands are sent with data stream. TELNET commands are identified by the use of special command IAC (Interpret as Command), with a decimal value of 255 (0xFF - in hexadecimals).
Check the following table to know about different commands used in TELNET.
| Command Name | Decimal Code | Description |
|---|---|---|
| SE | 240 | End of subnegotiation parameters. |
| NOP | 241 | No operation. |
| DM | 242 | Data Mark (part of the Synch function). Indicates the position of a Synch event within the data stream. |
| BRK | 243 | NYT character break. |
| 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? AYT is used to determine if the remote TELNET partner is still up and running. |
| EC | 247 | Erase character. Erase character is used to indicate 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. Go ahead is used in half-duplex mode to indicate the other end that it can transmit. |
| SB | 250 | Begin of subnegotiation |
| WILL | 251 | The sender wants to enable an option |
| WON’T | 252 | The sender do not wants to enable an option. |
| DO | 253 | Sender asks receiver to enable an option. |
| DONT | 254 | Sender asks receiver not to enable an option. |
| IAC | 255 | IAC (Interpret as Command) |
Telnet uses different options to perform negotiation between TELNET client and TELNET server. Some important TELNET options are listed in below table.
| Decimal Code | Option |
|---|---|
| 1 | Echo |
| 3 | Suppress Go Ahead |
| 5 | Status |
| 6 | Timing Mark |
| 8 | Output Line Width |
| 9 | Output Page Size |
| 10 | Output Carriage Return Disposition |
| 11 | Output Horizontal Tabstops |
| 12 | Output Horizontal Tab Disposition |
| 14 | Output Horizontal Tabstops |
| 15 | Output Vertical Tab Disposition |
| 18 | Logout |
| 24 | Terminal Type |
| 25 | End of Record |
| 26 | User Identification |
| 31 | Window Size |
| 32 | Terminal Speed |
| 33 | Remote Flow Control |
| 34 | Linemode |
| 35 | X Display Location |
| 36 | Environment Variables |
| 39 | Telnet Environment Option |