Telnet Integration

Telling Telnet clients to behave so the prompt renders cleanly

By default, Telnet leaves each client to decide whether to echo characters locally, whether to wait for a GO AHEAD before transmitting, and how line endings are encoded. None of those defaults are compatible with a CLI that expects character-at-a-time input and renders its own prompt. A short option negotiation, sent once per new connection, is sufficient to put the client into a usable mode.

Option Negotiation

const unsigned char options[] = {
    255, 253, 1,    /* IAC DO ECHO - request that the client echo received characters */
    255, 251, 1,    /* IAC WILL ECHO - server will echo received characters */
    255, 253, 3,    /* IAC DO SUPPRESS-GO-AHEAD - request that the client not send GO AHEAD before transmitting */
    255, 251, 3,    /* IAC WILL SUPPRESS-GO-AHEAD - server will not send GO AHEAD either */
};
send(client_fd, options, sizeof(options), 0);

IAC is the byte 255. DO is 253, meaning “the other side should perform this option”. WILL is 251, meaning “this side will perform this option”. ECHO is option 1 and SUPPRESS-GO-AHEAD is option 3.

The combined effect is that the client (e.g. PuTTY) echoes the user’s typed characters (which is what TinyCLI assumes) and that neither side waits for a GO AHEAD before transmitting. The latter is a line-oriented Telnet behavior that would otherwise break character-at-a-time input.

Line Endings

A line is terminated by a carriage return (\r), a line feed (\n), or a NUL (\0). The two-byte sequences \r\n, \r\0, and \n\r are each treated as a single terminator: the second byte is consumed rather than starting a new (empty) line. Telnet clients differ in which of these they transmit, but all of the above are handled, so no additional handling on the application side is required.

Per-Connection Flow

A typical sequence per accepted connection:

  1. Send the option-negotiation bytes shown above.
  2. Optionally, send a banner, version string, or pre-login prompt: anything that should appear before the first interactive prompt.
  3. Call tclie_init (or reuse a long-lived instance, taking care with the embedded state).
  4. Forward each received byte into tclie_in_char. The output callback should write to the corresponding socket.
  5. On disconnect, ignore any further input. The library does not track sockets itself.

Sharing a single TinyCLI instance between multiple Telnet clients is not supported. Each tcli_t holds the line state, history, and completion context of one user. One instance is required per connection.