Telnet can simply be a TCP connection between two endpoints. But it can also be a whole lot more. There is a protocol for in-band negotiation between the endpoints on how they should behave, standards for special character sequences that may trigger special behaviour and even special characters that might also trigger their own special behaviours.

There are a multitude of clients that can be used to make telnet connections, and for the most part they work fine. However, telnet clients are much like web browsers in one sense. It is unlikely that there are any telnet clients that implement all of the possible functionality, and the functionality they do implement is not guaranteed to be implemented correctly.

Character Mode

Scandum @ MUDBytes wrote: Server needs to send IAC WILL SGA and IAC WILL ECHO (in other words, server offers to suppress go ahead (assumed enabled by default) and handle local echoing for the client), client needs to send IAC DO SGA and IAC DO ECHO - after that client side character mode is assumed to be enabled.

External Links

TELNET - Wikipedia page on the TELNET protocol.
RFC854 - Telnet protocol specification.

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