The s_client binary does not implement all of the terminal controls in real telnet.
If you really want to use true telnet over TLS on the registered port of 992, then get a regular telnet client and telnet to a custom port on local host.
On that custom port, run stunnel in client mode. You can run this either in an inetd/socket activation setup, or in daemon mode that actively calls listen()
That stunnel should be configured to point at your telnet-ssl server (on port 992 if you want it accurately fingerprinted).
That telnet-ssl server should also be stunnel, and it should be configured to run telnetd in inetd mode.
That is the correct way to implement telnet-ssl. I would not trust any telnet client or server that was linked directly to a TLS library.
If you really want to use true telnet over TLS on the registered port of 992, then get a regular telnet client and telnet to a custom port on local host.
On that custom port, run stunnel in client mode. You can run this either in an inetd/socket activation setup, or in daemon mode that actively calls listen()
That stunnel should be configured to point at your telnet-ssl server (on port 992 if you want it accurately fingerprinted).
That telnet-ssl server should also be stunnel, and it should be configured to run telnetd in inetd mode.
That is the correct way to implement telnet-ssl. I would not trust any telnet client or server that was linked directly to a TLS library.