Na minha versão do telnet-ssl do Debian 0.17-41:
$ telnet
telnet> help
Commands may be abbreviated. Commands are:
[...]
set set operating parameters ('set ?' for more)
telnet> set ?
[...]
debug enable debugging
netdata enable printing of hexadecimal network data (debugging)
prettydump enable output of "netdata" to user readable format (debugging)
options enable viewing of options processing (debugging)
termdata enable (debugging) toggle printing of hexadecimal terminal data
telnet> set options
Will show option processing.
telnet> open 10.1.2.3
Trying 10.1.2.3...
Connected to 10.1.2.3.
Escape character is '^]'.
SENT WILL NEW-ENVIRON
SENT WILL AUTHENTICATION
SENT DO SUPPRESS GO AHEAD
SENT WILL TERMINAL TYPE
SENT WILL NAWS
SENT WILL TSPEED
SENT WILL LFLOW
SENT WILL LINEMODE
SENT DO STATUS
SENT WILL XDISPLOC
RCVD DO TERMINAL TYPE
RCVD DO TSPEED
RCVD DO XDISPLOC
RCVD DO NEW-ENVIRON
RCVD DONT AUTHENTICATION
[SSL not available]
RCVD WILL SUPPRESS GO AHEAD
RCVD DO NAWS
SENT IAC SB NAWS 0 80 (80) 0 24 (24)
RCVD DO LFLOW
RCVD DONT LINEMODE
RCVD WILL STATUS
RCVD IAC SB TERMINAL-SPEED SEND
SENT IAC SB TERMINAL-SPEED IS 38400,38400
RCVD IAC SB X-DISPLAY-LOCATION SEND
SENT IAC SB X-DISPLAY-LOCATION IS "mypc:0.0"
RCVD IAC SB ENVIRON SEND
SENT IAC SB ENVIRON IS VAR "DISPLAY" VALUE "mypc:0.0"
RCVD IAC SB TERMINAL-TYPE SEND
SENT IAC SB TERMINAL-TYPE IS "xterm-256color"
RCVD DO ECHO
SENT WONT ECHO
RCVD WILL ECHO
SENT DO ECHO
Debian GNU/Linux 9
stretch-amd64 login:
A opção debug (-d) não parece útil. Ele define uma opção de soquete SO_DEBUG e requer raiz.