TOPS-20 Telnet and Port Forwading issue

Peter Coghlan cctalk at
Tue Dec 13 10:56:27 CST 2016

> > Interestingly, when I telnet from _outside_ the network to my
> > firewall's port 2320, it works, but Telnet goes into line mode rather
> > than character mode!
> Probably, as I think someone said, telnet noticing it's not using the
> standard telnet port and not even trying to go character-at-a-time.
> I did a telnet myself, snooping the network traffic, and...
> me -> you SYN
> you -> me SYN|ACK
> me -> you ACK
> you -> me data: IAC WILL ECHO
> me -> you data: IAC DO ECHO
> you -> me data: \r\n DECTEN, PANDA TOPS-20...
> Nowhere, in particular, is there any attempt at LINEMODE negotiation.
> And, when I type ?, nothing is sent until I press RETURN, upon which my
> end sends ?\r\n.

I tried doing a similar test to Mouse for comparison given that my telnet
client seems to be producing the desired behaviour (whether by accident or
by design I am not sure).  Here are the results and my interpretation of
them which may not be correct as I am a bit rusty on this stuff:

me -> you SYN
you -> me SYN|ACK
me -> you ACK
you -> me data: fffb 0100 0000 (IAC WILL ECHO)
me -> you data: fffd 03ff fd01 (IAC DO SUPPRESS GO AHEAD, IAC DO ECHO)
you -> me data: fffb 0300 0000 (IAC WILL SUPPRESS GO AHEAD)
me -> you ACK
you -> me data: \r\n DECTEN, PANDA TOPS-20...

Peter Coghlan.

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