TOPS-20 Telnet and Port Forwading issue

Seth Morabito lists+cctalk at
Mon Dec 12 12:04:15 CST 2016

Hi folks,

I don't know if this is the best place to ask this question, but I don't
know of any better forums for PDP-10 discussion, so hopefully it's
on-topic enough.

I recently set up the KLH10 PDP-10 emulator on my network, running
TOPS-20 7.1. It's on a box in my network. TCP/IP works
great, I can telnet to it from within my network without any issue.

I also set up my firewall (a box running CentOS 7.1) to port forward
from external TCP port 2320 to internal TCP port 23 on the KLH10 box.

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! I can fix it with telnet escape (^]) by setting
character mode explicitly with "mode character", but that's kind of
annoying to do each time I connect.

It feels like Telnet is no longer doing line mode / character mode
negotiation when I'm port forwarding. It's most mysterious.

If you want to see for yourself, you can telnet to 2320, which is the KLH10 instance.

Does this ring any bells for anyone?

Seth Morabito
seth at

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