TOPS-20 Telnet and Port Forwading issue

Warner Losh imp at
Tue Dec 13 00:10:57 CST 2016

On Mon, Dec 12, 2016 at 11:04 AM, Seth Morabito
<lists+cctalk at> wrote:
> 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?

I'm guessing that this is the classic "Many telnet clients default to
line mode for only for port 23." issue.


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