TOPS-20 Telnet and Port Forwading issue

Warner Losh imp at
Tue Dec 13 00:15:54 CST 2016

On Mon, Dec 12, 2016 at 11:10 PM, Warner Losh <imp at> wrote:
> 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.

For a recent telnet in FreeBSD, I had to do a 'mode linemode' before
it would behave like you are describing. Connect, hit return, hit the
escape character (traditional one is ^]). mode linemode <return>. then
I saw local echo and things looked approximately like I'd expect.


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