TOPS-20 Telnet and Port Forwading issue

Peter Coghlan cctalk at
Tue Dec 13 06:02:51 CST 2016

On Mon, 12 Dec 2016 23:10:57 -0700, Warner Losh 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.
> Warner

I've tried connecting using the Multinet telnet client under VMS.  It seems to
work fine.  However, this telnet client does not seem to have a way to manually
set line mode versus character mode so I don't seem to have a way of simulating
the problem to ensure that I am not seeing it.

Typing a question mark on it's own without pressing return results in a list
of commands being displayed so I think I am in character mode and this is the
expected behaviour.

I would agree with what Warner said regarding many telnet clients, except
maybe he meant they default to line mode for ports other than port 23.  You
could test the theory by temporarily port forwarding external port 23 to the
emulator.  Bear in mind that you will probably get lots of botnets trying to
break in on that port while you are testing.

Another possibility is that if the firewall is trying to be clever, it might
be intercepting the telnet negotiations instead of just doing straightforward
port forwarding.  I used to come across a lot of commercial firewall products
generally sticking their nose in where it wasn't wanted and making a mess (of
SMTP for example) but it doesn't sound like you are using one of those devices.

Peter Coghlan

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