using cmd.exe as a telnet client

[Donnal Walter]

Several months ago I tried using the telnet module (on Windows XP) to
communicate with a proprietary host on our network. This was
unsuccessful due to problems with "option negotiation", and I gave up on
the project for a while. I still have need for this, however, so I
recently started thinking about alternatives. I suppose I could dig deep
enough into option negotiation to use the socket module (with telnet as
a guide), but I am hoping to find a way to use fewer synapses.

Using the Windows C:> prompt (cmd.exe) I can telnet into this host with
no problems (the Windows telnet client performs option negotiation just
fine). Is there a straightforward way of using os.popen() (or os.fork()
or os.exec*() or os.wait*()) to connect to the host from Python via the
Windows telnet client?

Thanks,
Donnal Walter
Arkansas Children's Hospital

 

[Grant Edwards]

Several months ago I tried using the telnet module (on Windows XP) to
communicate with a proprietary host on our network. This was
unsuccessful due to problems with "option negotiation", and I gave up on
the project for a while. I still have need for this, however, so I
recently started thinking about alternatives. I suppose I could dig deep
enough into option negotiation to use the socket module (with telnet as
a guide), but I am hoping to find a way to use fewer synapses.

You don't have to start from scratch. The telnet module has
hooks built-into it1 so that you can have it call your routines
to handle option negotiation. I did it once to impliment some
extra Telnet protocol features, and it wasn't difficult.

 

[Donnal Walter]

You don't have to start from scratch. The telnet module has
hooks built-into it1 so that you can have it call your routines
to handle option negotiation. I did it once to impliment some
extra Telnet protocol features, and it wasn't difficult.

Ok, you've inspired me to give it a try. First, I am assuming that you
mean writing a custom callback function to send to
Telnet.set_option_negotiation_callback(callback). Or did you mean
writing a subclass of Telnet? Can someone provide an example of a
callback function that I might use as a template for writing mine?

This is unfamiliar territory for me, but I tried to snoop a bit using
ethereal (also new to me) and as near as I can tell, the options in
question are:
0, binary transmission
1, echo
3, suppress go ahead
23, send location

And it may be that not all of these are critical, but if so, I don't
know how to tell which is which.

I think I have a much simpler solution for you guys..
Assuming you can run arbitrary code on the proprietary
server.

Actually I cannot. The server runs a second-party information system,
the primary access to which is third-party terminal emulator software
that uses telnet connections. What I am trying to do is emulate this
emulator.

cmd.exe is not a command line program. It's a terminal (emulator).
You might be able to use the telnet program, though.

Yes, I see that now. Thanks.

Before doing this,
I'd recommend looking at Twisted's telnet support (both
the version in 1.3 and the version that will be in 2.0),
which actually supports option negotiation in a
reasonable way.

I've been wanting to get acquainted with Twisted for awhile now, so this
might be a good time to do so. I think I will give Grant's idea a whirl,
but if I get bogged down there, I will definitely look at Twisted's
telnet support. BTW, do you know if Twisted's option negotiation uses a
callback function? I might download it just to take a look, even if I
don't use it directly.

Thanks,
Donnal Walter
Arkansas Children's Hospital

 

[Donnal Walter]

> You don't have to start from scratch. The telnet module has
> hooks built-into it1 so that you can have it call your routines
> to handle option negotiation. I did it once to impliment some
> extra Telnet protocol features, and it wasn't difficult.

Ok, you've inspired me to give it a try. First, I am assuming that you
mean writing a custom callback function to send to
Telnet.set_option_negotiation_callback(callback). Or did you mean
writing a subclass of Telnet? Can someone provide an example of a
callback function that I might use as a template for writing mine?

This is unfamiliar territory for me, but as I can tell, the options in
question are:
0, binary transmission
1, echo
3, suppress go ahead
23, send location

And it may be that not all of these are critical, but if so, I don't
know how to tell which is which.

> I think I have a much simpler solution for you guys..
> Assuming you can run arbitrary code on the proprietary
> server.

Actually I cannot. The server runs a second-party information system,
the primary access to which is third-party terminal emulator software
that uses telnet connections. What I am trying to do is emulate this
emulator.

> cmd.exe is not a command line program. It's a terminal (emulator).
> You might be able to use the telnet program, though.

Yes, I see that now. Thanks.

> Before doing this,
> I'd recommend looking at Twisted's telnet support (both
> the version in 1.3 and the version that will be in 2.0),
> which actually supports option negotiation in a
> reasonable way.

I've been wanting to get acquainted with Twisted for awhile now, so this
might be a good time to do so. I think I will give Grant's idea a whirl,
but if I get bogged down there, I will definitely look at Twisted's
telnet support. BTW, do you know if Twisted's option negotiation uses a
callback function? I might download it just to take a look, even if I
don't use it directly.

Thanks,
Donnal Walter
Arkansas Children's Hospital

 

[Donnal Walter]

> You don't have to start from scratch. The telnet module has
> hooks built-into it1 so that you can have it call your routines
> to handle option negotiation. I did it once to impliment some
> extra Telnet protocol features, and it wasn't difficult.

Ok, you've inspired me to give it a try. First, I am assuming that you
mean writing a custom callback function to send to
Telnet.set_option_negotiation_callback(callback). Or did you mean
writing a subclass of Telnet? Can someone provide an example of a
callback function that I might use as a template for writing mine?

This is unfamiliar territory for me, but as near as I can tell, the
options in question are:
0, binary transmission
1, echo
3, suppress go ahead
23, send location

And it may be that not all of these are critical, but if so, I don't
know how to tell which is which.

> I think I have a much simpler solution for you guys..
> Assuming you can run arbitrary code on the proprietary
> server.

Actually I cannot. The server runs a second-party information system,
the primary access to which is third-party terminal emulator software
that uses telnet connections. What I am trying to do is emulate this
emulator.

> cmd.exe is not a command line program. It's a terminal (emulator).
> You might be able to use the telnet program, though.

Yes, I see that now. Thanks.

> Before doing this,
> I'd recommend looking at Twisted's telnet support (both
> the version in 1.3 and the version that will be in 2.0),
> which actually supports option negotiation in a
> reasonable way.

I've been wanting to get acquainted with Twisted for awhile now, so this
might be a good time to do so. I think I will give Grant's idea a whirl,
but if I get bogged down there, I will definitely look at Twisted's
telnet support. BTW, do you know if Twisted's option negotiation uses a
callback function? I might download it just to take a look, even if I
don't use it directly.

Thanks,
Donnal Walter
Arkansas Children's Hospital

 

[Grant Edwards]

Ok, you've inspired me to give it a try. First, I am assuming
that you mean writing a custom callback function to send to
Telnet.set_option_negotiation_callback(callback).
Yes.

Or did you mean writing a subclass of Telnet?

That depends. If you need to _impliment_ a feature in addition
to doing the negotiation, then you may want to define a
subclass -- though it may easier to just grab the source for
telnetlib and add the feature.

Can someone provide an example of a callback function that I
might use as a template for writing mine?

I've got a negotiation function at home I'll try to remember to
post.

This is unfamiliar territory for me, but as near as I can tell, the
options in question are:
0, binary transmission
1, echo
3, suppress go ahead
23, send location

And it may be that not all of these are critical, but if so, I
don't know how to tell which is which.

That's the tricky part.

I guess I'd "enable" negotiation of each of the features one at
a time, and see what happens.

 

[Donnal Walter]

I've been wanting to get acquainted with Twisted for awhile
now, ... BTW, do you know if Twisted's option negotiation
uses a callback function? I might download it to take a look, ...

Sorry I did not do this earlier, before I posted. It is obvious (now
that I have downloaded it) that Twisted's Telnet implementation is based
on its own Protocol base class (nice design). The only documentation I
have been able to find, however, is in the protocols/telnet module
itself. It is still not immediately obvious (to me) how to use the iac*
methods to negotiate these options. Any help is much appreciated.

(I've not given up on writing my own funtion for Python's telnetlib, but
I'm trying to keep all my options open. Besides, I will probably have
other uses for Twisted later.)

Donnal Walter
Arkansas Children's Hospital