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Re: New package: suite3270-3.2.20-1

On Tue, 18 Nov 2003, Corinna Vinschen wrote:

> On Mon, Nov 17, 2003 at 02:51:15PM -0800, Peter A. Castro wrote:
> > After getting a successful login I did the following:
> > doctor@garfunkel[501] ~ $ od -c
> > <ret>
> > <ret>
> > <ret>
> > <ctrl-j>
> > <ctrl-j>
> > <ctrl-j>
> > <ctrl-d>
> > 0000000  \0  \n  \0  \n  \0  \n  \0  \n  \n  \n
> > 0000012
> >
> > Notice the \0's ?  From above, if the crlf option is turned off, the
> > client sends 0x0d,0x00.  It appears the telnet daemon is forwarding on
> > the 0x00 as part of the data stream.  According to RFC 854, a <cr><nul>
> > combination is supposed to be treated as a straight carriage-return, so
> > the daemon is supposed to turn that pair of characters into a single <cr>
> > character.  You can run c3270 in trace mode ("c3270 -trace <hostname>"
> Hmm, I had a look into telnetd and it actually does conform to RFC 854,
> AFAICS.  There's a function called "telrcv()", which apparently cares
> for receiving characters from the client side.  There's this stuff in
> the code:
> 	case TS_CR:
> 		state = TS_DATA;
> 		/* Strip off \n or \0 after a \r */
> 		if ((c == 0) || (c == '\n')) {
> 			break;
> 		}
> 		/* FALL THROUGH */
> 	case TS_DATA:
> 		[...]
> As it turns out, telnetd is operating in so-called "LINEMODE".  An
> explanation of linemode is given in telnetd/termstat.c though it's
> not entirely clear to me how it works, admittedly.  Linemode seems
> to speed up the connection at least.

The idea behind line-mode is that the client is suppose to do local
editing of the input and only transmit data when enter or some other
flush/sync character is received as input.

The other mode is typically called "character-at-a-time" or "kludge
linemode" in which all characters are sent, real time, and the client
does no editing or buffering.

> Anyway, if linemode is enabled (which is negotiated on server startup),
> telnetd never calls the above code, since a few lines below it turns a
> \r into a \n.  When compiling telnetd without -DLINEMODE, that telnetd
> is able to operate with c3270 as expected.  It's also able to work
> fine with a Linux and the Cygwin telnet client.

What's interest to me is that a c3270 trace shows that the Cygwin telnetd
is sending "DO LINEMODE" but c3270 is responding with "WONT LINEMODE"
which is supposed to mean that telnetd should not assume LINEMODE
semantics.  A normal telnet client (using the "toggle options" command)
flow often shows "DO LINEMODE", intially, but after some additional
options are exchanged, later flows a "DONT LINEMODE".  I haven't studied
the BSD code in detail, and so can't really say what's going on.

Alternatively, with other telnetd's (ie: Linux, AIX, Solaris) the telnet
flow is often "DONT LINEMODE" initially.  I tried starting the telnetd
with the -k option, but to no avail.

> On the other hand, telnetd on BSD systems is by default also build
> with linemode on!  So it's not quite clear to me what to do or if I
> should do anything at all.

I think this is just one of those odd peculiarities of telnet protocol
implementation and not something you need to address.  Again, c3270 isn't
really ment to be used as a normal NVT client anyways.

> Corinna

Peter A. Castro <> or <>
	"Cats are just autistic Dogs" -- Dr. Tony Attwood

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