Re: Ideas for Windows Ncat: non-blocking read from stdin

[David Fifield <david () bamsoftware com>]

On Sun, Mar 01, 2009 at 07:35:22PM -0700, David Fifield wrote:
> I saw this in the TODO and I want to share some ideas about it.
>
> o Ncat Windows issue where you make a connection and then take several
>   seconds to type in a line to the server, Ncat wrongly times out when
>   trying to write your line to the remote server. [David]
>
> I checked this out over the weekend. This is what I wrote to Fyodor:
>
> > I found the source of the problem. It is related to the fselect call.
> > When reading from a TTY, fselect says that stdin is readable when a key
> > is hit:
> >         if (stdtty) {
> >             if (_kbhit()) {
> >                 FD_SET(STDIN_FILENO, &rset);
> >                 fds_ready++;
> >             }
> >         }
> > So Nsock a read on stdin, expecting it to return immediately. But the
> > input is still being buffered by the console until you hit enter,
> > turning the read into a blocking read. When you finish typing, the read
> > finishes, and Ncat schedules a write. No time is supposed to have gone
> > by during the read, so nsock_tod (time of day) is not updated, and the
> > write timeout is set to two seconds after you *started* typing. If it
> > takes you more than two seconds to type, the write is timed out as soon
> > as it is created.
>
> This is my current idea: create a thread (with CreateThread) that does
> nothing but read stdin and write to an anonymous pipe (created with
> CreatePipe). Duplicate the pipe's handle in the main program so that
> when Ncat thinks it's reading from stdin, it's reading from the pipe.
> The thread takes care of line buffering and echoing, and ensures that
> the main process won't block when it reads from the pipe.

I did all this in r12437. It took a long day of reading MSDN docs but
the technique is easy to understand. There's a long comment at the top
of nbase/nbase_winunix.c with technical details for anyone interested.

I retained Kris's looping fselect implementation. I didn't find a way to
wait on a pipe until input is available; the best I found was checking
and returning instantly with the PeekNamedPipe function.

If you wondered why the simple example at
http://nmap.org/ncat/guide/ncat-usage.html#ncat-connect didn't work on
Windows, it was because of this issue. Those examples should work now.

David Fifield

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