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Hangs on password queries once SSH session is up

I wrote a telnet client for Windows some years ago, and I had always wanted
to add SSH to it. I finally got around to doing it, using libssh. Here's how
it went, with one problem below.

My first surprise was that the code required the SSL library. I guess it is
only used for its crypto code, not for the SSL protocol proper. Building it
went fine.

I had more trouble building libssh. First, I didn't have CMake, so I had to
install that.

ZLib is supposed to be optional, but I had quite a fight getting CMake to
stop asking for it. Once that was solved, CMake asked for doxygen. That was
enough for me :-). I took the problem from the other end, as I often do. I
just tried compiling the C code. That probably went faster than if had
continued installing stuff until CMake was happy. After some adjustments to
config.h, the following gave me what I needed (under Cygwin):

   gcc -c -DLIBSSH_STATIC -I../openssl-1.0.0/include/ -I/usr/include/w32api/ -Iinclude/ -Ibuild -D_WIN32  libssh/*.c
   ar rcs libssh.a *.o

Anyway, the biggest issue I had is that the library uses blocking IO. My
telnet client is non-blocking (event-driven). To keep my life simple, I
decided to access the libssh code (once the channel is up) thru (timer-based)
polling: if the socket says there is something available, I call into the
library. This works fine > 99% of the time, so I'm happy! But I would have
preferred a non-blocking API. Maybe I should look at libssh2 (but when things
work, there is little motivation).


However, I do have a problem in some scenarios. If, within an SSH session (to
Linux), I try something which requires a password (eg. su/sudo, ftp), then my
client hangs (does not return to its main event loop). The call chain at that
point is the following (the socket has said it has data to read):

    (my client)

The client is not consuming CPU, I assume it's blocked on a socket read.

Does anyone have any quick hint at why? What is special about such cases?
Echo is turned off for the password prompt, but I would have thought this is
entirely a server-side issue, why would that impact the client? Or is the ssh
protocol somehow involved (eg. using a protocol mechanism to fetch this
password, or in echo control)? One way to unblock the situation is to send
some output to the hung session from another one. Or the issue can be worked
around by pasting the password into the client (complete with LF) all at once
(so it goes out in a single channel_write()).

The bug is of course in how I use the library, not in the library itself.
I'm just hoping for a hint that might help me solve that one small issue.

Pierre Lewis

Re: Hangs on password queries once SSH session is upAris Adamantiadis <aris@xxxxxxxxxxxx>
Re: Hangs on password queries once SSH session is upchebrolu bhanuchandar <bhanuchebrolu@xxxxxxxxx>
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