Re: Hangs on password queries once SSH session is up

[Pierre Lewis <leware@xxxxxxxxxxxxxxxx>]

On Mon, Jun 21, 2010 at 02:57:09PM +0200, Aris Adamantiadis wrote:

> What happened here is that usually openssh sends empty messages
> (SSH_MSG_IGNORE) packets when you're writing a password or something in
> a echoless terminal. The goal is to protect against packet counting to
> guess the size of your password.

Good, this makes sense. I think the time has come to read the RFC!

Does the SSH server have code to identify such situations? I have
an editor which for some function, does not echo anything, yet
this does not hang, even if I type quite a bit. So does the
server keep an eye on output, looking for the string "password"
followed by echo off? Or is there some specific function used by
all these programs to query for passwords?

Btw, there is an easy way to unblock such a hung session. From
another session on same remote host: just type something like

   date > /dev/pty/nn

Where nn corresponds to the hung session. This recovers it.

> Now, this had an impact on your program because of a bug in the polling
> mechanism. Since a packet was available on the socket, you detected it
> and called ssh_poll. Libssh then processed the ignore packet and
> launched yet another poll which returned "data available" instead of
> "nothing available" (due to a bug).

Yesterday I ran some tests with client + 0.4.4.90 and I still see
the problem, but much less frequently than with 0.4.2. Now that I
know a bit more about what the problem might be, I'll add some
tracing to the libssh code and try to get some good run-time
traces again.

Cheers,
Pierre