Question on SSH_ASKPASS

[Jim Knoble]

On 2009-12-23 17:40, Jan Alphenaar wrote:

: setsid ssh -T jan@<dest_host> "/<DEFANGED_script_dir>/<DEFANGED_script>"
: 
: does work as expected, it launches the remote script.
: 
: I know ssh should support this out of the box, so I want to get rid of
: setsid.

Hi, Jan.

You are correct in that using 'setsid' works as expected.  The reason
is here, to quote the ssh(1) man page:

    If ssh needs a passphrase, it will read the passphrase from the
    current terminal if it was run from a terminal.  If ssh does not
    have a terminal associated with it but DISPLAY and SSH_ASKPASS are
    set, it will execute the program specified by SSH_ASKPASS and open
    an X11 window to read the passphrase.  This is particularly useful
    when calling ssh from a .xsession or related script.  (Note that
    on some machines it may be necessary to redirect the input from
    /dev/null to make this work.)

The problem is that, on many (most?) systems, to disassociate ssh from
its controlling terminal, you need to put it into a new "session".
Redirecting stdin from /dev/null ('ssh ... </dev/null') is not enough.

The setsid command you are using is pretty much the only way to do that;
it's pretty much a wrapper around the setsid(2) system call, which then
execs the command given on the command line (probably via execvp(3)).
This is a feature of POSIX (see
http://www.opengroup.org/onlinepubs/009695399/functions/setsid.html ).

Hope this helps.

--jim

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jim knoble  |  jmknoble at pobox.com  |  http://www.pobox.com/~jmknoble/
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